太赫兹波发射晶体的亚波长微棱锥增透结构的设计与实验研究

Theoretical design and experiment study of sub-wavelength antireflective micropyramid structures on THz emitters

光学整流方法产生太赫兹(THz)辐射常用的非线性发射晶体在THz波段都具有较高的折射率,使得很大一部分THz波由于晶体表面的菲涅尔反射而无法有效耦合输出.本文报道了GaP晶体THz波发射器输出表面上亚波长微棱锥增透结构的设计和实验研究.利用有效介质模型在理论上验证了亚波长光栅结构的增透效果,并进一步设计了适用于不同频段的增透结构的参数.实验中,通过微机械加工手段在GaP晶体输出端面刻划了多种亚波长微棱锥结构,验证了其增透效果及参数对增透频带的关系.理论与实验的符合证明该设计思想也可用于其他THz波发射晶体.

Nonlinear crystals commonly used in optical rectification for the generation of terahertz （THz） radiation have high refractive indices in the THz frequency range, and thus Fresnel reflection at the crystal-air output surface causes a large part of the generated THz wave to be reflected back into the crystals. Here we report on the design and experimental study of sub-wavelength antireflective micropyramid structures on GaP crystals. Effective medium theory is used to demonstrate the enhancement of THz output by the antireflective micropyramid structures, and further to design the antireflective structures at different frequencies. Several micropyramid structures are fabricated on the output surface of GaP crystals by micromachining, and the correlation between the THz output enhancement and the structure parameters is verified. The agreement between theory and experiment shows that our methodology is applicable to other THz emitters based on optical rectification.