Microbubble resonators for scattering-free absorption spectroscopy of nanoparticles.

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.

Nonlinear Optics in Microspherical Resonators.

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.

Parametrical Optomechanical Oscillations in PhoXonic Whispering Gallery Mode Resonators.

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.

Long Period Grating-Based Fiber Coupling to WGM Microresonators.

A comprehensive model for designing robust all-in-fiber microresonator-based optical sensing setups is illustrated. The investigated all-in-fiber setups allow light to selectively excite high-Q whispering gallery modes (WGMs) into optical microresonators, thanks to a pair of identical long period gratings (LPGs) written in the same optical fiber. Microspheres and microbubbles are used as microresonators and evanescently side-coupled to a thick fiber taper, with a waist diameter of about 18 µm, in between the two LPGs. The model is validated by comparing the simulated results with the experimental data. A good agreement between the simulated and experimental results is obtained. The model is general and by exploiting the refractive index and/or absorption characteristics at suitable wavelengths, the sensing of several substances or pollutants can be predicted.

Efficient frequency generation in phoXonic cavities based on hollow whispering gallery mode resonators.

We report on nonlinear optical effects on phoxonic cavities based on hollow whispering gallery mode resonators pumped with a continuous wave laser. We observed stimulated scattering effects such as Brillouin and Raman, Kerr effects such as degenerated and non-degenerated four wave mixing, and dispersive wave generation. These effects happened concomitantly. Hollow resonators give rise to a very rich nonlinear scenario due to the coexistence of several family modes.

Optical Microbubble Resonators with High Refractive Index Inner Coating for Bio-Sensing Applications: An Analytical Approach.

The design of Whispering Gallery Mode Resonators (WGMRs) used as an optical transducer for biosensing represents the first and crucial step towards the optimization of the final device performance in terms of sensitivity and Limit of Detection (LoD). Here, we propose an analytical method for the design of an optical microbubble resonator (OMBR)-based biosensor. In order to enhance the OMBR sensing performance, we consider a polymeric layer of high refractive index as an inner coating for the OMBR. The effect of this layer and other optical/geometrical parameters on the mode field distribution, sensitivity and LoD of the OMBR is assessed and discussed, both for transverse electric (TE) and transverse magnetic (TM) polarization. The obtained results do provide physical insights for the development of OMBR-based biosensor.

Resonance Frequency of Optical Microbubble Resonators: Direct Measurements and Mitigation of Fluctuations.

This work shows the improvements in the sensing capabilities and precision of an Optical Microbubble Resonator due to the introduction of an encaging poly(methyl methacrylate) (PMMA) box. A frequency fluctuation parameter σ was defined as a score of resonance stability and was evaluated in the presence and absence of the encaging system and in the case of air- or water-filling of the cavity. Furthermore, the noise interference introduced by the peristaltic and the syringe pumping system was studied. The measurements showed a reduction of σ in the presence of the encaging PMMA box and when the syringe pump was used as flowing system.

Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators.

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres can be thought as a store of energy with a huge variety of applications: compact laser sources, highly sensitive biochemical sensors and nonlinear phenomena. A protocol for the fabrication of both the microspheres and coupling system is given. The couplers described here are tapered fibers. Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility Χ((3)) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS). A proof of the cavity-enhanced phenomenon is given by the lack of correlation among the pump, signal and idler: a resonant mode has to exist in order to obtain the pair of signal and idler. In the case of hyperparametric oscillations (four wave mixing and stimulated anti-stokes Raman scattering), the modes must fulfill the energy and momentum conservation and, last but not least, have a good spatial overlap.

Generation of hyper-parametric oscillations in silica microbubbles.

Cavity resonant enhanced stimulated Raman scattering (SRS), four-wave mixing, and broadband hyper-parametric oscillation in silica microbubble whispering gallery mode resonators (WGMR) in forward and backward directions are reported in this Letter. We show that microbubbles can operate not only in a highly ideal two-photon emission regime, but also generate combs, both natively and multi-mode spaced. The nonlinear process is phase matched because of the interaction of different mode families of the resonator.

Confocal reflectance microscopy for determination of microbubble resonator thickness.

Optical Micro Bubble Resonators (OMBR) are emerging as new type of sensors characterized by high Q-factor and embedded micro-fluidic. Sensitivity is related to cavity field penetration and, therefore, to the resonator thickness. At the state of the art, methods for OMBR's wall thickness evaluation rely only on a theoretical approach. The purpose of this study is to create a non-destructive method for measuring the shell thickness of a microbubble using reflectance confocal microscopy. The method was validated through measurements on etched capillaries with different thickness and finally it was applied on microbubble resonators.

Stimulated anti-Stokes Raman scattering resonantly enhanced in silica microspheres.

Efficient stimulated anti-Stokes Raman scattering has been observed in silica microspherical resonators pumped by CW laser. This process is observed in the normal dispersion regime. The lack of correlation between stimulated anti-Stokes and Stokes scattering spectra indicates that the signal has to be resonant with the cavity.