Strain-driven anisotropic SHG and Raman responses in NbOBr2

Abstract

Due to the anisotropic structure with broken inversion symmetry, niobium oxide dihalide (NbOBr2) exhibits notable directional and nonlinear optical properties. However, experimental studies on controlling the anisotropic structure and nonlinear optical properties of NbOBr2 remain scarce. Here, the strain-dependent anisotropic second-harmonic generation (SHG) and Raman responses in NbOBr2 are revealed through the combination of SHG and Raman spectroscopy with first-principles calculations. A uniaxial strain along the c axis enhances the SHG intensity by approximately 1.5 times, whereas strain along the b axis suppresses the SHG intensity. First-principles calculations reveal that SHG variations originate from strain tunable Peierls distortion. The strain-mediated bond-length modifications well explain the anisotropic Raman response. The agreement between theoretical and experimental results conclusively validates the proposed strain-regulation mechanism of SHG and Raman spectra. This work suggests the promising application of NbOBr2 in flexible optoelectronics and strain tunable nonlinear optical devices.