Outgoing Wave Research Articles

Comparative Optical Behaviour of High and Low Index Cladding Contrast Bragg Fiber

We investigate and evaluate the optical performance of high- and low-RI cladding contrast Bragg fiber for two design wavelengths: 1550 nm (in the near-infrared region) and 632.8 nm (in the visible range). In order to establish a connection between the incoming and outgoing light waves using the transfer matrix technique, the boundary matching approach is used. Less periodic cladding layers are needed to create a perfect photonic band gap (PBG), and the measured PBG in high RI cladding contrast Bragg fiber (HRCCBF) are rather large. Both low refractive index and high refractive index spectra of PBG Cladding contrast Bragg fiber (LRCCBF) performance is very sensitive to the incoming light's optical path, or angle of incidence. One way to find out how sensitive the Bragg fiber is for sensing applications is to measure the PBG's thickness or spectral shift. As the design wavelength increases, the spectral bandwidth of PBG with core RI changes, and HRCCBF seems to be quite sensitive to this shift. When taking the Left Band Edge (LBE) and the Right Band Edge (RBE) into account, LRCCBF is much more sensitive than HRCCBF, making it the better choice for sensing applications.

Generalized Coherent Wave Control at Dynamic Interfaces

AbstractCoherent wave control is of key importance across a broad range of fields such as electromagnetics, photonics, and acoustics. It enables us to amplify or suppress the outgoing waves via engineering amplitudes and phases of multiple incidences. However, within a purely spatially (temporally) engineered medium, coherent wave control requires the frequency of the associated incidences to be identical (opposite). In this work, this conventional constraint is broken by generalizing coherent wave control into a spatiotemporally engineered medium is broken, i.e., the system featuring a dynamic interface. Owing to the broken translational symmetry in space and time, both the subluminal and superluminal interfaces allow interference between scattered waves regardless of their different frequencies and wavevectors. Hence, one can flexibly eliminate the backward‐ or forward‐propagating waves scattered from the dynamic interfaces by controlling the incident amplitudes and phases. The work not only presents a generalized way for reshaping arbitrary waveforms but also provides a promising paradigm to generate ultrafast pulses using low‐frequency signals. It has also implemented suppression of forward‐propagating waves in microstrip transmission lines with fast photodiode switches.

A stable decoupled perfectly matched layer for the 3D wave equation using the nodal discontinuous Galerkin method

In outdoor acoustics, the calculations of sound propagating in air can be computationally heavy if the domain is chosen large enough for the sound waves to decay. The computational cost is lowered by strategically truncating the computational domain with an efficient boundary treatment. One commonly used boundary treatment is the perfectly matched layer (PML), which dampens outgoing waves without polluting the computed solution in the inner domain. The purpose of this study is to propose and assess a new perfectly matched layer formulation for the 3D acoustic wave equation, using the nodal discontinuous Galerkin finite element method. The formulation is based on an efficient PML formulation that can be decoupled to further increase the computational efficiency and guarantee stability without sacrificing accuracy. This decoupled PML formulation is demonstrated to be long-time stable, and an optimization procedure for the damping functions is proposed to enhance the performance of the formulation.

Implementation of PMDL and DRM in OpenSees for Soil-Structure Interaction Analysis

It is widely acknowledged that the effects of soil-structure interaction (SSI) can have substantial implications during periods of intense seismic activity; therefore, accurate quantification of these effects is of paramount importance in the design of earthquake-resistant structures. The analysis of SSI is typically conducted using either direct or substructure methods. Both of these approaches involve the use of numerical models with truncated or reduced-order computational domains. To ensure effective truncation, it is crucial to employ boundary representations that are capable of perfectly absorbing outgoing waves and allowing for the consistent application of input motions. At present, such capabilities are not widely available to researchers and practicing engineers. In order to address this issue, this study implemented the Domain Reduction Method (DRM) and Perfectly Matched Discrete Layers (PMDLs) in OpenSees. The accuracy and stability of these implementations were verified through the use of vertical and inclined incident SV waves in a two-dimensional problem. In terms of computational efficiency, PMDLs require a shorter analysis time (e.g., with PMDLs, the analysis concluded in 35 min as compared to 250 min with extended domain method) and less computational power (one processor for PMDLs against 20 processors for the extended domain method) thus offering a balance between accuracy and efficiency. Furthermore, illustrative examples of the aforementioned implemented features are presented, namely the response analysis of single-cell and double-cell tunnels exposed to plane waves inclined at an angle.

Correspondence between grey-body factors and quasinormal modes

Quasinormal modes and grey-body factors are spectral characteristics corresponding to different boundary conditions: the former imply purely outgoing waves to the event horizon and infinity, while the latter allow for an incoming wave from the horizon, thus describing a scattering problem. Nevertheless, we show that there is a link between these two characteristics. We establish an approximate correspondence between the quasinormal modes and grey-body factors, which becomes exact in the high-frequency (eikonal) regime. We show that, in the eikonal regime, the grey-body factors of spherically symmetric black holes can be remarkably simply expressed via the fundamental quasinormal mode, while at smaller ℓ, the correction terms include values of the overtones. This might be interesting in the context of the recently observed connection between grey-body factors and the amplitudes of gravitational waves from black holes. The correspondence might explain why grey-body factors are more stable, i.e. less sensitive, than higher overtones to small deformation of the effective potential.

Boundary condition and reflection anomaly in $2+1$ dimensions

It is known that the 2+12+1d single Majorana fermion theory has an anomaly of the reflection, which is canceled out when 16 copies of the theory are combined. Therefore, it is expected that the reflection symmetric boundary condition is impossible for one Majorana fermion, but possible for 16 Majorana fermions. In this paper, we consider a reflection symmetric boundary condition that varies at a single point, and find that there is a problem with one Majorana fermion. The problem is the absence of a corresponding outgoing wave to a specific incoming wave into the boundary, which leads to the non-conservation of the energy. For 16 Majorana fermions, it is possible to connect every incoming wave to an outgoing wave without breaking the reflection symmetry. In addition, we discuss the connection with the fermion-monopole scattering in 3+13+1 dimensions.

Broadband and efficient terahertz helicity inversion by a reconfigurable reflective metasurface based on vanadium dioxide

We experimentally demonstrate the broadband and highly efficient inversion of the rotation direction of the terahertz circular polarization achieved by a dynamic metasurface with vanadium dioxide (VO2). This metasurface converts linear polarization into circular polarization. The rotational direction of the output circular polarization can be reversed based on the VO2 phase transition. The VO2 state can be dynamically switched by injecting direct current into an on-chip circuit. To improve the terahertz-wave utilization efficiency, the metasurface is operated at an incident angle of 45°, thereby eliminating the beam splitter typically used to separate the outgoing wave from the incident wave at normal incidence. To achieve broadband operation, we employ a dispersion-cancelation strategy that cancels the dispersive phase responses between orthogonal linear polarizations. This strategy was previously developed for static metasurfaces; we have applied it to the design of dynamic metasurfaces. At a center frequency of 0.99 THz, the experimental evaluation demonstrated helicity switching with conversion efficiencies of 92% and 87% for the insulating and metallic states of VO2, respectively. Dispersion cancelation is simultaneously achieved for the insulating and metallic states of VO2, resulting in a relative bandwidth of 0.26, which is 3.2 times broader than that of a previously developed efficient dynamic metasurface.

The possible KK¯* and DD¯* bound and resonance states by solving the Schrodinger equation

The Schrodinger equation with a Yukawa type of potential is solved analytically. When different boundary conditions are taken into account, a series of solutions are indicated as a Bessel function, the first kind of Hankel function and the second kind of Hankel function, respectively. Subsequently, the scattering processes of KK¯* and DD¯* are investigated. In the KK¯* sector, the f 1(1285) particle is treated as a KK¯* bound state, therefore, the coupling constant in the KK¯* Yukawa potential can be fixed according to the binding energy of the f 1(1285) particle. Consequently, a KK¯* resonance state is generated by solving the Schrodinger equation with the outgoing wave condition, which lies at 1417 − i18 MeV on the complex energy plane. It is reasonable to assume that the KK¯* resonance state at 1417 − i18 MeV might correspond to the f 1(1420) particle in the review of the Particle Data Group. In the DD¯* sector, since the X(3872) particle is almost located at the DD¯* threshold, its binding energy is approximately equal to zero. Therefore, the coupling constant in the DD¯* Yukawa potential is determined, which is related to the first zero point of the zero-order Bessel function. Similarly to the KK¯* case, four resonance states are produced as solutions of the Schrodinger equation with the outgoing wave condition. It is assumed that the resonance states at 3885 − i1 MeV, 4029 − i108 MeV, 4328 − i191 MeV and 4772 − i267 MeV might be associated with the Zc(3900), the X(3940), the χ c1(4274) and χ c1(4685) particles, respectively. It is noted that all solutions are isospin degenerate.

The Weyl expansion for the scalar and vector spherical wave functions

The Weyl expansion technique, also known as the angular spectrum expansion, expresses an outgoing spherical wave as a linear combination of plane waves. The scalar spherical waves are the solutions of the homogeneous Helmholtz equation and therefore have direct relation to the scalar multipole fields. This paper gives the Weyl expansion of multipole fields, scalar and vector, of any degree and order for spherical wave functions. The expressions are given in closed form for the scalar, , and vector, , , multipole fields, evaluated across a plane orthogonal to any given direction. In the case of scalar spherical multipoles, the spherical gradient operator has been used, while for the vector spherical multipoles, the vector spherical wave operator has been constructed.

Wigner time delay revisited

The concept of Wigner time delay was introduced in scattering theory to quantify the delay or advance of an incoming particle in its interaction with the scattering potential. The scattered-wave part of the wave packet, which describes the interacting particle, is delayed/advanced with respect to the non-interacting plane-wave packet. It has been widely assumed that this concept can be transferred to ionization considering it as a half-scattering process. In the present work we show, by analyzing the corresponding wave packets, that this assumption is incorrect. For ionization the outgoing wave packet is a superposition of the continuum states ψk(−)(r,t), which satisfy the incoming-wave boundary condition at large distances r from the origin and which for large time t→∞ form a plane-wave packet. The expansion coefficients in this wave packet are the ionization probability amplitudes. If these amplitudes depend on the scattering phase shift, the plane-wave part of the ionized wave packet may be modified, leading to an ionization analog of the Wigner time delay. However, in most cases such a Wigner time delay is absent for ionization. We illustrate this with two examples: (i) photoionization by a weak field of an atom with the electron bound by a zero-range potential and (ii) strong-field ionization.

Ultra–short-period WD Binaries Are Not Undergoing Strong Tidal Heating

Double white dwarf (WD) binaries are increasingly being discovered at short orbital periods where strong tidal effects and significant tidal heating signatures may occur. We assume that the tidal potential of the companion excites outgoing gravity waves within the WD primary, the dissipation of which leads to an increase in the WD’s surface temperature. We compute the excitation and dissipation of the waves in cooling WD models in evolving MESA binary simulations. Tidal heating is self-consistently computed and added to the models at every time step. As a binary inspirals to orbital periods less than ∼20 minutes, the WD’s behavior changes from cooling to heating, with temperature enhancements that can exceed 10,000 K compared with nontidally heated models. We compare a grid of tidally heated WD models to observed short-period systems with hot WD primaries. While tidal heating affects their T eff, it is likely not the dominant luminosity. Instead, these WDs are probably intrinsically young and hot, implying that the binaries formed at short orbital periods. The binaries are consistent with undergoing common envelope evolution with a somewhat low efficiency α CE. We delineate the parameter space where the traveling wave assumption is most valid, noting that it breaks down for WDs that cool sufficiently, where standing waves may instead be formed.

ULDM self-interactions, tidal effects and tunnelling out of satellite galaxies

It is well-known that Dark Matter (DM) inside a satellite galaxy orbiting a host halo experiences a tidal potential. If DM is ultra-light, given its wave-like nature, one expects it to tunnel out of the satellite — if this happens sufficiently quickly, then the satellite will not survive over cosmological timescales, severely constraining this dark matter model. In this paper, we study the effects of the inevitable quartic self-interaction of scalar Ultra-Light Dark Matter (ULDM) on the lifetimes of satellite galaxies by looking for quasi-stationary solutions with outgoing wave boundary conditions. For a satellite with some known core mass and orbital period, we find that, attractive (repulsive) self-interactions decrease (increase) the rate of tunnelling of DM out of it. In particular, for satellite galaxies with core mass ∼𝒪(107–108) M⊙ and orbital period ∼𝒪(1) Gyr, one can impose constraints on the strength of self-interactions as small as λ∼𝒪(10-92). For instance, for ULDM mass m = 10-22 eV, the existence of the Fornax dwarf galaxy necessitates attractive self-interactions with λ≲ -2.12 × 10-91.

Coherent perfect absorber and laser induced by directional emissions in the non-Hermitian photonic crystals

In this study, we propose the application of non-Hermitian photonic crystals (PCs) with anisotropic emissions. Unlike the ring of exceptional points (EPs) found in isotropic non-Hermitian PCs, the EPs of anisotropic non-Hermitian PCs appear as symmetrical lines about the Γ point. The formation of EPs is related to the non-Hermitian strength and the real spectrum appears in the ΓY direction. The PCs have been validated as the complex conjugate medium (CCM) by effective medium theory (EMT). Conversely, EMT indicates that the effective refractive index has a large imaginary part along the ΓX direction, which forms an evanescent wave inside the PCs. Consequently, coherent perfect absorption (CPA) and laser can be achieved in the directional emission of the ΓY. The outgoing wave in the ΓX direction is weak, which can significantly reduce the losses and electromagnetic interference. The non-Hermitian PCs enable many fascinating applications such as signal amplification, collimation, and angle sensors.

Quantum trajectories and the nuclear optical model

In the context of nuclear scattering, we use the bipolar reduction of the Schrödinger equation to examine the effects of optical model absorption on incoming and outgoing scattering waves. We compare the exact solutions for these waves, obtained using a bipolar quantum trajectory-based formalism, with their approximate WKB counterparts. Aside from reducing the magnitudes of the incoming and outgoing waves, absorption smooths the variation of the potential at the turning point, reducing reflection in this region. This brings the incoming exact solution and WKB approximation into closer agreement, but tends to worsen the agreement between the outgoing solutions. Inside the turning point, the WKB approximation overestimates the inward decaying solution. The exact solution also possesses an outward going component, solely due to reflection, with no WKB counterpart.

Analysis and application of a time-domain finite element method for the Drude metamaterial perfectly matched layer model

In this paper, we develop a finite element method (FEM) to solve the Drude metamaterial perfectly matched layer (PML) model by edge elements. Both the stability analysis and error estimate for the scheme are established. To our best knowledge, this is the first paper on developing and analyzing a FEM for this Drude PML model. Numerical results are presented to justify the theoretical error estimate and demonstrate the effectiveness of this PML in absorbing outgoing waves in the Drude metamaterial.

Emulating the Deutsch-Josza algorithm with an inverse-designed terahertz gradient-index lens.

An all-dielectric photonic metastructure is investigated for application as a quantum algorithm emulator (QAE) in the terahertz frequency regime; specifically, we show implementation of the Deustsh-Josza algorithm. The design for the QAE consists of a gradient-index (GRIN) lens as the Fourier transform subblock and patterned silicon as the oracle subblock. First, we detail optimization of the GRIN lens through numerical analysis. Then, we employed inverse design through a machine learning approach to further optimize the structural geometry. Through this optimization, we enhance the interaction of the incident light with the metamaterial via spectral improvements of the outgoing wave.

Quantum Big Bounce of the Isotropic Universe Using Relational Time

We analyze the canonical quantum dynamics of the isotropic Universe with a metric approach by adopting a self-interacting scalar field as relational time. When the potential term is absent, we are able to associate the expanding and collapsing dynamics of the Universe with the positive- and negative-frequency modes that emerge in the Wheeler–DeWitt equation. On the other side, when the potential term is present, a non-zero transition amplitude from positive- to negative-frequency states arises, as in standard relativistic scattering theory below the particle creation threshold. In particular, we are able to compute the transition probability for an expanding Universe that emerges from a collapsing regime both in the standard quantization procedure and in the polymer formulation. The probability distribution results similar in the two cases, and its maximum takes place when the mean values of the momentum essentially coincide in the in-going and out-going wave packets, as it would take place in a semiclassical Big Bounce dynamics.

Reflective one-to-multi-polarization conversion via separate control of phase and magnitude.

Polarization manipulation is a key issue in electromagnetic (EM) research. Research on 90° polarization rotators and circularly-polarized wave generators has been widely conducted. In this study, a polarization conversion metasurface that can shift one linearly-polarized EM wave into multi-polarization outgoing waves at certain frequencies is demonstrated, including co-, cross-, left-hand, and right-hand circular-polarization components. The surface was made of periodically arranged chiral meta-atoms. The polarization manipulation method is based on the independent control of phase and magnitude, in which the phase control is based on the Berry-phase theory of linearly-polarized EM waves, while the magnitude control is based on the cavity mode theory of the microstrip structure. Both eigenmode analysis (EMA) and characteristic mode analysis (CMA) were utilized for magnitude control, which was further verified by the surface current distribution. Finally, the metasurface was fabricated and measured, showing good agreement between the measured and simulated results. This research proposed what we believe to be a novel polarization method, which can be potentially applied in polarization manipulation, EM radiation, filters, wireless sensors, etc., over a frequency range from optics to microwave bands.

Acoustic Michelson interferometer based on a phononic crystal

A practical and highly sensitive acoustic Michelson interferometer with a small form factor is introduced. It involves two different types of phononic crystals composed of steel rods in water acting as a medium for self-collimated waves and mirrors for the reference and sample beams, as well as a beam splitter formed by modified scatterers arranged diagonally. Finite-element method simulations are employed to demonstrate its operation around 200 kHz. Equifrequency contour analysis reveals self-collimation of ultrasonic waves between 190 and 210 kHz. Introduction of the beam splitter and mirror phononic crystals is not detrimental to self-collimation where outgoing waves from the two interferometer arms interfere such that the output intensity varies in a cosine squared manner. Consequently, maximum sensitivity is achieved when the movable mirror displacement is either zero or half of the interferometer phononic crystal period. On small intervals in these ranges, micrometer-scale displacement resolution is achievable, as the output intensity drops by 0.2% per micrometer. Thus, displacements smaller than a percent of the wavelength are easily resolvable. Nanoscale resolution can be obtained with a scaled down interferometer design. Moreover, application to liquid concentration sensing by considering ethanol–water binary mixture is demonstrated. A percent increase in weight fraction of ethanol up to 10% in the mixture leads to an intensity drop as high as 2%. Thus, significantly higher sensitivities compared to sensing schemes based on resonance frequency shift are attainable. The proposed approach can be adapted for surface acoustic waves in strain measurement or biosensing.

Multi-point scattering measurements for effective property extraction from metamaterials with skin effects

Scattering experiments can be leveraged to extract the effective properties of a heterogeneous metamaterial slab based on multi-point measurements in surrounding media. In this technique, two measurements are made in the ambient media on each side of a finite thickness micro-structured slab to decompose incoming and outgoing waves. The method is applied to an example with locally resonant micro-structured inclusions while paying close attention to parameters that influence the extracted material parameters. It is observed that the extracted overall parameters converge to limiting values when the number of unit cells across the slab thickness or ambient media modulus are increased. Dependence of extracted material parameters on the number of cells through thickness or ambient media properties are attributed to the different response of boundary (skin) and interior cells. A method is presented which represents a finite array with different effective properties for the skin regions vs. the interior regions with great success in reproducing the scattering for different slab thicknesses, and through which the interior regions properties become independent of ambient media properties. In stop bands with lower material loss, challenges arise that are associated with extremely small transmission. The assumption of continuity in transmission phase is enforceable to remove phase ambiguity, although in certain cases (associated with nearly lossless specimens) this assumption appears to fail. In such cases, interference from coupled shear modes may lead to apparent higher longitudinal transmission within the stop band. This work provides a proof of concept for the development of an experimental methodology capable of extracting the effective properties and dispersion behavior of heterogeneous mechanical metamaterials without any knowledge of the internal fields.