Development of terahertz-wave synthesizer using nonlinear-optical effect

相干可调谐太赫兹（THz）源是太赫兹频域光谱和太赫兹成像等应用的关键器件。本研究的目的是开发一种基于非线性光学晶体差频产生（DFG）的太赫兹波合成器。4′-二甲氨基- n-甲基-4-二硝基苯磺酸盐（DAST）等n共轭有机晶体是利用DFG产生宽带太赫兹波的有前途的材料。我们合成了一种新的4′-二甲氨基- n-甲基-4-二硝基对氯苯磺酸盐（DASC）有机晶体，该晶体以对氯苯磺酸盐取代DAST的tosyate。与DAST一样，DASC晶体可以在甲醇溶液中使用缓慢冷却方法生长。采用双波长光学参量振荡器（OPO），以两个KTiOPO_4晶体作为输入光源，从DAST和DASC晶体产生2 ~ 30太赫兹的可调谐太赫兹波。OPO具有从1340到1550环的可调范围，并由一个频率加倍（532环）的调q Nd:YAG激光器泵浦，脉冲持续时间为12 ns。用0.5 mm厚的DAST和DASC晶体获得了超过500 nJ的太赫兹输出能量。使用改进的双波长OPO，在990附近用2mm厚的DAST晶体产生了0.3-2太赫兹的亚太赫兹波。广泛可调谐的-太赫兹波合成器可能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.