Gallery Mode Resonators Research Articles

Frequency comb generation in WGM microsphere based generators for telecommunication applications

We review the frequency comb generation process, main microresonator parameters such as free spectral range (FSR) and Q-factor, previously used optical frequency comb (OFC) generator parameters and resulting frequency combs, as well as the implementation of OFC for optical data transmission. An optical frequency comb is produced in a setup based on a tapered fibre and a SiO2 microsphere. The generated frequency comb has a frequency spacing of 2 nm or 257 GHz. During the fabrication of a tapered fibre from SMF28, use is made of the transmission signal to control the taper pulling process. The final measured tapered fibre transmission is ∼96%. A microsphere whispering gallery-mode resonator (WGMR), exhibiting a Q-factor of at least 2 × 107, is fabricated from an optical fibre with a thicker core than SSMF. Moreover, for future experiments, a frequency comb generator based on a free-space setup consisting of lenses, a prism, and a microsphere is developed, and the Q-factor dependence on different distances between the prism and the microsphere is investigated.

Millimeter-Wave WGM Resonator-Based Characterization of Continuous and Noncontinuous Ultrathin Cu Films

We analyze the experimental data on the evolution of the microwave (MW) response of the whispering gallery mode (WGM) resonator. The focus is on the thickness $d_{f}$ variation of the conducting endplates (CEPs) from $d_{f}$ exceeding skin layer thickness $\delta _{s}$ to $d_{f}\ll \delta _{s}$ down to $d_{f} = 0$ , i.e., with no CEPs. The analysis was performed by comparing the numerical simulation results obtained using COMSOL software with the authors’ experimental data on the response of the Ka -band sapphire WGM resonator. The nonmonotonic change in the WGM resonator response with decreasing thickness $d_{f}$ is explained using the metal–insulator percolation model proposed by Krupka et al. We use a different resonator (quasi-optical) technique and different mode polarization, namely, the HE $_{14~1~\delta }$ mode. The artificially introduced discontinuity of the ultrathin film, resulting in a sharp growth of the ${Q}$ -factor is analogous with the percolation material structure. The revealed effect may be useful for the development of MW devices with dynamically controlled properties of metamaterials.

Polymer Patterning via Electrohydrodynamic Instabilities

ABSTRACTElectrohydrodynamic (EHD) instabilities can be induced in polymers by placing a polymer film above its Tg in a strong electric field between two capacitor plates or electrodes. The polymer experiences an electrostatic stress at the interface between the polymer and air due to a mismatch in their dielectric constants. This stress, along with thermal fluctuations, induces small magnitude capillary waves in the polymer film, and the minima and maxima of those waves experience slightly different electric field strengths. In a sufficiently strong electric field, the capillary wave maxima, where the distances between the polymer film and the top electrode(s) are the smallest, are eventually drawn up to the top electrode. The wavelength of the instabilities in the film and the ability of the polymer to be drawn upward is a dependent on the competition between surface tension forces and the electrostatic stress imparted on the polymer. While EHD instabilities are typically used to pattern polymer surfaces on a nanometer-scale, instabilities have been induced in polymer films with air gaps up to 500 μm. Upper electrodes with non-planar structures have also been used to induce instabilities in polymer films, resulting in patterned polymer surfaces without contact. Size, shape, arrangement, and placement of the upper electrode relative to the polymer film and lower electrode, as well as the processing conditions such as temperature and applied voltage, can all be modified to produce a desired array of structures with tailored performance characteristics. Patterned polymer surfaces can provide high-index contrast over a periodic matrix with 3-dimensional element shapes. The dielectric contrast and array pitch and height can be tuned to control specular reflection and achieve specific scattering characteristics. Surfaces with tailored scattering characteristics in the aforementioned ranges could be useful in producing frequency-selective windows for glare reduction, anti-reflective solar cells with enhanced efficiency, surface waveguides and whispering gallery-mode resonator arrays for integrated photonics and sensors, and surfaces with controlled emissivity for directed heat dissipation.

Universal nonlinear scattering in ultra-high Q whispering gallery-mode resonators

Universal nonlinear scattering processes such as Brillouin, Raman, and Kerr effects are fundamental light-matter interactions of particular theoretical and experimental importance. They originate from the interaction of a laser field with an optical medium at the lattice, molecular, and electronic scale, respectively. These nonlinear effects are generally observed and analyzed separately, because they do not often occur concomitantly. In this article, we report the simultaneous excitation of these three fundamental interactions in mm-size ultra-high Q whispering gallery mode resonators under continuous wave pumping. Universal nonlinear scattering is demonstrated in barium fluoride and strontium fluoride, separately. We further propose a unified theory based on a spatiotemporal formalism for the understanding of this phenomenology.

Widely tunable optical parametric oscillator in periodically poled congruently grown lithium tantalite whispering gallery mode resonators

We demonstrate a tunable optical parametric oscillator in a periodically poled congruently grown lithium tantalite whispering gallery mode resonator. The resonator is mechanically polished to millimeter size, and the quality factor is approximately 107 at 1064 nm. Our experiments show that this kind of resonator is capable of reaching a very low threshold and having a wide tuning range. Combined with its narrow resonant linewidth, it is potentially used as a compact, widely tunable, and narrow-linewidth infrared to mid-infrared laser source.

Brillouin Lasing in Ultra-High Q Lithium Fluoride Disk-Resonators

We report the fabrication of an ultra-high- $Q$ monolithic whispering gallery-mode resonator using a lithium fluoride crystal, with a quality factor above $10^{8}$ at 1550 nm. We show that when pumped above a certain threshold, we can excite single Stokes Brillouin lasing in this resonator. We thereby demonstrate Brillouin lasing in a monofluoride crystal for the very first time, to the best of our knowledge. We provide a detailed theoretical investigation of Brillouin scattering in this resonator, using a time-domain model which tracks the dynamics of the Stokes and pump waves. We also perform a stability analysis which enables us to analytically determine the threshold pump power. We expect such crystalline Brillouin lasers to find potential applications for ultra-pure microwave and multi-wavelength generation, as well as for compact sensors.

On-Chip Fabrication of Glass Sphere Laser

Fabrication and application of glass spherical micro-cavity for lasing are reported. Surface-tension molding (StM) and localized-laser heating (LLH) techniques have been developed to fabricate glass super sphere, which is partially truncated spherical shape, and true spheres, respectively. Whispering gallery mode (WGM) resonances or laser oscillations from the spherical glasses were demonstrated. Super-spherical glasses possessed WGM resonances on its equatorial plane. The equatorial plane with high roundness (>0.99) serves a high quality factor to lead laser oscillation. LLH technique enables us to fabricate true spheres on a transparent substrate. Tellurite glass spheres prepared by the LLH technique showed laser oscillation with few-mW-order thresholds by direct pumping. StM and LLH technique are very suitable for both preparation and utilization of glass spheres for optical micro-cavity.

Optical rogue waves in whispering-gallery-mode resonators

We report a theoretical study showing that rogue waves can emerge in whispering gallery mode resonators as the result of the chaotic interplay between Kerr nonlinearity and anomalous group-velocity dispersion. The nonlinear dynamics of the propagation of light in a whispering gallery-mode resonator is investigated using the Lugiato-Lefever equation, and we evidence a range of parameters where rare and extreme events associated with a non-gaussian statistics of the field maxima are observed.

Novel optical sensor based on morphology-dependent resonances for measuring thermal deformation in microelectromechanical systems devices

We have reported a novel optical sensor based on whispering gallery mode (WGM) resonance for measuring thermal deformation in microelectromechanical systems (MEMS) devices. New asymptotic expressions for transverse electric and transverse magnetic waves are developed based on electromagnetic theory derivations for the large size parameter ( times diameter divided by wavelength of light) limits. The optical thermal deformation sensor is characterized both theoretically and experimentally by considering the fact that the size parameter of the microspheres is very large at optical wavelengths. As a prototype thermal deformation sensor, an optical fiber experimental setup with tunable laser diode has been used for realizing the effect of WGM resonances due to change in surrounding temperature of a dielectric microsphere made of BK-7 glass. The quality factor of experimental resonance spectra observed in the laboratory is calculated approximately on the order of 104, which is sensitive enough for detecting micro or nano level deformation changes in the surrounding medium. The novel optical sensor can measure the thermal deformation in the MEMS devices as small as the submicron or nanometer level. This sensor could potentially be used for nanotechnology, MEMS devices, biomedical applications, and other microdevices.

Optical resonant sensors: a method to reduce the effect of thermal drift

The use of a whispering gallery mode (WGM) resonator with an ultrahigh quality factor Q is promising in highly sensitive, label-free, lab-on-a-chip sensor applications. We investigated a novel method of using the differential frequency of TE and TM modes to reduce the thermal noise baseline, which commonly limits the sensitivity of WGM sensors. We studied the temperature dependence of WGM based sensors and experimentally demonstrated the reduction of temperature fluctuation and thus a significant improvement in the practical sensor detection limit.

High-order tunable filters based on a chain of coupled crystalline whispering gallery-mode resonators

We demonstrate experimentally a tunable third-order optical filter fabricated from the three voltage-controlled lithium niobate whispering gallery-mode resonators. The filter operates at 1550 nm with 30-MHz bandwidth and can be electrooptically tuned by 12 GHz in the linear regime with approximately 80-MHz/V tuning rate. With this filter, we have demonstrated 6-dB fiber-to-fiber insertion loss and 30-ns tuning speed, limited by the resonator buildup time.