Critical Coupling Research Articles

Linear Terahertz Frequency Conversion in a Temporal‐Boundary Metasurface

AbstractLinear frequency conversion enabled by a temporal boundary of a resonant cavity provides a promising alternative to generating new frequency components, where the temporal boundary can be introduced by rapidly modifying the refractive index of the cavity. However, the perturbed photon dynamics during the ultrafast linear process is still vague. Here, the photon dynamics based on temporal coupled‐mode theory are explored and the phenomena are demonstrated with a silicon metasurface via simulations and experiments in the terahertz regime. With the time‐varying real and imaginary parts of the refractive index, a periodic modulation (Td) of the amplification coefficient of the newly generated photons is observed where the optimized coefficient occurs in the first period at td = Td /2. It further establishes a dynamic critical coupling condition to improve the amplification coefficient in the dynamic process. The scheme of linear frequency conversion via temporal boundary can revolutionize the paradigm of parametric amplification from nonlinear to linear processes, and conversion efficiency will be improved in a high‐quality factor cavity. The ultrafast modulation will find applications in optical signal processing, modulators, and reconfigurable intelligent surfaces for development of the next‐generation communications.

Maximum chirality of THz metasurfaces with quasi-bound states in the continuum.

Metasurfaces hold great promise for terahertz (THz) chiral-optical devices. Here, we proposed a chiral THz metasurface with quasi-bound state in the continuum (BIC) for maximum chirality. By exploiting structural perturbations of the dipole displacement and the diverging angle for the THz metasurface, the symmetry-protected BIC transforms into quasi-BIC. The critical coupling condition is satisfied by the introduction of graphene, enabling the theoretical maximum absorption of the quasi-BIC. Subsequently, the perturbations are balanced to obtain maximum chirality. The numerical simulations show that the THz metasurface exhibits strong linear chirality with the circular dichroism (CD) of 0.99 at the quasi-BIC. Additionally, the chiral third harmonic generation (THG) is achieved, characterized by high efficiency up to 19% and strong THG-CD as high as 0.99. It is expected that the THz metasurfaces has great potential for applications in chiral sensing and imaging.

Krylov spaces for truncated spectrum methodologies

We propose herein an extension of truncated spectrum methodologies, a nonperturbative numerical approach able to elucidate the low energy properties of quantum field theories. TSMs, in their various flavors, involve a division of a computational Hilbert space, H, into two parts, one part, H1 that is “kept” for the numerical computations, and one part, H2, that is discarded or “truncated.” Even though H2 is discarded, truncated spectrum methodologies will often try to incorporate the effects of H2 in some effective way. In these terms, we propose to keep the dimension of H1 small. We pair this choice of H1 with a Krylov subspace iterative approach able to take into account the effects of H2. This iterative approach can be taken to arbitrarily high order and so offers the ability to compute quantities to arbitrary precision. In many cases it also offers the advantage of not needing an explicit UV cutoff. To compute the matrix elements that arise in the Krylov iterations, we employ a Feynman diagrammatic representation that is then evaluated with Monte Carlo techniques. Each order of the Krylov iteration is variational and is guaranteed to improve upon the previous iteration. The first Krylov iteration is akin to the next-to-leading order approach of Elias-Miró [NLO renormalization in the Hamiltonian truncation, ]. To demonstrate this approach, we focus on the (1+1d)-dimensional ϕ4 model and compute the bulk energy and mass gaps in both the Z2-broken and unbroken sectors. We estimate the critical ϕ4 coupling in the broken phase to be gc=0.2645±0.002. Published by the American Physical Society 2024

Conventional and Unconventional Dicke Models: Multistabilities and Nonequilibrium Dynamics.

The Dicke model describes the collective behavior of a subwavelength-size ensemble of two-level atoms (i.e., spin-1/2) interacting identically with a single quantized radiation field of a cavity. Across a critical coupling strength it exhibits a zero-temperature phase transition from the normal state to the superradiant phase where the field is populated and the collective spin acquires a nonzero x component, which can be imagined as ferromagnetic ordering of the atomic spins along x. Here we introduce a variant of this model where two subwavelength-size ensembles of spins interact with a single quantized radiation field with different strengths. Subsequently, we restrict ourselves to a special case where the coupling strengths are opposite (which is unitarily equivalent to equal-coupling strengths). Because of the conservation of the total spin in each ensemble individually, the system supports two distinct superradiant states with x-ferromagnetic and x-ferrimagnetic spin ordering, coexisting with each other in a large parameter regime. The stability and dynamics of the system in the thermodynamic limit are examined using a semiclassical approach, which predicts nonstationary behaviors due to the multistabilities. At the end, we also perform small-scale full quantum-mechanical calculations, with results consistent with the semiclassical ones.

Kinetics and functional consequences of BK channels activation by N-type Ca2+ channels in the dendrite of mouse neocortical layer-5 pyramidal neurons.

The back-propagation of an action potential (AP) from the axon/soma to the dendrites plays a central role in dendritic integration. This process involves an intricate orchestration of various ion channels, but a comprehensive understanding of the contribution of each channel type remains elusive. In this study, we leverage ultrafast membrane potential recordings (Vm) and Ca2+ imaging techniques to shed light on the involvement of N-type voltage-gated Ca2+ channels (VGCCs) in layer-5 neocortical pyramidal neurons' apical dendrites. We found a selective interaction between N-type VGCCs and large-conductance Ca2+-activated K+ channels (BK CAKCs). Remarkably, we observe that BK CAKCs are activated within a mere 500 μs after the AP peak, preceding the peak of the Ca2+ current triggered by the AP. Consequently, when N-type VGCCs are inhibited, the early broadening of the AP shape amplifies the activity of other VGCCs, leading to an augmented total Ca2+ influx. A NEURON model, constructed to replicate and support these experimental results, reveals the critical coupling between N-type and BK channels. This study not only redefines the conventional role of N-type VGCCs as primarily involved in presynaptic neurotransmitter release but also establishes their distinct and essential function as activators of BK CAKCs in neuronal dendrites. Furthermore, our results provide original functional validation of a physical interaction between Ca2+ and K+ channels, elucidated through ultrafast kinetic reconstruction. This insight enhances our understanding of the intricate mechanisms governing neuronal signaling and may have far-reaching implications in the field.

Volcano transition in a system of generalized Kuramoto oscillators with random frustrated interactions

In a system of heterogeneous (Abelian) Kuramoto oscillators with random or ‘frustrated’ interactions, transitions from states of incoherence to partial synchronization were observed. These so-called volcano transitions are characterized by a change in the shape of a local field distribution and were discussed in connection with an oscillator glass. In this paper, we consider a different class of oscillators, namely a system of (non-Abelian) SU(2)-Lohe oscillators that can also be defined on the 3-sphere, i.e. an oscillator is generalized to be defined as a unit vector in four-dimensional Euclidean space. We demonstrate that such higher-dimensional Kuramoto models with reciprocal and nonreciprocal random interactions represented by a low-rank matrix exhibit a volcano transition as well. We determine the critical coupling strength at which a volcano-like transition occurs, employing an Ott–Antonsen ansatz. Numerical simulations provide additional validations of our analytical findings and reveal the differences in observable collective dynamics prior to and following the transition. Furthermore, we show that a system of unit 3-vector oscillators on the 2-sphere does not possess a volcano transition.

Bilocal Field Theory for Composite Scalar Bosons.

We give a bilocal field theory description of a composite scalar with an extended binding potential that reduces to the Nambu-Jona-Lasinio (NJL) model in the pointlike limit. This provides a description of the internal dynamics of the bound state and features a static internal wave function, ϕ(r→), in the center-of-mass frame that satisfies a Schrödinger-Klein-Gordon equation with eigenvalues m2. We analyze the "coloron" model (single perturbative massive gluon exchange) which yields a UV completion of the NJL model. This has a BCS-like enhancement of its interaction, ∝Nc the number of colors, and is classically critical with gcritical remarkably close to the NJL quantum critical coupling. Negative eigenvalues for m2 lead to spontaneous symmetry breaking, and the Yukawa coupling of the bound state to constituent fermions is emergent.

Optical sensing based on phase interrogation with a Young's interference hologram using a digital micromirror device.

We propose a new holographic interferometric technique of phase interrogation for nanophotonic sensors, allowing to reach low phase noise and fluctuation by using a digital micromirror device spatial light modulator. With the spatial light modulator, both beam shaping and phase shifting interferometry can be simultaneously managed, hence enabling the interrogation of nanophotonic devices with a common-path heterodyne Young's interference experiment. The efficiency of the technique is illustrated in the particular case of temperature sensing using Tamm plasmon photonic crystals. The hologram sensor allows to probe resonant structures with deep attenuation at resonance, such as resonant structures at critical coupling or with phase singularities.

An analytic estimation for the largest Lyapunov exponent of the Rössler chaotic system based on the synchronization method

<abstract><p>In this paper, we propose an analytical approach to estimate the largest Lyapunov exponent (LLE) of a Rössler chaotic system, leveraging the synchronization method. This research focuses on establishing an analytical criterion for the synchronization of two identical Rössler chaotic systems through the linear coupling of state variables. This is crucial because the LLE of such systems can be estimated based on the critical coupling required for synchronization. Unlike previous studies, we first transform the synchronization error system between two identical Rössler chaotic systems into a set of Volterra integral equations by using the Laplace transform and convolution theorem. The critical coupling for synchronization is analytically derived using integral equation theory to solve the error system. As compared to the numerical results of the Rössler chaotic system's LLE, our analytical estimates demonstrate high accuracy. Our findings suggest that the challenge of estimating the Rössler chaotic system's LLE can be simplified to solving a cubic algebraic equation, offering a novel perspective on the analysis of how parameters influence the LLE's value in the Rössler chaotic system.</p></abstract>

Fabrication of gallium nitride waveguide resonators by high-power impulse magnetron sputtering at room temperature

In this work, we demonstrate gallium nitride (GaN) waveguide resonators by sputtering amorphous GaN films on the silicon-based substrate. With the aid of high-power impulse magnetron sputtering (HiPIMS), high-quality, high-deposition-rate, and high-flatness GaN films can be deposited directly onto the silicon substrate with a 4 μm buried oxide layer at room temperature. Waveguide resonators with a quality factor of up to 4 × 104 are demonstrated, and closely critical coupling is achieved at a 0.2 μm gap by optimizing the gap sizes, showing a high extinction ratio of waveguide resonators at ≈24 dB. The fabrication process of HiPIMS-GaN waveguide resonators utilizes CMOS-compatible techniques and operates at a low thermal budget. Compared to conventional GaN films fabricated using metal-organic chemical vapor deposition, this study offers the potential to produce low-cost GaN waveguides on amorphous substrates and realize integrated GaN photonics in optical communication, nonlinear photonics, and quantum photonics by high-quality HiPIMS films.

Solitary death in coupled limit cycle oscillators with higher-order interactions.

Coupled limit cycle oscillators with pairwise interactions are known to depict phase transitions from an oscillatory state to amplitude or oscillation death. This Research Letter introduces a scheme to incorporate higher-order interactions which cannot be decomposed into pairwise interactions and investigates the dynamical evolution of Stuart-Landau oscillators under the impression of such a coupling. We discover an oscillator death state through a first-order (explosive) phase transition in which a single, coupling-dependent stable death state away from the origin exists in isolation without being accompanied by any other stable state usually existing for pairwise couplings. We call such a state a solitary death state. Contrary to widespread subcritical Hopf bifurcation, here we report homoclinic bifurcation as an origin of the explosive death state. Moreover, this explosive transition to the death state is preceded by a surge in amplitude and followed by a revival of the oscillations. The analytical value of the critical coupling strength for the solitary death state agrees with the simulation results. Finally, we point out the resemblance of the results with different dynamical states associated with epileptic seizures.

Restoring strong cosmic censorship in Reissner–Nordström–de Sitter black holes via non-minimal electromagnetic-scalar couplings

We investigate whether the Strong Cosmic Censorship (SCC) Conjecture can be reinstated at the classical level in Reissner–Nordström–de Sitter (RNdS) black holes by introducing non-minimal couplings between the electromagnetic and scalar fields in Einstein–Maxwell-scalar (EMS) theory. By conducting numerical calculations, we find that the SCC can be restored within a specific range of the coupling constant. Notably, for a given value of the cosmological constant, there exists a critical coupling constant above which the EMS theory satisfies the SCC. These findings suggest that the non-minimal couplings between the electromagnetic and scalar fields may play a crucial role in the restoration of the SCC in RNdS spacetime. Additionally, under negative coupling constants, the low-lying modes of nearly extremal black holes exhibit β>1,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta > 1,$$\\end{document} enabling C2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C^2$$\\end{document} extensions beyond the Cauchy horizon and intensifying the violation of the SCC.

On the two-photon quantum Rabi model at the critical coupling strength

On the two-photon quantum Rabi model at the critical coupling strength

Maximally Efficient Biphoton Generation by Single Photon Decay in Nonlinear Quantum Photonic Circuits.

We develop a general nonperturbative formalism and propose a specific scheme for maximally efficient generation of biphoton states by parametric decay of single photons. We show that the well-known critical coupling concept of integrated optics can be generalized to the nonlinear coupling of quantized photon modes to describe the nonperturbative optimal regime of a single-photon nonlinearity and establish a fundamental upper limit on the nonlinear generation efficiency of quantum-correlated photons, which approaches unity for low enough absorption losses.

Biphoton routing in few-emitter chiral waveguide quantum electrodynamics ladders

We study the problem of two-photon routing in waveguide QED ladders where a few two-level quantum emitters (QEs) are simultaneously coupled with two chiral waveguides. We analyze the routing probability in two regimes, namely, under a purely plane wave approximation (scattering case) and in the presence of photon-photon bound state formation. Within the scattering case, we examine the two-photon routing in the presence of up to five QEs, considering two possibilities separately: ideal-symmetric coupling and the critical coupling scenario. We examine the photon routing up to the two QEs for the bound state situation and compare the photon redirection efficiency with the corresponding scattering case. Our findings show the potential of utilizing chiral light-matter interactions in multi-photon and multi-emitter-based quantum networking protocols where interlinking among spatially distant nodes is required.

High-speed metasurface modulator using perfectly absorptive bimodal plasmonic resonance

Free-space electro-optic (EO) modulators operating at gigahertz and beyond are attractive for a wide range of emerging applications, including high-speed imaging, free-space optical communication, microwave photonics, and diffractive computing. Here, we experimentally demonstrate a high-speed plasmonic metasurface EO modulator operating in a near-infrared wavelength range with a gigahertz modulation bandwidth. To achieve efficient intensity modulation of reflected light from an ultrathin metasurface layer, we utilize the bimodal plasmonic resonance inside a subwavelength metal–insulator–metal grating, which is precisely tuned to satisfy the critical coupling condition. As a result, perfect absorption of −27 dB (99.8%) and a high quality (Q) factor of 113 are obtained at a resonant wavelength of 1650 nm. By incorporating an EO polymer inside the grating, we achieve a modulation depth of up to 9.5 dB under an applied voltage of ±30 V. The 3-dB modulation bandwidth is confirmed to be 1.25 GHz, which is primarily limited by the undesired contact resistance and the output impedance of the driver. Owing to the high electrical conductivity of metallic gratings and a compact device structure with a minimal parasitic capacitance, the demonstrated device can potentially operate at several tens of gigahertz, which opens up exciting opportunities for ultrahigh-speed active metasurface devices in various applications.

Machine learning assisted design of acoustic metamaterials with broadband sound-absorbing and superior mechanical performance

It remains challenging to design lightweight metamaterials with superior sound-absorbing and load-bearing capacities. One promising approach is using multiple coherently coupled resonators to serve as sound absorbers and reinforced grid cores of sandwich panels. However, with too many design parameters each requiring careful tuning to reach the stringent critical coupling condition, achieving an optimal design based on experience alone is challenging. An autoencoder-like neural network is constructed that, once well trained, significantly promote the inverse design process thanks to the highly efficient computational speed. Particularly, a probabilistic model is inserted into the network to tackle ill-posed inverse problems requiring man-made and probably unreal spectrum as an input, and can provide multiple ultra-thin (32 mm) and broadband designs. We have fabricated and tested the optimized metallic sandwich structures, showing broadband capacity of using solely nine resonators to achieve quasi-perfect sound absorption (over 0.9) from 399 to 675 Hz. The static compression and dynamic impact testing also verify the superior mechanical performance with high yield strength (21.2 MPa) and large normalized energy absorption (0.29) at lower relative density (13.5%). We believe this work is encouraging to accelerate the design of multifunctional absorbers targeted especially for low-frequency noise control in engineering.

Dual-mode tunable absorber based on quasi-bound states in the continuum.

In this Letter, we propose a novel, to the best of our knowledge, dual-mode tunable absorber that utilizes quasi-bound states in the continuum (q-BIC) based on the periodically arranged silicon cylinders tetramer. By introducing asymmetry perturbation through manipulating the diameters of diagonal cylinders in the all-dielectric structure, the symmetry-protected BIC (SP-BIC) transforms into q-BIC, leading to the emergence of one transmission and one reflection Fano-like resonant mode. The relationship between the quality factor of each mode and the asymmetry parameter α is analyzed, revealing an exponential dependence with an exponent of -1.75, i.e., Q ∝ α-1.75. To explain the underlying physics, multipole decomposition analysis and Aleksandra's theory are applied. Subsequently, a monolayer graphene is introduced to the all-dielectric structure to demonstrate the application of the dual-mode tunable absorber. When the critical coupling condition is satisfied, each mode can achieve the theoretical maximum absorption, demonstrating the distinctive capability of our proposed absorber for tuning and efficient light absorption. This research provides valuable insights into light-matter interactions and opens up possibilities for optical modulation and the development of graphene-based devices.

Perfect Absorption and Reflection Modulation Based on Asymmetric Slot-Assisted Gratings without Mirrors.

As a perfect graphene absorber without any external mirrors, we proposed asymmetric slot-assisted grating structures supporting two degenerate resonant modes of the guided-mode resonances (GMR) and the quasi-bound states in the continuum (quasi-BIC). The GMR mode functions as an internal mirror in conjunction with the background scattering, while the quasi-BIC, which is responsible for perfect graphene absorption, stems from the horizontal symmetry breaking by an asymmetric slot. By properly shifting the slot center from the grating center, the leakage rate of quasi-BIC can be controlled in such a way as to satisfy the critical coupling condition. We provide a comprehensive study on the coupling mechanism of two degenerate resonant modes for a one-port system mimicking the resonance. We also numerically demonstrated that our proposed grating structures show an excellent reflection-type modulation performance at optical wavelength ranges when doped double-layer graphene is applied. Due to the perfect absorption at the OFF state, a high modulation depth of ~50 dB can be achieved via a small Fermi level variation of ~0.05 eV. To obtain the lower insertion loss at the ON state, the higher Fermi level is required to decrease the graphene absorption coefficient.

Exploring the Impact of Intermolecular Interactions on the Glassy Phase Formation of Twist-Bend Liquid Crystal Dimers: Insights from Dielectric Studies

The formation of the nematic to twist-bend nematic (NTB) phase has emerged as a fascinating phenomenon in the field of supramolecular chemistry, based on complex intermolecular interactions. Through a careful analysis of molecular structures and dynamics, we elucidate how these intermolecular interactions drive the complex twist-bend modulation observed in the NTB. The study employs broadband dielectric spectroscopy spanning frequencies from 10 to 2 × 109 Hz to investigate the molecular orientational dynamics within the glass-forming thioether-linked cyanobiphenyl liquid crystal dimers, namely, CBSC7SCB and CBSC7OCB. The experimental findings align with theoretical expectations, revealing the presence of two distinct relaxation processes contributing to the dielectric permittivity of these dimers. The low-frequency relaxation mode is attributed to an “end-over-end rotation” of the dipolar groups parallel to the director. The high-frequency relaxation mode is associated with precessional motions of the dipolar groups about the director. Various models are employed to describe the temperature-dependent behavior of the relaxation times for both modes. Particularly, the critical-like description via the dynamic scaling model seems to give not only quite good numerical fittings, but also provides a consistent physical picture of the orientational dynamics in accordance with findings from infrared (IR) spectroscopy. Here, as the longitudinal correlations of dipoles intensify, the m1 mode experiences a sudden upsurge in enthalpy, while the m2 mode undergoes continuous changes, displaying critical mode coupling behavior. Interestingly, both types of molecular motion exhibit a strong cooperative interplay within the lower temperature range of the NTB phase, evolving in tandem as the material’s temperature approaches the glass transition point. Consequently, both molecular motions converge to determine the glassy dynamics, characterized by a shared glass transition temperature, Tg.