Effects Of Coherent Coupling Research Articles

A multilayer snowflake resonator metamaterial for low-frequency broadband sound absorption

Abstract Low-frequency broadband sound-absorbing structures with smaller thicknesses are the focus of research in noise control. Based on the principle of Helmholtz resonant cavity and the internal spacer chamber division structure, a multilayer snowflake resonator metamaterial (MLSRCM) is proposed. The MLSRCM model is investigated using theoretical and simulation methods, and it is concluded that the MLSRCM has excellent sound absorption performance in the frequency range of 450–950 Hz with a thickness of only 28 mm. This mainly originated from the parallel connection of multi-units as well as the application of the weak resonance effect of coherent coupling and the full utilization of space. The broadband sound absorption mechanism in MLSRCM is theoretically investigated by analyzing the complex frequency plane. Subsequently, the sound absorption mechanism of the structure is further analyzed in terms of sound velocity and power dissipation density. The results show that the MLSRCM has excellent low-frequency broadband acoustic absorption performance, which provides a reference for the design of new acoustic-absorbing metamaterials with small thicknesses.

Transmission Characteristics Analysis of a Twin-Waveguide Cavity

The transmission spectrum of a twin-waveguide cavity is systematically analyzed based on coupled mode theory, using the transfer matrix method (TMM). The results show that the traveling-wave transmission spectra of the twin-waveguide cavity is entirely determined by the coherent coupling effect involving the parameters of the effective refractive indices of the upper and lower waveguides, the coupling length Lc, and the ratio of the cavity length L to the coupling length (L/Lc). Filters with single, double, or triple-notch filtering could be obtained by choosing an appropriate L/Lc value. When the facet reflection is taken into consideration, the traveling-wave transmission spectrum is modified by the Fabry––Perot (FP) resonance, making it a standing-wave transmission spectrum. As a result, resonance splitting has been observed in the transmission spectrum of twin-waveguide resonators with high facet reflectivity. Further analysis shows that such an abnormal resonance phenomenon can be attributed to the destructive interference between the two FP resonance modes of the upper and lower waveguide through coherent coupling. In addition, narrow bandwidth amplification has also been observed through asymmetric facet reflections. Undoubtedly, all these unique spectral characteristics should be beneficial to the twin-waveguide cavity, achieving many more functions and being widely used in photonic integration circuits (PICs).

Broadband quasi-perfect sound absorption by a metasurface with coupled resonators at both low- and high-amplitude excitations

This study presents an acoustic metasurface (AM) consisting of coupled Helmholtz resonators with extended necks (HRENs) to effectively absorb broadband sound waves at both low and high amplitudes. A nonlinear acoustic model is proposed to characterize the AM performance based on the equivalent medium theory and the transfer matrix method. The nonlinear effects due to the high-amplitude excitation are considered by a velocity-dependence equivalent density of the air in the neck. The acoustic characteristics of an AM with two coupled HREN units at the sound pressure level (SPL) ranging from 100dB to 140dB are analytically, numerically, and experimentally investigated. Results from the three approaches show good agreement. The AM achieves quasi-perfect absorption (absorption coefficient α>0.9) in a wide range of SPL, while each HREN unit alone only performs well at around 130dB. The superior absorption performance of the AM at relatively low-amplitude excitation originates from the coherent coupling effects between adjacent units. With the increase of the sound level, the coupling effects tend to deteriorate; fortunately, the sound-vortex conversions occur, contributing to additional acoustic dissipation and thereby sustaining the quasi-perfect absorption capability. Based on the absorption mechanisms, a broadband AM with multiple coupled resonators is designed and manufactured. The measured averaged absorption coefficient of the AM is larger than 0.91 at the SPL below 140 dB (α>0.97 at 100dB) within [700,1100]Hz, and the compact thickness is of 1/18 wavelength at the lowest operating frequency. The findings in this study offer a promising approach to achieve highly efficient broadband sound absorption in a wide-ranging noise level, which holds great potentials in practical applications for noise control.

Broadband low-frequency sound-absorbing metastructure based on an impedance-matching coiled-up cavity with continuously variable cross section

Proposed here is a metastructure based on a micro-perforated panel and an impedance-matching coiled-up cavity with continuously variable cross section, which achieves perfect absorption with a resonant frequency of 496 Hz and an absolute bandwidth (α ≥ 0.5) of 468 Hz. The structure thickness is ca. 1/13 of the operating wavelength λ in the deep subwavelength range. A relative bandwidth of 84.04%–111.67% is achieved through parametric studies. Physically, the continuous variation of the cavity cross section through which sound waves enter weakens the acoustic reflections generated by cross-sectional abruptness and enhances the impedance matching with the air. Furthermore, particle swarm optimization is coupled with a theoretical model to tailor the metastructure to realize the maximum absorption coefficient in the defined frequency range. It is shown theoretically that coherent coupling “weak resonance”—in which each unit individually exhibits imperfect absorption peaks—significantly improves the absorption performance in a broad frequency band through the coherent coupling effect. Finally, a hybrid metastructure using a parallel coupling sample is fabricated, and its acoustic properties are measured in an impedance tube. The average absorption coefficient of this metastructure is 0.934 in the quasi-perfect band (α > 0.9) from 400 to 650 Hz, and the thickness is only ca. λ/15. The unique innovation of a cavity with continuously variable cross section provides new ideas for designing broadband low-frequency sound-absorbing metastructures.

Cooperative-effect-induced one-way steering in open cavity magnonics

We propose to generate and control stationary one-way steering with strong entanglement between photon and magnon modes by the cooperative effect of coherent coupling and dissipative coupling. Due to the combination of two couplings, the system becomes a parity-time-like symmetric non-Hermitian system, and two exceptional points (EPs)-like appear in the real and imaginary parts of the eigenvalues. We demonstrate that the especially obvious quantum entanglement and perfect one-way steering can be obtained around two EPs-like. The continuous variable entanglement and steering produced by this cooperative effect show stronger robustness to environment temperature and system dissipation than that induced by nonlinearity. The one-way steering directivity can be controlled by the relative phase of cooperative dissipation and the frequency detuning of the magnon mode. Our work shows the controllability advantage of the open cavity magnonic system and may open up a platform for the generation of stationary one-way steering.

Acoustic metamaterial for highly efficient low-frequency impedance modulation by extensible design

We propose a novel class of acoustic metamaterial for low-frequency broadband sound absorption with high efficiency. The design principle is physically decoupled into low-frequency mitigation and working frequency widening. The embedded tubes with phase shift are employed to alleviate low-frequency noise, achieving a quasi-perfect absorption (α>0.9) at 240 Hz with the peak value of 0.998. The spider-web-like configuration brings the high-efficiency acoustic impedance modulation via manipulating the coherent coupling effect, and allows for constantly broadening the effective working frequency in lower frequency domain by taming the resonant characteristics and the scale ratio of original and extended units. The theoretical and numerical methods are greatly validated by measured data. We further demonstrate the broadband potentials of the metamaterial, with a continuous quasi-perfect absorption from 301 Hz to 534 Hz (i.e., 1/22 of the operating wavelength) achieved from analytical results. The underlying physics is explained via coherent coupling, acoustic impedance matching, and system damping in detail. All in all, our work provides a viable and effective scheme in overcoming extremely low-frequency broadband noise for lightweight structures.

Numerical computation for rogue waves in the coupled nonlinear Schrödinger equations with the coherent coupling effect

ABSTRACT We report some efficient and accurate numerical experiments to simulate the vector rogue waves of the coupled nonlinear Schrödinger equations with the coherent coupling coefficients, which describe the propagation of orthogonally polarized optical pulses in an isotropic medium. We develop the time-splitting finite difference/spectral methods and exponential wave integrator spectral methods to compute the vector rogue-wave solutions. Because of the slowly decaying ratios at far field of the rogue waves with nonzero backgrounds, the truncation errors of numerical solutions are analysed numerically. A small computation interval of errors is considered to efficiently simulate rogue waves and compare different numerical methods. We find that the homogeneous Neumann boundary condition is better than periodic boundary condition, and the latter is often used by researchers to simulate rogue waves. The time-splitting cosine spectral method has the highest accuracy among the above methods for simulating the rogue waves. The stability of rogue waves is numerically analysed quantitatively and qualitatively. The vector rogue-wave solutions are always unstable under small perturbations, while such rogue wave dynamics may have a certain degree of stability that it has potential to be observed in experiments.

Low-Drift Closed-Loop Fiber Optic Gyroscope of High Scale Factor Stability Driven by Laser With External Phase Modulation

In view of the poor scale factor stability of the interferometric fiber optic gyroscope (IFOG), it is a creative method to use laser to drive the IFOG for its better frequency stabilization characteristics instead of the broadband light source. As the linewidth of laser is narrow, the errors of coherent backscattering, polarization coupling, and Kerr effect are reintroduced which cause more noise and drift. This paper studies laser spectrum broadening based on external phase modulation of Gaussian white noise (GWN). The theoretical analysis and test results indicate that this method has a good effect on spectrum broadening and can be used to improve the performance of the laser-driven IFOG. In the established closed-loop IFOG, a four-state modulation (FSM) is adopted to avoid temperature instability of the multifunction integrated-optic chip (MIOC) and drift caused by the electronic circuit in demodulation. The experimental results show that the IFOG driven by broadened laser has the angular random walk noise of 0.003 8 °/√h and the drift of 0.017 °/h, which are 62% and 66% better than those without modulation respectively, of which the drift has reached the level of the broadband light source. Although the noise still needs further reduction, its scale factor stability is 0.38 ppm, which has an overwhelming advantage compared with the traditional IFOG.

Machine learning-assisted low-frequency and broadband sound absorber with coherently coupled weak resonances

An artificial broadband sound absorber composed of multiple components is of significant interest in the physics and engineering communities. The existence of coherently coupled weak resonances (CCWRs) makes it difficult to achieve optimal broadband sound absorption, especially in the presence of complex and aperiodic components. Here, we present and experimentally implement a machine learning-assisted subwavelength sound absorber with CCWRs using an improved Gauss–Bayesian model, which exhibits flexible, high-efficient, and broadband properties at low frequencies (<500 Hz). The proposed aperiodic structure comprises three parallel split-ring units, which enable a quasi-symmetric resonant mode to be generated and effectively dissipate energy because of the huge phase difference between each component at the coupled resonant frequency. With high algorithmic efficiency (no more than 80 iterations), the improved Gauss–Bayesian model inversely determines the optimal CCWRs, realizing a reconfigurable high sound absorption spectrum (α > 0.9) from 229 to 457 Hz. The optimal configuration of sound spectrum characteristics and the unit cell structure can be confirmed flexibly. Good agreement between numerical and experimental results verifies the effectiveness of the proposed method. To further exhibit broadband and multiparameter optimization, a nine-unit sound absorber (27 parameters) is numerically simulated and shown to achieve high acoustic absorption and a relatively broad bandwidth (44.8%). Our work lifts the restrictions on analytic models of complex and aperiodic components with coherent coupling effects, paving the way for combining machine learning with the optimal design of metamaterials.

Coupling and scaling effect for low-frequency broadband sound absorption via vertex-based hierarchy

On-demand noise remediation in the low-frequency broadband region remains a challenge. We present a hierarchical sound-absorbing meta-structure (HSM) to realize the desired low-frequency broadband absorptive performance at a subwavelength thickness. The physical mechanism underlying the superior performance is revealed through the coherent coupling effect and the hierarchical scaling effect that are both tied to the hierarchical characteristics. We experimentally validate the advocated absorptive merits of HSM and the predicting results. The quasi-perfect (α>0.9) bandwidth of the first-order HSM can be remarkably improved by 219% and 363% when it upgrades to the second- and third-order, respectively. This work may pave the way of designing acoustic meta-absorbers against low-frequency noise over a wide range.

Coherent coupling of laterally coupled quantum dot lasers

Coherent coupling between two laterally coupled quantum dot semiconductor lasers based on longitudinal modes in the weak coupling regime has been investigated. Considering the effect of homogeneous and inhomogeneous broadenings, the spectral behavior of lasing modes in the coupled lasers depends on the coupling coefficient and the frequency detuning between the lasers. For cavity lengths with a minute difference and specific homogeneous broadening, full phase-locking along with single-mode operation is guaranteed provided that the coupling coefficient reaches a critical value. This result can be generalized for various cavity lengths and homogeneous broadenings covering almost all practical temperatures in the intermediate homogeneous broadening regime. The effect of coherent coupling on the emission spectrum, especially at sufficiently low temperatures at which the lasing spectrum shows broadband emission in the absence of coupling, has also been discussed. For constant cavity lengths and relatively high temperatures, onset of full phase-locking occurs at lower values of the coupling strengths, while longer cavity lengths lead to higher critical coupling strengths at a constant temperature. From a practical view, coherent coupling based on longitudinal modes has been examined in the case of a differential bias condition, which proves the electronically controllable coherent coupling of optical output signals.

Extremely high Q-factor terahertz metasurface using reconstructive coherent mode resonance.

High Q-factor resonance has a pivotal role in wide applications for manipulating electromagnetic waves. However, high Q-factor resonance, especially in the terahertz (THz) regime, has been a challenge faced by plasmonic metamaterials due to the inherent ohmic and radiation losses. Here, we theoretically present a unique metasurface scheme to produce extremely high Q-factor Fano resonance of the reconstructive coherent mode in the THz regime. The THz metasurface is composed of periodically arranged vertical symmetric split ring resonators (SRRs), which can produce perfect reconstructive coherent coupling effect in the sense that dipole radiation is destructively suppressed. Under the polarized electric field perpendicular to SRR gap, the surface currents are out of phase for an individual SRR, leading to the cancellation of net dipole moment. The reconstructive coherent mode resonance can occur between each SRR and its neighboring SRRs, accompanied by destructive interference of the scattered fields of each SRR. This is due to the coupling between the localized resonance of individual particles and the Rayleigh anomaly of the array. The proposed metasurface can significantly suppress far-field radiation and perform an extremely high Q-factor beyond 104 level with large modulation depth in the THz region, which pushes the advancement of THz high Q-factor resonance. The extremely high Q-factor of reconstructive coherent mode is tunable by adjusting the geometry parameters. The design strategy is useful to develop ultra-sensitive sensors, narrow-band filters and strong interaction of field-matter in the THz regime.

Asymmetrical, rotational and ultra-high amplitude fundamental polarized optical rogue waves associated with the coherent coupling

The dynamics of the fundamental rogue waves of the coupled nonlinear Schrödinger equations with the coherent coupling effect in the isotropic nonlinear medium is investigated. Firstly, via the Darboux-dressing transformation, we show the asymmetrical fundamental rogue waves and the rotational dynamics of fundamental rogue waves. Secondly, via the SO(2) rotation, we construct the fundamental rogue waves of ultra-high peak amplitude. With such transformation, we study the relation between ultra-high peak-background ratio and energy exchange. The fundamental rogue-wave excitation with ultra-high peak-background ratio in a chaotic background is confirmed via numerical simulations. The results discussed in this paper might contribute to the understanding of fundamental rogue wave phenomena in a variety of complex systems, from nonlinear optics to Bose-Einstein condensates.

Orthogonally polarized tunable dual-wavelength femtosecond optical parametric oscillator.

We demonstrate a femtosecond optical parametric oscillator that can generate orthogonally polarized dual-wavelength femtosecond pulses. Two periodically poled lithium niobate (PPLN) crystals with mutually orthogonal crystal axes are pumped by a single femtosecond fiber laser. The central wavelength of the two orthogonally polarized signal pulses can be continuously tuned from 1387 to 1588 nm with a maximum frequency separation of 27 THz. Because of the orthogonal dual-crystal scheme, the system is immune to the coherent coupling effect, thus overcoming the limitation of minimum frequency separation.

Compact broadband acoustic sink with coherently coupled weak resonances

Broadband sound sink/absorber via a structure with deep sub-wavelength thickness is of great and continuing interest in physics and engineering communities. An intuitive technique extensively used is to combine components (resonators) with quasi-perfect absorption to piece together a broad absorbing band, but the requirement of quasi-perfect absorption substantially places a very strict restriction on the impedance and thickness of the components. Here, we theoretically and experimentally demonstrate that a compact broadband acoustic sink that quasi-perfectly absorbs broadband arriving sound waves can be achieved with coherently coupled “weak resonances” (resonant sound absorbing systems with low absorption peaks). Although each component exhibits rather low absorption peak alone, via manipulating the coherent coupling effect among the components, they collectively provide a remarkably improved performance over a wide frequency range with a significantly compressed thickness. To illustrate the design principle, a hybrid metasurface utilizing the coaction of parallel and cascade couplings is presented, which possesses an average absorption coefficient of 0.957 in the quasi-perfect band (α>0.9) from 870 to 3224 Hz with a thickness of only 3.9 cm. Our results open new avenues for the development of novel and highly efficient acoustic absorbers against low frequency noise, and more essentially, suggest an efficient approach towards on-demand acoustic impedance engineering in broadband.

Rabi-coupled two-component Bose-Einstein condensates: Classification of the ground states, defects, and energy estimates

We classify the ground states and topological defects of two-component Bose-Einstein condensates under the effect of internal coherent Rabi coupling. We present numerical phase diagrams which show the boundaries between symmetry breaking components and various vortex patterns (triangular, square, bound state between vortices). We estimate the Rabi energy in the Thomas-Fermi limit which allows us to have an analytical description of the point energy leading to the formation of the various vortex patterns.

Coherent Coupling in High-Mesa Semiconductor Directional Coupler

The effect of coherent coupling on the performance of a directional coupler is investigated for the first time. Coherent coupling is the interference between a guided mode in a core layer and an unguided mode in a cladding layer (cladding mode) in a waveguide. It was experimentally confirmed that the cladding mode exists in an actual high-mesa semiconductor waveguide and it can be represented by a single propagation constant. The effect of coherent coupling on coupling loss and coupling efficiency in a directional coupler is analyzed using a new technique based on a matrix method. The analyzed results reveal that a small periodic oscillation of coupling loss of the transmitted light that depends on coupler length occurs owing to the coherent coupling in a strong-coupled directional coupler, and its effect should be considered when we minimize propagation loss in high-mesa semiconductor directional couplers. It is also shown that it is possible to design a directional coupler with both the desired coupling efficiency and low loss on the basis of the periodic oscillation of coupling loss.

Soliton solutions via auxiliary function method for a coherently-coupled model in the optical fiber communications

A coherently-coupled nonlinear Schrödinger system in the optical fiber communications, with the mixed self-phase modulation (SPM), cross-phase modulation (XPM) and positive coherent coupling terms, is studied through the bilinear method with an auxiliary function. Solutions for that system are found to be of two types: singular and non-singular ones, and the latter appear as the soliton-typed. Vector bright one- and two-solitons are derived with the corresponding phase-shift parameter constraints. In virtue of computerized symbolic computation and asymptotic behavior analysis, elastic collision mechanisms of such vector solitons are investigated. With the aid of graphical simulation, vector solitons are displayed to be of the single- or double-hump profiles. The formation and collision mechanisms of the vector bright solitons for that system are generated based on the combined effects of SPM, XPM and coherent coupling. Only elastic collisions of the vector solitons occur for that system, which is a distinctive feature amid those of other coherently-coupled nonlinear Schrödinger systems.

Femtosecond optical Kerr effect measurement using supercontinuum for eliminating the nonlinear coherent coupling effect

A nonlinear coherent coupling effect (NCCE) was observed in a degenerate femtosecond optical Kerr effect (OKE) measurement, which disturbed the analysis of the dynamics of the OKE response. The NCCE in the femtosecond OKE measurement was dependent on the pump power. To eliminate this unwanted influence of the NCCE, we proposed a two-color femtosecond OKE measurement method using supercontinuum, and the feasibility of this method has been demonstrated.

Vector bright soliton behaviors associated with negative coherent coupling

With the introduction of an auxiliary function, a genuine bilinear system (in contrast to the published trilinear forms) is obtained for the two-coupled nonlinear Schrödinger equations with negative coherent coupling in the optical fiber communications. With symbolic computation, degenerate and nondegenerate vector solitons are derived associated with the corresponding phase-parameter constraints. In virtue of asymptotic analysis and graphical simulation, vector solitons of the single-hump, double-hump, or flat-top profiles are displayed, and the collision mechanisms of such vector solitons are revealed as well; namely, the collisions among degenerate solitons and among nondegenerate solitons are both elastic. The only possible inelastic collision, the collision in the degenerate-nondegenerate case, is pointed out, where a degenerate soliton interacts with a nondegenerate one. Results in this paper may be useful for the optical switching with the combined effects of self-phase modulation, cross-phase modulation, and negative coherent coupling.