Microresonator Structure Research Articles

Transmission and Reflection Spectra of a Bragg Microcavity Filled with a Periodic Graphene-Containing Structure

The transmission and reflection spectra of a one-dimensional microresonator structure with dielectric Bragg mirrors, the working cavity of which is filled with several “dielectric-graphene” or “semiconductor-graphene” periods with controlled material parameters, were obtained using transfer matrices and numerical methods. Carrier drift in graphene monolayers is created to achieve amplification, which makes it possible to use the hydrodynamic approximation to represent graphene conductivity in the terahertz range. The transformation of spectra is achieved both by changing the energy state of the graphene monolayers and by changing the external magnetic field. It is shown that amplification is observed in the region where the real part of the conductivity is negative as the chemical potential (Fermi energy) increases, and the coefficients T and R become substantially greater than unity. The results of the work may be of interest to developers of graphene-based controlled photonic devices.

Spectra of a Bragg Microresonator Filled with a Graphene-Containing Medium

Transmission spectra of a symmetric microresonator structure, with dielectric Bragg mirrors, are obtained. The working cavity of the structure is partially filled by a layer of a quarter-wave thickness of finely layered “graphene–semiconductor” medium, with material parameters controlled by external electric and magnetic fields. It is shown, that the transformation of the spectra is achieved both by changing the energy state of the graphene layers and by changing the external magnetic field. The spectral characteristics for the inverted and doped states of graphene layers are established.

Comprehensive Numerical Analysis of Temperature Sensitivity of Spherical Microresonators Based on Silica and Soft Glasses

In recent years, the use of optical methods for temperature measurements has been attracting increased attention. High-performance miniature sensors can be based on glass microspheres with whispering gallery modes (WGMs), as their resonant frequencies shift in response to the ambient parameter variations. In this work, we present a systematic comprehensive numerical analysis of temperature microsensors with a realistic design based on standard silica fibers, as well as commercially available special soft glass fibers (GeO2, tellurite, As2S3, and As2Se3). Possible experimental implementation and some practical recommendations are discussed in detail. We developed a realistic numerical model that takes into account the spectral and temperature dependence of basic glass characteristics in a wide parameter range. To the best of our knowledge, spherical temperature microsensors based on the majority of the considered glass fibers have been investigated for the first time. The highest sensitivity dλ/dT was obtained for the chalcogenide As2Se3 and As2S3 microspheres: for measurements at room temperature conditions at a wavelength of λ = 1.55 μm, it was as high as 57 pm/K and 36 pm/K, correspondingly, which is several times larger than for common silica glass (9.4 pm/K). Importantly, dλ/dT was almost independent of microresonator size, WGM polarization and structure; this is a practically crucial feature showing the robustness of the sensing devices of the proposed design.

Photon Spectra of a Bragg Microresonator with Bigyrotropic Filling

In this article, we have obtained the transmission spectra of a microresonator structure with Bragg mirrors, the working cavity of which is filled with a magnetically active finely layered ferrite-semiconductor structure with material parameters controlled by an external magnetic field. It is shown that a change in the external field and the size of the cavity (filling layer thickness) provokes a controlled rearrangement of the transmission spectrum of TM and TE waves. The polarization characteristics of the microcavity, their dependence on the external field, and the ratio of the thicknesses of the layers that make up the period of the ferrite-semiconductor structure are investigated.

Modeling of luminescence spectra in spherical microresonators with an emitting shell

The luminescence spectra of a microresonator structure consisting of a spherical core of small diameter (3.5-6 mcm) covered with a luminescent shell with a refractive index less than that of the core are modeled. Shell luminescence spectra, radial distribution of the whispering gallery mode (WGM) field, and mode parameters (wavelength, width, quality factor) are calculated using the expansion of the electromagnetic wave field in the basis of vector spherical harmonics and the method of spherical wave transfer matrices. The dependence of the luminescence spectra and WGM parameters on the geometric and optical parameters of the structure --- the shell thickness, the refractive index of the shell, and the core diameter --- is studied. Keywords: spherical microresonator, luminescent shell, whispering gallery modes, modeling of luminescence spectra.

A compound optical microresonator design for self-referencing and multiplexed refractive index sensing

We propose a new type of self-referencing and multiplexed refractive index (RI) sensor based on a compound optical microresonator structure consisting of Fabry-Pérot (FP) resonators coupled with microring resonators. The transmission spectra shows resonant features that are superimposed on a background defined by FP oscillations. The resonances have asymmetric Fano-like non-Lorentzian shapes, which are used as sensing peaks, while the FP oscillations are used as reference peaks for internal self-referencing. The sensing peaks shift linearly with the increased RI of the cladding in the microring resonator, while FP peaks stay constant. When the temperature is increased, both the FP peaks and the Fano resonances shift linearly at the same rate, which eliminates the temperature effect on RI measurements. We theoretically analyzed that the two-mirror FP resonator coupled with a single microring resonator and optimized its sensing performance through finite-difference time-domain simulations. A sensitivity value of 220 nm/RIU and a maximum figure of merit of 4400 RIU-1 were achieved. We also proposed two possible multiplexing schemes consisting of two-mirror and three-mirror FP resonators coupled with two microring resonators of different radii. The proposed sensor concept is simple, easy-to-fabricate, self-calibrating and can be used for simultaneous measurements of different samples.

Control of Photonic Spectrum of a Bragg Microresonator with a Magnetoactive Layer

Transmission spectra of a microresonator structure with Bragg mirrors are obtained. A quarter-wave plate made of extrinsic semiconductor is installed in the working cavity. The properties of such a semiconductor can be controlled using external magnetic field. It is shown that variations in the external field, cavity size, and position of the active layer lead to modification of the photonic spectrum of the TM and TE eigenwaves. Splitting of a defect mode at the center of the band gap into two modes takes place when the active layer is placed in the cavity. The evolution of such modes caused by variations in the magnetic field is studied. Dependences of the size and number of stopbands on the structural parameters, frequency, and field are determined, and maps of band gaps are obtained.

Моделирование спектров люминесценции в сферических микрорезонаторах с излучающей оболочкой

The luminescence spectra of a microresonator structure consisting of a spherical core of small diameter (3.5 – 6 mcm) covered with a luminescent shell with a refractive index less than that of the core are modeled. Shell luminescence spectra, radial distribution of the whispering gallery mode (WGM) field, and mode parameters (wavelength, width, quality factor) are calculated using the expansion of the electromagnetic wave field in the basis of vector spherical harmonics and the method of spherical wave transfer matrices. The dependence of the luminescence spectra and WGM parameters on the geometric and optical parameters of the structure - the shell thickness, the refractive index of the shell, and the core diameter - is studied.

Quartz-Enhanced Photoacoustic Spectroscopy Based on the Four-Off-Beam Acoustic Micro-Resonator

A four-off-beam acoustic micro-resonator (AmR) structure is proposed to improve the photoacoustic signal intensity in quartz-enhanced photoacoustic spectroscopy (QEPAS) for trace gas sensing. Two vertical-array-based double-off-beam AmR structures are arranged on each side of the quartz tuning fork (QTF) horizontally to construct a four-off-beam AmR structure using a rectangular prism. This QEPAS scheme based on the four-off-beam AmR is evaluated in the water-vapor detection of air at normal atmospheric pressure and room temperature (27 °C). Comparing with the bare QTF scheme, the new QEPAS configuration enhances the photoacoustic spectroscopy signal by a factor of 25.84 times, achieving a minimum detection limit of 278 ppbv with a normalized noise equivalent absorption coefficient of 1.27 × 10−9 W/Hz1/2 in water vapour detection at a wavelength of 1368.597 nm.

Characterization of Continuous Wave Laser-Induced Thermal Gradients in Magnetic Tunnel Junctions Integrated Into Microresonators via COMSOL Simulations

Spin caloritronics investigates static and dynamic effects on magnetic structures due to spin-currents generated by thermal gradients. Here, we present COMSOL simulation results using a 2-D heat transfer module applied to Co2FeAl/MgO/CoFeB magnetic tunnel junctions (MTJs) integrated into microcavity resonators. Microresonators are used in order to study the effects of temperature gradients on single micro-/nano-objects. We find that the thermal conductivity of the insulating barrier (MgO) plays a crucial role, influencing the overall temperature, as well as the thermal gradient over the barrier. Taking into account the microresonator structure around the MTJ, which is mainly made from copper, strongly affects the uniform heating of the overall stack. Nevertheless, the gradient over the barrier is relatively unaffected by the surrounding conditions. The simulation results provide insight into the temperature profile of the whole structure and show how modifying the structure of the surrounding materials may tune and optimize the thermal gradient magnitude and ultimately provide a path for quantifying spin-transfer torques induced by thermal gradients.

Electric field sensing with liquid-crystal-filled slot waveguide microring resonators.

We consider the operation principles of the sensor of an external electric field on the bases of microring resonators that consist of bent-strip waveguides with vertical or horizontal slots filled with nematic liquid crystal. The mode-field distribution and dispersion parameters of the bent-slot waveguides are calculated by using the algorithm based on the method of lines. The influence of the waveguide and microresonator structure (slot width, position and orientation, microresonator radius, etc.) on the sensor sensitivity is analyzed.

Realization of mode independent multichannel transmission filter by controlling the photon localization in symmetric cavities

Modern optical communication system requires multiple channel transmission in different spectral band for different applications. Here we are proposing a microresonator structure for closely spaced, mode independent multiple channel transmission in 3rd transmission window by controlling the localization of photons in its cavity. To design the proposed structure, the spectral filtering properties of a single cavity planar microresonator are studied initially. In conventional multilayer dielectric filter, the localization of light in the microcavity is generally controlled by controlling the number of layers. Here we have shown that application of electric field across the structure, designed with suitable materials can also affect the localization of light in the cavity as well as the Q values of the transmission spectra. The mode independent characteristic of the structure is found from the study of its transmittance characteristics at different angle of incidence of light. It is found that the results of single cavity structure hold well for multicavity resonator structure also, in the regime of optical communication wavelength range. A method to increase the number of transmission channels is also proposed here keeping the fabrication perspective in mind.

Patternable polymer microstructure for optical biosensing

Introduction: Applications of rapid sensing and detection of biological analytes are growing due to the significant environmental monitoring, health screening, cell growth and bio/chemical sensors. These factors influence the research and development on simple, cheap and sensitive functional biomolecular device. The interest in the label-free optical detection has been increased as the optical waveguide has a direct light interaction with surrounding analytes, easy integration with microfluidic system and the capability to provide specific interaction. Methods: In this work, we demonstrate the potential of microstructure as an optical biosensor. Visible wavelength is utilized because it is commonly used in biological and chemical sensing for both label and label-free sensing. The SU8 polymer microstructures waveguides were fabricated on 3.5 µm oxide. The microstructures are simulated using COMSOL Multiphysics. Simulation was recorded based on refractive index that mimic the bioanalytes solution and biological binding. Then the experimental setup is developed to control the optical component and manipulate the liquid samples. The fabricated devices were characterized by using the end-facet technique. Meanwhile, microfluidic channel system was also constructed in order to inject the liquid sample into the sensor surface. Results: The wavelength shift for microresonator structure approximately 41.2 nm for 0.1 increments of surrounding refractive index. The experiments demonstrate that the shift occurred approximately 22.5 nm. In other hand, simulated Bragg grating structure gained the shift at 63.2 nm. Meanwhile, the experimental results achieve approximately 20.3 nm. Conclusions: Thus, both simulation and experimental results strongly indicate that patternable polymer microstructure has a huge potential for optical biosensing applications at visible region.

Amplitude and phase transmission characteristics of parallel-coupled dual racetrack silicon microresonator structure

Parallel-coupled dual racetrack silicon microresonator structures are fabricated and characterized. With an integrated Mach–Zehnder interferometer, the full information of amplitude and phase of the structure is obtained experimentally. The spectral characteristics of the amplitude and phase are shown to be in reasonable agreement with simulation results, considering possible small structure variations in fabrication. The structure is potentially useful for developing modulators for advanced modulation formats.

Tunable frequency combs based on dual microring resonators.

In order to achieve efficient parametric frequency comb generation in microresonators, external control of coupling between the cavity and the bus waveguide is necessary. However, for passive monolithically integrated structures, the coupling gap is fixed and cannot be externally controlled, making tuning the coupling inherently challenging. We design a dual-cavity coupled microresonator structure in which tuning one ring resonance frequency induces a change in the overall cavity coupling condition. We demonstrate wide extinction tunability with high efficiency by engineering the ring coupling conditions. Additionally, we note a distinct dispersion tunability resulting from coupling two cavities of slightly different path lengths, and present a new method of modal dispersion engineering. Our fabricated devices consist of two coupled high quality factor silicon nitride microresonators, where the extinction ratio of the resonances can be controlled using integrated microheaters. Using this extinction tunability, we optimize comb generation efficiency as well as provide tunability for avoiding higher-order mode-crossings, known for degrading comb generation. The device is able to provide a 110-fold improvement in the comb generation efficiency. Finally, we demonstrate open eye diagrams using low-noise phase-locked comb lines as a wavelength-division multiplexing channel.

A primal/dual approach for the accurate evaluation of the electromechanical coupling in MEMS

This paper presents a primal/dual approach to solve the coupled electromechanical problem arising in the modelling of electrostatically actuated micro-electromechanical systems (MEMS). After a derivation of complementary energy functionals for both the mechanical and the electrical problems, an original coupling strategy using a displacement-based (primal) mechanical formulation and an electric vector potential-based (dual) electrostatic formulation are elaborated. A derivation of the electric force and the tangent stiffness matrix of the coupled problem is obtained, based on the virtual work principle. The new formulation has interesting properties with respect to error estimation and pull-in voltage calculation, which is highlighted on the finite element solution of a micro-resonator structure.

Parallel-coupled dual racetrack silicon micro-resonators for quadrature amplitude modulation

A parallel-coupled dual racetrack silicon micro-resonator structure is proposed and analyzed for M-ary quadrature amplitude modulation. The over-coupled, critically coupled, and under-coupled scenarios are systematically studied. Simulations indicate that only the over-coupled structures can generate arbitrary M-ary quadrature signals. Analytic study shows that the large dynamic range of amplitude and phase of a modulated over-coupled structure stems from the strong cross-coupling between two resonators, which can be understood through a delicate balance between the direct sum and the "interaction" terms. Potential asymmetries in the coupling constants and quality factors of the resonators are systematically studied. Compensations for these asymmetries by phase adjustment are shown feasible.

Mathematical simulation of self-oscillations in a fiber optic laser with a microresonator reflector

In this paper we obtain approximate equations that describe the dynamics of a fiber optic laser with an external reflector, constructed on the basis of microresonance structures under inhomogeneous pumping conditions. The results demonstrate the possibility of the existence of selfmodulation modes with frequencies equal to the frequency of natural vibrations of the microresonator structure. The parameter ranges in which self-oscillations are possible are calculated.

Coupling Whispering-Gallery-Mode Microcavities With Modal Coupling Mechanism

We theoretically describe and study coupling between two whispering-gallery-mode microcavities through a side coupled fiber taper. The two microcavities can couple to each other bidirectionally with only one fiber taper waveguide due to the modal coupling between a pair of whispering-gallery modes traveling in opposite directions. By choosing appropriate physical parameters, some interesting spectra, such as electromagnetically-induced transparency-like coherent transmission and band-like reflection, can be obtained in such a system. Slow light is discussed in this paper as a direct application of the coupled microresonator structure. This structure also has potential application in coupled cavity quantum electrodynamics.

Performances of a Fully Integrated All-Optical Pulse Reshaper Based on Cascaded Coupled Nonlinear Microring Resonators

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We propose a simple way to design a fully integrated and passive all-optical pulse reshaper in the aim of reamplifying and reshaping (2R) regeneration of optical digital signals. The device could be achieved from a coupled microresonator structure with Kerr nonlinearity. By using time-domain simulation, we show that we can obtain a clear nonlinear power transfer that is capable of improving signal-to-noise ratio and reducing bit error rate for digital signals. The reshaping performances are estimated by calculating <formula formulatype="inline"><tex>$Q$</tex> </formula>-factor enhancement. Since we take benefit from field enhancement at resonance, this integrated reshaper could be much smaller than other gates based on nonlinear fibers or waveguides. </para>