  Here we identify a new form of optomechanical coupling in gas-filled
hollow-core fibers. Stimulated forward Brillouin scattering is observed in air
in the core of a photonic bandgap fiber. A single resonance is observed at 35
MHz, which corresponds to the first excited axial-radial acoustic mode in the
air-filled core. The linewidth and coupling strengths are determined by the
acoustic loss and electrostrictive coupling in air, respectively. A simple
analytical model, refined by numerical simulations, is developed that
accurately predicts the Brillouin coupling strength and frequency from the gas
and fiber parameters. Since this form of Brillouin coupling depends strongly on
both the acoustic and dispersive optical properties of the gas within the
fiber, this new type of optomechanical interaction is highly tailorable. These
results allow for forward Brillouin spectroscopy in dilute gases, could be
useful for sensing and will present a power and noise limitation for certain
applications.
