Birefringent Dispersion Optimization to Achieve Superior Nonlinear Optical Phase Matching in Deeper Solar‐Blind UV Band from KH2PO4 to BeH3PO5

Abstract Nonlinear‐optical (NLO) crystals require birefringent phase matching (BPM), particularly in the solar‐blind ultraviolet (UV) (200–280 nm) and deep‐UV (100–200 nm) regions. Achieving BPM requires optimization of optical dispersion along with having large birefringence. This requirement is especially critical for structures with low optical anisotropy, including classical phosphate UV–NLO crystals like KH 2 PO 4 (KDP). However, there is a scarcity of in‐depth theoretical analysis and general design strategies based on structural chemistry to optimize dispersion. This study presents findings from a simplified dielectric model that uncover two vital factors to micro‐optimize transparent optical dispersion: effective mass ( m *) of excited states and effective number ( N *) of photo‐responsive states. Smoothing of dispersion occurs as m * increases and N * decreases. First‐principles analysis of deep‐UV KBe 2 BO 3 F 2 ‐family structures is used to confirm the conciseness and validity of the model. It further proposes substituting K + with Be 2+ to decrease N * and increase m * while enlarging bandgap. This will lead to improved dispersion and an overall enhancement of KDP's BPM capability. The existing BeH 3 PO 5 (BDP) is predicted to improve the shortest BPM wavelength for second‐harmonic generation, from 251 nm in KDP to 201 nm in BDP. BDP's extension into the broader UV solar‐blind waveband fully supports the proposed optimization strategy.