3R MoS₂ with Broken Inversion Symmetry: A Promising Ultrathin Nonlinear Optical Device

TLDR

Nonlinear 2D layered crystals are ideal platforms for ultrathin nonlinear optical devices, but their frequency‑conversion efficiency is limited by lattice symmetry and the number of layers. The study demonstrates that 3R MoS₂ with broken inversion symmetry, grown from monolayer to bulk‑like thickness, serves as an excellent nonlinear optical crystal. The authors propose a model combining bulk nonlinear contributions and interface interactions to explain the observed nonlinear behavior. The 3R MoS₂ crystals exhibit strong, thickness‑dependent second‑harmonic generation with polarization enhancement along the staggered stacking direction, confirming that broken inversion symmetry enables robust nonlinear optical performance across monolayer to bulk‑like scales and opening a pathway for ultrathin NLO devices.

Abstract

Nonlinear 2D layered crystals provide ideal platforms for applications and fundamental studies in ultrathin nonlinear optical (NLO) devices. However, the NLO frequency conversion efficiency constrained by lattice symmetry is still limited by layer numbers of 2D crystals. In this work, 3R MoS 2 with broken inversion symmetry structure are grown and proved to be excellent NLO 2D crystals from monolayer (0.65 nm) toward bulk‐like (300 nm) dimension. Thickness and wavelength‐dependent second harmonic generation spectra offer the selection rules of appropriate working conditions. A model comprising of bulk nonlinear contribution and interface interaction is proposed to interpret the observed nonlinear behavior. Polarization enhancement with two petals along staggered stacking direction appears in 3R MoS 2 is first observed and the robust polarization of 3R MoS 2 crystal is caused by the retained broken inversion symmetry. The results provide a new arena for realizing ultrathin NLO devices for 2D layered materials.