Coupled-mode Analysis Research Articles

Penrose型金属開口アレイによる共鳴透過のサイズ効果と開口配置依存性

Extraordinary optical transmission EOT through Penrose metal hole arrays PMHA in the terahertz region is investigated numerically by using coupled-mode analysis. Spectral change of the transmittance is studied at resonant frequencies by increasing the number of holes and comparing the spectra of PMHA and a periodic metal hole arrays MHA. While the resonant mode of MHA is excited in periodic order along one direction, the resonant mode on PMHA has a complicated shape. It seems that the propagating length of surface wave of PMHA is shorter than that of the MHA. Therefore, when the number of holes is increased, increasing rate of the resonant transmittance through PMHA is smaller than that through MHA. In addition, the transmission spectra for three kinds of generalized PHMA are analyzed numerically. Although the distributions of holes in these PMHA are different from each other in real space, the transmission spectra have almost the same shape. Consequently, the structure given by the aperture configuration of PMHA in reciprocal lattice space has stronger effects on EOT through metal hole arrays, rather than that in real space.

Numerical Analysis of Mode Coupling in Multimode Graded Index Optical Fibers with Bending

The palatability of food depends on various factors including the five human senses. The palatability of beverages can now be objectively and quantitatively evaluated owing to the development of various sensing devices including taste sensors. In this study, we carried out a sensory evaluation and physicochemical analyses using three sensing devices (a taste sensor, a creep meter, and a colorimeter) to objectively evaluate the palatability of foods simmered by different heating methods. The samples used for the evaluation were selected from daikon radishes simmered in large quantities using professional heating appliances. The results of the sensory evaluation were in good agreement with those of the analysis using each sensing device. It was clarified that the palatability, taste, texture, and color of the simmered food (daikon) placed at different positions (outer and middle regions of a pan) were affected by the heating method.

Numerical Analysis of Mode Coupling in Multimode Graded Index Optical Fibers with Bending

Numerical Analysis of Mode Coupling in Multimode Graded Index Optical Fibers with Bending

Enhancing optical isolator performance in nonreciprocal waveguide arrays.

We investigate the operation of optical isolators based on magneto-optics waveguide arrays beyond the coupled mode analysis. Semi-vectorial beam propagation simulations demonstrate that evanescent tail coupling and the effects of radiation are responsible for degrading the device's performance. Our analysis suggests that these effects can be mitigated when the array size is scaled up. In addition, we propose the use of radiation blockers in order to offset some of these effects, and we show that they provide a dramatic improvement in performance. Finally, we also study the robustness of the system with respect to fabrication tolerances using the coupled mode theory. We show that small, random variations in the system's parameters tend to average out as the number of optical guiding channels increases.

<formula formulatype="inline"><tex Notation="TeX">${\hbox {Q}}$</tex></formula> Factor Analysis of Reflectarray Elements Investigating the Effects From Angle of Incidence Using Floquet Modes

A theoretical approach to investigate the effects of incidence angle on reflection properties of reflectarray elements is developed for the first time. Utilizing coupled-mode theory and Floquet analysis, the dependence of reflection coefficient on incidence angles and plane of incidence is presented in terms of Q factors. A unit cell excited by a plane wave at different incidence angles is shown to have varying coupling conditions and reflection properties. Emphasis is laid on elements located on planes where extremities in performance tend to occur. Anomalous phase characteristics are observed for particular polarization and incidence angles. The antenna element properties are assessed in terms of maximum reflection loss and slope of the reflection phase. A thorough analysis is performed at K a band and the results obtained are verified using full-wave simulations. Reflection coefficients over a 749-element reflectarray aperture for a broadside radiation pattern are presented for a couple of cases and the effects of coupling conditions in conjunction with incidence angles are demonstrated. The presented theory provides explicit physical intuition and guidelines for efficient and accurate reflectarray design.

Non-Hermitian degeneracies and unidirectional reflectionless atomic lattices.

Light propagation in optical lattices of driven cold atoms exhibits non-Hermitian degeneracies when the first-order modulation amplitudes of real and imaginary parts of the probe susceptibility are manipulated to be balanced. At these degeneracies, one may observe complete unidirectional reflectionless light propagation. This strictly occurs with no gain and can be easily tuned and fully reversed as supported by the transfer-matrix calculations and explained via a coupled-mode analysis.

Coherent anti-Stokes Raman scattering in a high-finesse microcavity.

We report the measurement of degenerate coherent anti-Stokes Raman scattering (CARS) in a high-finesse optical microcavity, with an atmospheric gas as the nonlinear medium. Unlike the well-known bulk process in which index-dispersion compensation is required for phase-matching, efficient microcavity CARS involves a resonant coupling at the Stokes, pump, and anti-Stokes frequencies which can be described using coupled-mode analysis. We show how the interaction is thereby dramatically enhanced in a microscopic sample volume and illustrate the technique for the measurement of CO₂ in air.

Nonlinear rheology of dense colloidal systems with short-ranged attraction: A mode-coupling theory analysis

The nonlinear rheology of glass-forming colloidal suspensions with short-ranged attractions is discussed within the integration-through transients framework combined with the mode-coupling theory of the glass transition. Calculations are based on the square-well system, as a model for colloid-polymer mixtures. The high-density regime featuring reentrant melting of the glass upon increasing the attraction strength and the crossover from repulsive glasses formed at weak attraction to attractive glasses formed at strong attraction are discussed. Flow curves are found in qualitative agreement with experimental data, featuring a strong increase in the yield stress and for suitable interaction parameters, the crossover between two yield stresses. The yield strain, defined as the position of the stress overshoot under startup flow, is found to be proportional to the attraction range for strong attraction. At weak and intermediate attraction strength, the combined effects of hard-core caging and attraction-driven bonding result in a richer dependence on the parameters. The first normal-stress difference exhibits a weaker dependence on short-ranged attractions as the shear stress, since the latter is more sensitive than the short-wavelength features of the static structure.

Characterization of Distributed Bragg Reflectors

We propose and demonstrate a novel method to characterize the coupling coefficient and loss coefficient of a distributed Bragg reflector (DBR). The method is based on a coupled-mode analysis of periodic structures, and involves the linewidth comparison of a pair of DBR Fabry-Perot resonators. The method is shown to be independent of coupling efficiency and wavelength dependence of the DBR parameters.

Three-dimensional coupled mode analysis of internal-wave acoustic ducts.

A fully three-dimensional coupled mode approach is used in this paper to describe the physics of low frequency acoustic signals propagating through a train of internal waves at an arbitrary azimuth. A three layer model of the shallow water waveguide is employed for studying the properties of normal modes and their coupled interaction due to the presence of nonlinear internal waves. Using a robust wave number integration technique for Fourier transform computation and a direct global matrix approach, an accurate three-dimensional coupled mode full field solution is obtained for the tonal signal propagation through straight and parallel internal waves. This approach provides accurate results for arbitrary azimuth and includes the effects of backscattering. This enables one to provide an azimuthal analysis of acoustic propagation and separate the effects of mode coupled transparent resonance, horizontal reflection and refraction, the horizontal Lloyd's mirror, horizontal ducting and anti-ducting, and horizontal tunneling and secondary ducting.

Coupled-mode analysis of contra-directional coupling between two asymmetric photonic crystal waveguides

A self-contained coupled-mode theory for the coupled two asymmetric photonic crystal waveguides (PCWs) is presented. The first-order coupled-mode equations are derived under a weak coupling assumption. The coupling coefficients are obtained systematically by a matrix calculus using the modal solutions of each PCW in isolation. The coupled-mode equations are solved for contra-directional coupling between two asymmetric PCWs formed by a hexagonal lattice of circular air holes in a dielectric medium. The power transmission spectra at different output ports of the coupled PCWs are investigated. It is shown that the self-contained coupled-mode analysis is useful to characterize a peculiar feature of the contra-directionally coupled PCWs as a drop filter.

Mode-coupling analysis of residual stresses in colloidal glasses.

We present results from computer simulation and mode-coupling theory of the glass transition for the nonequilibrium relaxation of stresses in a colloidal glass former after the cessation of shear flow. In the ideal glass, persistent residual stresses are found that depend on the flow history. The partial decay of stresses from the steady state to this residual stress is governed by the previous shear rate. We rationalize this observation in a schematic model of mode-coupling theory. The results from Brownian-dynamics simulations of a glassy two-dimensional hard-disk system are in qualitative agreement with the predictions of the theory.

Circular dichroism of four-wave mixing in nonlinear metamaterials

Metamaterial engineering offers a route to combine unusual and interesting optical phenomena in ways that are rare or nonexistent in nature. As an exploration of this wide parameter space, we experimentally demonstrate strong cross-phase modulation and four-wave mixing in a chiral metamaterial, highlighting the interplay of nonlinearity and circular dichroism. Furthermore, we show that the magnitude of the nonlinear parametric interaction follows certain selection rules regarding the circular polarizations of the various interacting waves. Using a coupled-mode analysis and finite element simulations, we relate these selection rules to the metamaterial's internal symmetries as well as its circular dichroism in the linear regime.

Coupled-mode analysis for linear fiber Bragg grating made of metamaterial

We derive the coupled-mode equations for a metamaterial fiber Bragg gratings (FBGs). The coupled-mode theory describing wave propagation in periodic structures such as FBG is generalized for studying the transmission and reflection of electromagnetic waves through a FBG containing metamaterial layers. This approach shows that the band gap becomes broader in comparison with a conventional FBG. It is observed that the central frequency of the band gap has a shift in regard of changing metamaterial thickness. We verify this investigation through attaining the transmission and reflection spectra of a conventional FBG while we are setting related parameters. Furthermore, this proposed structure can be applied as a tunable filter.

Evidence of two distinct local structures of water from ambient to supercooled conditions

The liquid and supercooled states of water show a series of anomalies whose nature is debated. A key role is attributed to the formation of structural aggregates induced by critical phenomena occurring deep in the supercooled region; the nature of the water anomalies and of the hidden critical processes remains elusive. Here we report a time-resolved optical Kerr effect investigation of the vibrational dynamics and relaxation processes in supercooled bulk water. The experiment measures the water intermolecular vibrations and the structural relaxation process in an extended temperature range, and with unprecedented data quality. A mode-coupling analysis of the experimental data enables to characterize the intermolecular vibrational modes and their interplay with the structural relaxation process. The results bring evidence of the coexistence of two local configurations, which are interpreted as high-density and low-density water forms, with an increasing weight of the latter at low temperatures.

Extraordinary optical transmission through incommensurate metal hole arrays in the terahertz region

By using coupled-mode analysis, we investigate numerically the extraordinary optical transmission (EOT) of incommensurate metal hole arrays (IMHAs) in the terahertz region to study how the degree of long-range and short-range orders affect EOT. In IMHAs, the holes of one set of metal hole arrays (MHAs) are regarded to work as impurities to the holes of another set of MHAs. Therefore, the transmittance spectra are expected to be significantly modified from those of the constituent MHAs. It is found that the resonance transmission frequencies of IMHAs almost coincide with those of the composite MHAs despite the fact that the surface waves of each MHA are mutually perturbed strongly due to the presence of short-range disorder. This indicates that the presence of long-range order is essential to establish EOT. These observations allow more flexible design of narrow bandpass filters in the terahertz region.

Can carbon nanotube fibers achieve the ultimate conductivity?—Coupled-mode analysis for electron transport through the carbon nanotube contact

Recent measurements of carbon nanotube (CNT) fibers electrical conductivity still show the values lower than that of individual CNTs, by about one magnitude order. The imperfections of manufacturing process and constituent components are described as culprits. What if every segment is made perfect? In this work, we study the quantum conductance through the parallel junction of flawless armchair CNTs using tight-binding method in conjunction with non-equilibrium Green's function approach. Short-range oscillations within the long-range oscillations as well as decaying envelopes are all observed in the computed Fermi-level (low bias) conductance as a function of contact length, L. The propagation of CNTs' Bloch waves is cast in the coupled-mode formalism and helps to reveal the quantum interference nature of various behaviors of conductance. Our analysis shows that the Bloch waves at the Fermi-level propagate through a parallel junction without reflection only at an optimal value of contact length. For quite a long junction, however, the conductance at the Fermi level diminishes due to the perturbation of periodic potential field of close-packed CNTs. Thus, a macroscopic fiber, containing an infinite number of junctions, forms a filter that permits passage of electrons with specific wave vectors, and these wave vectors are determined by the collection of all the junction lengths. We also argue that the energy gap introduced by long junctions can be overcome by small voltage (∼0.04 V) across the whole fiber. Overall, developing long individual all-armchair metallic CNTs serves as a promising way to the manufacture of high-conductivity fibers.

Multicore Photonic Crystal Fiber Based 1 × 8 Two-Dimensional Intensity Splitters/Couplers

This study uses the beam propagation method to propose a new design of multicore photonic crystal fiber-based 1 × 8 two-dimensional splitters. An optical signal at a wavelength of 1.55 μm inserted into the central core is equally divided into eight other cores, each with 12.5% of the total power. In addition, the physical behavior of the coupling characteristics is obtained using coupled mode analysis. Numerical simulations demonstrate that the optical signal can be divided equally in a photonic crystal fiber structure having dimensions of 60 μm × 60 μm × 5.35 mm.

Bloch surface eigenstates within the radiation continuum

From detailed numerical calculations, we demonstrate that in simple photonic crystal structures, a discrete number of Bloch surface-localized eigenstates can exist inside the continuum of free-space modes. Coupling to the free space causes the surface modes to leak, but the forward and back-reflected leakage may interfere destructively to create a perfectly bound surface state with zero leakage. We perform analytical temporal coupled-mode theory analysis to show the generality of such phenomenon and its robustness from variations of system parameters. Periodicity, time-reversal invariance, two-fold rotational symmetry and a perfectly reflecting boundary are necessary for these unique states. Inside the periodic structure of a photonic crystal, photons behave analogously to electrons in solid-state materials. Localized light patterns can be found at the interface between a photonic crystal and the surrounding air. Such surface states can exist even when the light has a possibility of escaping into the air, as Marin Soljacic and co-workers at the Massachusetts Institute of Technology and Harvard University, USA, now show in a theoretical study. They demonstrate that under certain conditions, different leakage channels from the crystal surface interfere destructively and thus cancel each other completely. These findings should help in the design of photonic-crystal structures for applications such as sensing and spectroscopy, where strongly localized light states are desired.

Mode-coupling analysis and trench design for large-mode-area low-cross-talk multicore fiber

The complete analytical solutions of the mode-coupling dynamics for seven-core multicore fibers (MCFs) with identical cores is proposed. All the coupling coefficients C(mn) of adjacent cores and nonadjacent cores as a function of the structural parameters are investigated. It is shown that the coupling coefficients can decrease by adjusting the structural parameters. In addition, the trench-assisted structure could be used for suppressing cross talk in MCF, and the effective area A(eff) can be enlarged without degrading the crosstalk properties. Simulations suggest that low cross talk and/or large A(eff) could be achieved by adjusting the trench parameters. Large mode area could be obtained by utilizing small trench (small trench width c, small refractive index difference Δ(trench)) in the trench-assisted MCF (TA-MCF), whereas low cross talk could be achieved by utilizing larger trench (large trench width c, large Δ(trench)) in the TA-MCF.