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Nonlinear up-conversion detection of sub-terahertz waves via DFG and SFG in DAST crystal
Deepika Yadav1, Yuma Takida1, Kunio Ishida2, and Hiroaki Minamide1
1Tera-Photonics Research Team, RIKEN Center for Advanced Photonics, RIKEN, 519-1399 Aramaki-Aoba, Sendai, Miyagi, 980-0845 Japan
2School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan
Exploring beyond conventional applications, the THz band has proven instrumental in delving into the intricacies of advanced 2D structures and quantum materials [1,2], astronomical applications [3], and medical imaging [4]. However, the widespread adoption of sub-THz technology continues to encounter obstacles, primarily stemming from the absence of compact and highly sensitive detectors capable of operating at room temperature. THz frequency up-conversion detection utilizing organic nonlinear crystals such as DAST (4-dimethylamino-N’-methyl-4’-stilbazolium tosylate) [5,6] and OH1 [7] has been utilized in prior studies and has showcased exceptional detection sensitivity at room temperature, surpassing that of a 4K bolometer by three orders of magnitude. Earlier studies on THz detection using organic crystals predominantly focused on operating frequencies exceeding 1 THz. Conversely, for frequencies below 1 THz, research leaned towards the electro-optic effect, which often requires expensive femtosecond lasers [8]. Moreover, the detection of sub-THz waves employing inorganic crystals like KTP [9] and MgO:LiNbO3 [10,11] has demonstrated remarkable sensitivities. However, the need for precise optical alignment, owing to angle-dependent non-collinear phase matching geometries, poses a constraint on its straightforward usability. Leveraging the simplified optical alignment facilitated by collinear phase matching in organic nonlinear crystals, in this work, we investigate their efficacy at sub-THz frequencies. Utilizing a near-infrared (NIR) pump beam (1.064 μm, 0.46 ns, 100 Hz), we effectively convert 0.46 THz radiation into the NIR range through both sum frequency generation (SFG) and difference frequency generation (DFG) mechanisms in the DAST crystal.
Figure 1: (a) Schematic of the experimental setup, (b) spectrum measurements showing up-converted signal in the presence of THz wave (pink ink), and absence of THz wave (purple ink).
The schematic of the experimental setup is shown in Fig. 1(a). We used a passive Q-switched microchip Nd:YAG laser along with a two-stage Nd:YAG optical amplifier as a single longitudinal mode pump source. The measured spectral width (FWHM) of the amplified laser beam was 0.005 nm, which corresponds to the wavelength resolution limit of our optical spectrum analyzer (Yokogawa AQ6380). The pump beam is split into two beams using a polarizing beam splitter, one for the THz generation and the other for detection. THz-waves were generated by using injection-seeded backward THz-wave parametric oscillation (BW-TPO) in slant-stripe type periodically poled lithium niobate (PPLN) crystal, where a pump photon is down-converted into a pair of THz signal and NIR idler photons as per the energy conservation guided by quasi-collinear phase-matching scheme [12,13]. Sub-THz photons emitted as a backward wave from the PPLN are reflected using a thin lithium niobate (LN) substrate placed in front of the PPLN and are then focused onto the DAST crystal using a pair of Tsurupica lenses and one more thin LN substrate. The collimated pump beam of size 840 x 720 μm at FWHM is overlapped with the incoming THz photons at the DAST crystal of thickness 680 μm. To segregate the upconverted photons from the intense pump beam, we employed two Raman filters and a blazed diffraction grating.
When the NIR pump and THz photons coincide on the DAST crystal, and their temporal timing is synchronized, along with the polarizations of both the THz and pump beams aligning parallel to the crystal’s optical axis to fulfill the collinear phase matching requirement, the THz frequency is converted to the NIR region. This conversion enables the concurrent detection of DFG and SFG signals. The spectral measurements depicted in Fig. 1 (b) were obtained at a pump energy of 100 μJ and THz energy of 16 nJ using a real-time spectrum analyzer (Shimadzu SPG-V500). The measured wavelength of the up-converted SFG signal is λSFG = 1.0625 μm, and the DFG signal is λDFG = 1.0659 μm as shown in pink color. When the injection of sub-THz photons into the DAST crystal is obstructed by a metal plate, the generation of the up-converted signal ceases, resulting in the absence of both DFG and SFG signals, as depicted by the purple color in Fig. 1 (b). Also, the upconverted signal intensity goes down as we decrease the number of THz photons (not shown here), confirming the signal is generated by the frequency up-conversion of THz photons. Current efforts are centred on isolating the collinear up-converted signal from the intense pump beam by employing blazed diffraction gratings, to confirm the detection sensitivity using an avalanche photodiode (APD).
In conclusion, our study showcases the conversion of the BW-TPO signal at 0.46 THz to the NIR range using the SFG and DFG processes using a lab-grown organic nonlinear DAST crystal. These results underscore the potential of organic crystals for compact and sensitive room-temperature THz detection, particularly at sub-THz frequencies, and extend the application of organic nonlinear crystals in THz technologies.
Acknowledgments
This work was supported (in part) by Innovative Science and Technology Initiative for Security, ATLA, Japan (Grant No. JPJ004596). The authors acknowledge Ms. M. Saito and Dr. N. Yaekashiwa of RIKEN for growing DAST crystals, Prof. H. Ito of RIKEN/Tohoku University, and Prof. M. Kumano of Tohoku University for valuable discussions.
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