Electrically induced resonance shifts of whispering gallery resonators made of barium magnesium fluoride

Barium magnesium fluoride (BMF) is a ferroelectric crystal with a transparency range far beyond that of other optical materials. In particular, its low loss in the deep ultraviolet makes this material a unique candidate for frequency conversion in this spectral range. Due to its relatively weak second-order nonlinearity, a resonant configuration, such as an optical whispering gallery, would be beneficial. We show that femtosecond-laser based material processing enables the reliable fabrication of BMF whispering gallery resonators with quality factors beyond 107. Their resonance frequencies can be shifted linearly by applying electric voltages between the +c and -c faces of the crystal. The slope of the shift is -0.8 MHz/V. It seems that the origin of this shift is piezoelectricity, while the electro-optic effect is negligible. Our results pave the way for millimeter-sized frequency converters in the deep ultraviolet. Furthermore, they indicate that a careful determination of fundamental material properties is still necessary.