Spintronic terahertz emitters: Status and prospects from a materials perspective

Since the first development of terahertz time-domain spectroscopy (THz-TDS) in the 1980s,^1,2 the technique has found application in numerous areas of science and engineering. Today, it is being utilized for the characterization of materials,^3–5 for industrial process monitoring,^6–8 and to identify chemical compounds.^9–11 While all terahertz time-domain spectrometers utilize femtosecond pulses of laser light to create a sub-picosecond transient current or electronic polarization in order to generate pulses of broadband terahertz (THz) frequency radiation, there are a plethora of different materials employed for the THz generation process. THz emitters include photoconductive antennas (employing GaAs^12–21 or InGaAs^22–24), organic and inorganic nonlinear crystals (ZnTe,^25–32 GaP,^33–35 LiNbO₃,^36,37 DAST,^38,39 DSTMS,^39–42 OH1,^39,41,43 and BNA⁴⁴), and recently, air plasma,^45–49 liquids,⁵⁰ and metamaterials.⁵¹ The reason for the large diversity of THz emitters employed is because not one particular source offers all of the desirable characteristics, including: a high THz electric-field amplitude, a broad spectral bandwidth, a gapless spectral coverage, and the requirement of only modest nano-joule laser pulse energies to excite the emitter. The table shown in Fig. 1 compares the characteristics of commonly used THz emitters in THz-TDS.

Photoconductive antennas (PCAs), which consist of a gap typically several micrometers wide between metal contacts deposited onto a photoconductive semiconductor substrate, generate THz radiation by accelerating photoexcited carriers using an externally applied bias field. For an overview of THz PCA technology, see the comprehensive reviews by Burford and El-Shenawee⁵² and Isgandarov et al.⁵³ The drawbacks with PCAs include the high fabrication cost associated with microlithography, the added complexity of the requirement for an external bias voltage, and the difficulty in scaling to higher electric-field amplitudes. This difficulty arises from the limit placed on optical excitation fluences by the damage threshold of the photoconductive material. In addition, the polar semiconductors commonly used have strong optical phonon resonances across the THz spectral region, preventing a gapless spectral coverage. Here, a gapless coverage is particularly desirable due to the large number of important electronic and vibrational modes within the THz spectral region. Nevertheless, emission spectra extending up to 20 and 30 THz have been demonstrated using semi-insulating (SI)-¹⁵ and low-temperature grown (LTG)-¹⁶ GaAs photoconductive antennas, respectively. A 70 THz bandwidth was also recently demonstrated using a non-polar Ge photoconductive antenna.⁵⁴ In the 1990s, an electric-field strength of 150 kV cm⁻¹ was reported for a large-area LTG-GaAs PCA¹⁹ and 350 kV cm⁻¹ for a large-area SI–GaAs PCA.²⁰ However, Hattori et al.²¹ highlighted that the estimation of electric-field strength from the pulse energy measurements employed in Refs. 19 and 20 were indirect and unreliable. This is due to the finding that Hattori et al. were only able to achieve an electric-field strength of 5.7 kV cm⁻¹ from a large-area SI–GaAs PCA with the same electrode spacing. Recently, a high electric-field strength has been realized in a large-area photoconductive THz array structure,¹⁷ consisting of a thin LTG-GaAs active region transferred to an insulating substrate, with a reported value in excess of 120 kV cm⁻¹.

Nonlinear crystals, such as inorganic ZnTe and GaP, and organic DAST and DSTMS, are commonly used for THz generation in THz-TDS due to their large nonlinear susceptibilities. In such crystals, the driving laser pulse generates a transient polarization that in turn emits a broadband THz pulse. The main drawback with inorganic, nonlinear crystals is the requirement to match the optical group velocity of the driving laser pulse to the THz phase velocity for efficiency generation. This requirement restricts the photoexcitation wavelength that may be used and limits the spectral bandwidth of the source. As these nonlinear crystals are polar in nature, they also suffer from optical phonons, which produce a highly absorbing Reststrahlen band within the THz spectral region. This absorption either limits the bandwidth of the emitted THz pulse or produces a gap within the spectral content of the source. By using a thin crystal or a thin film to minimize the optical group- and THz phase-velocity walk-off, a 30 μm thick (110)-cut ZnTe crystal and a 10 μm thick (111)-grown ZnTe film have produced broadband emission with spectral content up to 40 THz.^26,27 However, these emission spectra demonstrate significant gaps across the spectrum. LiNbO₃ inorganic crystals are often used for the generation of high fields,^36,37 with electric-field strengths of up to 4.0 MV cm⁻¹ being reported, although LiNbO₃ suffers from requiring a complex tilted-pulse-front-pumping scheme for velocity matching.

Organic, nonlinear crystals have, in contrast, achieved some of the highest reported electric-field strengths. A value of 62 MV cm⁻¹ has been reported for the non-ionic polyene crystal OH1,⁴¹ and a value of 83 MV cm⁻¹ has been reported for the stilbazolium salt DSTMS.⁴¹ However, the spectra emitted from organic crystals are affected by a large number of vibrational absorption bands,^38,40 preventing a gapless spectral coverage over the 0.1–10 THz region. For a thorough review of organic crystals, see the work of Jazbinsek et al.,⁵⁵ and for a comprehensive comparison of high-field THz sources, see the review by Fülöp et al.⁵⁶ 

The continued desire for THz emitters that can generate an ultra-broadband THz spectrum, covering the far- and mid-infrared spectral region with gapless coverage, has been recently realized by using air-plasma generation. Such THz sources have demonstrated a bandwidth of more than 100 THz,^48,49 limited only by the pulse duration of the photo-exciting laser, and electric field strengths of 400 kV cm⁻¹.⁴⁷ However, the key drawback with air-plasma sources is the requirement for high photoexcitation pulse energies (on the order of hundreds of microjoules) for the generation of the plasma. Furthermore, air-plasma generation can also suffer with large shot-to-shot pulse fluctuations.

A promising new class of THz emitters have recently emerged that have already demonstrated a gapless broadband emission reaching up to 30 THz.⁵⁷ They require photoexcitation with only nano-joule energy pulses,⁵⁷ are cheap to manufacture, and have demonstrated scalability for the generation of higher THz field amplitudes (up to 300 kV cm⁻¹).⁵⁸ They also appear to be independent of the photoexcitation wavelength.⁵⁹ These so-called spintronic, or spin-based, emitters consist of bilayers of ferromagnetic (FM) and non-magnetic (NM) heavy metal thin films. The mechanism for THz emission, as shown in Fig. 2, relies on the generation a spin current, j_s, in the FM layer and subsequent conversion of this to a transient charge current, j_c, in the NM layer. Consequently, such emitters are referred to as spintronic, or spin-based, emitters.

Research to date has focused predominately upon maximizing the emission amplitude of the THz pulse by investigating a range of material combinations^57,60–63 and film thicknesses^57,60,64 for the FM/NM bilayer structure. So far, THz pulses with the highest electric field amplitudes have been observed when using CoFeB as the FM material in combination with a NM material that exhibits a large spin–orbit coupling, such as Pt.^{57,58,60–62,65} A detailed review of material combinations and thicknesses can be found in Secs. II A and II B. Further approaches investigated for increasing the amplitude of the emitted pulse include the use of multilayer⁶⁶ and trilayer films (W/CoFeB/Pt),^57,58,61 which take advantage of the flow of the spin current in both directions through the FM thin film to boost the spin-to-charge conversion efficiency.⁵⁷ Changing the material properties by post-deposition annealing has been shown to influence the amplitude of the THz pulse when using a CoFeB FM layer.^64,67 In addition, the observation of emission of pulses of THz radiation from spintronic heterostructures with broken interfaces,^68,69 detailed in Sec. III, has enthused a research drive to understand the influence of interfacial properties upon the interfacial spin-to-charge current conversion mechanism.⁷⁰ While it has been established that particular combinations of FM/NM materials can maximize the emitted THz electric field amplitude, a detailed understanding of the role that interfaces and material properties contribute to the generation process of THz frequency radiation in these systems is less well understood. In particular, the effects of varying the material properties, such as the roughness, crystal structure, and strain from the substrate, have not been widely reported. Further research will be necessary to inform the design of emitters that offer optimum performance required for scientific and industrial applications of THz-TDS. In this review, we discuss experimental and theoretical findings, which have led to the development of spintronic THz emitters. We begin to answer the vital question–what combination of materials and properties is required to produce an emitter that can generate pulses of THz radiation with a large spectral bandwidth and high electric field strength? By comparing ideas from the literature, we discuss possible mechanisms that contribute to the THz emission from spintronic sources and highlight how material and interfacial properties of the system contribute to the generation and emission process.

The physical processes associated with illuminating metallic thin films using picosecond laser pulses were predicted almost 50 years ago by Anisimov et al. who developed a numerical model for the emission of electrons from a metallic surface.⁷¹ This model was derived from the two competing phenomena: (1) the photoelectric effect and (2) the heating of the metallic surface, resulting in thermionic emission. At the time, these phenomena had been extensively investigated in metals using millisecond and nanosecond pulses,^72–75 allowing the photoelectric effect and the thermionic effect to be separated.⁷⁶ However, the separation of these effects for picosecond pulses was complex as the pulse duration was shorter than the characteristic time for the transfer of energy between electrons and the lattice in metals.⁷¹ Considering this, Anisimov et al. predicted an additional increase in electron temperature, T_e, with respect to the lattice temperature, T_i, of metals subjected to ultrafast heating, which was experimentally observed 8 years later by Eesley.⁷⁷ In order to observe non-equilibrium electron heating above the lattice temperature, Eesley used picosecond laser pulses to excite electrons in a 400 nm thick Cu film and to probe the thermally induced reflectivity, ΔR/R, at normal incidence from the film surface in a non-collinear pump–probe configuration. During the 40 ps delay between the initial heating pulses (with a photon energy of 1.92 eV) and the successive probe pulses (with tunable photon energies between 2.03 and 2.17 eV), Cu reflectivity transients were observed to have both fast and slow decay components, with the slow decay having a lifetime of ∼200 ps, as shown in Fig. 3. The rapid change in ΔR/R on a timescale of less than 15 ps was attributed to direct electron heating and cooling.⁷⁷ In resolving the Fermi level smearing components of ΔR/R (to extract T_e) and comparing these to the photon energies of the probe pulses, it was implied that T_e exceeded T_i during the heating pulse.⁷⁷ Due to the limited resolution of a picosecond time frame, the hot electron decay time, τ_e, could not be determined.⁷⁷ However, Eesley’s experiments confirmed that non-equilibrium heating of electrons was taking place in copper.

Following Eesley’s work, picosecond laser pump–probe experiments were performed on a range of metals, including W, Ag, Au, and Sn, in which the non-equilibrium heating of electrons was observed.^78–80 It was additionally shown by Agranat et al.⁷⁹ that the deviation of T_e from T_i could be substantial, reaching ∼4 × 10³ K in Ag.⁷⁹ Subsequently, the first picosecond pulsed laser experiments performed on ferromagnetic materials were conducted by Agranat et al. on Ni films⁸¹ and Vaterlaus et al. on Fe films⁸⁰ to investigate ultrafast, laser-induced magnetization dynamics. In particular, the characteristic time required to establish thermal equilibrium between the heated lattice and the spin system of ferromagnets, known as the spin-lattice relaxation time τ_is, was of technological interest at the time for determining speeds attainable in Curie-point writing for magneto-optical recording.⁸² Changes in the magnetization of 50–100 nm thick Ni films were therefore measured using the magneto-optical Kerr effect⁸³ after applying single, picosecond or nanosecond laser pulses with a fluence of 100 mJ cm⁻². It was found that heating the 50 nm Ni film using pulses of duration τ_p = 5–20 ps did not produce observable changes to the magnetization of Ni. However, using pulses of duration τ_p = 40 ns resulted in demagnetization of the heated section of the film as T_i was able to reach temperatures ∼2.5 times higher than the Curie temperature of Ni (T_C = 631 K).⁸¹ The spin-lattice relaxation time for Ni, within the vicinity of T_C, was determined to be within the interval 1 ns < τ_is < 40 ns.⁸¹ A similar behavior was observed in 5 mm thick Fe films by Vaterlaus et al.,⁸⁰ who conducted a spin-polarized photoemission experiment using a pulsed dye laser (with a photon energy of 2.15 eV) to illuminate the films with τ_p = 30 ps and 20 ns. The spin polarization of the photoelectrons emitted from the film remained constant over the duration of the 30 ps pulse, even as the laser fluence was increased to the value necessary to melt the Fe lattice, ∼90 mJ cm⁻². In contrast, during the 20 ns pulse, a decrease in polarization was observed for both solid and liquid Fe films, consistent with a demagnetization of the Fe at the point where T_i ≥ T_C.⁸⁰ It was therefore concluded that Fe could not be demagnetized within the duration of a 30 ps laser pulse due to τ_is > 30 ps.⁸⁰ In the following year, Vaterlaus et al. successfully measured the spin-lattice relaxation time in ferromagnetic Gd.⁸² To raise the temperature of the lattice, the 5 nm thick Gd film was illuminated using a laser pulse of 10 ns duration and a photon energy of 2.15 eV, which was less than the photothreshold of the sample, so as not to emit photoelectrons. Simultaneously, a synchronized laser pulse of 60 ps duration and photon energy of 3.2 eV was used to induce emission of electrons from the Gd, allowing the measurement of the time resolved spin-polarized photoemission.⁸² The variation of magnetization during the ns heating pulse was determined as a function of time, giving the characteristic time for the heat transfer from the lattice to the spin system of ferromagnetic Gd to be τ_is = 100 ± 80 ps.⁸² 

Until the beginning of the 1980s, ultrafast lasers were limited to the picosecond time resolution. However, by the mid-1980s, the development of high-repetition rate sources^84–86 made it possible to investigate non-equilibrium electron heating of metals in the femtosecond regime. Experiments undertaken in this regime have enabled critical temperature dynamics to be studied using femtosecond thermomodulation. This is a particularly powerful technique that separates electronic processes from phonon or lattice heating effects.⁸⁷ The first, direct measurement of the time taken, τ_e, for hot electrons to decay from a maximum to equilibrium temperature in the system was reported independently by Schoenlein et al. in 100 nm thick Au films⁸⁷ and by Elsayed-Ali et al. in 20 nm thick Cu films.⁸⁸ In both pump–probe experiments, high power femtosecond duration pulses were generated by a colliding pulse mode (CPM) locked laser (with a photon energy of 2.0 eV) and intensified using a copper vapor laser (CVL) amplifier.^87,88 Amplified pulses were focused into an ethylene glycol cell to generate continuum broadband pulses. Such pulses were subsequently used to probe the reflectivities of Au (for the wavelength range of 580–450 nm) and Cu (for the wavelength range of 560–640 nm) as a function of time, enabling the electron decay time to be resolved in Au, τ_e ≈ 2–3 ps,⁸⁷ and in Cu, τ_e ≈ 1–4 ps.⁸⁸ A positive correlation was observed between the heating pulse fluence (1.4–11.4 mJ cm⁻²) and the electron decay time in Cu. Elsayed-Ali et al. proposed that higher pulse fluences increased the temperature difference between T_e and T_i; hence, the increase in electron decay time was attributed to the greater number of electron–phonon collisions necessary for the Cu film to thermalize.⁸⁸ 

In 1996, Beaurepaire et al. performed the first femtosecond pulsed laser experiments on ferromagnetic Ni allowing demagnetization processes to be measured.⁸⁹ In contrast to previous experiments on ferromagnetic materials,^80–82 Beaurepaire et al. used a time scale in which the electron-spin and spin-lattice relaxation mechanisms could be resolved independently.⁸⁹ The 20 nm thick Ni films were illuminated using a CPM locked laser with a pulse duration of τ_p = 60 fs and a wavelength of 620 nm (amplified using a CVL) in order to induce and measure the electronic thermalization process. Simultaneously, the spin dynamics of Ni were measured using the time resolved longitudinal Magneto-Optical Kerr Effect (MOKE),⁸³ where a full magnetic hysteresis loop was recorded prior to illumination via the pump beam, and for each delay, Δt, between the pump and probe pulse.⁸⁹ It was discovered that when the film was illuminated by a pump of fluence of 7 mJ cm⁻², a striking decrease in the magnetization of the Ni could be observed at zero field, M_r, which occurred within Δt = 2 ps, followed by a relaxation to a plateau, shown in Fig. 4(a). The magnetization of the film decreased rapidly during the first picosecond, which indicated a fast increase in the spin temperature, T_S, of Ni.⁸⁹ However, from the calculation of electron and spin dynamics, T_e, T_S, and T_i, of the film over a range Δt = 0–5 ps, it was demonstrated that they behave differently after the first few picoseconds, shown in Fig. 4(b). In the intervening years since the seminal work by Beaurepaire,⁸⁹ significant research effort has been devoted to understanding the physical origins of this effect in a variety of materials. However, the microscopic mechanism by which these effects occur have yet to be unambiguously established.^90–94 In conclusion, the remarkable discovery that the demagnetization of Ni could be induced using femtosecond pulses of laser light⁸⁹ challenged the fundamental understanding of magnetism at the time, stimulating research interest in the ultrafast processes occurring in ferromagnetic materials.

An increased understanding of the all-optical generation of spin currents using ultrafast laser pulses^95–97 and of spin transport processes in ferromagnetic materials^98–100 has led to the development of ultrafast, THz spintronic emitters. Novel spintronic THz emitters, designed by Kampfrath et al. in 2013,¹⁰¹ consisted of ferromagnetic (FM)/non-magnetic (NM) bilayers, as shown in Fig. 2. Following the first experimental demonstration of the all-optical generation and detection of sub-picosecond, oscillating spin current,⁹⁵ laser pulses with femtosecond duration have been used to trigger the picosecond excitation process in spintronic emitters.^{57,61,64,101,102} Illuminating a spintronic emitter with intense, fs laser pulses creates excited, hot electrons in the ferromagnetic film, which undergo a superdiffusive spin transport process to the NM layer.^91,96,97,103 The superdiffusive process describes the motion of hot electrons, which is ballistic over short excitation times and diffusive (governed by Brownian motion) over longer excitation times.¹⁰³ In 2010, it was shown theoretically by Battiato et al.,¹⁰³ and demonstrated experimentally a year later by Melnikov et al.,⁹⁶ that the superdiffusive spin transport process could explain the mechanism of ultrafast demagnetization observed in 3d ferromagnets. After the absorption of a photon, electrons in the FM layer are excited from the d band to the sp bands above the Fermi level, increasing in mobility.¹⁰³ The asymmetry of the band structure gives rise to hot-electron lifetimes that are spin-dependent,⁹⁷ giving excited majority spins a longer mean free path than excited minority spins in the FM layer.¹⁰³ The subsequent depletion of majority spin carriers in the FM layer causes a decrease in the net magnetization of the ferromagnet.^97,103

Some studies have attributed the diffusive spin transport process to an ultrafast, spin-dependent Seebeck effect,^98,104,105 shown in Fig. 5. Experimental work to determine the timescales of these ultrafast processes indicates that the superdiffusive transport process and the ultrafast spin-dependent Seebeck effect occur over similar timescales of ∼1 ps.^101,106 The spin-dependent Seebeck effect, which has been demonstrated in lateral spin valve structures,¹⁰⁷ refers to the generation of a spin current, induced by a temperature gradient, in a ferromagnet. This spin current is generated because in the ferromagnet, the Seebeck effect, which describes the generation of a potential difference (voltage) due to a temperature gradient,¹⁰⁸ is spin-dependent.¹⁰⁷ The spin-dependent scattering rates and densities of conduction electrons in the ferromagnet give rise to individual Seebeck coefficients, S, for spin-up (↑) and spin-down (↓) electrons.¹⁰⁸ Therefore, when subjected to a temperature gradient, ∇T, or consequently, a heat current, ∇Q, a spin current, μ_↑–μ_↓, proportional to ∇T traverses the ferromagnet, where μ_↑ and μ_↓ are the electrochemical potentials for spin-up and spin-down electrons, respectively.¹⁰⁸ ∇Q is conserved across the NM/FM interface; therefore, ∇T is discontinuous between the ferromagnetic layer and the non-magnetic layer, as Q = −k_FM∇T_FM, where k_FM is the thermal conductivity of the ferromagnet.¹⁰⁷ As a result, the heat current induces a spin imbalance close to the interface proportional to μ_↑–μ_↓. This spin accumulation relaxes in the FM and NM layers on the length scale of their spin-relaxation lengths λ_FM and λ_NM, respectively.¹⁰⁷ 

The observation of thermally driven spin injection across a FM/NM interface^91,107–109 has opened up the possibility of utilizing the spin-dependent Seebeck effect as an alternative to electrical spin injection techniques for spin-transfer-torque (STT)-driven devices. In 2014, Choi et al.⁹⁸ demonstrated this concept experimentally in a Pt/FM₁/Cu/FM₂ heterostructure, as shown in Fig. 6. In this work, an 800 fs laser pulse was used to create a heat current in the Pt layer, which traverses the Pt/FM₁ interface.⁹⁸ This led to an ultrafast demagnetization of the FM₁ layer with perpendicular anisotropy, which was attributed to electron-magnon coupling.⁹⁸ Choi et al. reported that differences in the temperatures of electrons and magnons in the rapidly heated FM₁ layer enabled the conversion of the demagnetization to a thermally driven spin diffusive current, which traversed across the FM₁/Cu interface. This thermally driven current was then absorbed by the FM₂ layer, where a STT acting on the FM₂ layer led to the precession of the magnetic moment.⁹⁸ These findings have led to ongoing investigations of thermally driven spin currents as a mechanism for diffusive spin transport in spintronic THz emitters.^57,58,106

More recently, however, it has been shown by Rouzegar et al.¹¹⁰ that the laser-induced spin transport in a FM/NM spintronic THz emitter arises from the same force that drives ultrafast demagnetization in a FM thin film.¹¹⁰ This force is given by $Δμ̄s$ = μ_↑–μ_↓ and is referred to as a generalized spin voltage.¹¹⁰ Rouzegar et al. reported that $Δμ̄s$ is caused by a pump-induced increase in the temperature of photoexcited electrons in the FM thin film, which leads to a shift in μ_↑ and μ_↓ of majority- and minority-spin electrons in the FM layer, respectively. Alternatively to previous work,^98,104,105 Rouzegar et al. have explained their results, detailed in Ref. 110, without the assumption of a temperature difference between the FM and NM layers or a temperature difference between the majority and minority electrons in the FM layer. Therefore, the authors reported that the laser-induced spin transport in a FM/NM spintronic THz emitter is not the result of a spin-dependent Seebeck effect.¹¹⁰ 

While the mechanism driving the spin current is still under debate, it is widely accepted that a spin current, j_s, with polarization, σ, diffuses across a FM/NM layer interface and is converted into an ultrafast charge current density, j_c, in the NM layer via an inverse spin-Hall effect (ISHE).^99,100 It has been shown that the ISHE is driven by the spin–orbit interaction in the NM layer, which acts to deflect electrons traveling along the spatial direction of j_s, aligned parallel or antiparallel to the direction of σ.¹⁰⁰ Consequently, electrons with opposing spins are deflected in opposite directions,¹⁰⁰ inducing a charge current density transverse to j_s, as shown in Fig. 2.

The charge current due to the ISHE, first measured experimentally in a NiFe/Pt bilayer by Saitoh et al.¹⁰⁰ and in the Al layer of a lateral spin valve by Valenzuela and Tinkham⁹⁹ in 2006, can be expressed as¹⁰¹ 

where M denotes the magnetization in the FM layer, which is a function of the externally applied field, and α_H is the spin-Hall angle that characterizes the electron deflection¹⁰¹ and therefore the ISHE efficiency.¹¹¹ Due to their large spin-Hall angles, Pt and other metals with large spin–orbit coupling are chosen for the NM layer in spintronic THz emitters.^{57,58,60–62,65,101}

The transverse charge current density, j_c = α_Hj_s, acts as a source of THz radiation in the NM layer.^57,101 This leads to the emission of a linearly polarized THz pulse, with an electric field direction perpendicular to the magnetization, M, of the FM layer¹⁰¹ (shown in Fig. 2). Due to the relationship between charge current and M shown in Eq. (1), emission only occurs when M is in-plane. If M is brought out-of-plane, then this will result in no THz pulse being emitted. Throughout the literature, an external magnetic field is applied to saturate the FM layer in-plane, thus maximizing M and the corresponding THz output [see Eq. (1)]. However, the effect of operating at remanence has been investigated by Guo et al.¹¹² and Kampfrath et al.¹⁰¹ Guo et al.¹¹² demonstrated THz emission from W/CoFeB/Pt trilayer emitters in the absence of an applied field, resulting in an emitted THz pulse with an amplitude/bandwidth comparable to a typical ZnTe emitter. Here, the sample was saturated along a known easy axis prior to the emission experiment taking place. Kampfrath et al. (see the supplementary material for Ref. 101) investigated field-free emission from an Fe/Au bilayer and found that the resulting emitted THz pulse was half the amplitude of that observed when an in-plane saturating magnetic field was applied; they ascribe the reduction in emission to the formation of randomly oriented magnetic domains. The polarization of the THz pulse is independent of the polarization of the laser pump pulse.¹⁰¹ We direct the reader to Refs. 105, 110, and 113–115, which provide a more in-depth discussion of the underlying processes of THz emission from these systems.

The dependence of the polarization on M offers a unique potential to control the polarization profile of THz pulses,^102,113 as depicted in Fig. 7, with potential applications in THz-driven electron–hole recollisions (for high-order-sideband generation),¹¹⁶ ferroelectric domain switching,¹¹⁷ and rotatable polarization spectroscopy.¹¹⁵ Furthermore, the ability to generate unique THz polarization profiles, demonstrated by Hibberd et al.,¹⁰² could allow more complex modes, such as radial polarization,¹¹⁹ to be obtained. Such modes offer high-efficiency coupling of THz radiation to cylindrical wire waveguides¹²⁰ and, when focused, could provide the strong longitudinal electric field component desired for THz-driven particle acceleration.

A significant body of work has been devoted to exploring the impact of different material combinations on the emission from spintronic THz emitters. Figure 8 provides a table that shows the number of papers published where the THz emission has been investigated for the different material combinations. Figure 8 demonstrates that a range of materials have been investigated for the NM layer (layer 1), including topological insulators, semiconductors, and, in some cases, magnetic materials. These NM material choices are mapped against the range of magnetic materials that have been investigated for the magnetic (FM) layer (layer 2), which include ferromagnetic, ferrimagnetic, and antiferromagnetic thin films. The colormap gives an indication of the number of papers that have been published for each material combination. Also shown is whether bilayer (red circle), trilayer (blue triangle), or multilayer structures (green square) have been investigated for each material combination. Additionally, Fig. 8 shows a range of single layer magnetic materials from which THz emission has been observed, without the need for the NM layer. The impact of material choice on the THz emission is discussed in greater detail in Sec. II A. However, Fig. 8 demonstrates the use of a Pt NM layer in combination with a Co or CoFeB FM layer, which are the two popular avenues of exploration for spintronic THz emitters.

In 2013, Kampfrath et al.¹⁰¹ demonstrated that the temporal shape of the emitted THz pulse from a spintronic source can be manipulated using specifically designed FM/NM bilayers. In their work, 10 nm thick Fe films, evaporated onto glass substrates, were capped with a 2 nm thick layer of either Ru or Au to investigate how NM layers with a lower (Ru) or higher (Au) electron mobility could shape the emitted THz waveform. Using an 800 nm pump pulse with 20 fs duration to excite the Fe/Ru and Fe/Au bilayers, they observed significant differences in the emitted THz waveforms, as shown in Fig. 9(a).

The amplitude spectrum, ∣E_x(ω)∣, of the Fe/Ru bilayer was found to cover a lower frequency range from ω/2π = 0.3–4 THz, whereas the emission spectrum from the Fe/Au bilayer extended to frequencies greater than 15 THz, as shown in Fig. 9(b). This factor of four increase in bandwidth was attributed to the injection of hot electrons into the Au sp states, which have a high band velocity and greater mobility than Ru d-band states, which undergo more scattering due to a stronger electron–phonon coupling.¹⁰¹ As a result of strong coupling, a significant spin accumulation of ≈0.2 μ_B per Ru atom was determined to occur in the Ru layer at an absorbed pump fluence of 1.3 mJ cm⁻². This was an order of magnitude greater than spin accumulation in the Au layer, leading to a much longer spin current pulse in the Fe/Ru bilayer.¹⁰¹ These initial results demonstrated how the choice of NM material could influence the bandwidth of the THz emission from a FM/NM bilayer.

In agreement with Kampfrath et al.,¹⁰¹ it has been found that larger THz electric field amplitudes have been obtained when using NM materials that have a high electron mobility^60–62 and exhibit a large spin-Hall conductivity,⁵⁷ such as Pt. The THz pulse amplitudes emitted from various NM materials coupled to Co₂₀Fe₆₀B₂₀ have been directly compared in a study by Seifert et al.⁵⁷ as shown in Fig. 10.

The study by Seifert et al. showed that not only the amplitude of the THz waveform but also the polarity of the THz electric field correlates strongly with the amplitude and sign of the spin-Hall conductivity for the NM material chosen. Pt, for example, was found to produce pulses of THz radiation with waveform amplitudes an order of magnitude larger than Ta or Ir. However, choosing W for the NM layer was found to produce pulse amplitudes of similar magnitude to Pd, but of opposite polarity.⁵⁷ Similar results were found by Wu et al.,⁶¹ shown in Fig. 11, from NM/Co/SiO₂ heterostructures excited by a fs laser with a pulse energy of 220 μJ. Choosing Pt as the NM material was found to produce THz pulses with the largest peak-to-peak amplitude of the six NM materials investigated: Pt, Ir, Gd, Ru, Ta, and W, where the observed peak-to-peak amplitudes from Pt/Co/SiO₂ were ∼2 times larger than W/Co/SiO₂ and Ir/Co/SiO₂, and an order of magnitude greater than Ta/Co/SiO₂, Ru/Co/SiO₂, and Gd/Co/SiO₂. The Ta/Co/SiO₂ and W/Co/SiO₂ heterostructures also generated THz waveforms of opposite polarity, indicating that the charge currents, j_c, have a 180° phase shift in their oscillations.⁶¹ Given that the electric field component of THz emission is dominated by j_c (induced by the ISHE^57,101), the sign change in the THz pulse was attributed to the sign change of α_H.^57,61

More recently, an investigation of the emission of THz radiation from Al/Co heterostructures by Zhang et al.¹²¹ has demonstrated that while maximum THz electric field amplitudes have been obtained when using heavy metals with a large spin–orbit coupling for the NM layer,^{57,60–62,101} it may be worth revisiting light metals for cost-effective spintronic THz emitters. This is because despite the large difference between the spin-Hall angle of Al and Pt, where α_H = 0.03% and 3% (at 4.2 K), respectively,⁶¹ the maximum THz electric field amplitude of Al (4 nm)/Co (3 nm)/SiO₂ was still found to be ∼34% that of Pt (4 nm)/Co (3 nm)/SiO₂. This recently measured response is greater than previously observed THz pulse amplitudes emitted from FM/NM heterostructures, which have incorporated (heavy) NM metals with a lower spin-Hall conductivity, σ_H, such as Ta (3 nm) (σ_H ≈ −0.4 × 10⁵ Sm⁻¹) or Ir (3 nm) (σ_H ≈ 0.5 × 10⁵ Sm⁻¹), coupled to ferromagnetic Co₂₀Fe₆₀B₂₀ (3 nm).⁵⁷ These bilayers, shown in Fig. 10, generated only 10% and 20%, respectively, of the maximum amplitude of THz radiation emitted from a CoFeB (3 nm)/Pt (3 nm) bilayer, where σ_H ≈ 4.5 × 10⁵ Sm⁻¹ for Pt.⁵⁷ Similarly, the peak-to-peak THz pulse amplitudes emitted from Ta (4 nm)/Co (4 nm) heterostructures, investigated by Wu et al.,⁶¹ reached only ∼8% of the maximum amplitude of THz radiation emitted from a Pt (4 nm)/Co (4 nm) heterostructure, as shown in Fig. 11. However, curiously, in the comparative material study by Wu et al.,⁶¹ the Ir (4 nm)/Co (4 nm) heterostructure combination generated a maximum peak-to-peak THz pulse amplitude of ∼50% of the response from Pt (4 nm)/Co (4 nm) heterostructure despite the fact that its spin-Hall conductivity is comparable to Ta.⁵⁷ This interesting response, along with the considerable THz electric field amplitude emitted from the Al (4 nm)/Co (3 nm) based heterostructure with respect to the spin-Hall angle,¹²¹ prompts further questions. It suggests that our current understanding of the generation mechanism of THz radiation in spintronic emitters is not complete. While Zhang et al. were able to conclude that the strong emission of THz radiation from the Co (3 nm)/Al (4 nm) based heterostructure was a bulk effect occurring in the Al layer as a result of the ISHE, the physical origin was not clear, demonstrating the need for further experiments to uncover the underlying physics.¹²¹ 

One possibility for the unexpected range of THz electric field amplitudes emitted from varying NM materials could be related to the choice of FM material used for the heterostructure. The THz pulse amplitudes emitted from FM/NM heterostructures have been studied for a range of FM materials, including Fe, Co, Ni, and their binary alloys.^57,60,122 As demonstrated by Seifert et al.⁵⁷ in Fig. 12, FM materials (3 nm) coupled to Pt (3 nm) were found to emit THz pulses with similar amplitudes, with the exception of Ni. The alloy CoFeB, of the two studied compositions, produced the maximum THz pulse amplitude, whereas Ni yielded less than 20% of this maximum amplitude. This result has been hypothesized to be related to the low Curie temperature of Ni (627 K) in comparison to other FM materials (>1000 K).⁵⁷ 

In agreement with Seifert et al.,⁵⁷ Zhang et al.⁶⁰ have shown that in heterostructures consisting of Ta (3 nm)/Ru (5 nm)/FM (3 nm)/Ta (3 nm), where the FM material was CoFeB or CoFe, the resulting emitted THz pulse when using CoFeB yielded an ∼9% greater peak-to-peak amplitude compared to CoFe. While the reason for this small increase in THz pulse amplitude observed when using CoFeB for the FM material is not yet fully understood,^57,60 it has been hypothesized that it may be related to the higher spin polarization value of CoFeB (∼65%) in comparison to CoFe (∼47%).⁶⁰ This is because a higher spin polarization leads to the generation of a larger, superdiffusive spin current of hot electrons in the FM metal.

The dependence of the emitted THz radiation on the thickness of the FM and NM layers in spintronic heterostructures has provoked significant research interest. It has been demonstrated theoretically that the total thickness, d_tot, of the heterostructure, along with the impedance, Z(ω), spin-Hall angle, α_H, and spin current, j_s, can influence the emitted THz electric field amplitude, given by⁵⁷ 

While Eq. (2) suggests that the THz electric field amplitude scales with d_tot, experimentally, the opposite trend has been observed.^57,60,61 It has been shown by Zhang et al.⁶⁰ that reducing the thickness of the non-magnetic Pd layer from 10 to 5 nm in CoFeB (d_CoFeB nm)/Pd (d_Pd nm) based heterostructures, where d_CoFeB = 3–10 nm, increases the maximum amplitude of THz pulse emitted by ∼24%. Furthermore, decreasing the thickness of the ferromagnetic CoFeB layer from 20 to 3 nm, in the same CoFeB/Pd based heterostructures, increased the resulting THz pulse amplitude by ∼57%.⁶⁰ In Co₂₀Fe₆₀B₂₀/Pt bilayers, the total thickness, d_tot, dependence on the THz pulse amplitude has been further refined by Seifert et al.⁵⁷ For bilayers with approximately equal NM and FM layer thicknesses, the amplitude of the THz pulse emitted was measured to increase with decreasing d_tot, peaking at d_tot = 4 nm, before rapidly decreasing for a smaller d_tot,⁵⁷ as shown in Fig. 13. In similar work, a FM layer thickness dependence study was carried out on Pt (4 nm)/Co (d_Co nm) and W (4 nm)/Co (d_Co nm) based heterostructures.⁶¹ This showed that a peak THz pulse amplitude was measured when d_Co = 1–2 nm, before decreasing gradually with increasing FM layer thickness. Torosyan et al.¹²³ investigated the effect of layer thickness upon the emission of THz radiation from Fe (d_Fe nm)/Pt (d_Pt nm) bilayers. Here, the emission was measured for varying d_Pt and constant d_Fe = 12 nm and also for varying d_Fe and constant d_Pt = 3 nm. For both cases, a sharp increase in the THz emission was observed with increasing film thickness, with the emission peaking for film thicknesses of 2 nm for Fe and 3 nm for Pt, with the THz emission shown to fall off as the film thickness is increased further. The overall increase in peak THz pulse amplitude with decreasing thickness (to a minimum d_tot value) has been attributed to the laser-induced spin diffusion length of majority spins^60,61 and, most significantly, a photonic effect known as Fabry-Pérot resonance.^57,60 In detail, for a FM/NM bilayer structure, the metallic FM/substrate and NM/air interfaces act as two parallel reflecting surfaces, which effectively form a Fabry-Pérot cavity.⁶⁰ The length of this cavity is much smaller than the wavelength of the laser pump pulse (calculated as 300 nm for an 800 nm pump pulse inside the metal film⁶⁰) or the generated THz pulse; therefore, reflections will remain in-phase and interfere constructively. This resonant enhancement of the pump and THz pulses is proposed to increase for shorter cavity lengths⁵⁷ as more reflections go back-and-forth through the film before the light field has decayed. Hence, the THz pulse amplitude increases with decreasing total bilayer film thickness, as shown in Fig. 13. However, as the cavity reduces to a length smaller than some critical length, d < d_c, the reflection losses at the cavity interfaces dominate and no further benefit is gained from reducing the length to minimize the attenuation of the light field and maximize the number of reflections. The enhancement of the pump and THz light field is saturated at d < d_c and can no longer compensate for the reducing emitter volume, hence resulting in a sharp decrease in the THz pulse amplitude.⁶⁰ 

In contrast to results observed by Zhang et al. for CoFeB/Pd heterostructures,⁶⁰ when studying the thickness dependence of Pt (d_NM nm)/Co (4 nm) and W (d_NM nm)/Co (4 nm) heterostructures, Wu et al.⁶¹ observed that the maximum peak-to-peak THz pulse amplitude increased with increasing thickness of the NM layer, where d_NM = 2–10 nm.⁶¹ For this case, the increasing amplitude of THz pulse emitted was attributed to an increase in the spin accumulation in the heterostructure, which resulted from increasing d_NM.⁶¹ Interestingly, this result agrees with (2), as this relation suggests that the THz electric field amplitude scales with total sample thickness, d_tot.⁶¹ Alternatively, the recent work by Zhang et al.,¹²¹ which investigates the thickness dependence of Al (d_Al nm)/Co (3 nm) and Pt (d_Pt nm)/Co (3 nm) based heterostructures, revealed an optimum layer thickness of 4 nm and an optimum Pt layer thickness of 2–3 nm.¹²¹ These results follow the opposite trend for THz pulse amplitude vs layer thickness that was observed in previous work.^57,60,61 In order to understand the discrepancy between the contrasting results observed by Zhang et al.¹²¹ and Wu et al.,⁶¹ it is clear that additional material and structural properties, such as interfacial roughness, crystal structure, and strain from the substrate, need to be investigated and their impact further understood.

Understanding the impact of material properties on the generation of THz radiation produced by spintronic emitters is crucial in order to inform the design of emitters that exhibit optimal performance for commercial, scientific, and industrial applications. While the impact of FM/NM material combinations and their thickness dependence upon THz emission have been intensively researched over recent years, the impact of material properties, such as roughness, crystal structure, interfacial intermixing, and strain from the substrate, have not been widely explored. A systematic study of the emission of THz radiation from Co/Pt bilayers with varying material properties was undertaken by Li et al.¹²⁴ Here, Co (10 nm)/Pt (3 nm) bilayers were deposited onto glass and Cr-buffered MgO substrates using magnetron sputtering. The table shown in Fig. 14 presents results from this study.

The roughness of the Co/Pt interface was varied by changing the Ar sputtering pressure of the Co layer during the deposition. It was shown that decreasing the surface roughness of the Co layer from ∼0.2 to ∼0.1 nm resulted in a smoother Co/Pt interface and an increase in the measured THz electric field strength from 0.8 to 2.5 V cm⁻¹, as shown in Fig. 14. This factor of ∼3 increase in THz electric field amplitude was attributed to a decreasing spin–flip probability at the FM/NM interface with a reduction in the Co/Pt interfacial roughness.¹²⁴ Therefore, Li et al. reported that smoother interfaces become increasingly transparent to spin currents, j_s, and suggested that this leads to an increase in the strength of the charge current, j_c generated in the Pt layer via the ISHE.¹²⁴ In addition, the effect of varying chemical intermixing at the Co/Pt interface was explored by depositing a 1–2 nm thick Co_xPt_1−x alloy spacer layer between the Co and Pt films, where x = 0.25, 0.50, and 0.75. In comparison to the Co/Pt bilayer structure without a spacer layer, the peak THz pulse amplitudes emitted from these heterostructures were up to 4 times greater, where the emission was observed to increase with the level of intermixing, as shown in Fig. 14. It has been proposed that the increase in emission of THz radiation in this case could be generated by either (1) a higher influx of spin currents, due to a reduction of interfacial spin resistivity at the graded Co/Pt interface, or (2) a significant spin–orbit interaction in the Co_xPt_1−x spacer layer itself, which could contribute to the spin-to-charge current conversion.¹²⁴ Furthermore, the same authors investigated the role of crystal symmetry at the Co/Pt interface using fcc-Co(100)/Pt and hcp-Co(110)/Pt films, grown onto Cr-buffered single crystal MgO substrates, as shown in Fig. 14. The crystal symmetry at the Co/Pt interface was found to have only a small impact upon the THz electric field amplitude, where effects observed were attributed to the topography related to the crystal interfaces; polycrystalline hcp-Co was reported to form a rougher interface than epitaxial fcc-Co.¹²⁴ 

In addition to the impact of material properties on the emission of THz radiation, the related effect of post-deposition annealing warrants further investigation. This has been studied in CoFeB/W bilayers⁶⁷ and also in more complex Ta/CoFeB/MgO/Ta heterostructures⁶⁴ where, in general, annealing has been shown to enhance the emission of THz radiation from spintronic emitters.^64,67 Gao et al. demonstrated that annealing (post-deposition) CoFeB (2.0 nm)/W (2.2 nm) bilayers to a temperature of 280 °C, under a uniform external magnetic field of 1 T, led to an increase in peak-to-peak THz electric field amplitude by a factor of 3.⁶⁷ This increase in THz emission efficiency was coupled to a decrease in film resistivity, from ∼200 μΩ cm (unannealed film) to ∼10 μΩ cm (annealed film), and was therefore attributed to an increase in hot electron mean free path as a result of enhanced crystallization with annealing.⁶⁷ 

The effect of post-deposition annealing on the emission of THz radiation from multilayer Ta (5 nm)/CoFeB (t nm)/MgO (2 nm)/Ta (2 nm) heterostructures has been investigated by Sasaki et al.,⁶⁴ with results from this work shown in Fig. 15(a). The authors observed that for heterostructures with a 1 nm thick CoFeB layer, the amplitude of the emitted THz pulse increased by a factor of ∼1.5 after annealing at a temperature of T_a = 300 °C.

Sasaki et al. modeled the thickness dependence of the THz emission from Ta/CoFeB/MgO/Ta heterostructures, shown in Fig. 15(b), in order to explain the behavior observed experimentally. The model was based upon (1) the energy density contributed to the system from the pump pulse, (2) impedance matching between the air and the heterostructure, and (3) the attenuation of the spin current in the Ta (heavy metal) layers.⁶⁴ While the theoretical transmission intensity of the THz radiation as a function of t_CFB predicted a gradual change in the range t_CFB > 1 nm, the more rapid change that was observed experimentally (except for T_a = 400 °C) was attributed to possible backscattering and diffusion of hot electrons, which was not included in the model.⁶⁴ It was reported that the significant differences between the experimentally observed THz emission dependence on post-deposition annealing [Fig. 15(a)] and theoretically predicted behavior [Fig. 15(b)] indicates that annealing has a significant effect upon the attenuation of the spin current.⁶⁴ The emission dependence on annealing was therefore attributed to the possible spin scattering of electrons with boron atoms, where boron atoms were likely to have diffused into the Ta layer as a consequence of annealing.^64,125 Diffused boron atoms behaving as scattering centers in the adjacent Ta layer could increase the effective spin-Hall angle, α_H, and decrease the hot electron relaxation length, λ_NM, thus increasing the amplitude of the emitted THz radiation.⁶⁴ Interestingly, a study by Nenno et al.,¹²⁶ in which the performance of a Fe (2 nm)/Pt (3 nm) bilayer is modified by defect engineering, demonstrates that the hot electron scattering lifetime, τ_NM, can be decreased through increasing the number of defects present in the Pt layer. This result is in agreement with Sasaki et al.,⁶⁴ as it would lead to a decrease in λ_NM and a reduction in the magnitude of the spin current due to a larger scattering rate.¹²⁶ An alternative theory, put forward by Sasaki et al., is that the emission dependence on annealing could be attributed to an increase in velocity and mean free path of hot electrons traveling through an annealed FM CoFeB layer, with a resulting crystalline (001) structure, compared to an unannealed FM CoFeB layer, that exhibits an amorphous structure,⁶⁴ which agrees with the study undertaken upon CoFeB/W bilayers by Gao et al.⁶⁷ This dependence was found to be more significant for thicker films, where, for a post-deposition annealing temperature of T_a = 400 °C, the THz pulse amplitude for t_CFB = 10 nm increased by a factor of 5 compared to a factor of 2 when t_CFB = 2 nm.⁶⁴ 

Nenno et al.¹²⁶ reported that the differences in the local microstructure of Fe/Pt bilayers can have a dramatic influence on the THz emission. The microstructures of Fe (2 nm)/Pt (3 nm) bilayers grown onto MgO (001) were studied using high resolution zero-loss energy filtered transmission electron microscopy (HR-EFTEM), as shown in Fig. 16, for two cases: (a) a sample grown entirely at a deposition temperature of T_dep = 300 °C and (b) a sample grown at T_dep = 300 °C for the Fe layer and at room temperature (RT) for the Pt layer.

Comparing the two samples, it was shown that the deformation in the RT-Pt sample was substantially stronger than the Fe/Pt sample grown at T_dep = 300 °C, where deformation was defined by local nm-sized, twisted areas relative to the MgO substrate.¹²⁶ Alternatively, depositing the Pt layer at T_dep = 300 °C led to practically stress-free single crystal growth of Pt.¹²⁶ While the relaxation processes of the strained lattices led to local dislocations and vacancies in both samples, it was reported that a higher number of these local defects were present in the RT-Pt sample. The ability of the two samples to emit THz radiation was therefore attributed to the differences in the local microstructure of the Fe/Pt bilayers.¹²⁶ 

In conclusion, it is clear that material properties, such as interfacial roughness, chemical intermixing,¹²⁴ and post-deposition annealing,^64,67 have a significant impact upon the emission of THz radiation from spintronic emitters. The properties of many material systems have yet to be investigated, and it is not yet understood how specific materials, thicknesses, or substrates affect the generation of THz radiation. Thus, the impact of material properties on the emission process remains a key area of research interest.

While numerous studies on spintronic emitters based on ferromagnetic/non-magnetic bilayer material combinations have been reported, the THz emission from ferrimagnetic (FIM)/non-magnetic (NM) bilayers has also been explored. A ferrimagnet, which consists of two magnetically ordered, coupled sublattices, A and B, offers a unique functionality where the different exchange constants J_A and J_B lead to the magnetizations, M_A and M_B, having different temperature dependencies.¹²² This useful property enables the net magnetization (M_A + M_B) of the ferrimagnet to be effectively tuned by changing the temperature of the material,¹²² where a temperature change can be induced by exposure to femtosecond laser pulses.^62,127,128 FIM/NM THz emitters based upon Gd_xFe_1−x alloys have been explored^62,122 as they are archetypal rare-earth transition metal ferrimagnets, exhibiting composition and thickness dependent magnetic properties.^129,130 A recent study by Schneider et al.¹²⁷ has investigated the composition and magnetic field dependent THz emission from Gd_xFe_1−x (20 nm)/Pt (5 nm) heterostructures over the entire composition range (0 ≤ x ≤ 1). Figure 17 reveals how the THz emission varies with applied magnetic field strength. Here, THz pulses with the highest peak-to-peak amplitudes were observed for a maximum applied field of 500 mT.

This observation was attributed to the fully in-plane alignment of the Fe and Gd moments at large applied fields, where H_app = 500 mT was shown to saturate the samples with the composition x < 0.4.¹²⁷ In the case of smaller fields, H_app = 50 or 25 mT, which do not fully saturate the samples with a Gd content of x = 0.15–0.25, a significant reduction in peak-to-peak THz pulse amplitude¹²⁷ was observed during the transition of the magnetic easy axis from an in-plane (x ≤ 0.15) to out-of-plane (x ≥ 0.25) orientation. During this transition, the magnetic compensation point (x = 0.20) appears to have only a small influence on the emitted THz amplitude in comparison to the effect of a spin reorientation. Furthermore, it was found that samples with the composition range of 0.25 ≤ x ≤ 0.4, emitted inverted THz pulses, which is attributed to the generation of THz radiation at room temperature being dominated by Fe.¹²⁷ The authors reported that the 800 nm laser-induced spin polarization of heated electrons in the FIM layer is dominated by the 3d electrons of Fe as they are close to the Fermi energy, whereas the 4f electrons of Gd are 8–11 eV below the Fermi energy and therefore only contribute minimally to the generated THz response.¹²⁷ Consequently, the THz waveform flips in polarity when x ≥ 0.25 since the Fe magnetic moments are aligned antiparallel to the external magnetic field, whereas when x ≤ 0.2, the Fe magnetic moments are aligned parallel, inducing the ISHE in opposite directions.¹²⁷ 

Huisman et al.¹²² have similarly demonstrated that the emitted THz electric field amplitude from a GdFeCo (20 nm)/Pt (2 nm) heterostructure is dominated by the sensitivity of the FeCo magnetic sublattice to an applied magnetic field, rather than the net magnetization of the GdFeCo layer (M_FeCo + M_Gd).¹²² The dependence of the emitted THz electric field upon applied field, shown in Fig. 18, demonstrates that the emission of THz radiation from the heterostructure decreases when the applied field is stronger than 2 T. Similar to Schneider et al.,¹²⁷ the THz electric field was observed to undergo a polarity inversion below the compensation temperature of the GdFeCo.¹²² This behavior indicates that the THz electric field amplitude is proportional to a linear combination of the in-plane magnetic sublattice components, aM_FeCo + bM_Gd, where ∣a∣ >∣b∣ for the polarity inversion to occur.¹²² In agreement with Schneider et al., the variation in the sign and the value of a and b was attributed to the fact that the magnetization of the FeCo sublattice occurs from itinerant 3d electrons, while the magnetization of the Gd sublattice occurs from lower energy level 4f electrons, leading to a varying magnetic sublattice sensitivity with respect to the applied magnetic field.¹²² Subsequently, Huisman et al. demonstrated that this permits the generation of THz radiation from the GdFeCo/Pt bilayer by a spin-polarized current, even when its net magnetization is zero.¹²² 

This dependency of the THz emission upon net spin polarization, rather than net magnetization, has also been observed by Chen et al. in Co_1−xGd_x (7 nm)/Pt (6 nm) heterostructures, where x = 0–0.55.¹²⁸ At a Gd content of x = 0.26, almost identical to that measured by Schneider et al.,¹²⁷ the THz pulse was observed to undergo an abrupt polarity inversion without showing a fully quenched signal. This polarity inversion was attributed to the magnetic alignment of the Co sublattice as well as the spin polarization change in direction across the compensation temperature.¹²⁸ In addition to the Gd_xFe_1−x/Pt bilayers investigated by Schneider et al., a peak THz pulse amplitude was observed in Co_1−xGd_x/Pt bilayers when the Gd content was equal to zero.¹²⁸ THz pulse amplitudes were observed to decrease with increasing Gd content.¹²⁸ This observation and the thickness dependent properties of Co_1−xGd_x have yet to be fully understood.

More recently, the ability to change the polarity of the THz electric field by tuning the magnetic alignment of sublattices across the compensation temperature^127,128 has been used to control the THz output emission state.¹³¹ In 2020, Fix et al.¹³¹ developed a novel Pt/Gd₃₀Fe₇₀/W/Gd₁₀Fe₉₀/Pt heterostructure in which the varying composition of the two Gd_xFe_1−x alloys enabled parallel and antiparallel alignment of their Fe moments with changing temperature above and below the compensation temperature, T_comp, shown in Fig. 19. The non-magnetic layers, Pt and W, have opposing spin-Hall angles, which leads to the generation of a large, unidirectional charge current, j_c (see Sec. III A), when the Fe magnetic moments are aligned antiparallel at a temperature, T < T_comp. This results in the emission of high-amplitude THz pulses [Fig. 19(a)]. In comparison, when the Fe magnetic moments are aligned parallel at a temperature, T > T_comp, the charge current in the two Pt layers flows in opposite directions. Consequently, the THz pulse emitted by the three NM layers combined produces a lower amplitude pulse [Fig. 19(b)].¹³¹ These findings demonstrate potential for ferrimagnetic THz emitter systems, which can provide a function for controlled switching of the THz emission from high to low power.¹³¹ 

In addition to investigating the choice of thin film materials, research effort has been directed at maximizing the amplitude of the THz radiation pulse emitted from spintronic sources through the study of trilayer^57,58 and multilayer⁶⁶ heterostructures. It was first predicted by Seifert et al.⁵⁷ that the introduction of an additional, second non-magnetic layer (NM₂) layer at the second FM interface of a FM/NM₁ bilayer can boost the amplitude of emitted pulses of THz radiation. In a bilayer structure, only the forward-propagating half of the photoinduced spin current, j_s, can flow into the NM₁ layer, where it is converted into a charge current, resulting in a THz pulse being emitted (Fig. 2).⁵⁷ By introducing the NM₂ layer, it was anticipated that backward-flowing electrons propagating across the resulting NM₂/FM interface can be converted into an additional charge current. Provided that the NM₁ and NM₂ materials exhibit opposite spin-Hall angles, α_H, the charge currents j_c generated in each layer will flow in the same direction, resulting in the emission of a combined THz pulse of increased overall amplitude. Consequently, such a configuration would be expected to increase the efficiency of the spin-to-charge conversion mechanism and boost the amplitude of the THz radiation emitted.⁵⁷ Alternatively, if the spin-Hall angles of the two NM₁ and NM₂ materials are of the same sign, then destructive interference will take place due to the induced THz pulses being of opposite polarity. This would result in a reduced overall THz pulse amplitude being observed.

The effect of introducing a second non-magnetic layer was investigated experimentally by Seifert et al.,⁵⁷ who observed an increase in the amplitude of the THz pulse emitted from a W (2 nm)/Co₄₀Fe₄₀B₂₀ (1.8 nm)/Pt (2 nm) trilayer structure. Comparing the amplitude of the resulting THz pulse to that emitted from a Co₄₀Fe₄₀B₂₀/Pt bilayer counterpart structure of equal total thickness, the trilayer produced a THz pulse with a peak amplitude approximately twice that produced by the bilayer structure.⁵⁷ This key result has motivated ongoing optimization studies of trilayer spintronic emitters,⁵⁸ particularly those that combine W and Pt layers due to their large but opposite values of α_H.^57,58 Peak THz electric fields of 300 kV cm⁻¹ have since been measured from W (1.8 nm)/Co₂₀Fe₆₀B₂₀ (2.0 nm)/Pt (1.8 nm) trilayers, where the waveforms were observed to exhibit a gapless spectrum, extending from 1 to 10 THz at 10% of its amplitude maximum⁵⁸ as shown in Fig. 20. Here, the increase in the THz electric field amplitude was achieved by fabricating a large area emitter (7.5 cm diameter). The ability to increase the amplitude of the THz emission by increasing the area of the film offers distinct advantages, in terms of ease of fabrication and use, over traditional emitters such as photoconductor arrays where increased emission is achieved by fabricating arrays of microstructured antenna units.¹³² Note that the relatively flat spectral phase observed in Fig. 20 indicates that the THz pulse is transform-limited.

In addition to trilayer spintronic emitters, it has been demonstrated that cascaded multilayers can generate strong, broadband THz radiation.^66,114 It was theorized by Yang et al.⁶⁶ that integrating the transverse charge currents generated from several [NM/FM]_n structures can lead to an increase in emission intensity, provided that the transverse charge currents in all NM layers are approximately in-phase. To ensure that j_c was in-phase, structures were produced that consisted of repeating NM/FM layers, separated by thin insulating (I) layers, so that the photoinduced spin current in each FM layer is restricted to flow only into its neighboring NM layer.⁶⁶ Such a design promotes instantaneous excitation at each NM/FM interface in order to achieve in-phase integration of the THz emission.⁶⁶ Thus, Yang et al. demonstrated experimentally that [Pt (2 nm)/Fe (1 nm)/MgO (2 nm)]_n (n ≥ 1) cascaded multilayers could enhance the THz electric field strength up to 1.7 times that of a single Fe/Pt bilayer (n = 1), peaking when n = 3 before gradually decreasing as n is increased further, as shown in Fig. 21(a).⁶⁶ 

It was reported that when n > 3, the absorption of laser light illuminating the substrate side becomes increasingly significant, leading to a decrease in the number of photoexcited electrons in the top few Fe/Pt bilayers at the film/air interface.⁶⁶ The saturation of the THz emission at n = 3, thus, was attributed to this decrease in photoexcited electrons. Nevertheless, Yang et al. demonstrated [Fig. 21(b)] that the structure with n = 3 repeated layers was, surprisingly, capable of outperforming a standard 100 μm thick GaP nonlinear crystal.⁶⁶ Recently, Nenno et al.¹¹⁴ have theoretically investigated the terahertz generation efficiency in cascaded multilayer spintronic systems. In agreement with Yang et al.,⁶⁶ the authors have demonstrated numerically that the current conversion efficiency in an Fe (10 nm)/Pt (5 nm) bilayer can be optimized by using n = 3–4 repeated bilayers, separated by an insulating layer.¹¹⁴ However, in contrast to the experimental work by Yang et al.,⁶⁶ the insulating layers in the numerical study were modeled as anti-reflective coatings of thickness d = 200 nm, where 200 nm is equivalent to $λ4$ of the incident 800 nm laser pulse. Interestingly, the numerical optimization process revealed a factor of 2 increase in terahertz generation efficiency,¹¹⁴ which is similar to the experimental results obtained when using 2 nm thick MgO insulating layers.⁶⁶ 

Subsequently, Feng et al.¹³³ designed a spintronic heterostructure that combined the physical properties of both trilayers^57,58 and cascaded multilayers⁶⁶ to further improve conversion efficiency. The proposed heterostructures consisted of [Pt (1.8 nm)/Fe (1.8 nm)/W (1.8 nm)]_n trilayers (to utilize forward and backward-flowing spin currents), separated by SiO₂ dielectric layers (n = 1, 2, 3) to promote instantaneous excitation at each pair of NM₁/FM/NM₂ interfaces.¹³³ In order to tailor absorbance and reflection at multiple layer interfaces,⁶⁶ the thickness of the spacer layer was increased from d_Si = 2 to d_Si = 110 nm for n repeated layers. Feng et al.¹³³ observed that the heterostructure with n = 2 and d_Si = 110 nm emitted a THz pulse with a peak amplitude ∼1.3 times greater than THz pulse emitted from a single heterostructure (n = 1) with the same d_Si = 110 nm. Furthermore, for all heterostructures, the amplitude of the THz pulse emitted increased non-linearly with increasing d_Si, where this increment was larger with a greater number of repeated layers.¹³³ This phenomenon, in agreement with predictions of Yang et al.,⁶⁶ was attributed to the absorption, multiple reflection, and interference of THz radiation by the periodic trilayers in each heterostructure. Additionally, Feng et al. developed an analytical expression for the emission of THz radiation from the heterostructures, reporting that the emission from a structure with, for example, n = 2 repeating layers is dependent on (i) the forward-flowing emission from the first NM/FM/NM trilayer that experiences multiple reflections and interference, (ii) the direct forward-flowing emission from the second trilayer, and (iii) the backward-flowing emission from the second trilayer that experiences multiple reflections and interference.¹³³ This was expressed by considering the coefficients of transmission, t_τ, and reflection, r_τ, of THz radiation at film interfaces,

where E_1,2 = βA_1,2, which denotes absorbance, A, of the laser (β is a THz conversion constant), and 2ikd denotes the phase accumulation, dependent on the wave vector (k) and SiO₂ thickness (d), during reflection.¹³³ From this relation, Feng et al. demonstrated theoretically and experimentally how the conversion efficiency of [NM/FM/NM]_n cascaded trilayers can be improved by tuning the thickness of the dielectric layer,¹³³ showing promise for the use of these heterostructures in future THz applications.

So far, this review has focused upon the emission of helicity independent (HI) pulses of THz radiation from spintronic heterostructures, where the THz pulses are polarized perpendicular to the magnetization of the FM layer.¹⁰¹ However, in 2013, it was shown by Huisman et al.⁶⁵ that exciting a spintronic emitter using a circularly polarized fs laser pulse can lead to the emission of an additional helicity dependent (HD) pulse of THz radiation, polarized parallel to the magnetization of the FM layer.⁶⁵ In contrast to HI THz pulses, which are generated in the interior of a NM layer by the ISHE spin-to-charge current conversion mechanism,^99,100 it has been shown that HD pulses of THz radiation are generated by an interfacial spin-to-charge current conversion mechanism.^65,67,68,134 This interfacial conversion mechanism is known as the inverse Rashba–Edelstein effect (IREE)^70,135,136 and occurs at interfaces with broken inversion symmetry,⁷⁰ such as those between different metals.⁶⁵ It has been shown that the IREE is driven by the spin–flip scattering of electrons at these interfaces, known as Rashba interfaces, shown in Fig. 22(b).⁶⁸ Interfacial Rashba systems have spin-split energy bands; therefore, injection of a spin current, j_s, into the system leads to an unequal spin accumulation in the two sub-bands.⁶⁹ Consequently, the Fermi surface contours are shifted by Δk, shown in Fig. 22(a), leading to a non-equilibrium surface density of states.⁶⁸ A charge current, $jcIREE$⁠, carried by the interfacial states, is therefore generated at the Rashba interface, which can be expressed as¹³⁵ 

where α_R is the Rashba coefficient,¹³⁶ τ_s is the spin-lattice relaxation time, and λ^IREE has the dimension of a length.

Huisman et al. excited Co (10 nm)/Pt (2 nm) emitters using circularly polarized laser fs pulses and measured the HD emission, ∣E_y∣, polarized parallel to M and also the helicity independent (HI) emission, ∣E_x∣, polarized perpendicular to M, as shown in Fig. 23(a). It was found that when the helicity of the fs laser pump was reversed, a change of sign in ∣E_y∣ was observed. This demonstrated a dependence of the THz pulse polarization direction, generated by the interfacial IREE conversion mechanism, upon the helicity of the pump pulse,⁶⁵ shown in Fig. 23(b). In contrast, no change of sign in ∣E_x∣ was observed, confirming a helicity independence of the THz electric field polarized perpendicular to M. Further fundamental differences between the HD emission and HI emission have been demonstrated, including a contrasting dependence on the thickness of the Pt layer and the amplitude of the THz pulse emitted for the two cases.⁶⁵ Huisman et al. observed a negligible increase in the HD emission of THz radiation from Co (10 nm)/Pt (d_Pt) heterostructures with an increasing thickness of the Pt layer from d_Pt = 2 to 4 nm, which supports the theory that HD THz radiation is being generated at the interface of the heterostructure.⁶⁵ In contrast, the HI emission of THz radiation from the same structure was measured to increase from ∣E_x∣ = 25 to 100 V cm⁻¹ with an increasing thickness of the Pt layer from d_Pt = 2 to 4 nm, which is in agreement with the trend observed for Co/Pt emitters generating pulses of THz radiation by the ISHE conversion mechanism.^61,121 These results highlighted the individual nature of HI and HD THz emission and the differences between the ISHE and IREE spin-to-charge conversion mechanisms for the generation of THz radiation.

Following this work, there has been increased research interest in the use of topological insulators (TI), such as Ag/Bi,^68,69,135 as components of spintronic THz emitters, where FM/NM/TI layered systems have been shown to generate pulses of THz radiation by the IREE.^68,69,135 Initially, spin-pumping experiments were undertaken by Sánchez et al.¹³⁵ in order to understand the potential of an IREE spin-to-charge current conversion in NiFe (15 nm)/Ag (d_Ag nm)/Bi (8 nm) heterostructures. In their work, conventional ferromagnetic resonance (FMR) and electrical measurement techniques were used to probe the spin dependent properties of the heterostructures, shown in Fig. 24. By comparing results obtained from the NiFe/Ag/Bi heterostructure to those measured from NiFe (15 nm)/Ag (5 nm) and NiFe (15 nm)/Bi (8 nm) bilayers, Sánchez et al. reported that the generation of a large spin signal in the NiFe/Ag/Bi heterostructures could only be explained by the occurrence of an IREE at the Ag/Bi interfaces and not by the ISHE of Ag or Bi.¹³⁵ This is because the measured charge current, $jcIREE$⁠, was observed to decrease to a negligible value with the absence of the Bi layer [Fig. 24(a)], which indicated that the contribution from the ISHE in Ag could be neglected.¹³⁵ Furthermore, a comparatively lower $jcIREE$ was observed for the NiFe/Bi bilayer [Fig. 24(b)], highlighting the necessity of the Ag/Bi interface in generating a large spin signal.

In addition, Sánchez et al. observed that increasing the thickness of the Ag layer from d_Ag = 0 to 10 nm in the NiFe/Ag/Bi heterostructures did not decrease $jcIREE$⁠. This behavior was attributed to the long spin diffusion length of electrons in Ag, ∼150 nm at RT,¹³⁷ which exceeded the thickness of the Ag layer.¹³⁵ In conjunction with the relatively long, ∼50 nm, electron diffusion length in Bi, Sánchez et al. thus reported the spin absorption of Ag and Bi to be negligible (except in their interfacial states). Following this, a simplified relationship between spin current and effective spin mixing conductance was proposed: j_s ∝ $G↑↓eff$⁠, where $G↑↓eff$ describes how efficiently spin current can be transmitted from the FM layer to the NM₁/NM₂ interfacial layer states in a FM/NM₁/NM₂ heterostructure (taking into account the back flow of electrons).¹³⁵ 

The observations from Sánchez et al. have generated growing research interest in spintronic heterostructures with Rashba interfaces for the generation of THz radiation.^68,69 In 2018, the generation of broadband THz radiation was reported, simultaneously, by Jungfleisch et al.⁶⁸ from a CoFeB/Ag/Bi heterostructure, and by Zhou et al.⁶⁹ from a Fe/Ag/Bi heterostructure, where both samples exhibited the interfacial IREE mechanism. A bandwidth of up to 2.2 THz was exhibited by the CoFeB (2 nm)/Ag (2 nm)/Bi (2 nm) heterostructures (where bandwidth was defined as the frequency at which the intensity drops 10 dB from the maximum),⁶⁸ and a bandwidth of up to 4 THz (at 10% of its amplitude maximum) was observed from the Fe/Ag/Bi heterostructures.⁶⁹ In agreement with Sánchez et al.,¹³⁵ Jungfleisch et al.⁶⁸ showed that increasing the thickness of the Ag layer from d_Ag = 2 to d_Ag = 4 nm had a negligible effect on the amplitude of the THz radiation emitted. In contrast, when d_Ag was increased to 10 nm, the peak THz pulse amplitude was observed to reduce by a factor of 2; however, the reasons for this observation are not clear. The relationship between d_Ag and the peak THz pulse amplitude has been further explored by Jungfleisch et al. using circularly polarized laser pulses.⁶⁸ Here, a small HD THz electric field component, ∣E_y∣, parallel to the magnetization of the CoFeB layer was observed in CoFeB/Ag/Bi heterostructures, and this dependence was most pronounced when d_Ag = 2 and 4 nm. Interestingly, no helicity dependence was found in the control samples CoFeB/Bi and CoFeB/Pt,⁶⁸ which contrasts to the observation of helicity dependent THz radiation observed from similar Co/Pt structures by Huisman et al.⁶⁵ Furthermore, Zhou et al.⁶⁹ confirmed that the emission of HI, broadband THz radiation from Fe/Ag/Bi heterostructures excluded any effects from (i) demagnetization in the Fe film, (ii) ISHE in bulk Bi or Ag, and (iii) potential signals from other interface states. However, interestingly for these trilayers, when the Bi/Ag interface symmetry was reversed, it was found that the THz pulses emitted from Fe (2 nm)/Ag (2 nm)/Bi (3 nm) and Fe (2 nm)/Bi (3 nm)/Ag (2 nm) trilayers were not always 180° out of phase.⁶⁹ This result is unusual because a reversal of the interface symmetry, in the case of an ideal Rashba interface, would be expected to reverse the direction of the interfacial electric field, ẑ, as shown by Eq. (5) from Zhou et al.,⁶⁹ 

Thus, this finding was attributed to the fact that Eq. (5) is only valid if a perfect Rashba interface exists. Interference effects, including a strong spin–orbit coupling, were attributed to the possible surface alloying of AgBi at a thickness of 0 < d_Bi < 0.5 nm, which may have led to a non-negligible spin-Hall effect.⁶⁹ Zhou et al. subsequently reported a superposition of the measured THz signals by the ISHE and the IREE from heterostructures with a Bi thickness of 0 < d_Bi < 0.5 nm, which could be manipulated by controlling the symmetry of the Ag/Bi interface.⁶⁹ This result provided potential for increasing the efficiency of generation of THz radiation from a FM/NM₁/NM₂ heterostructure by utilizing the ISHE and IREE in the same device.⁶⁹ 

Increasing the efficiency of the IREE spin-to-charge current conversion mechanism has recently attracted increasing research interest.^69,134 To date, heterostructures with FM/Ag/Bi Rashba interfaces have achieved peak THz pulse amplitudes up to ∼25% of the amplitudes emitted from FM/NM bilayers,⁶⁸ which are favored for their large ISHE efficiency. Additional materials have therefore been investigated for the possibility of increasing the spin-to-charge conversion efficiency and hence $G↑↓eff$⁠. A recent study of the topological insulator Bi₂Se₃, incorporated into the heterostructure Bi₂Se₃/Co/SiO₂, revealed temperature independent (from 0 to 300 K) THz emission with a peak electric field amplitude of comparable magnitude to Pt/Co/SiO₂ heterostructures.¹³⁴ 

As shown in Fig. 25, the peak THz electric field emission from the Bi₂Se₃/Co heterostructure was measured to be ∼1.7 times that emitted from the Pt(2 nm)/Co (3 nm) heterostructure.¹³⁴ This striking result was attributed to the superior spin-to-charge conversion efficiency possessed by spin momentum-locked topological surface states.¹³⁴ However, this has yet to be fully understood in order to create highly efficient spintronic THz emitters. Overall, it can be seen in Fig. 25 that the peak THz electric field emitted from the Pt(6 nm)/Co (3 nm) bilayer still exceeds that of the Bi₂Se₃/Co heterostructure.

To further understand the role of interfaces upon photoinduced current and spin transport, the introduction of interlayers into bilayer or multilayer spintronic THz emitters has been investigated.^138,139 In 2018, Li et al.¹³⁸ used pulses of circularly polarized laser light to investigate the effect of including Cu and ZnO interlayers (denoted X) in Co (10 nm)/X (d_x nm)/Pt (2.6 nm) heterostructures upon the HD and HI THz emission. In their work, the interlayer thickness, d_x, was increased from 1 to 5 nm. It was found that the introduction of a Cu or a ZnO interlayer resulted in a decay in the integrated field strength of both the HD and HI components, ∣E_x∣ and ∣E_y∣, respectively, as the interlayer thickness was increased, shown in Fig. 26.¹³⁸ Note that the x and y components of the THz electric field are reversed for Fig. 26 in comparison to Fig. 23. In the case of the HD components, a gradual exponential decay was observed in ∣E_x∣ with increasing thickness of the Cu interlayer [Fig. 26(a)], which contrasts with a faster (exponential) decay observed in ∣E_x∣ with increasing thickness of ZnO [Fig. 26(b)]. Furthermore, a complete suppression of the HD component was measured when d_ZnO ≥ 2 nm [Fig. 26(b) inset].¹³⁸ These results were attributed to the strong dependence of HD THz emission on the quality of the interface between Co and Pt, highlighting the importance of a direct electric contact between the Co and Pt layers for the generation of photoinduced spin current in the helicity dependent case.¹³⁸ 

Additionally, Li et al. reported that increasing the thickness of the Cu interlayer leads to a gradual, near linear decay in the HI component, ∣E_y∣ [Fig. 26(c)]. However, increasing the thickness of the ZnO interlayer leads to a strong, exponential decay in the HI component, at a rate ∼28 times faster than Cu [Fig. 26(d)]. This rate was determined by comparing the HI spin diffusion lengths of the Co/ZnO/Pt and Co/Cu/Pt structures, measured to be l_ZnO = 0.4 nm and l_Cu = 11 nm). In contrast to the HD THz emission [Figs. 26(a) and 26(b)], Li et al. reported that the HI THz emission from Co/X/Pt heterostructures [Figs. 26(c) and 26(d)] is less sensitive to the interlayer thickness, as ∣E_y∣ was observable for d_x ≤ 5 nm [Fig. 26(d) inset] in contrast to the suppression of HD emission when d_ZnO ≥ 2 nm [Fig. 26(b) inset].¹³⁸ 

In contrast to these observations, a recent study by Yang et al.¹³⁹ indicates that including a Cu insertion layer in a W/Cu/CoFeB based heterostructure could lead to an enhancement of the emitted HD THz pulse. Using spin-torque FMR and spin-Hall magnetoresistance measurements, the effective spin mixing conductance, $G↑↓eff$⁠, of W (5 nm)/Co₄₀Fe₄₀B₂₀ (5 nm)/MgO (2 nm)/Ta (2 nm) multilayers was shown to increase by ∼25% with the insertion of a 0.52 nm Cu interlayer between the W and CoFeB layers.¹³⁹ In addition, an ∼10% enhancement in interfacial spin transparency, T, was observed with Cu insertion, where T takes into account the effects that lead to electrons being reflected instead of being transmitted at the NM/FM interface.¹⁴⁰ Similar to Li et al.,¹³⁸ Yang et al. reported that the enhancement of both $G↑↓eff$ and T indicates that Cu insertion into W/CoFeB based heterostructures can induce a better interface for the transmission of spin current and demonstrated, theoretically, that the spin–orbit torque efficiency could be improved by up to 54% using a Cu interlayer of 0.52 nm thickness.¹³⁹ Recently, Lu et al.¹⁴¹ have calculated the transmission and reflection of spin current at FM/NM interfaces for a range of material combinations. Using first principles calculations, the authors demonstrated that differences between the reflectivities of spin-up and spin-down electrons at particular FM/NM material interfaces can result in spin filtering, which can be used to improve the spin polarization of the electrons entering the NM layer. The authors reported that inserting a thin FM metal or an alloy with a high density of states above the Fermi energy, such as Fe, or an anti-reflective coating between the FM and NM layers to act as a “spin-filter,” can lead to an enhancement in the emission of THz radiation due to the increased spin polarization of the hot electrons entering the NM metal;¹⁴¹ however, this has yet to be investigated experimentally. It is clear from these studies^138,139,141 that the impact of introducing interlayers into spintronic heterostructures needs to be further investigated with respect to varying thicknesses, material and spin properties, in order to fully understand their potential for improving the performance of THz emitters.

One of the advantages of spintronic THz emitters is their ability to generate pulses of THz radiation that are polarized perpendicular to the magnetization, M, in the plane of the film and independent of the polarization of the pump laser.¹⁰¹ This can provide a useful function in allowing the tailoring of the polarization profile of the emitted THz pulse.¹⁰² However, in order to manipulate the polarization profile of the emitted THz pulse, an external magnetic field, H_app, is required to define the magnetic state of the FM layer.

However, recently, the emission of THz radiation has been observed from synthetic antiferromagnetic (SAF) structures, without the application of an external field.^142,143 SAF structures consist of two or more exchange-coupled ferromagnetic layers separated by metallic or dielectric spacer layers.^144–146 The interlayer exchange coupling in a SAF structure results from the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction,^145,146 which is responsible for an oscillatory short-range interaction that can promote ferromagnetic (parallel) alignment or antiferromagnetic (antiparallel) alignment of the FM layers, depending on the spacer layer thickness, d_s.^146,147

In 2020, Ogasawara et al.¹⁴² studied the emission of THz radiation from Ta (3 nm)/CoFeB (2 nm)/Ir (d_s)/CoFeB (2 nm)/Ta (3 nm) synthetic (anti)ferromagnetic structures. The thickness of Ir, d_s, was varied from 0.3 to 1.4 nm to align the magnetization of the two FM layers, M₁ and M₂, antiferromagnetically or ferromagnetically.¹⁴² In the absence of an external field, H_app, THz pulses with a maximum electric field amplitude were measured when M₁ and M₂ were aligned antiparallel (d_s = 0.5 nm), as shown in Fig. 27. In contrast, increasing d_s to 0.9 nm resulted in the parallel alignment of M₁ and M₂, with the consequence of an almost negligible emitted THz electric field amplitude.¹⁴² This finding was attributed to the variation in structural inversion symmetry between the antiparallel and parallel aligned synthetic (anti)ferromagnetic structures.¹⁴² For parallel alignment of M₁ and M₂, the magnetic and structural inversion symmetry in the synthetic (anti)ferromagnetic structure would, theoretically, lead to the generation of a zero net charge current, j_c, due to the positive and negative contributions from the ISHE spin-to-charge conversion mechanism [Fig. 27(a)].¹⁴² Consequently, the generation of THz radiation would require an external field to align M₁ and M₂. In contrast, when M₁ and M₂ are aligned in an antiparallel configuration, magnetic inversion symmetry is broken, leading to strong THz emission by the integration of in-phase charge currents in each NM layer of the synthetic antiferromagnetic structure.¹⁴² Ogasawara et al. subsequently demonstrated that applying an external field to the parallel aligned synthetic (anti)ferromagnetic structure, in order to break the magnetic inversion symmetry, leads to a small increase in the electric field amplitude of the THz pulses. Applying the same magnetic field to the antiparallel configuration resulted in a significant decrease in the THz electric field amplitude, as shown in Fig. 27(b).¹⁴² These results indicate a strong dependence of THz emission upon antiferromagnetic interlayer coupling and demonstrate the potential of SAF structures for field-free generation of THz radiation.

Also in 2020, Zhang et al.¹⁴³ reported the emission of THz radiation from Fe–Mn–Pt based SAF structures. However, this was achieved using a different mechanism to that proposed by Ogasawara et al. Until very recently, only two current conversion mechanisms for the generation of THz radiation from a spintronic source have been widely studied—the inverse spin-Hall effect^99,100 and the inverse Rashba–Edelstein effect.^70,135,136 However, in 2019, Zhang et al. demonstrated that THz radiation could be generated from a single layer ferromagnet by an additional mechanism known as the anomalous spin-Hall effect (AHE).¹⁴⁸ The AHE, shown in Fig. 28, acts as a mechanism for generating spin current in conducting ferromagnets with broken time-reversal symmetry.¹⁴⁹ When a current, I, flows into a FM layer in a longitudinal direction, spin-up (↑) and spin-down (↓) electrons are deflected to traverse in opposite directions through the FM layer.¹⁴⁹ This process is thought to occur due to underlying extrinsic mechanisms, such as skew scattering¹⁵⁰ and side jump,¹⁵¹ or intrinsic mechanisms that are related to the band structure of the material.¹⁵² In the FM layer, the asymmetry in density of states at the Fermi level gives rise to transverse charge current, j_c, and spin accumulation of electrons at the boundaries of the ferromagnet (Fig. 28).¹⁴⁹ This leads to the generation of an anomalous Hall voltage, V_H, across the FM layer, simultaneously with a spin current, j_s, perpendicular to the magnetization M.¹⁴⁹ 

The AHE conversion mechanism was initially utilized by Zhang et al.¹⁴⁸ to generate pulses of THz radiation from a single layer ferromagnet. In their study, a fully saturated (Fe₈₀Mn₂₀)₆₇Pt₃₃ (3 nm) ferromagnet, capped with dielectric MgO (4 nm), was irradiated using a femtosecond laser with a fluence of 0.55 mJ cm⁻². Following excitation, nonthermal, superdiffusive electrons were reflected at the substrate/ferromagnet and the ferromagnet/dielectric interfaces, which induced a backflow of current through the ferromagnet.¹⁴⁸ The amplitude and polarity of the backflow current were attributed to the material properties and roughness of the interfaces, where the smoother (substrate/ferromagnet) interface produced a larger backflow current than the rougher (ferromagnet/dielectric) interface in the structure.¹⁴⁸ This led to the formation of a net longitudinal current, I_N, in the FM layer, which, with the presence of the AHE, was converted to a transient, transverse current,¹⁴⁸ 

where α_AHE is the AHE angle. This resulted in the emission of pulses of THz radiation, dependent on (1) the direction of the applied magnetic field, where positive H_app led to the emission of positive E_THz; (2) the strength of the AHE in the FM layer, where greater THz pulse amplitudes were measured from (Fe₈₀Mn₂₀)₆₇Pt₃₃ and Ni₈₀Fe₂₀ with a larger AHE than Fe₈₀Mn₂₀ and Co₂₀Fe₆₀B₂₀; and (3) the thickness of the FM layer, where peak THz pulse amplitudes were observed when d_FM = 3–4 nm in (Fe₈₀Mn₂₀)₆₇Pt₃₃ films.¹⁴⁸ 

Zhang et al. reported the emission of THz radiation from (Fe₈₀Mn₂₀)₆₇Pt₃₃ (4 nm)/Ru (0.8 nm)/(Fe₈₀Mn₂₀)₆₇Pt₃₃ (4 nm)/MgO (4 nm) SAF structures, by the AHE mechanism,¹⁴³ where peak THz pulse amplitudes were almost double that of corresponding single layer emitters of the equivalent thickness.¹⁴³ In these SAF structures, the thickness of the Ru layer, d_s, promotes antiparallel alignment of the magnetization of two FM layers, M₁ and M₂, as shown in Fig. 29(a). Therefore, when irradiated by fs laser pulses, in the absence of an applied field, nonthermal, superdiffusive spin currents that were reflected from the substrate/ferromagnet and the ferromagnet/dielectric interfaces flow in opposite directions through each FM layer in the SAF structure. Subsequently, due to the opposite polarity of M₁ and M₂, shown in Fig. 29(a), both longitudinal currents flowing in each FM layer were converted into transverse charge currents with the same polarity via the AHE, with a net, transverse charge-current density j_c ∝ (M₁ + M₂) x ẑ, where ẑ is a unit vector along the z axis [Fig. 29(a)].¹⁴³ This was reported to lead to the emission of THz radiation from the antiparallel aligned SAF structure when H_app = 0, shown in Fig. 29(b), which was attributed to the constructive contribution of the charge currents generated in each FM layer.¹⁴³ 

In comparison, the application of an external field H = 200 Oe, which was strong enough to saturate the SAF structure so that M₁ and M₂ aligned in parallel, was found to reduce the electric field amplitude of the THz pulse emitted from the structure by ∼30%. As shown in Fig. 29(b), the electric field amplitude emitted from the parallel aligned SAF structure is proportional to the response measured from the original 5 nm thick single layer Fe–Mn–Pt ferromagnet.¹⁴⁸ By treating the parallel aligned SAF structure as a single layer emitter with an equivalent thickness of 8.8 nm, Zhang et al. demonstrated consistency between the results from the two studies, which demonstrate an AHE-driven emission of THz radiation.^143,148

More significantly, Zhang et al. demonstrated that incorporating a single layer ferromagnet¹⁴⁸ into an antiparallel aligned SAF structure will not only enhance the AHE-driven THz emission but also provide field-free THz emission from a spintronic heterostructure at room temperature.¹⁴³ In conjunction with the observation of field-free THz emission from a SAF structure by Ogasawara et al.,¹⁴² it is clear that these novel structures could have great potential for use in a large range of THz applications.

Since the discovery in 2013¹⁰¹ that novel thin film ferromagnetic (FM)/non-magnetic (NM) bilayers can be utilized to generate THz radiation, researchers have focused on investigating how different thin film materials affect the emission. A key goal has been to create structures that improve the emission amplitude of the THz pulse produced. In less than a decade, researchers have developed a coherent understanding of how the ISHE spin-to-charge current conversion mechanism results in the generation of sub-picosecond, linearly polarized pulses of THz radiation from such emitters.^99,100 Furthermore, a range of material, structural, and interfacial properties have been investigated, as well as the use of trilayer and multilayer structures, in order to understand what is required to optimize the performance of spintronic THz emitters.

Following the realization that the choice of NM material could greatly enhance the electric field amplitude and bandwidth of the pulse of THz radiation emitted from a FM/NM bilayer,¹⁰¹ the impact of different FM/NM material combinations on emission and their thickness dependence have been studied in detail.^{57,60–62,121} While the choice of FM material does not appear to have a significant impact on emission (with the exception of Ni due to its low Curie temperature, 627 K),^57,60 it is now well established that peak THz electric field amplitudes can be achieved by using NM materials with a high electron mobility and a large spin-Hall conductivity,^60,101 such as Pt.^{57,58,61,62,65,124} These findings have led to a large research focus on CoFeB/Pt,^57,58,133 Co/Pt,^{62,65,124,138} and Fe/Pt^66,126 based heterostructures as high performing THz emitters. Furthermore, thickness dependence studies on a range of FM/NM material combinations, including CoFeB/Pt,⁵⁷ CoFeB/Pd,⁶⁰ Co/Pt,^61,121 and Co/W,^57,61 have reported that peak THz pulse amplitudes and bandwidths are emitted when the FM layers and NM layers are 1–3 nm thick.^57,60,61,121 Note that the electric field amplitudes reported in these studies are difficult to compare directly due to the different characteristics of the individual measurement systems and the lack of coherency in the units used. As the development of spintronic emitters continues, there needs to be a move away from comparative measurements of the amplitude of THz waveforms, often presented in arbitrary units, to measurements that enable the merits of emitters developed by different groups to be directly compared. While there is no single figure-of-merit to characterize the performance of a terahertz emitter, as the important parameter often depends on the application, the measured peak electric field focused at the detector, the measured terahertz pulse energy, and the calculated optical-to-terahertz conversion efficiency are all widely used in comparing the performance of nonlinear crystals and photoconductive antennas¹⁵³ and should be employed. This information together with experimental details on the THz emitter area, the pump excitation fluence, and the collection and focusing optics employed can allow for quantitative comparisons between spintronic emitter designs. We note that the terahertz pulse energy and peak electric field emitted from a spintronic emitter have been measured by Seifert et al.,⁵⁸ where a THz pulse energy of 5.1 nJ and an electric field of 300 kV cm⁻¹ were obtained by photoexcitation of a large-area (7.5 cm diameter) trilayer structure using a 40 fs pulse with a central wavelength of 800 nm and a pulse energy of 5.5 mJ. Work reporting this level of detail is uncommon but will be essential in the future if progress is to be made.

Studies have shown that the magnitude and polarity of the emitted THz pulse correlate with the amplitude and sign of the spin-Hall angle of the NM material chosen.^57,61 This useful electrical property has been exploited to improve the ISHE spin-to-charge current conversion mechanism in the heterostructures by the development of NM₁/FM/NM₂ trilayer emitters, where NM₁ and NM₂ have large but opposing spin-Hall angles (such as W and Pt).^57,58,133 Peak THz pulse amplitudes, approximately twice those for bilayer structures, have been observed from trilayer spintronic emitters of equal total thickness.⁵⁷ This has motivated the ongoing study of trilayers⁵⁸ and cascaded trilayers, [NM₁/FM/NM₂]_n,^114,133 for improving the amplitude of spintronic emitters. It is possible that with effective optimization of the appropriate material properties and layer thicknesses,^114,133 these novel structures will have significant potential for use in a wide range of THz applications.

While the impact of different FM/NM material combinations on the THz emission has been well documented for some thin film structures, the effects of material properties, such as roughness, crystal structure, interfacial intermixing, and strain from the substrate, on emission have not been widely reported. Ongoing studies have, so far, demonstrated that these properties can have a critical impact upon the generation of THz radiation from particular systems including Co/Pt,¹²⁴ Ta/CoFeB,⁶⁴ and CoFeB/W based heterostructures.⁶⁷ In particular, it was shown by Li et al.¹²⁴ that a reduction in the roughness of the interface, as well as increased chemical intermixing at the interface can lead to up to a factor of 2 improvement in the THz pulse amplitude in a Co/Pt heterostructure due to a possible reduction of interfacial spin resistivity.¹²⁴ Furthermore, post-deposition annealing has been observed to significantly increase the intensity of the THz radiation emitted from heterostructures containing CoFeB.^64,67 The enhancement of the THz pulse amplitude with annealing has been attributed to either the diffusion of B atoms into the NM layer, leading to increased spin scattering and increased effective spin-Hall angle, α_H,^64,126 or the relaxation of strained lattices, leading to an increase in mean free path of hot electrons.^64,67,126 It is clear that further studies need to be undertaken to fully understand the significance of each mechanism and that a wider range of thin film heterostructures need to be explored to fully understand their influence on spin transport and the generation of THz radiation.

While the majority of research for high performing THz emitters has largely focused upon Pt based heterostructures,^{57,58,62,65,66,124,126,133,138} recently, light metals such as Al¹²¹ have been revisited as potential, cost-effective, alternative NM films in spintronic emitters. Despite the large difference between the spin-Hall angle of Al (0.03%) and Pt (3%), the maximum THz electric field amplitude of Al/Co bilayers was still observed to be significant, ∼34% of that found in Pt/Co bilayers.¹²¹ The physical origin of this result is not clear,¹²¹ highlighting the need for further experiments on these potentially cost-effective systems. Other systems investigated for application in room temperature spintronic THz emitters include ferrimagnetic (FIM)/NM bilayers, where optimization has focused largely upon tailoring the composition and thickness of the FIM layer.^{62,122,127,128} The realization of a polarity inversion in the THz electric field below the compensation temperature of Gd_xFe_1−x/Pt,¹²² Gd_xCo_1−x/Pt,¹²⁸ and GdFeCo/Pt¹²⁷ bilayers offers great potential for output control of spintronic THz emitting systems. Subsequently, a novel NM/Gd_xFe_1−x/NM/Gd_xFe_1−x/NM heterostructure has been proposed that demonstrates the ability to control the emitting state (high- or low-amplitude) of the THz radiation, depending on the relative alignment of the Fe moments.¹³¹ 

While the use of FIM materials offers the ability to control the polarity of the emitted THz radiation from a spintronic heterostructure, it has been shown that the polarization profile of THz pulses can also be manipulated by the shape of the applied magnetic field.¹⁰² The ability to produce polarization profiles with radial components could offer high efficiency coupling of THz radiation to cylindrical wire waveguides,¹²⁰ and such profiles could also provide enhanced longitudinal electric field components when focused,¹⁰² which has applications in particle acceleration.¹⁵⁴ Furthermore, the THz electric field emitted from a spintronic heterostructure can be manipulated by the helicity of light, which may provide opportunities for wireless spintronics applications.⁶⁸ The polarity of the HD THz emission from a Co/Pt bilayer has been shown to invert when the helicity of light is reversed,⁶⁵ which has been attributed to the generation of THz radiation by the interfacial IREE spin-to-charge current conversion mechanism.^65,68,69 Discovery of the IREE conversion mechanism in structures with broken symmetry interfaces^70,135,136 has stimulated a research drive to understand the influence of the interfacial properties upon the generation of THz radiation. Here, initial studies using FM/Ag/Bi trilayers with Rashba interfaces^68,69 have led to the significant measurement of a superimposed THz signal, generated by the ISHE and IREE simultaneously.⁶⁹ This demonstrates potential to create spintronic THz emitters with increased electric field amplitudes by utilizing both spin-to-charge current conversion mechanisms in the same device. However, this has yet to be explored experimentally. Following the reports of highly efficient IREE spin-to-charge current conversion in Bi₂Se₃/Co based heterostructures,¹³⁴ it is clear that additional materials need to be considered in order to understand the role of interfaces upon the generation of THz radiation.

Field-free emission of THz radiation has been observed from SAF structures at room temperature by both the ISHE and the AHE mechanisms.¹⁴³ This key development highlights the potential of spintronic emitters based on SAF structures for applications requiring tunable polarization.¹⁴³ The possibility to generate THz radiation without an external field makes such emitters easier to construct and use. Furthermore, it has been shown that the AHE mechanism, similar to the ISHE, leads to a dependency of the emitted THz pulse amplitude on the thickness of the FM layer.¹⁴⁸ However, the AHE conversion mechanism has yet to explored in more than a small number of heterostructures.^143,148

In summary, spintronic THz emitters have already demonstrated several desirable source characteristics, including a gapless broadband emission reaching up to 30 THz, the requirement for only nano-joule energy pulses,⁵⁷ scalability for the generation of high THz field amplitudes (up to 300 kV cm⁻¹),⁵⁸ and appearing to be independent of the photoexcitation wavelength.⁵⁹ 

It is evident that there are still a multitude of avenues to investigate in order to optimize spintronic heterostructures to create idealized emitters that can generate high amplitude, broadband pulses of THz radiation. Promising areas of investigation that have been highlighted from the research presented in this review include the following:

1. The optimization of material properties, such as tailoring the microstructure, strain, or defect engineering in trilayer or multilayer heterostructures. This has demonstrated potential for the improvement of spin-to-charge conversion efficiency. However, it is also important to consider strategic materials to use in these systems, where further studies into the material properties of light metal/FM based heterostructures potentially offer significant advantage.

2. Interface engineering of FM/NM (heavy metal) and FM/NM/NM Rashba systems in order to fully understand the role of interfaces upon the generation of THz radiation in a variety of materials. An increased understanding of interfacial phenomena could allow for simultaneous utilization of the ISHE and IREE conversion mechanisms in the same structure, ultimately leading to THz emitters with an increased emission amplitude.⁶⁹ 

3. The continued development of recently proposed THz emitters based upon synthetic antiferromagnetic structures for the field-free generation of THz radiation at room temperature. Exploring the geometry of emitters based upon SAF structures could enable the future development of easy to construct THz emitters with tunable polarizations.¹⁴³ 

4. The further investigation of thin film materials that exhibit a very large anomalous Hall effect for the generation of THz radiation utilizing the AHE conversion mechanism.¹⁴³ This category of materials includes magnetic Weyl semi-metals, which are the subject of significant current research activity, where the anomalous Hall angle is reported to be an order of magnitude greater than typically found in magnetic systems.^155,156 Recent theoretical work has also predicted that it may be possible to generate spin currents whose polarization can be in any direction.¹⁵⁷ If this could be realized experimentally, then it may be possible to create homogeneous structures that trigger even greater emission amplitudes.

Given the relatively low cost of spintronic THz emitters (in comparison to PCAs and nonlinear crystals) and their already competitive source parameters, together with the potential for further significant development, means that they are extremely promising sources for THz time-domain spectroscopy and its exploitation in scientific research and industrial applications.

This work was supported by the United Kingdom Engineering and Physical Sciences Research Council (Grant No. EP/S033688/1). We also wish to acknowledge the Ph.D. scholarship support for R. Ji by the China Scholarship Council (NSCIS Grant No. 201906120039).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.