Deep-Ultraviolet Transparent Nonlinear Optical Phosphates: A New Alternative to Conventional Borates

Deep ultraviolet (DUV) nonlinear optical (NLO) crystals with balanced performance are crucial for extending laser wavelengths into the DUV region, essential for various laser applications. However, developing ideal DUV crystals is challenging due to stringent requirements: strong second-harmonic generation (SHG) response, short cut-off wavelength, and effective phase-matching behavior. DUV NLO borates, which feature π-conjugated groups, have garnered attention for their higher SHG coefficients compared to phosphates. However, phosphates typically show short UV cut-off edges due to the large HOMO-LUMO gap (approximately 9.6 eV) of the [PO₄] units. To enhance the SHG effect while maintaining DUV transparency in phosphates, several strategies have been proposed: 1) using distorted polymerized P-O groups like isolated C₁-[P₃O₁₀]⁵^- and [P₂O₇]⁴^-; 2) aligning isolated [PO₄]^3- tetrahedra along the polar screw axis; 3) introducing additional NLO-active units; and 4) exploring new units derived from [PO₄]^3-, such as [PO₃F]^2- tetrahedra. These strategies have led to the successful development of various non-centrosymmetric phosphates, highlighting their potential as DUV NLO candidates. This review explores the relationship between their crystal structures and DUV NLO performance, and proposes future directions for developing ideal DUV NLO materials.