Simultaneous multi-frequency topological edge modes between one-dimensional photonic crystals

Sensing performance of Fano resonance induced by the coupling of two 1D topological photonic crystals

In this work, a realized Fano resonance due to the coupling between two 1DTPC is proposed for refractive index sensing with an ultra-high-quality factor of 106. The generated Fano can be assigned to the coupling between topological edge states of two 1D TPCs. The resulting Fano peak is characteristic with a high transmission value reach to 99% with high sensing performance parameters making the proposed sensor a novel detector for refractive index. The proposed coupling 1D TPCs show a high sensitivity value of 888.252 nm/RIU, ultra-high-quality factor and figure of merit value reach 106, and perfect detection limit value of 10−7. The proposed coupling 1D TPCs provides a straightforward platform for sensing refractive index applications with high performance.

Angle-insensitive topological interface states in hybrid one-dimensional photonic crystal heterostructures containing all-dielectric metamaterials

Topological interface states (TISs) in conventional one-dimensional (1D) photonic crystal (PhC) heterostructures strongly shift toward higher frequencies as the incident angle increases. This strong blueshift property of TISs intensively limits the operating angle ranges of TISs. Herein, we design two angle-insensitive photonic bandgaps (PBGs) in two hybrid 1D PhCs containing all-dielectric metamaterials. By cascading these two hybrid 1D PhCs to construct a hybrid 1D PhC heterostructure, we achieve an angle-insensitive TIS under transverse magnetic polarization. Empowered by the angle-insensitive property of the PBGs, the angular tolerance of the TIS reaches 69.65°, which is much higher than those of the TISs in conventional 1D PhC heterostructures. In addition, the angle-insensitive property of the TIS is robust against the layer thickness. Our work provides a viable route to achieving TISs with high angular tolerances and would facilitate the applications of photonic topological states.

Topological super-modes engineering with acoustic graphene plasmons

Acoustic graphene plasmons (AGPs) in a graphene-dielectric-metal structure possess extreme field localization and low loss, which have promising applications in strong photon-matter interaction and integrated photonic devices. Here, we propose two kinds of one-dimensional crystals supporting propagating AGPs with different topological properties, which is confirmed by the Zak phase calculations and the electric field symmetry analysis. Moreover, by combining these two plasmonic crystals to form a superlattice system, the super-modes exist because of the coupling between isolated topological interface states. A flat-like dispersion of super-modes is observed by designing the superlattice. These results should find applications in optical sensing and integrating photonic devices with plasmonic crystals.

Conjugated topological interface-states in coupled ring resonators

The optical properties of topological photonics have attracted much interest recently because its potential applications for robust unidirectional transmission that are immune to scattering at disorder. However, researches on topological series coupled ring resonators (T-SCRR) have been much less discussed. The existence of topological interface-states (TIS) in the T-SCRR is described for the first time in this article. An approach has been developed to achieve this goal via the band structure of dielectric binary ring resonators and the Zak phase of each bandgap. It is found that an ultra-high-Q with complete transmission is obtained by the conjugated topological series coupled ring resonators due to the excitation of conjugated topological interface-states, which is different from those in conventional TIS. Furthermore, the problem of transmission decreases resulting from high-Q increases in the traditional photonic system is significantly improved by this approach. These findings could pave a novel path for developing advanced high-Q filters, optical sensors, switches, resonators, communications and quantum information processors.

Enhancement of graphene Faraday rotation in the one-dimensional topological photonic crystals

One-dimensional topological photonic crystals (TPCs) with graphene sheet have been proposed to enhance the Faraday rotation (FR). Because of the strong localized field of the topological interface state, the enhanced FR angle with high transmittance has been confirmed. The effects of external magnetic field, unit cell number and multiple interface states of multilayers on FR angle and transmittance are studied. As a result, the FR is raised, which shows a field enhancement constraint at the interface between the TPCs with graphene. The FR angle can reach 16.2° with the high transmission (70%). By constructing multiple interface states, multiple transmission peaks and FR angles are further achieved. Our result would give a fresh idea, which could be applied in nonreciprocal photonic device or optical communication systems.

Robust high-Q filter with complete transmission by conjugated topological photonic crystals

High quality factor (High-Q) and transmission optical devices are required for various applications in the fields of physics and engineering. Critical for these applications is the realization of a structure with high-Q, complete transmission and small volume. A robust high-Q filter with complete transmission by conjugated topological photonic crystals (CTPC) is presented. The study shows that an ultra-high-Q of more than 108 with complete transmission is obtained by the CTPC with 2 μm long due to the excitation of conjugated topological edge-states (CTES). It is also found that even though the quality factor of resonances increases as the periodic number of multilayers increases, these resonances are still complete transmission. A novel concept of CTES is first proposed in this study and investigated the effect of its topological phenomenon on high quality factor via CTPC. We theoretically realize the robust high-Q and complete transmission in the CTPC, which is different from those in periodic, quasi-periodic, Fabry-Perot photonic crystals and traditional topological photonic crystals (TPC).

Enhanced nonlinear optical responses of graphene in multi-frequency topological edge modes

We propose a hybrid structure where graphene is inserted to the interface of two one-dimensional photonic crystals (1D PCs). The two PCs are designed to have opposite topological properties, and at the interface, topological edge modes exist. The edge modes exist at both the fundamental frequency (FF) and the third harmonic (TH) frequency. This double resonant structure will enhance the nonlinear responses of graphene greatly, including Kerr nonlinearity and TH generation. We discuss these two kinds of nonlinearities both at terahertz (THz) and near-infrared (NIR) frequencies. The influence of Kerr nonlinearity on the resonant frequencies is considered, when we calculate the TH generation. At THz frequency, low-threshold bistability (about 8MW/cm²) is obtained and the TH generation efficiency of 2.5% is achieved with incident intensity of 10MW/cm². At NIR frequency, the nonlinear conductivities of graphene are about 7 orders lower. Bistability is unlikely to happen with incident intensity below 1GW/cm². The TH generation efficiency is only about 5×10^-6 with incident intensity of 25MW/cm². The proposed structure is more suitable to work as a low-threshold saturable absorber at NIR frequency. These results may be helpful both for a better understanding of graphene's nonlinear responses in a double resonant structure and for potential applications in THz nonlinear devices and NIR nanophotonics.

Wide-angle ultrasensitive biosensors based on edge states in heterostructures containing hyperbolic metamaterials

Edge states in photonic heterostructures composed of metal layers and all-dielectric one-dimensional photonic crystals (1DPCs) will shift toward short wavelengths (blueshift) with the increase in the incident angle for both transverses magnetic (TM) and transverse electric (TE) polarizations. However, we achieve redshift edge states for TM polarization and blueshift edge states for TE polarization in heterostructures composed of metal layers and 1DPCs containing layered hyperbolic metamaterials. Owing to the opposite wavelength shift of the edge states for two orthogonal polarizations, the ellipsometric phase will change dramatically around the edge state wavelength in a broad angle range. Based on this wide-angle phase singularity property, we propose a biosensor which can work with high refractive index resolution in a broad angle range.

Topological Phase Transition in a One-Dimensional Elastic String System

We show that topological interface mode can emerge in a one-dimensional elastic string system which consists of two periodic strings with different band topologies. To verify their topological features, Zak-phase of each band is calculated and reveals the condition of topological phase transition accordingly. Apart from that, the transmittance spectrum illustrates that topological interface mode arises when two topologically distinct structures are connected. The vibration profile further exhibits the non-trivial interface mode in the domain wall between two periodic string composites.

Band structure and topological phase transition of photonic time crystals

We investigate the band structure and topological phase transition of photonic time crystals (PTC)-systems in which the physical parameter varies periodically in time. We find that the topological phase transition of the PTC system can be revealed by the wave vector gap (k-gap) size, which was induced by the temporal refraction and reflection. Interestingly, a temporal zero-averaged refractive index k-gap is obtained when the PTC system includes a dispersive medium. This special k-gap is invariant with modulation time scaling at a given modulation frequency.

Zak phase and topological plasmonic Tamm states in one-dimensional plasmonic crystals

The Zak phase and topological plasmonic Tamm states in plasmonic crystals based on periodic metal-insulator-metal waveguides are systematically investigated. We reveal that robust topological interfacial states against structural defects exist when the Zak phase between two adjoining plasmonic lattices are different in a common band gap. A kind of efficient admittance-based transfer matrix method is proposed to calculate and optimize the configuration with inverse symmetry. The topologically protected states are favorable for the spatial confinement and enhancement of electromagnetic fields, which open a new avenue for topological photonic applications.

Ultra-slow light in one-dimensional Cantor photonic crystals

Ultra-slow light and complete transmission properties in one-dimensional Cantor photonic crystals are presented. In contrast to traditional dielectric photonic crystals, the proposed structure has large group delay, slower group velocity, and a high quality factor within the same layers and materials. This study shows that larger than 1 μs group delay and slower than 1 m/s group velocity are achieved in the fifth-order Cantor photonic crystal with 52.75 μm length. This ultra-slow-light structure is very promising for application in advanced slow-light devices. A high quality factor of 10⁹ and multiband filters with complete transmission can also be obtained by using this approach.

Nonlinear frequency up-conversion via double topological edge modes

We study the nonlinear frequency up-conversion in a plasmonic thin film sandwiched between one-dimensional photonic crystals (PCs) of different Zak phases by rigorous numerical time-domain nonlinear hydrodynamic calculations. We show that the proposed hetero-structure can support robust fundamental and high-order topological edge modes that simultaneously enhance the third-harmonic generation. Numerical simulations also show that femtosecond pulses can excite double topological edge modes through optical tunneling in band gaps, leading to a large nonlinear response. The obtained third harmonic generation (THG) conversion efficiency of the hetero-structure is three orders of magnitude larger than that of a single plasmonic film. The results presented here may open new avenues for designing high-efficiency nonlinear photonic devices.

Tamm-plasmon polaritons in one-dimensional photonic quasi-crystals

We present an investigation to ascertain the existence of Tamm-plasmon-polariton-like modes in one-dimensional (1D) quasi-periodic photonic systems. Photonic bandgap formation in quasi-crystals is essentially a consequence of long-range periodicity exhibited by multilayers and, thus, it can be explained using the dispersion relation in the Brillouin zone. Defining a "Zak"-like topological phase in 1D quasi-crystals, we propose a recipe to ascertain the existence of Tamm-like photonic surface modes in a metal-terminated quasi-crystal lattice. Additionally, we also explore the conditions of efficient excitation of such surface modes along with their dispersion characteristics.

Zak phase induced multiband waveguide by two-dimensional photonic crystals

Interface states in photonic crystals provide efficient approaches to control the flow of light. Photonic Zak phase determines the bulk band properties of photonic crystals, and, by assembling two photonic crystals with different bulk band properties together, deterministic interface states can be realized. By translating each unit cell of a photonic crystal by half the lattice constant, another photonic crystal with identical common gaps but a different Zak phase at each photonic band can be created. By assembling these two photonic crystals together, multiband waveguide can thus be easily created and then experimentally characterized. Our experimental results have good agreement with numerical simulations, and the propagation properties of these measured interface states indicate that this new type of interface state will be a good candidate for future applications of optical communications.