In recent years, there has been growing interest in optical sensors based on microcavities due to their advantages of size reduction and enhanced sensing capability. In this paper, we aim to give a comprehensive review of the field of photonic crystal nanobeam cavity-based sensors. The sensing principles and development of applications, such as refractive index sensing, nanoparticle sensing, optomechanical sensing, and temperature sensing, are summarized and highlighted. From the studies reported, it is demonstrated that photonic crystal nanobeam cavities, which provide excellent light confinement capability, ultra-small size, flexible on-chip design, and easy integration, offer promising platforms for a range of sensing applications.

A new class of hybrid systems that couple optical and mechanical nanoscale devices is under development. According to their interaction concepts, two groups of opto-mechanical systems are summarized as mechanically tunable and radiation pressure-driven optical resonators. On account of their high-quality factors and small mode volumes as well as good on-chip integrability with waveguides/circuits, photonic crystal (PhC) cavities have attracted great attention in sensing applications. Benefitting from the opto-mechanical interaction, a PhC cavity integrated opto-mechanical system provides an attractive platform for ultrasensitive sensors to detect displacement, mass, force, and acceleration. In this review, we introduce basic physical concepts of opto-mechanical PhC system and describe typical experimental systems for sensing applications. Opto-mechanical interaction-based PhC cavities offer unprecedented opportunities to develop lab-on-a-chip devices and witness a promising prospect to further manipulate light propagation in the nanophotonics.

High-precision detection of electric charge is critical for physical, chemical, and biological measurements. Nanophotonic optomechanical system confines the optical field at the nanoscale and enables a strong interaction between optical cavity and mechanical resonator. Its high optical quality factor cavity and strong optomechanical coupling are promising for precision sensing applications. Here an integrated optomechanical electrometer is proposed for the electric charge sensing using a zipper cavity with a suspended photonic crystal nanobeam (PCN) acting as a movable mechanical resonator. As the electrostatic force arising from the electric voltage to be measured interacts with the mechanical motion of the movable PCN and modulates its resonance through electrostatic stiffening effect, optomechanical coupling transduces the mechanical motion to the optical field with enhanced sensitivity. The resonance shift of the mechanical resonator can be monitored to detect the electric voltage with a sensitivity of 0.007 Hz / m V 2 . Moreover, the sensing performance can be further enhanced with the operation of the optomechanical electrometer in the self-sustained oscillation above threshold power. Owing to the narrow-linewidth of detector radio frequency (RF) spectrum with a large peak-to-noise floor ratio (up to 73.5 dB), the enhanced electrical sensitivity of 0.014 Hz / m V 2 is achieved with a high resolution of 1.37 m V 2 H z − 1 / 2 . A theoretical minimal detectable electrostatic charge is calculated as 1.33 × 10 − 2 eH z − 1 / 2 by converting the measured electric voltage versus RF shift to an approximatively linear relationship. This on-chip optomechanical electrometry scheme provides a powerful solution to the ultrasensitive determination of charged nanoparticles in biological and chemical applications.

Optomechanical nanocavities open a new hybrid platform such that the interaction between an optical cavity and mechanical oscillator can be achieved on a nanophotonic scale. Owing to attractive advantages such as ultrasmall mass, high optical quality, small mode volume and flexible mechanics, a pair of coupled photonic crystal nanobeam (PCN) cavities are utilized in this paper to establish an optomechanical nanosystem, thus enabling strong optomechanical coupling effects. In coupled PCN cavities, one nanobeam with a mass m_eff~3 pg works as an in-plane movable mechanical oscillator at a fundamental frequency of π${\mathsf{\Omega }}_{\mathrm{m}}/2\mathsf{\pi }=4.148 \mathrm{MHz}$. The other nanobeam couples light to excite optical fundamental supermodes at $1542.858$ and 1554.464 nm with a ${\mathrm{Q}}_{\mathrm{o}}$ larger than 4 × 10⁴. Because of the optomechanical backaction arising from an optical force, abundant optomechanical phenomena in the unresolved sideband are observed in the movable nanobeam. Moreover, benefiting from the in-plane movement of the flexible nanobeam, we achieved a maximum displacement of the movable nanobeam as 1468 ${\mathrm{fm}/\mathrm{Hz}}^{1/2}$. These characteristics indicate that this optomechanical nanocavity is capable of ultrasensitive motion measurements.

Cavity optomechanics in the photonic devices enables a light-matter interaction in the chip-scale systems via the optical force driven mechanical motion. In this paper, a pump-probe scheme consisting of dual optomechanical cavities is proposed for the first time, where one optical cavity driven by a high input power act as a pump channel and the other one is used as a probe channel to detect the dynamical motion of mechanical resonator. Moreover, modulation of the optical force in the pump channel is achieved to observe the amplification of mechanical motions. As the light input of the pump channel increases, the dynamical displacement of the mechanical resonator is theoretically actuated to 4.8 nm $/\!\sqrt {\text{Hz}} $ at the pump power of 0.75 mW. This all-optical amplification of mechanical motion based on the pump-probe scheme potentially paves the way for the development of optomechanical actuation and switch for the photonic integrated circuits.