Ruan Jiawei, Chan Yang-Hao, Louie Steven G
Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States.
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
Nano Lett. 2024 Dec 11;24(49):15533-15539. doi: 10.1021/acs.nanolett.4c03434. Epub 2024 Nov 18.
Excitons are vital in the photophysics of materials, especially in low-dimensional systems. The conceptual and quantitative understanding of excitonic effects in nonlinear optical (NLO) processes is more challenging compared to linear ones. Here, we present an ab initio approach to second-order NLO responses, incorporating excitonic effects, that employs an exciton-state coupling formalism and allows for a detailed analysis of the role of individual excitonic states. Taking monolayer h-BN and MoS as two prototype 2D materials, we calculate their second harmonic generation (SHG) susceptibility and shift current conductivity tensor. We find strong excitonic enhancement in the NLO responses requires that the resonant excitons are not only optically bright themselves but also able to couple strongly to other bright excitons. Our results explain the occurrence of two strong peaks in the SHG of monolayer h-BN and why the A and B excitons of MoS unexpectedly exhibit minimal excitonic enhancement in both SHG and shift current generation.
激子在材料的光物理中至关重要，特别是在低维系统中。与线性光学过程相比，对非线性光学（NLO）过程中激子效应的概念性和定量理解更具挑战性。在此，我们提出一种纳入激子效应的二阶NLO响应的从头算方法，该方法采用激子态耦合形式，并允许对各个激子态的作用进行详细分析。以单层h-BN和MoS作为两种典型的二维材料，我们计算了它们的二次谐波产生（SHG）极化率和位移电流电导率张量。我们发现，NLO响应中的强激子增强要求共振激子本身不仅具有光学活性，而且还能够与其他明亮激子强烈耦合。我们的结果解释了单层h-BN的SHG中出现两个强峰的原因，以及为什么MoS的A和B激子在SHG和位移电流产生中意外地表现出最小的激子增强。
