Cavity optomechanical liquid level meter using a twin-microbottle resonator

Cavity optomechanical devices can be made to have good compatibility with optical fiber technology by utilizing fiber-based waveguides and cavities and can be used in high-performance optical sensor applications. Such optomechanical microsensors have a great potential for exploring the properties of liquids, such as density, viscosity, and masses of included nanoparticles. However, as yet, there is no cavity optomechanical architecture that can be used to sense the liquid's shape, e.g., liquid level. In this paper, we report a demonstration of a liquid-level meter using a twin-microbottle resonator that can make measurements at arbitrary positions and depths in the liquid. The twin-microbottle resonator has a maximum diameter of 68 $\mu$m and length of 800 $\mu$m. By immersing one part of it in water and keeping the other part in air, the mechanical radial breathing mode can be read out sensitively while maintaining a high optical quality factor of the optical whispering gallery mode regardless of the water immersion. This high mechanical displacement sensitivity provides a frequency resolution that is high enough to measure the mechanical frequency shift due to the water immersion and resolves the water level to 2.6$\pm$0.9 pm. This unique liquid-level meter based on a highly sensitive cavity optomechanical setup can be used to detect tiny fluctuations of various air-liquid and liquid-liquid interfaces

Near-field optomechanical transduction enhanced by Raman gain

Raman-gain-enhanced near-field optomechanical transduction between a movable optical cavity and SiN-membrane resonator is demonstrated. The Raman gain compensates for the intrinsic loss of the cavity and amplifies the optomechanical transduction, through which the membrane vibration is sensed using a high-Q whispering-gallery-mode optical cavity evanescently. The optical Q of the cavity resonance is improved with respect to the optical pump power, which results in an increase in the optomechanically transduced vibration signals of the mechanical resonator. Our near-field optomechanical coupling approach with optical gain realizes highly sensitive displacement measurement in nano- and micro-mechanical resonators consisting of arbitrary materials and structures