Coupled-mode Theory Research Articles

Terahertz dual-tunable absorber based on hybrid gold-graphene-strontium titanate-vanadium dioxide configuration

A terahertz dual-tunable polarization-independent metamaterial absorber based on hybrid gold-graphene-strontium titanate (STO)-vanadium dioxide (VO2) configuration is proposed. The results show that the absorption rate of absorber can achieve 98.3% at 0.2 THz. Moreover, both the absorption frequency and absorption rate of the absorber can be dual-tuned by varying the chemical potential of graphene and temperature of STO and VO2. Additionally, the performance of the absorber is theoretically analyzed by using the coupled mode theory (CMT) and impedance matching theory (IMT). Finally, the changes in the absorber's absorption spectra are discussed when the depth of STO and VO2 layers is modified. This work provides a theoretical basis for the designs of dual-tunable absorbers and filters, and also offers a new method for switching and modulation of THz radiation.

Triple plasmon-induced transparency and polarization-insensitive optical switch based on monolayer patterned graphene metamaterial

This study introduces a single-layer patterned graphene metamaterial, composed of a graphene ring (GR), two parallel graphene strips (TPGSs), two vertical graphene strips (TVGSs), and a graphene block (GB), which achieves triple plasmon-induced transparency (TPIT) in the terahertz (THz) frequency range using coupled mode theory (CMT) and the finite-difference time-domain (FDTD) technique. Moreover, a synchronized electro-optical switch with four modulation modes is implemented by dynamically adjusting graphene’s Fermi level, exhibiting modulation degrees of amplitude (MDA) of 88.3%, 94.7%, 86.9%, and 89.0% at 1.69[Formula: see text]THz, 3.14[Formula: see text]THz, 3.95[Formula: see text]THz, and 4.67[Formula: see text]THz, respectively. In addition, the modes excited by TPGSs and TVGSs interconvert due to the proposed structure’s symmetry, accounting for the polarization insensitivity of TPIT. The enhanced MDA and polarization insensitivity of the electro-optical switch presented in this study significantly surpass those of comparable electro-optical switches. As a result, the polarization insensitivity of multiple optical switches to incident light fields offers a promising approach for designing optoelectronic devices.

High-performance plasmonic graphene-based multiplexer/demultiplexer

This paper proposes the use of two innovative single-mode plasmonic resonator-based filters to design a two-channel multiplexer/demultiplexer that demonstrates high transmission efficiencies and small sizes below 0.042 μm2. The proposed filters utilize slits in the center of rectangular and square-shaped resonators which results in a single-mode filtering effect. The study employs numerical and theoretical analyses, specifically the finite difference time domain method and coupled-mode theory, to evaluate the designed structures. The simulation results indicate that the rectangular-based demultiplexer has a cross-talk of −34 dB, a maximum transmission efficiency of 0.93, and an amplitude modulation of 0.44 dB, while the square-shaped resonator-based demultiplexer has a maximum transmission efficiency of 0.83, an amplitude modulation of 0.1 dB, and a cross-talk of −39.5 dB. These findings suggest that the proposed structures hold potential for application in optical communication systems.

Observation of the bound states in the continuum supported by mode coupling in a terahertz metasurface.

Metasurface supporting bound states in the continuum (BIC) provides a unique approach for the realization of intense near-field enhancement and high quality factor (Q-factor) resonance, which promote the advancement of various applications. Here we experimentally demonstrate a Friedrich-Wintgen BIC based on the mode coupling in the terahertz metasurface, which produces BIC by the coupling of the LC mode and dipole mode resonances. The transition from ideal BIC to quasi-BIC is caused by the mismatch of the coupling, and the mode decay rate during this process is analyzed by temporal coupled mode theory. The Q-factor and the electric field enhancement of the quasi-BIC resonance are significantly increased, which provides enormous potential in sensing, nonlinear optics, and topological optics.

Acoustic chiral mode switching by dynamic encircling of exceptional points

Chiral mode switching initiated by dynamic encircling of an exceptional point (EP) has shown an extraordinary ability in wave controlling. In this work, we study the chiral mode transfer for acoustic waves in the coupled waveguide system that supports the non-adiabatic evolution of eigenstates. The system comprises a finite number of structural elements, which are constructed according to parametric conditions in the loop enclosing the EP and then stacked such that acoustic propagation in the waveguide system is equivalent to the dynamic encircling of the EP. An analytic model based on the spatial coupled-mode theory is developed, which provides a practical guide to design the system and makes predictions for dynamic evolution. Numerical simulation of the waveguide system is conducted to demonstrate the chiral mode switching for sounds.

Quadruple Plasmon-Induced Transparency and Dynamic Tuning Based on Bilayer Graphene Terahertz Metamaterial.

This study proposes a terahertz metamaterial structure composed of a silicon-graphene-silicon sandwich, aiming to achieve quadruple plasmon-induced transparency (PIT). This phenomenon arises from the interaction coupling of bright-dark modes within the structure. The results obtained from the coupled mode theory (CMT) calculations align with the simulations ones using the finite difference time domain (FDTD) method. Based on the electric field distributions at the resonant frequencies of the five bright modes, it is found that the energy localizations of the original five bright modes undergo diffusion and transfer under the influence of the dark mode. Additionally, the impact of the Fermi level of graphene on the transmission spectrum is discussed. The results reveal that the modulation depths (MDs) of 94.0%, 92.48%, 93.54%, 96.54%, 97.51%, 92.86%, 94.82%, and 88.20%, with corresponding insertion losses (ILs) of 0.52 dB, 0.98 dB, 1.37 dB, 0.70 dB, 0.43 dB, 0.63 dB, 0.16 dB, and 0.17 dB at the specific frequencies, are obtained, achieving multiple switching effects. This model holds significant potential for applications in versatile modulators and optical switches in the terahertz range.

Coupling-Assisted Quasi-Bound States in the Continuum in Heterogeneous Metasurfaces

In this paper, we present a Bound states in the continuum (BIC) metamaterial in heterogeneous structures based on the coupled mode theory. We find the more general physical parameters to represent BIC, which are the resonant frequencies and corresponding phases of metamaterial structures. Therefore, BIC metamaterial comes from the equal value of the resonant frequencies and phases of metamaterial structures which are not only for homogeneous structures. Meanwhile, if one of the metamaterial structure's resonant frequency and phase vary by varying geometry, we can obtain the quasi-BIC instead of the broken symmetry of homogeneous structures. In this paper, we provide the BIC and quasi-BIC with one example of two heterogeneous structures which are cut wire (CW) and Split-Ring Resonator (SRR) and it widely extends the metamaterial BIC beyond common sense. Furthermore, we demonstrate the simulation results and experimental results to proof our idea.

Analytical investigation of unidirectional reflectionless phenomenon near the exceptional points in graphene plasmonic system.

We propose a two-dimensional array made of a double-layer of vertically separated graphene nanoribbons. The transfer matrix method and coupled mode theory are utilized to quantitatively depict the transfer properties of the system. We present a way to calculate the radiative and the intrinsic loss factors, combined with finite-difference time-domain simulation, conducting the complete analytical analysis of the unidirectional reflectionless phenomenon. By adjusting the Fermi energy and the vertical distance between two graphene nanoribbons, the plasmonic resonances are successfully excited, and the unique phenomena can be realized at the exceptional points. Our research presents the potential in the field of optics and innovative technologies to create advanced optical devices that operate in the mid-infrared range, such as terahertz antennas and reflectors.

Dynamical heterogeneity in active glasses is inherently different from its equilibrium behavior

Activity-driven glassy dynamics, while ubiquitous in collective cell migration, intracellular transport, dynamics in bacterial and ant colonies, etc., also extends the scope and extent of the as-yet mysterious physics of glass transition. Active glasses are hitherto assumed to be qualitatively similar to their equilibrium counterparts at an effective temperature, [Formula: see text]. Here, we combine large-scale simulations and an analytical mode-coupling theory (MCT) for such systems and show that, in fact, an active glass is inherently different from an equilibrium glass. Although the relaxation dynamics can be equilibrium-like at a [Formula: see text], effects of activity on the dynamic heterogeneity (DH), which is a hallmark of glassy dynamics, are quite nontrivial and complex. With no preexisting data, we employ four distinct methods for reliable estimates of the DH length scales. Our work shows that active glasses exhibit dramatic growth of DH and systems with similar relaxation times, and thus, [Formula: see text] can have widely varying DH. To theoretically study DH, we extend active MCT and find good qualitative agreement between the theory and simulation results. Our results pave avenues for understanding the role of DH in glassy dynamics and can have fundamental significance even in equilibrium.

Higher-order exceptional points using lossfree negative-index materials

Negative-index materials (NIMs) support optical anti-parity-time (anti-) symmetry even when they are lossless. Here we prove the feasibility in achieving higher-order exceptional points (EPs) in lossfree waveguide arrays by utilizing the anti- symmetry induced by NIM. Numerical simulation about a third-order EP fits well with the coupled-mode theory. A scheme of achieving fourth-order EPs is also discussed. This work highlights the great potential of NIM in overcoming the obstacles of ordinary non-Hermitian optics, and the possibilities of combining anti-, , and Hermitian couplings for various purposes.

Acoustic properties, elasticity, and equation of state of glycerol under pressure.

We employed high-pressure Brillouin scattering to study the pressure dependencies of acoustic modes of glycerol up to 14GPa at 300K. We observed longitudinal acoustic velocities and transverse acoustic velocities for the first time from 5 to 14GPa. The results allow the determination of a complete set of elastic properties and an accurate determination of the pressure-volume (P-V) equation of state (EOS). EOS parameters, K0 = 14.9 ± 1.8GPa and K'0 = 5.6 ± 0.5, were determined from fits to the data from ambient pressure to 14GPa. Direct volume measurements of the P-V EOS are consistent with those determined by Brillouin scattering. A deviation from a Cauchy-like relationship for elastic properties was observed, and the pressure dependencies of the photoelastic constants and relaxation times were documented from 5 to 14GPa. These results have broad implications for glass-forming liquids, viscoelastic theory, and mode coupling theory.

Integrated Bragg grating filters based on silicon-Sb2Se3 with non-volatile bandgap engineering capability.

Integrated optical filters show outstanding capability in integrated reconfigurable photonic applications, including wavelength division multiplexing (WDM), programmable photonic processors, and on-chip quantum photonic networks. Present schemes for reconfigurable filters either have a large footprint or suffer from high static power consumption, hindering the development of reconfigurable photonic integrated systems. Here, a reconfigurable hybrid Bragg grating filter is elaborately designed through a precise, modified coupling mode theory. It is also experimentally presented by integrating non-volatile phase change material (PCM) Sb2Se3 on silicon to realize compact, low-loss, and broadband engineering operations. The fabricated filter holds a compact footprint of 0.5 µm × 43.5 µm and maintains a low insertion loss of < 0.5 dB after multiple levels of engineering to achieve crystallization. The filter is able to switch from a low-loss transmission state to the Bragg reflection state, making it a favorable solution for large-scale reconfigurable photonic circuits. With a switching extinction ratio over 30 dB at 1504.85 nm, this hybrid filter breaks the tradeoff between insertion loss and tuning range. These results reveal its potential as a new candidate for a basic element in large-scale non-volatile reconfigurable systems.

Acoustic scattering in lined panel cavities with membrane interfaces.

Noise control strategies are extremely important in industry. Acoustic liners and elastic membranes, which absorb or attenuate sound waves, are key components in ducting systems for active and passive noise reduction. In this article, we design and examine a reactive liner panel cavity with flexible interfaces. The lined panel cavity system is comprised of elastic membranes at the interfaces. The aim is to couple the incoming duct modes with the flexible components and then with the localized modes in the lined region. The governing equations for the lined panel cavity system are solved using a mode-matching technique that assures continuity of the normal velocities at the interfaces and can handle a range of strong coupling and higher-order edge conditions in contrast to the coupled mode theory. The aim is to investigate the effects of reactive liners and elastic membrane interfaces on wave scattering through the proposed acoustic enclosure design. The scattering performance of the proposed lined panel cavity is evaluated in terms of the reflected and transmitted energy flux as well as transmission loss. Analysis shows that the resonances at the membrane interfaces coupled to the linear cavities have a direct impact on power variations and the maximum transmission loss.

Thickness insensitive nanocavities for 2D heterostructures using photonic molecules

Abstract 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.

Critical coupling in cavity-resonant integrated-grating filters (CRIGFs).

We experimentally demonstrate critical coupling in miniature grating-coupled resonators known as cavity-resonant integrated-grating filters (CRIGFs). Using previously proposed asymmetric grating coupler designs for non-linear CRIGFs, and introducing a dedicated variant of a coupled-modes theory model to estimate physical properties out of the measured reflection and transmission characteristics of these resonators, we demonstrate fine control over the in-and out-coupling rate to the resonator while keeping constant both the internal losses and the resonant wavelength. Furthermore, the critical coupling condition is also observed to coincide with the maximum enhancement of the second harmonic generation signal.

Lock-in phenomena in non-Hermitian coupled systems

Coupled-mode theory has been widely employed in various branches of photonics, such as nano-photonics and non-Hermitian optics. Here we show that it can also be used as an effective physical model for clarifying various optical mode lock-in phenomena, which have been revealed to essentially originate from non-Hermitian degeneracies. We discuss the lock-in effects in physical systems through prototypical examples of optical feedback self-injection frequency lock-in technology and lock-in phenomena of ring laser gyroscopes (RLGs). Seemingly unrelated effects have been unified through one general non-Hermitian mode coupling model. Furthermore, based on our theoretical framework, we have uncovered the underlying mechanism of lock-in in RLGs and analyzed how this effect can be systematically suppressed or even eliminated

Ultrastrong Coupling Enhancement with Squeezed Mode Volume in Terahertz Nanoslots.

Metallic nanogaps have emerged as a versatile platform for realizing ultrastrong coupling in terahertz frequencies. Increasing the coupling strength generally involved reducing the gap width to minimize the mode volume, which presents challenges in fabrication and efficient material coupling. Here, we propose employing terahertz nanoslots, which can efficiently squeeze the mode volume in an extra dimension alongside the gap width. Our experiments using 500 nm wide nanoslots integrated with an organic-inorganic hybrid perovskite demonstrate ultrastrong phonon-photon coupling with a record-high Rabi splitting of 48% of the original resonance (Ω = 0.48ω0), despite having a gap width 5 times larger than previously reported structures with Ω = 0.45ω0. Mechanisms underlying this effective light--matter coupling are investigated with simulations using coupled mode theory. Moreover, bulk polariton analyses reveal that our results account for 68% of the theoretical maximum Rabi splitting, with the potential to reach 82% through further optimization of the nanoslots.

A dual-purpose sensor with a sawtooth U-shaped cavity and a rectangle-shaped cavity in a MIM waveguide structure

To improve the performance of subwavelength refractive index and temperature sensors, this paper proposes a subwavelength metal-insulator-metal (MIM) waveguide structure consisting of a sawtooth U-shaped cavity and a rectangular cavity based on surface plasmon polaritons. The transmission spectrum of the system is simulated using the finite element method (FEM) and verified with multi-mode interference coupled-mode theory (MICMT). The results demonstrate excellent sensing characteristics for the system, with a refractive index sensitivity of 1300 nm RIU−1, a figure of merit (FOM*) of 191.262, and a temperature sensitivity of 0.525 nm/°C. This indicates that the nano-plasma system is highly significant in refractive index and temperature sensing.

Multi-frequency polarization and electro-optical modulator based on triple plasmon- induced transparency in monolayer graphene metamaterials

In this paper, a dynamically tunable triple plasmon-induced transparency (PIT) is numerically investigated in a simple monolayer graphene- patterned structure. The structure is composed of five graphene strips and a silicon substrate. Electric field distributions show that the triple PIT effect is originated from the strong destructive interference between three bright modes and one dark mode, coupled mode theory (CMT) is utilized to confirm the finite-difference-time-domain (FDTD) simulation. Results show the maximum figure of merit (FOM) and sensitivity can reach 15.88 and 0.91 THz/RIU. A on-to-off electro-optical modulator is realized by tuning the Fermi levels of the graphene, the modulation degrees of amplitude are 79.3 %, 87.1 %, 70.5 %, 79.2 %, and 84.3 %, corresponding to 2.35 terahertz (THz), 3.12THz, 3.58 THz, 3.91THz, and 4.31 THz, respectively. In addition, the effect of polarized angle on the transmission spectra was studied in detail. A multi-frequency polarization modulator can also be realized with modulation degrees of amplitude (MDA) of 77.6 %, 85.9 %, and 93.7 % at 2.09 THz, 2.82 THz, and 3.59 THz, respectively, and the corresponding polarization extinction ratio (PER) are 6.5 dB, 8.5 dB, and 12.1 dB, respectively. Therefore, the results of the paper are significant to the design of multi-frequency, terahertz sensors, slow light, and modulators.

Multifrequency on-off modulation and slow light characterization of the patterned black phosphorus metamaterial based on dual plasmon-induced transparency.

We present a monolayer patterned black phosphorus (BP) metamaterial for generating a tunable dual plasmon-induced transparency (PIT). We have derived the expression for the theoretical transmittance by introducing the coupled mode theory (CMT), and the calculated results of the expression highly overlap with the simulation results. The quarterly frequency synchronous switch with two different operating bands is designed by the carrier density and scattering rate on the dual PIT modulation effect. Two parameters were selected as important markers to show the performance of the optical switch: the modulation depth (MD) and the insertion loss (IL). The theoretical analysis of this structure shows that the higher modulation depth (5.45d B<M D<12.06d B) and lower insertion loss (0.60d B<I L<0.22d B) of these switches are of good application. In addition, we found the slow light properties of the structure were excellent with a group index of up to 219. This work provides a theoretical basis to prepare multifrequency optical switch and optical buffer devices.