Optical Spring Effect in Micro-Bubble Resonators and Its Application for the Effective Mass Measurement of Optomechanical Resonant Mode

Optical Whispering-Gallery-Mode Microbubble Sensors

Whispering-gallery-mode (WGM) microbubble resonators are ideal optical sensors due to their high quality factor, small mode volume, high optical energy density, and geometry/design/structure (i.e., hollow microfluidic channels). When used in combination with microfluidic technologies, WGM microbubble resonators can be applied in chemical and biological sensing due to strong light–matter interactions. The detection of ultra-low concentrations over a large dynamic range is possible due to their high sensitivity, which has significance for environmental monitoring and applications in life-science. Furthermore, WGM microbubble resonators have also been widely used for physical sensing, such as to detect changes in temperature, stress, pressure, flow rate, magnetic field and ultrasound. In this article, we systematically review and summarize the sensing mechanisms, fabrication and packing methods, and various applications of optofluidic WGM microbubble resonators. The challenges of rapid production and practical applications of WGM microbubble resonators are also discussed.

Microbubble Resonators for All-Optical Photoacoustics of Flowing Contrast Agents

In this paper, we implement a Whispering Gallery mode microbubble resonator (MBR) as an optical transducer to detect the photoacoustic (PA) signal generated by plasmonic nanoparticles. We simulate a flow cytometry experiment by letting the nanoparticles run through the MBR during measurements and we estimate PA intensity by a Fourier analysis of the read-out signal. This method exploits the peaks associated with the MBR mechanical eigenmodes, allowing the PA response of the nanoparticles to be decoupled from the noise associated with the particle flow whilst also increasing the signal-to-noise ratio. The photostability curve of a known contrast agent is correctly reconstructed, validating the proposed analysis and proving quantitative PA detection. The experiment was run to demonstrate the feasible implementation of the MBR system in a flow cytometry application (e.g., the detection of venous thrombi or circulating tumor cells), particularly regarding wearable appliances. Indeed, these devices could also benefit from other MBR features, such as the extreme compactness, the direct implementation in a microfluidic circuit, and the absence of impedance-matching material.

Noise suppression of mechanical oscillations in a microcavity for ultrasensitive detection

Optical microcavities have been widely applied as sensitive detectors due to ultrahigh quality factors and small mode volumes. Besides considering the optical mode as the sensing signal, the optomechanical oscillations induced by the optical spring effect also perform as an elegant sensing signal. However, the minimal size of a detectable analyte is limited by the relatively weak light-matter interaction compared to the experimental noises. To improve the detection limit, many methods have been developed to either enhance device sensitivities or suppress experimental noises. In this work, we present a way to lower the detection limit by suppressing experimental noises of the mechanical frequency by 3 orders of magnitude. Utilizing a fiber tip as a benchmark analyte attaching onto the cavity, the mechanical frequency shift reflects the changes of the optical mode detuning of the cavity, predicting an effective tool for ultrasensitive detection.