外差式相干探测时域光谱仪对磷化铟（InP）晶片的超宽频带太赫兹光谱的探测

磷化铟（InP）属于Ⅲ-Ⅴ族化合物半导体材料, 在毫米波的应用中展示出了高性能，在非线性太赫兹器件应用上具有很大的潜力。以前关于InP的研究主要集中于太赫兹频率在0.1～4 THz的频率范围内，在4～10 THz频率范围内InP的太赫兹光学数据还是空白。该研究利用空气等离子体相干探测太赫兹波的时域光谱系统研究了无掺杂的InP晶片在超宽THz频率范围（0.5～18 THz）内的光学特性, 实验中用电离的空气作为太赫兹的发射器和探测器, 利用可以调制的局部偏压诱导二次谐波产生，使在气体中太赫兹波的相干探测成为可能，明显提高了系统的动态范围和灵敏度。产生的太赫兹频谱宽度主要被激光脉冲持续时间所限制，太赫兹脉冲通过InP晶片后相对于参考脉冲会延迟，同时振幅会降低。另外，太赫兹信号的频谱振幅在6.7～12.1 THz范围内下降到本底噪声。同时还可以看出InP晶片在6.7～12.1 THz频率范围内不透光，在0.8～6.7 THz以及12.1～18 THz频率范围内InP的吸收系数相对较低，特别是在15～17.5 THz范围内吸收系数很低并且保持相对稳定，与此同时它的折射率单调增加。这些发现将有助于基于InP晶片的非线性太赫兹器件设计。

The Detection of Ultra-Broadband Terahertz Spectroscopy of InP Wafer by Using Coherent Heterodyne Time-Domain Spectrometer

Indium Phosphide (InP) has attracted great physical interest because of its unique characteristics and is indispensable to both optical and electronic devices .However ,the optical property of InP in the terahertz range (0.110 THz) has not yet been fully characterized and systematically studied .The former researches about the properties of InP concentrated on the terahertz frequency between 0.1 and 4 THz .The terahertz optical properties of the InP in the range of 4~10 THz are still missing .It is fairly necessary to fully understand its properties in the entire terahertz range ,which results in a better utilization as efficient ter-ahertz devices .In this paper ,we study the optical properties of undoped (100) InP wafer in the ultra-broad terahertz frequency range (0.5~18 THz) by using air-biased-coherent-detection (ABCD) system ,enabling the coherent detection of terahertz wave in gases ,which leads to a significant improvement on the dynamic range and sensitivity of the system .The advantage of this method is broad frequency bandwidth from 0.2 up to 18 THz which is only mainly limited by laser pulse duration since it uses ionized air as terahertz emitter and detector instead of using an electric optical crystal or photoconductive antenna .The terahertz pulse passing through the InP wafer is delayed regarding to the reference pulse and has much lower amplitude .In addition ,the frequency spectrum amplitude of the terahertz sample signal drops to the noise floor level from 6.7 to 12.1 THz .At the same time InP wafer is opaque at the frequencies spanning from 6.7 to 12.1 THz .In the frequency regions of 0.8~6.7 and 12.1~18 THz it has relativemy low absorption coefficient .Meanwhile ,the refractive index increases monotonously in the 0.8~6.7 THz region and 12.1~18 THz region .These findings will contribute to the design of InP based on nonlinear terahertz devices .