Highly Efficient Acousto-Optic Modulation Driven by Ultra-Low Power in Integrated Photonic–Phononic Waveguides

Abstract

Acousto-optic modulation (AOM) can tailor acoustic and optical domains, promising applications from signal processing to quantum transduction. Advances in integrated circuits technologies have facilitated minimization and integration of acousto-optic devices, paving ways for their applications in large-scale hybrid integrated photonic-phononic systems. Recently, integrated AOM is demonstrated with promising performance on various integrated piezoelectric materials. However, their optical conversion efficiencies remain significantly below unity. Achieving high efficiency necessitates strong mode-coupling over long interaction space, which requires strong confinement and minimizing losses for both acoustic and optical fields in integrated structures. This is challenging for most integrated material platforms. In this work, efficient AOM is demonstrated using semi-insulating gallium nitride (GaN) on sapphire. By leveraging the sub-wavelength confinement of both optical and acoustic fields within GaN waveguides, while minimizing optical (0.35 dB ${\mathrm{cm}}^{-1}$) and acoustic (0.4 dB ${\mathrm{mm}}^{-1}$) propagation losses, complete optical mode conversion of AOM driven by a 1.67-mW radio frequency power, setting a benchmark for integrated acousto-optical modulators, is achieved. Furthermore, the AOM non-reciprocity is also performed with high non-reciprocal contrast ($>$10 dB) across a 4-GHz optical bandwidth. The work offers a robust and efficient acousto-optic platform, opening new opportunities for integrated quantum transduction, signal processing, and non-magnetic optical isolation.