Strain-induced giant second-harmonic generation in monolayered 2 H -MoX₂ (X = S, Se, Te)

Dynamic second-order nonlinear susceptibilities, χ(2)(2ω,ω,ω) χ(2)(ω), are calculated here within a fully first-principles scheme for monolayered molybdenum dichalcogenides, 2H-MoX₂ (X = S, Se, and Te). The absolute values of χ(2)(ω) across the three chalcogens critically depend on the band gap energies upon uniform strain, yielding the highest χ(2)(0)∼140 pm/V for MoTe₂ in the static limit. Under this uniform in-plane stress, 2H-MoX₂ can undergo direct-to-indirect transition of band gaps, which in turn substantially affects χ(2)(ω). The tunability of χ(2)(ω) by either compressive or tensile strain is demonstrated especially for two important experimental wavelengths, 1064 nm and 800 nm, where resonantly enhanced non-linear effects can be exploited: χ(2) of MoSe₂ and MoTe₂ approach ∼800 pm/V with -2% strain at 1064 nm.