Critical Coupling Research Articles

Superconducting multi-vortices and a novel BPS bound in chiral perturbation theory

We derive a novel BPS bound from chiral perturbation theory minimally coupled to electrodynamics at finite isospin chemical potential. At a critical value of the isospin chemical potential, a system of three first-order differential field equations (which implies the second-order field equations) for the gauge field and the hadronic profile can be derived from the requirement to saturate the bound. These BPS configurations represent magnetic multi-vortices with quantized flux supported by a superconducting current. The corresponding topological charge density is related to the magnetic flux density, but is screened by the hadronic profile. Such a screening effect allows to derive the maximal value of the magnetic field generated by these BPS magnetic vortices, being Bmax = 2, 04 × 1014 G. The solution for a single BPS vortex is discussed in detail, and some physical consequences, together with the comparison with the magnetic vortices in the Ginzburg-Landau theory at critical coupling, are described.

Numerical Verification of a Polarization-Insensitive Electrically Tunable Far Infrared Band-Stop Meta-Surface Filter

Tunable filters have many potential applications in diverse fields, including high-capacity communications, dynamic beam shaping and spectral imaging. Although providing a high-performance solution for actively tunable devices, metasurface combined with tunable materials faces the great challenges of limited tuning range and modulation depth. Here, we propose a far-infrared tunable band-stop filter based on Fabry-Perot (FP) resonators and graphene surface plasmons. By switching the wavelength of the critical coupling condition of the filter via the gate voltage applied on graphene, achieving the dynamically tunable band-stop filtering at the central wavelengths from 12.4 μm to 14.1 μm with a modulation depth of more than 99%. Due to the symmetry of the proposed meta-atoms, the filter is insensitive to the polarization direction of the incident light. And it can realize more than 85% filtering efficiency within 60° angle of incidence around the vertical direction. By adjusting the geometry of the meta-atoms structure, it is feasible to move the operational range from the near-infrared to terahertz bands.

Quasi-bound state in the continuum enhancing background limited infrared detectivity

Quasi-bound states in the continuum (quasi-BIC) have tunable high Q factors and thus have many potential applications in optoelectronics. Here, we propose to construct a quasi-BIC with a perturbated elliptical-cylinder photonic crystal slab, and then develop a new way based on the quasi-BIC to address the background noise problem of infrared detectors. By integrating the photonic crystal slab with a quantum well infrared photodetector (QWIP), and by pushing the quasi-BIC system with absorption to the critical coupling state, an ultra-narrow photoresponse band (Q factor equal to 1150) with a high peak quantum efficiency (over 60%) is achieved in the long-wavelength infrared range. The critical coupling state is realized by matching the radiation Q factor, which is mainly controlled by the quasi-BIC, to the absorption Q factor, which is mainly controlled by the doping of the quantum wells (QWs). This device rejects most background radiation and thus significantly suppresses the background noise, resulting in a background limited specific detectivity (DBLIP∗) 11.85 times that of a conventional ideal photoconductor. A detailed formula is employed for calculation of DBLIP∗, incorporating the wavelength and incident angle dependence of the quantum efficiency. The ultra-narrow photoresponse band with a high peak responsivity can be tuned over the photosensitive wavelength range of the QWs by simply modifying the in-plane structural parameters. This work opens a new avenue to greatly enhance the specific detectivity for infrared detectors based on the joint effect of quasi-BIC and critical coupling.

Minimum-dissipation principle for synchronized stochastic oscillators far from equilibrium

We prove a linear stability-dissipation relation (SDR) for q-state Potts models driven far from equilibrium by a nonconservative force. At a critical coupling strength, these models exhibit a synchronization transition from a decoherent into a synchronized state. In the vicinity of this transition, the SDR connects the entropy production rate per oscillator to the phase-space contraction rate, a measure of stability, in a simple way. For large but finite systems, we argue that the SDR implies a minimum-dissipation principle for driven Potts models as the dynamics selects stable nonequilibrium states with least dissipation. This principle holds arbitrarily far from equilibrium, for any stochastic dynamics, and for all q. Published by the American Physical Society 2024

Enhancing circular dichroism with anisotropic heterogeneous-structure based on MQBIC resonance

Metasurfaces based on quasi-bound states in the continuum (QBIC) typically require careful tuning of structural parameters to meet critical coupling condition, thereby maximizing circular dichroism (CD). In this study, we propose an innovative heterogeneous-structure comprising four pairs of symmetry-breaking silicon dielectric rods embedded in an anisotropic polymer. The resonance of magnetic QBIC (MQBIC) within the dielectric rods generates a CD response, while the polymer effectively modulates the chirality of the heterogeneous-structure and enhances the CD. This enhancement occurs because the resonance of the MQBIC in the diagonal direction of the heterogeneous-structure is confined, leading to a break in mirror symmetry. Further investigations reveal that even when critical coupling condition are not satisfied, CD can still be maximized through rotational modulation. Additionally, the critical coupling condition for the resonance of MQBIC in the heterogeneous-structure are more readily achievable, allowing the CD response to remain unconstrained by lattice constants. This research is anticipated to have a significant impact on the fields of biosensing and medical imaging in the future.

A perfect absorber induced by critical coupling between phonon polaritons of molybdenum trioxide and photons of photonic crystal

A perfect absorber induced by critical coupling between phonon polaritons of molybdenum trioxide and photons of photonic crystal

Topological data analysis of monopole current networks in $U(1)$ lattice gauge theory

In 44-dimensional pure compact U(1)U(1) lattice gauge theory, we analyse topological aspects of the dynamics of monopoles across the deconfinement phase transition. We do this using tools from Topological Data Analysis (TDA). We demonstrate that observables constructed from the zeroth and first homology groups of monopole current networks may be used to quantitatively and robustly locate the critical inverse coupling \beta_{c}βc through finite-size scaling. Our method provides a mathematically robust framework for the characterisation of topological invariants related to monopole currents, putting on firmer ground earlier investigations. Moreover, our approach can be generalised to the study of Abelian monopoles in non-Abelian gauge theories.

Microwave field sensor based on cold cesium Rydberg three-photon electromagnetically induced spectroscopy

Abstract We present the electromagnetically induced transparency (EIT) spectra of cold Rydberg four-level cascade atoms consisting of the 6S1/2 → 6P3/2 → 7S1/2 → 60P3/2 scheme. A coupling laser drives the Rydberg transition, a dressing laser couples two intermediate levels and a weak probe laser probes the EIT signal. We numerically solve the Bloch equations and investigate the dependence of the probe transmission rate signal on the coupling and dressing lasers. We find that the probe transmission rate can display an EIT or electromagnetically induced absorption (EIA) profile, depending on the Rabi frequencies of the coupling and dressing lasers. When we increase the Rabi frequency of the coupling laser and keep the Rabi frequency of the probe and dressing laser fixed, flipping of the EIA to EIT spectrum occurs at the critical coupling Rabi frequency. When we apply a microwave field coupling the transition 60P3/2 → 61S1/2, the EIT spectrum shows Autler–Townes splitting, which is employed to measure the microwave field. The theoretical measurement sensitivity can be 1.52 × 10−2 nV⋅cm−1⋅Hz−1/2 at the EIA–EIT flipping point.

High-Q all-dielectric metasurface perfect absorber powered by quasi-bound states in the continuum

All-dielectric perfect absorbers powered by quasi-bound states in the continuum (-BIC) are of great importance for developing high-performance optoelectronic devices because they provide high Q-factor absorption. However, these structures usually require the addition of metal back reflectors or degenerate critical coupling to achieve high absorption, which often introduces problems such as Joule heat output and sensitivity to geometrical parameters. In this work, we present an all-dielectric high Q-factor metasurface perfect absorber (MPA) with a cell structure consisting of split Si elliptical disks. By adjusting the tilt angle of the gap, the metasurface can excite a single quasi-BIC resonance in a highly reflective background, corresponding to the electric quadrupole (EQ) mode. Due to the asymmetric coupling of the EQ mode, the proposed metasurface easily breaks the 50% absorption limit. At the wavelength of 894.645 nm, the metasurface achieves a perfect absorption of more than 99% and has a Q-factor of up to 1955. In addition, the structure shows excellent tolerance to geometrical parameters while ensuring high absorption performance. By adjusting the polarization angle, we have also achieved an arbitrary tuning of the absorption efficiency without frequency shift. This work provides a viable scheme for the design of tunable, large-tolerance, and high-Q all-dielectric MPAs, which have a broad potential application in the fields of optical filtering, optical switching, and polarization detection.

T-matrix of piezoelectric shunt inclusions on a thin plate

Developing piezoelectric-based plate-like metamaterials necessitates an effective modeling method to elucidate the omnidirectional wave properties of piezoelectric coupled inclusions on a thin plate. The commonly used methods, such as the transfer-matrix method and finite element method, are inadequate for analyzing the transmission and reflection of omnidirectional waves in a two-dimensional elastic medium. Unlike the existing methods, the multiple scattering method employs Bessel functions as the displacement-basis methods which can accurately describe the propagation and scattering characteristics of omnidirectional waves. This paper develops a novel T-matrix formulation to support the multiple scattering method, representing the input-output relationship between incident and reflected waves from a piezoelectric shunt inclusion on a host thin plate. The piezoelectric shunt inclusion comprises a varying-thickness substrate bonded with piezoelectric shunting patches. The T-matrix of the piezoelectric shunt inclusion is formulated by integrating the wave-based method with Rayleigh-Ritz method. The derived T-matrix is then used to semi-analytically analyze the far-field scattering and reflection properties of a single inclusion. Numerical results capture the scattering properties resulting from trapped mode resonances of the piezoelectric shunt inclusion. Additionally, the capability of the piezoelectric shunt damping to design and tune multiple critical coupling conditions for axisymmetric modes of thin plates is parametrically investigated by varying the values of inductors and resistors.

Scattering of vortices with excited normal modes

We consider head-on collisions at critical coupling of vortices modeled by the Abelian-Higgs model. We investigate the two-vortex scattering, whereby the vortices are excited by the shape mode causing fluctuations in the gauge-invariant quantities. When the vortices are excited with a sufficiently large amplitude the moduli space approximation fails, and we observe an interesting behavior in which the vortices can become trapped in a quasibound state with multiple bounces. We perform a detailed investigation on the behavior of these excited vortices and sample a phase space of solutions. Interestingly, we find a fractal structure dependent on the initial phase of the mode and velocity of the vortices. Published by the American Physical Society 2024

Graphene-based hybrid metasurfaces with quasi-bound states in the continuum for manipulating terahertz absorption

We present a graphene-based hybrid metasurface with quasi-bound states in the continuum (BICs) for manipulating terahertz (THz) wave absorption. By the strategic application of structural perturbations for the THz metasurface, the symmetry-protected BICs transforms into quasi-BICs. The incorporation of graphene adeptly satisfies the critical coupling condition, thereby facilitating the attainment of the theoretical maximum absorption of 0.5 for the quasi-BICs in the THz region. And the Q-factor of the quasi-BIC can be up to 2755. Moreover, the metasurface exhibits a distinctive ability for unique tuning and efficient absorption of terahertz waves by varying the asymmetry parameter and Fermi levels. This work provides promising strategies for manipulating terahertz wave absorption.

Achieving Ultra‐High Background‐Limited Detectivity by a Brillouin Zone Folding Induced Quasi‐Bound State in the Continuum

AbstractDuring infrared detection, the thermal radiation from the background generates substantial photon noise and thus severely limit the capability of an infrared detector to identify a target. Going beyond this limitation has been a long‐standing challenge in the development of infrared detectors. This paper proposes to break this limitation by creating a narrow photoresponse band with a high peak responsivity to reject the background radiation and enhance the responsivity to the target with characteristic emission lines. This scheme is numerically demonstrated in a dimerized grating integrated quantum well infrared photodetector, based on critical coupling with a Brillouin zone folding induced quasi‐bound state in the continuum (BIC). The asymmetric deformation of the grating structure folds the photonic band and generates a quasi‐BIC with a tunable high radiation Q factor (QR) at the Γ point. By reducing the doping concentration of the quantum wells for a high absorption Q factor (QA) and tuning the QR to make QR = QA for critical coupling, a narrowband photoresponse with a high peak responsivity is achieved and the background‐limited specific detectivity of 4.55 × 1012 cm Hz1/2 W−1 is obtained for a 2π field of view, surpassing the ideal‐photoconductor limit by 92 times.

One dimensional staggered bosons, clock models, and their noninvertible symmetries

We study systems of staggered boson Hamiltonians in a one dimensional lattice and in particular how the translation symmetry by one unit in these systems is in reality a noninvertible symmetry closely related to T-duality. We also study the simplest systems of clock models derived from these staggered boson Hamiltonians. We show that the noninvertible symmetries of these lattice models together with the discrete ZN symmetry predict that these are critical points with a U(1) current algebra at c=1 and radius 2N whenever N>4. We also present an independent computation of this value that arises directly from the staggered boson variables and does not use these additional symmetries. We also present a theoretical estimate of the values of critical coupling constants away from the self-dual symmetry point in these clock models. Published by the American Physical Society 2024

Spectral wall in collisions of excited Abelian Higgs vortices

We find a spectral wall in collisions of two vortices in the Abelian Higgs model at the critical coupling. It occurs if the out-of-phase mode of initially separated vortices is excited. Published by the American Physical Society 2024

Long-distance strong coupling of magnon and photon: Effect of multi-mode waveguide

Coupled mode theory predicts that the long-distance coupling between two distant harmonic oscillators is in the weak coupling regime. However, a recent experimental measurement observed strong coupling of magnon and critically-driven photon with a distance of over two meters. To explain the discrepancy between theory and experiment, we study long-distance coupling of magnon and photon mediated by a multi-mode waveguide. Our results show that strong coupling is achieved only when both critical coupling and multi-mode waveguide are involved. The former reduces the damping while the latter enhances the coupling strength by increasing the pathways of coupling magnon and photon. Our theory and results pave the way for understanding the long-distance coherence and designing the magnon-based distributed quantum networks.

Third order interactions shift the critical coupling in multidimensional Kuramoto models

The study of higher order interactions in the dynamics of Kuramoto oscillators has been a topic of intense research. Previous works have demonstrated that such interactions can give rise to interesting new phenomena such as multi-stability and synchronization even if the interaction between oscillators is repulsive. Here we consider higher order interactions in the multidimensional Kuramoto model where pairs (1-simplex), triplets (2-simplex) and quadruples (3-simplex) of oscillators interact simultaneously with different coupling strengths, k1, k2 and k3, respectively. For the types of asymmetric interactions considered, we show that three body terms shift the critical coupling for synchronization towards higher values, except in 2 dimensions where a cancellation occurs. However, after the transition, three and four body interactions combine to facilitate synchronization. We also show that, for fixed values of k2 and k3, and fully connected networks, the behavior of the order parameter r(k1) is described by a universal curve given by its value at k2=k3=0, shifted along the k1 axis. Similar to the 2-dimensional case with asymmetric interactions, bi-stability and hysteresis develop for large enough higher order interactions. Multi-stability, typical of symmetric higher order interactions, is not found. We show simulations in three and four dimensions to illustrate the dynamics.

Resonance-induced synchronization in coupled phase oscillators with bimodal frequency distribution and periodic coupling.

Synchronization behaviors in globally coupled phase oscillators with symmetric bimodal frequency distribution and periodic coupling are studied. It is found that by a proper setting of the frequency of the periodic coupling, the synchronization propensity of the oscillators can be markedly improved. Specifically, we show that when the frequency of the periodic coupling matches the distance of the central frequencies in the distribution, the critical coupling characterizing the onset of synchronization can be substantially decreased. The mechanism behind the phenomenon of periodic-coupling-enhanced synchronization is analyzed by the methods of Ott-Antonsen ansatz and synchronization transition tree, and it is revealed that the synchronization enhancement is attributed to the resonance between the synchronization clusters and the periodic coupling.

Experimental Demonstration of High-Efficiency Harmonic Generation in Photonic Moiré Superlattice Microcavities.

Integrated photonic microcavities have demonstrated powerful enhancement of nonlinear effects, but they face a challenge in achieving critical coupling for sufficient use of incident pump power. In this work, we first experimentally demonstrate that highly efficient third-harmonic generation (THG) and detectable second-harmonic generation (SHG) can be produced from high-Q photonic moiré superlattice microcavities, where a critical coupling condition can be achieved via selecting a magic angle. Furthermore, at the magic angle of 13.17°, critical coupling is satisfied, resulting in a normalized THG conversion efficiency of 136%/W2 at a relatively low peak pump power of 6.8 MW/cm2, which is 3 orders of magnitude higher than the best results reported previously. Our work shows the power of photonic moiré superlattices in enhancing nonlinear optical performances through flexible and feasible engineering resonant modes, which can be applied in integrated frequency conversion and generation of quantum light sources.

Disorder free many-body localization transition in two quasiperiodically coupled Heisenberg spin chains

Disorder free many-body localization (MBL) can occur in interacting systems that can dynamically generate their own disorder. We address the thermal-MBL phase transition of two isotropic Heisenberg spin chains that are quasiperiodically coupled to each other. The spin chains are incommensurate and are coupled through a short-range exchange interaction of the XXZ type that decays exponentially with the distance. Using exact diagonalization, matrix product states, and a density matrix renormalization group, we calculate the time evolution of the entanglement entropy at long times and extract the inverse participation ratio in the thermodynamic limit. We show that this system has a robust MBL phase. We establish the phase diagram with the onset of MBL as a function of the interchain exchange coupling and of the incommensuration between the spin chains. The Ising limit of the interchain interaction optimizes the stability of the MBL phase over a broad range of incommensurations above a given critical exchange coupling. Incorporation of interchain spin flips significantly enhances entanglement between the spin chains and produces delocalization, favoring a prethermal phase whose entanglement entropy grows logarithmically with time. Published by the American Physical Society 2024