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.
Interband and Quantum Cascade Lasers are key sources for MIR molecular sensing. Understanding their noise features and stabilizing their emission is of fundamental importance for applications like precision spectroscopy and metrology. High-Q crystalline Whispering Gallery Mode Resonators have proven to be powerful tools for characterization and stabilization of lasers from the UV to the MIR. Here, we report our recent results on Whispering Gallery Mode Resonators used for frequency characterization, stabilization and linewidth narrowing of Interband and Quantum Cascade Lasers. These results pave the way to new classes of compact MIR sources usable in Space missions, Metrology and Fundamental Physics.
We report on a metrological-grade mid-IR source with a 10–14 short-term instability for high-precision spectroscopy. Our source is based on the combination of a quantum cascade laser and a coherent radiation obtained by difference-frequency generation in an orientation-patterned gallium phosphide (OP-GaP) crystal. The pump and signal lasers are locked to an optical frequency comb referenced to the primary frequency standard via an optical fiber link. We demonstrate the robustness of the apparatus by measuring a vibrational transition around 6 μm on a metastable state of CO molecuels with 11 digits of precision.
Orientation-patterned gallium phosphide (OP-GaP) crystals are used here for the first time for the generation of continuous-wave coherent mid-infrared radiation around 5.85 μm by difference frequency generation (DFG) of a Nd:YAG laser at 1064 nm and a diode-laser at 1301 nm. We provide the first characterization of the linear, thermo-optic and nonlinear properties of OP-GaP in a DFG configuration, and we derive an effective nonlinear coefficient deff = 17 pm/V (3) for first-order quasi-phase-matched OP-GaP. This novel nonlinear material can be used to referenced the mid-IR light to a frequency standard by locking the pump and signal laser to a near-IR optical frequency comb.
The realization of a first-order interferometric autocorrelator in a nearly collinear geometry is reported as an
alternative method to measure the minimum pulse duration of femtosecond deep-UV laser pulses. The Fourier
limited duration of 257-nm femtosecond pulses is measured with high accuracy, and compared to what obtained by
spectrally measuring the bandwidth of the pulses. The agreement between the two methods is excellent, thus
indicating the interferometric autocorrelator as a useful tool to extract the chirp of femtosecond UV pulses when
used together with a second-order autocorrelator to measure the actual pulse duration.
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