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

TL;DR

This study demonstrates that barium magnesium fluoride whispering gallery resonators can have their resonance frequencies linearly shifted by electric fields, primarily due to piezoelectric effects, enabling potential deep ultraviolet frequency conversion.

Contribution

The paper introduces a method to fabricate high-quality BMF whispering gallery resonators and characterizes their electrically-induced resonance shifts, highlighting piezoelectricity as the main mechanism.

Findings

Resonance frequency shifts linearly with electric field (-0.8 MHz/V/mm)

Quality factors exceed 10^7 in fabricated resonators

Piezoelectricity, not electro-optic effect, causes the resonance shifts.

Abstract

Barium magnesium fluoride (BMF) is a ferroelectric crystal with a transparency range far beyond the one of other optical materials. In particular, its low loss in the deep ultraviolet makes this material an 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 $10^7$. Their resonance frequencies can be shifted linearly by applying electric fields between the $+c$ and $-c$ faces of the crystal. The slope of the shift is $-0.8$~MHz/(V/mm). 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.