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Whispering-Gallery Mode (WGM) microresonators have become popular in photonic systems thanks to their ease of fabrication, high optical Q-factor and ultra-small mode volume. Here, we illustrate the modelling and experimental activities derived from light coupling mechanisms to passive WGM microresonators based on free-space scattering without using any prism or fiber waveguide. This has been carried out for cavities made of liquid and solid materials, for which we report applications and potential use in optical sensing, machine learning and spectroscopy. In particular, angular momentum matching, i.e. light coupling via scattering, is obtained only in a strict interval of alignment conditions exhibiting WGM spectra populated with a variety of peaks with diverse quality factors. We devised an optical feedback loop based on a spatial light modulator that tailors the phase of a laser beam and, thanks to a random algorithm optimizes the alignment maximizing the scattered light-ring pattern. This allows to harness the strong mode confinement and power amplification of the microresonator to observe opto-mechanical and radiation-pressure effects. Free-space WGMs is also an appealing platform for artificial intelligence architectures. Taking a further step from this optical setup, we started implementing a photonic learning machine whereby the SLM acts as an optical encoder while the WGM spectrum provides the optical readout. Finally, we developed a novel room-temperature radiation sensor based on a free-space laser locked on the resonance of a silica microsphere. Thanks to silica strong absorption in the IR, we showed that the microsphere element enables detection of electromagnetic radiation from the mid-IR (MIR) up to the THz spectral range proving also suitable for absorption spectroscopy.
D’Ambrosio et al. (Tue,) studied this question.