Evanescent Light Coupling Research Articles

Temperature Sensor Based on Whispering Gallery Modes of Metal-Filled Side-Hole Photonic Crystal Fiber Resonators

In this study, we propose and analyze a temperature sensor based on whispering gallery modes (WGMs) in a cylindrical microresonator formed by metal-filled side-hole photonic crystal fiber (mSH-PCF). WGMs in the mSH-PCF microresonator are excited by evanescent light coupling using a tapered single-mode silica fiber. The application of temperature to the mSH-PCF microresonator causes a change in both the geometric shape of the microresonator and the refractive index produced by the thermal expansion of the filler metal, resulting in a linear shift of the WGM resonances in which the WGM quality factors do not decrease significantly during temperature changes. We also demonstrate that the temperature sensitivity of this platform is governed by the interaction between the microresonator and the fiber taper. Bismuth and indium were used to examine the effect of metal composition on temperature sensitivity. The measurements show that a maximum temperature sensitivity of 18.72 pm/°C can be achieved with the indium-filled PCF. The experimental results are in good agreement with the numerical simulations. WGM shifts were found to depend mainly on cavity deformation.

Evanescent coupling to silicon waveguides using surface plasmon polaritons.

In this work, we discuss evanescent light coupling to silicon waveguides using surface plasmon polaritons (SPPs) in the Otto configuration. The setup for excitation of SPPs includes a fused-silica prism, a 100nm silver layer with a 650nm airgap between them, and a tapered silicon waveguide located across from the silver layer. A $p$p-polarized light of 1550nm wavelength is incident at plasmonic angle of 44° to the interface of the fused silica-airgap to excite the SPP.A 2D Lumerical FDTD Solutions simulation shows a coupling efficiency of 54%. The fabricated device on a silicon-on-insulator substrate shows 10% light transmission for $p$p-polarized light.

Controlling nanoscale air-gaps for critically coupled surface polaritons by means of non-invasive white-light interferometry

We report an experimental method to control large-area air-gaps in the nanometer range for evanescent coupling of light to surface waves such as surface plasmon polaritons or surface phonon polaritons. With the help of spectrally resolved white-light interferometry, we are able to stabilize and tune the gap with nanometer precision and high parallelism. Our technique is non-invasive and leaves the coupling area unobstructed, and the setup delivers reference-free real-time readout up to a distance of 150 μm between the coupling prism and the sample. Furthermore, we demonstrate the application to prism coupled surface polariton excitation. The active gap control is used to determine the dispersion of a critically coupled surface phonon polariton over a wide spectral range in the mid infrared region.

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°.

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.

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.

Strain-induced spectral tuning of the whispering gallery modes in a cylindrical micro-resonator formed by a polymer optical fiber

A mechanical strain assisted technique for spectral tuning of whispering gallery modes (WGM) in a cylindrical micro-resonator formed by a polymer optical fiber (POF) is investigated. WGMs in the POF-based micro-cylinder are excited by evanescent light coupling using a tapered single mode silica fiber. WGMs observed in the transmission spectrum of the silica fiber taper have a high extinction ratio of up to 19 dB and a quality factor (Q-factor) of up to 2.64×104. The application of tensile axial strain (μϵ) in the range from 0 to 1746 μϵ (0.17%) to the POF micro-resonator results in a linear shift of the WGM spectrum with a sensitivity of 0.66 pm/μϵ. Under the influence of the applied strain, the WGMs undergo a blue shift and return to their initial spectral positions after the strain is decreased. The proposed strain tunable POF micro-resonator has potential applications in fiber optic sensing and tunable micro lasers.

Design and Analysis of 2-μm InGaSb/GaSb Quantum Well Lasers Integrated Onto Silicon-on-Insulator (SOI) Waveguide Circuits Through an Al2O3 Bonding Layer

GaSb-based quantum well (QW) laser diode, with emission wavelength ∼2 μm, integrated onto a silicon-on-insulator (SOI) waveguide circuit through a high-thermal-conductivity Al2O3 bonding layer has been designed and analyzed. Prior to bonding, the fabricated Fabry–Perot GaSb QW laser worked under continuous wave operation at room temperature, with a low threshold current of 37 mA at the emission wavelength of 2019 nm, demonstrating high material quality. A tapered structure has been used for evanescent coupling of light from the GaSb laser to the underlying Si waveguide. Instead of using SiO2 for direct bonding or Benzocyclobutene for adhesive bonding, the use of Al2O3 to directly bond GaSb lasers onto SOI wafers is proposed. The optical mode distribution simulations by a beam propagation method software show that light can be coupled efficiently to the underlying Si waveguide through the tapered structure without compromise in optical coupling efficiency. Furthermore, there is a significant reduction (∼70%) in the total thermal resistance compared with the same structure using a SiO2 bonding layer. Our results suggest that the Al2O 3 bonding layer could be a promising candidate for III–V lasers integrated on SOI circuits, where thermal dissipation is very critical.

Near-Field Fano-Imaging of TE and TM Modes in Silicon Microrings

A deep-subwavelength imaging of the optical-guided modes localized in silicon microring resonators, obtained with a polarization-sensitive Fano-imaging technique, is demonstrated. We merge together near-field scanning optical microscopy and resonant forward scattering spectroscopy, leading to near-field hyperspectral imaging without the need of embedded light emitters or evanescent light coupling into the microring. The combined analysis of the observed Fano-like spectral line shapes and of the near-field intensity spatial distributions, supported by accurate numerical calculations, gives a clear discrimination between the TE and the TM modes.

Vectorial near-field imaging of a GaN based photonic crystal cavity

We report a full optical deep sub-wavelength imaging of the vectorial components of the electric local density of states for the confined modes of a modified GaN L3 photonic crystal nanocavity. The mode mapping is obtained with a scanning near-field optical microscope operating in a resonant forward scattering configuration, allowing the vectorial characterization of optical passive samples. The optical modes of the investigated cavity emerge as Fano resonances and can be probed without the need of embedded light emitters or evanescent light coupling into the nanocavity. The experimental maps, independently measured in the two in-plane polarizations, turn out to be in excellent agreement with numerical predictions.

Controlling thermo-optic response in microresonators using bimaterial cantilevers

We demonstrate a novel platform to control the thermo-optic sensitivity in nanophotonic devices by evanescent coupling of light with bimaterial cantilevers. The cantilever can be designed to provide a negative thermal feedback to passively compensate for the positive thermo-optic effect in the waveguide core. We demonstrate athermal operation over 14deg in cantilever coupled Silicon ring resonators, limited only by fabrication tolerances. We also show how the same platform can provide positive thermal feedback and overcome the material thermo-optic limit for increasing sensitivity of resonant detectors and thermal imagers.

Quantum-Dot Double Layer Polymer Waveguides by Evanescent Light Coupling

In this work we analyze numerically and experimentally new active waveguides based on a bilayer structure composed by a passive polymer and an active poly(mehtyl methacrylate) (PMMA) film doped with CdSe colloidal quantum dots (QDs), namely a nancomposite. In a first bilayer structure a planar PMMA layer is deposited on top of the nanocomposite, where the signal beam intensity is enhanced because this cladding layer is able to collect radiated emission of QDs. Moreover, the pump beam is also propagating through the cladding without limitation by QD absorption. These results are extended to a second bilayer structure, where ridge patterns of a commercially available resist (SU-8) are deposited on the top of the nanocomposite active layer. These SU-8 patterns are also able to guide with low absorption losses both pump and signal beams. The optimum geometrical parameters of the bilayer structures were properly designed to optimize the light waveguiding, previous to their fabrication and optical characterization. For this purpose, a spontaneous emission model has been developed and programmed into an active beam propagation method. This technology can be the base for developing integrated photonics on silicon at visible and telecom wavelengths.

Characterization of azimuthal and longitudinal modes in rolled-up InGaAs/GaAs microtubes at telecom wavelengths

We report on theoretical and experimental investigation of azimuthal and longitudinal modes in rolled-up microtubes at telecom wavelengths. These microtubes are fabricated by selectively releasing a coherently strained InGaAs/GaAs bilayer. We apply planar waveguide method and a quasi-potential model to analyze the azimuthal and longitudinal modes in the microtubes near 1550 nm. Then we demonstrate these modes in transmission spectrum by evanescent light coupling. The experimental observations agree well with the calculated results. Surface-scattering-induced mode splitting is also observed in both transmission and reflection spectra at ~1600 nm. The mode splitting is in essence the non-degeneracy of clockwise and counter-clockwise whispering-gallery modes of the microtubes. This study is significant for understanding the physics of modes in microtubes and other microcavities with three-dimensional optical confinement, as well as for potential applications such as microtube-based photonic integrated devices and sensing purposes.

Bottle microresonator with actively stabilized evanescent coupling

The evanescent coupling of light between a whispering-gallery-mode bottle microresonator and a subwavelength-diameter coupling fiber is actively stabilized by means of the Pound-Drever-Hall technique. We demonstrate the stabilization of a critically coupled resonator with a control bandwidth of 0.1 Hz, yielding a residual transmission of (9±3)×10(-3) for more than an hour. Simultaneously, the frequency of the resonator mode is actively stabilized.

Toward Automatic Label-Free Whispering Gallery Modes Biodetection with a Quantum Dot-Coated Microsphere Population

We explore a new calibration-free approach to biodetection based on whispering gallery modes (WGMs) without a reference measure and relative shifts. Thus, the requirement to keep track of the sensor position is removed, and a freely moving population of fluorophore-doped polystyrene microspheres can now fulfill this role of sensing resonator. Breaking free from fixed surface-based biosensing promotes adhesion between the microsphere sensors and the analytes since both can now be thoroughly mixed. The 70-nm-wide spectrum of green fluorescent microbeads allows us to monitor over 20 WGMs simultaneously without needing evanescent light coupling into the microspheres, hence enabling remote sensing. Since the exact radius of each microsphere is unknown a priori, it requires algorithmic analyses to obtain a reliable result for the refractive index of a solution. We first test our approach with different solutions of alcohol in water obtaining 3 × 10−4 precision on the refractive index at lower concentrations. Then, the solutions of bacterial spores in water yield clear evidence of biodetection in the statistical analysis of WGMs from 50 microspheres. To extend the fluorescence spectral range of our WGM sensors, we present preliminary results on coating microspheres with CdSe/ZnS quantum dots.

Whispering gallery mode spectra of channel waveguide coupled microspheres

Evanescent coupling of light from glass channel waveguides into the whispering gallery modes of glass microspheres of radius 15?m and 100?m is studied experimentally at wavelengths near 1550 nm. Fitting the positions, widths and heights of resonances in the experimental spectra to the characteristic equation for microsphere modes and to universal coupled microresonator theory, we establish sphere radius and index, identify mode numbers, and determine losses. The results provide detailed information for the design of optical devices incorporating microsphere resonators in planar lightwave circuits.

High-Speed Optical Interconnect Coupler Based on Soft Lithography Ribbons

In this paper, high-speed optical ribbon couplers for card-to-backplane interconnect applications are presented. The ribbon couplers are based on evanescent coupling between flexible multimode waveguide arrays. A soft lithographic technique is utilized to fabricate the ribbons. A flexible nonterminating optical data bus has been developed. Using BeamPROP software, we simulated the evanescent light coupling between two closely spaced ribbon waveguides to study the effects of waveguides separation, interaction length, and misalignment on coupling efficiency. Further experimental analysis and tests have been performed to quantify these effects. To investigate data transmission performance, a 12-channel optical interconnect link has been assembled. Experimental results demonstrated successful evanescent coupling; facilitating auto alignment coupling between card and backplane ribbon waveguides at data speeds as high as 10 Gb/s per channel. The evident high-speed interconnect performance and rapid ribbon prototyping approach can result in overall lower cost coupler fabrication for prospective optical interconnect applications.