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Whispering Gallery Mode (WGM) hollow microcavities turn out to be the site of an extremely rich and complex phenomenological scenario when pumped with a continuous-wave laser source. The coexistence of numerous non-linear and optomechanical effects have been reviewed in this paper. In our previous works we have investigated and described non-linear emissions as the stimulated Brillouin and Raman scattering, the degenerated and non-degenerated Kerr effects, such as four wave mixing (FWM). These effects happened concomitantly to parametric optomechanical oscillations which are the consequence of the radiation pressure. We have confirmed the regenerative oscillation of acoustic eigenmodes of the cavity leading to parametric instabilities and the activation of optomechanical chaotic oscillations. Finally, we have demonstrated that the blue-side excitation of WGM resonances lead to the chaos transition with a spectral evolution depending on the cavity size.

We report the activation of optomechanical chaotic oscillations and chaos transfer from a strong pump to a very weak probe in microbubble resonators, both signals follow the same route to chaos.

We present a proof-of-concept experiment where the absorbance spectra of suspensions of plasmonic nanoparticles are accurately reconstructed through the photothermal conversion that they mediate in a microbubble resonator. This thermal detection produces spectra that are insensitive towards light scattering in the sample, as proved experimentally by comparing the spectra of acqueos gold nanorods suspensions in the presence or absence of milk powder. In addition, the microbubble system allows for the interrogation of small samples (below 40 nl) while using a low-intensity beam (around 20 µW) for their excitation. In perspective, this system could be implemented for the characterization of turbid biological fluids through their optical absorption, especially when considering that the microbubble resonator naturally interfaces to a microfluidic circuit and may easily fit within portable or on-chip devices.

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.

We report on the experimental analysis of parametrical optomechanical oscillations and photo-acoustical applications such as flow cytometers in hollow phoxonic whispering gallery mode resonators. Both phenomena can be enchanced or suppressed and showed chaotic behavior.

We developed an all-optical photoacoustic (PA) sensing platform based on whispering gallery mode (WGM) hollow resonators able to detect plasmonic nanoparticles in microfluidics. A silica microbubble resonator (MBR) made on a capillary and exhibiting high optical and mechanical Q factors acts as a unique ultrasound transducer for photoacoustic characterization of metallic nanoparticles. The system was tested with gold nanorods (GNR) placed inside the MBR and acoustic detection was achieved by monitoring the enhanced modulation of the optical resonances at the microbubble mechanical eigenfrequencies.

We report on chemical, biochemical and physical sensors based on whispering gallery mode resonators. We characterized the resonators, improved the mitigation of fluctuations, and developed different types of optical sensors, labelled and label-free sensors for chemical and biochemical sensing, and pressure, temperature and nanoparticle sensing., Franco Cosi is gratefully acknowledged for his help in the device fabrication. Franco Quercioli and Pablo Loza-Alvarez are gratefully acknowledged for allowing us to use their nonlinear microscopes.

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.

We report on the experimental analysis of parametrical optomechanical oscillations and nonlinear frequency generation in hollow phoxonic whispering gallery mode resonators. Both phenomena can be enchanced or suppressed and showed chaotic behavior.

A variety of studies have been performed on WGM microresonators made of different materials and exploiting very high quality factors. This feature has allowed their use in a broad variety of applications including micro-lasers, optical filtering and switching, frequency conversion through non-linear effects, RF photonics and sensing. The easiest way to shape a glass spheroid with high quality factor is to use the surface tension during the thermal reflow of a highly transparent amorphous dielectric. Depending on the application, alternatives shapes like micro-bubbles or micro-bottles can be implemented in order to obtain specific performances. This manuscript reports the results we obtained on micro-laser sources in erbium doped microspheres, parametric frequency conversion in silica microspheres, stimulated Brillouin scattering in silica microbubbles, and non-linear effects in polymer coated or fluorophore filled glass microcavities.

Optical microresonators with high quality factors are key in photonic circuits requiring fine spectral filtering or resonant storage of optical power. Silicon (Si) photonics provides high-performance optoelectronic circuits but yields planar Si microresonators with rather low quality factors ( Q < 10 5 ). On the other hand, bulk resonators achieve exceptionally high quality factors, Q > 10 7 . Si photonic waveguides and bulk resonators have very different sizes and refractive indices that preclude efficient coupling. Here, we show an efficient method to couple bulk resonators and Si waveguides based on subwavelength metamaterial engineering. We demonstrate up to 99% light coupling efficiency for microspheres and microdisks made of silica, lithium niobate, and calcium fluoride, with 0.3 − 3.6 m m diameter. This achievement could enable the heterogeneous integration of bulk resonators and silicon photonic circuits, with potential applications in sensing, communications, and quantum information.

We present and discuss an analytical model for the propagation of light in a glass microsphere resonator based on wave optics and Maxwell's equations in the formulation of vector and scalar Debye's potential. References and connections with geometrical optics and quantum mechanics theory are given in order to better understand the electromagnetic phenomenon of whispering-gallery modes (WGMs). The problem of light coupling into a glass microsphere and the various strategies usually adopted for this purpose are also detailed.

An optical resonator like a fiber ring (FR) or a whispering gallery mode (WGM) resonator with two couplers along its loop is referred to be in the add-drop configuration, in analogy with the add-drop multiplexer in telecom networks. Both for practical applications as well as in several fundamental studies involving high-Q resonators, this configuration is of great interest and the assessment of the intrinsic properties of the resonator and of its interaction with the coupling systems is extremely important. We developed an original method able to fully characterize high-Q resonators in an add-drop configuration. The method is based on the study of the two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We implemented the model describing the two CRD profiles from which a best fit process of the measured profiles allows deducing the key parameters. We successfully validated the model with an experiment based on a FR resonator of known characteristics. Finally, we fully characterized a high-Q, home-made, MgF2 WGM disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.

Whispering gallery mode resonators (WGMR) have attracted a great interest in the last decade. WGMR have been fabricated in different geometries, solid and hollow, spherical, toroidal, and bottled shaped. Hollow spherical WGMR or microbubble resonators (MBR) are the last arrived in the family of resonators. The approach used for their fabrication is based on surface tension driven plastic deformation on a pressurized capillary, similar to glassblowing. Using such technique we are able to fabricate large surface area and thin spherical shells with high quality factor (Q). MBR are efficient phoxonic cavities that can sustain both optical photons and acoustic phonons. It has been demonstrated that MBR can be used to study Turing comb patterns (Kerr modulation) and Stimulated Brillouin Scattering (SBS). Radiation pressure is another mechanism that also leads to excitation of acoustic phonons with lower frequencies, in the range of hundreds of kHz to tens of MHz in the case of silica MBR. The frequency of such oscillations occurs very close to the mechanical eigenfrequencies of the cavity. We have studied the temporal behavior of the cavity, the coexistence and the suppression of the oscillation while generating Turing comb patterns. The observed phenomenology can be explained by the geometrical characteristics of a MBR. MBRs are spheroidal WGM resonators with quite dense spectral characteristics. The total dispersion of MBR is anomalous and large, as expected for very large MBR. Thus, Kerr comb formation is allowed for all MBR used in this work.

Coating of high-Q whispering gallery mode micro-resonators is typically performed in order to add the functionalities of the coating material to the unique properties of this type of resonators. Silica microspheres or microtoroids are typically used as high-Q cavity substrate on which a functional film is deposited. In order to effectively exploit the coating properties a critical step is the efficient excitation of WGMs mainly contained inside the deposited layer. We developed a simple method able to assess whether or not these modes are selectively excited. The method is based on monitoring the thermal shift of the excited resonance, which uniquely depends on the thermo-optic coefficient and on the thermal expansion coefficient of the material in which the mode is embedded. We applied this technique to the case of a SU-8 layer deposited on a silica microsphere. Main tests were performed around the wavelength of 770 nm because of potential application in biochemical sensing requiring low light absorption in aqueous environment. We show that by using integrated waveguides made with SU-8 polymer (rather than silica fiber tapers) we can fulfill the proper phase matching conditions thus exciting the fundamental WGM mainly confined in the coating. A further proof of the validity of the approach is obtained assessing the free spectral range of the excited modes which depends on the refractive index of the material in which the mode is confined.

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.

We implement a whispering-gallery-mode microbubble resonator (MBR) as a multifunctional component for all-optical photoacoustic inspection of materials in a platform combining microfluidic design and high sensitivity to spectral absorbance. In our setup, the MBR acts both as the acoustic transducer and as the vial for a sample causing the photoacoustic event, thus dictating its operational frequency and removing any need for impedance-matching media. We challenge our setup in the assessment of the photostability of gold nanorods to demonstrate its validity. We develop a numerical model to qualitatively interpret the experimental signal as a transient shift of the resonance condition of the MBR. Possible implementations of the MBR in photoacoustic sensors are discussed.

The paper illustrates both review and original simulation results obtained via the modelling of different set-ups based on optical microresonators for applications in optical sensing, lasing and spectroscopy. Passive microbubbles and microspheres coupled via long period fiber gratings (LPGs) and tapered fibers are designed and/or constructed for sensing of biological fluids in the near infrared (NIR) wavelength range. Rare earth doped chalcogenide glass integrated microdisks are designed for active sensing in the medium infrared (MIR) wavelength range. A home-made numerical code modelling the optical coupling and the active behavior via rate equations of ion population is employed for a realistic design, by taking into account the most important active phenomena in rare earths, such as the absorption rates, the stimulated emission rates, the amplified spontaneous emission, the lifetime and branching ratios, the ion-ion energy transfers and the excited state absorption. Optical coupling is obtained by employing ridge waveguides, for micro-disks, and tapered fibers, for microspheres and microbubbles. Different dopant rare earths as Erbium (Er3+) and Praseodymium (Pr3+) are considered.

Nonlinear frequency generation requires high intensity density which is usually achieved with pulsed laser sources, anomalous dispersion, high nonlinear coefficients or long interaction lengths. Whispering gallery mode microresonators (WGMRs) are photonic devices that enhance nonlinear interactions and can be exploited for continuous wave (CW) nonlinear frequency conversion, due to their capability of confine light for long time periods in a very small volume, even though in the normal dispersion regime. All signals must be resonant with the cavity. Here, we present a review of nonlinear optical processes in glass microspherical cavities, hollow and solid.

A system consisting of a microbubble coupled with a tapered fiber is designed in order to detect different concentrations of an aqueous sodium chloride (NaCl) solution. The microbubble whispering gallery modes WGMs are selectively excited by means of a pair of identical long period fiber gratings (LPGs) placed on both taper ends. The simulation of the set-up gives a numerical proof-of-concept toward the exploitation of sensing applications in medical diagnostic and environment monitoring. The response of the microbubble, in terms of the different WGMs resonance detuning shifts, to an increasing NaCl concentration is investigated. The simulated results promise interesting applications.

International audience; An original method able to fully characterize high-Q resonators in an add-drop configuration has been implemented. The method is based on the study of two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We first developed a model describing the two CRD output signals and a fitting program able to deduce the key parameters from the measured profiles. We successfully validated the model with an experiment based on a fiber ring resonator of known characteristics. Finally, we characterized a high-Q, home-made, MgF2 whispering gallery mode disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.

Surface tension induced whispering gallery mode (WGM) micro-resonators can be made in glass with very high quality factor Q. In fact, low losses amorphous glassy dielectrics can be easily shaped in high-surface-quality spheroids by thermal reflow. Since the pioneering works on fused silica microspheres showing several orders of magnitude higher Qs compared to previous findings, a large number of studies have been performed in the last years on glass based microresonators. Main results include frequency conversion through non-linear effects and micro-lasers, filtering and optical switching, RF photonics and sensing. Besides spheres, alternatives shapes like micro-bottles and micro-bubbles have been implemented to improve the resonator performances depending on the application. Other glasses rather than silica have been considered in order to enhance properties like transparency windows and non-linear effects. This presentation will review the main results we obtained on micro-laser sources in erbium doped microcavities, parametric conversion in silica microspheres, and stimulated Brillouin scattering in silica microbubbles. Potentials of coated silica microspheres implemented to add the functionalities of the coating material will be also presented.

Whispering Gallery Mode (WGM) micro-resonators like microspheres or microtoroids are typically used as high-Q cavity substrate on which a functional film coating is deposited. In order to exploit the coating properties a critical step is the efficient excitation of WGMs mainly contained inside the deposited layer. We developed a simple method able to assess whether or not these modes are selectively excited. The method is based on monitoring the thermal shift of the excited resonance, which uniquely depends on the thermo-optic coefficient and on the thermal expansion coefficient of the material in which the mode is embedded.

Over recent years, photoacoustic concepts have drawn much attention for new applications and innovative solutions in biomedical optics. In particular, photoacoustic imaging is penetrating the clinical arena as a non-invasive and versatile solution to combine optical contrast and ultrasonic depth of penetration and scalability of resolution. [1,2] The peculiar features of photoacoustic imaging have been demonstrated in a notable variety of setups, ranging from microscopy to tomography, that are already mature and competitive for translational developments. Most photoacoustic platforms rely on piezoelectric transducers that were developed within the venerable context of ultrasonography. However, the generation and propagation of acoustic signals in photoacoustics and ultrasonics differ in fundamental aspects that deserve innovative solutions and yield new opportunities. Here, we disclose our design and preliminary results for an all-optical photoacoustic flow spectro-cytometer that may be exploited in a broad variety of applicative domains, such as the detection of individual circulating cells in bio fluids as well as the inspection of colloidal suspensions of artificial particles in sols or aerosols. This setup rests on the implementation of an optical microcavity resonator [3] that may be coupled to a microfluidic interface and filled with any sample of interest. Then, its excitation with short optical pulses triggers a photoacoustic conversion within the cavity, which imparts a transient deformation of the glass resonator and its dielectric landscape, thus shifting its optical resonances. The advantages of this setup include its inherent feasibility for miniaturization and workability in air rather than water, as occurs with piezoelectric transducers for ultrasonography. We illustrate these features in the analysis of the spectral fingerprints of colloidal suspensions of plasmonic particles with volumes in the nL range and repetition rates in the kHz domain. We gratefully acknowledge funding from the bilateral project CNR-CONACYT (2017-2019, "All optical morphogenesis of nanostructures characterized by photo-acoustic microscopy").

We report on nonlinear optical phenomena on phoxonic cavities based on microbubble whispering gallery mode resonators pumped with a continuous wave laser. We observed simultaneous excitation of Brillouin, Raman and Kerr effects in silica MBR.

Microresonator structures are very suitable for sensing application and the investigation of nonlinear effects, due to their enormous quality factor and small mode volume. These properties can be extended into the mid-infrared region by creating microresonators from chalcogenide glasses, which have a wide window of transparency and a large third-order optical nonlinearity. We present the analysis of the nonlinear effects observation in chalcogenide microspheres created by inert gas heating.

LiNbO 3 crystals are already widely used for THz application [1], The main drawback of using this material is represented by its high absorbance in this electromagnetic wavelength range [2], In this work, we propose to exploit its high absorbance for thermal conversion of the incident wave, By means of the high sensitivity to temperature variations of WGM resonators, it would be possible to convert absorbed power in a thermal shift of the resonance, In our model, the microdisc is represented in a mono-dimensional picture as a h=1 mm thick slab between a perfect insulator (air, with thermal conductivity k=0) and a perfect heat sink (k=0) that acts as mirror for the THz radiation, Under this conditions, the time constant could be written as τ = ρ h2/k C = 0.74 s, (1) where C and k are the Specific Heat and Conductivity of LiNbO 3 , C=0.63 J/(g K) and k=3.92 W/(m K) respectively, Considering the LiNbO 3 thermo-optic coefficient equal to α tot = α to + α te – 5.82 GHz/K, a working wavelength of 1500 nm and a WGM resonator with a 108 Q factor [3], the minimum precision available for temperature will be: ΔT lim = δv/α tot = 0.1 MHz/5.82 MHz/K = 17 μK. (2)