In this thesis we use quartz tuning fork resonators to probe properties of normal
and superfluid ⁴He and ³He. Our main goal is to study both quantum turbulence and acoustic emission of tuning forks in liquid helium.
By employing a multi-frequency lock-in amplifier we contrast single and multi-
frequency methods of measuring tuning forks in the linear regime. In the non-linear response of tuning forks during turbulence we create multi-frequency excitations called intermodulation products which are used to find the non-linear forces that created them. We apply this technique to quantum turbulence in superfluid ⁴He-II and find that the retarding in-phase force on the fork increases at a critical velocity for turbulence nucleation. We also observe that the out-of-phase non-linear force increases, which we attribute to energy loss via vortex ring emission by the fork.
Superfluid ³He is a fermionic condensate of Cooper pairs of ³He atoms. At
ultra-low temperatures of 120 μK thermally excited unpaired quasiparticles travel ballistically through the condensate. We beam quasiparticles from a black body source towards a 5 × 5-pixel camera and observe that the excitations follow photonic-like trajectories. We apply the source-camera configuration to non-invasively detect and even image quantum vortices, that are topological defects in the superfluid.
Lastly, we explore the frequency dependent damping of quartz tuning forks in
liquid ³He. We find that at high frequencies the fork damping is governed by acoustic emission. Furthermore, we show that existing models developed for sound emission in ⁴He can be used to predict observed acoustic damping in ³He. The results also suggest that devices for ³He experiments should be placed in cavities or designed to operate at low frequencies.