States In Photonic Crystals Research Articles

Observation of Landau levels and chiral edge states in photonic crystals through pseudomagnetic fields induced by synthetic strain

Observation of Landau levels and chiral edge states in photonic crystals through pseudomagnetic fields induced by synthetic strain

Topological protection of dual-polarization biphoton states in photonic crystals

Polarization control is a major issue in topological quantum optics that limits reliable generation and transmission of quantum states. This study presents what we believe to be a novel topological photonic crystal design that provides topological protection for biphoton pairs for both TE and TM polarization. By well-designed cell configurations within the lattice, two topological boundaries emerge that can accommodate TM and TE modes at the same time. By adjusting the dispersion curves, we can further design nonlinear four-wave mixing processes within the topological photonic crystals and provide theoretical explanations for the entanglement of the dual-polarization biphoton states. Numerical results confirm the robust transport of entangled photon pairs, even when subjected to sharp bending. Moreover, combining the dual-polarization topological photonic crystal with a polarization beam splitter enables the preparation of polarization-encoded maximally entangled states. Our work exhibits significant potential for applications in robust optical quantum information processing and quantum secure communication.

Tailoring coupled topological corner states in photonic crystals via symmetry breaking induced by defects

In this paper, we present an effective approach to tailor the optical properties of coupled corner states in photonic crystal (PhC) supercell arrays by symmetry breaking. Defects are strategically introduced into unit cells to break the in-plane C4v symmetry, and numerical simulations confirm the preservation of the band gap and two-dimensional Zak phases, in spite of the presence of defects. Notably, the size of defects is demonstrated to play a significant role in tuning the eigenfrequencies, quality (Q) factors, and band structures of the coupled corner states. Furthermore, these defects exhibit the ability to transform topological dark states into bright states, enabling efficient excitation by external plane waves and polarization-dependent Q factors. The resonant origin of the coupled corner states is comprehensively elucidated using a multipole decomposition analysis. This study further releases the potential of the topological light-matter interaction, holding significant implications for the design and advancement of topological photonic devices with on-demand tunability and multifrequency regions for various applications such as lasing, sensing, and quantum information processing. Published by the American Physical Society 2024

Multiple topological states in photonic crystals with generalized Kekulé modulation

We theoretically proposed a feasible way to design multi-topological states in one two-dimensional photonic crystal (PC). By sliding and rotating the meta-atoms of its unit cell, the topological phase of the PC can be continuously tuned, but without a closing of bandgap; it is similar to the Kekulé modulation in honeycomb lattices. We first show the existence of helical topological edge states in conventional topological insulator, which intersect together and form a Dirac cone in the two-dimensional synthetic space of the PC. Then, in full synthetic space, the PC behaves high-order topological insulator, and the corner state is then constructed in a heterostructure that consists of three different PCs. Furthermore, we demonstrate the photonic crystal with non-uniform modulation can behaves as a splitter and a rainbow trapper. Our proposal may provide interesting insight for designing of multiple topological states, which have potential in photonic on-chip devices.

Actively Reconfigurable Valley Topological Edge and Corner States in Photonic Crystals Based on Phase Change Material Ge2Sb2Te5

Actively Reconfigurable Valley Topological Edge and Corner States in Photonic Crystals Based on Phase Change Material Ge2Sb2Te5

Topological light guiding and trapping via shifted photonic crystal interfaces

The exploration of topological states in photonic crystals has inspired a number of intriguing discoveries, which in turn provide new mechanisms for the manipulation of light in unprecedented ways. Here, we show that light can be effectively guided and trapped at the shifted photonic crystal interfaces (SPCIs). The projected bandgap of SPCIs, which depends on the shift parameter, is characterized by a Dirac mass. Interestingly, the SPCI with zero Dirac mass is a glide-symmetric waveguide featured with gapless interface states that exhibit excellent transmission performance even in the presence of disorders and sharp corners. Moreover, placing two SPCIs with opposite Dirac mass together results in a photonic bound state due to the Jackiw–Rebbi theory. Our work provides an alternative way toward the design of ultracompact photonic devices such as robust waveguides and cavities as well as the cavity-waveguide coupled systems that can serve as high-performance building blocks of miniature integrated topological photonic circuits.

Tailoring topological corner states in photonic crystals by near- and far-field coupling effects

We explore the behaviors of optically coupled topological corner states in supercell arrays composed of photonic crystal rods, where each supercell is a second-order topological insulator. Our findings indicate that the coupled corner states possess nondegenerate eigenfrequencies at the Γ point, with coupled dipole corner states excited resonantly by incident plane waves and displaying a polarization-independent characteristic. The resonance properties of coupled dipole corner states can be effectively modulated via evanescently near-field coupling, while multipole decomposition shows that they are primarily dominated by electric quadrupole moment and magnetic dipole moment. Furthermore, we demonstrate that these coupled corner states can form surface lattice resonances driven by diffractively far-field coupling, leading to a dramatic increase in the quality factor. This work introduces more optical approaches to tailoring photonic topological states, and holds potential applications in mid-infrared topological micro-nano devices.

Realization of edge and corner states in photonic crystals with kagome lattices through topological insulator generators

High-order topological insulators can be realized via two-dimensional photonic crystals (PCs) with different lattices including honeycomb, square and kagome lattices. In this paper, we utilize the kagome lattices to build a two-dimensional all-dielectric PCs platform with two kinds of topological insulator generators, obtaining the topological edge states in different frequency bands, and realizing the dual-band single channel waveguide and frequency divider. In addition, we research the origins of the two types of corner states in the combined triangle and parallelogram box-shaped structures, which consist of nontrivial PCs and trivial PCs, realizing the superimposed corner states and further enhancing the localized energy intensity in the PCs platform. These works have great potential for application in integrated nanophotonic devices.

Free-electron–light interactions in nanophotonics

When impinging on optical structures or passing in their vicinity, free electrons can spontaneously emit electromagnetic radiation, a phenomenon generally known as cathodoluminescence. Free-electron radiation comes in many guises: Cherenkov, transition, and Smith–Purcell radiation, but also electron scintillation, commonly referred to as incoherent cathodoluminescence. While those effects have been at the heart of many fundamental discoveries and technological developments in high-energy physics in the past century, their recent demonstration in photonic and nanophotonic systems has attracted a great deal of attention. Those developments arose from predictions that exploit nanophotonics for novel radiation regimes, now becoming accessible thanks to advances in nanofabrication. In general, the proper design of nanophotonic structures can enable shaping, control, and enhancement of free-electron radiation, for any of the above-mentioned effects. Free-electron radiation in nanophotonics opens the way to promising applications, such as widely tunable integrated light sources from x-ray to THz frequencies, miniaturized particle accelerators, and highly sensitive high-energy particle detectors. Here, we review the emerging field of free-electron radiation in nanophotonics. We first present a general, unified framework to describe free-electron light–matter interaction in arbitrary nanophotonic systems. We then show how this framework sheds light on the physical underpinnings of many methods in the field used to control and enhance free-electron radiation. Namely, the framework points to the central role played by the photonic eigenmodes in controlling the output properties of free-electron radiation (e.g., frequency, directionality, and polarization). We then review experimental techniques to characterize free-electron radiation in scanning and transmission electron microscopes, which have emerged as the central platforms for experimental realization of the phenomena described in this review. We further discuss various experimental methods to control and extract spectral, angular, and polarization-resolved information on free-electron radiation. We conclude this review by outlining novel directions for this field, including ultrafast and quantum effects in free-electron radiation, tunable short-wavelength emitters in the ultraviolet and soft x-ray regimes, and free-electron radiation from topological states in photonic crystals.

Topologically protected energy-time entangled biphoton states in photonic crystals

The concepts of topological phases have been widely exploited in quantum optics in recent years. Here we demonstrate a topological insulator implementing topological protection of correlated biphoton states. A degenerate four-wave mixing (FWM) process of pseudospin states propagating along the topological interface is numerically simulated. Strikingly, the signal and idler photons generated from the FWM process are clarified to be entangled between two degrees of freedom—the frequencies of photon pairs and their time of arrival. The topological edge states of the pump, signal, and idler are robust against the sharp bends and defects, revealing the topological protection of energy-time entangled biphoton states. These findings could pave the way for unprecedented topological quantum devices.

Manipulation of photon states in photonic crystals based on silicon

Coupling between the active centers generated by localized states and the two-dimensional photonic crystals (2D-PCs) has been used to select model in the nano-laser and the optical interconnects on silicon chip. The photonic crystals have been investigated in the simulating calculation, and the physical model has been built in the manipulation of photon states. New quantum phenomena appear in the photonic crystals as well as in the atomic crystals. We have numerically studied on the band structure of 2D-PCs with different types of lattice and various shapes in the optical communication windows. The simulating calculation results show that the band gap of photon states can be effectively manipulated in 2D-PCs by using the competition between the quantum confinement effect and the lattice symmetry effect, in which the band gap width of the photon states will obviously increase by changing the lattice symmetry and by increasing the filling rate of the air hole to restrain photons in 2D-PCs on silicon. It is interesting to make a comparison between the transverse electric (TE) polarization and the transverse magnetic (TM) polarization, in which a wider band gap occurs in the TM mode. By adjusting the defect mode in 2D-PCs, the photon states can be localized into the second and third windows of optical communication for selecting modes and resonating on silicon chip.

Realization of Edge and Corner States in Photonic Crystals with Kagome Lattices Through Topological Insulator Generators

Realization of Edge and Corner States in Photonic Crystals with Kagome Lattices Through Topological Insulator Generators

Realization of Edge and Corner States in Photonic Crystals with Kagome Lattices Through Topological Insulator Generators

Realization of Edge and Corner States in Photonic Crystals with Kagome Lattices Through Topological Insulator Generators

Dynamically reconfigurable topological states in photonic crystals with liquid crystals.

Dynamically tunable and reconfigurable topological states are realized in higher-order topological insulators with the liquid crystal (LC). By changing the loading voltage of the LC, the eigenfrequency of the edge and corner states can be tuned, but even more important is that the edge state and corner state with the same frequency are realized. Based on this reconfigurability of topological states, optical routers and lasers with multiple topological states can be realized. Our results may be applied to topological optical circuits and provide new ideas for optical field localization and manipulation.

Interface states in the rectangular lattice photonic crystals with identical dielectric rods

Interface states in photonic crystals (PCs) have attracted attention for the special properties, such as high transmission efficiency in bend waveguides, and their generation related to the topological phase. Previous works on interface states in PCs were mainly based on the square lattice, the honeycomb lattice or the triangular lattice, but with different materials, shapes, or sizes of rods resulting in the complicated structure. In this paper, an interface is constructed by two 2D PCs with different rectangular lattices, but the same materials, shapes, and sizes of dielectric rods, which generates interface states. The interface states are analyzed with respect to Zak phases and surface impedances. The retainability of the interface states in rectangular lattice PCs is investigated by studying the relationship between the length-width ratio of the rectangular lattice and the Zak phase. It is found that, when the interface states are generated by changing the length-width ratio of the rectangular lattice, the retainability of the interface states is related to the positions of the photonic bandgaps or the Zak phases of the bands. A more detailed examination indicates that these conclusions are applicable to the rectangular lattice PCs with other materials, shapes, and sizes of dielectric rods. These results can lead to new ways to generate interface states easily, with only one kind of dielectric rod. In addition, these outcomes may contribute to the understanding of the relationship between the geometry and the interface state.

Direct observation of topological edge states in silicon photonic crystals: Spin, dispersion, and chiral routing

Topological protection in photonics offers new prospects for guiding and manipulating classical and quantum information. The mechanism of spin-orbit coupling promises the emergence of edge states that are helical, exhibiting unidirectional propagation that is topologically protected against back scattering. We directly observe the topological states of a photonic analog of electronic materials exhibiting the quantum spin Hall effect, living at the interface between two silicon photonic crystals with different topological order. Through the far-field radiation that is inherent to the states' existence, we characterize their properties, including linear dispersion and low loss. We find that the edge state pseudospin is encoded in unique circular far-field polarization and linked to unidirectional propagation, thus revealing a signature of the underlying photonic spin-orbit coupling. We use this connection to selectively excite different edge states with polarized light and directly visualize their routing along sharp chiral waveguide junctions.

Optical properties of globular photonic crystals with different particle sizes

The article reports the results of experimental studies of the optical properties of globular photonic crystals filled with various chemical liquids. It opens up the possibility of creating highly sensitive refractive sensors of chemical compounds based on the introduction of these substances into the pores of opal matrices. The optical properties of opal matrices with different sizes of globules have been analyzed. Different spectral positions of the stop bands are present in the reflectance spectrum of globular photonic crystals, depending on the size of the particles. The sharp increase in the density of photonic states in photonic crystals opens up the possibility of increasing the efficiency of Raman scattering and nonlinear-optical processes in materials of this kind.

One-way propagation of bulk states and robust edge states in photonic crystals with broken inversion and time-reversal symmetries

The valley is a flexible degree of freedom for light manipulation in photonic systems. In this work, we introduce the valley concept in magnetic photonic crystals with broken inversion symmetry. One-way propagation of bulk states is demonstrated by exploiting the pseudo-gap where bulk states only exist at one single valley. In addition, the transition between Hall and valley-Hall nontrivial topological phases is also studied in terms of the competition between the broken inversion and time-reversal symmetries. At the photonic boundary between two topologically distinct photonic crystals, we illustrate the one-way propagation of edge states and demonstrate their robustness against defects.

Direct observation of valley-polarized topological edge states in designer surface plasmon crystals

The extensive research of two-dimensional layered materials has revealed that valleys, as energy extrema in momentum space, could offer a new degree of freedom for carrying information. Based on this concept, researchers have predicted valley-Hall topological insulators that could support valley-polarized edge states at non-trivial domain walls. Recently, several kinds of photonic and sonic crystals have been proposed as classical counterparts of valley-Hall topological insulators. However, direct experimental observation of valley-polarized edge states in photonic crystals has remained difficult until now. Here, we demonstrate a designer surface plasmon crystal comprising metallic patterns deposited on a dielectric substrate, which can become a valley-Hall photonic topological insulator by exploiting the mirror-symmetry-breaking mechanism. Topological edge states with valley-dependent transport are directly visualized in the microwave regime. The observed edge states are confirmed to be fully valley-polarized through spatial Fourier transforms. Topological protection of the edge states at sharp corners is also experimentally demonstrated.

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.