## 1 Introduction  
One of the challenging technologies in optics and photonics is the manipulation of light propagation, and it has a direct effect on optical communications, sensing and imaging [1, 2], Micro/nano-opto-electro-mechanical systems (M/NOEMS) offer unprecedented opportunities to control the flow of light in nanophotonic structures with efficient and compact architecture, high operation speed and low power consumption [3]. Numerous research works have been developed to establish methods to dynamically control the interaction between photonics and mechanics [4]. Research on opto-mechanical interaction is attracting considerable interests due to its potential for highly efficient and low crosstalk for signal transduction. For opto-mechanical interaction-based sensing applications, mechanical displacement arising from perturbated factors can be converted into optical transmission signals. One of the general laws of electromagnetism to dynamically control photonic devices is achieved by displacing the dielectric boundaries, also known as mechanically tunable photonics [5]. It has been proved that both the mechanical displacement of dielectric boundary or variation of refractive index (RI) has a crucial effect on the cavity resonance, thus they can serve as a sensitive tool to measure mechanical factors [6]. Electrical actuation is widely implemented by exerting an electrostatic force to obtain moving boundaries or deforming structures, and it has led to the development of M/NOEMS integrated photonic devices. Current on-chip M/NOEMS for photonic integrated circuits (PICs)have many competing edges such as a small footprint, easy fabrication process and compatibility of integration with PIC integration [7]. For the purpose of sensing application, with the advantages of flexible mechanical configurations and integration with nano-/micro-electromechanical systems (N/MEMS), mechanically tunable photonic crystal (PhC) cavities have widely developed into a variety of sensors, such as displacement [8; 9; 10] and stress sensors [11; 12], magnetic [13; 14] and electric field [15] sensors. In addition, miniaturization of tunable photonics, which localizes light into sub-wavelength mode volumes in optomechanical resonators, has reinforced the interplay between photons and mechanics to an unprecedented level [16; 17]. As light exerts forces, radiation pressure from the light reflected from an optical resonator can induce mechanical movement of device. This mechanical displacement, in turn, can change the optical modes of cavity and induce dynamically optomechanics phenomena. Unlike MOEMS tunable photonics under electrical driving, cavity optomechanics enables opto-mechanical interaction by the radiation pressure of light. Some milestone experiments have demonstrated, including ground-state cooling [18; 19] and other quantum mechanical phenomena [17]. Understanding the behaviors of radiation force-driven interaction beyond the standard quantum limit [20], cavity optomechanics is also exploited in engineering metrology for fine control of nanophotonic optomechanical devices for sensing applications [21; 22]. Microcavities in opto-mechanical interactions consist of Fabry-Perot (F-P) cavities [23], whispering gallery mode (WGM) cavities [24] and photonic crystal (PhC) cavities [25]. Among typical cavities, the PhC cavity has been demonstrated as an outstanding platform due to its high Q-factors and small mode volume (ultra-high Q/V) [26]. Besides, with good on-chip compatibility with PICs, PhC cavity facilitates the enhancement of opto-mechanical interaction. For the past decade, PhC cavities based on opto-mechanical interaction have developed quickly, and there have already been many reviews, but they mainly focus on PhC cavity sensing applications [27; 28; 29; 30], cavity optomechanics [31; 32; 17; 24; 33; 34] or their applications [21; 22; 35; 36]. In this review, we pay attention to the work on sensing application of on-chip PhC cavities based on opto-mechanical interaction.