Coupling Of Whispering Gallery Modes Research Articles

Fano-Like Spectrum With a Quasi-Independent Tuning of Slope Ratio Based on HE Mode Components Coupled Whispering Gallery Mode

Narrow bandwidth Fano-like spectrum was demonstrated with tunable slope ratio, stable intensity extremums and high extinction ratio based on HE mode components coupled whispering gallery mode. The theory was presented with calculations to analyze the spectra profiles with slope ratio and extremums of the transmission included. In the experiment, spectra with a modified positive slope ratio from 49.2 dB/GHz to 197.0 dB/GHz, and a negative slope ratio from −71.9 dB/GHz to −230.2 dB/GHz were achieved with the extremums varied less than 10%. The configuration could be widely applied in optical sensing, optical switching and dispersion control.

Morphology-dependent resonance induced optical forces in a multiple-sphere system.

Optical fields and forces can be greatly enhanced for a microparticle when the whispering gallery modes (WGMs) are excited. In this paper, by solving the scattering problem using the generalized Mie theory, the morphology-dependent resonances (MDRs) and resonant optical forces derived from the coherent coupling of WGMs are investigated in multiple-sphere systems. When the spheres approach each other, the bonding and antibonding modes of MDRs emerge and correspond to the attractive and repulsive forces, respectively. More importantly, the antibonding mode is good at propagating light forward, while the optical fields decay rapidly for the bonding mode. Moreover, the bonding and antibonding modes of MDRs in the PT-symmetric system can persist only when the imaginary part of the refractive index is small enough. Interestingly, it is also shown that for a PT-symmetric structure, only a minor imaginary part of the refractive index is required to generate a significant pulling force at MDRs, making the whole structure move against the light propagation direction. Our work deepens the understanding of the collective resonance behavior of multiple spheres and paves the way for potential applications in particle transportation, non-Hermitian systems, integrated optical devices, etc.

Rod and slit photonic crystal microrings for on-chip cavity quantum electrodynamics

Abstract Micro-/nanocavities that combine high quality factor (Q) and small mode volume (V) have been used to enhance light–matter interactions for cavity quantum electrodynamics (cQED). Whispering gallery mode (WGM) geometries such as microdisks and microrings support high-Q and are design- and fabrication-friendly, but V is often limited to tens of cubic wavelengths to avoid WGM radiation. The stronger modal confinement provided by either one-dimensional or two-dimensional photonic crystal defect geometries can yield sub-cubic-wavelength V, yet the requirements on precise design and dimensional control are typically much more stringent to ensure high-Q. Given their complementary features, there has been sustained interest in geometries that combine the advantages of WGM and photonic crystal cavities. Recently, a “microgear” photonic crystal ring (MPhCR) has shown promise in enabling additional defect localization ( > $ > $ 10× reduction of V) of a WGM, while maintaining high-Q ( ≈ 1 0 6 ) $(\approx 1{0}^{6})$ and other WGM characteristics in ease of coupling and design. However, the unit cell geometry used is unlike traditional PhC cavities, and etched surfaces may be too close to embedded quantum nodes (quantum dots, atomic defect spins, etc.) for cQED applications. Here, we report two novel PhCR designs with “rod” and “slit” unit cells, whose geometries are more traditional and suitable for solid-state cQED. Both rod and slit PhCRs have high-Q ( > 1 0 6 ) $( > 1{0}^{6})$ with WGM coupling properties preserved. A further ≈10× reduction of V by defect localization is observed in rod PhCRs. Moreover, both fundamental and 2nd-order PhC modes co-exist in slit PhCRs with high Qs and good coupling. Our work showcases that high-Q/V PhCRs are in general straightforward to design and fabricate and are a promising platform to explore for cQED.

Whispering gallery mode excitation using exposed-core fiber.

Whispering gallery modes (WGMs) in micro-resonators are of interest due to their high Q-factors. Ultra-thin fiber tapers are widely deployed to couple light into micro-resonators but achieving stable and practical coupling for out-of-lab use remains challenging. Here, a new WGM coupling scheme using an exposed-core silica fiber (ECF) is proposed, which overcomes the challenge of using fragile fiber tapers. Microspheres are deposited onto the exposed channel for excitation via the evanescent field of the fiber's guided modes. The outer jacket of the ECF partially encapsulates the microspheres, protecting them from external physical disturbance. By varying the mode launching conditions in this few-mode ECF, in combination with a Fano resonance effect, we demonstrate a high degree of tunability in the reflection spectrum. Furthermore, we show multi-particle WGM excitation, which could be controlled to occur either simultaneously or separately through controlling the ECF mode launching conditions. This work can bring value towards applications such as optical switches and modulators, multiplexed/distributed biosensing, and multi-point lasing, integrated in a single optical fiber device that avoids fiber post-processing.

Laser-Controlled Fano Resonance Sensing Based on WGM Coupling in Eccentric Hole Fibers Integrated With Azobenzene

In this article, we propose two laser-controlled Fano resonance refractive index (RI) sensors based on eccentric hole fibers (EHFs) integrated with photo-sensitive azobenzene (azo) by exploiting whispering gallery mode (WGM) coupling between the eccentric hole and the EHF microresonators. Azo coating is coated over the external surface of the EHFs to achieve laser tuning of the RI sensing range. Fano resonance would emerge when the phase matching condition of the WGMs that respectively propagate through the azo coating and the eccentric hole is satisfied. Fano resonance sensing with tunable RI range by adjusting the laser power density is theoretically demonstrated. The EHF-based Fano resonance sensor having the silica-based eccentric hole infiltrated with liquid sample of RI between 1.61364 and 1.61436 achieves a RI sensitivity of 875.185 nm/RIU, while the RI sensitivity of the one with a higher index layer serving as the internal surface of the eccentric hole turns to be 1106.351 nm/RIU for a RI range of 1.33748 to 1.33805. More significantly, simulation results indicate that as 5 mW/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> laser is applied, the RI sensing ranges of the proposed Fano resonance sensors respectively show 101.39% and 101.75% increases compared with the cases when no laser illumination is applied. The EHF-based Fano resonance sensors provide promising candidates for applications in bio/chemical sensing with laser-controlled extended RI sensing range. And moreover, the proposed laser tuning method paves a new way toward developing tunable photonic devices and all-optical networking systems with desired operation wavelength ranges.

GaN microdisk with direct coupled waveguide for unidirectional whispering-gallery mode emission.

Microdisks are excellent whispering-gallery mode (WGM) optical resonators, but their emissions are invariably in-plane isotropic due to their circularities and thus difficult to be extracted efficiently. In this work, a waveguide with a width of 0.16 µm directly coupled to a microdisk with a diameter of 10 µm is fabricated on a 0.77 µm thick GaN thin film containing InGaN/GaN multi-quantum wells. This eliminates the need for precision patterning required by evanescent coupling schemes in which coupling gaps of the order of tens of nanometers must be maintained. The fabrication was carried out using nanosphere and nanowire lithography. Non-evanescent coupling of WGMs to the waveguide from the microdisk is successfully demonstrated.

Periodical focusing mode achieved through a chain of mesoscale dielectric particles with a refractive index near unity

Abstract At present three main mechanisms of optical propagation with periodically focused modes in the chains of dielectric spherical particles with a refractive index near n = 1.59 are considered in the literature: evanescent whispering gallery mode coupling, photonic nanojet-induced modes, and the propagation of transverse magnetic modes under conditions close to Brewster angle geometry. In this paper we show that quasi-periodical focusing modes exist in a range of near unity refractive indices for chains of dielectric particles in both spherical and cubic forms. We also show that such a medium can be considered a gradient index waveguide with a parabolic law of refractive index variation.

A scatterer-assisted whispering-gallery-mode microprobe

Abstract A fiber-based whispering-gallery-mode (WGM) microprobe, combining both the high optical field enhancement of the WGMs and the compact structure of the optical fiber, is highly desired for sensing and imaging. Here we report a WGM microsphere resonator coupled to a single-mode fiber interfaced by a graded-index lens. By scattering a focused laser beam through a nano-scatterer, with the help of a two-step focusing technique as well as Purcell effects, the efficient far-field coupling of WGMs with an efficiency as high as 16.8% has been demonstrated in our system. With the feature of both input and output of the probe light propagating along the same fiber, such a scatterer-assisted WGM microprobe will serve as a convenient tool for sensing/imaging applications.

Three-Dimensional Printed Polymer Waveguides for Whispering Gallery Mode Sensors

Whispering gallery mode (WGM) microresonators are widely exploited for highly sensitive sensing applications due to their ultrahigh quality $(Q)$ factor enabling single-particle detection. The WGM surface-confined resonant light recirculates around the resonator, allowing the light to interact with analytes over several round trips, which enhances the sensitivity of detection. However, the stability of efficient coupling of WGMs to ingress-egress waveguides for lab-on-chip has been problematic. The typical way to achieve WGM coupling is to use a tapered optical fiber or a prism, which makes the device either fragile or bulky, and is not conducive to sensor integration that can lead to robust lab-on-chip devices. In this letter, we demonstrate three-dimensional (3D) fabricated polymer waveguides that can be integrated and provide evanescent coupling to silica WGM microspheres without sacrificing $Q$ factor. By mechanically connecting the waveguides to the microcavity, we can control the coupling to the resonator and the mechanical stability of the 3D lab-on-chip sensor is greatly improved. This provides a pathway to enhance the stability of WGM coupling for sensor integration.

Plasmon-Coupled Whispering Gallery Modes on Nanodisk Arrays for Signal Enhancements

Metallic nanostructures including single and double nanodisks are successfully used to enhance the localized electric field in vicinity of microcavity in whispering gallery mode (WGM) sensor. We demonstrate numerical calculations of plasmonic coupling of WGMs to single and double nanodisk arrays on a planar substrate. We then experimentally confirmed that the resonance wavelength of WGM sensor was dramatically shifted by adoption of single and double nanodisks on the surface of microcavity in the WGM sensor. Thus, our approach provides the tunable sensitivity of WGM sensor, and has a great potential to be used in numerous areas where the single biomolecule, protein-protein folding and biomolecular interactions are involved.

Whispering gallery mode hybridization in photonic molecules

AbstractThis 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. image

Whispering Gallery Mode Lasing from Self-Assembled Hexagonal Perovskite Single Crystals and Porous Thin Films Decorated by Dielectric Spherical Resonators

Lasing in self-assembled hybrid organic–inorganic lead halide perovskites semiconductors has attained intensive research for low cost and high performance optoelectronic devices due to their inherent outstanding optical response. However, to achieve the controllable laser action from a small single crystal remains as a challenging issue. Here, we present a novel technique to fabricate self-assembled high-quality hexagonal perovskite single crystals for realizing room-temperature near-infrared whispering-gallery-mode (WGM) laser action. Quite interestingly, the lasing spectrum for an individual CH3NH3PbI3 hexagonal single crystals encompasses the aspects of high quality factor (Q) and low threshold WGM lasing around 1200 and 26.8 μJ/cm2, respectively. In addition, we demonstrate that when the porous perovskite thin films were decorated with dielectric spheres, the laser oscillation can be achieved through the coupling of WGM with perovskite gain material. We found that the lasing spectra can be well manipu...

Acoustooptic-Mode-Coupling-Based Whispering Gallery Mode Excitation in Silica-Capillary Microresonator

An acoustooptic-mode-coupling-based scheme is proposed for excitation of whispering gallery modes (WGMs) in a silica-capillary microresonator. By exploiting acoustooptic mode coupling in a cladding-etched optical fiber, the evanescent field is strengthened by the cladding modes coupled from the core mode through axial flexural wave modulation, leading to the significant enhancement of the mode coupling between evanescent field and WGMs in a silica-capillary microresonator. WGM resonance peaks have been experimentally acquired by employing a cladding-reduced optical fiber with a diameter of 20.68 μ m, which is more robust than the coupling fibers of a few microns in diameter used in conventional WGM excitation schemes. Our proposed WGM coupling approach has desirable merits such as compactness, ease of fabrication, improved WGM coupling efficiency, and robustness, which is anticipated to find potential applications in optical filtering, microresonator laser, fiber sensor, as well as signal processing. Additionally, by controlling the electric driving signals to apply acoustic modulation, it would be convenient to achieve flexible tuning of WGMs based on acoustooptic coupling approach.

Tunable Autler–Townes Splitting Observation in Coupled Whispering Gallery Mode Resonators

In this paper, we experimentally demonstrated an Autler–Townes splitting (ATS) effect in a cascaded optical whispering gallery mode resonator system. Although electromagnetically induced transparency (EIT) has been investigated as an interesting Fano resonance in the recent decades, the critical resonance matched condition of a high-quality factor resonator and a low-quality factor resonator makes EIT difficult to realize and hardly tunable. In contrast, ATS requires relatively simple system configurations, and the effect can be preserved over a large tuning range. In our approach, a theoretical model based on coupled mode theory is developed, and the methodology of creating and controlling the ATS modes is demonstrated. In our experiment, the optical resonators have a loaded quality factor of about 3 × 105 and cascaded in a strong coupling condition. For the experimental demonstration, a thermo-optical mechanism is utilized to control the splitting modes in our system. The asymmetric shape of a splitting resonances shift has a perfect fit with the theoretical calculations and numerical modeling results. Utilizing the precise tenability of splitting modes, the ATS has an extra degree of freedom to perform resonance tuning, which has become an ideal method to overcome the current detection limit.

Radiative rate modification in CdSe quantum dot-coated microcavity

Whispering gallery modes (WGMs) of the microparticles with spherical or cylindrical symmetry have exceptionally high quality factors and small mode volume. Quantum dots (QDs) are zero dimensional systems with variable band gap as well as luminescent properties with applications in photonics. In this paper, the WGMs have been observed in the luminescence spectra of CdSe QD-coated single silica microspheres. Theoretical estimations of variation of resonance frequency, electric field, and Q-values have been done for a multilayer coating of QDs on silica microspheres. Observed WGMs have been identified for their mode number and polarization using Mie theory. Broadening of modes due to material absorption has been observed. Splitting of WGMs has also been observed due to coherent coupling of counter propagating waves in the microcavity due to the presence of QDs. At room temperature, the time-resolved study indicates the modification of the radiative rate due to coupling of WGMs of the microcavity-QD hybrid system.

Light coupling into the Whispering Gallery Modes of a fiber array thin film solar cell for fixed partial Sun tracking

We propose the use of whispering gallery mode coupling in a novel configuration based on implementing a thin film cell on the backside of an array of parallel fibers. We performed numerical calculations using the parameters of a thin film organic cell which demonstrate that light coupling becomes more effective as the angle for the incident light relative to the fiber array normal increases up to an optimal angle close to 55 deg. At this angle the power conversion efficiency of the fiber array solar cell we propose becomes 30% times larger than the one from an equivalent planar cell configuration. We demonstrate that the micro fiber array solar cell we propose may perform an effective partial tracking of the sun movement for over 100 degrees without any mechanical help. In addition, in the event that such fiber array cell would be installed with the adequate orientation on a vertical façade, an optimal photon-to-charge conversion would be reached for sunlight incident at 55 deg with respect to the horizon line, very close to the yearly average position for the sun at Latitude of 40 deg.

Broadband Light‐Trapping Enhancement in an Ultrathin Film a‐Si Absorber Using Whispering Gallery Modes and Guided Wave Modes with Dielectric Surface‐Textured Structures

An embedded nanosphere dielectric structure on an a-Si ultrathin film improves weighted absorption from 23.8% to 39.9%. The PMMA embedding layer offers a guided wave mode as well as mechanical robustness, in addition to the resonant whispering gallery modes coupling. Broadband light-trapping enhancements are observed by dielectric surface textured structures of hemispheres, nanocones, nanospheres, or embedded nanospheres.

Acoustic whispering gallery mode coupling with Lamb waves in liquid

Acoustic whispering gallery modes (WGMs) in a micro-quartz tube immersed in liquid medium, excited by the evanescent wave of a Lamb wave device, are demonstrated. These excited WGMs exhibit characteristics similar to those of optical WGMs, such as narrow periodic transmission dips and a high quality factor. These results show the possibility to build acoustic sensors, filter and acoustic resonators working in liquid with improved performance.

Surface Plasmon Coupled Whispering Gallery Mode for Guided and Free-Space Electromagnetic Waves

We demonstrate theoretically that plane wave propagating in free space can be used to excite the whispering gallery mode in dielectric microresonators grown on the top of nanoplasmonic structures, with the assistance of surface plasmon wave. We have demonstrated the coupling modes using both localized and propagating surface plasmon-supporting nanostructure surfaces.

Coupled whispering gallery mode resonators in the Terahertz frequency range

We report on coupling of two whispering gallery mode resonators in the Terahertz frequency range. Due to the long wavelength in the millimeter to submillimeter range, the resonators can be macroscopic allowing for accurate size and shape control. This is necessary to couple specific modes of two or more resonators. Sets of polyethylene (PE) and quartz disk resonators are demonstrated, with medium (loaded) quality (Q)-factors of 40-800. Both exhibit coinciding resonance frequency spectra over more than ten times the free spectral range. Loading effects of single resonators are investigated which provide strong Q-factor degradation and red-shifts of the resonances in the 0.2% range. By coupling two resonators of the same size, we observe mode splitting, in very good agreement with our numerical calculations.