Showing papers on "Coupled mode theory published in 2022"

Terahertz spin-selective perfect absorption enabled by quasi-bound states in the continuum

Abstract: Spin-selective absorption is broadly applicable to numerous photonic devices. Here, based on a stereoscopic full metallic resonator array, a terahertz chiral metasurface with a single-layer structure is proposed and numerically demonstrated. By employing the coupled-mode theory, we demonstrate that the chiral metasurface can near-perfectly absorb one circularly polarized wave in the quasi-bound states in the continuum-induced critical coupling region but non-resonantly reflect its counterparts. Interestingly, the linewidths and handedness of the proposed chiral metasurface can be flexibly controlled by an in-plane symmetry perturbation. Our designs might offer an alternative strategy to develop chiral metasurfaces apart from conventional methods and might stimulate many potential applications for emerging terahertz technologies.

Investigation of bound states in the continuum in dual-band perfect absorbers.

Abstract: Enhancing the light-matter interaction of two-dimensional materials in the visible and near-infrared regions is highly required in optical devices. In this paper, the optical bound states in the continuum (BICs) that can enhance the interaction between light and matter are observed in the grating-graphene-Bragg mirror structure. The system can generate a dual-band perfect absorption spectrum contributed by guided-mode resonance (GMR) and Tamm plasmon polarition (TPP) modes. The optical switch can also be obtained by switching the TE-TM wave. The dual-band absorption response is analyzed by numerical simulation and coupled-mode theory (CMT), with the dates of each approach displaying consistency. Research shows that the GMR mode can be turned into the Fabry-Pérot BICs through the transverse resonance principle (TRP). The band structures and field distributions of the proposed loss system can further explain the BIC mechanism. Both static (grating pitch P) and dynamic parameters (incident angle θ) can be modulated to generate the Fabry-Pérot BICs. Moreover, we explained the reason why the strong coupling between the GMR and TPPs modes does not produce the Friedrich-Wintgen BIC. Taken together, the proposed structure can not only be applied to dual-band perfect absorbers and optical switches but also provides guidance for the realization of Fabry-Pérot BICs in lossy systems.

Triple plasmon-induced transparency and dynamically tunable electro-optics switch based on a multilayer patterned graphene metamaterial.

Abstract: A terahertz-band metamaterial composed of multilayer patterned graphene is proposed and triple plasmon-induced transparency is excited by coupling three bright modes with one dark mode. The Lorentz curve calculated by the coupled-mode theory agrees well with the finite-difference time-domain results. Dynamic tuning is investigated by changing the Fermi level. Multimode electro-optics switching can be designed and achieved, and the amplitude modulations of four resonance frequencies are 94.3%, 92.8%, 90.7%, and 93%, respectively, which can realize the design of synchronous and asynchronous electro-optics switches. It is hoped that these results can provide theoretical support and guidance for the future design and application of photonic and optoelectronic devices.

Independently tunable dual resonant dip refractive index sensor based on metal–insulator–metal waveguide with Q-shaped resonant cavity

Abstract: A plasmonic resonator system consisting of a metal–insulator–metal waveguide and a Q-shaped resonant cavity is proposed in this paper. The transmission properties of surface plasmon polaritons in this structure are investigated by using the finite difference in time domain (FDTD) method, and the simulation results contain two resonant dips. The physical mechanism is studied by the multimode interference coupled mode theory (MICMT), and the theoretical results are in highly consistent with the simulation results. Furthermore, the parameters of the Q-shaped cavity can be controlled to adjust the two dips, respectively. The refractive index sensor proposed in this paper, with a sensitivity of 1578 nm/RIU and figure of merit (FOM) of 175, performs better than most of the similar structures. Therefore, the results of the study are instructive for the design and application of high sensitivity nanoscale refractive index sensors.

High-Q localized surface plasmon resonance based on bound states in the continuum for enhanced refractive index sensing

Abstract: Nanophotonics based on localized surface plasmon resonance (LSPR) has emerged as a vibrant arena for research into enhanced light-matter interactions with potential applications in imaging, sensing, and computing. However, the low quality (Q) factor of LSPR is a significant barrier to comprehensive device applications. Here, we demonstrate that coupling the LSPR of a gold nanowire array with the optical bound states in the continuum (BIC) of a dielectric double-layer grating can significantly increase the Q factor of LSPR. We realize two hybrid modes with Q factors of up to 111 at 558 nm and 83 at 582 nm, which are about 14 and 10 times larger than those of an uncoupled gold nanowire array. Based on temporal coupled-mode theory, we further show that the resonance frequencies and Q factors of the hybrid modes can be modulated and optimized by varying relevant structural parameters. This coupled system provides a new platform for improving the figures of merit (FoMs) of LSPR-based refractive index sensors, and the concept of LSPR-BIC coupling can be extended to other similar nanosystems.

Independently tunable dual resonant dip refractive index sensor based on metal-insulator-metal waveguide with Q-shaped resonant cavity

Abstract: A plasmonic resonator system consisting of a metal-insulator-metal waveguide and a Q-shaped resonant cavity is proposed in this paper. The transmission properties of surface plasmon polaritons in this structure are investigated using the finite difference in time domain (FDTD) method, and the simulation results contain two resonant dips. And the physical mechanism is studied by the multimode interference coupled mode theory (MICMT), the theoretical results are in highly consistent with the simulation results. Furthermore, the parameters of the Q-shaped cavity can be controlled to adjust two dips respectively. The refractive index sensor with a sensitivity of 1578nm/RIU and figure of merit (FOM) of 175, performs better than most of the similar structures. Therefore, the results of the study are instructive for the design and application of high sensitivity nanoscale refractive index sensors.

Can a Terahertz Metamaterial Sensor be Improved by Embedding it in a High-Q Resonator and Operating the System in the Ultra-Strong Coupling Regime?

Abstract: When a metamaterial (MM) is embedded in a one-dimensional photonic crystal (PC) cavity, the ultra-strong coupling between the MM plasmons and the photons in the PC cavity gives rise to two new polariton modes with high quality factor. Here, we investigate by simulations whether such a strongly coupled system working in the terahertz (THz) frequency range has the potential to be a better sensor than a MM (or a PC cavity) alone. Somewhat surprisingly, one finds that the shift of the resonance frequency induced by an analyte applied to the MM is smaller in the case of the dual resonator (MM and cavity) than that obtained with the MM alone. However, the phase sensitivity of the dual resonator can be larger than that of the MM alone. With the dielectric perturbation theory - well established in the microwave community - one can show that the size of the mode volume plays a decisive role for the obtainable frequency shift. The larger frequency shift of the MM alone is explained by its smaller mode volume as compared with the MM-loaded cavity. Two main conclusions can be drawn from our investigations. First, that the dielectric perturbation theory can be used to guide and optimize the designs of MM-based sensors. And second, that the enhanced phase sensitivity of the dual resonator may open a new route for the realization of improved THz sensors.

Imaginary couplings in non-Hermitian coupled-mode theory: Effects on exceptional points of optical resonators

Abstract: Exceptional point (EP) degeneracies in coupled cavities with gain and loss provide on-chip photonic devices with unconventional features and performance. However, such systems with realistic structures often miss the exact EPs even in simulation, and the mechanism of this EP disruption has yet to be thoroughly identified. Here, we extend the coupled-mode theory of one-dimensional non-Hermitian resonator arrays to study the effects of the imaginary part of the inter-cavity coupling, which is a second-order term and attributed to material amplification, absorption, and radiation. By taking an appropriate gauge for the model, we clarify that the imaginary coupling components have a symmetric form in the effective Hamiltonian and hence represent non-Hermiticity. These additional factors can lift the gain- and loss-based EP degeneracies. However, they are proportional to the sum of the imaginary permittivities for involved cavity pairs. Thus, when the amplification and absorption of adjacent cavities are balanced, their contribution to the imaginary coupling is canceled, and the EP singularity can be restored. Radiation-induced imaginary couplings measure the change in net radiation loss by the interference between cavity modes. Their impact on the EP can also be counteracted by small cavity resonance detuning even in loss-biased cases. We show and analyze eligible simulation examples based on photonic crystal nanocavities, and highlight design of an ideal EP degeneracy that is protected by generalized PT symmetry and induced by radiation.

Enhanced nanoparticle sensing by mode intensity in a non-reciprocally coupled microcavity

Abstract: Optical microcavities operating at exceptional points have a strong mode splitting response to small perturbations such as nanoparticles. The detection limit is susceptible to mode linewidth so that small nanoparticles cannot induce a mode splitting in the transmission or reflection spectrum. Here, we propose a sensing mechanism to avoid the limitation of mode linewidth on the detection limit. We show that a microcavity with two Bragg gratings generates bright and dark modes due to completely non-reciprocal coupling. Since the two modes are not degenerate initially, utilizing the dark mode intensity as a readout scheme significantly reduces the detection limit for small perturbations. This work opens up the way toward a new class of ultrasensitive nanoparticle sensor.

Tunable mechanical-mode coupling based on nanobeam-double optomechanical cavities

Abstract: Tunable coupled mechanical resonators with nonequilibrium dynamic phenomena have attracted considerable attention in quantum simulations, quantum computations, and non-Hermitian systems. In this study, we propose tunable mechanical-mode coupling based on nanobeam-double optomechanical cavities. The excited optical mode interacts with both symmetric and antisymmetric mechanical supermodes and mediates coupling at a frequency of approximately 4.96 GHz. The mechanical-mode coupling is tuned through both optical spring and gain effects, and the reduced coupled frequency difference in non-Hermitian parameter space is observed. These results benefit research on the microscopic mechanical parity–time symmetry for topology and on-chip high-sensitivity sensors.

Ab initio spatial coupled-mode theory of Fano resonances in optical responses of multilayer interference resonators

Abstract: To reveal origins of resonance characteristics of multilayer interference structures, we developed an ab initio spatial coupled-mode theory using the approximations of general electromagnetic theory of wave propagation in stratified media. In contrast to the conventional coupled-mode theory, the coefficients of developed coupled-mode models, which describe the Fano resonance behavior of interference field enhancement, are given by analytical functions of structural and optical parameters of the resonance systems. The results of analytical modeling of low- and high-loss resonator systems supporting waveguide, Fabry-P\'erot, symmetric, and antisymmetric plasmonic normal modes agree very well with electromagnetic numerical simulations. We demonstrate also that the conventional spatial phenomenological coupled-mode theory is accurate only for low-loss structures.

Efficient 4.95 µm-8.5 µm dual-band grating coupler with crosstalk suppression capability.

Abstract: In many integrated optics systems, grating couplers are a key component of interfacing the external light source with in-plane photonic devices. Grating couplers with dual-band capability are often desired for expanding the operation spectrum of photonic systems. Here, we propose and theoretically investigate, for the first time, a 4.95 µm-8.5 µm dual-band grating coupler on a Ge-on-SOI platform. In addition to conventional structures, Bragg gratings are introduced to two wavelength division directions for crosstalk suppression. With this design, the simulated coupling efficiencies have respectively reached 59.93% and 46.38% for the 4.95 µm and 8.5 µm bands. This mid-infrared dual-band grating coupler may be useful for defense and environmental monitoring applications.

Lineshape Modulation and Sensing Characteristics of Resonator Assembled With Liquid-Metal Core Edge-Coupled With Microfiber Coupler

Abstract: The resonant response of the hollow microcapillary resonator (HMCR) assembled with liquid-metal core edge-coupled with microfiber coupler (MFC) was theoretically and experimentally investigated. Primarily, for the first time, the analogy electromagnetically induced transparency (EIT) phenomenon and Fano resonance was proposed and discovered in this coupling system. Different from the traditional method, the discrepancy in temperature characteristics of waveguide super-mode and whispering gallery mode (WGM) leads to the evolution of EIT and Fano resonances. This feature has great application value for fiber optic sensors, filters, and switches due to its sharp and asymmetric lineshape. Secondly, the temperature response characteristics were improved by assembling a liquid metal core inside the HMCR. Benefitting from the high thermal expansion-coefficient material, this solution reduces externally imposed energy consumption during the tuning process. In addition to all that, to realize the multi-parameter tuning characteristics, a fiber Bragg grating (FBG) unit was constructed to perform strain tuning. This research has significant implications for sensor measurement, optical filters, optical switches, and optical modulators.

Refractive Index Sensor Based on a Metal-Insulator-Metal Bus Waveguide Coupled with a U-Shaped Ring Resonator

Abstract: In this study, a novel refractive index sensor structure was designed consisting of a metal-insulator-metal (MIM) waveguide with two rectangular baffles and a U-Shaped Ring Resonator (USRR). The finite element method was used to theoretically investigate the sensor’s transmission characteristics. The simulation results show that Fano resonance is a sharp asymmetric resonance generated by the interaction between the discrete narrow-band mode and the successive wide-band mode. Next, the formation of broadband and narrowband is further studied, and finally the key factors affecting the performance of the sensor are obtained. The best sensitivity of this refractive-index sensor is 2020 nm/RIU and the figure of merit (FOM) is 53.16. The presented sensor has the potential to be useful in nanophotonic sensing applications.

Double exceptional points in grating coupled metal-insulator-metal heterostructure.

Abstract: In this work we theoretically study the exceptional points and reflection spectra characteristics of a grating coupled metal-insulator-metal heterostructure, which is a non-Hermitian system. Our results show that by selecting suitable geometrical parameters with grating periodicity @150 nm, that satisfy zero reflection condition, double exceptional points appear in a mode bifurcation regime. Furthermore, the thickness of partition metal layer between two cavities plays an important role in controlling the reflection properties of the heterostructure. There is a clear mode splitting when the partition layer allows strong coupling between the two cavity modes. Conversely, in weak coupling regime the mode splitting becomes too close to be distinguished. Moreover, the vanishing of reflection leads to unidirectional reflectionless propagation, which is also known as unidirectional invisibility. With grating periodicity ≥400nm, the transmissions for forward and backward incident directions are no longer the same due to the generation of diffraction. High contrast ratio (≈1) between the two incident directions leads to asymmetric transmission. This work lays the basis for designing double exceptional points and asymmetric transmission in coupled non-Hermitian photonics system. The proposed heterostructure can be a good candidate for new generation optical communications, optical sensing, photo-detection, and nano-photonic devices.

Effect of symmetry breaking on bound states in the continuum in waveguide arrays

Abstract: We developed the theoretical framework based on the coupled-mode theory which describes spectral and scattering properties of the photonic analog of an extended Fano-Anderson model---a waveguide array with two additional side-coupled waveguides. The structure supports a rich spectrum of eigenmodes, including bound state in the continuum (BIC) and other bound and leaky modes, which can be classified according to the relation between the self-coupling coefficients and eigenvalues. We focus on the structures with broken vertical symmetry with their band structures revealing interesting phenomena, such as exceptional points and level repulsion, and offer a lossless platform for $\mathcal{PT}$-symmetry phase transition. We interpreted the resonant features in the scattering spectra through a generalized Weierstrass factorization. The resonance related with quasi-BIC arises from the interference between two leaky modes: one of them representing a continuum spectrum and the other (quasi-BIC) discrete state. The reflectance near the resonance can be rewritten into the form of the Fano formula where the shape parameter $f$ can be expressed in terms of the poles associated with the two modes. Our approach provides a flexible framework which allows to interpret and to engineer the resonant properties of more complex systems.

Bifunctional MIM device with narrowband filtering and high-performance sensing

Abstract: We propose a fundamental plasmonic system consisting of a metal-insulator-metal (MIM) waveguide and a semi-ring resonator. This system can achieve Fano resonances to realize high-performance sensing and narrowband filtering. A pair of horizontal cavities are placed above and below the waveguide, respectively. When light propagates along the waveguide, resonances are excited in the upper and lower cavities simultaneously, and then couple with each other in the semi-ring. Changing the relative position of the upper and lower cavities can produce Fano resonances. We use finite-difference time-domain (FDTD) for the simulation and coupled-mode theory(CMT) theory to demonstrate the principles of the structure. The structure has very high sensitivity of 1025 nm/RIU and FOM value of 36.1 dB.

Spectral emissivity modeling in multi-resonant systems using coupled-mode theory.

Abstract: The ability to design multi-resonant thermal emitters is essential to the advancement of a wide variety of applications, including thermal management and sensing. These fields would greatly benefit from the development of more efficient tools for predicting the spectral response of coupled, multi-resonator systems. In this work, we propose a semi-analytical prediction tool based on coupled-mode theory. In our approach, a complex thermal emitter is fully described by a set of coupled-mode parameters, which can be straightforwardly calculated from simulations of unit cells containing single and double resonators. We demonstrate the accuracy of our method by predicting and optimizing spectral response in a coupled, multi-resonant system based on hBN ribbons. The approach described here can greatly reduce the computational overhead associated with spectral design tasks in coupled, multi-resonant systems.

Impact of non-Hermitian mode interaction on inter-cavity light transfer

Abstract: Understanding inter-site mutual mode interaction in coupled physical systems is essential to comprehend large compound systems, as this local interaction determines the successive multiple inter-site energy transfer efficiencies. In the present study, we demonstrate that only the non-Hermitian coupling can correctly account for the light transfer between two coupled optical cavities. We also reveal that the non-Hermitian coupling effect becomes crucial as the system dimension decreases. Our results provide important insight for handling general-coupled devices in the subwavelength regime.

Enhancing light absorption of graphene with dual quasi bound states in the continuum resonances

Abstract: Graphene is promising for next-generation optoelectronic devices owing to its unique optical and electronic properties whereas relatively weak light-matter interaction limits its performance. In this work, light can be absorbed totally by graphene with the aid of single layer all dielectric metasurface in the near infrared region. The metasurface is composed of periodically arranged amorphous Silicon nanocuboids quadrumer suspended in air which supports resonances governed by symmetry protected bound states in the continuum. Symmetry perturbation is introduced to the metasurface to induce quasi bound states in the continuum resonances by elements pair movement method in x or/and y direction. Light absorption performance of the hybrid structure can be enhanced to 50% at single-mode critical coupling state and 100% at dual-mode degenerate critical coupling state. Temporal coupled mode theory is used to analyze physical mechanism of max absorption Amax for both single mode and dual mode condition in two-port system as an analog of transparent hybrid structure. Furthermore, absorption performance can be tuned by varying the elevation between graphene and metasurface. Effects of metasurface's geometric parameters including scaling factor and thickness on absorption characteristics are also explored. Our results may provide a new route for enhancing light absorption of graphene or other 2D materials which may facilitate to design advanced modulators and photodetectors.

Compact polarization-independent quasi-adiabatic 2×2 3 dB coupler on silicon

Abstract: We demonstrate a quasi-adiabatic polarization-independent 2×2 3 dB coupler based on the silicon-on-insulator platform. Using a quasi-adiabatic taper design for the mode evolution/coupling region, the TE mode evolution is accelerated, and the TM mode coupling is achieved at a short coupling length. The measured working bandwidth is 75 nm with a compact mode evolution/coupling region of 11.7 μm.

Dynamic Fano resonance and enhanced harmful gas measurement sensitivity in a universal multimode waveguide-microcavity model

Abstract: We investigate an approach to measuring harmful gases by enhanced Fano resonance generated in a universal multimode waveguide-microcavity model. Dynamic Fano resonance is theoretically described and experimentally proved to be associated with the phase shift between two waveguide modes as well as their amplitude ratio and coupling coefficient. The spectra can be engineered to form a Lorentz dip, various Fano lineshape, and Lorentz peak by controlling the coupling point in both microbottle and surface nanoscale axial photonics platforms. In principle, the model can be applied to any class of whispering gallery mode microcavity device. With sharp asymmetric lineshape, our model can improve the sensitivity by 51.5 times in theory when measuring the harmful gas refractive index change, which may open up opportunities for advancements in the harmful gas leakage detection applications.

Supermode Switching in Coherently-Coupled Vertical Cavity Surface Emitting Laser Diode Arrays

Abstract: We report and analyze the supermode switching with current injection found for coherently-coupled dual-element vertical cavity surface emitting laser arrays. The output power, spectral characteristics, and far-field mode profile are measured for a range of bias current operating conditions. The coupling-induced enhanced output power from an in-phase or out-of-phase supermode is confirmed when approximately equal currents are injected into each array element. We observe that the coherent supermode switches from out-of-phase to in-phase supermode with increasing equal current injection. The switching is found to be consistent with coupled mode theory and a 1-dimensional spatial supermode model. The calculated modal gain of the out-of-phase and in-phase array supermodes are consistent with the observed switching of the coherent supermodes.

Resonance and sensing characteristics of horn-shaped cavity-coupled MIM waveguide

Abstract: The resonant coupling of optical microcavities to waveguides is important in photonic devices. In this paper, a horn-shaped cavity structure is designed on the side of the metal–dielectric–metal waveguide, and the coupling between the cavity and the waveguide is simulated by the finite-difference time-domain method and the coupled mode theory. It is found that the cavity and local modes appear in the horn-shaped cavity. Second, the geometric parameters of the cavity structure are changed, and the influence of the structural parameters on the transmission spectrum is obtained by theoretical analysis. Third, the maximum refractive index sensitivity of the structure is calculated to be 1750 nm/RIU, and the temperature sensitivity is 2.455 nm/°C. Ultrafine particles are placed between the tips of the horn-shaped cavity structure, and the sensitivity of the wavelength shift of the localized mode and the change in the transmittance of the trapezoidal cavity mode to the particle size and the refractive index of the particles are obtained; the nanoparticle sensor is designed by using this characteristic. The horn-shaped resonator structure proposed in this paper provides a high-performance cavity choice for the design and application of micro-nano sensor devices.

Characterization of PT-symmetric quantum interference based on the coupled mode theory.

Abstract: In this paper, we propose a comprehensive quantum theoretical framework to formulate the quantum interference inside the parity-time (PT) symmetric waveguide system which is formed by two coupled optical waveguides with unequal losses. Based on the theory, the expression for the well-known Hong-Ou-Mandel (HOM) dip is derived, which is in an exact agreement with the published results. What's more, a novel one-photon quantum interference phenomenon is predicted according to the model, which suggests a quantum interference process similar to the HOM effect can be observed for the one-photon state, while the other photon is lost due to the waveguide attenuation. Such phenomenon cannot occur in a Hermitian system or in the system formed by the waveguides with equal losses.

Mode-Coupling Induced Crosstalk Optimization in a Graded-Index Six-Mode Fiber

Abstract: We thoughtfully investigate the feasibility of reducing the mode-coupling caused crosstalk by optimizing the practical parameters of the few-mode fiber (FMF), based on the linear coupling theory. According to our simulation results, the low crosstalk FMF could be achieved with the small fiber core, the low refractive-index (RI) related parameters, the long operating-wavelength and the low core-eccentricity (CE). Moreover, the transmission behaviors over the fixed coupling and the random coupling scenarios are also discussed. The power distribution and the signal to interference plus noise ratio (SINR) evolutions along FMF reveal the impacts from the mode-coupling induced crosstalk in the spatial division multiplexing (SDM) transmission system. The random nature of the mode coupling suggesting the turbulent results are obtained in our simulation, confirmed the observation in the SDM test. And only a 1.23 dB SINR degradation of 16-QAM signals can be realized after 100 km transmission with the optimized CE of lower than 4 × 10−3.

Mode splitting and multiple-wavelength managements of surface plasmon polaritons in coupled cavities

Abstract: Resonance cavity is a basic element in optics, which has wide applications in optical devices. Coupled cavities (CCs) designed in metal-insulator-metal (MIM) bus waveguide are investigated through the finite difference time domain method and coupled-mode theory. In the CCs, the resonant modes of the surface plasmon polaritons (SPPs) split with the thickness decreasing of the middle baffle. Through the coupled-mode theory analysis, it is found that the phase differences introduced in opposite and positive couplings between two cavities lead to mode splitting. The resonant wavelength of positive coupling mode can be tuned in a large range (about 644 nm) through adjusting the coupling strength, which is quite different from the classical adjustment of the optical path in a single cavity. Based on the resonances of the CCs in the MIM waveguide, more compact devices can be designed to manipulate SPPs propagation. A device is designed to realize flexible multiple-wavelength SPPs routing. The coupling in CC structures can be applied to the design of easy-integrated laser cavities, filters, multiple-wavelength management devices in SPPs circuits, nanosensors, etc.

Generalized temporal coupled-mode theory for a P T-symmetric optical resonator and Fano resonance in a P T-symmetric photonic heterostructure.

Abstract: We have proposed generalized temporal coupled-mode theory for P T-symmetric optical resonator, and on this basis we have explained the Fano resonance in P T-symmetric photonic heterostructure. Our theoretical predictions agree very well with the simulated results obtained by transfer matrix method, which confirms the correctness of our theory. Compared with conventional Fano resonance in optical resonator with time-reversal symmetry, in this Fano resonance the amplitudes of scattering coefficients can be tuned in much larger range, which can be much larger than one, and tend to infinity at singular scattering point, where the rates of dissipation and accumulation are equal to each other and the difference of the phases of the coupling coefficients between output fields and resonant mode is equal to ±π/2. Not only that, the quality factor Q here can be negative out of accumulation, and approaches infinity at this singular scattering point. The phases of reflections jump π in the vicinity of the minima of corresponding amplitudes. We believe that we open a new door to study Fano resonance in non-Hermitian optics and inspire relevant study in other non-Hermitian wave systems.

Coupled topological edge states in one-dimensional all-dielectric heterostructures.

Abstract: We theoretically propose a coupled-topological-edge-state waveguide (CTESW), which is composed of stacked binary one-dimensional (1D) photonic crystals with opposite topological properties. The CTESW modes originate from the coupling between a sequence of topological edge states (TESs), which can be verified by the coupled mode theory (CMT). Based on finite element method (FEM), the tunable multiple transmission peaks due to CTESW modes are obtained, and the optical properties of the system can be modulated by the geometric parameters. Besides, the CTESW modes can also be tuned by changing incident angle from 0° to 60° under TE and TM polarization. Moreover, considering the relationship between channel spacing and the frequency spectrum utilization, a dense wavelength division multiplex (DWDM) filter with 50 GHz channel spacing based on CTESW is designed in communication band.