Whispering gallery mode profiles in a coated microsphere

The properties of whispering gallery modes (WGM) of a microsphere can be tailored by coating the resonator with a thin film. If the coating has a higher refractive index than the microsphere core, the resulting structure will have a configuration which has similarities both with a classical WGM resonator and with a planar waveguide. By choosing the film thickness and refractive index we can tailor the degree of confinement of the WGMs inside the sphere core, coating and surrounding medium, which will in turn influence the WGMs contact with its surroundings. We present a study of the effect of the coating on the WGM mode profiles in particular of a silicon coating on a silica sphere.     

Whispering gallery mode hybridization in photonic molecules

This work takes inspiration from chemistry where the spectral characteristics of the molecules are determined by hybridization of electronic states evolving from the individual atomic orbitals. Based on analogy between quantum mechanics and the classical electrodynamics, we sorted dielectric microspheres with almost identical positions of their whispering gallery mode (WGM) resonances. Using these microspheres as classical photonic atoms, we assembled them in a wide range of structures including linear chains and planar photonic molecules. We studied WGM hybridization effects in such structures using side coupling by tapered microfibers as well as finite difference time domain modeling. We demonstrated that the patterns of WGM spectral splitting are representative of the symmetry, number of constituting atoms and topology of the photonic molecules which in principle can be viewed as “spectral signatures” of various molecules. We also show new ways of controlling WGM coupling constants in such molecules. Excellent agreement was found between measured transmission spectra and spectral signatures of photonic molecules predicted by simulation.

     

Whispering gallery mode biosensors in the low-Q limit

Whispering gallery modes (WGM) of dye-doped polystyrene beads with diameters from 20 down to 1.5 μm are studied with respect to their appearance and linewidth by excitation of the entire mode spectrum within the emission range of the dye. The lowest order (q=1) modes, which travel most closely to the inner particle surface, are assigned to their individual quantum numbers by means of a least-square-fit, resulting in a precise determination of particle radius and eccentricity. On this basis, the suitability of these microscopic cavities for applications in optical (bio-)sensing is explored. Due to the low quality (Q) factors of these small cavities, particles with diameters below 6 μm exhibit only q=1 modes, thereby causing a drastic simplification of the WGM spectrum. In such spectra, the shift in the WGM positions upon molecular adsorption can be easily monitored, as we demonstrate for the adsorption of bovine serum albumin as well as multiple layers of polyelectrolytes onto the surface of particles with 2 μm diameter. Mie simulations are used to confirm our findings. With a mass sensitivity limit of 3 fg, these microscopic sensors are highly competitive in the field of label-free detection techniques. Moreover, their small size and the simplified, dye-mediated excitation and detection scheme may pave the way to remote in-vitro biosensing in the future. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. PACS 07.07.Df; 42.60.Da; 42.70.Jk     

Whispering gallery mode emission from microtube cavity

Optical properties were studied of a novel microtube cavity of ∼8 μm diameter prepared by vacuum-assisted filtration of aluminosilicate xerogel using micro-channel glass matrix followed by thermal treatment. Periodic very narrow peaks of the emission spectra corresponding to orthogonally polarized whispering gallery modes were detected. The spectral position of different modes were analyzed using Lorenz-Mie theory. The mode assignment permits calculation of the spectral dependence of cavity Q values associated with observed peaks. Intensity-dependent, time-resolved stimulated emission experiments were performed on the single microtube cavity. The results show a strong decrease in emission lifetime with increasing excitation intensity, consistent with a mechanism of amplified stimulated emission. The text was submitted by the authors in English.     

Whispering gallery mode microresonators as polarization converters

A ring resonator coupled to a waveguide can be used as a highly efficient polarization converter when the input is properly polarized. We model this phenomenon and verify the predictions with a demonstration of very efficient polarization conversion (>90%) on a silica microsphere coupled to a tapered optical fiber.     

Whispering gallery modes in a conical resonator

A conical closed resonator with perfectly conducting walls is considered. The eigenmodes of the resonator are determined in the first order of perturbation theory, in which the cone angle is a small quantity. A semiclassical approximation is constructed for an arbitrary cone angle; it is shown that the spectrum of whispering gallery modes in this approximation differs insignificantly from the eigenmode spectrum of a cylindrical cavity even for a large cone angle.     

Whispering gallery mode selective reflector with a high azimuthal index for the input cavity of a gyroklystron

A selective reflector in the form of a resonance extension is suggested for the input cavity of a 8-mm-wave gyroklystron operating at whispering gallery mode H _{m, 1} with an azimuthal index of several tens. Simulation data for the selective reflector are presented.     

Whispering gallery modes in nonideal disk resonators

We consider the problem of the whispering gallery mode in resonators having the shape of a circular segment close to a disk or a half-disk. The calculations are performed using perturbation theory methods.     

Whispering gallery mode microlasers and refractive index sensing based on single polymer fiber

The realization of whispering gallery mode (WGM) lasing in polymer fibers is hindered by an appropriate method to dissolve the polymer and the gain material. In this work, microfibers fabricated by directly drawing from a dye doped polymer solution are exhibited as high quality microlasers and microsensors. Multi‐mode and even single‐mode lasing is observed from the fiber under optical pumping at room temperature. The linewidth of lasing mode is narrower than 0.09 nm. The lasing mechanism is unambiguously verified by comprehensive spectroscopic analysis and ascribed to WGMs. Diameter‐ and polarization‐dependent lasing characteristics are systematically investigated, showing good agreement with the theoretical calculation. Particularly, application of the fiber laser for refractive index sensing based on resonant shift of lasing mode is demonstrated and the sensitivity up to about 300 nm/RIU is achieved. The promising potential of high quality polymer microfibers as optical sensors and multi‐function components for flexible photonic integrated systems is highly expected.     

Whispering gallery modes in silicon-nanocrystal-coated silica microspheres

Silica microspheres were deposited into two-dimensional periodic arrays and coated with a thin layer of silicon nanocrystals. The luminescence from the silicon nanocrystals coupled into the whispering gallery modes of the spheres, with Q factors that depended on a range of parameters including sphere size, position on the sphere, viewing direction, and thickness of the nanocrystal coating. Scattering from the film-sphere and/or the sphere-substrate contacts resulted in a lower Q for modes that intersect these regions. The highest Q factors obtained in this work were ∼1500. The results suggest that silica microspheres may be promising candidates for high-Q cavities that incorporate silicon nanocrystals for cavity QED or nonlinear optical effects.     

Whispering gallery mode dielectric resonators in EMR spectroscopy above 150 GHz: Problems and perspectives

At frequencies above 150 GHz conventional microwave techniques become unsuitable for electron magnetic resonance (EMR) spectroscopy applications. Quasi-optical techniques are the general frame into which the different approaches to high-field high-frequency EMR (HF²-EMR) have been developed. Beyond the single-pass approach the existing HF²-EMR spectrometers make use of Fabry-Perot resonators. In this paper the use of an alternative resonating structure on the basis of the use of whispering gallery mode dielectric resonators is shown. The physics of these resonators and their implementation in a HF²-EMR probe head are described. Their use in HF²-EMR experiments performed on standard samples is described and the results of these experiments critically discussed in order to have insight into the open problems of the employed spectrometer; the more promising solutions to these problems, actually under development, are briefly outlined.     

Whispering gallery modes in nanosized dielectric resonators with hexagonal cross section

The size dependence of whispering gallery modes in dielectric resonators with hexagonal cross section has been observed within the visible spectral range for cavity diameters comparable to the light wavelength. As a model system single, tapered, high aspect ratio zinc oxide nanoneedles were analyzed. They enable systematic investigations as a function of the resonator diameter down to the nanometer regime. A simple plane wave interference model without free parameter describes the spectral positions and the linewidths of the modes in good agreement with the experiment.     

Whispering gallery modes in a hemispherical isotropic dielectric resonator with a perfectly conducting planar surface

The eigenmodes of a hemispherical dielectric resonator placed on a perfectly conducting planar surface are studied. The resonator is shown to support independent E-and H-type oscillations. For E and H oscillations, the sums n + m of the polar and azimuthal indices are, respectively, even and odd. Accordingly, E and H oscillations exhibit, respectively, n-and (n + 1)-fold frequency degeneracy in azimuthal index m. The distributions of the whispering gallery mode fields on the surface of the hemisphere and on the external conducting plane are obtained. The energy distribution among the eigenmodes with polar index n=36 is studied. The polar angle that corresponds to the maximum of the whispering gallery mode field and the width of the energy localization region in terms of the polar angle are calculated. The number of field variations in the spherical coordinates are correlated with the indices of the resonator’s eigenmodes. The eigenfrequencies and Q factors of the resonator are plotted versus the permittivity of the isotropic environment in the 8-mm wavelength range.     

Whispering gallery resonators for studying orbital angular momentum of a photon

We propose a simple method for generation and detection of photons with nonzero angular momentum. The method utilizes high-quality factor ring resonators that transform a plane electromagnetic wave into a wave with nonzero angular momentum, and vice versa. We show that the method is especially promising for studying high-order Bessel beams, unreachable by other techniques.     

Whispering gallery mode propagation in photonic crystals in front of subwavelength slit arrays: Interplay with extraordinary transmission

We numerically study the propagation of light in 2D photonic crystals (PC) made of infinite cylinders of high refractive index, via the excitation of their morphological dependent resonances, in the presence of metallic arrays of subwavelength slits, either corrugated or not, that may produce extraordinary transmission. In this way, we confirm and illustrate previous theoretical findings on PCs and show new effects when combined with slits. Among them, we show that the whispering gallery mode excitation in the PC couples dominates the transmission of the slit arrays alone. Appropriate design of the system parameters and the illumination conditions, selects the transport and confinement of energy.     

Tunable optical frequency comb with a crystalline whispering gallery mode resonator

We report on the experimental demonstration of a tunable monolithic optical frequency comb generator. The device is based on four-wave mixing in a crystalline calcium fluoride whispering gallery mode resonator. The frequency spacing of the comb is given by an integer number of the free spectral range of the resonator. We select the desired number by tuning the frequency of the pumping laser with respect to the corresponding resonator mode. We also observe a rich variety of optical combs and high-frequency hyperparametric oscillation, depending on the experimental conditions. A potential application of the comb for generating tunable narrow band frequency microwave signals is demonstrated.     

Polarization-modified Fano line shape spectrum with a single whispering gallery mode

Using the HE11Xand HE11Ymodes in a tapered fiber(TF)and the whispering gallery mode(WGM)in a microsphere resonator,Fano line-shape spectra were theoretically described and experimentally observed in this study.The line shapes of the spectra can be tuned to form a Lorentz notch,various Fano line shapes,and the Lorentz peak by controlling the polarizations at the input and output ports of the TF.The relative position or the size of the waveguide and WGM resonator do not need to be accurately controlled to produce these effects.The proposed configuration is suitable for mass production and will improve the performance of the devices in which it is applied.     

Adjustable electro-optic microlens with two concentric ring electrodes

A theoretical model of a new variable-focal-length lens design that uses an electro-optic wafer with two double-sided concentric ring electrodes is proposed on the basis of an original technique for calculating the induced refractive index. It is shown that the relationship between ring radii and wafer thickness produces the required distribution of the refractive index to create converging focusing elements.     

Photoinduced whispering gallery mode microcavity resonator in a chalcogenide microfiber

We demonstrate an approach to creating localized whispering gallery mode (WGM) microcavities by exploiting the photosensitivity of a chalcogenide (As2S3) microfiber. A highly prolate WGM microcavity with cavity quality factors (Q) exceeding 2×10(5) is fabricated and characterized. Without the need for geometrical shaping, our approach enables the cavity properties to be monitored during fabrication for the first time.     

Low-threshold optical parametric oscillations in a whispering gallery mode resonator

In whispering gallery mode (WGM) resonator light is guided by continuous total internal reflection along a curved surface. Fabricating such resonators from an optically nonlinear material one takes advantage of their exceptionally high quality factors and small mode volumes to achieve extremely efficient optical frequency conversion. Our analysis of the phase-matching conditions for optical parametric down-conversion (PDC) in a spherical WGM resonator shows their direct relation to the sum rules for photons' angular momenta and predicts a very low parametric oscillation threshold. We realized such an optical parametric oscillator (OPO) based on naturally phase-matched PDC in lithium niobate. We demonstrated a single-mode, strongly nondegenerate OPO with a threshold of 6.7 μW and linewidth under 10 MHz. This work demonstrates the remarkable capabilities of WGM-based OPOs.