Light-Mediated Control of Superfluid Flow

Abstract

Optical forces are widely used in microphotonic systems. By driving the motion of mechanical elements, they enable applications ranging from precision sensing and metrology to quantum information and fundamental science. To date, the primary approaches to optical forcing exploit either the direct radiation pressure from light [1], or photothermal forces [2, 3] where optical heating causes mechanical stress and subsequent deformation of a mechanical element. Photothermal forces have the advantage of allowing significantly stronger actuation capabilities, but to-date have proved incompatible with the cryogenic conditions required to reach the quantum regime. In this work we demonstrate a new approach to optical forcing that allows strong microphotonic forces to be achieved in cryogenic conditions. The approach utilises, for the first time in a microphotonic context, the well-known fountain effect in superfluid helium [4], whereby superfluids flow convectively towards a heat source. In our case, the heat is generated by optical absorption in the vicinity of a mechanical element. A force is exerted when the fluid reaches this element and imparts momentum onto it. We experimentally achieve, within a cryogenic environment, microphotonic forces that are an order of magnitude stronger than their radiation pressure counterparts. As a demonstration of the utility of our technique, we use the superfluid photoconvective force to feedback cool a mechanical mode of a microtoroidal resonator to temperatures as low as 137 mK. Depending on geometry of a microphotonic system, photoconvective flow can be utilised to exert a wide range of forces, including, for instance, torques and compression, providing a versatile tool for cryogenic actuation of microphotonic systems.