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Mid-infrared electronic wavelength tuning through intracavity difference-frequen

Mid-infrared electronic wavelength tuning through intracavity difference-frequency mixing in Cr:ZnSe lasers

Masaki Yumoto, Kentaro Miyata, Yasushi Kawata & Satoshi Wada
Scientific Reports volume 12, Article number: 16576 (2022) Cite this article


Mid-infrared tunable coherent light sources are used in various laser applications, such as trace gas detection, laser processing, and biomedical diagnostics. This study demonstrates mid-infrared generation in the 8.3–11 µm (i.e., 900–1200 cm−1) spectral range by configuring intracavity difference-frequency generation (DFG) using ZnGeP2 (ZGP) in an electronically tuned Cr:ZnSe laser. The broad tunability is achieved with the maximum pulse energies exceeding 100 μJ by combining the electronic wavelength tuning with sligh angle adjustments (Δθ < 0.5°) of ZGP under the spectral noncritical phase-matching condition of the nonlinear material. The proposed DFG method is generalized to give access to a significant fraction of the molecular fingerprint region by utilizing selenide compounds (e.g., AgGaSe2, CdSe) in addition to ZGP, revealing the remarkable potential of ultrabroadband electronic mid-infrared scanning for numerous spectroscopic applications.

The mid-infrared (IR) region has two distinct regions (3–5 and 8–13 µm), which are known as windows of transparency in the Earth’s atmosphere and are hard to be affected by the influence of water vapor absorption1. The molecular fingerprint region in the 6.6–20 µm range (i.e., 500–1500 cm−1) contains intense and distinctive spectral patterns of molecules2. Thus, the tunable laser sources in the 8–13 µm spectral region, where both the Earth’s atmospheric window and the fingerprinted region overlap, provide considerable advantages for applications in remote sensing and trace gas detection of various gas molecules3,4,5,6. Among such coherent light sources, the nanosecond pulsed mid-IR lasers with high brightness per wavelength and high wavelength controllability provide high sensitivity and a high signal-to-noise ratio for trace gas detection in cavity-ringdown spectroscopy (CRDS) and photoacoustic spectroscopy (PAS). Combined with microscopic and imaging techniques, the light sources also enable label-free biosensing of cells and tissues7,8,9.
For the realization of tunable nanosecond pulsed laser sources in the 8–13 µm range, nonlinear frequency conversion schemes, including difference-frequency generation (DFG) and optical parametric oscillators (OPOs), provide prominent advantages for continuous and broad mid-IR tunability. Since oxide crystals (e. g., KTiOPO4, KTiOAsO4, and LiNbO3) exhibit strong multiphonon absorption beyond 5 µm10, non-oxide semiconductor crystals including AgGaS2 (AGS), AgGaSe2 (AGSe), CdSe, and ZnGeP2 (ZGP) are generally used for the nonlinear processes pumped with 1–2 μm lasers11.
For the OPO systems, Miyamoto et al. have obtained a mid-IR tunability of 5–10 µm and a sub-mJ pulse energy at 7.7 µm by pumping ZGP with the idler output of a galvano-controlled double-crystal KTP OPO12. Boyko et al. have achieved a much broader tunability in the 5.8–18 µm range with the maximum pulse energy of 171 µJ at 11.5 µm by configuring an AGSe OPO that is pumped with a Rb:PPKTP OPO output at 1.85 µm13. Yang et al. recently reported a watt-level mid-IR CdSe OPO operating in the 10–11 µm range by using a Ho:YAG master-oscillator and power amplifier system as a pump source, giving ~ 1 mJ idler pulse energy14. For the DFG systems, Haidar et al. have demonstrated an idler tunability in the 5–12 µm range with the maximum pulse energy of 25 μJ at ~ 8 μm by mixing the signal and idler outputs of a Nd:YAG laser-pumped KTP OPO in ZGP15. Mennerat has established a much higher energy operation (up to 10 mJ) in the 5.8–24 µm range by mixing the signal and idler outputs of a Nd:YAG laser-pumped LiNbO3 OPO in CdSe, GaSe, and Tl3AsSe316. However, all these systems require angle tuning of the nonlinear crystal and/or input-wavelength tuning by rotating a diffraction grating, a prism, etc. to obtain the tunable idler outputs, resulting in a low scanning speed. The temperature tuning of the nonlinear crystal is also possible (e.g., see17), but with an even lower scanning speed.
To obtain rapid scanning of the idler wavelength, we have previously introduced an electronically-tuned solid-state laser18 into the DFG system19,20. The method, so-called electronic wavelength tuning, utilizes the acousto-optic tunable filter (AOTF) to enable mid-IR wavelength tuning without a mechanical rotation of the nonlinear optical crystal and wavelength tuning elements. In addition to the high wavelength-tuning speed, electronic wavelength tuning provides distinct features such as rapid wavelength switching in pulse-to-pulse, random-access switching speed, and high wavelength repeatability compared to other wavelength tuning methods as demonstrated in Refs.20,21. Continuous scanning and random-access switching in a 9–12 µm range were realized through the extra-cavity AgGaS2-DFG by using an electronically tuned dual-wavelength Ti:Al2O3 laser as a pump source. The pulse energy was, however, limited to below 0.2 µJ, owing to the low available pump energy as well as the low damage threshold of the nonlinear crystal at the near-IR signal and pump wavelengths19,20. Electronic wavelength tuning in a similar spectral range was also demonstrated by Zakel et al. by integrating an intracavity CdSe OPO into a Cr:ZnSe laser22. Despite the highest pulse energy exceeding 200 µJ at 8.25 µm, the resulting tuning range was very limited, i.e., 8.2–8.8 µm, due to the narrow spectral acceptance of the nonlinear material used. Therefore, the mid-IR tuning range of the previous electronic-tuning methods has been so far limited when operating with high pulse energy.

In this study, we propose intracavity DFG in an electronically tuned Cr:ZnSe (ET-Cr:ZnSe) laser for the realization of rapid wavelength tuning in the 8–13 µm range with the maximum pulse energy exceeding 100 μJ. To eliminate the need for two input laser sources in the general DFG process, the ET-Cr:ZnSe laser field pumped by a Tm:YAG laser is used as the signal beam and subsequently frequency-mixed with the residual pump source in a nonlinear crystal for the mid-IR generation. ZGP was selected as a frequency downconverter for proof of concept because, compared to other commercially available nonlinear materials like AGS, AGSe, and CdSe, it provides the much larger nonlinearity (d36 (9.6 μm) = 75 pm/V) together with the excellent thermal conductivity (35–36 W/mK)23 despite the rather poor IR transparency at the idler wavelength above ~ 8.5 μm. Here, we apply the electronic wavelength tuning of the signal beam under the spectral noncritical phase-matching (NCPM) condition realized in ZGP to enable broadband mid-IR spectral tuning by combining the electronic tuning with a slight angle adjustment of the nonlinear material. Thus, this intracavity DFG in the ET-Cr:ZnSe laser provides a simple, cost effective solution for designing practical mid-IR coherent sources having broadband rapid tunability together with power scalability.