Simultaneous Enhancement and Preservation of Valley-Polarized Second-Harmonic Generation in Monolayer WS<sub>2</sub> via Mie Resonances.

Abstract

Second-harmonic generation (SHG) with rigorous polarization preservation is essential for next-generation optical information processing. Monolayer transition-metal dichalcogenides offer an attractive platform for atomically thin, on-chip light sources owing to their valley-dependent polarization selection rules for SHG. However, their atomic-scale thickness severely limits conversion efficiency. To overcome this challenge, nanophotonic structures capable of simultaneously enhancing signal intensity and maintaining high-purity polarization states are required for practical device applications. Herein, we demonstrate the simultaneous resonant enhancement and high-fidelity preservation of valley-polarized SHG in monolayer WS₂ coupled with Mie-resonant silicon nanospheres. We show that the valley polarization state is spectrally modulated by the Mie modes of silicon nanospheres, achieving circular polarization retention of ∼80% within the enhanced spectral regime. These findings establish a robust strategy for manipulating polarization degrees of freedom in integrated nonlinear valley photonics.