High Q silica microbubble resonators fabricated by arc discharge

Axially slow-variation microbubble resonators fabricated by an improved arc discharge method for strain sensing applications

In this paper, we proposed an axially slow-variation microbubble resonator fabricated by an improved arc discharge method and applied to axial strain sensing. The prepared resonators are characterized by ultra-thin wall thickness and axial slow-variation. The wall thickness was experimentally measured to reach 938 nm and maintain a quality factor of an optical mode as large as 7.36 ×107. The main factors affecting the strain sensitivity of the microbubble resonators are investigated theoretically and experimentally. Experimentally, the maximum sensitivity measured was 13.08pm/µε, which is three times higher than the microbubble resonators without this method. The device is simple to prepare and possesses ultra-thin wall thickness. It is promising for applications in high-precision sensing, such as single molecule and biological sensing.

Sensitivity Equalization and Dynamic Range Expansion with Multiple Optofluidic Microbubble Resonator Sensors

A novel multi-optofluidic microbubble resonator (OMBR) sensitivity equalization method is presented that equalizes the sensing signal from different OMBRs. The method relies on the fact that the ratio of the wavelength shifts to the bulk refractive index sensitivity (BRIS) does not depend on the physical dimensions of the OMBR. The proof of concept is experimentally validated and the sensing signals from individual OMBRs can be directly compared. Furthermore, a wide dynamic range of sensing with favorable consistency and repeatability is achieved by piecing together signals from 20 OMBRs for HIV-1 p24 antigen detection from 50 fg/mL to 100 ng/mL (2.1 fM to 4.2 nM), indicating significant potential for practical applications, such as in drug screening and disease diagnosis.

Glyphosate Separating and Sensing for Precision Agriculture and Environmental Protection in the Era of Smart Materials

The present article critically and comprehensively reviews the most recent reports on smart sensors for determining glyphosate (GLP), an active agent of GLP-based herbicides (GBHs) traditionally used in agriculture over the past decades. Commercialized in 1974, GBHs have now reached 350 million hectares of crops in over 140 countries with an annual turnover of 11 billion USD worldwide. However, rolling exploitation of GLP and GBHs in the last decades has led to environmental pollution, animal intoxication, bacterial resistance, and sustained occupational exposure of the herbicide of farm and companies' workers. Intoxication with these herbicides dysregulates the microbiome-gut-brain axis, cholinergic neurotransmission, and endocrine system, causing paralytic ileus, hyperkalemia, oliguria, pulmonary edema, and cardiogenic shock. Precision agriculture, i.e., an (information technology)-enhanced approach to crop management, including a site-specific determination of agrochemicals, derives from the benefits of smart materials (SMs), data science, and nanosensors. Those typically feature fluorescent molecularly imprinted polymers or immunochemical aptamer artificial receptors integrated with electrochemical transducers. Fabricated as portable or wearable lab-on-chips, smartphones, and soft robotics and connected with SM-based devices that provide machine learning algorithms and online databases, they integrate, process, analyze, and interpret massive amounts of spatiotemporal data in a user-friendly and decision-making manner. Exploited for the ultrasensitive determination of toxins, including GLP, they will become practical tools in farmlands and point-of-care testing. Expectedly, smart sensors can be used for personalized diagnostics, real-time water, food, soil, and air quality monitoring, site-specific herbicide management, and crop control.

Compact on-chip crystalline resonator integration with etching of tapered fiber waveguide

Whispering-gallery-mode crystalline resonators currently maintain the best quality factor (Q) record; however, compact on-chip packaging is still a challenge, although various coupling architectures have been developed. Here, a chemical etching method is proposed to fabricate a miniaturized tapered fiber waveguide on silicon substrate. The Marangoni effect is implemented to reduce the surface roughness of the cone region. The optical loss of 0.1 dB/mm is obtained, and the Q of the on-chip crystalline resonator exceeds 10⁸. Additionally, thermoelectric cooler (TEC) is implanted in the package to actively customize the temperature, and the temperature response of 18 pm/°C is consistent with the theoretical calculation.

Optical Whispering-Gallery-Mode Microbubble Sensors

Whispering-gallery-mode (WGM) microbubble resonators are ideal optical sensors due to their high quality factor, small mode volume, high optical energy density, and geometry/design/structure (i.e., hollow microfluidic channels). When used in combination with microfluidic technologies, WGM microbubble resonators can be applied in chemical and biological sensing due to strong light–matter interactions. The detection of ultra-low concentrations over a large dynamic range is possible due to their high sensitivity, which has significance for environmental monitoring and applications in life-science. Furthermore, WGM microbubble resonators have also been widely used for physical sensing, such as to detect changes in temperature, stress, pressure, flow rate, magnetic field and ultrasound. In this article, we systematically review and summarize the sensing mechanisms, fabrication and packing methods, and various applications of optofluidic WGM microbubble resonators. The challenges of rapid production and practical applications of WGM microbubble resonators are also discussed.

A Review of Optical Fibre Ethanol Sensors: Current State and Future Prospects

A range of optical fibre-based sensors for the measurement of ethanol, primarily in aqueous solution, have been developed and are reviewed here. The sensing approaches can be classified into four groups according to the measurement techniques used, namely absorption (or absorbance), external interferometric, internal fibre grating and plasmonic sensing. The sensors within these groupings can be compared in terms of their characteristic performance indicators, which include sensitivity, resolution and measurement range. Here, particular attention is paid to the potential application areas of these sensors as ethanol production is globally viewed as an important industrial activity. Potential industrial applications are highlighted in the context of the emergence of the internet of things (IoT), which is driving widespread utilization of these sensors in the commercially significant industrial and medical sectors. The review concludes with a summary of the current status and future prospects of optical fibre ethanol sensors for industrial use.

Oxygen Interactions with Covalently Grafted 2D Nanometric Carboxyphenyl Thin Films—An Experimental and DFT Study

Surface modification is a hot topic in electrochemistry and material sciences because it affects the way materials are used. In this paper, a method for covalently attaching carboxyphenyl (PhCOOH) groups to a gold electrode is presented. These groups were grafted onto the electrode surface electrochemically via reduction of aryldiazonium salt. The resulting grafted surface was characterized using cyclic voltammetry (CV) before and after the functionalization procedure to validate the presence of the grafted layer. The grafting of PhCOOH groups was confirmed by analyzing electrode thickness and composition by ellipsometry and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations indicated that the grafted layers provide a stable platform and resolved, for the first time, their interactions with oxygen.

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.

Microbubble Resonators for All-Optical Photoacoustics of Flowing Contrast Agents

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.

Three-Dimensional Microtubular Devices for Lab-on-a-Chip Sensing Applications

The rapid advance of micro-/nanofabrication technologies opens up new opportunities for miniaturized sensing devices based on novel three-dimensional (3D) architectures. Notably, microtubular geometry exhibits natural advantages for sensing applications due to its unique properties including the hollow sensing channel, high surface-volume ratio, well-controlled shape parameters and compatibility to on-chip integration. Here the state-of-the-art sensing techniques based on microtubular devices are reviewed. The developed microtubular sensors cover microcapillaries, rolled-up nanomembranes, chemically synthesized tubular arrays, and photoresist-based tubular structures via 3D printing. Various types of microtubular sensors working in optical, electrical, and magnetic principles exhibit an extremely broad scope of sensing targets including liquids, biomolecules, micrometer-sized/nanosized objects, and gases. Moreover, they have also been applied for the detection of mechanical, acoustic, and magnetic fields as well as fluorescence signals in labeling-based analyses. At last, a comprehensive outlook of future research on microtubular sensors is discussed on pushing the detection limit, extending the functionality, and taking a step forward to a compact and integrable core module in a lab-on-a-chip analytical system for understanding fundamental biological events or performing accurate point-of-care diagnostics.

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.

A Single-Ended Ultra-Thin Spherical Microbubble Based on the Improved Critical-State Pressure-Assisted Arc Discharge Method

Hollow core microbubble structures are good candidates for the construction of high performance whispering gallery microresonator and Fabry-Perot (FP) interference devices. In the previous reports, most of interest was just focused on the dual-ended microbubble, but not single-ended microbubble, which could be used for tip sensing or other special areas. The thickness, symmetry and uniformity of the single-ended microbubble in previous reports were far from idealization. Thus, a new ultra-thin single-ended spherical microbubble based on the improved critical-state pressure-assisted arc discharge method was proposed and fabricated firstly in this paper, which was fabricated simply by using a commercial fusion splicer. The improvement to former paper was using weak discharge and releasing pressure gradually during the discharging process. Thus, the negative influence of gravity towards bubble deformation was decreased, and the fabricated microbubble structure had a thin, smooth and uniform surface. By changing the arc discharge parameters and the fiber position, the wall thicknesses of the fabricated microbubble could reach the level of 2 μm or less. The fiber Fabry-Perot (FP) interference technique was also used to analyze the deformation characteristic of microbubble under difference filling pressures. Finding the ends of the microbubbles had a trend of elongation with axial compression when the filling pressure was increasing. Its sensitivity to the inner pressure of microbubble samples was about ~556 nm/MPa, the bubble wall thickness was only of about 2 μm. Besides, a high whispering gallery mode (WGM) quality factor that up to 107 was realized by using this microbubble-based resonator. To explain the upper phenomenon, the microbubble was modeled and simulated with the ANSYS software. Results of this study could be useful for developing new single-ended whispering gallery mode micro-cavity structure, pressure sensors, etc.

Electric-arc-induced strength-controllable weak polarization mode coupling in polarization maintaining fiber

An electric-arc-based scheme to generate strength-controllable weak polarization mode coupling (PMC) points in polarization maintaining fiber (PMF) is described. The resulting PMC strengths can be readily controlled to be in the very weak range of -60 to -40  dB. In this range, excellent mechanical strength combined with high return loss is achieved. An experimental quasi-distributed temperature sensor is formed by three separate PMC points in a single PMF using the electric arc method.

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.

Highly Sensitive Label-Free Detection of Small Molecules with an Optofluidic Microbubble Resonator

The detection of small molecules has increasingly attracted the attention of researchers because of its important physiological function. In this manuscript, we propose a novel optical sensor which uses an optofluidic microbubble resonator (OFMBR) for the highly sensitive detection of small molecules. This paper demonstrates the binding of the small molecule biotin to surface-immobilized streptavidin with a detection limit reduced to 0.41 pM. Furthermore, binding specificity of four additional small molecules to surface-immobilized streptavidin is shown. A label-free OFMBR-based optical sensor has great potential in small molecule detection and drug screening because of its high sensitivity, low detection limit, and minimal sample consumption.

Whispering-Gallery Mode Resonators for Detecting Cancer

Optical resonators are sensors well known for their high sensitivity and fast response time. These sensors have a wide range of applications, including in the biomedical fields, and cancer detection is one such promising application. Sensor diagnosis currently has many limitations, such as being expensive, highly invasive, and time-consuming. New developments are welcomed to overcome these limitations. Optical resonators have high sensitivity, which enable medical testing to detect disease in the early stage. Herein, we describe the principle of whispering-gallery mode and ring optical resonators. We also add to the knowledge of cancer biomarker diagnosis, where we discuss the application of optical resonators for specific biomarkers. Lastly, we discuss advancements in optical resonators for detecting cancer in terms of their ability to detect small amounts of cancer biomarkers.

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.

Optical Microbottle Resonators for Sensing

Whispering gallery mode (WGM) optical microresonators have been shown to be the basis for sensors able to detect minute changes in their environment. This has made them a well-established platform for highly sensitive physical, chemical, and biological sensors. Microbottle resonators (MBR) are a type of WGM optical microresonator. They share characteristics with other, more established, resonator geometries such as cylinders and spheres, while presenting their unique spectral signature and other distinguishing features. In this review, we discuss recent advances in the theory and fabrication of different kinds of MBRs, including hollow ones, and their application to optofluidic sensing.

Precise measurement of micro bubble resonator thickness by internal aerostatic pressure sensing

We develop a new, simple and non-destructive method to precisely measure the thickness of thin wall micro bubble resonators (MBRs) by using internal aerostatic pressure sensing. Measurement error of 1% at a bubble wall thickness of 2 μm is achieved. This method is applicable to both thin wall and thick wall MBR with high measurement accuracy.

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.

Biosensing by WGM Microspherical Resonators

Whispering gallery mode (WGM) microresonators, thanks to their unique properties, have allowed researchers to achieve important results in both fundamental research and engineering applications. Among the various geometries, microspheres are the simplest 3D WGM resonators; the total optical loss in such resonators can be extremely low, and the resulting extraordinarily high Q values of 10⁸–10⁹ lead to high energy density, narrow resonant-wavelength lines and a lengthy cavity ringdown. They can also be coated in order to better control their properties or to increase their functionality. Their very high sensitivity to changes in the surrounding medium has been exploited for several sensing applications: protein adsorption, trace gas detection, impurity detection in liquids, structural health monitoring of composite materials, detection of electric fields, pressure sensing, and so on. In the present paper, after a general introduction to WGM resonators, attention is focused on spherical microresonators, either in bulk or in bubble format, to their fabrication, characterization and functionalization. The state of the art in the area of biosensing is presented, and the perspectives of further developments are discussed.

Lasing of whispering gallery modes in optofluidic microcapillaries

This paper demonstrates lasing of the whispering gallery modes in polymer coated optofluidic capillaries and their application to refractive index sensing. The laser gain medium used here is fluorescent Nile Red dye, which is embedded inside the high refractive index polymer coating. We investigate the refractometric sensing properties of these devices for different coating thicknesses, revealing that the high Q factors required to achieve low lasing thresholds can only be realized for relatively thick polymer coatings (in this case ≥ 800 nm). Lasing capillaries therefore tend to have a lower refractive index sensitivity, compared to non-lasing capillaries which can have a thinner polymer coating, due to the stronger WGM confinement within the polymer layer. However we find that the large improvement in signal-to-noise ratio realized for lasing capillaries more than compensates for the decreased sensitivity and results in an order-of-magnitude improvement in the detection limit for refractive index sensing.

Dispersion analysis of whispering gallery mode microbubble resonators

This paper examines the opportunities existing for engineering dispersion in non-silica whispering gallery mode microbubble resonators, for applications such as optical frequency comb generation. More specifically, the zero dispersion wavelength is analyzed as a function of microbubble diameter and wall thickness for several different material groups such as highly-nonlinear soft glasses, polymers and crystalline materials. The zero dispersion wavelength is shown to be highly-tunable by changing the thickness of the shell. Using certain materials it is shown that dispersion equalization can be realized at interesting wavelengths such as deep within the visible or mid-infrared, opening up new possibilities for optical frequency comb generation. This study represents the first extensive analysis of the prospects of using non-silica microbubbles for nonlinear optics.

Arc-discharge effects on residual stress and refractive index in single-mode optical fibers

Arc-discharge effects on the residual stress and refractive index in single-mode optical fibers are investigated using a previously developed three-dimensional concurrent stress-index measurement method. Using commercial optical fibers and a commercial fusion splicer, the residual stress and refractive index perturbations caused by weak electrical arc discharges in single-mode fibers were measured. Refractive index changes greater than 10^-4 and longitudinal perturbation lengths of less than 500 μm were shown to be possible. The subsequent prospects for arc-induced long-period fiber gratings are analyzed, and a typical transmission resonance is predicted to have a depth of 56 dB and a bandwidth of 0.08 nm at a wavelength of 1585 nm. The results of this investigation will be useful in modeling device performance and optimization of arc-induced long-period fiber grating fabrication.

Dispersion in silica microbubble resonators

We explore the scope for engineering dispersion in whispering gallery mode silica microbubbles for nonlinear applications, such as optical frequency comb generation. In particular, the zero dispersion wavelength is shown to be highly tunable by changing the thickness of the shell. Using a small diameter and small wall thickness, dispersion equalization within the visible is predicted. This opens up the possibility of realizing visible frequency combs for a range of different applications.

Liquid Core ARROW Waveguides: A Promising Photonic Structure for Integrated Optofluidic Microsensors

In this paper, we introduce a liquid core antiresonant reflecting optical waveguide (ARROW) as a novel optofluidic device that can be used to create innovative and highly functional microsensors. Liquid core ARROWs, with their dual ability to guide the light and the fluids in the same microchannel, have shown great potential as an optofluidic tool for quantitative spectroscopic analysis. ARROWs feature a planar architecture and, hence, are particularly attractive for chip scale integrated system. Step by step, several improvements have been made in recent years towards the implementation of these waveguides in a complete on-chip system for highly-sensitive detection down to the single molecule level. We review applications of liquid ARROWs for fluids sensing and discuss recent results and trends in the developments and applications of liquid ARROW in biomedical and biochemical research. The results outlined show that the strong light matter interaction occurring in the optofluidic channel of an ARROW and the versatility offered by the fabrication methods makes these waveguides a very promising building block for optofluidic sensor development.

High-Q, ultrathin-walled microbubble resonator for aerostatic pressure sensing

Sensors based on whispering gallery resonators have minute footprints and can push achievable sensitivities and resolutions to their limits. Here, we use a microbubble resonator, with a wall thickness of 500 nm and an intrinsic Q-factor of 10(7) in the telecommunications C-band, to investigate aerostatic pressure sensing via stress and strain of the material. The microbubble is made using two counter-propagating CO(2) laser beams focused onto a microcapillary. The measured sensitivity is 19 GHz/bar at 1.55 μm. We show that this can be further improved to 38 GHz/bar when tested at the 780 nm wavelength range. In this case, the resolution for pressure sensing can reach 0.17 mbar with a Q-factor higher than 5 × 10(7).

Packaged optofluidic microbubble resonators for optical sensing

A microbubble resonator (MBR) coupled with a fiber taper is packaged with low-index polymer. The cladding polymer serves as a protective matrix for the coupling system to avoid environmental disturbance. The packaged structure is portable and provides good performance to maintain high Q factors for a long working period. The hollow structure of the MBR makes the packaged system useful for practical chemical and biomedical sensing applications. To evaluate the performance of the packaged MBRs-based sensor, we carry out bulk refractive index and surface-sensing measurements with achieved sensitivities of 18.8 nm/RIU and 31.29 pm/nm, respectively.

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.

Quasi-distributed and wavelength selective addressing of optical micro-resonators based on long period fiber gratings

A novel all-in-fiber method for coupling light to high-Q silica whispering gallery mode (WGM) optical micro-resonators is presented, which is based on a pair of long period fiber gratings (LPGs) written in the same silica fiber, along with a thick fiber taper (15-18 μm in waist) in between the LPGs. The proposed coupling structure is robust and can be replicated many times along the same fiber simply cascading LPGs with different bands. Typical Q-factors of the order of 10(8) and total coupling efficiency up to 60% were measured collecting the resonances of microspheres or microbubbles at the fiber end. This approach uniquely allows quasi-distributed and wavelength selective addressing of different micro-resonators along the same fiber.

2-D optical/opto-mechanical microfluidic sensing with micro-bubble resonators

In this paper a new sensing scheme by simultaneously measuring optical refractive index change and sound speed change in an optofluidic thin wall micro-bubble resonator is reported. Sensitivity of sound speed is 4.2-6.8 MHz/ (km/s) for 3 types of mechanical modes. A 2-D optical/opto-mechanical sensing map is plotted by detecting both the whispering gallery mode resonance shift and the optomechanical resonance shift. This novel scheme provides a supplementary support to optical sensing when analytes do not respond to refractive index (RI) change.

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.

Optofluidic approaches for enhanced microsensor performances

Optofluidics is a relatively young research field able to create a tight synergy between optics and micro/nano-fluidics. The high level of integration between fluidic and optical elements achievable by means of optofluidic approaches makes it possible to realize an innovative class of sensors, which have been demonstrated to have an improved sensitivity, adaptability and compactness. Many developments in this field have been made in the last years thanks to the availability of a new class of low cost materials and new technologies. This review describes the Italian state of art on optofluidic devices for sensing applications and offers a perspective for further future advances. We introduce the optofluidic concept and describe the advantages of merging photonic and fluidic elements, focusing on sensor developments for both environmental and biomedical monitoring.

Long period grating-based fiber coupler to whispering gallery mode resonators

We present a new method for coupling light to high-Q silica whispering gallery mode resonators (WGMs) that is based on long period fiber gratings (LPGs) written in silica fibers. An LPG allows selective excitation of high-order azimuthally symmetric cladding modes in a fiber. Coupling of these cladding modes to WGMs in silica resonators is possible when partial tapering of the fiber is also implemented in order to reduce the optical field size and increase its external evanescent portion. Importantly, the taper size is about one order of magnitude larger than that of a standard fiber taper coupler. The suggested approach is therefore much more robust and useful especially for practical applications. We demonstrate coupling to high-Q silica microspheres and microbubbles detecting the transmission dip at the fiber output when crossing a resonance. An additional feature of this approach is that by cascading LPGs with different periods, a wavelength selective addressing of different resonators along the same fiber is also possible.

Observation of two-photon fluorescence for Rhodamine 6G in microbubble resonators

We report an observation of multi-photon excitation of organic chromophores in microbubble whispering gallery mode resonators. High-Q microbubble resonators were formed by heating a pressurized fused silica capillary to form a hollow bubble that was then filled with liquid. In this work, the microbubble was filled with a solution of Rhodamine 6G dye. The resonator and dye were excited by evanescently coupling continuous wave (CW) light from a 980 nm laser diode using a tapered optical fiber. The two-photon fluorescence of the dye can be seen with pump powers as low as 700 μW.

Quasi-droplet microbubbles for high resolution sensing applications

Optical properties and sensing capabilities of fused silica microbubbles were studied numerically using a finite element method. Mode characteristics, such as quality factor (Q) and effective refractive index, were determined for different bubble diameters and shell thicknesses. For sensing applications with whispering gallery modes (WGMs), thinner shells yield improved sensitivity. However, the Q-factor decreases with reduced thickness and this limits the final resolution. Three types of sensing applications with microbubbles, based on their optimized geometrical parameters, were studied. Herein the so-called quasi-droplet regime is defined and discussed. It is shown that best resolution can be achieved when microbubbles act as quasi-droplets, even for water-filled cavities at the telecommunications C-band.

Brillouin cavity optomechanics with microfluidic devices

Cavity optomechanics allows the parametric coupling of phonon- and photon-modes in microresonators and is presently investigated in a broad variety of solid-state systems. Optomechanics with superfluids has been proposed as a path towards ultra-low optical- and mechanical-dissipation. However, there have been no optomechanics experiments reported with non-solid phases of matter. Direct liquid immersion of optomechanics experiments is challenging, as the acoustic energy simply leaks out to the higher-impedance liquid surrounding the device. Here we confine liquids within hollow resonators to circumvent this issue and to enable optical excitation of mechanical whispering-gallery modes at frequencies ranging from 2 to 11,000 MHz. Our device enables optomechanical investigation with liquids, while light is conventionally coupled from the outer dry side of the capillary, and liquids are provided by means of a standard microfluidic inlet.

Kerr parametric oscillations and frequency comb generation from dispersion compensated silica micro-bubble resonators

Silica micro-bubble resonators (MBRs) with cavity quality factor as high as Q = 5 × 10(7) are fabricated. The total dispersion of MBRs is analyzed. The thin-wall structure opens a new anomalous dispersion window and thus supports the dispersion compensation for hyper-parametric frequency conversion processes. Experimentally, Kerr parametric oscillation is observed in a 136 μm diameter MBR, frequency comb generation is also realized. Meanwhile the same nonlinear process is not allowed in solid silica spheres with size smaller than 150 μm.