Quantum Geometric Origin of Strain-Tunable Giant Second-Harmonic Generation in Bi$_2$O$_2$X (X=S, Se, Te)

TL;DR

This study predicts that applying strain to 2D Bi$_2$O$_2$X materials significantly enhances their second-harmonic generation, driven by quantum geometric effects, with potential for tunable optoelectronic applications.

Contribution

It reveals the quantum geometric origin of strain-tunable giant SHG in Bi$_2$O$_2$X and demonstrates strain-induced bandgap and electronic phase transitions.

Findings

Strain enhances SHG susceptibilities to ~1 nm/V.

Quantum geometry underpins the giant SHG response.

Strain induces a semiconductor to metal transition in Bi$_2$O$_2$Te.

Abstract

Two-dimensional (2D) materials with giant nonlinear optical (NLO) responses are essential for the development of advanced on-chip NLO devices. Using first-principles calculations, we predict a remarkable strain-induced enhancement of second-harmonic generation (SHG) in the high-performance 2D semiconductors Bi$_2$O$_2$X (X = S, Se, Te). The SHG susceptibilities of Bi$_2$O$_2$X under strain are on the order of 1~nm/V, rivalling the highest values reported among 2D materials. This giant SHG response originates from gauge-invariant geometric quantities, including the quantum metric, shift vector, and triple phase product. The strain also induces a bandgap variation in Bi$_2$O$_2$X. Intriguingly, in Bi$_2$O$_2$Te, strain-induced bandgap tuning drives a transition from a semiconductor to a half-metal, and ultimately to a polar metal. Our findings present a unique platform that combines strain-tunable bandgap engineering with exceptional NLO properties, while also highlighting the crucial role of quantum geometry in enhancing SHG.