high-Q Whispering Gallery Mode Research Articles

Interaction of a microresonator with a nanoscatterer

We present a theoretical analysis of the effects induced on the high-Q whispering-gallery modes of a microsphere resonator by a small metallic scatterer. We study the near-field tip-microsphere interaction varying the distance between the two spheres, in correspondence of the most perturbed modes of the microsphere. The obtained results show that this kind of interaction can be used to achieve an active control of whispering gallery resonances.

Low-threshold parametric nonlinear optics with quasi-phase-matched whispering-gallery modes

We propose to fabricate a toroidal dielectric cavity from a periodically poled χ(2) nonlinear material (e.g., LiNbO3) to achieve efficient interaction among high-Q whispering-gallery modes. We show that the periodic poling allows for suppression of both material and cavity dispersion. Such a cavity might be a basic element of a family of efficient nonlinear devices operating at a broad range of optical wavelengths.

High-Q whispering gallery modes in a spherical shell dielectric resonator: Characterization at microwave frequencies

We present a numerical and experimental characterization of high-Q whispering gallery resonant modes excited in a fused quartz spherical shell operated in the 18-26.5 GHz band; easy coupling and selection of modes is a most advantageous feature of the proposed structure with respect to ordinary bulk resonators. The numerical results, based on a finite element algorithm, well match experimental data, which also show the capability of our device to perform gaseous spectroscopy measurements.

Whispering-gallery-mode electro-optic modulator and photonic microwave receiver

We report on the experimental observation of efficient all-resonant three-wave mixing using high-Q whispering-gallery modes. The modes were excited in a millimeter size toroidal cavity fabricated from LiNbO3. We implemented a low-noise resonant electro-optic modulator based on this wave mixing process. We observe an efficient modulation of light with coherent microwave pumping at 9 GHz with applied power of approximately 10 mW. Used as a receiver, the modulator allows us to detect nanowatt microwave radiation. Preliminary results with a 33-GHz modulator prototype are also reported. We present a theoretical interpretation of the experimental results and discuss possible applications of the device.

Dispersion compensation in whispering-gallery modes.

We show that manipulation by a spatial profile of the refractive index of a circularly symmetric dielectric cavity results in a novel way of fine tuning frequency separations as well as spatial localizations of high-Q whispering-gallery modes excited in the cavity. The method permits dispersion compensation in the modes (spectrum equalization), diminishes the quality-factor limitation by surface roughness and contamination, and allows critical coupling to ultra-high-Q modes without maintaining an air gap with evanescent couplers.

Sub-microwatt photonic microwave receiver

We report efficient photonic reception of nanowatt microwave signals by their direct upconversion into optical domain using high-Q whispering gallery modes in a millimeter size toroidal cavity fabricated from LiNbO/sub 3/.

Highly nondegenerate all-resonant optical parametric oscillator

We show that a nondegenerate multifrequency parametric oscillator has different properties compared with the usual three-wave parametric oscillator. We consider, as an example, a scheme for a resonant cw monolithic microwave-optical parametric oscillator based on high-Q whispering gallery modes excited in a nonlinear dielectric cavity. Such an oscillator may have an extremely low threshold and stable operation, and may be used in spectroscopy and metrology. The oscillator mimics devices based on resonant Χ ( 3 ) nonlinearity and can be utilized for efficient four-wave mixing and optical comb generation. Moreover, the oscillator properties are important to better understand the stability conditions of long-baseline interferometers with movable mirrors that are currently used for gravity wave detection.

Towards controlled coupling between a high-Q whispering-gallery mode and a single nanoparticle

We discuss our recent experiments that aim at the realization of coupling between a nano-emitter that is placed at the extremity of a sharp glass-fiber tip and a high-Q whispering-gallery mode. We quantify the influence of the tip using different probes and modes of a microsphere with different quality factors and mode extensions. Our measurements show that a micron-sized tip results in a substantial perturbation of the modes. On the contrary, by using a tip of diameter about 100 nm it should be possible to couple a nanoparticle to the most-confined modes of a microsphere without spoiling quality factors even as high as 108.

Mapping and manipulating whispering gallery modes of a microsphere resonator with a near-field probe.

We report high spatial resolution mapping of high-Q whispering gallery modes in microsphere resonators with a near-field probe. We present experimental results on the effect of Q-factor degradation when the probe interacts with the evanescent field and discuss future applications of our experimental set-up for realization of novel nanolasers and nano light-emitting-diodes.

Photons confined in hollow microspheres

Incorporation of CdSe quantum dots into a thin (<1 μm) surface shell of polymer microspheres (R∼2–4 μm) is achieved. The room-temperature emission spectra of single, hollow microcavities show several, spectrally well-separated cavity modes in the red-orange spectral range which have been assigned to high-Q whispering gallery modes (WGM) with radial quantum number n=1 and high angular quantum number l. An enhancement of the cavity finesse Q by a factor of about 10 with respect to CdSe-doped bulk polymer microspheres is found.

Resonant-enhanced evanescent-wave fluorescence biosensing with cylindrical optical cavities

We show that the artificial resonances of dielectric optical cavities can be used to enhance the detection sensitivity of evanescent-wave optical fluorescence biosensors to the binding of a labeled analyte with a biospecific monolayer. Resonant coupling of power into the optical cavity allows for efficient use of the long photon lifetimes (or equivalently, the high internal power) of the high-Q whispering gallery modes to increase the probability of photon absorption into the fluorophore, thereby enhancing fluorescence emission. A method to compare the intrinsic sensitivity between resonant cavity and waveguide formats is also developed. Using realistic estimates for dielectric cylindrical cavities in both bulk and integrated configurations, we can expect sensitivity enhancement by at least an order of magnitude over standard waveguide evanescent sensors of equivalent sensing geometries. In addition, the required sample volume can be reduced significantly. The cylindrical cavity format is compatible with a large variety of sensing modalities such as immunoassay and molecular diagnostic assay.

Rayleigh scattering in high-Q microspheres

The Rayleigh scattering has to be largely suppressed in high-Q whispering-gallery modes in microspheres because of restrictions imposed on scattering angles by cavity confinement. Earlier estimates of the fundamental limit for the quality factor in fused-silica microspheres are revisited, and Q≃1012 is predicted in few-millimeter-size fused-silica spheres, if the surface hydration problem is ovecome. Particular effects of surface scattering losses are analyzed, and the manifestation of scattering in the form of intermode coupling is calculated. The predominant effect of counterpropagating mode coupling (intracavity backscattering) is analyzed in the presence of a mode-matched traveling-wave coupler. As much as 100% resonance reflection regime is shown to be feasible.

High-Q whispering gallery mode dielectric resonator bandpass filter with microstrip line coupling and photonic bandgap mode-suppression

We describe a novel bandpass filter based on using whispering gallery (WG) modes in high-density dielectric resonators (DR). Microstrip line mode-coupling and photonic bandgap (PEG) suppression techniques are also features of this design. Based on this approach, we demonstrate a filter with center frequency of 13.7 GHz, Q of 2000, insertion loss of 7 dB, and attenuation of 30 dB.

Analytic theory of coupling from tapered fibers and half-blocks into microsphere resonators

Coupling from tapered fibers and polished half-block couplers into the high-Q whispering gallery modes of microsphere resonators is investigated analytically. Numerous formulas are derived to predict the external coupling Q values, and intrinsic whispering gallery loss, for arbitrary structures, and for any sphere mode. Phase-mismatch due to the differences in propagation constants between input and sphere modes is taken into account. These formulas are strictly mechanical once a simple characteristic equation is solved which relates the spherical mode orders to the resonant wave vector. Results are in very good agreement with values that are calculated by different numerical methods.

Optical microsphere resonators: optimal coupling to high-Q whispering-gallery modes

A general model is presented for coupling of high-$Q$ whispering-gallery modes in optical microsphere resonators with coupler devices possessing discrete and continuous spectrum of propagating modes. By contrast to conventional high-Q optical cavities, in microspheres independence of high intrinsic quality-factor and controllable parameters of coupling via evanescent field offer variety of regimes earlier available in RF devices. The theory is applied to the earlier-reported data on different types of couplers to microsphere resonators and complemented by experimental demonstration of enhanced coupling efficiency (about 80%) and variable loading regimes with Q>10^8 fused silica microspheres.

High-Q whispering gallery modes from a composite system of GaAs quantum well and fused silica microsphere

We demonstrate a composite system consisting of a GaAs quantum well structure placed in the evanescent field of a fused silica microsphere and show evanescent coupling between excitons in the quantum well and high-Q whispering gallery modes of the composite system. The composite system allows separate engineering of photonic and electronic confinement and can take advantage of high-Q factor and small mode volume of fused silica microspheres.

Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper

We show that high-Q whispering-gallery modes in fused-silica microspheres can be efficiently excited by an optical fiber taper. By adjusting the taper diameter to match the ropagation constant of the mode in the taper with that of the resonant mode of interest, one can couple more than 90% of the light into the sphere. This represents a significant improvement in excitation efficiency compared with other methods and is, we believe, the most efficient excitation of a high- Q microcavity resonance by a monomode optical fiber yet demonstrated.

Directional emission from asymmetric resonant cavities

Asymmetric resonant cavities with highly noncircular but convex cross sections are predicted theoretically to have high-Q whispering gallery modes with highly anisotropic emission. We develop a ray dynamics model for the emission pattern and present numerical and experimental confirmation of the theory.

Ray chaos and Q spoiling in lasing droplets.

It is proposed that the spoiling of high-Q whispering gallery modes in deformed dielectric spheres can be understood as a transition to chaotic ray dynamics which can no longer be confined by total internal reflection. This is a Kolmogorov-Arnold-Moser/Lazutkin transition for light. The theory is applied to explain the lasing properties of droplets.

Mapping whispering-gallery modes in microspheres with a near-field probe

We demonstrate the use of a near-field probe to map the angular dependence of high-Q whispering-gallery modes in fused-silica microspheres. The mapping is performed by placing a micrometer-sized tip formed on the end of a monomode fiber into the evanescent f ield at the microsphere surface, causing light to be coupled from the microsphere resonance into the fiber guided mode. The light output of the fiber is then measured while the tip is moved to different points on the microsphere surface. We have used this method to investigate the lifting of spherical degeneracy in the system.