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Metasurfaces are sub‐wavelength nanostructures with the potential to overcome the limitations of traditional optics. Existing dielectric metasurfaces, however, are often limited to geometric primitives, which hamper their application in emergent hybrid metasurfaces. Here, a simple fabrication scheme for non‐primitive metasurfaces that addresses this limitation is introduced. Due to their broken out‐of‐plane symmetry, these nanostructures provide a new dimension in the design space. Taking inspiration from bio‐photonic systems in nature, using these elements complex hybrid metasurfaces are designed that encompass an ordered and a disordered phase. The capabilities of this hybrid optical systems are illustrated by generating configurable structural colors with extra ordinary resolution. Furthermore, the local control of light over a broad bandwidth is demonstrated and a maximum coupling efficiency of more than 97% is achieved. [ABSTRACT FROM AUTHOR]

This paper presents the results of a detailed theoretical study of the bending properties of original all-glass leakage channel microstructured optical fibers (LC MOFs) over a bending radius range from 3 cm to 11 cm. These LC MOFs contain two layers of fluorine-doped silica glass elements with reduced refractive index, different diameters, and different distances between them. We determined the spatial distributions of the electric field components of different modes in addition to the usual parameters such as effective refractive indices, bending losses, and spatial intensity distributions. A detailed analysis showed that three modes for each polarization have to be considered to correctly calculate the bending losses. Two pairs of these three modes couple in two distinct bending radius ranges, specifically near 3.68 cm and near 5.95 cm, and the mode coupling in these pairs is resonant. The resulting bending losses of the LC MOF for two polarizations are very close to each other and have two maxima at bending radii of 3.68 cm and 5.95 cm. However, the nature of these maxima is not resonant; they are caused by the combined influence of all three modes, each of which has specific dependencies of losses and other parameters on the bending radius that exhibit quasi-resonant behavior near the corresponding bending radii. [ABSTRACT FROM AUTHOR]

Abstract We present the results of studies on the non-uniform frequency shift of spin wave spectrum in a two-dimensional magnonic crystal of cobalt/permalloy under the influence of external magnetic field changes. We investigate the phenomenon of coupling of modes and, as a consequence, their hybridization. By taking advantage of the fact that compressing the crystal structure along the direction of the external magnetic field leads to an enhancement of the demagnetizing field, we analyse its effect on the frequency shift of individual modes depending on their concentration in Co. We show that the consequence of this enhancement is a shift in the coupling of modes towards higher magnetic fields. This provides a potential opportunity to design which pairs of modes and in what range of fields hybridization will occur.

Higher-dimensional communications are of interest for multiple reasons, including increasing the classical transmission capacity and, more recently, the quantum state transfer through fibers using the many modes within the fiber. For quantum communications, this enables an increase in the number of bits per photon, increasing quantum fidelity, increasing error thresholds and enabling hyperentanglement transfer, among other possibilities. A high-dimensional quantum state transfer can be transported through multimode fiber using the many modes available. However, this transfer of information through multimode optical fiber is limited by attenuation and mode coupling among the various spatial and polarization modes. Here, we consider how this mode coupling impacts the transfer process. We consider the fiber’s modal properties, including orbital angular momentum, modal group numbers, and principal modes. We also investigate and propose input and output optical components, as well as fiber properties, which better mitigate the deleterious effects of mode coupling. We use the WKB approximation to the scaler wave equation as a guidance to quantify this coupling and then implement corrections to this approximation using exact solutions to the scaler wave equation. We consider methods to circumvent this mode coupling using optical fiber designs, holographic optical components and devices that are commercially available today. Some of these components, such as the holographic gratings and lenses, could be implemented using flat optics.

The metal/dielectric/metal (MDM) multilayer resonator is of increasing interest because of its functionality of surface plasmons generated at the interfaces. We aimed to control the EL spectra of OLED with an MDM electrode by actively utilizing the plasmon-microcavity coupling. The change in EL spectra of OLED with MDM electrode was simulated depending on the design of MDM structures by FDTD simulations. The strength of the plasmon-microcavity coupling can be controlled by the thickness of the Ag film in AZA, which can determine the tuning range of the EL peak position. [ABSTRACT FROM AUTHOR]

Turbulence implies nonlinear wave–wave coupling, and determining cause and effect of either is important to understand mixing responsible for enhanced number, momentum, or energy (NME) transport. To explain the identification of parent and daughter modes via a look-up table, we sketch the framework of bispectral analysis without repeating the mathematical formalism of earlier bispectrum researchers. We then apply this technique to a test signal and plasma fluctuation data from the WVU-Q machine, where the inhomogeneous energy density-driven spectrum exhibited a degree of coupling to lower frequencies that was absent in the case of the related, single-eigenmode, current-driven spectrum. [ABSTRACT FROM AUTHOR]

Higher-dimensional communications are of interest for multiple reasons, including increasing the classical transmission capacity and, more recently, the quantum state transfer through fibers using the many modes within the fiber. For quantum communications, this enables an increase in the number of bits per photon, increasing quantum fidelity, increasing error thresholds and enabling hyperentanglement transfer, among other possibilities. A high-dimensional quantum state transfer can be transported through multimode fiber using the many modes available. However, this transfer of information through multimode optical fiber is limited by attenuation and mode coupling among the various spatial and polarization modes. Here, we consider how this mode coupling impacts the transfer process. We consider the fiber's modal properties, including orbital angular momentum, modal group numbers, and principal modes. We also investigate and propose input and output optical components, as well as fiber properties, which better mitigate the deleterious effects of mode coupling. We use the WKB approximation to the scaler wave equation as a guidance to quantify this coupling and then implement corrections to this approximation using exact solutions to the scaler wave equation. We consider methods to circumvent this mode coupling using optical fiber designs, holographic optical components and devices that are commercially available today. Some of these components, such as the holographic gratings and lenses, could be implemented using flat optics. [ABSTRACT FROM AUTHOR]

A number of previous papers have used the coupled-mode method to assess three-dimensional scattering by cylindrically symmetric anomalies (seamounts, hills) or anomalies which are invariant in a horizontal direction (wedges, ridges). This paper makes an extension to combinations of these two anomaly types. The upper and lower depth boundaries of the anomalies may be flat or irregular, and the sound source may be anywhere in the medium. After a discretization of the anomalies of the two types with laterally homogeneous rings and strips, respectively, an adaptation of the coupled-mode method yields the solution of the pertinent Helmholtz equation. The adaptation involves a combination of Fourier-series summations to handle the ring anomalies and adaptive wavenumber integrations to handle the strip structure. For each anomaly, recursively computed reflection matrices relate the expansion coefficients for incoming and outgoing normal modes. Iterative solution of a linear equation system for the amplitudes of the scattered cylindrical waves from the ring anomalies, involving formulas for transformation between plane and cylindrical waves, yields an expansion of the field. Related expansions allow isolation of partial waves, multiply scattered among the anomalies. The paper includes examples from underwater acoustics and atmospheric acoustics. [ABSTRACT FROM AUTHOR]

In this paper, we construct a near-infrared Fabry–Perot cavity composed of two sodium (Na) layers and an antimony trisulfide (Sb2S3) layer. By cascading two Fabry–Perot cavities, the transmittance peak splits into two transmittance peaks due to the coupling between two Fabry–Perot modes. We utilize a coupled oscillator model to describe the mode coupling and obtain a Rabi splitting of 60.0 meV. By cascading four Fabry–Perot cavities, the transmittance peak splits into four transmittance peaks, leading to a near-infrared transparent band. The near-infrared transparent band can be flexibly tuned by the crystalline fraction of the Sb2S3 layers. In addition, the effects of the layer thickness and incident angle on the near-infrared transparent band and the mode coupling are investigated. As the thickness of the Na layer increases, the coupling strength between the Fabry–Perot modes becomes weaker, leading to a narrower transparent band. As the thickness of the Sb2S3 layer increases, the round-trip propagating of the Sb2S3 layer increases, leading to the redshift of the transparent band. As the incident angle increases, the round-trip propagating of the Sb2S3 layer decreases, leading to the blueshift of the transparent band. This work not only provides a viable route to achieving tunable near-infrared transparent bands, but also possesses potential applications in high-performance display, filtering, and sensing. [ABSTRACT FROM AUTHOR]

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The microcavity is a promising sensor platform, but any perturbation will disturb its linewidth, cause resonance shift or splitting. However, such sensing resolution is limited by the cavity's optical quality factor and mode volume. Here, an on‐chip integrated microcavity system is proposed and demonstrated that the resolution of a self‐referenced sensor can be enhanced with multi‐mode coupling. In experiments, inter‐mode coupling strength is carefully optimized with a pulley waveguide and observed a resolution improvement of nearly 3 times in the frequency domain. While experiencing small refractive index change tuned by temperature, mode‐coupled system shows a 7.2 times sensitivity enhancement that is than the uncoupled system on the same chip and a very significant lineshape contrast ratio change as great reference for minor frequency shifts. This approach will help design microcavity sensors to improve detection sensitivity and resolution under limited manufacture precision. [ABSTRACT FROM AUTHOR]

We study theoretically and experimentally the acoustic Purcell effect induced by quasi-bound states in the continuum (quasiBICs). A theoretical framework describing the acoustic Purcell effect of a resonant system is developed based on the system’s radiative and dissipative factors, which reveals the critical emission condition for achieving optimum Purcell factors. We show that the quasiBICs contribute to highly confined acoustic field and bring about greatly enhanced acoustic emission, leading to strong Purcell effect. Our concept is demonstrated via two coupled resonators supporting a Friedrich–Wintgen quasiBIC, and the theoretical results are validated by the experiments observing emission enhancement of the sound source by nearly two orders of magnitude. Our work bridges the gap between the acoustic Purcell effect and acoustic BICs essential for enhanced wave-matter interaction and acoustic emission, which may contribute to the research of high-intensity sound sources, high-quality-factor acoustic devices and nonlinear acoustics.

This paper presents the results of a detailed theoretical study of the bending properties of original all-glass leakage channel microstructured optical fibers (LC MOFs) over a bending radius range from 3 cm to 11 cm. These LC MOFs contain two layers of fluorine-doped silica glass elements with reduced refractive index, different diameters, and different distances between them. We determined the spatial distributions of the electric field components of different modes in addition to the usual parameters such as effective refractive indices, bending losses, and spatial intensity distributions. A detailed analysis showed that three modes for each polarization have to be considered to correctly calculate the bending losses. Two pairs of these three modes couple in two distinct bending radius ranges, specifically near 3.68 cm and near 5.95 cm, and the mode coupling in these pairs is resonant. The resulting bending losses of the LC MOF for two polarizations are very close to each other and have two maxima at bending radii of 3.68 cm and 5.95 cm. However, the nature of these maxima is not resonant; they are caused by the combined influence of all three modes, each of which has specific dependencies of losses and other parameters on the bending radius that exhibit quasi-resonant behavior near the corresponding bending radii.

A giant electric field on a subwavelength scale is highly beneficial for boosting the light–matter interaction. In this paper, we investigated a hybrid structure consisting of a hemispheric dimer array and a gold film and realized resonant mode coupling of the surface lattice resonance (SLR) and surface plasmon polariton (SPP). Mode coupling is demonstrated by observing anti-crossing in reflection spectra, which corresponds to Rabi splitting. Although the resonance coupling does not enter the strong coupling regime, an improved quality factor (Q~350) and stronger electric field enhancement in the gap region of the dimer (i.e., hot spot) in our hybrid structure are obtained compared to those of the single dimer or dimer array only. Remarkably, the magnitude of electric field enhancement over 500 can be accessible. Such high field enhancement makes our hybridized structure a versatile platform for the realization of ultra-sensitive biosensing, low-threshold nanolasing, low-power nonlinear optical devices, etc. [ABSTRACT FROM AUTHOR]

Turbulence implies nonlinear wave–wave coupling, and determining cause and effect of either is important to understand mixing responsible for enhanced number, momentum, or energy (NME) transport. To explain the identification of parent and daughter modes via a look-up table, we sketch the framework of bispectral analysis without repeating the mathematical formalism of earlier bispectrum researchers. We then apply this technique to a test signal and plasma fluctuation data from the WVU-Q machine, where the inhomogeneous energy density-driven spectrum exhibited a degree of coupling to lower frequencies that was absent in the case of the related, single-eigenmode, current-driven spectrum.

Two-dimensional (2D) heterostructures integrated into nanophotonic cavities have emerged as a promising approach towards novel photonic and opto-electronic devices. However, the thickness of the 2D heterostructure has a strong influence on the resonance frequency of the nanocavity. For a single cavity, the resonance frequency shifts approximately linearly with the thickness. Here, we propose to use the inherent non-linearity of the mode coupling to render the cavity mode insensitive to the thickness of the 2D heterostructure. Based on the coupled mode theory, we reveal that this goal can be achieved using either a homoatomic molecule with a filtered coupling or heteroatomic molecules. We perform numerical simulations to further demonstrate the robustness of the eigenfrequency in the proposed photonic molecules. Our results render nanophotonic structures insensitive to the thickness of 2D materials, thus owing appealing potential in energy- or detuning-sensitive applications such as cavity quantum electrodynamics.

Simultaneous localization of light to extreme spatial and spectral scales is of high importance for testing fundamental physics and various applications. However, there is a longstanding trade-off between localizing a light field in space and in frequency. Here we discover a new class of twisted lattice nanocavities based on mode locking in momentum space. The twisted lattice nanocavity hosts a strongly localized light field in a 0.048 λ3 mode volume with a quality factor exceeding 2.9 × 1011 (∼250 μs photon lifetime), which presents a record high figure of merit of light localization among all reported optical cavities. Based on the discovery, we have demonstrated silicon-based twisted lattice nanocavities with quality factor over 1 million. Our result provides a powerful platform to study light-matter interaction in extreme conditions for tests of fundamental physics and applications in nanolasing, ultrasensing, nonlinear optics, optomechanics and quantum-optical devices.

In a printer fuser assembly, unpleasant noise caused by friction-induced vibrations can occur owing to friction between the pad and the inner surface of the fixing sleeve. However, the vibration mode of the fuser assembly involved in the generation of friction-induced vibration has not yet been identified, and several aspects of the noise-generation mechanism remain unexplained. This study aimed to clarify the mechanism of noise generation due to friction-induced vibration in the printer fuser assembly and propose countermeasures against this noise. The relationship between the tendency of noise occurrence, friction characteristics of the fixing sleeve, and the generation mechanism of noise generation was investigated by examining the actual operating vibration modes of the fixing sleeve and pressure roller during noise generation. The results confirmed that the fixing sleeve is the main vibration source during noise generation and that the fixing sleeve’s vibration modes in the radial and circumferential directions are coupled. The effect of negative velocity gradient characteristics on the critical friction coefficient at the onset of frictional self-excited vibration caused by mode coupling was examined using a 2-degree-of-freedom lumped mass model. The friction characteristics between the sleeve and pad and the actual operating vibration mode of the sleeve during noise generation revealed that the noise generation mechanism is characterized by mode-coupling type self-excited vibration and that the noise can occur at low friction due to the negative gradient characteristic of the friction coefficient. Finally, the countermeasure was proposed against noise based on the characteristics of self-excited vibration caused by mode coupling. The authors found that placing the spacer on the trailing side and increasing the surface pressure distribution on the trailing side could control the friction-induced noise without changing the negative velocity gradient characteristics of the friction coefficient.

This article describes a photonic crystal fiber (PCF) with two cores as refractive index sensor that was built using COMSOL Multiphysics software by considering elliptical shaped air holes within the cladding. The two cores of the PCF are formed by an elliptical air hole at center, which represents two independent waveguides. The approach is intrinsically suited to investigate the sensor's performance, a full-vector Finite Element Method technique is employed. According to mathematical evidence, the proposed PCF based sensor has maximum sensitivities 9000 nm/RIU and 10,000 nm/RIU for x-polarized and y-polarized respectively. The sensing range of analyte is 1.35–1.39. The proposed sensor that has excellent sensitivity reveals an unmatched capacity for detecting chemicals, agents that cause cancer, biomolecules, and other analytes. [ABSTRACT FROM AUTHOR]

In this paper, a long-range hybrid waveguide for subwavelength confinement based on double SPP coupling is proposed. The hybrid waveguide consists of a metal-based cylindrical hybrid waveguide and a silver nanowire. There are two coupling regions in the waveguide structure that enhance mode coupling. Strong mode coupling enables the waveguide to exhibit both a small effective mode area (0.01) and an extremely long transmission length (700 μm). The figure of merit (FOM) of the waveguide can be as high as 4000. In addition, the cross-sectional area of the waveguide is only 500 nm × 500 nm, allowing optical operation in the subwavelength range, which helps enhance the miniaturization of optoelectronic devices. The excellent characteristics of the hybrid waveguide make it have potential applications in photoelectric integrated systems. [ABSTRACT FROM AUTHOR]

Abstract—A broadband (35–1000 Hz) sound field formed by a point source in a shelf zone with inhomogeneous bottom sediment structure is studied using numerical modeling. The shelf depth is about 30 m and the maximum distance is 10 km. The transitional zone from the bottom with a sound speed of 1400 m/s to the bottom with a speed of 1600 m/s is chosen as the model inhomogeneity. The normal mode theory and wide-angle parabolic equations are used for sound field calculations. Numerical experiments show that the manifestation of horizontal refraction is noticeable at low frequencies (below 100 Hz). It leads to an increase in the amplitude of the low-frequency sound pulse propagating along the transitional zone by more than 10 dB in comparison with a similar waveguide with a homogeneous bottom. At frequencies above 100 Hz, the dominant effect is the mode coupling, causing the appearance of quasi-periodic oscillations of modal amplitude in the frequency domain. The conclusions from the simplified model are confirmed by calculations for the real structure of bottom sediments in the Kara Sea. [ABSTRACT FROM AUTHOR]

Fiber optic communications are inevitable to achieve higher data rates of modern telecom networks. After utilization of Wavelength division multiplexing, higher order modulations and polarization multiplexing, mode division multiplexing is a new dimension to achieve higher transmission capacity for optical fiber communication links. Different spatial distributions of optical energy along cross sectional area of optical fiber allows simultaneous transmission of data by considering each mode as an independent channel. During such simultaneous transmissions, possibility of mixing of signals amongst modes causes signal degradations and acts as limiting factor for bandwidth -- distance product of the link. This effect of mode coupling has been explored in this article by presenting its mathematical formulations. A simulation has been performed to study the impact of fiber constructional parameters on mode coupling using optical wavelengths used for telecommunication systems. The observations help to develop fiber for reduced mode coupling for particular group of modes and operating wavelengths. This article paves the way forward for study of mode coupling in micro and macro bending conditions for forthcoming research endeavours. [ABSTRACT FROM AUTHOR]

In this paper, we construct a near-infrared Fabry–Perot cavity composed of two sodium (Na) layers and an antimony trisulfide (Sb2S3) layer. By cascading two Fabry–Perot cavities, the transmittance peak splits into two transmittance peaks due to the coupling between two Fabry–Perot modes. We utilize a coupled oscillator model to describe the mode coupling and obtain a Rabi splitting of 60.0 meV. By cascading four Fabry–Perot cavities, the transmittance peak splits into four transmittance peaks, leading to a near-infrared transparent band. The near-infrared transparent band can be flexibly tuned by the crystalline fraction of the Sb2S3 layers. In addition, the effects of the layer thickness and incident angle on the near-infrared transparent band and the mode coupling are investigated. As the thickness of the Na layer increases, the coupling strength between the Fabry–Perot modes becomes weaker, leading to a narrower transparent band. As the thickness of the Sb2S3 layer increases, the round-trip propagating of the Sb2S3 layer increases, leading to the redshift of the transparent band. As the incident angle increases, the round-trip propagating of the Sb2S3 layer decreases, leading to the blueshift of the transparent band. This work not only provides a viable route to achieving tunable near-infrared transparent bands, but also possesses potential applications in high-performance display, filtering, and sensing.

Wavelength dependence of coupling coefficient and inter-core crosstalk in a homogeneous 9-core Multi-core Optical Fiber (MCF) are investigated analytically. The analysis is further extended to evaluate the mean crosstalk power at the output of any core with light launched into the center core of the MCF. Analysis shows that the mean crosstalk power increases with the increase in operating wavelength. Propagation length dependence of crosstalk power is also investigated using Coupled Power Theory. It is noticed that the crosstalk increases exponentially with the increase in propagation length (z), and it is higher with the higher coupling coefficient at a particular propagation distance. The crosstalk is also not periodic with the propagation distance, z. The behavior of relative crosstalk power with respect to the operating wavelength within a span of C and L band (1530–1620 nm) is also investigated analytically. It is noticed that the relative crosstalk increases almost linearly with core pitch as well as with operating wavelength. It is also seen that the relative crosstalk power (in dB) is more in the MCF with less number of cores when it is varied with respect to wavelength. From the analysis, it appears that the inter-core coupling induced crosstalk increases with respect to the operating wavelength even when the core density is reduced. [ABSTRACT FROM AUTHOR]

Internal solitary waves evolving with time in shallow water are known to affect sound propagation significantly. Unlike prior work studying the acoustic effects of individual internal-wave properties separately, this paper elucidates and evaluates the influence of a complete evolution process of internal waves on acoustic fields both theoretically and by the coupled ocean-acoustic simulation. Two evolving wave properties considered here are shape deformations including the variations of wave amplitudes and widths and packet dispersion manifested as the increasing wavelength (i.e., the distance between successive solitons). The acoustic modal intensity expressed by the Dyson series solution is reformulated to explicitly reveal the modulation effects induced by the deformation and dispersion of internal waves. Dispersion leads to modal interference and causes the intensity envelope to oscillate with the varying wavelength. Deformation modulates intensity in a non-oscillatory manner that is less predictable due to the complexity of amplitude and width variations. In the environment reconstructed from the field observations of internal waves in the South China Sea, the modal intensity simulated by the parabolic-equation model exhibits pronounced modulation effects, where the modal interference due to dispersion dominates the intensity-envelope shape, and deformation affects the extremum positions of envelopes. [ABSTRACT FROM AUTHOR]

This paper proposes a new circular hydrostatic pressure sensor based on a dual-core photonic crystal fiber (DC-PCF) with a graphene layer that can be used in industrial and medical applications. Numerical simulations are performed using COMSOL Multiphysics software, which solves problems using the finite element method (FEM). The coupling length and confinement loss of the proposed DC-PCF are calculated at a wavelength of 1.55 μm, which are 1.291 mm and 2.3362 × 10–3 dB/m, respectively. A linear relationship is observed between the applied pressure and the peak wavelength of the transmission spectrum. When pressure is applied to the proposed DC-PCF from 0 to 1000 MPa, the spectrum transmission shifts to blue wavelengths. The results show that the proposed DC-PCF with a length of 6 cm has the highest sensitivity of − 30 pm/MPa. A very low value of the limit of detection (LOD) parameter of 0.785 pPa is calculated. High sensitivity and low LOD make the sensor able to measure pressure changes with high accuracy. It is also shown that adding a graphene layer to the center of the DC-PCF sensor increases the birefringence, confinement loss, and stress component values. [ABSTRACT FROM AUTHOR]

We have studied the nonlinear interaction between two different eigen modes in suspended carbon nanotube resonators. It was found that the mode coupling in suspended carbon nanotubes was dominated by single electron-tunneling process. For suspended carbon nanotube resonator at low temperatures using quantum dot embedded in the nanotube as detector, the interaction was found between two different eigen modes. For nanotube resonators in the coulomb blockade regime the nonlinear modal interaction was dominated by single electron tunneling as opposed to displacement induced tension. A strongly enhanced mode coupling was observed in the coulomb blockade regime which was of the order of magnitude larger than in conventional micro resonators. The obtained results found in good agreement with previously obtained results. [ABSTRACT FROM AUTHOR]

The effect of crosstalk due to inter-core coupling on the bit error rate (BER) performance of a fiber optic communication link with a homogeneous multi-core optical fiber (MCF) and a direct detection receiver with optical preamplifier is analytically evaluated and presented in this paper. Coupling coefficients due to inter-core coupling in a 7-core MCF is evaluated numerically with as a function of core-to-core distance (pitch) and relative refractive index contrast. Coupling length is determined against various index contrasts and at different core pitches. Crosstalk power, Signal to Crosstalk plus Noise Ratio (SCNR), BER and power penalty at any output core with light launched into other core of a 7-core MCF are determined analytically. The results show that there is mentionable deterioration in BER performance due to coupling induced crosstalk, and system suffers distinct power penalty at a given BER. For example, power penalty at BER of 10e-9 is found to be 4.0 dB and 4.3 dB for a MCF link of 10 km corresponding to relative refractive index difference of 0.0036 and 0.0030, respectively, for a pitch value of 30 µm and core radius of 4.5 µm. [ABSTRACT FROM AUTHOR]

In this paper, a spring system symmetrically arranged around a circular plate compliant to out-of-plane oscillation is proposed. The spring system consists of single serpentine springs mutually coupled in a plane. Three theoretical mechanical models for evaluating the stiffness of the spring system are built, which are based on the flexural beam, Sigitta, and serpentine spring theories and equivalent mechanical spring structure models. The theoretically calculated results are in good agreement with numerical solutions using the finite element method, with errors less than 10% in the appropriate dimension ranges of the spring. Compared to similar spring structures without mechanical coupling, the proposed mechanically coupled spring shows advantage in suppressing the mode coupling.

With the miniaturization and high-frequency requirements of quartz crystal sensors, microscopic issues affecting operating performance, e.g., the surface roughness, are receiving more and more attention. In this study, the activity dip caused by surface roughness is revealed, with the physical mechanism clearly demonstrated. Firstly, the surface roughness is considered as a Gaussian distribution, and the mode coupling properties of an AT-cut quartz crystal plate are systematically investigated under different temperature environments with the aid of two-dimensional thermal field equations. The resonant frequency, frequency–temperature curves, and mode shapes of the quartz crystal plate are obtained through the partial differential equation (PDE) module of COMSOL Multiphysics software for free vibration analysis. For forced vibration analysis, the admittance response and phase response curves of quartz crystal plate are calculated via the piezoelectric module. The results from both free and forced vibration analyses demonstrate that surface roughness reduces the resonant frequency of quartz crystal plate. Additionally, mode coupling is more likely to occur in a crystal plate with a surface roughness, leading to activity dip when temperature varies, which decreases the stability of quartz crystal sensors and should be avoided in device fabrication. [ABSTRACT FROM AUTHOR]

Fiber-optic surface plasmon resonance (FOSPR) sensing technology has become an appealing candidate in biochemical sensing applications due to its distinguished capability of remote and point-of-care detection. However, FOSPR sensing devices with a flat plasmonic film on the optical fiber tip are seldom proposed with most reports concentrating on fiber sidewalls. In this paper, we propose and experimentally demonstrate the plasmonic coupled structure of a gold (Au) nanodisk array and a thin film integrated into the fiber facet, enabling the excitation of the plasmon mode on the planar gold film by strong coupling. This plasmonic fiber sensor is fabricated by the ultraviolet (UV) curing adhesive transferring technology from a planar substrate to a fiber facet. The experimental results demonstrate that the fabricated sensing probe has a bulk refractive index sensitivity of 137.28 nm/RIU and exhibits moderate surface sensitivity by measuring the spatial localization of its excited plasmon mode on Au film by layer-by-layer self-assembly technology. Furthermore, the fabricated plasmonic sensing probe enables the detection of bovine serum albumin (BSA) biomolecule with a detection limit of 19.35 μM. The demonstrated fiber probe here provides a potential strategy to integrate plasmonic nanostructure on the fiber facet with excellent sensing performance, which has a unique application prospect in the detection of remote, in situ, and in vivo invasion. [ABSTRACT FROM AUTHOR]

The nonlinear interaction between the m / n = 2 / 1 beta-induced Alfvén eigenmode (BAE) and the m / n = 2 / 1 tearing mode (TM) observed in the HL-2A experiment is systematically studied with the hybrid kinetic-magnetohydrodynamic code M3D-K. It is found that the nonlinear growth of TM can lead to a gradient buildup on the magnetic island (MI) edge, and then triggers the destabilization of the linearly-stable BAE. Then, the triggered BAE together with TM can produce a significant redistribution of energetic particles. For BAE linearly-dominant-unstable case, the TM activity results in the inward motion of BAE mode structure, and the BAE can have a delay effect on the MI saturation. Furthermore, a high-frequency axisymmetric m / n = 0 / 0 'breathing' mode is generated by the mode coupling of BAE and TM, agreeing well with the experimental observation, and causes the synchronized periodic oscillation of MI width. [ABSTRACT FROM AUTHOR]

During the fabrication of quartz crystal resonators (QCRs), parallelism error is inevitably generated, which is rarely investigated. In order to reveal the influence of parallelism error on the working performance of QCRs, the coupled vibration of a non-parallel AT-cut quartz crystal plate with electrodes is systematically studied from the views of theoretical analysis and numerical simulations. The two-dimensional thermal incremental field equations are solved for the free vibration analysis via the coefficient-formed partial differential equation module of the COMSOL Multiphysics software, from which the frequency spectra, frequency–temperature curves, and mode shapes are discussed in detail. Additionally, the piezoelectric module is utilized to obtain the admittance response under different conditions. It is demonstrated that the parallelism error reduces the resonant frequency. Additionally, symmetry broken by the non-parallelism increases the probability of activity dip and is harmful to QCR's thermal stability. However, if the top and bottom surfaces incline synchronously in the same direction, the influence of parallelism error is tiny. The conclusions achieved are helpful for the QCR design, and the methodology presented can also be applied to other wave devices. [ABSTRACT FROM AUTHOR]

The Langevin equation (LE) is used to evaluate mode coupling in multimode step-index polymer optical fiber (SI POF) that is both unstrained and strained. The numerical solution of the LE matches the numerical solution of the power flow equation (PFE). Strain-induced mode coupling is noticeably stronger in strained fiber than in unstrained fiber of the same types. Therefore, compared to similar lengths for unstrained fibers, the coupling length of the equilibrium mode distribution (EMD) is attained and the length of fiber required to produce a steady-state distribution (SSD) are both much shorter for strained fibers. We have demonstrated that the mode coupling in strained and unstrained multimode SI POFs that comes from the random perturbations (RPs) of the fiber can be successfully treated by the LE. The study's findings can be used to improve communication and sensory systems that use multimode SI POFs under different bending circumstances. Additionally, it is crucial to be able to compute the modal distribution of the SI POFs used in the optical fiber sensory system at a specific length and under various bending scenarios.

A giant electric field on a subwavelength scale is highly beneficial for boosting the light–matter interaction. In this paper, we investigated a hybrid structure consisting of a hemispheric dimer array and a gold film and realized resonant mode coupling of the surface lattice resonance (SLR) and surface plasmon polariton (SPP). Mode coupling is demonstrated by observing anti-crossing in reflection spectra, which corresponds to Rabi splitting. Although the resonance coupling does not enter the strong coupling regime, an improved quality factor (Q~350) and stronger electric field enhancement in the gap region of the dimer (i.e., hot spot) in our hybrid structure are obtained compared to those of the single dimer or dimer array only. Remarkably, the magnitude of electric field enhancement over 500 can be accessible. Such high field enhancement makes our hybridized structure a versatile platform for the realization of ultra-sensitive biosensing, low-threshold nanolasing, low-power nonlinear optical devices, etc.

Nonlinear modeling of mitigation of the edge localized mode (ELM) with resonant magnetic perturbation (RMP) is performed for the HL-2A tokamak, utilizing the three-dimensional (3D) magnetohydrodynamic code JOREK. Based on the 3D equilibrium established after application of the $n = 1$ ( $n$ is the toroidal mode number) RMP at $4.9\,\,{\text{kAt}}$ coil current with odd parity, ELM mitigation is successfully simulated consistent with the experimental result. Nonlinear simulations show strong mode coupling among toroidal Fourier harmonics, allowing redistribution of the magnetic energy such that the most unstable toroidal mode saturates at a lower level. This magnetic energy cascade offers an explanation of the RMP-induced ELM mitigation achieved in HL-2A. Detailed examination of the simulation results shows persistent resonant field screening even during the ELM mitigation phase. Finite plasma resistivity however does enable partial penetration of the resonant field thus modifying the edge magnetic topology and characteristics of the edge transport. Plasma radial profiles undergo pronounced changes around the pedestal region, when the magnetic energy of the most unstable toroidal mode reaches the maximum value. Systematic scans of the applied RMP coil current with the JOREK simulations find a threshold value of around $4.5\,\,{\text{kAt}}$ required for achieving the ELM mitigation on HL-2A.

We report our analytical description to predict and interpret the observed light transmission characteristics through a system of concatenated dissimilar optical fibers having core diameter mismatch while devising cladding mode-based refractive-index sensing devices. The configuration consists of a pair of identical multimode fibers (MMFs) in which a short segment of conventional single-mode fiber (SMF) is spliced as a sensing zone. Because of the core diameter mismatch, the cladding of the SMF guides light and makes the device sensitive to external perturbation/change in refractive index of the surrounding medium. The device can operate at different wavelengths, which makes it attractive for diverse applications. Using our analysis, we evaluate the performance of a few configurations of the sensor and compare the results with those obtained experimentally or known otherwise. We demonstrate that our model is general and can be applied to devise sensor with any transverse profile MMF to explore new applications. [ABSTRACT FROM AUTHOR]

This paper presents and numerically analyzes a refractive index sensor based on side-polished D-shaped two-channel photonic crystal fiber (PCF) and surface plasmon resonance (SPR). The effects of pore duty ratio, polishing depth, and thickness of a Nano-Titania sensitizing layer on sensor performance are studied, and the sensor performance is analyzed and optimized. The results show that the sensitivity of the Nano-Titania sensitized sensor can reach 3392.86 nm/RIU and temperature sensitivity of the sensor is increased to 1.320 nm/K, and the amplitude sensitivity of the unsensitized sensor can reach 376.76 RIU−1. In addition, the influence of titanium dioxide layer on the mode field diameter of PCF fiber core is also studied. It is found out that the sensor with a 50 nm thick titanium dioxide film has a larger mode fiber diameter, and is more conducive to coupling with single-mode fiber. Our detailed results contribute to the understanding of SPR phenomena in hexagonal PCF and facilitate the implementation and application of SPR-PCF sensors. [ABSTRACT FROM AUTHOR]

This article presents the twin-core photonic crystal fiber (TC-PCF) for temperature sensing application. The twin solid cores in the cross-section of TC-PCF structure, separated by one vertical elliptical air hole, act as two independent waveguides. This novel arrangement of circular and elliptical air holes in our proposed structure gives an edge over other existing structures to attain high sensitivity. The proposed sensor is highly birefringent and operates on the principle of mode coupling between the two fiber cores. It is practically possible to realize this TC-PCF based temperature sensor by coupling one fiber core to a broadband source on the input end while the other fiber core to an optical spectrum analyzer on the output end. The finite element method has been employed to simulate and quantitatively analyze the proposed TC-PCF temperature sensor. Numerical simulation shows that, the 3 cm long TC-PCF sensor has been optimized to have a high temperature sensitivity of around 21.5 pm/°C over a wide temperature sensing range of 0–1200 °C. Also the impact of diameter variation of air holes on sensitivity of proposed model is studied. [ABSTRACT FROM AUTHOR]

Mutations are the cause of several diseases as well as the underlying force of evolution. A thorough understanding of their biophysical consequences is essential. We present a computational framework for evaluating different levels of mutual information (MI) and its dependence on mutation. We used molecular dynamics trajectories of the third PDZ domain and its different mutations. Nonlinear MI between all residue pairs are calculated by tensor Hermite polynomials up to the fifth order and compared with results from multivariate Gaussian distribution of joint probabilities. We show that MI is written as the sum of a Gaussian and a nonlinear component. Results for the PDZ domain show that the Gaussian term gives a sufficiently accurate representation of MI when compared with nonlinear terms up to the fifth order. Changes in MI between residue pairs show the characteristic patterns resulting from specific mutations. Emergence of new peaks in the MI versus residue index plots of mutated PDZ shows how mutation may change allosteric pathways. Triple correlations are characterized by evaluating MI between triplets of residues. We observed that certain triplets are strongly affected by mutation. Susceptibility of residues to perturbation is obtained by MI and discussed in terms of linear response theory. [ABSTRACT FROM AUTHOR]

Abstract Long-period waveguide grating based filters have attracted attention due to their flexible fabrication, a variety of materials and structures, low back reflection, low insertion loss, and excellent performance in the tuning range and temperature sensitivity. To our knowledge, for the first time, a two-segment polymer long-period waveguide grating was cascaded to implement a filter with a narrower bandwidth. Experimental results showed that the device had a maximum extinction ratio of 24 dB at 1 577 nm, and the 12 dB bandwidth was 10 nm. The temperature sensitivity of the fabricated device was 1.79 nm/°C.

• Novel application of FSQP method for coupled vibrations in prestress-strained FBAR. • Determination of frequency spectrograms for predicting and controlling mode couplings. • Observation of biaxial shift enhancement of frequency spectral curves by initial strains. • Significant tunability of resonance frequencies via strain-stiffening or -softening effect. The method of frequency spectrum quantitative prediction (FSQP) is extended to investigate high-frequency and mode-coupling vibrations of piezoelectric film bulk acoustic resonators (FBARs) subject to initial stress-strain biasing fields for the first time. In numerical examples, we explore the cases of uniaxial compressive and tensile initial stresses along the in-plane and thickness directions, respectively. Derived from the nonlinear electroelastic theory, the governing and constitutive equations for piezoelectric films under complex stress-strain biasing states are formulated. Based on these formulations, the first step of the FSQP involves obtaining exact dispersion curves of bulk waves propagating in FBARs with different stress-strain biasing fields through the classical displacement method. Subsequently, mode-coupling solutions of physical fields are constructed for the prestressed FBARs operating with the thickness-extensional mode by superimposing relevant eigenmodes in dispersion curves. The second step of the FSQP involves deriving Hamilton's principle of piezoelectric film with initial stress-strain biasing fields using the perturbation method. Finally, the frequency spectrograms describing coupling vibration intensities between the thickness-extensional mode and unwanted eigenmodes are obtained by substituting mode-coupling solutions into Hamilton's principle, which verifies the effectiveness of the extended FSQP method for addressing dynamic problems in FBARs with biasing fields. The influences of both the amplitudes and orientations of initial stresses on the frequency spectral curves are examined. Mode-shape diagrams and displacement distributions of mutually coupled eigenmodes are presented to illustrate diverse mode-coupling behaviors in thickness-extensional FBARs under complex stress-strain biasing states. Numerical results indicate that the stress-strain biasing fields significantly affect the electromechanical properties of piezoelectric films, including effective elastic, piezoelectric, and dielectric constants. Consequently, these stress-strain biasing states exert substantial changes in frequencies and propagation wavenumbers of various mode branches within frequency ranges of the thickness-extensional mode branch. Furthermore, due to changes in propagation wavenumber, frequency spectral curves experience remarkable horizontal shifts along the length-to-thickness ratio axis, significantly altering mode-coupling behaviors of FBARs. Induced initial strains can enhance shift amplitudes of frequency spectral curves caused by initial stresses. In addition, stress-strain biasing fields result in significant shifts of frequency spectral curves along the frequency axis through the strain-stiffening or -softening effect, which can be harnessed to modulate resonance frequencies of FBARs. This study offers a solid foundation for frequency tunability, mode-coupling control, and structural designs in FBAR devices with residual stresses. [Display omitted] [ABSTRACT FROM AUTHOR]

• Solved trapped modes of non-axisymmetric blockages in air-filled cylindrical pipe. • Derived a 3D modal coupling analysis for acoustic response on a robotic platform. • Validated analytical models with comparison to numerical/experimental results. Acoustic sensing system deployed on an autonomous platform (also referred to as robot) for accurate condition monitoring and fault detection in pipes requires the knowledge of wave scattering from various in-pipe faults or the robot itself. Existing solutions to estimate wave scattering tend to either be constrained to the plane wave regime or be computationally expensive outside this range. There has been a lack of work to apply analytical modal coupling methods to study wave scattering from a non-symmetric cross-sectional change in a pipe beyond the plane wave regime. This paper proposes an efficient three-dimensional (3D) modal coupling method to predict wave scattering from a cross-sectional change in a pipe in the frequency range beyond the plane wave regime. The trapped modes induced by a 3D axisymmetric or non-axisymmetric cross-sectional change in an air-filled pipe are estimated using modal coupling analysis. The derived analytical model is validated against numerical simulations and measurements. It agrees with a finite element simulation with Comsol Multiphysics in the 0.01< kR <4 frequency range (k being the wavenumber and R being the pipe radius) within 15 %, but it is approximately 600 times faster than the Comsol simulation making it attractive for the deployment on sensors with limited computer power that can be used for autonomous inspection of buried pipes. [ABSTRACT FROM AUTHOR]

• A deep learning method is demonstrated to determine the wavelengths of mode coupling. • The mode coupling recognition method is efficient and effective. • The method will favor the development of optics communication, sensing, and detection. Understanding the transmission of light waves in optical fibers and accurately determining the locations of mode coupling are crucial for enhancing the efficiency of optical devices and advancing innovative technologies such as fiber optic sensors, lasers, and modulators. This study utilizes deep learning and image recognition techniques to identify the wavelengths at which mode coupling occurs in optical fibers. Our research findings show that using the ResNet-18 model allows for the rapid and accurate identification of the wavelengths at which mode coupling occurs in optical fibers, as well as the modes involved, achieving an accuracy close to 100 %. We experimented with sampling the dataset at 5 nm and 10 nm intervals to create smaller training and validation sets. Despite the reduced data volume, high accuracy rates were maintained, exceeding 99 % and 97 % respectively. This study provides new insights into the use of deep learning for precise localization of mode coupling points and tracking of transmission modes in optical fibers. [ABSTRACT FROM AUTHOR]