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 second harmonic generation (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.