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.
We present a new method for detecting single nanoparticles using a sensor based on a whispering gallery mode resonator submerged in aqueous solutions. A free-space diode laser excites whispering-gallery mode resonances by focusing it on the edge of the microresonator. Its emission frequency is then locked to a resonant mode in order to track any change induced by the interaction of the microsphere with nanoparticles, which can be suspended in the surrounding liquid medium. A theoretical analysis based on some seminal work, together with preliminary noise source evaluation, indicates that frequency shifts down to the order of hundreds of kHz are measurable, thus allowing to detect single nanoparticles. Further upgrades of the experimental scheme aimed at precise nanoparticle sizing and positioning are discussed.
Miniaturization of Fourier transform spectrometers must necessarily deal with the problem of the mechanical scan of an interferometer, which is the core element of this technology. Several small-scale FT spectrometers with no moving parts have been demonstrated over the last years, although they all rely on advanced integrated photonics technology. We have shown previously that spectrum of an unknown radiation can be accurately measured with the simplest of the optofluidic systems: a droplet evaporating on a partially reflective substrate. This principle allows to devise a miniaturized FT spectrometer with a spontaneous evaporation-driven scan. Here, we show that such optofluidic spectrometer can be employed for the readout of a plasmonic nanostructured optical fiber sensor, allowing to devise a plasmonic sensing setup where either radiation source, sensor and readout spectrometer are all integrated in an optical fiber. The system performance is analogous to that of a macroscopic plasmonic setup, but it can be obtained with a dramatically reduced cost and dimensions.
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