Significantly Enhanced Infrared Second Harmonic Generation in 1T-Phase Janus PdXY (X, Y = S, Se, Te) Monolayers

In this letter, we construct a family of intrinsically noncentrosymmetric Janus PdXY (X, Y = S, Se, Te) monolayers and reveal their significantly enhanced SHG responses across the entire infrared region via ab-initio calculations. Comprehensive energetic and vibrational analyses further confirm that Janus PdXY monolayers are thermodynamically stable and experimentally accessible. These systems possess pronounced conduction-band separation characteristics, confining the optical transitions to a limited set of low-energy bands and leading to a resonantly enhanced SHG response in the infrared region. The in-plane SHG susceptibility reaches up to 1740.42 pm/V in the infrared region, surpassing that of representative bulk nonlinear crystals and 2D materials by 1-2 orders of magnitude. The enhanced infrared SHG response is mainly dominated by intraband transitions with a highly symmetric and spatially localized distribution in momentum space. The Janus PdXY monolayers exhibit anisotropic polarization-resolved SHG patterns, with symmetry and intensity determined by the excitation geometry and SHG susceptibility, as captured by the proposed analytical model. These findings provide a rational guideline for designing high performance nonlinear infrared materials, paving the way for the application of Janus structures in next-generation on-chip infrared photonic and optoelectronic devices.