Enhancement of second harmonic generation in monolayer WS₂ by feedback-based wavefront shaping

Two-dimensional transition metal dichalcogenides are of great interest for second harmonic generation due to their large second-order susceptibility χ(2), atomically thin structure, and relaxed phase-matching conditions. Such materials are also promising candidates for miniaturizing nonlinear optical devices for versatile applications in photon manipulation, quantum emission and sensing, and nanophotonic circuits. However, their strong second harmonic response is limited by nanometer-scale light–matter interaction and material impurities. Although there is considerable work toward engineering these materials for enhancing their nonlinear responses, all-optical methods are still in the exploration stages. We incorporate, to the best of our knowledge, the first experimental demonstration of feedback-based wavefront shaping techniques in atomically thin media to reveal and enhance the weak second harmonic generation of monolayer WS₂. Phase tuning of the incident wavefront leads to the increase in the intensity of the second harmonic generated in the target regions up to an order of magnitude. We enhance the local nonlinear signal conversion from monolayer WS₂ up to 41× using phase-only modulation. Furthermore, by introducing a shift in the transverse phase structure, we achieve observable second harmonic generation at the destructively interfering grain boundaries of polycrystalline monolayers. This method allows for all-optical tuning of transition metal dichalcogenides’ nonlinear responses, opening up possibilities for dynamic signal routing and on-demand enhancement in nanoscale photonic systems.