## 2 Photonic Crystal Cavities for Opto-Mechanical Interaction
### Experimental Opto-Mechanical PhC Cavities
#### 2.2.2 Two-Dimensional Photonic Crystal Cavity in Opto-Mechanical System  
For 2D PhC cavities, mechanical tuning effects on the cavity resonance can also be classified based on the working mechanisms of probe perturbation, deformable cavity and coupled cavities, as outlined in Table 2. First, the shape of planar 2D PhC slab cavity in the mechanical tunable system can be modulated under the applied mechanical forces. [PERSON] et al. [72; 73] investigated the property of a ring 2D PhC cavity based on the stress-induced deformation of microcantilever. When an external force was applied on the cantilever tip, holes inside the PhC cavity were deformed, thereby resulting in the resonance shift. In addition to the cavity deformation, the presence of probe perturbation could alter the resonance of 2D PhC slab cavity. For example, an electrostatically driven bimorph probe was self-aligned to the holes of 2D PhC cavity, so the propagation properties of PhC cavity could be altered based on the proximity of tips to holes [74]. The presence of a probe accounted for a variation of effective RI in 2D PhC cavity, thereby mechanically modulating the photonic band structure of the cavity. Second, a tunable air-slot 2D PhC slab was developed with in-plane motion driven by an integrated MEMS actuator [75; 76]. Due to the dielectric discontinuity in the air-slot, optical mode was locally confined and enhanced inside the air-slot gap. Therefore, motion tuning of the width of air-slot gap could exert an intensive impact on the resonance and Q-factor of cavity. Integrating a nanocantilever inside a 2D PhC cavity was numerically and experimentally studied [77; 78], wherein the nanocantilever acted as a tunable photonic defect in the 2D PhC slab cavity. Strong opto-mechanical interaction between nanocantilever and defect-mode field resulted in a good sensing performance of resonance dispersion to surface stress-induced out-of-plane deflection. Finally, a vertically coupled bilayer 2D PhC cavity was also experimentally demonstrated [79; 80]. The electrostatic actuating out-of-plane motion-controlled separation gap between two membranes, as a result, a resonant wavelength tuning range of 13 nm was obtained at a lower temperature of 8K [79].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Classification** & **Planar PhC** & **Air-Slot PhC** & **Cantilever-PhC** & **Slot Embedded** & **Vertically** \\  & **Slab Cavity** & **Cavity** & **Cavity** & **PhC Cavity** & **Coupled Bilayer** \\ \hline \hline \end{tabular}
\end{table}
Table 2: Typical configurations of 2D PhC cavities for opto-mechanical interactions.  
Similarly, optical configurations of 2D PhC cavity in the radiation pressure-driven system are almost same as those in the mechanically tunable system in Table 2. 2D planar PhC cavities can support photonic and phononic band gaps simultaneously. The simplest way is to create an optomechanical structure from a single planar PhC slab [81, 84]. In this structure [81], the optical modes of a L3 cavity were strongly coupled to localized mechanical modes, which could be used for exciting photon-phonon conversions. The air-slot PhC structure [82, 85] was established by confining the cavity field into the air-slot between two suspended and flexible membranes. This system could activate the dynamic backaction to realize mechanical amplification or cooling. It played a significant role in exploiting sensing platform utilizing optomechanical backaction with reductive thermal noise and low-noise optical read-out. Another nanomechanical slot embedded PhC cavity was achieved by defining three missing holes [83, 86]. The ultra-small dimension and femtogram mass of nanomechanical resonators with high mechanical and optical Q-factors were promising for ultrasensitive mechanical measurements and mass sensing of nanoparticles.