  We report on the first experimental demonstration of widely-tunable
parametric sideband generation in a Kerr microresonator. Specifically, by
pumping a silica microsphere in the normal dispersion regime, we achieve the
generation of phase-matched four-wave mixing sidebands at large frequency
detunings from the pump. Thanks to the role of higher-order dispersion in
enabling phase matching, small variations of the pump wavelength translate into
very large and controllable changes in the wavelengths of the generated
sidebands: we experimentally demonstrate over 720 nm of tunability using a
low-power continuous-wave pump laser in the C-band. We also derive simple
theoretical predictions for the phase-matched sideband frequencies, and discuss
the predictions in light of the discrete cavity resonance frequencies. Our
experimentally measured sideband wavelengths are in very good agreement with
theoretical predictions obtained from our simple phase matching analysis.
