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Terahertz (THz) waves have been widely investigated recently due to their ability to reflect the fingerprint characteristics of samples. As a promising method, THz technology has aroused great interest in various applications, especially biological imaging, environmental monitoring, non-destructive evaluation, spectroscopy and molecular analysis. In order to reveal the intramolecular vibration/rotation information of various compounds, the linewidths of their absorption lines are usually in a range of GHz or even MHz, and THz waves with wide tunability, narrow linewidth, high frequency accuracy, and high power stability are required. Currently, the linewidth with GHz level and low SNR at higher frequency still limit its further applications in reveal intramolecular information. In this work, the thermal distribution characteristics of DAST crystals based on diamond substrates under continuous laser pumping conditions are theoretically studied by COMSOL Multiphysics, and the effectiveness of diamond substrates in dissipating heat from DAST crystals is experimentally verified. Then, a narrow-linewidth and tunable organic-crystal continuous-wave terahertz source is demonstrated. Two narrow-linewidth CW fiber lasers are used as the pump sources for generating difference frequency. The terahertz wave is continuously tunable in a range of 1.1–3 THz. The maximum output power of 3.39 nW is obtained at 2.493 THz. The power fluctuation in 30 min is measured to be 2.19%. In addition, the generated THz wave has a high polarization extinction ratio of 9.44 dB. Using this CW-THz source for high-precision spectral detection of air with different humidity, the results correspond well with the gas absorption spectral lines in the Hitran database, proving that the CW-THz source has narrow linewidth, high frequency accuracy and stability. Therefore, it can promote the practical application of tunable CW-THz source, thus having good potential in THz high-precision spectroscopic detection and multispectral imaging.
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图 1 DAST晶体热分布仿真结果 (a) DAST; (b) Diamond-DAST
Figure 1. Simulation results of heat distribution of organic crystals: (a) Without and (b) with diamond substrate.
图 2 晶体温度随泵浦功率的变化曲线 (a)最高温度; (b)最低温度
Figure 2. Curves of crystal temperature variation with pump power: (a) Maximum temperature; (b) minimum temperature.
图 3 (a)热损伤实验示意图; (b) DAST晶体热损伤表面; (c) Diamond-DAST内表面; (d) Diamond -DAST晶体内部; (e) Diamond -DAST晶体外表面
Figure 3. (a) Schematic diagram of thermal damage experiment; (b) thermal damage surface of DAST crystal; (c) surface of Diamond-DAST crystal; (d) inside the diamond-DAST crystal; (e) outer surface of the Diamond-DAST crystal.
图 4 连续太赫兹辐射源实验装置图(插图为Diamond -DAST晶体实物照片)
Figure 4. Schematic diagram of the CW-THz source. The inset is DAST crystal with diamond substrate.
图 5 (a)双波长泵浦光偏振特性; (b)波长稳定性 (插图为功率调谐特性)
Figure 5. (a) Polarization characteristics of the dual-wavelength pump; (b) wavelength stability and the inset is power tuning characteristics.
图 6 (a)连续太赫兹辐射源调谐输出特性; (b)输出频率2.493 THz时太赫兹波功率稳定性(插图太赫兹波偏振特性)
Figure 6. (a) Tunable characteristics of the CW-THz source; (b) stability of the generated THz power over a time frame of 30 min, and the inset is polarization characteristics of the THz wave @ 2.493 THz.
图 7 太赫兹输出强度与(a)双波长泵浦总功率以及(b) λ1, λ2泵浦功率的关系
Figure 7. Relationship of the THz wave intensity with (a) the dual-wavelength pump power and (b) the pump power of λ1 or λ2.
图 8 不同湿度空气1.90—2.85 THz透射特性
Figure 8. Transmission characteristics of air with different humidity of 1.90–2.85 THz.
表 1 有/无金刚石衬底DAST晶体热损伤情况与泵浦功率关系表
Table 1. Dependence of pump power and thermal damage of DAST crystal with/without diamond substrate.
晶体无形变 热应力导致晶体内部发生可恢复
微小形变(降低功率可复原)热应力导致晶体内部发生不可
恢复形变(降低功率不可复原)晶体熔化 DAST P<0.45 W 0.45 W≤P<0.75 W 0.75 W≤P<1.20 W P≥1.20 W Diamond-DAST P<1.10 W 1.10 W≤P<1.70 W 1.70 W≤P<2.65 W P≥2.65 W 
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