## 5 Summary and Outlook  
As discussed above, on-chip tunable PhC devices integrated with N/MEMS generally have been demonstrated for various sensing applications on mechanical measurements. With further advancements in nanofabrication and device integration techniques, dynamically tunable PhC systems with ultra-precise features are expected to achieve a better sensing performance. A significant technical breakthrough in the mass production of a 5-nm semiconductor device comes out in 2020, which further enables the manufacturing ability of micro-nano on-chip sensors and networks sensors with smaller features and lower fabrication errors in future applications. Moreover, coupling techniques for the integrated on-chip device are the key and challenging factors to achieve practical opto-mechanical devices, playing an important role in sensing applications of PhCs.  
For radiation pressure-driven PhCs, as the field of cavity optomechanics is still under booming development, it is expanding rapidly in both basic physics and engineering applications. When it comes to sensing applications, cavity optomechanics possesses certain advantages and challenges. Compared with most N/MEMS devices with limited operational bandwidth, optomechanical PhC devices allow both optical actuation and readout to provide faster sampling and scanning rate. Besides, optomechanical devices can be operated at high frequencies up to GHz, and thus they can directly link the signal domains at microwave frequencies in optical cavities for signal conversion [60; 183; 184]. In addition, optomechanical PhC cavities can offer the unique resource of strong optical backaction effects, which allows dynamic control of the mechanical resonance frequency and bandwidth. It should be noted that most realized optomechanical PhC systems till now have selected the device material with a large Young's modulus, such as silicon, silicon nitride and silica, etc., mainly considering their compatibility with nanofabrication technology and optical properties. However, the displacement of these materials driven by optical force is at most of the order of tens of nm, thereby it always reduces their mechanical stiffness and affects optomechanical performances. To achieve larger sensitivity and tunability, it is meaningful to exploit mechanically compliant materials such as polymers [185; 186], which can reduce cost and can be easily combined with microfluidic platforms.  
In summary, PhC cavities with the interplay of optical and mechanical resonators for sensing application are reviewed. The basic physics, experimental systems, and sensing applications in the context of mechanically tunable and radiation pressure-driven cavities are presented. Mechanically tunable PhC sensors have the characteristics of the simple system, easy fabrication, high precision, and dynamic control capability. Meanwhile, radiation pressure-driven PhC cavities reveal significant improvements in the performance of enhanced sensitivity and faster operation frequency. However, it also raises the stringent requirements for experimental testing, for example, vacuum environment and  
Figure 28: Proposal scheme of opto-mechanical PCN cavity with graphene integrated.  
low adiabatic temperature are required in most optomechanical measurements. In the future, cavity optomechanics will bring a new interaction between the optics-mechanics interface. It embarks on the long journey towards miniaturization and integration of signal processing and on-chip devices for multi-function applications.  
[PERSON] conducted the review work and write this manuscript; [PERSON] and [PERSON]. ([PERSON]) checked and edited language and formal; [PERSON]. ([PERSON]) and [PERSON] supervised this work. All authors have read and agreed to the published version of the manuscript.  
This research was funded by MOE Tier2 project, grant number R-265-000-694-112.  
The authors declare no conflict of interest.