Broadband and tunable time-resolved THz system using argon-filled hollow-core photonic crystal fiber

Terahertz time-domain spectroscopy (THz-TDS) is a broadband optical characterization technique increasingly applied in many fields for non-invasive imaging and identification of molecules.^1–8 In condensed matter research, this technique is also used to trace ultrafast dynamics of low-energy excitations when combined with a pump pulse to excite non-equilibrium states inside materials such as semiconductors, strongly correlated materials, superconductors, and topological insulators.^8–18 Common THz-TDS systems, including most of the ones commercially available, are now able to resolve with great sensitivity the spectral range covering 0.5–4 THz. One of the next frontiers in THz photonics is therefore the development of efficient schemes for expanding this spectral window beyond 4 THz, so as to allow access to both a wider range of molecular resonances for sensing applications and new microscopic interactions in condensed matter. Some experimental schemes have already been reported for achieving ultra-broadband THz spectroscopy. They are based on nonlinear optical generation and detection in laser-induced gas plasmas (THz wave air photonics),^19–22 GaP or several-micron-thick ZnTe crystals^23–25 and birefringent LiGaS₂ (LGS),^26,27 GaSe,^27–31 and organic crystals such as DAST.^32,33 Although these configurations rely on different types of nonlinear media, they all share an essential common component: an ultrafast near-infrared (NIR) laser capable of delivering broadband femtosecond pulses. Such an optical source is crucial for accessing the high THz frequency range since THz radiation is generated by nonlinear difference frequency mixing of NIR pulses, which means that the highest generated THz frequencies are determined by the spectral bandwidth of the NIR pulses. Furthermore, efficient time-resolved THz detection requires ultrashort NIR pulses with a duration shorter than the oscillation cycle of the highest THz frequency components to achieve broadband detection. These two conditions impose stringent requirements on the NIR ultrafast source. As a result, expensive and bulky optical systems are often necessary for broadband THz-TDS. We propose an alternative setup for generating broadband THz radiation, one that can be used with a compact and stable MHz laser system delivering pulses of sub-microjoule energy and a few hundreds of femtoseconds in duration. We use a commercial Yb:KGW ultrafast amplifier in combination with a gas-filled kagomé-type hollow-core photonic crystal fiber (kagomé-PCF) to achieve efficient broadband THz generation and detection.³⁴ Gas-filled hollow-core photonic crystal fiber (HC-PCF) is one of the most efficient and versatile nonlinear platforms for spectral and temporal structuring of NIR pulses.^35–39 It provides weak anomalous dispersion that can be counter-balanced by the normal dispersion of the gas filling the fiber, allowing propagation of ultrashort pulses with minimal temporal distortion. In contrast to solid-core fibers or highly nonlinear materials, the linear and nonlinear properties of the system can be adjusted simply by changing the species and the pressure of the gas filling the HC-PCF. Here, we take advantage of this unique feature to broaden the spectrum covered by the THz-TDS system out to ∼6 THz, limited only by the choice of the nonlinear crystal for time-resolved detection. More importantly, the general concept of using a HC-PCF pulse shaper in combination with a relatively narrow spectral linewidth (<4 THz) laser could be extended to other schemes based on different generation and detection crystals such as LGS or GaSe, which would extend further the spectral coverage of THz-TDS.

The experimental configuration is sketched in Fig. 1. The optical source is a commercial Yb:KGW amplifier delivering 185 fs pulses at a central wavelength of 1035 nm, an average power of 1 W, and a repetition rate of 1.1 MHz. The emitted pulses are launched into an Ar-filled HC-PCF with 75% coupling efficiency. The fiber, a 55 cm-long kagomé-PCF with a 34 μm-diameter core, is entirely placed inside a gas cell within which the Ar pressure can easily be adjusted. This scheme allows us to change the properties of the optical medium and tune the effects of self-phase modulation (SPM) broadening and restructuring the NIR pulse spectrum. A pair of identical chirped mirrors (CMs), providing a total dispersion of −2500 fs², is placed after the HC-PCF to compensate for the positive chirp resulting from SPM and to ensure that the pulses are nearly Fourier-transform-limited. A standard THz-TDS configuration is then used to generate and detect the THz radiation.³⁵ Briefly, the NIR beam is split into two paths. In the first path, phase-locked THz radiation is generated by difference frequency mixing inside a 220 μm thick 110-oriented GaP crystal. The second path is used as an optical gate for time-resolved electro-optical detection. An identical GaP nonlinear crystal is used for detection.

The NIR pulse properties are measured after the CMs using a USB spectrometer and a home-made autocorrelator based on second harmonic generation in a 150-μm-thick beta-barium borate (BBO) crystal. As the Ar pressure (P_Ar) is increased from 0 to 10 bar, the NIR spectrum gradually broadens from a full-width at half-maximum (FWHM) of 3.5 to 8.7 THz [Fig. 2(a)]. The spectral broadening manifests itself mainly in two sidelobes separated by Δν_SL = 3.1 THz at 7.5 bar and 4.7 THz at 10 bar. The corresponding autocorrelation traces are shown in Fig. 2(b) from which, assuming that the structured NIR pulses have a sech² temporal shape, the original pulse duration can be recovered. The pulse duration is observed to decrease gradually, from 185 fs to 65 fs (FWHM), as P_Ar is increased. The fact that the spectral bandwidth increases by a factor of 2.5 while the temporal duration decreases by 2.8 indicates that the pulses are close to Fourier-transform-limited at all the pressures used in the experiment.

Figure 2(c) shows the simulated spectra at the fiber output at different Ar pressures for a 185 fs (FWHM) Gaussian pulse with 0.85 μJ energy. The simulations are based on a unidirectional field equation⁴¹ and approximate the fiber dispersion by that of a narrow-bore capillary.⁴² Over the pressure range used, the NIR pulses lie in the anomalous dispersion range within the fiber. For these spectral bandwidths, however, the fiber dispersion is insufficient to compensate for the positive chirp resulting from SPM, which therefore requires further compensation using negatively chirped mirrors after the fiber. Figure 2(d) shows the simulated temporal profiles at the fiber output after introducing 2000 fs² negative chirp (as in the experiment). As the argon pressure increases from 0 to 10 bar the temporal FWHM decreases from 189 fs to 68 fs, which is in excellent agreement with the experiments. The simulations show no contribution related to pulse-induced gas ionization over the range of parameters used in the experiments.

The NIR pulses measured in Figs. 2(a) and 2(b) are injected in the THz-TDS scheme for phase-locked THz generation and detection by electro-optic sampling. Figure 3(a) shows the resulting time-resolved THz field. Simply by adjusting the gas pressure, the peak THz amplitude can be increased by a factor of ∼4. This increase is caused by temporal compression of the NIR pulses, leading to higher peak powers and, consequently, to more efficient nonlinear frequency down-conversion. The corresponding THz spectral amplitudes are shown in Fig. 3(b). Distinctly different behavior is observed above and below a frequency of 1 THz: the amplitude of higher spectral components is enhanced as P_Ar is increased, while no significant change is observed in the sub-1-THz portion of the spectrum. As a result, the THz bandwidth can be increased from 2.3 THz to 4.5 THz (FWHM). The sudden drop in the spectral amplitude at 6.2 THz is related to the restricted phase-matching conditions in the two 220 μm-thick GaP crystals used for THz generation and detection, which ultimately limit the attainable THz bandwidth. The results agree well with the calculated phase-matching cut-off frequency at 6.6 THz.⁴³ 

Interestingly, SPM in kagomé-PCF produces an unevenly distributed spectrum in the NIR pulses, which will have a direct impact on the generated THz spectrum. At P_Ar > 5 bar the NIR spectrum departs from a Gaussian-like distribution, displaying two side-lobes separated by Δν_SL. Since the THz radiation is produced by difference frequency mixing between NIR pulse components, these side-lobes are expected to enhance THz generation around Δν_SL, resulting in distinct peaks in the spectra. However, in the experiment, the electro-optic detection process prevents us from clearly distinguishing this peak since the detection efficiency is not homogeneous over the whole spectral bandwidth.^44,45 Due to the time-resolved configuration, the amplitude of the lowest and highest THz frequencies is under-estimated: Low frequencies have a larger spot size on the detection crystal and do not overlap as well with the focused gating pulse, while high THz frequencies suffer from a phase mismatch with the gating pulse inside the detection crystal. The peak in the measured THz spectra is therefore mostly determined by the time-resolved detection response rather than the spectral shape of the generated THz radiation.

A pressure-tunable pulse shaper based on gas-filled kagomé-PCF can be used to prepare NIR pulses for efficient broadband THz-TDS. As the Ar pressure is increased in the PCF, spectral broadening and temporal compression of the NIR pulses allow the bandwidth of the measured THz spectrum to be broadened by a factor of 2, the highest frequency component at ∼6 THz being limited only by phase-matching conditions in the experiment. This scheme could also be used for accessing higher THz frequencies if the argon pressure is increased beyond 10 bar and different nonlinear generation and detection crystals are used, such as GaSe, LGS, DAST, or AgGaS₂. In brief, a single fiber-based module, combined with an ultrafast source with relatively narrow linewidth (<4 THz), can be used for broadband THz-TDS, paving the way to the design of more compact and cost-effective THz-TDS setups capable of reaching high THz frequencies without the need for complex optical sources based on ultrashort Ti-sapphire amplifiers, synchronized fiber lasers, or optical parametric chirped-pulse amplifiers. Since HC-PCFs are robust and able to guide extremely high peak powers, they may also enable the use of high power and high repetition rate lasers for THz-TDS.^46,47

J.-M. Ménard acknowledges support from the National Sciences and Engineering Research Council (NSERC), Canada foundation for innovation (CFI), and Ministry of Research and Innovation’s (MRI) Ontario Research Fund.