## 3 Mechanically Tunable PhC Cavity Sensors
### Sensing Applications of Mechanically Tunable PhC Cavities
#### 3.2.1 Deformable Cavity-Based PhC Sensors  
Under the operation of external physical forces, the deformation of cavity structure can modify the cavity modes and lead to cavity resonance shifts and varying quality factors. Research efforts have been carried out to achieve the mechanical sensing based on mechanically tunable PhC cavities. 1D PCN cavities have attracted great attention in optomechanical sensing owing to their smaller size as well as the higher design flexibility. Split PCN cavity is a typical example of reconfigurable or deformable cavities, as shown in Figure 3. The split PCN cavity consisted of an air-slot in the middle of a nanocavity, which separated the nanobeam cavity into two symmetric parts. The movable side was mechanically controlled by the comb-drive, and the width of air-slot was tuned by varying the voltage applied to the actuator. By changing the axial separation of two aligned cantilevers (see Figure 3b), Q-factor of cavity was largely tuned and yet, while resonant frequency was barely affected [65]. Longitudinally split nanobeam cavities could also be mechanically tuned by varying the offset laterally in-plane (see Figure 3c) [66] and out-of-plane (see Figure 3d) [67] while maintaining the constant separation gap. Another ladder-like cavity was transversely split by an air-slot into two symmetric parts, as shown in Figure 3e. By controlling both movable structures by their individual comb-drives, the width of air-slot affected the performance of cavity resonance. In the reported experiments [68; 69], a resonance shift of up to 17 nm of the second-order mode was achieved with an air-slot width increment of 26 nm while the Q-factor could be maintained. Both these two split PCN cavities were mechanically tuned to deform their optical cavities and then greatly affected optical resonance, therefore tunable configuration based on this split PCN cavity was sensitive to exploit sensing applications on physical displacement, force and vibration etc. Taking a compact and geometrically isolated actuator (also called nanobender) as an example, it composed of metal electrodes on a single layer of piezoelectric thin-film, as demonstrated in Figure 4[96]. Combined with a sliced PhC cavity for displacement sensing, a wavelength tunability of an optical resonance by \(\sim\)5 nm/V with a large tuning linewidth ranging from 1520 to 1560 nm at the applied voltage of 4 V. Additionally, several nanobender-integrated schemes were proposed to detect the displacement induced by physical actuations operating in-plane, out-of-plane and twisting motions [96]. Besides 1D PhC cavities, reconfigurable cavities can also be more effectively realized with 2D PhC cavities. First, the geometrical deformation of cavity was introduced based on 2D PhC slabs. [PERSON] et al. presented a pressure sensor with a shoulder-coupled PhC cavity located on a suspended silicon bridge [97]. As shown in Figure 5a, when the pressure was applied on the PhC cavity, longitudinal deformation of air holes and defect length could shift the resonant wavelength of PhC cavities. It was demonstrated that the minimum detectable force and vertical deflection reached up to 0.25 N and 20-25 nm, respectively. Moreover, it pointed out that the relative position variation of air holes in the deformed PhC cavity played a major role in contribution to the behavior of optical resonance. On this basis, [PERSON] et al. investigated a hexagonal nano-ring PhC slab by using a silicon cantilever (see Figure 5b) [73]. A roughly linear relationship between the applied strain and resonance shift was observed. When the ring PhC was embedded onto a 50 \(\upmu\)m \(\times\) 15 \(\upmu\)m planar cantilever, the minimum detectable force and strain for ring PhC were 75.7 nN and 0.0023%, respectively. Compared with shoulder-coupled PhC configuration [97], this cantilever design greatly enhanced the sensitivity.