A Rare Earth Chalcogenide Nonlinear Optical Crystal KLaGeS 4 : Achieving Good Balance among Band Gap, Second Harmonic Generation Effect, and Birefringence.

Midinfrared nonlinear optical (NLO) rare earth chalcogenides have attracted extensive research interest in recent several decades. Employing charge-transfer engineering strategy in the early stage, rigid tetrahedral [GeS 4 ] was introduced into rare-earth sulfides to synthesize KYGeS 4 , which had an enlarged band gap while maintaining a strong second harmonic generation (SHG) effect. Based on KYGeS 4 , La was equivalently substituted to successfully synthesize KLaGeS 4 with a stronger SHG effect ( d ij = 1.2 × AgGaS 2 ) and lower cost. Meanwhile, a larger band gap ( E g = 3.34 eV) was retained and realized phase matching (Δ n = 0.098 @ 1064 nm). KLaGeS 4 enabled an effective balance among band gap, SHG effect, and birefringence, making it a promising candidate for infrared NLO optical materials among various rare-earth sulfides.