  In recent years, optical control of mechanical oscillators has emerged as a
critical tool for everything from information processing to laser cooling.
While traditional forms of optomechanical cooling utilize systems comprised of
discrete optical and mechanical modes, it has recently been shown that cooling
can be achieved in a chip-based system that possesses a continuum of modes.
Through Brillouin-mediated phonon-photon interactions, cooling of a band of
traveling acoustic waves can occur when anti-Stokes scattered photons exit the
system more rapidly than the relaxation rate of the mechanical waves -- to a
degree determined by the acousto-optic coupling. Here, we demonstrate that a
continuum of traveling wave phonons can be cooled within an optical fiber,
extending this physics to macroscopic length scales. Leveraging the large
acousto-optic coupling permitted within a liquid-core optical fiber, heterodyne
spectroscopy reveals power-dependent changes in spontaneous Brillouin
scattering spectra that indicate a reduction of the thermal phonon population
by 21K using 120 mW of injected laser power.
