Development of terahertz-wave synthesizer using nonlinear-optical effect

相干可调谐太赫兹（THz）光源是THz频域光谱和THz成像等应用的关键器件。本研究计画旨在研制一种基于非线性光学晶体差频产生的太赫兹波合成器。N-共轭有机晶体如4 ′-二甲氨基-N-甲基-4-芪对甲苯磺酸盐（DAST）是利用差频产生宽带THz波的理想材料。与DAST晶体一样，DASC晶体也可以在甲醇溶液中采用缓冷法生长，利用双波长光学参量振荡器（OPO），以KTiOPO 4晶体为输入光源，从DAST和DASC晶体中产生了2 ~ 30 THz的宽调谐THz波。OPO的可调谐范围为1340 ~ 1550 rim，由倍频（532 rim）调Q Nd：YAG激光器泵浦，脉宽为12 ns。用0.5 mm厚的DAST和DASC晶体获得了超过500 nJ的THz输出能量。使用改进的双波长OPO近990 nm和2 mm厚的DAST晶体，也产生了0.3-2 THz范围内的Sub-THz波。可调谐太赫兹波合成器可用于实时太赫兹成像，太赫兹测距和光谱应用。

A coherent tunable terahertz (THz) source is a key device for applications such as THz frequency-domain spectroscopy and THz imaging. The purpose of this research project is to develop a THz-wave synthesizer based on difference-frequency generation (DFG) using nonlinear optical crystals. The n-conjugated organic crystals such as 4'-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) are promising materials for wideband THz-wave generation using DFG We synthesized a new organic crystal of 4'-dimethylamino-N- methyl-4-stilbazolium p-chlorobenzenesulfonate (DASC), which possessed p-chlorobenzenesulfonate instead of the tosylate of DAST. DASC crystals can be grown using a slow-cooling method in methanol solution, the same as DAST.Widely tunable THz waves ranging from 2 to 30 THz were generated from DAST and DASC crystals using a dual-wavelength optical parametric oscillator (OPO) with two KTiOPO_4 crystals as the input light source. The OPO has a tunable range from 1340 to 1550 rim and is pumped by a frequency-doubled (532 rim), Q-switched Nd:YAG laser with a pulse duration of 12 ns. The THz output energy exceeding 500 nJ was obtained with a 0.5-mm-thick DAST and DASC crystal. Sub-THz waves in the range of 0.3-2 THz were also generated using an improved dual-wavelength OPO near 990 run with 2-mm-thick DAST crystals. The widely tunable -THz-wave synthesizer could potentially be used for real-time THz-imaging, THz-ranging, and spectroscopic applications.