Daniele Farnesi

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 OMBRs 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.

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.

Non-linear fluorescence excitation of Rhodamine 6G and TRITC labeled IgG in whispering gallery mode microresonators

We report the non linear fluorescence real-time detection of labeled IgG covalently bonded to the surface of a microspherical whispering gallery mode resonator (WGMR). The immunoreagents have been immobilized onto the surface of the WGMR sensor after being activated with an epoxy silane and an orienting layer. The developed immunosensor presents great potential as a robust sensing device for fast and early detection of immunoreactions. We also tested the potential of microbubbles as nonlinear enhancement platform. The dyes used in these studies are tetramethyl rhodamine isothiocyanate and Rhodamine 6G. All measurements were performed in a modified confocal microscope.

Cladding modes fiber coupling to silica micro-resonators based on long period gratings

A novel method based on long period fiber gratings (LPGs) for coupling light to high-Q silica whispering gallery mode (WGM) resonators is presented. An LPG couples the fundamental mode of a fiber to higher order LP cladding modes at selected frequencies. At an adiabatically tapered section of the fiber following the LPG we demonstrated effective coupling of these cladding modes to WGMs both in silica microspheres and microbubbles. The taper is about one order of magnitude thicker than standard tapers used for the same purpose. Therefore this new method offers improved robustness for practical applications.

Long Period Grating-Based Fiber Coupling to WGM Microresonators for Sensing Applications

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.

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.

Glass-based microresonators

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.

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.

Microspherical resonators for multicolor laser light emission

Silica microspheres are artificial resonant cavities that show great potential for hosting a wide range of nonlinear effects in low-power and continuous-wave (CW) regimes.1 They can be used to study fundamental light–matter interactions and can even become tunable laser sources at visible wavelengths. Such laser sources can be used in compact spectroscopic applications and ‘lab-on-chip’ systems, and they may pave the way to nonclassical light generation.2 In addition, these cavities are capable of sustaining high-quality resonant optical modes, e.g., whispering gallery modes (WGMs).3 WGM resonators trap light within sub-millimeter-sized, dielectric, spherical cavities through the process of internal reflection. The light also becomes resonantly amplified in these devices. This strong spatial confinement of the resonant light leads to very high circulating optical intensities— for long intra-cavity times—that can be exploited for nonlinear wave generation at low-CW power levels. To generate nonlinear waves effectively, however, it is also necessary to fulfill the phase and mode-matching conditions and the energy conservation law. In the case of WGM resonators that are made of silica glass (i.e., which exhibit inversion of symmetry), second-order nonlinear interactions are forbidden.4 In these cases, the elemental nonlinear interaction is caused by third-order susceptibility ( 3) effects, in which four photons are coupled. Efficient generation of visible light via third-order sum-frequency generation (TSFG)—also known as four-wave mixing—and third-harmonic generation (THG) in silica microspheres, however, has not previously been investigated in a thorough manner. In our work, it is therefore our aim to study a variety of 3 nonlinear interactions in silica microspheres. Such microspheres can easily be fabricated directly on the tip of a standard Figure 1. Schematic diagram of the experimental setup. TDL: Tunable diode laser. EDFA: Erbium-doped fiber amplifier. Att: Attenuator. DET: Photodiode detector. MMF: Multimode fiber. OSA: Optical spectrum analyzer. SCOPE: Oscilloscope.