Understanding and controlling nonlinear optical generation in transition metal dichalcogenides (TMDs) is critical for developing scalable photonic devices. Here, we investigate second-harmonic generation (SHG) in Au-supported 2H-WSe2 and noncentrosymmetric 3R-WS2 flakes and explore the impact of dielectric capping with hexagonal boron nitride (hBN). Using SHG microscopy and finite-element modeling, we demonstrate that optical-cavity resonances in Au-supported TMDs strongly enhance SHG, with multilayer 2H-WSe2 flakes exhibiting up to a 40-fold increase in SHG intensity at resonant thicknesses compared to monolayer WSe2 on the same Au substrate. Sequential deposition of hBN layers onto multilayer WSe2/Au optical cavities further enhances SHG by an average of 350% across a ∼70 nm spectral bandwidth by partially impedance matching WSe2 to air and, thereby, improving in and out-coupling at the fundamental and second-harmonic wavelengths. Extending this approach to 3R-WS2, we observe similar optical cavity-mediated enhancement, with SHG intensities up to 700 times that of monolayer WS2 on Au. These results demonstrate cavity engineering and dielectric capping as promising strategies for boosting broadband nonlinear optical responses in both centrosymmetric and noncentrosymmetric TMD flakes, providing design principles for future layered photonic platforms.
Munson et al. (Fri,) studied this question.