Development of real time terahertz imaging system using optical mixing effect

本文从理论和实验两方面研究了有机DAST和GaP晶体中光学与太赫兹（THz）波的非线性混频转换。使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晶体产生2 ~ 30太赫兹的可调谐太赫兹波。OPO具有1340 ~ 1550 nm的可调范围，由脉冲持续时间为12 ns的倍频调q Nd: YAG激光器泵浦。用1 mm厚的DAST晶体在11.3太赫兹处获得了超过550 nJ的太赫兹输出能量（峰值功率为46 W）。在990 nm附近使用改进的双波长OPO与2 mm厚的DAST和15 mm厚的GaP晶体结合，也产生了0.3-3 THz范围的亚太赫兹波。太赫兹波到光波的频率转换可以通过和频产生（SFG）实现。研究了DAST、GaP、ZnTe和GaAs晶体的相匹配条件。将11.3 THz波段的太赫兹光波与1478 nm波段的光波混合，在DAST晶体中产生1400nm的和频光波。这些太赫兹系统由可调谐太赫兹源和SFG探测系统组成，可以潜在地用于实时太赫兹成像、太赫兹测距和光谱应用。

Frequency conversion between optical and terahertz (THz) waves using nonlinear optical mixing in an organic DAST and GaP crystal was investigated theoretically and experimentally. Widely tunable THz waves ranging from 2 to 30 THz were generated from DAST 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 nm and is pumped by a frequency-doubled, Q-switched Nd : YAG laser with a pulse duration of 12 ns. The THz output energy exceeding 550 nJ (peak power : 46 W) at 11.3 THz was obtained with a 1-mm-thick DAST crystal. Sub-THz waves in the range of 0.3-3 THz were also generated using an improved dual-wavelength OPO near 990 nm in combination with 2-mm-thick DAST and 15-mm-thick GaP crystals.Frequency conversion from THz-wave to optical wave can be achieved by sum frequency generation (SFG). Phase-matching conditions were studied for DAST, GaP, ZnTe and GaAs crystals. Sum frequency optical wave at 1400nm was generated from DAST crystal by mixing the THz wave at 11.3 THz and the optical wave at 1478 nm. These THz system consisting of the tunable THz source and the SFG detection system could potentially be used for real-time THz-imaging, THz-ranging, and spectroscopic applications.