  Presented in this paper are measurements of an optomechanical device in which
various acoustic modes of a sample of superfluid helium couple to a fiber-based
optical cavity. In contrast with recent work on the paraxial acoustic mode
confined by the cavity mirrors, we focus specifically on the acoustic modes
associated with the helium surrounding the cavity. This paper provides a
framework for understanding how the acoustic modes depend on device geometry.
The acoustic modes are observed using the technique of optomechanically induced
transparency/amplification. The optomechanical coupling to these modes is found
to be predominantly photothermal.
