Theoretical investigation on the balance between large band gap and strong SHG response in BMO4 (M = P and As) crystals

Abstract

Nonlinear optical (NLO) materials with outstanding performances are particularly important in laser science. To design an efficient NLO material with an optimal balance between the optical band gap and the NLO coefficient is still a huge challenge. In this paper, the electronic structures and optical properties of two isostructural non-centrosymmetric BMO₄ (M = P and As) crystals exclusively containing tetrahedral groups have been studied by the first-principles method. Our calculated results show the compounds exhibit excellent properties with a suitable balance of large band gap and large NLO coefficient for BPO₄ (10.4 eV and 0.64 pm V⁻¹) and BAsO₄ (7.8 eV and 1.58 pm V⁻¹). The unanticipated large second-harmonic generation (SHG) response can be understood by the relatively strong s–p hybridization in BMO₄ (M = P and As). It is found that substitution of PO₄ groups for AsO₄ groups can increase the SHG effect and the birefringence but decrease the band gap. Whether the redshift of bandgap will be responsible for the change of SHG effect between BMO₄ (M = P and As) is explored by energy correction method. The increased polarizability anisotropy of tetrahedral groups explained the enhanced birefringence.