  We design high quality factor photonic crystal nanobeam cavities formed by
two mechanically isolated cantilevers. These "split-beam" cavities have a
physical gap at the center, allowing mechanical excitations of one or both of
the cavity halves. They are designed by analyzing the optical band structures
and mode profiles of waveguides perforated by elliptical holes and rectangular
gaps, and are predicted to support optical resonances with quality factors
exceeding 1E6 at wavelengths of ~ 1.6 um.
