Optomechanics with phononic modes in superfluid thin-film helium

In recent years, nanomechanical oscillators in thin films of superfluid helium-4 have attracted attention in the field of optomechanics due to their exceptionally low mechanical dissipation and optical scattering. Here, we study surface waves – so-called ‘third sound’ – in superfluid helium thin films self-assembled on silicon nanobeam optical cavities. By embedding nanobeam cavities in helium-4 gas and cooling the system below the superfluid transition temperature, nanometer thin films of superfluid helium form on the resonator surface. Surface waves in the superfluid helium film couple dispersively to the optical cavity mode by spatial modulation of the refractive index. We characterize the mechanical properties and optomechanical coupling of third sound modes on the nanobeam surface by measuring time-resolved oscillations of the optical cavity resonance and homodyne detection of the mechanical spectrum. We find mechanical mode frequencies in the range of 1 – 50 MHz in agreement with finite-element simulations. This work is a step towards realizing strong mechanical non-linearities that were recently theoretically predicted in superfluid thin-film phononic crystals [1].

[1] npj Quantum Inf. 7(1), 1-12 (2021)