  We demonstrate an optomechanical phase shifter. By electrostatically
deflecting the nanofabricated mechanical structure, the effective index of a
nearby waveguide is changed and the resulting phase shift is measured using an
integrated Mach-Zehnder interferometer. Comparing to thermo-optical phase
shifters, our device does not consume power in static operation and also it can
operate over large frequency, wavelength, and power ranges. Operation in the
MHz range and sub-$\mu$s pulses are demonstrated.
