Complete property tensors of some low-symmetry borate NLO crystals

Several low-symmetry borate crystals, such as α-BiB₃O₆ (BiBO), YCa₄O(BO₃)₃ (YCOB) and La₂CaB₁₀O₁₉ (LCB) show not only promising nonlinear optical (NLO) properties but also other favorable physical properties like large electro-optic (EO) coefficients and exceptionally large piezoelectric constants. Those materials may serve as high temperature sensors and electro- or acoustic- modulators beyond their traditional applications in high power laser generations. The knowledge of their full property tensors are indispensable for their efficient and practical usages. Due to the low-symmetry nature, more none-zero matrix elements are present and those elements most likely are combined when measuring them. Therefore, experimental determinations of the full tensor elements are not always easy especially for those high rank tensors. In fact, though being discovered over several decades, some of the property tensor matrices are still missing. Recently it is shown that by applying a modified post-DFT LD (density functional theory + London dispersion) correction based on linear combination of atomic orbitals (LCAO) and B3LYP functional, many important material property tensors of BiBO crystal are calculated in unprecedented precisions. Among them, theoretical calculation of the refractive indices in the terahertz (THz) range was firstly achieved. The calculation also confirms that BiBO has an exceptional large piezoelectric constant d₂₂ = 40 pC/N and largest free EO coefficients on the order of 10 pm/V among borate crystals. The same calculation has been extended to the other low-symmetry borate NLO crystals, including YCOB and LCB. With those calculated material constants, practical devices may be designed with preferable figure of merits over existing materials.