Valley Photonic Crystals Research Articles

Polarization-independent Topological Corner States Based on All-dielectric Valley Photonic Crystals

Polarization-independent Topological Corner States Based on All-dielectric Valley Photonic Crystals

Streamlined line-defect taper design for broadband and efficient mode coupling: connecting a silicon strip with a planar valley photonic crystal zigzag interface.

The integration of planar valley photonic crystal (VPC) interfaces into high-speed data communication chips markedly improves data rates and system robustness. This Letter presents a novel, to the best of our knowledge, edge coupler, termed the line-defect taper, which is crucial for efficient and broadband light delivery to planar VPC interfaces via silicon strip waveguides. The coupling performance of the line-defect taper is evaluated through full-wave three-dimensional finite-element simulations. The results demonstrate a -3 dB transmission bandwidth of 65.5 nm, covering 41.2% of the topological bandgap, and a -1 dB transmission bandwidth of 16.3 nm, accounting for 10.3%. With its compact design (only 3.6 µm in length), simplicity, and scalability, the line-defect taper is a promising candidate for integration into densely packed chips, highlighting its potential in advancing on-chip devices.

Thermally tunable add-drop filter based on valley photonic crystals for optical communications

Abstract Valley photonic crystals (VPCs) provide an intriguing approach to suppress backscattering losses and enable robust transport of light against sharp bends, which could be utilized to realize low-loss and small-footprint devices for on-chip optical communications. However, there are few studies on how to achieve power-efficient tunable devices based on VPCs, which are essential for implementing basic functions such as optical switching and routing. Here, we propose and experimentally demonstrate a thermally tunable add-drop filter (ADF) based on VPCs operating at telecommunication wavelengths. By leveraging the topological protection of the edge state and the distinct property of negligible scattering at sharp bends, a small footprint of 17.4 × 28.2 μm2 and a low insertion loss of 2.7 dB can be achieved for the proposed device. A diamond-shaped microloop resonator is designed to confine the light and enhance its interaction with the thermal field generated by the microheater, leading to a relatively low power of 23.97 mW needed for switching the output signal from one port to the other. Based on the thermally tunable ADF under the protection of band topology, robust data transmission is implemented with an ultrahigh data rate of 132 Gb/s. Our work shows great potential for developing high-performance topological photonic devices with the thermally tunable silicon-based VPCs, which offers unprecedented opportunities for realizing topologically protected and reconfigurable high-speed datalinks on a chip.

Topological slow light and rainbow trapping of surface wave in valley photonic crystal bounded by air

Abstract Topological slow light and rainbow trapping tend to rely on large-scale interface structure in previous research work, which have restricted further miniaturization. In this work, we propose a method to realize slow light and rainbow trapping at the zigzag edge of a single valley photonic crystals (VPCs) bounded by air, which is very different from previous studies where rainbow trapping is supported at the interface separating two VPCs with inversion symmetry. By constructing the VPC–air boundaries and VPC–VPC interfaces experimentally, we have observed the topologically protected rainbow trapping simultaneously at the external and internal boundary. This work provides a feasible platform for the miniaturized optical communication devices such as optical buffers, optical storage and optical routing.

Ultrabroadband valley transmission and corner states in valley photonic crystals with dendritic structure

In photonic crystal systems, topologically protected edge states and corner states can be achieved by breaking spatial inversion symmetry, which is expected to be applied to topologically protected lasers, optical communication and integrated photonics. However, designing ultrabroadband topological photonic crystals is still a challenge. In this work, we propose a valley photonic crystal composed of dendritic structures, which can realize valley transmission with a relative bandwidth up to 59.65%. Compared with the previously reported two-dimensional broadband photonic crystals with 32.02% bandwidth, the relative bandwidth of the proposed valley transmission is increased by almost 100%. Theoretical analysis, numerical simulation and experimental measurement all confirm flexible manipulation of electromagnetic wave propagation paths. Ultrabroadband topological waveguides with the zigzag and armchair interface are demonstrated, which can achieve experimentally 58.71% and 36.78% relative bandwidth, respectively. In addition, several topological channel intersections are designed. Finally, two types of corner states with valley switchability and selectivity are demonstrated.

Topological valley-locked waveguides with C4 impurity

Abstract Heterostructures play a pivotal role in the design of valley-locked waveguides, facilitating the manipulation of width as an additional degree of freedom. Through this design, we demonstrate the extension of the topological guided modes from the domain wall of topologically nontrivial valley photonic crystals (VPCs) into the trivial VPCs. We propose a C4 impurity to control the states of the light wave transmission in topological valley-locked waveguides through the intervalley scattering of defects in Quantum Valley Spin Hall topological insulators. By rotating the C4 structure, the ON/OFF (0°/45°) state of the valley-locked waveguides can be controlled, effectively serving as a switch component. Furthermore, many unique applications could be devised based on the introduced impurity. Examples include the development of coding channels with arbitrary output ports and energy concentrators with enhanced secondary concentration. The proposed topological valley-locked waveguides with C4 impurity will be beneficial for on-chip integrated photonic networks.

Dual-polarization valley-like transport in dielectric photonic crystals

Valley photonic crystal (VPC) waveguides possess strong immunity to backward scattering and robust transmission, originating from topologically protected edge states. Here, we propose a VPC/air/VPC sandwiched structure, and VPC possesses the common valley band gaps for TM and TE polarizations. By modifying the radii of the cylinders on both sides of the air channel, finite element method (FEM) simulations show the valley-locked waveguide modes with high flux are achieved, for both polarizations. Based on VPC/air/VPC sandwiched structure, we further design waveguide couplers and zigzag waveguide bends, and immunity to backward scattering and robust transmission are also verified. Our work finds a new method of designing photonic crystal waveguides, which support valley-like transport with enhanced flux and dual-polarization, making topological photonic devices easily to be integrated with conventional optical waveguides.

Design of a temperature sensor based on a valley photonic crystal Mach–Zehnder interferometer

With the development of photonics device integration technology, Mach–Zehnder interferometers (MZIs) are widely applied as sensors because they are extremely sensitive to environmental parameters. Conventional MZI sensors are generally large and unsuitable for high-density integration. MZIs based on photonic crystals (PCs) can significantly improve compactness and are suitable for integration. However, PC MZIs experience substantial optical loss due to scattering. Based on the spin-valley locking effect that can achieve defect-immune unidirectional transmission of topological edge states in valley photonic crystals (VPCs), we propose an MZI sensor based on a VPC structure and apply it in temperature sensing for what we believe is the first time. The interference peaks redshift with an increase in the temperature, which allows accurate sensing of the temperature shift with a high sensitivity of 0.06 nm/K in the temperature range of 100 K–750 K. The ultracompact device has a small footprint of 9.26×7.99µm2 and a high forward transmittance of 0.88. The design is suitable for the current complementary metal-oxide semiconductor (CMOS) fabrication technique. Thus, it will find broad applications in integrated photonics, optical communication, and integrated temperature sensing.

Ultrafast Unidirectional On-Chip Heat Transfer.

Effective and rapid heat transfer is critical to improving electronic components' performance and operational stability, particularly for highly integrated and miniaturized devices in complex scenarios. However, current thermal manipulation approaches, including the recent advancement in thermal metamaterials, cannot realize fast and unidirectional heat flow control. In addition, any defects in thermal conductive materials cause a significant decrease in thermal conductivity, severely degrading heat transfer performance. Here, the utilization of silicon-based valley photonic crystals (VPCs) is proposed and numerically demonstrated to facilitate ultrafast, unidirectional heat transfer through thermal radiation on a microscale. Utilizing the infrared wavelength region, the approach achieves a significant thermal rectification effect, ensuring continuous heat flow along designed paths with high transmission efficiency. Remarkably, the process is unaffected by temperature gradients due to the unidirectional property, maintaining transmission directionality. Furthermore, the VPCs' inherent robustness affords defect-immune heat transfer, overcoming the limitations of traditional conduction methods that inevitably cause device heating, performance degradation, and energy waste. The design is fully CMOS compatible, thus will find broad applications, particularly for integrated optoelectronic devices.

Wide-mode-area slow light waveguides in valley photonic crystal heterostructures

We designed slow-light waveguides with a wide mode area based on slab-type valley photonic crystal (VPhC) heterostructures which are composed of a graphene-like PhC sandwiched by two topologically distinct VPhCs. The group velocity of the topological guided mode hosted in a VPhC heterostructure can be slowed down by shifting the VPhC lattice toward the graphene-like PhC at the domain interfaces. Simultaneously, the mode width of the slow-light topological guided mode can be widened by increasing the size of the graphene-like PhC domain. We found that employing the graphene-like structure at the center domain is crucial for realizing a topological single-guided mode in such heterostructures. Furthermore, the impact of random fluctuations in air-hole size in the graphene-like domain was numerically investigated. Our simulation results demonstrate that the transmittance for the slow-light states can be kept high as far as the size fluctuation is small although it drops faster than that for fast-light states when the disorder level increases. The designed wide-mode-area slow-light waveguides are based on hole-based PhCs, offering novel on-chip applications of topological waveguides.

Topological Edge State Ring Resonator for Mid-Infrared(MI) Refractive Indices Biosensor for detection of Brain Tumors

The unique properties of light underlie the perspectives of quantum photonic technologies, optical interconnects, and a wide range of new sensors.Some of the most dangerous and deadly diseases are tumors, cancers, and brain lesions, which are expensive to detect and treat. Therefore, a low-cost and accurate method to diagnose them can prevent and treat the progress of this disease. In this work, we used the precise topological valley photonic crystal (TVPC) method for detection. TVPC is an important method for transmitting and controlling light in the optical device. The valley-spin locking in the topology state provides robust transfer and low propagation loss at the desired path.In order to design, by finding the edge band within the first bulk band gap of 146.4 THz–155.9 THz, the model and design of the topological ring resonator (TRS)were realized. We proposed a topological biosensorbyusing the hexagonal lattice air holes in the silicon slab with a compact size of 22.95 μm × 10 μm. the quality factor and sensitivity at best value are 2.905 × 104 and 9021 nm RIU−1 respectively. This design can be implemented on the Complementary Metal-Oxide-Semiconductor (CMOS) technology as a high-sensitivity optical device.

Tailored Triggering of High-Quality Multi-Dimensional Coupled Topological States in Valley Photonic Crystals

The combination of higher-order topological insulators and valley photonic crystals has recently aroused extensive attentions due to the great potential in flexible and efficient optical field manipulations. Here, we computationally propose a photonic device for the 1550 nm communication band, in which the topologically protected electromagnetic modes with high quality can be selectively triggered and modulated on demand. Through introducing two valley photonic crystal units without any structural alteration, we successfully achieve multi-dimensional coupled topological states thanks to the diverse electromagnetic characteristics of two valley edge states. According to the simulations, the constructed topological photonic devices can realize Fano lines on the spectrum and show high-quality localized modes by tuning the coupling strength between the zero-dimensional valley corner states and the one-dimensional valley edge states. Furthermore, we extend the valley-locked properties of edge states to higher-order valley topological insulators, where the selected corner states can be directionally excited by chiral source. More interestingly, we find that the modulation of multi-dimensional coupled photonic topological states with pseudospin dependence become more efficient compared with those uncoupled modes. This work presents a valuable approach for multi-dimensional optical field manipulation, which may support potential applications in on-chip integrated nanophotonic devices.

Dielectric topological metasurfaces based in-plane control of light propagation by using valley degrees of freedom

Metasurface has garnered widespread attention due to its remarkable ability of precise light manipulation. The emergence of topological photonics has brought novel topological phenomena and holds the promise of introducing new optical functionalities to the metasurfaces. In this paper, by introducing valley degrees of freedom into metasurfaces, we design and numerically simulate topological-nanostructure-enabled on-chip chiral light propagation and multistage beam splitting, as well as holographic display by integrating a beam-splitting metasurface with valley photonic crystals (VPCs). Importantly, the proposed on-chip topological optical devices exhibit broadband response covering 1480–1660 nm and low transmission losses with the maximum efficiency reaching 82 %. Our work is expected to bring new optical functionalities to the metasurface based devices and can be widely applied in fields of optical fiber communications, on-chip optical transportation and smart wearable devices, which require high-efficiency, broadband and highly-integrated optoelectronic elements and devices.

Investigation of the properties of photonic crystal resonant cavities based on valley spin reversal

Resonators have been treated as essential elements in optics because of their capacity to store and enhance light and exhibit a wide range of applications such as semiconductor lasers and optical communication components. In this article, we reveal a new mechanism of light field confinement in an optical cavity composed of different valley photonic crystals. The electromagnetic field of light is localized because of the valley spin states contrasted between the inner and outer regions, which leads a high Q-factor and a small model volume of the resonator. Furthermore, the whispering-gallery-mode modulated vortex phase distribution is demonstrated in the proposed structure, which offers a new method for manipulating the light field. The energy spectrum as well as the light field distributions show the simultaneous appearance of both bulk and edge states. Such effect becomes pronounced or diminished when the domain wall changes, and can be explained by the location of the edge states in the shared bandgap. Our findings offer a novel mechanism of light field confinement and phase modulation, which may pave the way for a new type of topological device and provide broad applications in the areas of micro-lasers, optical communications, and other light-matter interaction systems.

Quantifying robustness against sharp bending in an integrated topological interface of valley photonic crystals

Abstract Sharp-bending waveguide is a key element for ultra-compact and densely integrated photonic devices, which is promising to enlarge the capability of modern information processing in a single chip. Topological photonics manifest the nature of robust propagation against sharp bending and such robustness has been experimentally demonstrated in topological integrated interfaces. It is important to quantify the bending loss of topological interface but has remained exclusive. In this work, we report on the characterization of sharp-bending robustness in the integrated topological interface of valley photonic crystals (VPCs) by experimentally quantifying the ultralow bending loss. The VPCs are designed on a standard silicon-on-insulator platform with the inversion-symmetry broken in honeycomb lattice, and four types of topological interfaces can be constructed by two topologically-distinct VPCs. As one of the representative cases, zigzag-AA interface is applied to demonstrate the robust propagation along sharp bending. In experiment, we fabricate a series of VPC interfaces with different turn number and the same transmission distance, which perform the ultralow bending loss less than 0.02 dB per 120-deg turning. Furthermore, we experimentally characterize the propagation loss in the integrated interfaces. Our approach not only shows the ability of VPC topological interfaces to suppress backscattering stemming from sharp bending, but also paves the way for topological nanophotonic dense integration.

Multifrequency and Multimode Topological Waveguides in a Stampfli‐Triangle Photonic Crystal with Large Valley Chern Numbers

AbstractThe multifrequency quantum valley Hall effect (QVHE) has been realized to significantly improve the transmission capacity of topological waveguides, and the multimode QVHE with a large valley Chern number has been realized to increase the mode density of topological waveguides. However, multifrequency and multimode QVHEs have not been realized simultaneously. In this work, using tight‐binding model calculations and numerical simulations, a valley photonic crystal (VPC) consisting of a Stampfli‐triangle photonic crystal is constructed, and its multiple degeneracies in the low‐frequency and high‐frequency bands split simultaneously to realize the QVHE with multiple topological edge states (TESs). The multifrequency and multimode topological transmission with two low‐frequency modes and four high‐frequency modes is realized by means of simulations and experiments through a Z‐shaped waveguide constructed using two VPCs with opposite valley Chern numbers to prove the realization of a large valley Chern number in the two frequency bands. The two low‐frequency modes are successfully distinguished with position‐dependent selective excitations, which experimentally demonstrate the occurrence of a large valley Chern number. A frequency‐dependent multimode beam splitter is theoretically proposed for high‐performance integrated photonic device applications. These results provide new ideas for high‐efficiency and high‐capacity optical transmission and communication devices and their integration; furthermore, they broaden the application range of TESs.

Efficient light couplers to topological slow light waveguides in valley photonic crystals.

We numerically and experimentally demonstrate efficient light couplers between topological slow light waveguides in valley photonic crystals (VPhCs) and wire waveguides. By numerical simulations, we obtained a high coupling efficiency of -0.84 dB/coupler on average in the slow light regime of a group index ng = 10 - 30. Experimentally, we fabricated the couplers in a Si slab and measured the transmitted power of the devices. We realized a high coupling efficiency of approximately -1.2 dB/coupler in the slow light region of ng = 10 - 30, which is close to the result from the numerical simulations. These demonstrations will lay the groundwork for low-loss photonic integrated circuits using topological slow light waveguides.

Ultra-compact electro-optic phase modulator based on a lithium niobate topological slow light waveguide.

Electro-optic modulators (EOMs) are essential devices of optical communications and quantum computing systems. In particular, ultra-compact EOMs are necessary for highly integrated photonic chips. Thin film lithium niobate materials are a promising platform for designing highly efficient EOMs. However, EOMs based on conventional waveguide structures are at a millimeter scale and challenging to scale down further, greatly hindering the capability of on-chip integration. Here, we design an EOM based on lithium niobate valley photonic crystal (VPC) structures for the first time. Due to the high effective refractive index introduced by the strong slow light effect, the EOM can achieve an ultra-compact size of 4 μm×14 μm with a half-wave voltage of 1.4 V. The EOM has a high transmittance of 0.87 in the 1068 nm because of the unique spin-valley locking effect in VPC structures. The design is fully compatible with current nanofabrication technology and immune to fabrication defects. Therefore, it opens a new possibility in designing lithium niobate electro-optic modulators and will find broad applications in optical communication and quantum photonic devices.

Topological edge and corner states of valley photonic crystals with zipper-like boundary conditions

We present a stable valley photonic crystal (VPC) unit cell with C 3v symmetric quasi-ring-shaped dielectric columns and realize its topological phase transition by breaking mirror symmetry. Based on this unit cell structure, topological edge states (TESs) and topological corner states (TCSs) are realized. We obtain a new type of wave transmission mode based on photonic crystal zipper-like boundaries and apply it to a beam splitter assembled from rectangular photonic crystals (PCs). The constructed beam splitter structure is compact and possesses frequency separation functions. In addition, we construct a box-shaped triangular PC structures with zipper-like boundaries and discover phenomena of TCSs in the corners, comparing its corner states with those formed by other boundaries. Based on this, we explore the regularities of the electric field patterns of TESs and TCSs, explain the connection between the characteristic frequencies and locality of TCSs, which helps better control photons and ensures low power consumption of the system.

Topological light transport in low-symmetry valley photonic crystals

Valley photonic crystals represent a cornerstone in the field of topological photonics, which promotes the development of cutting-edge photonic waveguides. These waveguides support robust transmission by using valley-dependent edge states. This innovation marks a great leap forward in enhancing transmission efficiency, (especially in sharp bends), thus opening up a new way for efficient optical information transmission. However, although the role of symmetry in topology and photonic crystals cannot be exaggerated, it is worth noting that valley photonic crystals provide a unique platform for exploring the interplay between symmetry and topological phenomena. An intriguing analogy between valley photonic crystals and the quantum valley Hall effect is an example, which will be shown when the symmetry of spatial inversion is broken. At present, the characteristic of most valley photonic crystals is <i>C</i><sub>3</sub>-rotational symmetry, which leads to an interesting study, that is, whether crystals with lower symmetry can also support topological light transmission. In order to solve this problem head-on, our work focuses on constructing and characterizing valley photonic crystals with low symmetry by carefully adjusting the unit cell morphology. Through theoretical analysis and numerical simulation, we unveil the remarkable ability of these low-symmetry valley photonic crystals to facilitate topological light transport. Initially, we analyze the bulk bands of these low-symmetry crystals, observing a narrowed photonic band gap and a shift in the irreducible Brillouin zone compared with <i>C</i><sub>3</sub>-rotation symmetric crystals. To examine edge state transmission, we calculate dispersion relations and electric field distributions, revealing two edge states with opposite phase chirality at the same frequency. Using this point, we achieve unidirectional excitation of edge states. Additionally, we manipulate the refractive index of the surrounding medium and explore various scenarios of external light beam coupling. Moreover, we investigate the robust transmission of edge states, demonstrating smooth passage of light through sharp corners in <i>Z</i>-shaped bend waveguides without backscattering. In conclusion, our findings underscore the pivotal role played by edge states in facilitating unidirectional excitation and robust transmission in low-symmetry valley photonic crystals. By enriching the diversity of topological photonic structures and providing valuable insights into the behavior of topological light transport in structures with lower symmetry, our work contributes to the ongoing quest for novel photonic platforms with enhanced functions and performance.