A Broadband THz-TDS System Based on DSTMS Emitter and LTG InGaAs/InAlAs Photoconductive Antenna Detector

We demonstrate a 4-f terahertz time-domain spectroscopy (THz-TDS) system using an organic crystal DSTMS as the THz emitter and a low temperature grown (LTG) InGaAs/InAlAs photoconductive antenna as the receiver. The system covers a frequency range from 0.2 up to 8 THz. The influences of the pump laser power, the probe laser power and the azimuthal angle of the DSTMS crystal on the time-domain THz amplitude are experimentally analyzed. The frequency accuracy of the system is verified by measuring two metamaterial samples and a lactose film in this THz-TDS system. The proposed combination of DSTMS emission and PC antenna detection realizes a compact and low-cost THz-TDS scheme with an ultra-broad bandwidth, which may promote the development and the applications of THz-TDS techniques.

tens of femtoseconds. By use of ultrashort laser pulses the generation of THz radiation by OR process has been reported to reach over 41 THz 18 .
Besides the consideration on the center frequency and the bandwidth, in recent years, more and more potential applications require optical-fiber-friendly THz-TDS systems pumped at 1550 nm, where LTG InGaAs/ InAlAs based PC antennas and organic crystals such as DAST, DSTMS are used as promising THz sources [19][20][21] and receivers 22,23 . The band coverage of LTG InGaAs/InAlAs based PC antennas can reach up to 5 THz but the center frequency is mainly less than 0.8 THz [24][25][26] . In contrast, the center frequency and the bandwidth of DSTMS emitters and detectors are much better. DSTMS has good velocity match at a wide range of wavelengths (from 720-1650 nm) 19,[27][28][29] . When pumped with IR radiation between 1.3 and 1.5 μ m, the radiated THz wave can fill the entire THz gap and the center frequency is up to 2 THz 30 . However, the PC antenna is much easier to integrate with the fiber technique, especially at the detection side, where as the optics of EO sampling is nearly impossible to be directly coupled with a fiber. Thus to realize an optical fiber coupled THz-TDS with a broad bandwidth and a high center frequency, a combination of DSTMS OR emission and LTG InGaAs/InAlAs PC sampling is strongly suggested. So far, this combination is still lack of experimental verification.
In this letter, we demonstrate a typical free-space 4 f THz-TDS system with a DSTMS crystal as the emitter and a LTG InGaAs/InAlAs PC antenna as the detector, which has a frequency coverage up to 8 THz with a center frequency at 1.44 THz. To verify the frequency accuracy of the proposed system, two different metamaterial samples: U-shape split-ring resonators (SRRs) and rectangle hole arrays, as well as a lactose film are tested in the system. The proposed DSTMS along with PC antenna scheme not only realizes a compact and low-cost ultra-broadband THz-TDS, but also inherits the fiber coupling capability of the PC antenna. The demonstrated system here may pave a new way for the development of ultra-broadband THz systems, thus promoting the applications of THz-TDS techniques in security check, drug prevention and medical research.

Experimental setup
Since the first discovery of DAST, it has been used as a broadband and high-energy THz source. The organic crystal DSTMS is derived from DAST but has a lower absorption around 1 THz and a faster crystal growth rate 13,31,32 . What's more, DSTMS has the best performances when pumped in the IR regime. For example, DSTMS has a nonlinear coefficient as high as χ = 490pm/V 111 (2) when pumped with IR radiation between 1.3 and 1.5 μ m, as well as a good phase match. Therefore, DSTMS has become a favored potential emitter for assembling all fiber THz-TDS system and has been widely studied. In this work, a commercial DSTMS crystal from Rainbow Photonics is utilized as the THz emitter, which has a total area of 4 × 5 mm 2 with a thickness of 0.3 mm. Although DSTMS has been utilized as detector in several previous works, its giant birefringence and EO sampling based detection optics hinder its utilization as a fiber-coupled THz receiver. Thus the detection of THz pulses in this work was accomplished by a commercial LTG InGaAs/InAlAs PC antenna (TERA15-DP25) from Menlo Systems GmbH. This is a typical H-type dipole antenna, which has a 4 × 4 mm 2 total area and a 0.35 mm thickness with a 25 μ m long dipole arm and a 10 μ m wide PC gap. Compared with bare LTG InGaAs grown lattice-matched to InP substrate, beryllium-doped LTG InGaAs/InAlAs has a higher resistivity up to 10 6 Ω/sq, a suppressed dark conductivity and a reduced relaxation time 33,34 . A 4.5 THz response bandwidth and a 60 dB dynamic range are specified by the manufacturer.
The schematic of the DSTMS emission and LTG InGaAs/InAlAs PC antenna detection based THz-TDS system is shown in Fig. 1. The source is a 100 fs fiber amplifier (TOPTICA, FemtoFiber Pro NIR) operating at 1560 nm with an 80 MHz repetition rate. The output is split into two beams by using a beam splitter consisting of a λ /2 wave plate and a cubic polarizer, and the powers in each beam can be tuned by rotating the wave plate. One beam is used to pump the DSTMS crystal and the other one with an average power of 25 mW is used as the probe beam of the PC antenna. In front of the emitter there is a lens with a 50 mm focal length to adjust the beam size. The pump power measured in front of the crystal is 172.9 mW, which is the net average power used to pump the

Results and Discussion
Figure 2(a) shows the THz signals in the time domain measured under conditions of different humidity. A humidity of 44% causes a decrease in the THz amplitude and strong oscillations after the main peak. As the humidity is reduced to 3.6%, this disorder decreases and the peak amplitude of the THz pulse increases by 42.6%. Figure 2(b) shows the frequency spectra and the power spectrum calculated by Fourier transform, which shows an unexpected frequency coverage up to 8 THz and 4.4 THz (full width at 1/10 of maximum) bandwidth with a center frequency at 1.44 THz. The dynamic range obtained from the power spectrum in Fig. 2(b) is larger than 50 dB. This ultra-broad bandwidth, as far as we know, is far beyond the usually reported frequency range of 5 THz observed in InGaAs/InAlAs PC antenna. In the spectra, there are obvious absorption peaks around 1.15, 1.31, 1.42, 1.69, 1.81 THz due to ambient water vapor, which agrees with the values reported in other references 35,36 . There are two spectral valleys centered at 0.61 and 0.98 THz even in a dry-air environment. This is mainly caused by the characteristic absorption in the DSTMS crystal 13,37 , which is responsible for the time-domain oscillations following the main peak of the signal. However, in the spectrum there is a relatively broad dip around 2.6 THz that needs subsequent investigation in the future work to clarify its origin.
Though usually water vapor absorption lines are used to test the spectral accuracy of a THz-TDS, water vapor absorption under large humidity can not be clearly recognized at high frequencies due to the too weak THz spectral amplitude. So to verify the high frequency accuracy of this system, two metamaterial samples with a few designed frequency resonances were fabricated. One is a U-shape SRR array with typical LC and high-order plasmonic resonances. The other is a rectangular hole array with surface plasmonic polarization (SPP) resonance. These two 10 × 10 cm 2 -sized aluminum patterns were thermally evaporated on a 500-μ m-thick semi-insulating silicon substrate. The unit cells of the SRR and the hole array structures with their geometrical parameters are shown in Fig. 3(a),(b). In the experiment, the samples are placed midway between the two parabolic mirrors to ensure a nearly plane wave incidence. A piece of the blank silicon wafer, which is the same as the sample substrate, is used as the reference. The THz pulses propagate through the metamaterial samples at normal incidence with the same polarization of the electric field as the one in Fig. 3(a). The amplitude transmission t(ω ) is defined as where ω E ( ) sam and ω E ( ) ref are the Fourier amplitudes of the transmitted THz fields through the sample and reference, respectively. In Fig. 4(a) the red line is the result measured with this system while the black line is obtained by a commercial FTIR system. The FTIR system, which has essentially different THz emission and detection mechanisms but covers nearly the whole THz band can objectively verify the spectral accuracy of our THz-TDS system. As shown in Fig. 4(a) there are three resonances at 0.74, 2.13 and 3.41 THz corresponding to three modes of the SRR sample: the LC, the second-order, and the third-order SPP resonances 38 . These resonances coincide with the results from the FTIR measurement by employing a commercial THz polarizer, considering the 0.06 THz frequency accuracy of the FTIR apparatus. Figure 4(b) shows the measured frequency-dependent amplitude transmission of the hole array sample. The resonance at 5.53 THz is primarily attributed to the resonant excitation of the SPP mode at the metal-dielectric interface. The measured result is also verified by the FTIR measurement. Besides the artificial metamaterials, pure lactose film on the semi-insulating silicon wafer was also measured in our system. The well-known absorption peaks of lactose at 0.54, 1.36, 1.82, 2.56, 2.89, 3.24, 3.91 and 4.25 THz are clearly observed as shown in Fig. 4(c), which coincides with the reported results in Refs [39,40]. It demonstrates that the DSTMS emission plus PC antenna detection scheme proposed here has an accurate frequency response and the frequency coverage up to 8 THz is experimentally reliable.
As mentioned above, the pump laser in our THz-TDS is a fiber femtosecond amplifier with small volume, low cost and fiber-coupling capability. But for feasible devices in realistic applications, a small pump power and low nonlinear effects in the fiber are of the same importance as the fiber integration capability. To investigate the pump power dependence of this system, we changed the power of the pump beam by two neutral density filters and measured the amplitude of the THz time-domain pulses, while the power of the probe beam remained at 25 mW. As shown in Fig. 5(a) the amplitude linearly increases by 8 times as the pump power rises from 20-180 mW, which is verified by linear fitting. The results are entirely in accordance with the second-order nonlinear feature of the OR process. Besides the pump power on the crystal, the obtained amplitude of the THz signal   also changes when the DSTMS crystal is rotated along its normal, which originates from the polarization dependence of OR emission and antenna detection. Based on these two factors, we fit the azimuthal angle dependence of our system, as shown in Fig. 5(b), where the azimuthal angle θ was defined as the angle between the b-axis of the crystal (The pump beam incidents along the c-axis of the crystal onto its ab surface) and the polarization of the pump beam (the same direction with the dipole of the antenna). As we can see in Fig. 5(b), the fit is in good accordance with the experimental result. Two maximum amplitudes were obtained when the a-axis of the crystal is parallel to the polarization of the pump beam. Besides the pump power and the azimuthal angle dependences, the power of the probe beam also strongly influences the whole response of the THz-TDS system by determining the detection efficiency, which is investigated by changing the power of the probe beam from 5 mW to 35 mW (the breakdown limitation power) while keeping the pump power at 177 mW. Figure 5(c) demonstrates that the amplitude increases linearly as the probe power is raised from 5 mW to 20 mW. But after that, the amplitude turns to saturation when the probe beam power continues to rise [41][42][43] .
Even though in this work the PC antenna was still sampled by free space optics, its fiber coupling capability has been demonstrated by previous works 33,34 . The DSTMS emission plus PC antenna detection scheme proposed here has a promising future in developing compact ultra-broadband THz-TDS systems, which is urgently needed in many important areas. Firstly, both DSTMS and LTG InGaAs/InAlAs PC antenna have very good performances around 1550 nm, where pumping lasers, propagation devices and modulation components are relatively compact, flexible, low-costly and fiber-technique integratable, making the scheme much easier to be accepted by the society 33,34 . Secondly, the frequency coverage up to 8 THz is broader than most THz-TDS systems commercially available today, which means our proposal has a higher spectral signature identification capability. However, before the realistic application of this scheme, the remaining issues such as the echo from the substrate, the absorption of the InP substrate, the SNR of the whole system and the reason of the fine structures in the spectrum need more through studies.

Conclusion
We presents a new scheme to generate and detect broadband THz wave. The proposed THz-TDS with a DSTMS crystal as the source and an LTG InGaAs/InAlAs PC antenna as the detector shows a frequency response up to 8 THz with a central frequency of 1.44 THz. The transmission of two metamaterial samples with different structures measured in this system coincides with the results measured by FTIR. And the absorption signatures of lactose tested in this system are in good accordance with the previously reported results. The performance dependence on the pump power, the azimuthal angle of the crystal and the probe excitation power is experimentally analyzed. This demonstration paves a way to realize a compact, low-cost and fiber technology friendly THz-TDS with a much broader frequency range in future.