Topological Waveguide Research Articles

Magnon-magnon coupling mediated by topological edge states

The topology of the photonic bath shows excellent potential to engineer the intriguing interaction properties between light and matter. Here, we study the dielectric resonator array with a zigzag geometry, an analogy of the Su-Schrieffer-Heeger model equipped with peculiar freedom to manipulate the nearest-neighbor coupling strength by the internal interaction. A staggered coupling strength of s-type pillar modes is experimentally achieved via the photonic spin-orbit coupling by employing the zigzag dielectric resonator chain. As a result, we observe that the robust edge states of the zigzag chains manifest themselves with linear polarization for an even number of dielectric resonators but elliptical polarization for an odd number. In addition, by coupling magnons to the topological waveguide, we observe resonant magnon–magnon–edge-state coupling, whose coupling strength is topologically protected. More broadly, our work shows that topological waveguide-QED systems may provide the potential for synthesis and study of many-body states with attractive long-range interaction. Published by the American Physical Society 2024

Deep learning improves performance of topological bending waveguides.

This study introduced design informatics using deep learning in a topological photonics system and applied it to a topological waveguide with a sharp bending structure to further reduce propagation loss. The sharp bend in the topological waveguide composed of two photonic crystals wherein dielectrics having C6v symmetry were arranged in triangle lattices of hexagons, and the designing of parameters individually for 6 × 6 unit cells near the bending region using deep learning resulted in an output improvement of 60% compared to the initial structure. The proposed structural design method has high versatility and applicability for various topological photonic structures.

Robust and Reconfigurable Waveguide Design in Valley-Topological Phononic Crystals

As an analogy of topological insulators and superconductors, “topological phononics”, which applies the concept of band topology to acoustic dispersion, has attracted increasing attention in recent years. We present design of topological acoustic/elastic waveguides in phononic crystals. Topological waveguides are designed from the phonon dispersion analyses by finding edge modes appearing at interfaces between phononic crystals with different band topologies. As a prototype model, we first designed the topological waveguides in kHz regimes. Experimental validation of the designed waveguide has been performed in the frequency region via laser-doppler measurements. The robustness of the waveguide propagation against defects, corners, and structural inaccuracy in the waveguide has been quantitatively evaluated. We also introduced a structural transition of local symmetry inversion in the phononic crystal to implement a reconfigurability into the waveguide .Further development toward GHz regime will pave the way to the development of next-generation information devices using the proposed structures as an alternative or complimentary approach.

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.

Waveguide–Cavity Coupling System Based on Topological Edge States and Corner States in Kagome Photonic Crystals

AbstractIn recent years, the topological edge states (TESs) and topological corner states (TCSs) based on high‐order topological photonic insulators have been researched and applied in various fields. Among them, the combination of topological photonics and waveguide–cavity coupling system has attracted much attention of researchers. In this paper, TESs and TCSs based on Kagome lattice photonic crystals (PCs) with rectangular dielectric columns are obtained. In addition, a topological waveguide–cavity coupling system is proposed, realizing the energy coupling between the TES waveguide and the TCS cavity. To further improve the performance of the system, a sandwich‐like waveguide and an extended waveguide are used to replace the original TES waveguide. Interestingly, the proposed extended waveguide–cavity coupling system cannot only achieve the excitation of two TCSs at different frequencies, but also has stronger electric field intensity, narrower resonance bandwidth, and higher quality factor than the other structures. This work provides a new path to design high‐performance micro‐nanointegrated optical devices such as filters, lasers, and resonators.

Reconfigurable higher-order topological electromechanical metamaterial

Higher-order topological insulators (HOTIs) exhibit hierarchical and multidimensional features of topological states, expanding the applications of topological insulators (TIs). Reconfigurable mechanical metamaterial-based HOTIs for the manipulation of elastic waves are highly desirable. Here, we propose a reconfigurable electromechanical metamaterial with first-order topological edge states and second-order topological corner states for the control of Lamb waves. The metamaterial consists of an aluminum layer sandwiched between two piezoelectric layers shunted through negative capacitance circuits. The topology of the metamaterial can be reconfigured by switching on or off the connected circuits, and the metamaterial is also tunable by changing the electrical parameters. Hierarchical and multidimensional topological waveguides are constructed and their reconfigurability is demonstrated. The metamaterial is continuous and thus facilitates waveguide miniaturization, such as on-chip device design.

Valley-conserved topological integrated antenna for 100-Gbps THz 6G wireless.

The topological phase revolutionized wave transport, enabling integrated photonic interconnects with sharp light bending on a chip. However, the persistent challenge of momentum mismatch during intermedium topological mode transitions due to material impedance inconsistency remains. We present a 100-Gbps topological wireless communication link using integrated photonic devices that conserve valley momentum. The valley-conserved silicon topological waveguide antenna achieves a 12.2-dBi gain, constant group delay across a 30-GHz bandwidth and enables active beam steering within a 36° angular range. The complementary metal oxide semiconductor-compatible valley-conserved devices represent a major milestone in hybrid electronic-photonic-based topological wireless communications, enabling terabit-per-second backhaul communication, high throughput, and intermedium transport of information carriers, vital for the future of communication from the sixth to X generation.

The perspective of topological photonics for on-chip terahertz modulation and sensing

Terahertz (THz) technology has seen significant advancements in the past decades, encompassing both fundamental scientific research, such as THz quantum optics, and highly applied areas like sixth-generation communications, medical imaging, and biosensing. However, the progress of on-chip THz integrated waveguides still lags behind that of THz sources and detectors. This is attributed to issues such as ohmic losses in microstrip lines, coplanar and hollow waveguides, bulky footprints, and reflection and scattering losses occurring at sharp bends or defects in conventional dielectric waveguides. Inspired by the quantum Hall effects and topological insulators in condensed matter systems, recent discoveries of topological phases of light have led to the development of topological waveguides. These waveguides exhibit remarkable phenomena, such as robust unidirectional propagation and reflectionless behavior against impurities or defects. As a result, they hold tremendous promise for THz on-chip applications. While THz photonic topological insulators (PTIs), including wave division, multiport couplers, and resonant cavities, have been demonstrated to cover a wavelength range of 800–2500 nm, research on tunable THz PTIs remains limited. In this perspective, we briefly reviewed a few examples of tunable PTIs, primarily concentrated in the infrared range. Furthermore, we proposed how these designs could benefit the development of THz on-chip PTIs. We explore the potential methods for achieving tunable THz PTIs through optical, electrical, and thermal means. Additionally, we present a design of THz PTIs for potential on-chip sensing applications. To support our speculation, several simulations were performed, providing valuable insights for future THz on-chip PTI designs.

Topological acoustic waveguide with high-precision internal-mode-induced multiband

In this paper, we propose a topological acoustic waveguide which can transmit high-precision and multiband sound energy with backscattering suppression and no energy loss. It is a kind of acoustic waveguide with spatial lattice uniformity without destroying lattice position. This acoustic structure is obtained by integrating two kinds of multi-stage resonators arranged in a triangular lattice pattern in the air environment. On one hand, the introduction of internal modes greatly improves the transmission accuracy of topological acoustic waveguide compared with the Bragg scattering alone under the same overall structure size. On the other hand, it greatly improves the utilization rate of the internal space of the structure and makes full use of the multi-modal in structure to construct more sound transmission bands. After introducing the rotational scattering mechanism, the numerical and experimental results show that up to seven high-precision acoustic transmission channels with backscattering suppression and no energy loss below 14 kHz are obtained along the sonic crystal interface with different topological states in this topological acoustic waveguide. This research paves the way for engineering applications of high-precision acoustic transmission, multi-band acoustic antennas, acoustic logic control, covert sound transmission and other multifunctional acoustic devices.

Tunable bifunctional acoustic logic gates based on topological valley transport

Valley degree of freedom has attracted great interest in the realization of topological edge states in acoustic systems owing to its rich valley-contrasting physics and great potential applications. However, the practice of valley acoustic topological insulators (ATIs) in designing tunable multifunctional devices without changing their structures still remains a great challenge. Here, we show that the antisymmetric and symmetric distribution nature of valley edge states in the valley ATIs with two different domain walls can be utilized to design tunable robust acoustic logic gates (ALGs). We experimentally demonstrate two types of tunable bifunctional ALGs (denoted as ALG-I and ALG-II), in which ALG-I is composed of a single domain wall, and ALG-II is constructed by a bent topological waveguide containing two domain walls. For ALG-I, the functions of logical inclusive OR and logical exclusive OR (denoted as OR and XOR, respectively) can be switched by actively tuning the phases of two input sound sources without changing the structure. For ALG-II, the logic functions OR and XOR can be implemented through the left and right incidences, respectively, of a pair of sound sources. Similarly, the switching of the logic functions OR and XOR on both sides of ALG-II can be realized by simply adjusting the phases of two sound sources. The designed ALGs have the advantages of simple structure, high robustness, as well as active tunability, leading to a wide range of potential applications in integrated acoustics, acoustic communications, and information processing.

A Topological Directional Coupler Fed by Microstrip Line With Configurable Coupling Coefficient

Topological waveguides have been extensively studied for their robust transmission properties immune to defects and their application potentials for microwave and terahertz integrated circuits. In this paper, by using grounded planar valley-Hall photonic topological insulators, a high-efficiency topological directional coupler fed directly by microstrip line with configurable coupling coefficient is theoretically proposed and experimentally demonstrated. The topological directional coupler consists of two coupled topological waveguides, which can be directly integrated with microstrip circuits. Different coupling coefficients were achieved by configuring the coupling between the two topological waveguides. Both simulation and measurement demonstrated the proposed designs. In addition, the proposed design is applicable in both microwave and terahertz bands.

Robust topological valley-locked waveguide transport in photonic heterostructures

Topological valley-locked edge state has arisen a rapidly developing research topic in photonics for its gapless dispersion and immunity against intervalley scattering. However, valley-locked edge state is usually confined around the domain walls of valley photonic crystals (VPhCs) with different topological invariants, which decreases the flexibility of valley photonic devices and limits its application in on-chip integration. Here we propose a topological valley-locked waveguide (TVLW) with a width degree of freedom (DOF) by using a photonic heterostructure where a VPhC characterized by Dirac cones is sandwiched by two VPhCs with opposite valley Chern numbers. The TVLW is available for the transmission of waveguide states which maintain the properties of momentum-valley locking and robustness to defects. Taking advantage of these properties of TVLWs, we design a topological energy concentrator for field enhancement and a topological power splitter with an arbitrary splitting ratio by introducing the rotation angle of channels as a new DOF. Compared with conventional waveguides, the proposed TVLWs with tunable mode-width are more flexible to interface with the existing photonic devices, which provide broad application prospects in on-chip communication and photonic integrated networks.

Demonstration of a highly efficient topological vertical coupler.

A defect structure is proposed for enhancing the coupling efficiency of vertically incident circularly polarized light in a topological waveguide. In the topological edge-state waveguide based on triangle lattices of hexagons consisting of six nanoholes respecting C6v symmetry in a silicon optical circuit, the vertical coupling rate is improved by removing the nanoholes from one hexagonal cell near the line. The coupling efficiency was evaluated with and without the defect structure. The introduced defect structure operates suitably for focused beams of left- and right-handed circularly polarized light, enhancing the optical communication wavelength bandwidth by up to 10 dB.

Defect‐Adjusted Valley Edge States and Rainbow Trapping of Elastic Waves in 2D Topological Phononic Crystals

Topological phononic crystals (PnCs) with topologically protected boundary states have important applications in the fields of acoustic wave transmission and control. However, previous studies based on solid‐state PnC systems are mostly limited by fixed structures, resulting in the difficulty to deform the edge states, which partly limits its practical applications. Herein, a 2D solid topological PnC coupled with the defect is designed to achieve the adjustable valley edge state and rainbow trapping. First, by breaking the spatial inversion symmetry, the valley Hall phase transition of elastic wave is realized and valley edge states are obtained. Next, by introducing defects of different widths between the two different valleys’ topological PnCs, both the defect‐adjusted valley edge state and defect state are achieved. Then, by designing different topological PnCs waveguides, the robust transport characteristics of the two above states are compared. Subsequently, a new power divider based on the defect‐adjusted valley edge state is designed, which is found to possess various manners of operation such as equal and unequal power divisions. Finally, based on defect adjustment of the edge states, a rainbow trapping is implemented. This research provides an important guidance for ultrasonic devices, such as waveguides, energy harvesters, and power dividers.

A topological gap waveguide based on unidirectional locking of pseudo-spins

Photonic topological insulators have been widely studied due to the robustness of energy transport via supported edge modes immune to structural disorder. In this work, a topological gap waveguide is constructed by introducing line defect into a topological photonic crystal structure and combining it with a gap waveguide structure, the design of which, therefore, combines the advantages of both topological and gap waveguides. Not only does it give high transmission efficiency but it also enables high robustness for energy transmission under structural defects and sharp bends. Our proposed topological waveguide design can be implemented with conventional semiconductor technology and integrated into optical circuits for communication systems.

Enhanced Magneto‐Optical Effects in Epsilon‐Near‐Zero Indium Tin Oxide at Telecommunication Wavelengths

AbstractEpsilon‐near‐zero (ENZ) materials exhibit near‐zero permittivity and induce various intriguing linear and nonlinear optical phenomena. Magneto‐optical (MO) effects, which are notoriously weak in the optical domain, have been predicted to be significantly enhanced in ENZ materials, which can facilitate the realization of compact nonreciprocal devices. However, to date, enhanced MO effects have been predominantly observed in effective ENZ media, which are commonly based on complex combinations of photonic nanostructures. It is difficult for these effective media to achieve isotropic ENZ responses, which severely limits their use in the development of ENZ‐MO devices. Here, enhanced MO effects in pristine indium tin oxide (ITO) with ENZ properties at technologically important telecommunication wavelengths are demonstrated. MO transmission (reflection) spectroscopy of ITO films with different ENZ wavelengths reveal Faraday (Kerr) rotation peaks around the respective ENZ (EN‐one) wavelengths, demonstrating that these observations are due to the intrinsic properties of the ITO materials. The demonstrated mechanism of the MO effect enhancement is universal and can be applied to various ENZ materials, including those incorporating ferromagnetic materials. Native ENZ materials with enhanced MO responses will greatly expand the opportunities for the development of novel nonreciprocal nanophotonic devices, such as on‐chip optical isolators and one‐way topological waveguides.

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.

Uncovering and Experimental Realization of Multimodal 3D Topological Metamaterials for Low-Frequency and Multiband Elastic Wave Control.

Topological mechanical metamaterials unlock confined and robust elastic wave control. Recent breakthroughs have precipitated the development of 3D topological metamaterials, which facilitate extraordinary wave manipulation along 2D planar and layer-dependent waveguides. The 3D topological metamaterials studied thus far are constrained to function in single-frequency bandwidths that are typically in a high-frequency regime, and a comprehensive experimental investigation remains elusive. In this paper, these research gaps are addressed and the state of the art is advanced through the synthesis and experimental realization of a 3D topological metamaterial that exploits multimodal local resonance to enable low-frequency elastic wave control over multiple distinct frequency bands. The proposed metamaterial is geometrically configured to create multimodal local resonators whose frequency characteristics govern the emergence of four unique low-frequency topological states. Numerical simulations uncover how these topological states can be employed to achieve polarization-, frequency-, and layer-dependent wave manipulation in 3D structures. An experimental study results in the attainment of complete wave fields that illustrate 2D topological waveguides and multi-polarized wave control in a physical testbed. The outcomes from this work provide insight that will aid future research on 3D topological mechanical metamaterials and reveal the applicability of the proposed metamaterial for wave control applications.

Topological edge state assisted dynamically tunable microwave propagations in photonic crystals

Topological photonics has emerged as a cutting-edge and intriguing field that permits electromagnetic waves to transmit with minimal losses even when they come in contact with sharp turns or imperfections or defects. In this study, we validate a strategy to realize dynamically amplitude tunable light propagation in a topological waveguide based on a three-dimensional printed valley Hall photonic crystal structure operating in the microwave frequency region. Here, tunable light propagation is facilitated by inserting an active defect in the form of a semiconductor (silicon) material slab across the domain boundary of the topological waveguide. In this configuration, dynamic variation of transmission amplitude is realized via photoexcitation of the semiconductor defect using an external laser of wavelength, nm. This results in active amplitude tunability of dB in topological wave propagation under a photoexcitation of 100 mW. Our demonstrated route can lead to the design of dynamically controlled topological photonic devices such as optical modulators, switches, optical buffers, etc which are important for the development of 6G communication systems.

Active control of localized mode and transmission in topological phononic waveguides by non-Hermitian modulation

We demonstrate the switching behavioral differences between lossy and nearly lossless edge-mode propagation by non-Hermitian modulation based on the phononic band design of a C 3v symmetric, two-dimensional phononic crystal with a unit cell composed of three air-filled circular holes in polydimethylsiloxane. We numerically show that strong loss effects lead to the extinction of the localized modes. This mechanism is analogous to the bound-to-unbound transition in non-Hermitian quantum systems. This result suggests that large variations in non-Hermitian modulation can be used for the active control of edge-mode propagation along topological interfaces.