Giant Enhancement and Directional Second Harmonic Emission from Monolayer WS on Silicon Substrate via Fabry-Pérot Micro-Cavity.

Two-dimensional transition metal dichalcogenides (TMDs) possess large second-order optical nonlinearity, making them ideal candidates for miniaturized on-chip frequency conversion devices, all-optical interconnection, and optoelectronic integration components. However, limited by subnanometer thickness, the monolayer TMD exhibits low second harmonic generation (SHG) conversion efficiency (<0.1%) and poor directionality, which hinders their practical applications. Herein, we proposed a Fabry-Pérot (F-P) cavity formed by coupling an atomically thin WS film with a silicon hole matrix to enhance the SH emission. A maximum enhancement (∼1580 times) is achieved by tuning the excitation wavelength to be resonant with the microcavity modes. The giant enhancement is attributed to the strong electric field enhancement in the F-P cavity and the oscillator strength enhancement of excitons from suspended WS. Moreover, directional SH emission (divergence angle ∼5°) is obtained benefiting from the resonance of the F-P microcavity. Our research results can provide a practical sketch to develop both high-efficiency and directional nonlinear optical devices for silicon-based on-chip integration optics.