Deka Jayanta, Dandu Medha, Krishna A S Lal, Menon Sruti, Jyothsna K M, Biswas Rabindra, Majumdar Kausik, Raghunathan Varun
Opt Express. 2020 Nov 23;28(24):35695-35707. doi: 10.1364/OE.405524.
We demonstrate polarization-independent resonant-enhancement of second harmonic generation (SHG) from multilayer Gallium Selenide (GaSe) on a silicon-based resonant metasurface. Two-dimensional hexagonal photonic lattice with circularly symmetric silicon meta-atoms are designed to achieve resonant field enhancement at the fundamental wavelength independent of the incident polarization direction. Such structures are however found to exhibit strong resonant field depolarization effects at the fundamental excitation fields resulting in modified nonlinear polarization components when compared to the native GaSe layer. Furthermore, the sub-wavelength metasurface designed to have resonances at the fundamental wavelengths act as a higher order diffraction grating at the second harmonic wavelength. Nonlinear wave propagation simulations show that the higher order diffracted SHG exhibit strong polarization dependent enhancement with characteristics very different from the native GaSe layer. In this context, polarization independent enhancement of the second harmonic signal is achieved only for the zeroth order diffracted component. Experimental study of second harmonic generation from the GaSe layer integrated with the silicon metasurface shows maximum nonlinear signal enhancement on-resonance with polarization dependence identical to the native GaSe layer by selectively detecting the zeroth-order diffracted component. This work shows that it is not sufficient to use symmetric meta-atoms in such 2D material integrated resonant metasurfaces for achieving polarization independent nonlinear optical enhancement. Depolarization of the resonant fields and higher-order diffraction at the nonlinear signal wavelength need to be considered as well.
我们展示了在基于硅的共振超表面上多层硒化镓（GaSe）产生的二次谐波（SHG）的偏振无关共振增强。设计具有圆对称硅元原子的二维六角形光子晶格，以在与入射偏振方向无关的基波波长处实现共振场增强。然而，与原生GaSe层相比，发现这种结构在基波激发场处表现出强烈的共振场去极化效应，从而导致非线性偏振分量发生改变。此外，设计为在基波波长处具有共振的亚波长超表面在二次谐波波长处充当高阶衍射光栅。非线性波传播模拟表明，高阶衍射的SHG表现出强烈的偏振依赖增强，其特性与原生GaSe层非常不同。在此背景下，仅对零阶衍射分量实现了二次谐波信号的偏振无关增强。对与硅超表面集成的GaSe层产生的二次谐波进行的实验研究表明，通过选择性检测零阶衍射分量，在共振时实现了最大的非线性信号增强，其偏振依赖性与原生GaSe层相同。这项工作表明，在这种二维材料集成的共振超表面中使用对称元原子不足以实现偏振无关的非线性光学增强。还需要考虑共振场的去极化以及非线性信号波长处的高阶衍射。
