A Rare Earth Chalcogenide Nonlinear Optical Crystal KLaGeS₄: Achieving Good Balance among Band Gap, Second Harmonic...

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 [GeS₄] was introduced into rare-earth sulfides to synthesize KYGeS₄, which had an enlarged band gap while maintaining a strong second harmonic generation (SHG) effect. Based on KYGeS₄, La was equivalently substituted to successfully synthesize KLaGeS₄ with a stronger SHG effect (d = 1.2 × AgGaS₂) and lower cost. Meanwhile, a larger band gap (E_g = 3.34 eV) was retained and realized phase matching (Δn = 0.098 @ 1064 nm). KLaGeS₄ 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.