nanoparticles [@Liang2015]. They are also the basis for optomechanical devices such as split-beam cavities (SBC’s) [@Hryciw:13; @ref:wu2014ddo; @zhang2016chip; @lin2015design], where a complete gap is introduced into the centre of the photonic crystal, and paddle nanocavities (PNC’s) [@Healey:15; @Kaviani:15], where a nano-mechanical resonator is inserted into the gap of the split-beam cavity and interacts strongly with the optical mode. In these devices, the electric fields concentrated in the air and overlapping with the boundaries of the nanomechanical resonator allow sensitive transduction of the mechanical motion and enable studies of magnetometry [@Wu:17] and nonlinear optomechanics [@Kaviani:15].

To date, one dimensional air-mode PCC’s have typically been fabricated from high refractive index silicon (Si, $n_\text{Si}~\sim~3.5$). There is significant interest however in creating air mode silicon nitride devices (Si$_{3}$N$_{4}$, $n_\text{Si$_{3}$N$_{4}$}~\sim~2.0$) [@Baets:16]. The large internal tensile stress of Si$_{3}$N$_{4}$ films on Si ($\sim$ 1 GPa) allows for realization of high mechanical quality factor $Q_m$ &gt; $10^6$ nanobeam mechanical resonators [@Verbridge:06; @Verbridge:08; @ghadimi2018elastic]. It also enables creating optomechanical structures [@Camacho:09; @Krause:12; @Krause:15; @norte2016mechanical; @reinhardt2016ultralow; @tsaturyan2017ultracoherent; @ghadimi2018elastic] suspended over large length scales without warping or stiction compared with materials such as Si. This allows fabrication of long, thin supports suitable for suspending nanomechanical elements with enhanced sensitivity to 