  We describe the design, fabrication, and characterization of a
one-dimensional silicon photonic crystal cavity in which a central slot is used
to enhance the overlap between highly localized optical and mechanical modes.
The optical mode has an extremely small mode volume of 0.017
$(\lambda_{vac}/n)^3$, and an optomechanical vacuum coupling rate of 310 kHz is
measured. With optical quality factors up to $1.2 \cdot 10^5$, fabricated
devices are in the resolved-sideband regime. The electric field has its maximum
at the slot wall and couples to the in-plane breathing motion of the slot. The
optomechanical coupling is thus dominated by the moving-boundary effect, which
we simulate to be six times greater than the photoelastic effect, in contrast
to most structures, where the photoelastic effect is often the primary coupling
mechanism.
