Microscopic Origin of Nonlinear Optical Properties of 2D Materials: A First Principles Study

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have provided a unique materials platform with a variety of interesting optoelectronic properties and great potential for device applications. Janus 2D TMDCs represent a new class of 2D materials whose crystalline symmetry and physical properties can be tailored via Janus structure engineering. Here we present our first-principles study of nonlinear optical properties in Janus 2D TMDCs. Electronic structures such as linear and nonlinear optical properties were calculated using first-principles density functional theory and analyzed in combination with group theory. The microscopic origin of these nonlinear optical properties of Janus TMDCs is elaborated by k-point resolved optical absorption, shift current, and shift vector. We found that the absence of horizontal mirror plane in Janus 2D materials enables the out-of-plane second harmonic generation (SHG) and other nonlinear optical phenomena, such as shift photocurrent and circular photocurrent. Janus 2D materials, therefore, offer a unique platform for exploring nonlinear optical phenomena and designing configurable layered nonlinear optical materials.