Development of Optomechanical Sensors Using Capillary-based Microfluidic Ring Resonators

In current state-of-art optical microresonators, light travels more than 106 roundtrips before decaying out and demonstrates enhanced light-matter interactions that are challenging to achieve with traditional optics. Intensity of the circulating light in microresonators accumulates and induces nonlinear effects such as high order harmonic generation and optomechanical vibration. Furthermore, owing to evanescent optical field and elongated optical path, sensors based on optical microresonators respond to small perturbations in resonance wavelength due to samples.
In this work, we utilize the radiation pressure of circulating light inside microfluidic capillary based whispering gallery mode (WGM) ring resonators to optically monitor the mechanical properties of non-solid samples. Parametrical coupling between optical and mechanical modes in microfluidic ring resonators optically generated mechanical vibrations up to 100 MHz with viscous liquid inside the resonator. The change in mechanical properties of liquids were optically monitored by the shift of mechanical eigen-frequencies. Furthermore, we characterized surface sensitivity of a microfluidic ring resonator by layer-by-layer removal of SiO2 molecules from the resonator's inner surface. A mechanical eigen-frequency downshift was observed with a surface sensitivity of 1.2 Hz/(pg/mm2) and a detection limit of 83 pg/mm2.
We also present the feasibility of using optical ring resonators for air-coupled ultrasound detection by studying the interaction between externally generated acoustic waves and optical modes within the ring resonator. We theoretically analyzed and experimentally demonstrated non-contact detection of air-coupled ultrasound using high optical Q (~107) mFOMRRs. Detectable noise equivalent pressures of 6 mPa and 16.5 mPa for 50 kHz and 800 kHz in air, respectively, were observed. Furthermore, detection of air-coupled photoacoustic pulses optically generated from a 200 nm thick Chromium film is demonstrated