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We report photoluminescence and photoionization properties of Sm2+ ions generated by X-irradiation of nanocrystalline CaF2:Sm3+ prepared by coprecipitation. The nanocrystals were of 46 nm average crystallite size with a distribution of ±20 nm and they were characterised by XRD, TEM and SEM-EDS. At room temperature, the X-irradiated sample displayed broad electric dipole allowed Sm2+ 4f55d (A1u) → 4f6 7F1 (T1g) luminescence at 725 nm that narrowed to an intense peak at 708 nm on cooling to ∼30 K. The narrow f–f transitions of Sm3+ were also observed. The X-irradiation-induced reduction of Sm3+ + e− → Sm2+ as a function of X-ray dose was investigated over a very wide dynamic range from 0.01 mGy to 850 Gy by monitoring the photoluminescence intensities of both Sm2+ and Sm3+ ions. The reverse Sm2+ → Sm3+ + e− photoionization can be modelled by employing dispersive first-order kinetics and using a standard gamma distribution function, yielding an average separation of 13 A between the Sm2+ ions and the hole traps (e.g. oxide ion impurities). The present results point towards potential applications of Sm doped CaF2 nanocrystals in the fields of dosimetry and X-ray imaging.

Graded-index optical fiber probes suitable for focusing or collimating the output of an optical fiber at a wavelength of 1.3 µm have become an enabling technology in optical coherence tomography imaging applications for in vivo bioimaging. Such fiber tips however remain uncommon in other photonics applications. This paper provides the first numerical study of graded-index fiber tips covering a broad range of wavelengths spanning from the UV to short-infrared. The wavelength dependency and the influence of probe geometry on performance characteristics such as far-field divergence angle, spot size and working distance are analyzed. The paper yields easily accessible design guidelines for the fabrication of collimating or focusing fiber tips. Fiber collimators have considerable potential for use in free-space systems and could benefit a range of devices such as variable attenuators, dynamic wavelength equalisers and large 3D optical cross-connect switches, whereas focusing fiber tips have applications in high-resolution imaging. Refereed/Peer-reviewed

In recent decades, storage phosphors have found widespread applications in the fields of dosimetry and computed radiography due to their impressive properties including large dynamic range and passive storage mode. Despite this success, further development of highly efficient X-ray and UV sensitive luminescent materials is still desirable, in par-ticular for applications in dosimetry. In this Article we report X-ray and UV storage properties of nanocrystalline Ba-LiF3 doped with Sm3+. Nanocrystalline BaLiF3:Sm3+ was prepared mechanochemically by ball milling and the resulting product was characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The re-sponse of the phosphor to X-ray and UV radiation was investigated by measuring room temperature photolumines-cence spectra before and after irradiation for a wide range of doses. The lifetime of the Sm2+ emission at 694 nm was determined for both the X-ray and UV irradiated powder samples. The results indicate a potential fo...

We report on the potential of BaLiF3 as a host matrix for the upconversion sensitizer (Yb3+)-activator (Er3+) pairs as prepared by ball milling. Specifically, the infrared (980 nm)-excited upconversion luminescence (UCL) of BaLiF3:Yb3+, Er3+ samples before and after annealing in air is reported, and it is demonstrated that the upconversion efficiency is enhanced by similar to 2 orders of magnitude via a postannealing treatment. The UCL of BaLiF3:Yb3+, Er3+ compared favorably with that of the efficient NaYF4:Yb3+, Er3+ UCL nanophosphors. In addition, the X-ray storage capability of BaLiF3:Yb3+, Er3+ was explored. It was observed that the photoluminescence intensities of Yb3+ (similar to 980 nm) and Er3+ (similar to 650 nm) ions decrease gradually with cumulative X-ray doses due to X-ray-induced reduction of Yb3+ to Yb2+ and Er3+ to Er2+, and as a result, the upconversion luminescence decreases. The Yb2+ photoluminescence spectrum upon X-irradiation was observed at 480 nm in accord with earlier reports for the BaLiF3 host. In a unique development, 2D X-ray imaging based on the UCL signal is reported for the first time. These results demonstrate the X-ray recording capability of mechanochemically synthesized BaLiF3:Yb3+, Er3+ nanocrystals and point to possible applications in combining X-ray dosimetry and bioimaging. Refereed/Peer-reviewed

Persistent spectral hole-burning is shown to be suitable for multiple bit per point optical data storage at room temperature. In a novel approach employing bespoke tailoring of the effective inhomogeneous line width and line shape, by using a physical mixture of chemically different matlockite nanocrystals, a relatively broad fluorescence emission line with a top-hat distribution can be realized at the diffraction limit and at room temperature. This top-hat profile enables multiple spectral holes to be burnt, all having the same baseline. We demonstrate this with two spectral holes burnt in a diffraction limited spot, with the prospects for multihole-depth or multilevel encoding to increase the number of bits stored per spectral hole. Using thermoelectrically cooled temperatures down to 175 K, a larger number of holes is also demonstrated. The technique presented is predicted to allow for optical data densities approaching terabits/cm2 by exploiting the unique properties of these inorganic phosphors. Refereed/Peer-reviewed

This paper describes the design and characterization of miniaturized optofluidic devices for sensing based on integrating collimating optical fibers with custom microfluidic chips. The use of collimating graded-index fiber (GIF) tips allows for effective fiber-channel-fiber interfaces to be realized when compared with using highly-divergent standard single-mode fiber (SMF). The reduction in both beam divergence and insertion losses for the GIF configuration compared with SMF was characterized for a 10.0 mm channel. Absorption spectroscopy was demonstrated on chip for the measurement of red color dye (Ponceau 4R), and the detection of thiocyanate in water and artificial human saliva. The proposed optofluidic setup allows for absorption spectroscopy measurements to be performed with only 200 mu L of solution which is an order of magnitude smaller than for standard cuvettes but provides a comparable sensitivity. The approach could be integrated into a lab-on-a-chip system that is compact and does not require free-space optics to perform absorption spectroscopy. Refereed/Peer-reviewed

There is great interest in the use of highly-efficient all-inorganic halide perovskites CsnPbBr2+n for optoelectronic applications. There however remains considerable debate as to the origins of the green luminescence in the zero-dimensional phase of the perovskite Cs4PbBr6, with theories suggesting it originates either from defects in the Cs4PbBr6 lattice or CsPbBr3 impurities/inclusions. The confusion has arisen due to the two phases being miscible and typically co-existing. Moreover, low impurity levels of CsPbBr3 in Cs4PbBr6 are difficult to detect by XRD measurements, yet have much stronger photoluminescence than bulk CsPbBr3 that exhibits quenching, further contributing to the confusion as to the origins of the green photoluminescence. With the rise of significant debate and misconceptions, we provide conclusive evidence that the green emission from Cs4PbBr6 is indeed due to nanocrystalline CsPbBr3 impurities. This is demonstrated by undertaking cathodoluminescence and EDX measurements on samples prepared mechanochemically by ball-milling. Cathodoluminescence imaging clearly shows the presence of small crystals embedded in/or between larger crystallites of Cs4PbBr6 and they emit around 520 nm. EDX shows that the smaller crystal inclusions have a Pb : Br ratio that is approximately 2 times higher, confirming the CsPbBr3 phase, which has the expected size-dependent shift to shorter wavelengths (about 528 to 515 nm). These studies make significant inroads into understanding these lead halide perovskites for their use in a variety of optoelectronic and photovoltaic applications.

In recent years, mechanochemistry has experienced a massive resurgence allowing for solvent-free preparation of many important materials with minimal energy requirements. This paper provides a review of the mechanochemical preparation of nanocrystalline metal halides for applications as inorganic phosphor materials. The review puts strong emphasis on our recent work on optical and X-ray storage phosphors such as the matlockite BaFCl:Sm3+. In addition, previously unpublished results are presented including the effect on the samarium oxidation state when using ball-milling, as well as results on other rare earth-doped matlockites. We outline how mechanochemical methods can be applied to synthesise, without the need for solvents and high temperatures, a wide range of halides ranging from the most important commercial X-ray storage phosphor BaFBr:Eu2+ to lead perovskites of the formula APbX3 with A = Cs+, CH3NH3+, etc., and X = F, Cl, Br, I or a mixture thereof. We also demonstrate that a wide variety of solid solutions of the general formula $$ {\text{M}}_{x}^{1} {\text{M}}_{1 - x}^{2} {\text{FX}}_{y}^{1} {\text{X}}_{1 - y}^{2} $$ (with M1 and M2 = Ba, Sr, Ca; X1, X2 = Cl, Br, I) that can be suitable hosts for luminescent activator ions, can be prepared by mechanochemical methods. Importantly, for prolonged grinding times with a high-energy ball-mill, crystallites on the nanoscale can be obtained as can be confirmed by Rietveld refinements of powder XRD patterns and electron microscopy.

We propose a two-dimensional distributed sensing scheme using few-mode exposed-core fibre and optical frequency domain reflectometry. This scheme aims to measure the size and position of particles using the evanescent field of different guided modes. © 2020 The Author(s)

The increase in Internet data demand has resulted in the development of novel optical fibers. Ultrafast laser inscription is a powerful tool to create 3D waveguide circuits that can interface with these new fiber types. Refereed/Peer-reviewed

To meet future data traffic demand, the telecommunications industry is developing optical fiber network architectures that exploit space division multiplexing (SDM). Femtosecond laser written photonics is a promising platform for SDM end-of-fiber components. Refereed/Peer-reviewed

Fiber-optic Fabry-Perot interferometers developed using collimating graded-index fiber tips are shown to allow for dramatic improvements in fringe visibility of up to 10-fold for long pathlength, small FSR, cavities

We report a novel method for detecting ionizing radiation by employing the phenomenon of upconversion luminescence. Nanocrystalline SrFCl:Yb 3+ /Er 3+ was prepared by ball-milling and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The photoluminescence properties of nanocrystalline SrFCl:Yb 3+ , SrFCl:Er 3+ and SrFCl:Yb 3+ /Er 3+ before and after X-irradiation were investigated. The results demonstrate that both Yb 3+ and Er 3+ ions in the SrFCl host are reduced to their divalent state upon X-ray exposure. Under 980 nm infrared excitation, SrFCl:Yb 3+ /Er 3+ nanocrystals displayed efficient upconversion luminescence. The upconversion luminescence intensity gradually decreased with increasing X-irradiation in a double exponential fashion with rate constants of k 1 = 0.08 Gy -1 and k 2 = 0.01 Gy -1 . In comparison with other X-ray storage phosphors, the present system shows a much higher stability of stored information since it is not subject to photobleaching in the read-out process. This is the first report on detecting ionizing radiation by upconversion luminescence, with the potential for improved read-out performance over traditional storage phosphors. Possible applications of the present phosphor include bioimaging and in vivo cell-level X-ray dose monitoring. Refereed/Peer-reviewed

We investigate crosstalk fluctuations between cores and modes in a few-mode-MCF, observing almost 20dB power fluctuations for CW signals. We then measure increased crosstalk penalty for LP 11 modes compared to the more confined LP 01 modes.

Space-division multiplexing (SDM) in few-mode multi-core fibers (FM-MCF) allows the transmission of multi-petabit/s data-rates over a single optical fiber. Recently, transmission of more than 1.2 Pbit/s was demonstrated in a FM-MCF with a cladding diameter of 160 μm, compatible with the standard coating diameter of 250 μm [1]. When increasing the core density in FM-MCF or the transmission distance, inter-core crosstalk can limit the achievable data rates. Inter-core crosstalk is strongly wavelength-dependent and higher-order modes in FM-MCF are expected to show a larger crosstalk penalty compared to the fundamental mode, as their mode fields extend farther into the cladding [2]. In this paper, we present an experimental investigation of the crosstalk-induced Q-factor degradation of 16-Quadrature amplitude modulation (16-QAM) signals, transmitted over a four-core three-mode fiber Refereed/Peer-reviewed

Microcapillaries are inherently fluidic sensing platforms that can, under appropriate conditions, support lasing. We propose and demonstrate a lasing microcapillary sensor based on "star" and "triangle" interferences arising from reflection and refraction at the inner and outer microcapillary walls. These interferences lead to distinct modal trajectories that sample the channel medium inside the microcavity structure. Both star and triangle resonances are therefore sensitive to the nature of the channel medium, yielding theoretical and experimental sensitivities significantly exceeding those found for sensors based on conventional whispering-gallery modes. Additional enhancement can be achieved because the two main resonance families shift in an opposite sense, yielding a differential sensitivity that can reach several thousand nanometers per refractive index "unit." These devices represent a robust, yet easily fabricated optical sensor and capillary-based lasing system for microfluidic sensing. Refereed/Peer-reviewed

With the capacity limits of standard single-mode optical fiber fast approaching, new technologies such as space-division multiplexing are required to avoid an Internet capacity crunch. Few-mode multicore fiber (FM-MCF) could allow for a two orders of magnitude increase in capacity by using the individual spatial modes in the different cores as unique data channels. We report the realization of a monolithic mode-selective few-mode multicore fiber multiplexer capable of addressing the individual modes of such a fiber. These compact multiplexers operate across the S + C + L telecommunications bands and were inscribed into a photonic chip using ultrafast laser inscription. They allow for the simultaneous multiplexing of the LP01, LP11a and LP11b modes of all cores in a 3-mode, 4-core fiber with excellent mode extinction ratios and low insertion losses. The devices are scalable to more modes and cores and therefore could represent an enabling technology for practical ultra-high capacity dense space-division multiplexing.

The development of a fast and reliable whispering gallery mode (WGM) simulator capable of generating spectra that are comparable with experiment is an important step forward for designing microresonators. We present a new model for generating WGM spectra for multilayer microspheres, which allows for an arbitrary number of concentric dielectric layers, and any number of embedded dipole sources or uniform distributions of dipole sources to be modeled. The mode excitation methods model embedded nanoparticles, or fluorescent dye coatings, from which normalized power spectra with accurate representation of the mode coupling efficiencies can be derived. In each case, the emitted power is expressed conveniently as a function of wavelength, with minimal computational load. The model makes use of the transfer-matrix approach, incorporating improvements to its stability, resulting in a reliable, general set of formulae for calculating whispering gallery mode spectra. In the specific cases of the dielectric microsphere and the single-layer coated microsphere, our model simplifies to confirmed formulae in the literature. Refereed/Peer-reviewed

We present results demonstrating the prospects of samarium-doped nanocrystals for use in ultra-high capacity multilevel optical data storage. Optical data storage is demonstrated through fluorescence tuning of MeFCl:Sm (Me: Ba, Sr) in the deep UV with a 7 orders of magnitude linear dynamic range.

A method for determining the diameters and shell thicknesses of microbubble resonators is presented; it entails simulating whispering gallery mode (WGM) spectra using a newly developed finite-difference time-domain (FDTD)-based toolkit. Spectra for a range of shell thicknesses are simulated using FDTD, assuming a linear dependence of the free spectral range on the diameter, and the free spectral ranges and positions of the prominent modes are matched to those of the measured spectrum. This method improves upon existing techniques for extracting the diameter and thickness, such as SEM imaging, which typically require the microbubble to be dissected or otherwise rendered unusable for subsequent use. The model allows a variety of methods of mode excitation to be simulated. Dye coatings are simulated by placing a layer of dipole sources on the surface of the resonator, yielding mode couplings comparable to those measured in experiments. The model is tested for a smalldiameter silica glass microbubble, with the free spectral range being simulated for a range of diameters and shell thicknesses. The numerically simulated spectra are then compared to the experimentally measured spectrum. The ability to determine the geometric parameters of such resonators directly from their WGM spectra represents a step forward in the characterization of microbubble resonators. Furthermore, the model opens the way to previously unstudied spectral behavior of microbubbles with small diameters and thin shell thicknesses. Refereed/Peer-reviewed

Although whispering gallery mode (WGM) biosensors have shown tremendous potential, they are still yet to find practical use as biomedical diagnostic tools. This is primarily due to the nature of the interrogation mechanism itself which relies on indirect measurement of the binding of a specific biomolecule onto the sensor through the associated refractive index change. Since nonspecific binding cannot be differentiated from the specific interaction of interest, this can result in a high rate of false positive readings when the detection is performed in complex biological samples. Here we show that this inherent limitation can be solved using a relatively simple approach. This approach involves the development of a self-referenced biosensor consisting of two almost identically sized dye-doped polystyrene microspheres placed on adjacent holes at the tip of a suspended core optical fiber. Here self-referenced biosensing is demonstrated with the detection of Neutravidin in undiluted human serum samples. The fiber allows remote excitation and collection of the WGMs of the microspheres in a dip sensing setting. By taking advantage of surface functionalization techniques, one microsphere acts as a dynamic reference, compensating for nonspecific binding events, while the other microsphere is functionalized to detect the specific interaction. The almost identical size allows the two spheres to have virtually identical refractive index sensitivity and surface area. This ensures their responses to nonspecific binding and environmental changes are almost identical, whereby any specific changes such as binding events, can be monitored via the relative movement between the two sets of WGM peaks. Refereed/Peer-reviewed

Whispering gallery mode (WGM) resonators have become increasingly diverse in terms of both architecture and applications, especially as refractometric sensors, allowing for unprecedented levels of sensitivity. However, like every refractometric sensor, a single WGM resonator cannot distinguish temperature variations from changes in the refractive index of the surrounding environment. Here, we investigate how breaking the symmetry of an otherwise perfect fluorescent microsphere, by covering half of the resonator with a high-refractive-index (RI) glue, might enable discrimination of changes in temperature from variations in the surrounding refractive index. This novel approach takes advantage of the difference of optical pathway experienced by WGMs circulating in different equatorial planes of a single microsphere resonator, which induces mode-splitting. We investigated the influence of the surrounding RI of the microsphere on mode-splitting through an evaluation of the sphere&rsquo, s WGM spectrum and quality factor (Q-factor). Our results reveal that the magnitude of the mode-splitting increases as the refractive index contrast between the high-refractive-index (RI) glue and the surrounding environment increases, and that when they are equal no mode-splitting can be seen. Investigating the refractive index sensitivity of the individual sub modes resulting from the mode-splitting unveils a new methodology for RI sensing, and enables discrimination between surrounding refractive index changes and temperature changes, although it comes at the cost of an overall reduced refractive index sensitivity.

Novel approaches for digital data storage are imperative, as storage capacities are drastically being outpaced by the exponential growth in data generation. Optical data storage represents the most promising alternative to traditional magnetic and solid-state data storage. In this paper, a novel and energy efficient approach to optical data storage using rare-earth ion doped inorganic insulators is demonstrated. In particular, the nanocrystalline alkaline earth halide BaFCl:Sm is shown to provide great potential for multilevel optical data storage. Proof-of-concept demonstrations reveal for the first time that these phosphors could be used for rewritable, multilevel optical data storage on the physical dimensions of a single nanocrystal. Multilevel information storage is based on the very efficient and reversible conversion of Sm³⁺ to Sm²⁺ ions upon exposure to UV-C light. The stored information is then read-out using confocal optics by employing the photoluminescence of the Sm²⁺ ions in the nanocrystals, with the signal strength depending on the UV-C fluence used during the write step. The latter serves as the mechanism for multilevel data storage in the individual nanocrystals, as demonstrated in this paper. This data storage platform has the potential to be extended to 2D and 3D memory for storage densities that could potentially approach petabyte/cm³ levels. Refereed/Peer-reviewed

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. Refereed/Peer-reviewed

Whispering gallery modes (WGMs) have been widely studied over the past 20 years for various applications, including biological sensing. While the WGM-based sensing approaches reported in the literature have shown tremendous performance down to single molecule detection, at present such sensing technologies are not yet mature and still have significant practical constraints that limit their use in real-world applications. Our work has focused on developing a practical, yet effective, WGM-based sensing platform capable of being used as a dip sensor for in-vivo biosensing by combining WGM fluorescent microresonators with silica Microstructured Optical Fibers (MOFs).We recently demonstrated that a suspended core MOF with a dye-doped polymer microresonator supporting WGMs positioned onto the tip of the fiber, can be used as a dip sensor. In this architecture the resonator is anchored to one of the MOF air holes, in contact with the fiber core, enabling a significant portion of the evanescent field from the fiber to overlap with the sphere and hence excite the fluorescent WGMs. This architecture allows for remote excitation and collection of the WGMs. The fiber also permits easy manipulation of the microresonator for dip sensing applications, and hence alleviates the need for a complex microfluidic interface. More importantly, it allows for an increase in both the excitation and collection efficiency compared to free space coupling, and also improves the Q factor.In this paper we present our recent results on microstructured fiber tip WGM-based sensors and show that this sensing platform can be used in clinical diagnostics, for detecting various clinically relevant biomarkers in complex clinical samples. Refereed/Peer-reviewed

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. Refereed/Peer-reviewed

Whispering gallery modes (WGMs) within microsphere cavitiesenable highly sensitive label-free detection of changes in the surroundingrefractive index. This detection modality is of particular interest forbiosensing applications. However, the majority of biosensing work utilizingWGMs to date has been conducted with resonators made from either silicaor polystyrene, while other materials remain largely uninvestigated. Byconsidering characteristics such as the quality factor and sensitivity of theresonator, the optimal WGM sensor design can be identified for variousapplications. This work explores the choice of resonator refractive indexand size to provide design guidelines for undertaking refractive indexbiosensing using WGMs. Refereed/Peer-reviewed

Whispering gallery modes (WGMs) have been exploited for a broad range of sensing applications. However, the vast majority of WGM sensors consist of passive resonators, requiring complex interrogation systems to be employed, ultimately limiting their practicality. Active resonators containing a gain medium, allowing remote excitation and collection of the WGM-modulated fluorescence spectra, have emerged as an alternative to passive resonators. Although research is still in its infancy, recent progress has reduced the performance gap between the two paradigms, fueled by the potential for new applications that could not previously be realized. Here, recent developments in sensors based on active WGM microresonators are reviewed, beginning with a discussion of the theory of fluorescence-based and lasing WGMs, followed by a discussion of the variety of gain media, resonator architectures, and emerging sensing applications. We conclude with a discussion of the prospects and future directions for improving active WGM sensors. (Figure presented.). Refereed/Peer-reviewed

We report on the highly efficient generation of stable Sm²⁺ in nanocrystalline BaFCl:Sm³⁺ by exposure in the deep UV (UV-C) below 250 nm. The generated Sm²⁺ can be read out via its distinct fluorescence signature which is efficiently excited around 425 nm by the relatively strong f-d transitions with ε ≈400 (l mol-1 cm-1). The generation of Sm²⁺ can also be reversed, erasing the fluorescence signal via two-photon ionization by increasing the power at 425 nm from ≤ 1 mW/cm2 to ~100 mW/cm2. It follows that the switching mechanism is based on oxygen impurities that are in close proximity to the Sm³⁺ ions. The photoionization kinetics indicate that the average Sm³+- oxide impurity separation is a few interionic spacings. The level of Sm³⁺- Sm²⁺ conversion in BaFCl is shown to be tunable over a large dynamic range, and therefore could serve as a platform for rewritable ultra-high density multi-level optical data storage. The present study also sheds light on BaFCl:Sm³⁺ as a photoluminescent X-ray storage phosphor. Refereed/Peer-reviewed

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. Refereed/Peer-reviewed

This paper investigates the Q-factor limits imposed on the far-field detection of the whispering gallery modes of active microspherical resonators. It is shown that the Q-factor measured for a given active microsphere in the far-field using a microscope is significantly lower than that measured using evanescent field collection through a taper. The discrepancy is attributed to the inevitable small asphericity of microspheres that results in mode-splitting which becomes unresolvable in the far-field. Analytic expressions quantifying the Q-factor limits due to small levels of asphericity are subsequently derived. Refereed/Peer-reviewed