  Optomechanical crystals provide coupling between phonons and photons by
confining them to commensurate wavelength-scale dimensions. We present a new
concept for designing optomechanical crystals capable of achieving
unprecedented coupling rates by confining optical and mechanical waves to deep
sub-wavelength dimensions. Our design is based on a dielectric bowtie unit cell
with an effective optical/mechanical mode volume of $7.6 \times 10^{-3}
{(\lambda/n_{\textrm{Si}})}^3$/$ 1.2 \times 10^{-3}
{\lambda_{\textrm{mech}}}^3$. We present results from numerical modeling,
indicating a single-photon optomechanical coupling of 2.2 MHz with
experimentally viable parameters. Monte Carlo simulations are used to
demonstrate the design's robustness against fabrication disorder.
