Second harmonic generation and electronic properties in the septuple-atomic-layer MA2Z4 family

Abstract

Two-dimensional (2D) Janus monolayers represent a novel class of materials characterized by their unique structures and exceptional properties. The excitonic characteristics, along with the linear and nonlinear optical responses of the 2D MA₂Z₄ family, have been systematically investigated through first-principles calculations. The findings indicate that monolayers of MSiGeN₄ (M = Mo or W) exhibit a pronounced second harmonic generation (SHG) response and notable infrared transmission capabilities, due to the breaking of mirror symmetry in these materials. A comparative analysis of the SHG response of intrinsic MSi₂N₄ (M = Mo and W) monolayers demonstrates that Janus MSiGeN₄ (M = Mo and W) monolayers significantly enhance the SHG response, with enhancement factors of 5.5 and 3.8 at infrared wavelengths (1064 nm). The Janus monolayers also exhibit an additional out-of-plane response, thus enhancing the efficiency of incident light utilization from all directions. Specifically, the out-of-plane response of Janus WSiGeN₄ monolayers is 1.1 pm/V greater than the in-plane response of intrinsic WSi₂N₄. Furthermore, the dependence of SHG polarizability indicates that the out-of-plane polarizability significantly modifies the SHG response as a function of the incident angle θ. As a derivative of the 2D MA₂Z₄ family, MSiGeN₄ (M = Mo and W) expands the scope of 2D Janus materials, and these findings may inform the design and synthesis of innovative 2D nonlinear optical materials.