  Optical forces in guided-wave nanostructures have recently been proposed as
an effective means of mechanically actuating and tuning optical components. In
this work, we study the properties of a photonic crystal optomechanical cavity
consisting of a pair of patterned silicon nitride nanobeams. Internal stresses
in the stoichiometric silicon nitride thin-film are used to produce inter-beam
slot-gaps ranging from 560 to 40nm. A general pump-probe measurement scheme is
described which determines, self-consistently, the contributions of
thermo-mechanical, thermo-optic, and radiation pressure effects. For devices
with 40nm slot-gap, the optical gradient force is measured to be 134fN per
cavity photon for the strongly coupled symmetric cavity supermode, producing a
static cavity tuning greater than five times that of either the parasitic
thermo-mechanical or thermo-optic effects.
