Recently, it was predicted that the energy transfer between normal modes of open systems with a specific type of degeneracy known as an exceptional point (EP) could be achieved by topological operations, and that the outcome of these operations would be non-reciprocal.<sup>1</sup> Here we demonstrate this topological transfer of energy between two vibrational modes of a cryogenic optomechanical device using a tunable laser. We also show the non-reciprocity of the energy transfer process. These results open up new directions in system control, as well as other dynamical processes of multimode systems that are robust against small perturbations.
In cavity optomechanics the state of a mechanical element can be manipulated by interfacing it with light via radiation pressure, electrostriction, or related phenomena. The majority of mechanical elements employed in optomechanical systems to date are solid objects (membranes, nanowires, mirrors, etc); however fluids can also be used as a mechanical element. Compared to solids, fluids have an advantage: they readily achieve precise alignment with the optical cavity, as the fluid can conformally fill or coat the optical cavity. However, almost all optomechanical systems need to be cooled to sub-Kelvin temperatures in order for quantum effects to be observed. Liquid helium is the only fluid that doesn't solidify under its own pressure at these temperatures. Additionally, helium has almost no optical absorption, high thermal conductivity and very low acoustic loss at cryogenic temperatures. We have developed an optomechanical system in which the mechanical mode is a standing density wave in superfluid helium inside a 70 μm long Fabry-Perot cavity. The optical mode is also a mode of the same cavity. Thus, the system is completely self-aligned. In this system, we used electrostriction to drive the mechanical mode with light by modulating the optical intensity. We also observed the mode's undriven Brownian motion and from that extracted it mean phonon number. We measured phonon number as low as <i>n</i><sub>ac</sub>=11. The optomechanical effects of optical spring and optical damping were observed, and agreed well with the predictions of conventional optomechanical theory.
We present measurements of an optomechanical system in which the mechanical element is inside the cavity, and
couples dispersively to the intracavity field. This geometry makes it easier to simultaneously achieve high optical finesse
and high mechanical quality factor in an optomechanical device. We measured the linear optical properties of a such a
device in which the mechanical element is a 50 nm thick silicon nitride membrane. We find that the device's finesse,
resonant transmission and resonant reflection can be explained with a simple model which allows us to extract the
membrane's optical loss. Our results indicate that it should be possible to increase the finesse of these devices to 5 × 105