Superfluid Optomechanics with Nanofluidic Geometries

In this work, a novel implementation of superfluid optomechanics is developed, exploiting nanofluidic confinement within a sonic crystal geometry. The aim of which is to enhance the optomechanical coupling strength while preserving the intrinsic properties of superfluid He-4, via limitation of radiative acoustic effects. These nanostructures are designed and fabricated using cleanroom techniques, with focus on the development of a direct wafer bonding technique. The cleanroom fabricated chips are then integrated into superconducting microwave cavities. For measurements, a dilution refrigerator (capable of mK cooling) was refurbished for modern microwave measurements and work with superfluid He-4. COMSOL Multiphysics simulations are used to design the optomechanical system, both the acoustics of the sonic crystal and the chip-cavity microwave environment. From these simulations a predicted vacuum optomechanical coupling is calculated giving g_0/2pi = 0.63 mHz; between a superfluid mode of frequency 1.34 MHz, and a microwave cavity mode of 4.2 GHz. Phase sensitive homodyne measurements of the chip-cavity system in the presence of He-4 found mechanical signals close to the predicted resonance frequency, however these signals were insufficient in strength or consistency to determine the optomechanical parameters of the system.