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We report on the experimental and theoretical analysis of parametrical optomechanical oscillations in hollow spherical phoxonic whispering gallery mode resonators due to radiation pressure. The optically excited acoustic eigenmodes of the phoxonic cavity oscillate regeneratively leading to parametric oscillation instabilities.

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.

The foremost advanced photoacoustic (PA) transport theory, dealing with image formation, relates to target volumes of shallow depths. It assumes the medium as homogeneous with negligible PA amplitude attenuation. Causal changes in the velocity distribution spectrum, as described in Debye’s theory, and related to propagation distance and sample's density, are also neglected. However, these are relevant for imaging targets at larger depths and improving image resolution of PA images of thick biological tissues. Here we introduce some concepts for extending the PA transport model. These are theoretical and experimental considerations for analysing PA attenuation and the significance of spectral dispersion; and in consequence, disclose those conditions at which they should be included as part of the PA transport theory. Departing from the PA Heaviside telegraph equation and causality conditions, we obtain analytic expressions for associated attenuation and dispersion coefficients. As part of the analysis, we propose expressions for the PA group and PA phase velocities, and for the group velocity dispersion parameter; those in analogy with optical fields. In this way we get a refined description for the spectral dispersion. As proof of consistency, the introduced expressions are tested with experimental data extracted from homogeneous colloid samples. The observed performance is compared against already known general acoustic dispersion theory.

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