Dramatically Enhanced Second Harmonic Generation in Janus Group-III Chalcogenide Monolayers

Abstract

2D materials are excellent platforms for nonlinear optical (NLO) response, especially second harmonic generation (SHG), due to its large surface to volume ratio and sub-nanometer thickness. The SHG susceptibility strongly relies on the symmetry of materials. Constructing Janus structures can break the out-of-plane mirror symmetry, bringing about asymmetric charge distribution and leading to a built-in electric field. Consequently, SHG response along the out-of-plane direction can be improved. In the current work, combining first-principles calculations with independent particle approximation, the SHG response of nine Janus group-III chalcogenide monolayers (M₂XX', MM'X₂) are systematically evaluated. Both extraordinary in-plane and out-of-plane SHG response are revealed in all the Janus structures. Besides, cation MM'X₂ Janus structures exhibit systemically higher SHG response than that of anion M₂XX’ due to stronger dipole. Among them, GaInTe₂ possesses extremely high out-of-plane SHG response with d₃₁ up to 10490.4 pm V⁻¹ at photon energy (PE) of 4.7 eV, enabling promising applications in ultraviolet NLO devices. The SHG intensity polar plots from Janus structures display unusual rotational symmetry at different PEs allowed by the out-of-plane SHG components. This work provides theoretical guidelines for further experimental explorations in 2D group-III monochalcogenide Janus structures and paves the way for their utilization in NLO devices.