Evanescent Coupling Research Articles

An opto-electro-mechanical system based on evanescently-coupled optical microbottle and electromechanical resonator

Evanescent coupling between a high-Q silica optical microbottle and a GaAs electromechanical resonator is demonstrated. This coupling offers an opto-electro-mechanical system which possesses both cavity-enhanced optical sensitivity and electrical controllability of the mechanical motion. Cooling and heating of the mechanical mode are demonstrated based on optomechanical detection via the radiation pressure and electromechanical feedback via the piezoelectric effect. This evanescent approach allows for individual design of optical, mechanical, and electrical systems, which could lead to highly sensitive and functionalized opto-electro-mechanical systems.

A comprehensive experimental study of whispering gallery modes in a cylindrical microresonator excited by a tilted fiber taper

AbstractWhispering gallery modes (WGMs) excitation in a cylindrical microresonator formed by a section of silica optical fiber has been studied. Evanescent light coupling into the microresonator is realized using a tapered optical fiber, fabricated by a microheater brushing technique. Several types of silica fibers with different diameters are studied as microresonators, and the influence of the resonator's diameter on the excitation of WGMs is investigated. The excitation of WGMs in a cylindrical fiber resonator were studied with changes to the tilt angle between the microcylinder and the fiber taper in the range of angles from a perpendicular position (0°) to large tilt angles (24°). The evolution of the fiber taper transmission spectrum with the change of the tilt angle results in changes in the intensity, broadening of and a blue shift in the WGM resonance spectra. Overall losses in the taper transmission spectrum decrease with the increase of the taper tilt angle from its perpendicular position, followed by a complete disappearance of the WGM resonances at large tilt angles greater than 20°.

Identification of Dielectric, Plasmonic, and Hybrid Modes in Metal-Coated Whispering-Gallery-Mode Resonators

Making available and accessing in a controlled manner optical modes with largely disparate properties in a given system constitutes a prime challenge for different applications. Here, we propose, realize, and optically characterize a high-Q polymeric wedge-like whispering-gallery-mode resonator coated with a thin silver layer that supports pure surface plasmon polariton modes, pure dielectric modes, and hybrid photonic–plasmonic modes with Q-factors larger than 1000 and modal volumes as small as only a few cubic micrometers. We demonstrate both theoretically and experimentally that all three distinct kinds of cavity eigenmodes can be efficiently excited in the infrared via evanescent coupling to a tapered fiber. Performing finite-element simulations and coupled-mode theory, we develop an experimental procedure based on mode filtering to unambiguously identify the resonances observed in fiber transmission spectra. By controlling both the position of the tapered fiber with respect to the resonator and the i...

A Coated Spherical Microresonator for Measurement of Water Vapor Concentration at PPM Levels in Very Low Humidity Environments

This paper presents a novel approach to measurements of low relative humidity (RH) values based on the whispering gallery modes (WGMs) effect in a small silica microsphere coated with a thin layer of Agarose hydrogel. The light from a narrow-linewidth tunable laser with a few GHz tuning range is launched into an adiabatic tapered fiber, which excites WGMs in the Agarose layer via evanescent coupling. Adsorption and desorption of water vapor by the Agarose layer in response to changes in ambient humidity lead to changes in the layer's refractive index, which in turn leads to a spectral shift of the WGM resonances. We experimentally demonstrate the WGMs spectral shift over a range of RH values from 1%RH to 25%RH. The proposed sensor displays linear response and is capable of measurement of low concentrations of water vapor (corresponding to about 11.7 ± 0.32 ppm water molecules in air). In addition, the proposed sensor offers the advantages of a very small form factor and good repeatability. Detailed studies of the sensor stability, cross sensitivity to temperature, response times, and hysteresis are also presented.

Localization of light in an optical microcapillary induced by a droplet

Sensing with optical whispering gallery modes (WGMs) is a rapidly developing detection method in modern microfluidics research. This method explores the perturbations of spectra of WGMs propagating along the wall of an optical microcapillary to characterize the liquid medium inside it. Here we show that WGMs in a silica microcapillary can be fully localized (rather than perturbed) by evanescent coupling to a water droplet and, thus, form a high-quality-factor microresonator. The spectra of this resonator, measured with a microfiber translated along the capillary, present a hierarchy of resonances that allow us to determine the size of the droplet and variation of its length due to the evaporation. The resolution of our measurements of this variation equal to 4.5 nm is only limited by the resolution of the optical spectrum analyzer used. The discovered phenomenon of complete localization of light in liquid-filled optical microcapillaries suggests a new type of microfluidic photonic device as well as an ultraprecise method for microfluidic characterization.

Efficient higher-order nonlinear optical effects in CdSe nanowaveguides

Stimulating higher-order nonlinear optical (HO-NLO) response from individual semiconductor nanostructures is challenging due to the low nonlinear coefficients and the small number of molecules within the nanostructures. In this work, we demonstrate efficient third harmonic generation and multi-photon luminescence in CdSe nanowaveguides by means of evanescent wave coupling technique. Under appropriate conditions, a coupling efficiency of 70% can be achieved from an optical microfiber to a single CdSe nanowaveguide, leading to the enhanced HO-NLO effects. Provided a high signal-to-noise ratio, we thus observe a fourth order excitation power dependence of 3-photon luminescence, and we attribute it to surface defect mechanism based on the recombination of free carriers. This work provides an alternative for efficient excitation for HO-NLO, which also makes these hard-to-produce signals more feasible in the applications of nonlinear optical devices.

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.

Low-loss prism-waveguide optical coupling for ultrahigh-Q low-index monolithic resonators

While compact and low-loss optical coupling to ultrahigh-quality-factor (Q) crystalline resonators is important for a wide range of applications, the major challenge for achieving this coupling stems from the relatively low refractive index of the crystalline resonator host material compared to those of the standard waveguide coupling materials. We report the first demonstration of a single-mode waveguide structure (prism-waveguide coupler) integrated on a low-loss compact silicon nitride platform resulting in low-loss and efficient coupling to magnesium fluoride crystalline resonators by achieving the phase-matched and the mode-matched evanescent wave coupling. The coupling is characterized with 1 dB loss at 1550 nm wavelength. We further present a photonic integrated chip containing a pair of waveguides successfully coupling light into and out of the resonator, demonstrating a planar-waveguide-coupled crystalline resonator with a loaded Q of 1.9×109. We assemble this waveguide-coupled resonator and a distributed-feedback-laser chip into a butterfly package to realize a miniature Kerr optical frequency comb source using self-injection locking of the distributed feedback laser to the waveguide-coupled crystalline resonator.

Compact Polarization Beam Splitter Based on a Three-Waveguide Asymmetric Coupler With a 340-nm-Thick Silicon Core Layer

A compact polarization beam splitter (PBS) is presented for the case with a 340-nm-thick silicon core layer by using an asymmetric evanescent coupling system. It consists of a narrow input waveguide, a narrow output waveguide, and a wide middle waveguide between them. In this design, the widths of these three waveguides are chosen optimally, so that the phase matching condition is satisfied for the fundamental (TM0) mode in the narrow input/output waveguide and the first higher order (TM1) mode in the wide middle waveguide. Meanwhile, there is moderate phase mismatch for TE polarization. Then, the length of the coupling region is also optimized, so that the launched TE 0 mode gets out from the through port while the launched TM0 mode is coupled completely to the cross port with the assistance of the first higher order (TM1) mode in the wide middle waveguide. The designed PBS has a compact footprint of 38 × 3.5 μm2. In theory, the designed PBS works very well with a low excess loss ( 15 dB) in a broad band of 1510–1570 nm. For the fabricated PBS, the extinction ratios are >10 dB and >20 dB in the wavelength range of 1535–1610 nm, for TM and TE polarizations, respectively.

Lead Halide Perovskite Based Microdisk Lasers for On‐Chip Integrated Photonic Circuits

AbstractOrganic–inorganic lead halide perovskites (MAPbX3, MA = CH3NH3, X = Br, Cl, I) are intensively studied due to their unique properties and potential applications in optoelectronic devices. However, up to now, the MAPbX3 microdevices such as solar cells, lasers, and photodetectors are studied individually and their potentials in optical integration are rarely explored. Herein it is experimentally demonstrated that MAPbBr3 microdisks can be directly coupled to MAPbBr3 bus waveguides, enabling the possibility of all‐perovskite photonic networks for the first time. The MAPbBr3 microdisks with bus waveguides are fabricated with a well‐controlled top‐down fabrication technique and their laser characteristics are determined through optical excitations. In contrast to an isolated device, the lasers in perovskite microdisks can be coupled to bus waveguides via the evanescent coupling. In addition, it is shown that the outputs along bus waveguide can be switched from bidirectional emissions to unidirectional emission by introducing internal chirality to optical resonances. Considering the wide applications of perovskites in microlasers, waveguides, solar cells, and even photodetectors, this research shall pave a new way to all‐perovskite‐based self‐powered on‐chip photonic circuits.

Studies of geometrical profiling in fabricated tapered optical fibers using whispering gallery modes spectroscopy

Abstract This paper experimentally demonstrates a method for geometrical profiling of asymmetries in fabricated thin microfiber tapers with waist diameters ranging from ∼10 to ∼50 µm with submicron accuracy. The method is based on the analysis of whispering gallery mode resonances excited in cylindrical fiber resonators as a result of evanescent coupling of light propagating through the fiber taper. The submicron accuracy of the proposed method has been verified by SEM studies. The method can be applied as a quality control tool in fabrication of microfiber based devices and sensors or for fine-tuning of microfiber fabrication set-ups.

Spectral shaping of lasing in vertically aligned coupled nanowire lasers

We use numerical simulations to study the effects of nanowire geometry on the emission spectra of nanowire-based lasers. The studied nanowire lasers are made from gallium-nitride and the simulations are performed using finite difference time domain method. We show that changes in the diameter of the nanowire lasers also change the effective refractive index of their optical guided modes, which allows control over their Fabry-Perot spectrum. In addition, we show that evanescent coupling of two vertically standing nanowire lasers, having different cross section sizes, leads to a Vernier effect, which results in single mode emission from the small footprint coupled lasers system.

C-band optical filters with micromechanical tuning

Tunable optical filters that cover the entire range of the C-band (1530–565 nm) are designed by utilizing the Vernier effect, i.e. series coupling of microring resonators of different sizes, and the micromechanical tuning method. The micromechanical tuning method employs lateral comb-drive actuators to control evanescent coupling between the resonators and index modulators. Single crystalline silicon is used as the material for all of the main components including bus waveguide cores, resonators, index modulators, and comb-drive actuators. A finite-difference time-domain method is used for optical analysis of the filter. The simulation results show good agreement with those by analytical methods, previously reported. The width of the index modulator is found to play an important role to the filter characteristics. A wider modulator (e.g., width: 100 nm) can cover the full tuning range of 35 nm without switching among different bands owing to stronger effective index change effect, but induces significant loss to the filter, especially when it is brought close to the resonator. While a narrow modulator (e.g., width: 50 nm), on the other hand, induces moderate loss to the filter, it requires hopping among multiple bands to cover the full range since the effective index change incurred is again moderate. In order to achieve linear tuning characteristics in the cascaded-resonator filters, the shaped-finger comb-drive actuator design method is applied. The design method based on two-dimensional slice approximation is further examined by three-dimensional finite element analysis for verification. It is shown that the design method can also work for the cascaded-resonator filters, even for the one that requires band hopping. Effects of fabrication imperfections to the designed device characteristics are studied as well.

Far-field single nanoparticle detection and sizing

Whispering gallery mode based optical microcavities are important for highly sensitive optical sensing. However, the current experimental realizations are strongly dependent on high-resolution tunable lasers and evanescent coupling, which are too cumbersome and too expensive for portable devices. Herein we experimentally demonstrate a cost-effective and robust approach to detect and size a single nanoparticle with far-field laser emissions. By placing a limacon microdisk close to a spiral microdisk, chiral resonances have been successfully generated. In contrast to previous research, here the internal chirality is strongly correlated with the far-field patterns (FFPs) and thus can be transduced to far-field emissions. Once a nanoparticle is attached to the limacon microdisk, the asymmetrical backscattering at the notch of the spiral can be averaged by the symmetrical scattering of the nanoparticle. Consequently, the internal chirality and the corresponding FFPs are changed. By measuring a far-field directional laser emission, nanoparticles with a radius of ∼50 nm have been successfully detected and sized without employing any spectral information. As a narrow-linewidth tunable laser is not used in our experiment and microdisks lasers may be electrically driven, this research will provide a new path to cost-effective, portable, highly sensitive optical sensors.

Effects of whispering gallery mode in microsphere super-resolution imaging

Whispering Gallery modes have been presented in microscopic glass spheres or toruses with many applications. In this paper, the possible approaches to enhance the imaging resolution by Whispering Gallery modes are discussed, including evanescent waves coupling, transformed and illustration by Whispering Gallery modes. It shows that the high-order scattering modes play the dominant role in the reconstructed virtual image when the Whispering Gallery modes exist. Furthermore, we find that the high image resolution of electric dipoles can be achieved, when the out-of-phase components exist from the illustration of Whispering Gallery modes. Those results of our simulation could contribute to the knowledge of microsphere-assisted super-resolution imaging and its potential applications.

Ultra-compact polarization rotation in integrated silicon photonics using digital metamaterials.

Polarization controlling devices such as polarization splitters and rotators are critical elements in integrated-photonic circuits that function via polarization-diversity schemes. Here, we present the design of an ultra-compact nanophotonic-polarization rotator (NPR) that rotates the polarization state from TE to TM with a simulated extinction ratio of 23dB over a coupling length of 5µm and an operating bandwidth of 40nm. This all-silicon device can be fabricated in a single lithography step and we have fabricated and characterized a preliminary device exhibiting 9dB extinction ratio. To emphasize the generality of our methodology, we also designed a NPR that can rotate the polarization state from TM to TE as well. A small device footprint is enabled by the evanescent coupling of guided modes enabled by computationally designed digital metamaterials.

Whispering gallery germanium-on-silicon photodetector.

We design and demonstrate, to the best of our knowledge, the first whispering gallery germanium-on-silicon photodetector with evanescent coupling from a silicon bus waveguide in a CMOS-compatible process. The small footprint (63.6 μm2), high responsivity (∼1.04 A/W at 1530nm), low bias voltage (-1 V), low dark current (2.03nA), and large optoelectric bandwidth (32.9GHz) of the detector enable simultaneous wavelength filtering and power detection, ideal for handling large network data traffic. In addition, with the resonant nature of the detector, we also optimize the design to enable long-wavelength detection, achieving a separate device with a detection range of up to 1630nm with a >0.45 A/W responsivity, making it an important building block for optical communication networks.

Mode-Locked Thulium-Doped Fiber Laser Using a Single-Layer-Graphene-Covered Tapered Fiber

For the first time, to the best of our knowledge, we demonstrate an all-fiber passively mode-locked thulium-doped fiber ring laser using evanescent field coupling in a tapered fiber with a single-layer graphene as a saturable absorber. The modulation depth limitation of a single-layer graphene is overcome by incorporating the graphene layer along the fiber longitudinal plane, effectively extending the interaction length between the single-layer graphene and the evanescent field. The fiber oscillator produces stable 0.98 ps soliton pulses with a pulse energy of 544 pJ, a 3 dB spectral bandwidth of 4.6 nm, and an RF signal-to-noise ratio of 70 dB.

Evanescent coupling between refillable ring resonators and laser-inscribed optical waveguides.

We investigated theoretically and experimentally the evanescent coupling between photonic waveguides of arbitrary shapes and refillable optical ring resonators on the same chip. The resonator hosts were designed to facilitate whispering gallery modes and etched by using a single-mask standard lithography process, whereas the waveguides were imprinted in the proximity of the ring resonator by using 3D ultrafast laser-writing technology. Finite element analysis in conjunction with coupled-mode theory revealed a coupling Q-factor (QC) of approximately 106. The polymer core ring resonator exhibited a loaded Q-factor (QT) as high as 5.4×104 and a free spectral range (FSR) of 406pm at a center wavelength of 775nm. Long-term stability of the ring resonator was repeatedly tested by examining the spectral location of optical resonances and the constancy of Q-factors and FSRs under ambient laboratory conditions for 1 month. We recorded consistent Q-factors and repeatable FSRs for all measurements. Renewability of the polymer core was demonstrated by removing and redepositing the polymer in the cavity, followed by measurements of Q-factors and FSRs. This work promises to enable reconfigurable and renewable photonic devices for on-chip lasers, 3D integrated optical signal processing, chip-scale molecular sensing, and the investigation of new optical phenomena.

Magnetic field sensing using whispering-gallery modes in a cylindrical microresonator infiltrated with ferronematic liquid crystal.

An all-fiber magnetic field sensor based on whispering-gallery modes (WGM) in a fiber micro-resonator infiltrated with ferronematic liquid crystal is proposed and experimentally demonstrated. The cylindrical microresonator is formed by a 1 cm-long section of a photonic crystal fiber infiltrated with ferronematic materials. Both ferronematics suspensions are prepared based on the nematic liquid crystal 1-(trans-4-Hexylcyclohexyl)-4-isothiocyanatobenzene (6CHBT) doped with rod-like magnetic particles in the first case and with spherical magnetic particles in the second case. WGMs are excited in the fiber microresonator by evanescent light coupling using a tapered fiber with a micron-size diameter. The Q-factor of the microresonator determined from the experimentaly measured transmission spectrum of the tapered fiber was 1.975 × 103. Under the influence of an applied magnetic field the WGM resonances experience spectral shift towards shorter wavelengths. The experimentally demonstrated sensitivity of the proposed sensor was -39.6 pm/mT and -37.3 pm/mT for samples infiltrated with rod like and spherical like ferromagnetic suspensions respectively for a magnetic field range (0-47) mT. Reducing the diameter of the cylindrical micro-resonator by tapering leads to enhancement of the magnetic field sensitivity up to -61.86 pm/mT and -49.88 pm/mT for samples infiltrated with rod like and spherical like ferromagnetic suspensions respectively for the magnetic field range (0-44.7) mT.