Giant Second-Order Susceptibility in Monolayer WSe2 via Strain Engineering

Monolayer WSe₂ (ML WSe₂) exhibits high second harmonic generation (SHG) efficiency under single 1-photon (1-p) or 2-photon (2-p) resonance conditions due to enhanced second-order susceptibility compared with the off-resonance state {lin2021narrow,wang2015giant}. Here, we propose a novel strain engineering approach to dramatically boost the in-plane second-order susceptibility (χyyy) by tuning the biaxial strain to shift two K-valley excitons (the A-exciton and a high-lying exciton (HX)) into double resonance. We first identify the A-exciton and HX from the 2D Mott-Wannier model and compare the enhanced χyyy at single 1-p and 2-p resonance states. The strongest enhancement arises from the 2-p resonance state of HX. By applying a small uniform biaxial strain (0.16%), we observe an excitonic double resonance state (EHX = 2EA, EHX and EA are the exciton absorption energies). Strain-tuning can yield up to a $50$X enhancement in χyyy compared to the single 2-p resonance state of HX, resulting in a further three orders of magnitude enhancement in SHG efficiency. Further exploration of strain states near 0.16% reveals double resonance also occurs at other wavevectors near the K valley, leading to a further increase in χyyy, confirming that strain engineering is an effective methodology for enhancing χyyy. Our findings suggest new avenues for strain engineering the optical properties of 2D materials for novel nonlinear optoelectronics applications.