Breaking through the “3.0 eV wall” of energy band gap in mid-infrared nonlinear optical rare earth chalcogenides by charge-transfer engineering

Abstract

Increasing the energy band gap under the premise to maintain a large nonlinear optical (NLO) response is a challenging issue for the exploration and molecular design of mid-infrared nonlinear optical crystals. Utilizing a charge-transfer engineering method, we designed and synthesized a rare earth chalcogenide, KYGeS₄. With an NLO effect as large as that in AgGaS₂, KYGeS₄ breaks through the limitation of energy band gap, i.e., the "3.0 eV wall", in NLO rare earth chalcogenides, and thus exhibits an excellent comprehensive NLO performance. First-principles electronic structure analysis demonstrates that the large band gap in KYGeS₄ is ascribed to the decreased covalency of Y-S bonds by transferring charge from [YS₇] to [GeS₄] polyhedra. The charge-transfer engineering strategy would have significant implications for the exploration of good-performance NLO crystals.