Polar Heterolayered Oxychalcogenides Enable Strong Nonlinear Optical Effect and Dual Midinfrared Transparent Windows

Midinfrared (mid-IR) nonlinear optical (NLO) crystals are essential for laser and photonic technologies, yet existing materials frequently suffer from either weak NLO response or narrow IR transparency. Herein, we propose a polar heterolayered design strategy to overcome this limitation in oxychalcogenides by integrating cooperative layer polarization, optical anisotropy, and phonon decoupling. Guided by this approach, we identify LaGaOSe2, which features a polar heterolayered structure. This material exhibits a strong NLO response comparable to that of AgGaS2, a large birefringence of 0.2, and a bandgap of 2.36 eV. Remarkably, vibrational decoupling between the heterogeneous sublayers produces a pronounced phonon gap in LaGaOSe2, giving rise to predicted dual IR transparent windows within the one-phonon approximation. This work identifies LaGaOSe2 as a promising mid-IR NLO crystal and provides an intriguing structural strategy for designing high-performance IR-NLO materials through cooperative polarization and vibrational engineering in heterolayered systems.