Magnetically tunable and stable deep-ultraviolet birefringent optics using two-dimensional hexagonal boron nitride

Birefringence is a fundamental optical property that can induce phase retardation of polarized light. Tuning the birefringence of liquid crystals is a core technology for light manipulation in current applications in the visible and infrared spectral regions. Due to the strong absorption or instability of conventional liquid crystals in deep-ultraviolet light, tunable birefringence remains elusive in this region, notwithstanding its significance in diverse applications. Here we show a stable and birefringence-tunable deep-ultraviolet modulator based on two-dimensional hexagonal boron nitride. It has an extremely large optical anisotropy factor of 6.5 × 10^-12 C² J^-1 m^-1 that gives rise to a specific magneto-optical Cotton-Mouton coefficient of 8.0 × 10⁶ T^-2 m^-1, which is about five orders of magnitude higher than other potential deep-ultraviolet-transparent media. The large coefficient, high stability (retention rate of 99.7% after 270 cycles) and wide bandgap of boron nitride collectively enable the fabrication of stable deep-ultraviolet modulators with magnetically tunable birefringence.