  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.
