Enhancing resonant second harmonic generation in bilayer WSe2 by layer-dependent exciton-polaron effect

Two-dimensional (2D) materials serve as exceptional platforms for controlled second harmonic generation (SHG), an important nonlinear optical phenomenon with diverse applications. Current approaches to SHG control often depend on non-resonant conditions or symmetry breaking via single-gate control. Here, we employ dual-gate bilayer WSe2 to demonstrate a new SHG enhancement concept that leverages strong exciton resonance and layer-dependent exciton-polaron effect. By selectively localizing injected holes within one layer, we induce exciton-polaron states in the hole-filled layer while maintaining normal exciton states in the charge-neutral layer. The distinct resonant conditions of these layers effectively break interlayer inversion symmetry, thereby promoting resonant SHG. Our method achieves a remarkable 40-fold enhancement of SHG at minimal electric field, equivalent to conditions near the dielectric-breakdown threshold but using only ~3% of the critical breakdown field. Our findings also reveal significant sensitivity of resonant SHG to carrier density and carrier type, with distinct enhancement and quenching observed across different gating regimes. This advancement offers an innovative approach to manipulating SHG in 2D excitonic materials and provides a potent spectroscopic tool for probing layer-dependent quantum states.