Terahertz (THz) technology has seen significant advancements in the past decades, encompassing both fundamental scientific research, such as THz quantum optics, and highly applied areas like sixth-generation communications, medical imaging, and biosensing. However, the progress of on-chip THz integrated waveguides still lags behind that of THz sources and detectors. This is attributed to issues such as ohmic losses in microstrip lines, coplanar and hollow waveguides, bulky footprints, and reflection and scattering losses occurring at sharp bends or defects in conventional dielectric waveguides. Inspired by the quantum Hall effects and topological insulators in condensed matter systems, recent discoveries of topological phases of light have led to the development of topological waveguides. These waveguides exhibit remarkable phenomena, such as robust unidirectional propagation and reflectionless behavior against impurities or defects. As a result, they hold tremendous promise for THz on-chip applications. While THz photonic topological insulators (PTIs), including wave division, multiport couplers, and resonant cavities, have been demonstrated to cover a wavelength range of 800–2500 nm, research on tunable THz PTIs remains limited. In this perspective, we briefly reviewed a few examples of tunable PTIs, primarily concentrated in the infrared range. Furthermore, we proposed how these designs could benefit the development of THz on-chip PTIs. We explore the potential methods for achieving tunable THz PTIs through optical, electrical, and thermal means. Additionally, we present a design of THz PTIs for potential on-chip sensing applications. To support our speculation, several simulations were performed, providing valuable insights for future THz on-chip PTI designs.

Terahertz (THz) radiation encompasses a wide range within the electromagnetic spectrum, spanning from microwaves to the far infrared, typically ranging from 100 GHz to 30 THz. Since Auston et al.’s first demonstration of THz generation and detection,1 the applications of THz technology have rapidly expanded across various fields, including astronomy,2–5 quantum optics,6–9 biotechnology,10–12 medical imaging,13–16 climate monitoring,17,18 sixth-generation (6G) communications,19–23 and more. The development of THz science and technology has been extensively reviewed in 201724 and 2023,25 providing valuable insights into its progress over the past half century. In this perspective, our focus centers on two key areas of THz on-chip technology: modulation and biosensing. Given the exponential growth in data transmission demands, THz communications play a pivotal role in shaping the future of 6G communications. Therefore, the development of modulation techniques for on-chip THz photonic topological insulators (PTIs) becomes indispensable in promoting on-chip THz communications. Driven by the widespread use of artificial intelligence (AI) in various aspects of daily life, the requirements of heavy data transmission among advanced processing units, such as graphics processing units (GPUs) and tensor processing units (TPUs), lead to a disparity known as the “interconnect gap.”26,27 The THz band, with its high carrier frequency and rich bandwidth resource, can support ultrafast rate data transmission, effectively addressing the issue of “last-centimeter communication.” Tunable PTI devices hold the promise of unlocking a multitude of potential applications in on-chip THz applications. However, current research in this field remains limited. The primary hurdle is finding a way to modulate PTI devices without compromising their topological states. THz biosensing also presents intriguing opportunities, as THz radiation can interact with medium- and long-range intermolecular interactions, including lattice phonon modes and water relaxation motions, as well as vibrations of biomacromolecules. THz imaging of cancer tissues, such as skin cancer or breast cancer, has demonstrated its potential in medical applications.28–31 However, scaling down THz biosensing to the cellular level remains challenging for two primary reasons: 1. THz wavelengths significantly differ from the size of cells; and 2. Polar solvents, like water, heavily attenuate THz signals. Integrating microfluidics with THz metamaterials can utilize localized THz waves near the metamaterial’s surface to enable trace measurements of tiny samples. Yet, existing THz systems, including both far-field and near-field setups, still have substantial volumes. The development of a THz on-chip biosensing system, incorporating the source, detector, waveguide, and microfluidic design on a single chip, is highly desirable. By focusing on THz on-chip modulation and biosensing, we aim to shed light on the immense potential of THz on-chip technology, opening up new possibilities in 6G communications and advancing the capabilities of medical biosensing at the cellular level.

The emergence of PTIs has demonstrated extraordinary properties, including backscattering-free propagation and robustness against impurities or defects. This breakthrough opens new possibilities for THz integrated device design and accelerates the application of THz technology in 6G communications and biological techniques. The topological property arises from non-zero local Berry curvatures, as defined by
Ωnk=AγkxAγky,
where An = −iun|k|un⟩ is the Berry connection, with un being the periodic part of the Bloch wave function of the nth band. The Chern number associated with an energy band is given by the integration of Berry curvature over the Brillouin zone,
Cn=1/2πBZπnd2k.

The theory of PTIs has been thoroughly explained and summarized in previous studies.32–35 In this perspective, our focus will not delve deeply into the theory of PTIs. Instead, we will concentrate on exploring the functional devices enabled by PTIs for on-chip THz applications.

PTI is rooted in ideas that were initially developed to understand the topological phases of matter in solid-state physics. The first two theoretical works predicted that two-dimensional spatially arranged periodic structures, consisting of time-reversal-breaking magneto-optical materials, could generate photonic bands with nontrivial topological invariants.36,37 As a result, these photonic systems support robust chiral states that propagate along the edge of the system at frequencies within the photonic bandgap. Shortly afterward, the theoretical predictions were experimentally realized using a two-dimensional magnetoptical photonic crystal structure in the microwave domain [Fig. 1(a)],38 representing the photonic counterpart of the electronic quantum Hall (QH) effect.

FIG. 1.

Experimental demonstrations of PTIs based on different principles. (a) The first experimental observation of PTIs based on the quantum Hall (QH) effect. Reproduced with permission from Wang et al., Nature 461(7265), 772–775 (2009). Copyright 2009 Springer Nature. (b) Experimental observation of topological edge state robust transport based on the quantum spin Hall effect (QSH) in a synthetic gauge field. The figure in (b) is reproduced with permission from Hafezi et al., Nat. Photonics 7(12), 1001–1005 (2013). Copyright 2013 Springer Nature. (c) The schematic diagram of a honeycomb lattice waveguide array with straight waveguides. Adapted from Rechtsman et al., Nat. Photonics 7, 153–158 (2013). Copyright 2013 Springer Nature.39 (d) Schematic of an array of helical waveguides. Adapted from Rechtsman et al., Nature 496(7444), 196–200 (2013). Copyright 2013 Springer Nature. (e) Microscope image of the input facet of a honeycomb lattice waveguide array with straight waveguides. Adapted from Rechtsman et al., Phys. Rev. Lett. 111, 103901 (2013). Copyright 2013 American Physical Society.40 (f) Quantum valley hall (QVH) effect-based energy band maps of PTIs. The images are reproduced with permission from T. Ma and G. Shvets, New J. Phys. 18(2), 025012 (2016). Copyright 2016 Author(s), licensed under a Creative Commons Attribution 4.0 License.

FIG. 1.

Experimental demonstrations of PTIs based on different principles. (a) The first experimental observation of PTIs based on the quantum Hall (QH) effect. Reproduced with permission from Wang et al., Nature 461(7265), 772–775 (2009). Copyright 2009 Springer Nature. (b) Experimental observation of topological edge state robust transport based on the quantum spin Hall effect (QSH) in a synthetic gauge field. The figure in (b) is reproduced with permission from Hafezi et al., Nat. Photonics 7(12), 1001–1005 (2013). Copyright 2013 Springer Nature. (c) The schematic diagram of a honeycomb lattice waveguide array with straight waveguides. Adapted from Rechtsman et al., Nat. Photonics 7, 153–158 (2013). Copyright 2013 Springer Nature.39 (d) Schematic of an array of helical waveguides. Adapted from Rechtsman et al., Nature 496(7444), 196–200 (2013). Copyright 2013 Springer Nature. (e) Microscope image of the input facet of a honeycomb lattice waveguide array with straight waveguides. Adapted from Rechtsman et al., Phys. Rev. Lett. 111, 103901 (2013). Copyright 2013 American Physical Society.40 (f) Quantum valley hall (QVH) effect-based energy band maps of PTIs. The images are reproduced with permission from T. Ma and G. Shvets, New J. Phys. 18(2), 025012 (2016). Copyright 2016 Author(s), licensed under a Creative Commons Attribution 4.0 License.

Close modal

The magnetic properties of materials tend to degrade with frequency, which poses a challenge for their application at THz and optical frequencies. To circumvent the need for external magnetic fields, quantum spin Hall (QSH) PTI was developed, preserving time-reversal symmetry compared to QH PTI. In photonic systems, the bosonic nature prohibits the existence of direct photonic analogs of the QSH PTIs. Therefore, to create a photonic counterpart of QSH PTI, an artificial spin degree of freedom needs to be introduced. The theory proposes the use of a synthetic magnetic field for photons by controlling the differential optical paths in a network of coupled resonator optical waveguides (CROWs).41 A pseudospin of 1/2 degree of freedom for photons is created when light propagates in opposite clockwise and counterclockwise directions within the CROW, allowing photons to hop between neighboring sites in two opposite directions. Subsequently, an experimental demonstration was achieved using a two-dimensional array of coupled silicon ring resonators [Fig. 1(b)].42 Various optical degrees of freedom have been utilized as pseudospins in the context of PTIs, including polarization in bi-anisotropic metamaterials,43 transverse electric and transverse-magnetic modes of photonic crystals,44,45 and p and d orbitals of artificial photonic atoms.46,47 By leveraging these pseudospins, topologically protected modes can be realized. Another class of PTI, known as Floquet PTI, can be achieved by applying periodic modulation either in space or time.48,49 Experimental realization of Floquet PTI in the optical regime has been reported by introducing a helical structure in a 2D honeycomb lattice of coupled waveguides [Figs. 1(c)1(e)].50 Furthermore, there is the valley Hall (VH) PTI, which serves as the photonic analog of valleytronic materials, hosting valley-dependent chiral edge states.51 VH PTIs do not break time-reversal symmetry, which hinders their ability to facilitate unidirectional transportation. Nevertheless, their topological states exhibit remarkable stability even in the presence of minor perturbations in system parameters and when subjected to sharp bending.52 In integrated circuits, VH PTI offers the advantage of robust light transport in a highly compact structure [Fig. 1(f)].53,54 When compared to existing THz waveguides, such as microstrip lines, coplanar waveguides, and hollow waveguides, the THz topological waveguide employing dielectric materials demonstrates both low-loss characteristics and robustness.55,56 These developments highlight the versatility and potential of various pseudospin degrees of freedom in creating photonic topological insulators, opening up new possibilities for on-chip THz applications and integrated optical circuits.

In this perspective, we begin by providing a concise review of representative works concerning tunable PTIs at different frequencies. We categorize these works based on their tuning methods, which include optical, electrical, and thermal approaches. By doing so, we aim to showcase the recent progress and diversity in the field of tunable PTIs. Next, we present an outlook on the potential application of THz on-chip sensors using integrated microfluidic devices with PTIs. This novel research area has seen limited publications, and we are excited to contribute to it by sharing some of our latest research results within this perspective. Acknowledging that tunable PTIs are still in their early stages of exploration, we hope that our findings will prove beneficial to this growing field.

In PTIs, breaking either temporal inversion symmetry (TRS) or spatial inversion symmetry (SRS)37,46 allows for the opening of a photonic bandgap at the Dirac cones. Within this bandgap, electromagnetic waves propagate along the edges of these topologically nontrivial photonic systems, demonstrating immunity to defects and backscattering even under large-angle bending,54,57,58 as described by the bulk-edge correspondence principle.59 However, breaking TRS traditionally involves introducing a magnetic field, which poses limitations as the magnetic properties of materials degrade with increasing frequency, such as THz or optical frequencies. Therefore, alternative approaches are being sought to realize PTIs in these frequency ranges without relying on magnetic fields.60 In this perspective, we focus on discussing THz on-chip applications based on valley Hall (VH) PTIs. VH PTIs have already been demonstrated in the THz band (Fig. 2).55 These PTIs break the SRS by reducing the C6 symmetry of the honeycomb lattice to C3. As a result, the degeneracy of Dirac points leads to non-vanishing valley-dependent Berry curvature, giving rise to a topologically protected photonic forbidden band. One of the advantages of VH PTIs lies in their compatibility with integrated photonic platforms. Additionally, VH PTIs have a compact structure and low out-of-plane radiation loss, making them promising candidates for on-chip THz applications.61,62 Other PTIs, such as quantum spin Hall (QSH) and Floquet PTIs, have not yet been reported in the THz range. However, they hold promise for bringing new applications in the future, further expanding the capabilities of on-chip THz technology. For example, QSH holds great promise for designing on-chip THz spin-splitters and signal routers, while Floquet PTIs offer non-reciprocal edge states with robust and backscattering-free propagation of light. A notable recent advancement is the chip-scale Floquet PTI designed for 5G wireless systems, as reported in Ref. 63. Operating at 0.5 GHz, this programmable device is compatible with a Complementary-Metal-Oxide-Semiconductor (CMOS) chip, making it an excellent candidate for on-chip THz applications. With further development of its core device, the N-path switched-capacitor, to operate in the THz band, it has the potential to revolutionize THz on-chip technologies.

FIG. 2.

Topological on-chip communication device. (a) The optical image of the fabricated twisted domain wall. (b) The simulated electric field distribution in the on-chip valley photonic crystal at 0.335 THz. The figures in (a) and (b) are reproduced with permission from Yang et al., Nat. Photonics 14(7), 446–451 (2020). Copyright 2020 Springer Nature.

FIG. 2.

Topological on-chip communication device. (a) The optical image of the fabricated twisted domain wall. (b) The simulated electric field distribution in the on-chip valley photonic crystal at 0.335 THz. The figures in (a) and (b) are reproduced with permission from Yang et al., Nat. Photonics 14(7), 446–451 (2020). Copyright 2020 Springer Nature.

Close modal

The photonic band is influenced by the refractive index of the substrate and cladding materials. By utilizing a refractive index tunable material, it becomes possible to shift the photonic band, covering or uncovering the targeted frequency, which gives rise to the concept of reconfigurable PTIs. Within PTIs, the transmitted THz light in the waveguide remains unaffected by defects but can be attenuated or modulated due to the presence of free carriers adhering to the waveguide structure. These characteristics form the basis for our subsequent discussion on optical, electrical, and thermal-induced tunable THz PTIs.

High-resistivity silicon (HR-Si) is a widely utilized material for THz devices due to its remarkable attributes, such as low-loss and non-dispersion features across a wide THz band. Moreover, it exhibits excellent compatibility with other photoelectronic devices on the CMOS platform. The first experimentally demonstrated waveguide in the THz band was manufactured using HR-Si with VH PTIs.55 Subsequently, on-chip THz taper-free waveguides, multiport couplers, wave division, and whispering gallery mode resonators in the THz band were experimentally demonstrated.64–72 However, there is a greater desire for tunable PTIs in the THz band for future applications.

One possible approach to design tunable PTIs is by adjusting the refractive index of the substrate, which can effectively shift the photonic band of PTIs. This concept was successfully demonstrated at infrared frequencies using an extremely strong femtosecond laser (fluence of 18.1 mJ/cm2) to photoexcite a silicon-on-insulator (SOI) wafer, resulting in a refractive index change of Δn = −0.02 + 0.0013i. As a consequence, the change in the substrate’s refractive index led to a blue shift of the transmission spectrum by up to 20 nm and a reduction in transmission of ∼85% [Fig. 3(a)].73 In the THz band, employing this method encounters two challenges: 1. The footprint of THz PTIs is considerably larger than that in the infrared band, causing the femtosecond laser spot to be unable to fully cover the entire PTI structure. 2. Additionally, the THz signal experiences attenuation due to photoexcited free carriers, effectively switching off the PTIs’ waveguide. Therefore, in the THz band, utilizing the optical pumping method to induce a refractive index change in the substrate for tunable PTIs is not feasible. However, a switchable THz PTI can be accomplished by photoexcited free carriers.56 A VH PTI utilizing HR-Si and irradiated by a continuous laser (λ = 447 nm) demonstrated a 19 dB attenuation to the THz wave, covering a bandwidth of 20 GHz with a 3 dB switching bandwidth of 60 kHz,74 as shown in Fig. 3(b). Kumar et al.75 reported on the active tuning of topological waveguides coupled with cavities through continuous laser pumping. They achieved a remarkable 60 dB intensity modulation at a frequency of f = 300 GHz with a modulation speed of up to 10 kHz [Fig. 3(c)].

FIG. 3.

Experimental demonstrations of pumped light regulating PTIs. (a) Adjusting the refractive index of silicon based topological waveguides by pump beam irradiation. The image is reproduced with permission from Shalaev et al., Optica 6(7), 839 (2019). Copyright 2019 Author(s), licensed under a Creative Commons Attribution 4.0 License. (b) Experimental setup for topology optical switching using a continuous laser. The image is reproduced with permission from Liu et al., Photonics Res. 10(4), 1090 (2022). Copyright 2022 Author(s), licensed under a Creative Commons Attribution 4.0 License. (c) Optical control of a THz topological cavity-waveguide chip achieved through a continuous laser. The image is reprinted with permission from Kumar et al., Adv. Mater. 34(27), 2202370 (2022). Copyright 2022 John Wiley and Sons.

FIG. 3.

Experimental demonstrations of pumped light regulating PTIs. (a) Adjusting the refractive index of silicon based topological waveguides by pump beam irradiation. The image is reproduced with permission from Shalaev et al., Optica 6(7), 839 (2019). Copyright 2019 Author(s), licensed under a Creative Commons Attribution 4.0 License. (b) Experimental setup for topology optical switching using a continuous laser. The image is reproduced with permission from Liu et al., Photonics Res. 10(4), 1090 (2022). Copyright 2022 Author(s), licensed under a Creative Commons Attribution 4.0 License. (c) Optical control of a THz topological cavity-waveguide chip achieved through a continuous laser. The image is reprinted with permission from Kumar et al., Adv. Mater. 34(27), 2202370 (2022). Copyright 2022 John Wiley and Sons.

Close modal

The long lifetime of carriers in HR-Si substrates poses a limitation on the switch rate achievable through photoexcitation methods. To overcome this limitation, there are two potential approaches: 1. Shortening the lifetime of HR-Si carriers through ion-implantation. This technique was previously employed in the 1980s to generate THz time-domain signals using a photoconductive antenna (PCA) on silicon on a sapphire substrate (SOS);76 2. Utilizing semiconductor materials with inherently short carrier lifetimes, such as gallium arsenide (GaAs) or indium phosphide (InP).77–80 For the first approach, ion-implantation on the surface of an HR-Si substrate is not feasible, as the photoexcited free carriers would diffuse into the substrate. In the SOI structure, the silicon dioxide (SiO2) layer acts as an effective barrier, preventing the diffusion of photoexcited free carriers and confining them within the top silicon layer. This confinement within the top silicon layer accelerates carrier recombination, effectively shortening their lifetime, as shown in Fig. 4. In the case of an SOI substrate, the ultrathin SiO2 insulation layer can be neglected, thereby requiring the parameters of the unit cell of VH PTIs to remain the same.

FIG. 4.

Scheme of optical modulation of PTIs based on SOI wafer. The photoexcited free carriers are confined within the top Si layer, which accelerates their recombination and enhances the switch rate.

FIG. 4.

Scheme of optical modulation of PTIs based on SOI wafer. The photoexcited free carriers are confined within the top Si layer, which accelerates their recombination and enhances the switch rate.

Close modal

An SOS wafer is also applicable to enhance the switch rate due to its sub-picosecond carrier lifetime.76,81,82 The thickness of silicon in SOS is ultrathin; the parameters of the unit cell for PTIs with SOS need to be recalculated based on the optical constants of sapphire in the THz band (SOS, εsapphire = 10.5).83 The second approach involves using semiconductors with short lifetimes of free carriers, such as GaAs (εGaAs = 12.9)84 or InP (εInP = 12.5).85 For instance, low-temperature grown GaAs (LT-GaAs) exhibits a femtosecond-level carrier lifetime and is commonly used in fabricating THz photoconductive antennas (PCA).86 When these materials are clad with air, the parameters of the unit cell for THz VH PTIs are simulated and depicted in Fig. 5. In the simulation, the targeted frequency range is the J-Band (220–325 GHz), which aligns with the IEEE channelization standards (IEEE Std. 802.15.3d-2017) newly approved band (252.72–321.84 GHz). Additionally, the outcome of the World Radiocommunication Conference (WRC 2019) has established a regulatory framework for this specific band,87 which is expected to pave the way for significant advancements in future THz communication systems within this frequency range. In this frequency band, the dispersion and absorption of the four intrinsic materials (HR-Si, sapphire, GaAs, and InP) can be neglected. The cell cycle is set to 340 µm, and each cell unit consists of two triangular air holes. By adjusting the length difference of the equilateral triangle air holes, the topological photonic bandgap is achieved. Specifically, the simulated side lengths of the two triangular air holes are 245 µm (A) and 141 µm (B). The energy band with different substrate materials is illustrated in Fig. 5(a). For this study, the thickness of the silicon substrate is set at h = 170 µm, which has minimal influence on the transmission of PTIs and mainly leads to a shift in the operating band. Consequently, the substrate can be set to a specific thickness based on the frequency of the THz source. Figure 5(b) shows the bandwidth of the photonic band for VH PTIs using the four aforementioned substrate materials, which are almost the same.

FIG. 5.

(a) Photonic energy band of VH PTIs with four different substrate materials. The cell cycle is 340 µm, the side length of triangular air hole A is 245 µm, and air hole B is 141 µm. (b) The results of the photonic bandgap width of four materials in simulation.

FIG. 5.

(a) Photonic energy band of VH PTIs with four different substrate materials. The cell cycle is 340 µm, the side length of triangular air hole A is 245 µm, and air hole B is 141 µm. (b) The results of the photonic bandgap width of four materials in simulation.

Close modal

The fabrication of PTI devices using materials like GaAs, InP, and sapphire can be more challenging and expensive compared to silicon. However, anisotropic dry etching with depths exceeding 100 µm is commercially available with these materials, providing researchers with flexibility in selecting substrate materials based on specific requirements.

The development of topological insulator lasers offers several key advantages, including robustness against fabrication variations and defects, high emission slope efficiency, a high Q-factor, and the ability to achieve single-mode lasing.88–91 These attributes open up new opportunities for the creation of fully functional photonic integrated circuits. Recently, there has been a report on an electrically pumped topological THz quantum cascade laser (QCL) featuring valley edge modes92 that operate around 3.2 THz [Fig. 6(a)]. In this QCL design, a straight valley edge-state waveguide is introduced below the bottom arm of the triangular loop cavity, along with gratings on the left and right ends to outcouple laser light. This electrically pumped QCL can serve as a THz source for ultrafast on-chip communication, but directly modulating the QCL source to achieve Gbps or even Tbps data rates is extremely challenging and necessitates a high-rate external modulator. One promising approach to tackle this challenge is to couple the QCL laser into a topologically protected waveguide [Fig. 6(b)] instead of using the outcoupling grating. This waveguide can be fabricated using LT-GaAs, and its modulation can be achieved by an external pumping light. The femtosecond carrier lifetime in the LT-GaAs substrate allows for the possibility of achieving data transmission rates in the range of hundreds of Gbps or even Tbps, with the modulation speed being determined by the pumping light’s characteristics. This approach holds great potential for enabling the development of fully functional THz integrated circuits, particularly for applications in high-speed data transmission.

FIG. 6.

(a) Electrically pumped THz topological QCL with valley edge modes. The image is reprinted with permission from Zeng et al., Nature 578(7794), 246–250 (2020). Copyright 2020 Springer Nature. (b) Scheme of an optically pumped PTI waveguide based on LT-GaAs, with carrier lifetime in femtosecond level.

FIG. 6.

(a) Electrically pumped THz topological QCL with valley edge modes. The image is reprinted with permission from Zeng et al., Nature 578(7794), 246–250 (2020). Copyright 2020 Springer Nature. (b) Scheme of an optically pumped PTI waveguide based on LT-GaAs, with carrier lifetime in femtosecond level.

Close modal

Instead of using direct optical pumping on the substrate material, which typically requires high-power femtosecond lasers, the PTIs can be more effectively tuned at infrared frequencies using a thin film of transparent conducting oxides (TCO).93, Figure 7 illustrates an array of coupled ring resonators, where the coupling between these resonators generates a synthetic gauge magnetic field, resulting in topologically protected states of photons. To achieve active tuning of the refractive index, a thin film made of aluminum-doped zinc oxide (AZO), a TCO material, is deposited on the “link” couplers. The refractive index can be dynamically adjusted by applying a pumping light to excite free carriers. In the THz band, the optical conductivity of certain TCO materials, such as indium-tin oxide (ITO), is high, rendering them similar to metal-like materials.94 Consequently, introducing these TCO materials can directly break the topological state in the THz band. However, there are phase transition materials, such as vanadium dioxide (VO2),95,96 Ge2Sb2Te5 (GST) alloy,97 and germanium telluride (GeTe),98 that exhibit a wide tunable range of their refractive index within the THz band. These materials provide a promising alternative for achieving active tuning of photonic systems in the THz frequency range. Among the materials mentioned, VO2 undergoes a metal–insulator phase transition at ∼68 °C. However, this transition is volatile and requires external energy to maintain its conductive state. On the other hand, GST and GeTe exhibit reversible and nonvolatile phase transitions. GST, in particular, has been widely used in optical memory applications, such as CDs and DVDs, since the 1970s. Its phase transition can be altered by an external pumping light-induced temperature change. At around 150 °C, amorphous GST transforms into a cubic phase, which remains stable at room temperature until further heating is applied. It is important to note that the phase transitions of these materials are primarily dependent on thermal effects, which will be discussed in the section on thermally induced tunable PTIs in this paper.

FIG. 7.

(a) Real and imaginary parts of the refractive index of the aluminum-doped zinc oxide (AZO) film at two regimes, without pump and with pump. (b) Cross-section of the AZO integrated tunable link waveguide: on a silicon dioxide substrate. (c) Fundamental mode distribution inside a tunable link waveguide. (d) The unit cell used within transfer matrix formalism for strongly coupled microring resonators. (e) “Short-edge” and “Long-edge” topologically protected bands without and with pump. (f) Field distribution inside the photonic lattice without pump for “Short-edge” and “Long-edge” bands. (g) Field distribution inside the photonic lattice with pump for “Short-edge” and “Long-edge” bands. The figures in (a)–(g) are reproduced with permission from Kudyshev et al., ACS Photonics 6(8), 1922–1930 (2019). Copyright 2019 American Chemical Society.

FIG. 7.

(a) Real and imaginary parts of the refractive index of the aluminum-doped zinc oxide (AZO) film at two regimes, without pump and with pump. (b) Cross-section of the AZO integrated tunable link waveguide: on a silicon dioxide substrate. (c) Fundamental mode distribution inside a tunable link waveguide. (d) The unit cell used within transfer matrix formalism for strongly coupled microring resonators. (e) “Short-edge” and “Long-edge” topologically protected bands without and with pump. (f) Field distribution inside the photonic lattice without pump for “Short-edge” and “Long-edge” bands. (g) Field distribution inside the photonic lattice with pump for “Short-edge” and “Long-edge” bands. The figures in (a)–(g) are reproduced with permission from Kudyshev et al., ACS Photonics 6(8), 1922–1930 (2019). Copyright 2019 American Chemical Society.

Close modal

One potential approach to achieve an electrically reconfigurable topological photonic crystal device is by altering the refractive index of the cladding material. This method has been previously demonstrated in the microwave and millimeter frequencies using liquid crystals (LCs).99 When an external voltage is applied, the orientation of LC molecules can be rotated, resulting in a change in the refractive index of the background medium and a corresponding shift in the spectral position of the photonic bandgap.100,101 In addition to changing the refractive index of the background medium, LCs have also been utilized to fill the photonic crystal rods. By varying the applied voltage on these rods, the orientation of the LCs within the silicon cyclic rings can be rotated, allowing for the tuning of topological edge states.102 In the THz band, the tunable range of refractive index for LCs typically lies between 1.5 and 1.7.103 However, electrically tunable PTIs have not been previously reported in this frequency range. In this perspective, we propose a novel design for tunable PTIs utilizing electrically controlled LCs in the THz band, as illustrated in Fig. 8. Our design is based on VH PTIs, with HR-Si selected as the substrate material. The top and bottom electrodes consist of ITO glass and are separated from the PTI by two spacers, which are thick enough to avoid interfacing with the evanescent wave of transmitted THz light in the PTI waveguide along the z-direction. To investigate the tunable characteristics, we conducted simulations by varying the refractive index of the LCs between 1.5 and 1.7. The simulation results are presented in Fig. 9, revealing the relationship between the refractive index of the background material and the spectral position of the photonic bandgap. Within the frequency range of 260–270 GHz, represented with gray rectangular in Fig. 10, the simulation demonstrates a transmitted intensity difference of ∼25 dB, suggesting that an effective switch of the THz PTIs can be achieved with this proposed design.

FIG. 8.

(a) Schematic view of the reconfigurable PTIs utilizing LCs as the refractive index tunable background medium. (b) The relationship between the axial orientation of liquid crystal molecules and electric field direction and the energy density distribution of transmitted electromagnetic waves at two voltages. The figures in (a) and (b) are reproduced with permission from Shalaev et al., New J. Phys. 20(2), 023040 (2018). Copyright 2018 Author(s), licensed under a Creative Commons Attribution 4.0 License.

FIG. 8.

(a) Schematic view of the reconfigurable PTIs utilizing LCs as the refractive index tunable background medium. (b) The relationship between the axial orientation of liquid crystal molecules and electric field direction and the energy density distribution of transmitted electromagnetic waves at two voltages. The figures in (a) and (b) are reproduced with permission from Shalaev et al., New J. Phys. 20(2), 023040 (2018). Copyright 2018 Author(s), licensed under a Creative Commons Attribution 4.0 License.

Close modal
FIG. 9.

(a) Schematic of reconfigurable PTIs based on HR-Si substrate VH PTIs surrounded by LCs. Two ITO glass plates act as the electrodes to apply voltage for rotating the orientation of LCs. Two spacers separate the ITO glass from the HR-Si platform. (b) Topological energy band diagrams of this design with a tunable refractive index of LCs. In the tunable range of refractive index of LCs, representative values of 1.5, 1.6, and 1.7 were selected for calculations.

FIG. 9.

(a) Schematic of reconfigurable PTIs based on HR-Si substrate VH PTIs surrounded by LCs. Two ITO glass plates act as the electrodes to apply voltage for rotating the orientation of LCs. Two spacers separate the ITO glass from the HR-Si platform. (b) Topological energy band diagrams of this design with a tunable refractive index of LCs. In the tunable range of refractive index of LCs, representative values of 1.5, 1.6, and 1.7 were selected for calculations.

Close modal
FIG. 10.

Transmission spectra of HR-Si substrate VH PTIs corresponding to the refractive index of LCs of 1.5, 1.6, and 1.7. The gray rectangle highlights the frequency range of 260–270 GHz. Within this range, the transmitted intensity exhibits an abrupt change, which corresponds to the refractive index change of LCs.

FIG. 10.

Transmission spectra of HR-Si substrate VH PTIs corresponding to the refractive index of LCs of 1.5, 1.6, and 1.7. The gray rectangle highlights the frequency range of 260–270 GHz. Within this range, the transmitted intensity exhibits an abrupt change, which corresponds to the refractive index change of LCs.

Close modal

Another approach to achieving a tunable PTI is through reconfiguring the unit cell. One of the earliest demonstrations of this method was conducted by Darabi et al.,104 who utilized a graphene-like polylactic acid (PLA) layer as the host medium in an electroacoustic PTI. They employed negative capacitance circuits to alter the mechanical impedance of the shunted piezoelectric (PZT) disks, resulting in a reconfigurable PTI in the acoustic domain, as depicted in Fig. 11(a). In the electromagnetic domain, a programmable metasurface was integrated with PTI theory to design reconfigured plasma PTIs [Fig. 11(b)].105 In this system, each lattice cell within the programmable metasurface PTI could be independently encoded using a positive-intrinsic-negative (PIN) diode, enabling the reconfiguration of the electromagnetic wave’s propagation path. The fabrication of PTIs using printed circuit board (PCB) technology offers compatibility with PCB-based on-chip optoelectronic integrated circuits. Nevertheless, directly applying this design to the THz band presents several challenges: first, commercially available PIN diodes that operate in the THz band are still lacking; second, PCB boards exhibit high loss at THz frequencies. In the future, as semiconductor technology continues to develop, it is expected that PIN diodes suitable for the THz band will become more readily available, enabling their integration with PTIs on a single chip. At present, a promising alternative for reconfigurable PTIs in the THz band is the use of a two-dimensional electron gas (2DEG) structure-composited metasurface, which has already been employed as an active THz amplitude106,107 and phase108,109 modulator (Fig. 12), potentially replacing the role of PIN diodes.

FIG. 11.

(a) Schematic of the phononic crystal formed by hexagonal unit cells with graphene-like polylactic acid (PLA) as the host layer and attached circular piezoelectric (PZT) patches connected to external circuits. Reproduced with permission from Darabi et al., Proc. Natl. Acad. Sci. U. S. A. 117(28), 16138–16142 (2020). Copyright 2020 Author(s), licensed under a Creative Commons Attribution 4.0 License. (b) Schematic of an ultrafast reprogrammable plasmonic topological insulator. Reproduced with permission from You et al., Nat. Commun. 12(1), 5468 (2021). Copyright 2021 Author(s), licensed under a Creative Commons Attribution 4.0 License.

FIG. 11.

(a) Schematic of the phononic crystal formed by hexagonal unit cells with graphene-like polylactic acid (PLA) as the host layer and attached circular piezoelectric (PZT) patches connected to external circuits. Reproduced with permission from Darabi et al., Proc. Natl. Acad. Sci. U. S. A. 117(28), 16138–16142 (2020). Copyright 2020 Author(s), licensed under a Creative Commons Attribution 4.0 License. (b) Schematic of an ultrafast reprogrammable plasmonic topological insulator. Reproduced with permission from You et al., Nat. Commun. 12(1), 5468 (2021). Copyright 2021 Author(s), licensed under a Creative Commons Attribution 4.0 License.

Close modal
FIG. 12.

(a) Cross-sectional diagram of the high electron mobility transistor (HEMT) controlled metamaterial. (b) Schematic of the metamaterial array. (c) Schematic of the staggered netlike metamaterial THz modulator. The figures in (a)–(c) are reproduced with permission from Zhao et al., Nano Lett. 19(11), 7588–7597 (2019). Copyright 2019 American Chemical Society.

FIG. 12.

(a) Cross-sectional diagram of the high electron mobility transistor (HEMT) controlled metamaterial. (b) Schematic of the metamaterial array. (c) Schematic of the staggered netlike metamaterial THz modulator. The figures in (a)–(c) are reproduced with permission from Zhao et al., Nano Lett. 19(11), 7588–7597 (2019). Copyright 2019 American Chemical Society.

Close modal

In comparison with the photoexcitation method, the electrical method offers several advantages. First, it eliminates the need for bulky lasers, making the setup more compact and efficient. Second, the electrical method is highly compatible with integrated circuits and CMOS platforms. It can be seamlessly integrated into existing electronic systems, allowing for the development of more versatile and multifunctional devices.

GST alloys110 have garnered significant interest due to their excellent temperature-sensitive properties, which include fast switching, outstanding scalability, and thermal stability.111,112 By precisely manipulating the temperature, GST alloys can undergo rapid and reversible transitions between the amorphous and crystalline phases. These distinct states display substantial variations in dielectric constants, making GST alloys an optimal candidate for tunable devices. Based on the properties described earlier, a proposal was put forth for a PTI λ = 2174 nm that was successfully achieved utilizing the phase change material GST,113 as illustrated in Fig. 13(a). This design involved regulating the temperature of GST rods, each with a thickness of 100 nm, by applying a bias voltage. This temperature control led to a phase transition from the amorphous state to the crystalline state, resulting in a significant change in its refractive index. Other phase change materials, such as Sb2S3 and Sb2Se3, have also been utilized for designing PTIs,114 as depicted in Fig. 13(b). Sb2S3 offers a wider bandgap and relatively fast switching time without requiring continuous temperature control. This allows for a temperature-tuned topology edge-state and corner-state switching. However, so far, the proposed tunable PTIs using phase change materials remain theoretical designs without experimental implementation. In the THz band, designing PTIs based on GST differs from that in the infrared band,113 mainly due to the required height of the GST rod, which would need to be hundreds of micrometers. Unfortunately, such a height is impractical for fabrication purposes. In this perspective, we introduce a practical PTI waveguide utilizing GST, which can be feasibly manufactured with current technology, as illustrated in Fig. 14(a). Our simulations demonstrate that by manipulating its phase state, the PTI waveguide can be effectively switched on/off, enabling efficient control of the waveguide’s functionality, as shown in Fig. 14(b). The simulation parameters are the same as those in Fig. 5. HR-Si was taken as the substrate material. The size of the phase transition material is 5 × 5 mm2 at a height of 200 nm. In future experimental demonstrations, phase transition materials like GST and VO2 can be deposited using a magnetron sputtering system, ensuring good integration with the substrate.96,97,115,116 GST can be treated as a dielectric material with a conductivity of 200 S/m at the THz band in the amorphous state, while in the crystalline state, its conductivity exhibits an abrupt change to 75 000 S/m.97,117

FIG. 13.

Two proposed reconfigurable PTIs utilizing temperature-dependent phase change materials. (a) The GST pillars in a topological 2D photonic crystal structure, enabling the switching of light propagation “on/off” by applying voltage. The schematic diagram showcases the overall device, the unit cell, and the distribution of electric field transmission strength in both the amorphous and crystalline states. Reproduced with permission from Cao et al., Sci. Bull. 64(12), 814–822 (2019). Copyright 2019 Elsevier. (b) Topological energy band diagrams corresponding to amorphous and crystalline forms of the phase transition material Sb2S3 at different temperatures and reversible transitions of topological edge and angular states at phase transition temperatures. Reproduced with permission from Zhang et al., Ann. Phys. 534(1), 2100293 (2021). Copyright 2021 John Wiley and Sons.

FIG. 13.

Two proposed reconfigurable PTIs utilizing temperature-dependent phase change materials. (a) The GST pillars in a topological 2D photonic crystal structure, enabling the switching of light propagation “on/off” by applying voltage. The schematic diagram showcases the overall device, the unit cell, and the distribution of electric field transmission strength in both the amorphous and crystalline states. Reproduced with permission from Cao et al., Sci. Bull. 64(12), 814–822 (2019). Copyright 2019 Elsevier. (b) Topological energy band diagrams corresponding to amorphous and crystalline forms of the phase transition material Sb2S3 at different temperatures and reversible transitions of topological edge and angular states at phase transition temperatures. Reproduced with permission from Zhang et al., Ann. Phys. 534(1), 2100293 (2021). Copyright 2021 John Wiley and Sons.

Close modal
FIG. 14.

(a) Schematic of reconfigurable PTI based on phase transition material and HR-Si substrate VH PTIs. The size of the phase transition material is 5 × 5 mm2 at a height of 200 nm. (b) Topological transmission spectra with different phase states of GST. It includes an amorphous state (a-GST) with a conductivity of 200 S/m and a crystalline state (c-GST) with a conductivity of 75 000 S/m.

FIG. 14.

(a) Schematic of reconfigurable PTI based on phase transition material and HR-Si substrate VH PTIs. The size of the phase transition material is 5 × 5 mm2 at a height of 200 nm. (b) Topological transmission spectra with different phase states of GST. It includes an amorphous state (a-GST) with a conductivity of 200 S/m and a crystalline state (c-GST) with a conductivity of 75 000 S/m.

Close modal

The metal–insulator transition of VO2 introduces a significant change in its conductivity, making it an ideal candidate to modulate the transport of topological edge states at infrared frequencies.118 As depicted in Fig. 15(a), a topological grating heterostructure consisting of VO2/SiO2 was successfully employed to achieve on/off modulated transmission of one-dimensional topological edge states, capitalizing on the thermotropic phase transition property of VO2. Similarly, in Fig. 15(b), a theoretical design of a metal rod VH PTI in the THz band is presented, where VO2 is used as a replacement for the periodic structures.119 By controlling the temperature of the VO2 rod, theoretical simulations demonstrate the possibility of achieving on/off switching of THz topological edge states. However, it is important to note that the implementation of this design faces practical challenges, primarily due to the required height of the VO2 column, which is 250 µm, rendering it difficult to realize in practice.

FIG. 15.

One- and two-dimensional reconfigurable PTIs based on phase change material, VO2. (a) One-dimensional topological photonic crystal transmission intensities corresponding to metallic and insulating phases due to different conductivities of VO2. Reproduced with permission from Li et al., Adv. Opt. Mater. 6(4), 1701071 (2018). Copyright 2018 John Wiley and Sons. (b) Transport intensity and electric field intensity distribution of 2D PTIs at different conductivities. Reproduced with permission from Li et al., IEEE Photonics J. 14(3), 4633206 (2022). Copyright 2022 Author(s), licensed under a Creative Commons Attribution 4.0 License.

FIG. 15.

One- and two-dimensional reconfigurable PTIs based on phase change material, VO2. (a) One-dimensional topological photonic crystal transmission intensities corresponding to metallic and insulating phases due to different conductivities of VO2. Reproduced with permission from Li et al., Adv. Opt. Mater. 6(4), 1701071 (2018). Copyright 2018 John Wiley and Sons. (b) Transport intensity and electric field intensity distribution of 2D PTIs at different conductivities. Reproduced with permission from Li et al., IEEE Photonics J. 14(3), 4633206 (2022). Copyright 2022 Author(s), licensed under a Creative Commons Attribution 4.0 License.

Close modal

In addition to its applicability with GST, our proposed design in Fig. 14(a) can also be used with a VO2 thin film. In practical applications, VO2 can be exposed to infrared light to increase its temperature, but it is the thermal effect that ultimately triggers the desired tunability. This approach enables efficient control of the PTI properties and facilitates the realization of dynamic and reconfigurable photonic devices, offering promising opportunities for advanced optoelectronic technologies.

THz waves exhibit unique characteristics, including low photon energy, high penetration through non-conductive materials, and label-free detection.120 As a result, THz technology has found diverse applications across various fields, such as material analysis,121 biomedical science,122 food safety,123 smart agriculture,124 environmental monitoring,125 and security inspection.126 Despite their wide-ranging applications, conventional THz sensing technologies typically rely on bulky and expensive systems, making them unsuitable for portable applications.

PTIs exhibit robust edge transport with significantly suppressed backscattering caused by the disorder and sharp bends, making them highly suitable for on-chip sensors compared to effective media cladded waveguides127 or dielectric photonic crystal waveguides.128,129 In the domain of THz sensing, PTIs have shown promise in detecting various parameters. For instance, PTIs have been utilized to measure the thickness of polyimide film and the distance between the film and the PTI structure130 [Fig. 16(a)]. These findings highlight the immense potential of PTIs for achieving ultrahigh sensitivity sensing. Furthermore, recent research by Navaratna et al. showcased a topological waveguide cavity with high Q factors, demonstrating its applicability for biomolecule detection and leaf-hydration monitoring131 [Fig. 16(b)]. In the biosensing experiments, different solutions were loaded onto absorbent paper and detected using the evanescent wave in the topological cavity. These studies effectively underscore the capabilities of THz PTIs in biosensing applications. However, to further enhance the efficiency of PTI-based biosensors, there are two important considerations. Firstly, the need for precise sample placement in relation to the PTI platform limits the detection speed. Secondly, it is preferable to directly detect the biosample using the PTI structure rather than loading the sample onto absorbent paper.

FIG. 16.

THz PTIs for sensing: (a) A high Q THz PTIs used for detecting the thickness of a polyimide film and the distance between the films and the cavity. Reproduced with permission from Kumar et al., Appl. Phys. Lett. 121(1), 011101 (2022). Copyright 2022 AIP Publishing. (b) A high Q THz PTIs used for leaf-hydration monitoring and solvent detection. Reproduced with permission from Navaratna et al., Appl. Phys. Lett. 123(3), 033705 (2023). Copyright 2023 AIP Publishing.

FIG. 16.

THz PTIs for sensing: (a) A high Q THz PTIs used for detecting the thickness of a polyimide film and the distance between the films and the cavity. Reproduced with permission from Kumar et al., Appl. Phys. Lett. 121(1), 011101 (2022). Copyright 2022 AIP Publishing. (b) A high Q THz PTIs used for leaf-hydration monitoring and solvent detection. Reproduced with permission from Navaratna et al., Appl. Phys. Lett. 123(3), 033705 (2023). Copyright 2023 AIP Publishing.

Close modal

Microfluidic devices have gained significant attention in the realm of THz biosensing, offering diverse applications in DNA detection,132 protein analysis,133 and cell characterization.134 These microfluidic-based biosensors leverage the advantages of combining microfluidics with THz time-domain spectroscopy systems135 or metamaterial devices.136 By incorporating microfluidic channels, these biosensors can effectively suppress the attenuation caused by water, leading to increased sensitivity in detecting solvated biomolecules, cells, or other biological samples. This suppression of water interference is crucial in THz biosensing, as it enables the detection of minute changes or interactions occurring at the cellular level within the samples.

Due to the relatively long wavelength of THz waves, which reaches the level of hundreds of micrometers, THz microfluidic chambers require a large volume of sample to effectively interact with the THz radiation137 [Fig. 17(a)]. To address this challenge and enhance sensitivity, researchers have explored innovative approaches, such as integrating microfluidic channels directly onto the THz metasurface. For instance, Geng et al.138 demonstrated a split-ring resonator metasurface integrated with microfluidics for bio-sensing applications [Fig. 17(b)]. By placing the microfluidic channel in close proximity to the THz metasurface, sample consumption can be reduced, leading to improved sensitivity in biosensing applications. Similarly, Serita et al.139 developed a THz-microfluidic chip based on a non-linear optical crystal (NLOC) to measure ultra-trace amounts of solution samples [Fig. 19(c)]. The integration of microfluidics with THz technology in this design enables precise detection and analysis of small sample volumes. However, it is important to note that metasurface-integrated microfluidic designs still require the coupling of free-space THz light to the metasurface, which can result in bulky setups.

FIG. 17.

THz microfluidic: (a) A THz microfluidic sensor consisted of capillaries, chamber, and ports on the silicon wafer; two metal plates used to clamp the PDMS coupling structures in place. Reproduced with permission from Baragwanath et al., J. Appl. Phys. 108(1), 013102 (2010). Copyright 2010 AIP Publishing. (b) A split-ring resonator metasurface integrated microfluidics. Reproduced with permission from Geng et al., Sci. Rep. 7(1), 16378 (2017). Copyright 2017 Author(s), licensed under a Creative Commons Attribution 4.0 License. (c) A non-linear optical crystal (NLOC)-based THz-microfluidic chip. Reproduced with permission from Serita et al., Photonics 6(1), 12 (2019). Copyright 2019 Author(s), licensed under a Creative Commons Attribution 4.0 License.

FIG. 17.

THz microfluidic: (a) A THz microfluidic sensor consisted of capillaries, chamber, and ports on the silicon wafer; two metal plates used to clamp the PDMS coupling structures in place. Reproduced with permission from Baragwanath et al., J. Appl. Phys. 108(1), 013102 (2010). Copyright 2010 AIP Publishing. (b) A split-ring resonator metasurface integrated microfluidics. Reproduced with permission from Geng et al., Sci. Rep. 7(1), 16378 (2017). Copyright 2017 Author(s), licensed under a Creative Commons Attribution 4.0 License. (c) A non-linear optical crystal (NLOC)-based THz-microfluidic chip. Reproduced with permission from Serita et al., Photonics 6(1), 12 (2019). Copyright 2019 Author(s), licensed under a Creative Commons Attribution 4.0 License.

Close modal

The similarities between the scheme of a microfluidic channel and the scheme of a waveguide in PTIs have inspired researchers to explore the possibility of integrating PTIs with microfluidic channels, as shown in Figs. 18(a) and 18(b). This integration aims to combine the advantages of THz light, the robustness and flexibility of PTI waveguides, and the powerful fluid handling capabilities of microfluidics into a single device. The resulting hybrid device holds tremendous potential and offers numerous benefits, such as high sensitivity, portability, fast detection, and high compactness.

FIG. 18.

(a) and (b) Waveguide route in THz PTIs and liquid channels in microfluidic are both marked by red lines. The outlines of these highlighted regions exhibit some similarities in their shape. The figure shown in (a) is reproduced with permission from Yang et al., Nat. Photonics 14(7), 446–451 (2020). Copyright 2020 Springer Nature. The figure shown in (b) is reproduced with permission from Fan et al., Biosens. Bioelectron. 145, 111730 (2019). Copyright 2019 Elsevier.

FIG. 18.

(a) and (b) Waveguide route in THz PTIs and liquid channels in microfluidic are both marked by red lines. The outlines of these highlighted regions exhibit some similarities in their shape. The figure shown in (a) is reproduced with permission from Yang et al., Nat. Photonics 14(7), 446–451 (2020). Copyright 2020 Springer Nature. The figure shown in (b) is reproduced with permission from Fan et al., Biosens. Bioelectron. 145, 111730 (2019). Copyright 2019 Elsevier.

Close modal

The integration of microfluidic components into THz PTIs requires careful consideration of the materials used for manufacturing. Various polymeric materials, such as polydimethylsiloxane (PDMS),140 cyclic olefin polymer (COP),141 polyethylene terephthalate (PET),142 polymethyl methacrylate (PMMA),143 and copolymers of cycloolefin (COC),144 are commonly employed in microfluidic device fabrication. A PDMS, in particular, has gained significant attention due to its gas-permeability, which provides oxygen and buffering capabilities, making it suitable for manipulating living cells,145 which enables the manipulation of living cells. PDMS also possesses a refractive index of around 1.6 and exhibits relatively low absorption in the low THz frequency range (Fig. 19, as measured by the authors). This refractive index is close to that of liquid crystals (LCs) in the THz band, making PDMS a viable choice for integration with HR-Si as a substrate to realize a topologically protected edge-state. Furthermore, thermoplastic materials like COP and PMMA have refractive indices around 1.5 and exhibit low absorption at THz frequencies,141,143 making them potential candidates for THz PTI microfluidic devices when integrated with HR-Si substrates.

FIG. 19.

The refractive index of PDMS (90% Sylgard 184 prepolymer, 10% curing agent, cured at 65 °C for 6 h, 4.03 ± 0.02 mm thick) at 0.1–1.1 THz. The THz spectra of PDMS were measured using a commercial time-domain terahertz spectrometer (TERA K15, Menlo System, Germany).

FIG. 19.

The refractive index of PDMS (90% Sylgard 184 prepolymer, 10% curing agent, cured at 65 °C for 6 h, 4.03 ± 0.02 mm thick) at 0.1–1.1 THz. The THz spectra of PDMS were measured using a commercial time-domain terahertz spectrometer (TERA K15, Menlo System, Germany).

Close modal

The structure of the microfluidic chip can be tailored to specific needs, offering a wide range of applications, including bioassay chips,146 cell and organ chips,147,148 droplet microfluidic chips,149 drug screening chips,145 medical application chips,150 and more. In our previously published THz PTI waveguide, the width of the defect line is ∼56 µm,56 allowing for the integration of a microfluidic liquid channel with a cross-section size of 30 × 30 μm2 to form a closed channel on the waveguide [the inset in Fig. 20(b)]. This channel size is capable of accommodating single cells, making our design potentially suitable for THz single-cell detection. With proper microfluidic structure design, THz-powered cell sorting could also be a viable option in the future. Moreover, PTIs with high Q cavities can be utilized for ultrahigh-sensitive biosensing applications. By incorporating a resonant tunneling diode (RTD), a compact solid-state electronic device, as the THz source and detector and integrating it onto the PTI device, a genuine on-chip THz biosensor can be realized. This integration would lead to a powerful and self-contained platform for THz biosensing. Such integrated platforms could revolutionize biosensing capabilities, enabling new frontiers in single-cell analysis, cell sorting, and ultrahigh-sensitive detection and paving the way for groundbreaking advancements in biomedicine and related fields.

FIG. 20.

The proposed PTIs integrated microfluidics in this paper: (a) Bent THz waveguide. (b) THz PTIs with a high Q cavity. The HR-Si substrate VH PTIs is surrounded by PDMS, and a liquid channel was fabricated on the surface of PTIs. The liquid channel intersects with the THz light waveguide. The inlet and outlet ports allow sample injection, which passes through the channel and interacts with the THz light.

FIG. 20.

The proposed PTIs integrated microfluidics in this paper: (a) Bent THz waveguide. (b) THz PTIs with a high Q cavity. The HR-Si substrate VH PTIs is surrounded by PDMS, and a liquid channel was fabricated on the surface of PTIs. The liquid channel intersects with the THz light waveguide. The inlet and outlet ports allow sample injection, which passes through the channel and interacts with the THz light.

Close modal

In this perspective, we review a few examples of tunable PTIs in the infrared frequency range. While the development of tunable PTIs in the infrared frequency range provides valuable insights, directly applying these tuning methods to the THz range proves to be challenging. This challenge can be attributed to at least two significant factors: differences in device footprints and variations in material properties between these two frequency ranges. Here, we have explored various methods to achieve tunable THz PTIs, including electrical, optical, and thermal approaches, as well as discussed THz on-chip biosensors based on PTIs. The PTIs offer a highly compact and versatile platform for developing functional THz devices. Meanwhile, it is impossible to list all potential tuning methods and applications for THz PTIs in this paper due to the length limit. Among the tuning methods, optical pumping appears to be readily applicable in practice. Demonstrations of directly photoexciting HR-Si substrate for tunable THz PTIs have been successful, and with careful substrate material selection, achieving ultrafast switchable THz PTIs is highly feasible in the future. For electrical tuning, the most challenging aspect lies in achieving direct contact between electrodes and PTIs. While rotating LC molecules with an external electric field avoids direct contact, the tuning speed is limited by the thickness of the LC layer. A desirable future goal is to discover a method for direct electric contact with PTIs without breaking the topologically protected states. The thermal approach relies on the phase transition of materials like GST or VO2, with our simulation results showing effective switching off of the PTI waveguide. Moreover, the thermal phase transition can be triggered by external light, simplifying the technical challenges of implementing this tuning method. To achieve a more compact device, an electrical heating design on the PTI platform is preferred in the future. Our proposed integration of THz PTIs with microfluidics presents exciting prospects for THz on-chip biosensing. By combining the flexibility of designing the optical route in PTIs with the fluid channel in microfluidics, empowered by the THz spectrum, this integration could address challenges in ultra-trace measurements, single-cell detection and sorting, and more. In conclusion, PTIs open up new opportunities or even a revolution in THz on-chip devices. The potential of PTIs in enabling tunability, flexibility, and high sensitivity makes them a promising candidate for advancing various THz applications and devices. Continued research and development in this field are likely to unveil even more exciting possibilities for the future.

The authors gratefully acknowledge partial financial support for this work from the National Natural Science Foundation of China (Grant Nos. 61975135, 61805148, and U2330114), the Natural Science Foundation of Guangdong Province (Grant Nos. 2019A1515010869 and 2021A1515012296), the Key Field Special Project on Biomedicine and Health of Guangdong Province (Grant No. 2022ZDZX2053), and the Medical-Engineering Interdisciplinary Research Foundation of Shenzhen University.

The authors have no conflicts to disclose.

Yiwen Sun: Conceptualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Zhijie Mei: Writing – original draft (equal); Writing – review & editing (equal). Xuejiao Xu: Formal analysis (equal); Writing – original draft (equal); Writing – review & editing (equal). Qingxuan Xie: Writing – original draft (equal). Shuting Fan: Funding acquisition (equal); Resources (equal). Zhengfang Qian: Resources (equal). Xudong Liu: Conceptualization (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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