Topological Photonic Crystals Research Articles

Topological electromagnetic waves in dispersive and lossy plasma crystals

Topological photonic crystals, which offer topologically protected and back-scattering-immune transport channels, have recently gained significant attention for both scientific and practical reasons. Although most current studies focus on dielectric materials with weak dispersions, this study focuses on topological phases in dispersive materials and presents a numerical study of Chern insulators in gaseous-phase plasma cylinder cells. We develop a numerical framework to address the complex material dispersion arising from the plasma medium and external magnetic fields and identify Chern insulator phases that are experimentally achievable. Using this numerical tool, we also explain the flat bands commonly observed in periodic plasmonic structures, via local resonances, and how edge states change as the edge termination is periodically modified. This work opens up opportunities for exploring band topology in new materials with non-trivial dispersions and has potential radio frequency (RF) applications, ranging from plasma-based lighting to plasma propulsion engines.

Realizing the topological rainbow based on cavity-coupled topological edge state

Topological photonic crystals (PCs) can support topologically protected states of light. These states are immune to backscattering and can be used to construct a variety of optical devices. One promising application of topological PCs is the formation of a topological rainbow. We propose a method for realizing a topological rainbow based on cavity-coupled topological edge state (TES) in a topological photonic crystal waveguide (TPCW). This is a new type of cavity coupled TPCW, in which the cavity is located inside the waveguide at the interface between two topologically distinct PCs. We demonstrated that by varying the cavity length in the TPCW, the separation of different frequencies of topological states can be controlled. Moreover, we showed that the topological rainbow is unaffected by the cavity shape inside the TPCW. This method is more robust and versatile than previous methods, as it is unaffected by the cavity shape. This makes it possible to create topological rainbow in a wide variety of high-performance devices. For example, it could be used to build topological rainbow in integrated circuits, which would be very difficult if the cavity shape was precisely controlled. We hope our findings will inspire further research in this area and lead to the development of new and innovative optical devices.

Topologically originated optical analog of electromagnetically induced reflectance in Dirac semi-metal based photonic structure

The optical analog of the electromagnetically induced reflectance (EIR) effect was theoretically studied in an active topological photonic structure comprising Dirac semi-metal and topological photonic crystal. The destructive interference between the optical Tamm state and topological edge state results in an induced reflection. It was known that the EIR-like effect occurs in a system having a radiative state and a metastable state. Topological protection is used here to achieve a metastable state, so an effective design of the EIR-like effect was possible. The observed EIR-like effect was modeled as a coupled oscillator system. The use of bulk Dirac semi-metal makes this an active photonic system at terahertz frequencies where the Fermi energy can act as a tunable and controlling parameter through which the induced transparency can be varied.

A low-threshold single-mode higher-order topological photonic crystal nanowire array laser

A higher-order topological photonic crystal GaAs/AlGaAs/GaAs nanowire array laser based on 0D corner modes is proposed and analyzed. The structure is composed of a nanowire trivial photonic crystal sandwiched by two topological photonic crystals. Compared with the conventional deformed honeycomb lattice photonic crystal structure, an additional mode selection mechanism is introduced by adjusting the coupling strength of topological cavities formed at two topological interfaces. By optimizing the structure parameters, a large side-mode suppression ratio of 24 dB and a high-quality factor of 7935 are obtained. Due to the strong mode confinement, lasing is achievable at an ultrasmall diameter of 86 nm, with a high confinement factor of 7.96 and low threshold gain of 230 cm−1. Moreover, the laser exhibits a small beam divergence of 6.6°. This work may pave a way for the development of low-threshold single-mode miniaturized lasers.

Multi-topological state via the Brillouin zone overlap for nonlinear frequency conversion.

Multiband topological edge states (TESs) or topological corner states (TCSs) in photonic crystals provide effective ways to manipulate the nonlinear frequency conversions. However, the deliberate design and the limited number of multibands lead to the difficulty of experimental realization of the topological nonlinear frequency conversion or higher harmonic generation. Here, we propose an effective method to achieve multiple TESs and TCSs by combining the Brillouin zones of multiple different systems. It is shown that the spectra of the subsystems disperse into different energy levels due to the inter-system hopping. Based on this approach, we construct a topological photonic crystal based on the Brillouin zone overlapped SSH model, which enables the overlapped TCSs to participate in nonlinear frequency conversion. Our scheme can provide a significant way to realize the topological nonlinear frequency conversion with double resonances or multiple resonances.

Disentangled Higher‐Orbital Bands and Chiral Symmetric Topology in Confined Mie Resonance Photonic Crystals

AbstractTopological phases based on tight‐binding models have been extensively studied in recent decades. By mimicking the linear combination of atomic orbitals in tight‐binding models based on the evanescent couplings between resonators in classical waves, numerous experimental demonstrations of topological phases have been successfully conducted. However, in dielectric photonic crystals, the Mie resonances' states decay too slowly as , leading to intrinsically different physics between tight‐binding models and dielectric photonic crystals. Here, a confined Mie resonance photonic crystal is proposed by embedding perfect electric conductors between dielectric rods, creating the chiral symmetric band structure which ideally matches tight‐binding models with nearest‐neighbour couplings. As a consequence, disentangled band structure spanned by higher atomic orbitals is observed. Moreover, the result provides an effective route to achieve a 3D photonic crystal with a complete photonic bandgap and third‐order topology. The implementation offers a versatile platform for studying exotic higher‐orbital bands and achieving tight‐binding‐like 3D topological photonic crystals.

Topological slow light enhanced second harmonic generation in double-resonant topological photonic crystal

Generally, the second-harmonic generation and slow light of multi-band topological edge states (TESs) have been studied separately. Therefore, the influence of simultaneous slow light and topology protection on second-harmonic generation (SHG) is deficient. Here, we propose a high-efficiency SHG using dual-frequency TESs in topological photonic crystals (TPCs) with slow-light conditions. The wave vector matching condition and energy conservation condition (frequency doubling) can be achieved by adequately adjusting the overall structural parameters of TPCs. The double-resonant nonlinear interaction between two TESs is enabled using a square lattice TPC. Due to the topological localization of the TES and the long interaction time of slow-light effect, the energy densities of the fundamental wave and SHG are significantly increased. Consequently, the high intrinsic efficiency of SHG can be obtained in the order of 7.40 × 10−4. Our work opens new avenues for using topological protected and slow light enhanced nonlinear frequency conversion in a TPC system.

Simulated annealing algorithm with neural network for designing topological photonic crystals

In this work, we utilize simulated annealing algorithm with neural network, to achieve rapid design of topological photonic crystals. We firstly train a high-accuracy neural network that predicts the band structure of hexagonal lattice photonic crystals. Subsequently, we embed the neural network into the simulated annealing algorithm, and choose the on-demand evaluation functions for optimizing topological band gaps. As examples, designing from the Dirac crystal of hexagonal lattice, two types of valley photonic crystals with the relative bandwidth of bandgap 26.8% and 47.6%, and one type of pseudospin photonic crystal with the relative bandwidth of bandgap 28.8% are obtained. In a further way, domain walls composed of valley photonic crystals (pseudospin photonic crystals) are also proposed, and full-wave simulations are conducted to verify the valley-locked (pseudospin-locked) edge states unidirectionally propagates under the excitation of circularly polarized source. Our proposed method demonstrates the efficiency and flexibility of neural network with simulated annealing algorithm in designing topological photonic crystals.

Electrically Tunable Topological Notch Filter for THz Integrated Photonics

AbstractElectromagnetic filtering is essential for emerging integrated photonic technologies and widely adaptable processing of high‐bandwidth signals. The conventional electro‐optic modulator‐based photonic approach for signal filtering at microwave frequencies cannot be implemented at terahertz (THz) frequencies due to the unavailability of the THz signal‐driven optical modulator. Here, an electrically tunable on‐chip THz photonic notch filter based on the topologically protected valley hall waveguide‐cavity platform is demonstrated. The device shows a significantly large notch suppression depth of more than 20 dB with a return loss of 13 dB in the entire tuning range of notch frequency. The feedback control circuit enables precise control of the notch frequency shift with a minimum step size of 7 MHz. This work extends the application of topological photonic crystals in developing THz‐integrated photonic devices for transformative technologies, including sixth‐generation (6G) communication and high‐resolution spectral sensing.

All-optical self-manipulation of light flow in on-chip topological waveguides based on synthetic dimension.

Topological photonic crystals provide a new platform for designing nanophotonic devices with robustness. Especially, all-optical devices, which use the light controlling light, based on nonlinear topological photonic crystals, have not been reported yet. In this article, we numerically investigate the robust self-manipulation of light flow in silicon topological photonic crystal waveguides based on the Kerr nonlinearity of silicon and topological edge states of photonic crystal waveguides. By adjusting the intensity of incident light at a communication wavelength of 1550 nm, the transmission path of the light flow in waveguides can be effectively controlled, and such manipulation is immune to some disturbances of nanostructures and thus shows the robustness. The results indicate that nonlinear topological photonic crystals have potential applications in on-chip integrated all-optical photonic devices.

Robust Topological Edge States in C6 Photonic Crystals

The study of photonic crystals has emerged as an attractive area of research in nanoscience in the last years. In this work, we study the properties of a two-dimensional photonic crystal composed of dielectric rods. The unit cell of the system is composed of six rods organized on the sites of a C6 triangular lattice. We induce a topological phase by introducing an angular perturbation ϕ in the pristine system. The topology of the system is then determined by using the so-called k.p perturbed model. Our results show that the system presents a topological and a trivial phase, depending on the sign of the angular perturbation ϕ. The topological character of the system is probed by evaluating the electromagnetic energy density and analyzing its distribution in the real space, in particular on the maximal Wyckoff points. We also find two edge modes at the interface between the trivial and topological photonic crystals, which present a pseudospin topological behavior. By applying the bulk-edge correspondence, we study the pseudospin edge modes and conclude that they are robust against defects, disorder and reflection. Moreover, the localization of the edge modes leads to the confinement of light and the interface behaves as a waveguide for the propagation of electromagnetic waves. Finally, we show that the two edge modes present energy flux propagating in opposite directions, which is the photonic analogue of the quantum spin Hall effect.

Near-field observation of mid-infrared edge modes in topological photonic crystals

Topological photonic crystals inherit the unique properties of topological insulators, including topologically protected energy transfer and unidirectional propagation, which offer an excellent platform for exploring exotic physics and developing photonic devices. However, topological photonic crystals possessing mid-infrared edge modes that have potential applications in infrared imaging, biosensing, thermal radiation energy transfer, etc., are seldom brought into focus. In this work, we study the topological properties of a photonic crystal slab (PCS) consisting of silicon square veins in the mid-infrared, which is intended to mimic the two-dimensional Su–Schrieffer–Heeger model. By interfacing topologically trivial and nontrivial PCSs, mid-infrared edge modes can appear at domain wall, according to the principle of bulk-edge correspondence. It is also demonstrated high-efficiency mid-infrared light transport can be achieved by these edge modes. In addition, adjusting the vertical offset near the interface can manipulate the bandwidth for various applications and turns the connected PCS structure to a photonic realization of Rice–Mele model. We further fabricate the PCS and provide an experimental observation of transverse-electric-like edge modes in mid-infrared by using the scattering-type scanning near-field optical microscope. Additionally, we integrate it with phase change material of nanoscale thickness, Ge2Sb2Te5, to realize an ultrafast and switchable topological waveguide with zero static power. This work not only enriches the fundamental understanding of topological physics in mid-infrared optical settings, but also shows promising prospects in compact devices for energy transfer and information processing for light sources in these wavelengths, for instance, thermal radiation.

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.

Topological photonic crystal nanowire array laser with edge states.

A topological photonic crystal InGaAsP/InP core-shell nanowire array laser operating in the 1550 nm wavelength band is proposed and simulated. The structure is composed of an inner topological nontrivial photonic crystal and outer topological trivial photonic crystal. For a nanowire with height of 8 µm, high quality factor of 4.7 × 104 and side-mode suppression ratio of 11 dB are obtained, approximately 32.9 and 5.5 times that of the uniform photonic crystal nanowire array, respectively. Under optical pumping, the topological nanowire array laser exhibits a threshold 27.3% lower than that of the uniform nanowire array laser, due to the smaller nanowire slit width and stronger optical confinement. Moreover, the topological NW laser exhibits high tolerence to manufacturing errors. This work may pave the way for the development of low-threshold single-mode high-robustness nanolasers.

Hybrid topological photonic crystals

Topologically protected photonic edge states offer unprecedented robust propagation of photons that are promising for waveguiding, lasing, and quantum information processing. Here, we report on the discovery of a class of hybrid topological photonic crystals that host simultaneously quantum anomalous Hall and valley Hall phases in different photonic band gaps. The underlying hybrid topology manifests itself in the edge channels as the coexistence of the dual-band chiral edge states and unbalanced valley Hall edge states. We experimentally realize the hybrid topological photonic crystal, unveil its unique topological transitions, and verify its unconventional dual-band gap topological edge states using pump-probe techniques. Furthermore, we demonstrate that the dual-band photonic topological edge channels can serve as frequency-multiplexing devices that function as both beam splitters and combiners. Our study unveils hybrid topological insulators as an exotic topological state of photons as well as a promising route toward future applications in topological photonics.

Combining sensitivity and robustness: EIT-like characteristic in a 2D topological photonic crystal.

The study of topological photonics has gained significant attention due to its potential application for robust and efficient light manipulation. In this work, we theoretically investigate a two-dimensional photonics crystal that exhibits a topological edge state (TES) and a topological corner state (TCS). Furthermore, we also achieve a coupling between a topological corner state and a trivial cavity (TC), resulting in a phenomenon similar to the electromagnetically induced transparency (EIT) effect. To verify the stability of the EIT-like effect, disorders around TES and TCS are introduced, and the theoretical results show that this structure is immune to the disorders. The achievement of the coupling between topological states can have potential applications in the areas of waveguiding, sensing, and logic gates. It is hoped that this work will contribute to the ongoing efforts in the exploration and utilization of topological photonics.

Topology Optimization-Based Design of Topological Edge-State Wavelength Division Multiplexer

Previous research processes on topological insulators were mainly based on the manual adjustment of a large number of fine parameters to achieve the desired properties. Here, we propose to treat the design process of photonic crystals as an objective function through topological optimization theory, allowing the desired properties to arise spontaneously with the optimization. Through topology optimization, we have designed and implemented three topological photonic crystals with different topological characteristics and dual-band protection common bandgap. The results show that functional edge modes with desired propagation properties can be generated within these two bandgaps by constructing domain walls between photonic crystal structures with different topological phases. The unidirectional transmission characteristics of the wave can also be observed, which provides an easy way to implement a wavelength division multiplexer.

Multichannel valley topological beam splitter based on different types of domain walls

Topological photonics has made great progress from physical concept verification to new technical applications, and valley topological photonic crystal (TPCs) are one of the most important candidates for future applications in functional devices because of large bandwidth and lossless optical transport. However, due to the limitations of the design method and structure arrangement, the multichannel valley topological beam splitter (BS) has not yet been much explored. Here, we reveal the different robustness of four types of domain walls in valley TPCs. Benefiting from the differences in domain walls, we numerically present and experimentally demonstrate a highly integrated multichannel valley topological BS in the microwave regime. Compared with traditional BSs, it has the advantages of being more robust and compact and having more output ports and higher integration. The reported multichannel topological BS opens an avenue to engineer the flow of light and offers effective design approaches for integrated photonic device miniaturization.

Compact topological polarization beam splitter based on all-dielectric fishnet photonic crystals.

Conventional polarization beam splitters (PBSs) suffer energy loss and signal distortion due to backscattering caused by disturbances. Topological photonic crystals provide backscattering immunity and anti-disturbance robustness transmission owing to the topological edge states. Here, we put forward a kind of dual-polarization air hole-type fishnet valley photonic crystal with a common bandgap (CBG). The Dirac points at the K point formed by different neighboring bands for transverse magnetic and transverse electric polarizations are drawn closer via changing the filling ratio of the scatterer. Then the CBG is constructed by lifting the Dirac cones for dual polarizations within a same frequency range. We further design a topological PBS using the proposed CBG via changing the effective refractive index at the interfaces which guide polarization-dependent edge modes. Based on these tunable edge states, the designed topological PBS (TPBS) achieves efficient polarization separation and is robust against sharp bends and defects, verified by simulation results. The TPBS's footprint is approximately 22.4 × 15.2 μ m 2, allowing high-density on-chip integration. Our work has potential application in photonic integrated circuits and optical communication systems.

Unique Huygens-Fresnel electromagnetic transportation of chiral Dirac wavelet in topological photonic crystal

Light propagates in various ways depending on environment, including uniform medium, surface/interface and photonic crystals, which appears ubiquitously in daily life and has been exploited for advanced optics technology. We unveiled that a topological photonic crystal exhibits unique electromagnetic (EM) transport properties originating from the Dirac frequency dispersion and multicomponent spinor eigenmodes. Measuring precisely local Poynting vectors in microstrips of honeycomb structure where optics topology emerges upon a band gap opening in the Dirac dispersion and a p-d band inversion induced by a Kekulé-type distortion respecting C6v symmetry, we showed that a chiral wavelet induces a global EM transportation circulating in the direction counter to the source, which is intimately related to the topological band gap specified by a negative Dirac mass. This brand-new Huygens-Fresnel phenomenon can be considered as the counterpart of negative refraction of EM plane waves associated with upwardly convex dispersions of photonic crystals, and our present finding is expected to open a new window for photonic innovations.