  The synthesis of ultra-long lived acoustic phonons in a variety of materials
and device geometries could enable a range of new coherent information
processing and sensing technologies; many forms of phonon dissipation pose a
barrier to this goal. We explore linear and nonlinear contributions to phonon
dissipation in silica at cryogenic temperatures using fiber-optic structures
that tightly confine both photons and phonons to the fiber-optic core. When
immersed in helium, this fiber system supports nearly perfect guidance of 9 GHz
acoustic phonons; strong electrostrictively mediated photon-phonon coupling (or
guided-wave stimulated Brillouin scattering) permits a flexible form of
laser-based phonon spectroscopy. Through linear and nonlinear phonon
spectroscopy, we isolate the effects of disorder-induced two-level tunneling
states as a source of phononic dissipation in this system. We show that an
ensemble of such two-level tunneling states can be driven into
transparency--virtually eliminating this source of phonon dissipation over a
broad range of frequencies. Experimental studies of phononic self-frequency
saturation show excellent agreement with a theoretical model accounting for the
phonon coupling to an ensemble of two-level tunneling states. Extending these
results, we demonstrate a general approach to suppress dissipation produced by
two-level tunneling states via cross-saturation, where the lifetime of a
phonons at one frequency can be extended by the presence of a high intensity
acoustic beam at another frequency. Although these studies were carried out in
silica, our findings are quite general, and can be applied to a range of
materials systems and device geometries.
