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Phonon-frequency-tunable optomechanical interactions are demonstrated in shaped bulk acoustic resonators. Non-collinear all-optical coupling enables access to phonons with novel mode-selection rules, high quality factors (>10⁷), and frequency tunability over 10 GHz. 

Abstract Surface acoustic waves (SAW) and associated devices are ideal for sensing, metrology, and hybrid quantum devices. While the advances demonstrated to date are largely based on electromechanical coupling, a robust and customizable coherent optical coupling would unlock mature and powerful cavity optomechanical control techniques and an efficient optical pathway for long-distance quantum links. Here we demonstrate direct and robust coherent optical coupling to Gaussian surface acoustic wave cavities with small mode volumes and high quality factors (>10⁵measured here) through a Brillouin-like optomechanical interaction. High-frequency SAW cavities designed with curved metallic acoustic reflectors deposited on crystalline substrates are efficiently optically accessed along piezo-active directions, as well as non-piezo-active (electromechanically inaccessible) directions. The precise optical technique uniquely enables controlled analysis of dissipation mechanisms as well as detailed transverse spatial mode spectroscopy. These advantages combined with simple fabrication, large power handling, and strong coupling to quantum systems make SAW optomechanical platforms particularly attractive for sensing, material science, and hybrid quantum systems. 

A continuously tunable microwave-photonic filter with ultranarrow bandwidth is enabled by forward inter-modal Brillouin interactions with a fundamental acoustic mode of a fiber taper. Sub-MHz bandwidth is demonstrated over >10 GHz, limited by current components. 

We demonstrate strong (~ 300 W^-1m^-1) and ultranarrow linewidth (~100 kHz) stimulated intermodal forward Brillouin scattering in a homogeneous few-mode optical fiber taper. This unique combination of parameters can enable record performance Brillouin-based microwave-photonic devices. 

Stimulated Brillouin-like optomechanical coupling to Gaussian surface acoustic wave (SAW) cavity modes with record-low losses, is predicted and observed, enabling contact-free optical control of SAW cavities for a wide range of frequencies and material platforms. 

Traveling-wave optomechanical interactions, known as Brillouin interactions, have now been established as a powerful and versatile resource for photonic sources, sensors, and radio-frequency processors. However, established Brillouin-based interactions with sufficient interaction strengths involve short phonon lifetimes, which critically limit their performance for applications, including radio-frequency filtering and optomechanical storage devices. Here, we investigate a new paradigm of optomechanical interactions with tightly confined fundamental acoustic modes, which enables the unique and desirable combination of high optomechanical coupling, long phonon lifetimes, tunable phonon frequencies, and single-sideband amplification. Using sensitive four-wave mixing spectroscopy controlling for noise and spatial mode coupling, optomechanical interactions with long $><\#comment/>2\text{µ<\#comment/>}\mathrm{s}$phonon lifetimes and strong $><\#comment/>400{\mathrm{W}}^{-<\#comment/>1}{\mathrm{m}}^{-<\#comment/>1}$coupling are observed in a tapered fiber. In addition, we demonstrate novel phonon self-interference effects resulting from the unique combination of an axially varying device geometry with long phonon lifetimes. A generalized theoretical model, in excellent agreement with experiments, is developed with broad applicability to inhomogeneous optomechanical systems.