Electro-optic effects of lithium borate crystals studied by first-principles method

•Full material properties related to electro-optic effect of LiB3O5 and Li2B4O7 crystals were obtained by first-principles method.•The large EO coefficients of Li2B4O7 crystal is mainly originated from piezoelectric induced effect.•The smallness of EO effect of LiB3O5 is due to the cancellation of electronic and vibrational contributions.•Li2B4O7 crystal has a quite large AOM figure of merit and may find applications as AO modulators. In respect to the electro-optic (EO) effect, LiB3O5 (LBO) and Li2B4O7 (LB4) contain the same elements but represent two extremes in the family of nonlinear optical (NLO) borate crystals: one with large second harmonic generation (SHG) coefficients but very small reported EO coefficients, the latter has extremely small SHG coefficients yet EO coefficients were found even larger than the practical EO modulator crystal β-BaB2O4 (BBO). The puzzle was unraveled through first-principles studies of the two crystals and it is found that the vibrational and electronic contributions to the EO effect were canceled in LBO and the major contributions are from piezoelectric induced effect for LB4 with electronic contributions negligible. In addition to the EO and NLO coefficients, dielectric, elastic, piezoelectric and elasto-optic constants are all obtained simultaneously. More importantly, fully determining the elastic, elasto-optic constants, all 4th rank tensors, can be adopted to calibrate the experimental uncertainties and help to design practical surface acoustic wave (SAW) or acoustic optic modulator (AOM) devices.