  Periodically structured materials can sustain both optical and mechanical
modes. Here we investigate and observe experimentally the optomechanical
properties of a conventional two-dimensional suspended photonic crystal defect
cavity with a mode volume of $\sim$$3(\lambda/n)^{3}$. Two families of
mechanical modes are observed: flexural modes, associated to the motion of the
whole suspended membrane, and localized modes with frequencies in the GHz
regime corresponding to localized phonons in the optical defect cavity of
diffraction-limited size. We demonstrate direct measurements of the
optomechanical vacuum coupling rate using a frequency calibration technique.
The highest measured values exceed 250 kHz, demonstrating strong coupling of
optical and mechanical modes in such structures.
