## 3 Mechanically Tunable PhC Cavity Sensors
### Sensing Applications of Mechanically Tunable PhC Cavities
#### 3.2.2 Near-Field Probe Perturbation-Based PhC Sensors  
Resonance control of optical resonators can also be established using the evanescent field perturbation method. For 1D PCN cavities, their tuning configurations are illustrated in Figure 8a. An external nano-scale probe driven by a NEMS comb-drive actuator acted as a perturbation factor to the PCN cavity. To enhance the probe interaction with the evanescent optical field, [PERSON] et al. [8, 42] reported a rectangular tip to tune a resonance shift of 1.1 nm with the decreasing gap between tip and cavity from 750 to 100 nm. In comparison, a meniscus-like tip was used to tune the resonance shift of 2.25 nm. However, tuning of a rectangular tip probe caused a significant drop of Q-factor and transmission of cavity, compared with a moderate degradation of Q-factor and transmission using the meniscus-like tip probe. The near-field perturbation tuning method was further developed with a nano-scale multi-tip probe [42] to reach a larger tuning range of 5.4 nm. Besides 1D PhC cavities, probe  
Figure 6: Schematics of the displacement sensors: (**a**) 2D-slot PhC with fixed and moving segments [9]; (**b**) schematic of the magnetic field sensor based on slot PhC slab cavity [13].  
Figure 7: Schematics of the strain detection system based on cantilever-PhC cavity: (**a**) deflection of cantilever in the center of cavity (inset: the cross-section view for the electromechanical tuning) [77]; (**b**) cantilever inserted into a PhC nanobeam [78].  
perturbation based 2D PhC cavities also has been developed for sensing applications. As shown in Figure 8b, a torsion-free pressure sensor was presented with the structure of a piston-rod side-coupled with the PhC membrane nanocavity [89]. The piston-rod was located on the center of PhC cavity and acted as a probe tip. It was attached to a silicon strip which can be deformed by applied strain. The stress displacement of silicon strip could change the depth of piston-rod inserted in the central nanohole of PhC nanocavity. A sensitivity reaching up to 0.50 nm/nN was observed and its minimum detection force was estimated to be 0.68 nN. Furthermore, [PERSON] et al. [100] proposed a three-dimensional force sensor. A shoulder-coupled PhC cavity was designed to detect forces in the horizontal and vertical directions, and a piston-rod PhC was used to detect the force in the upright direction. With a well-designed and optimized PhC structure, high sensitivities of 8.2 nm/\(\mu\)N, 12.5 nm/\(\mu\)N, and 10.9 nm/\(\mu\)N were obtained in the horizontal, vertical and upright directions, respectively; the detection limits for the three directions were 24 nN, 16 nN, and 18 nN, respectively. However, the resonance tuning range obtained in the near-field probe perturbation was generally not very large, and additional energy losses that negatively affect the Q factor and degraded optical transmission. Another sensing configuration based on the dielectric perturbation method in the PCN cavity sensors was present by [PERSON] et al. [101] for gas pressure measurement, as shown in Figure 9. In the PCN sensor, a polydimethylsiloxane (PDMS) membrane was coated on the functional silicon device area, which acted as a perturbation factor. As there was a pressure difference between the sealed holes and vacuum chamber, the deformation of PDMS could move close or away to the air holes of PCN cavity and then shift the resonant wavelength with the linear pressure sensitivity of 8.8 nm/MPa.