  We demonstrate slot-mode optomechanical crystals, a class of device in which
photonic and phononic crystal nanobeam resonators separated by a narrow slot
are coupled through optomechanical interactions. In these geometries, nanobeam
pairs are patterned so that a mechanical breathing mode is confined at the
center of one beam, and a high quality factor (Qo>10^5) optical mode is
confined in the slot between the beams. Here, we produce slot-mode devices in a
stoichiometric Si3N4 platform, with optical modes in the 980 nm band, coupled
to breathing mechanical modes at 3.4 GHz, 1.8 GHz, and 400 MHz. We exploit the
high Si3N4 tensile stress to achieve slot widths down to 24 nm, which leads to
enhanced optomechanical coupling, sufficient for the observation of
optomechanical self-oscillations at all studied frequencies. We utilize the
slot mode concept to develop multimode optomechanical systems with triple-beam
geometries, in which two optical modes are coupled to a single mechanical mode,
and two mechanical modes are coupled to a single optical mode. This concept
allows great flexibility in the design of multimode chip-scale optomechanical
systems with large optomechanical coupling at a wide range of mechanical
frequencies.
