Deep-ultraviolet second-harmonic generation in a three-coherence-length SrB₄O₇ microcavity

We demonstrate deep-ultraviolet second-harmonic generation at 249.5 nm from a reflective vertical microcavity containing a SrB₄O₇ layer three coherence lengths thick. With the fundamental beam blocked between settings, the second-harmonic signal showed near-quadratic power scaling. During uninterrupted upward power sweeps, the device exhibited a steeper, sweep-dependent response and reached a maximum observed average-power-normalized efficiency of 409% W^-1- approximately 157 times that of our previous 199 nm device using the same laser system [Appl. Phys. Express17, 082004 (2024)]. Modeling based on atomic force microscopy measurements linked thickness variations to spectral broadening, while position-dependent measurements revealed larger-scale nonuniformity. Thermo-optic analysis indicated sensitivity to fundamental-wave resonance tuning. These results represent a milestone toward practical microcavity-based deep-ultraviolet and vacuum-ultraviolet sources.