Optically isotropic longitudinal piezoelectric resonant photoelastic modulator for wide angle polarization modulation at megahertz frequencies

Polarization modulators have a broad range of applications in optics. The acceptance angle of a free-space polarization modulator is crucial for many applications. Polarization modulators that can achieve a wide acceptance angle are constructed by attaching a piezoelectric transducer to an isotropic material, and utilize a resonant transverse interaction between light and acoustic waves. Since their demonstration in the 1960s, the design of these modulators has essentially remained the same with minor improvements in the following decades. In this work, we show that a suitable single crystal with the correct crystal orientation, functioning as both the piezoelectric transducer and the acousto-optic interaction medium, could be used for constructing a highly efficient free-space resonant polarization modulator operating at megahertz frequencies and exhibiting a wide acceptance angle. We construct the modulator using gallium arsenide, an optically isotropic and piezoelectric crystal, and demonstrate polarization modulation at 6 MHz with an input aperture of 1 cm in diameter, acceptance angle reaching $\pm30^\circ$, and modulation efficiency exceeding 50%. Compared to state-of-the-art resonant photoelastic modulators, the modulator reported in this work exhibits greater than 50 fold improvement in modulation frequency for the same input aperture, while simultaneously reducing the thickness by approximately a factor of 80. Increasing the modulation frequency of photoelastic modulators from the kilohertz to the megahertz regime and substantially reducing their thickness lead to significant performance improvements for various use cases. This technological advancement also creates opportunities for utilizing these devices in new applications.