Prashanta Kharel

The interaction between light and mechanical motion has been harnessed for a variety of scientific and technological applications ranging from studies of decoherence to precision metrology and quantum information.  Building on these accomplishments, optomechanical systems show great potential for various classical and quantum applications, including ultra-low-noise oscillators and high-power lasers to quantum transducers and quantum memories.

Central to these goals of optomechanics, and more generally of quantum information science, is harnessing long-lived phonons while minimizing thermal noise. Often times, this means achieving coherent control of high-frequency mechanical modes. Acoustic modes having high Q-factors and high frequencies are less sensitive to thermal noise as they are more decoupled from their thermal environment.

A variety of microscale and nanoscale optomechanical systems use wavelength-scale structural control to access long-lived phonons at high frequencies. These GHz-frequency oscillators can be readily initialized in their quantum ground states using bulk refrigeration techniques, paving the way for impressive demonstrations ranging from non-classical mechanical states to remote entanglement between mechanical resonators. However, spurious laser absorption within these miniaturized systems (modal mass ~picogram), continue to threaten robust ground state operation. This is because even miniscule amounts of light absorbed at the material boundaries yield excess dissipation for light and add thermal noise.

In this context, macroscale systems based on bulk acoustic wave (BAW) resonators are intriguing resources for optomechanics. At cryogenic temperatures, these resonators support long-lived phonons within devices geometries that mitigate surface interactions by orders of magnitude over their microscale counterparts. So far, electromechanical coupling has been used to access such long-lived phonons, enabling various scientific and technological applications ranging from tests of Lorentz symmetry to low-noise oscillators. However, if we could access such phonons with light it could open new avenues for sensitive metrology, materials spectroscopy, high-performance lasers, and quantum information processing.

In this thesis, we demonstrate the optical control of long-lived, high-frequency phonons within BAW resonators.  We utilize Brillouin interactions to engineer tailorable coupling between free-space laser beams and high Q-factor phonon modes supported by a plano-convex bulk acoustic resonator. Analogous to the Gaussian beam resonator design for optics, we present analytical guidelines, numerical simulations, and novel microfabrication techniques to create stable acoustic cavities that support long-lived bulk acoustic phonons.

For efficient optical control of bulk acoustic phonons, we utilize resonant multimode interactions by placing the bulk crystal inside an optical cavity. Resonant interactions permit us to dramatically enhance the optomechanical coupling strength. Utilizing enhanced optomechanical interactions in a system where we can select between Stokes and the anti-Stokes process, we demonstrate cooling and parametric amplification of bulk acoustic modes as a basis for ultra-low-noise oscillators and high-power lasers.

Finally, we enhance the optomechanical coupling strength to be larger than the optical and mechanical decoherence rates, creating hybridized modes that are part light and part sound. Deterministic control of long-lived bulk acoustic phonons with light in this so-called strong coupling regime opens the door to applications ranging from quantum transduction to quantum memories.