Large Group Index Research Articles

Pseudospin-polarized slow light waveguides with large delay-bandwidth product

Delay-bandwidth product (DBP) is a key metric in slow light waveguides, requiring a balance between a large group index and broad bandwidth—two parameters that often involve a trade-off. Here, we propose and demonstrate a slow light waveguide with large DBP using a pseudospin-polarized transverse electromagnetic mode. This waveguide features a folded edge configuration that supports a 200% relative bandwidth from quasistatic limit (zero frequency) and an arbitrarily large group index. Owing to the pseudospin-polarized design, the dense folding would not introduce backscattering and the associated group velocity dispersion (GVD). The resulting gapless linear dispersion and pulse transmission behavior in folded edge waveguide are observed in microwave experiments. Our scheme provides a way to overcome the trade-off between group index and working bandwidth in slow light waveguide, which has potential applications in broadband optical buffering, light-matter interaction enhancement, terahertz radiation source and time domain processing.

Enhanced superluminal pulse propagation via chiral quantum dot system

The control and enhancement of superluminal pulse propagation via a chiral quantum-dots (QDs) system is presented in this article. The proposed system is a double chiral QDs four-level system, that gets excited by pump and probe fields, along with inter-dot tunneling strength. It is observed that the strength of inter-dot tunneling strongly influences the left-circular polarized (LCP) and right-circular polarized (RCP) superluminal pulse propagation. By appropriately modulating this strength, optimal coherence within the system can be achieved, thereby contributing to enhanced superluminal pulse propagation. We have examined the characteristics of RCP and LCP beams and observed the large negative group index and advance time. The observation of a large negative group index and advanced time shows strong evidence for enhanced superluminal pulse propagation in the proposed chiral QDs system. This study has important applications in the field of optical information processing, temporal cloaking, quantum communication, and improving computer chip speed.

Superluminal propagation of surface plasmon polaritons via hybrid chiral quantum dots system

We present a novel methodology for enhancing superluminal surface plasmon polaritons (SPPs) propagations within a hybrid nanostructure configuration consisting of gold (Au) metal and chiral quantum dots (CQDs) medium. The arrangement of CQDs and metal hybrid nanostructures enables the production of SPPs when exposed to incident light. The resonances of SPPs within a hybrid nanostructure are determined through analytical calculations using Maxwell’s equations under specified boundary conditions, while the dynamics of the CQDs system are calculated using the density matrix approach. It is demonstrated that the propagation of SPPs is significantly influenced by both right-circularly polarized (RCP) and left-circularly polarized (LCP) SPPs. Additionally, we investigate the enhancement of superluminal SPPs propagation by varying the electron tunneling strength and the intensity of the control field within the hybrid system. The characteristics of RCP and LCP SPPs have been investigated, indicating a large negative group index and advancement in time. The observation of a large negative group index and advancement in time provides strong evidence for enhanced superluminal SPPs propagation within the proposed hybrid nanostructure. The results have potential applications in the fields of optical information processing, temporal cloaking, quantum communication, and the advancement of computer chip speed.

Sensing and slow light applications of graphene plasmonic terahertz structure

In this work, Ansys FDTD is used to design and simulate a terahertz metamaterial structure based on periodic continuous pattern graphene monolayer, and the high-quality PIT phenomena are obtained by continuously adjusting structural parameters. To validate the designed structure, the simulated transmission curve (reflection curve) obtained is compared with the theoretical transmission curve (reflection curve) derived from coupled-mode theory. It is observed that these two results exhibit a remarkably high degree of overlap. The resonant frequency and Fermi energy reveals a perfect linear correlation between them with the resonant frequency increasing proportionally with Fermi energy increasing. Dynamic tuning of PIT can be realized by adjusting the Fermi energy of graphene. For a more in-depth study of its sensing characteristics, the structure is placed in different environments. As the refractive index of the detection medium increases, the resonant frequency gradually decreases, demonstrating a redshift phenomenon. By manipulating the resonant frequency of the PIT sensor, the selective detection of specific target can berealized. After analyzing the sensitivity and FOM values of the structure, it is found that the maximum sensitivity is 1.457 THz/RIU. At a resonant frequency of 6.8174 THz, FOM reaches 30.5652. In summary, the sensor structure designed in this work has dual frequency sensing characteristics and can be used for dual frequency detection. Moreover, compared with other sensor structures, it demonstrates superior sensing performance. Additionally, in studying the slow light effect of the structure, it is found that as the Fermi energy increases, the group index and phase shift at the transparency window continue to increase. At the Fermi energy of 1.2 eV, the group index reaches a high value of 584. This is because in the PIT phenomenon, transparent peaks are formed due to multimodal coupling. This coupling will significantly improve the dispersion characteristics near the transparent peak, resulting in a large group index near the transparent peak. Furthermore, with the increase of carrier mobility, the group index and phase shift of the structure also gradually increase. At a carrier mobility of 0.75 m²/(V·s), the group refractive index is 456, and reaches 1010 at 2.0 m²/(V·s). In this study, the slow-light performance of graphene structure can be optimized through jointly adjusting the Fermi energy and carrier mobility. This research provides theoretical support and methods for designing advanced graphene-based sensors and devices for slow-light applications.

Topological slow light waveguide in photonic valley-locked heterostructures

In this work, a photonic valley-locked heterostructure is proposed, which is composed of a Dirac photonic crystal (DPC) and two valley photonic crystals (VPC), with opposite valley Chern numbers. By modifying the size of rods nearest to the domain walls, the topological valley waveguide state (TVWS) with large group index (n g = 100) can be found, which is called topological slow-light waveguide mode (TSWM). The simulation results based on finite element method demonstrate that the TSWM supports high energy capacity in the proposed heterostructure waveguide, which is suitable for integrating with the existing waveguides. Besides, TSWM is also valley-locked and immune to backscattering. Our finding paves a way of topological slow light, enrich the research of VPCs, and has new applications in optical communication devices.

An Enhanced High Q-Factor Resonance of Quasi-Bound States in the Continuum With All-Dielectric Metasurface Based on Multilayer Film Structures

Confinement of light is very important to the field of optics, and it is theoretically possible to achieve the perfect confinement for optical modes located in the continuous domain radiation state. Achieving perfect confinement is possible for bound states in the continuum (BIC), which localize waves that lie in the continuous spectrum of the radiative state and remain in a perfect localized state. In this article, we propose a metasurface that consists of arrays of silicon nanoellipsoidal pairs and calculate its transmission spectra and energy bands. Based on an all-dielectric metasurface, a dielectric multilayer film is incorporated to improve the quality factor of the array composed of ellipsoidal table pairs by an order of magnitude. By breaking the symmetry of ellipsoidal table pairs, quasi-BIC resonance modes excited by electric quadrupole (EQ) and magnetic dipoles (MDs) are realized using asymmetric silicon nanoellipsoid surfaces, where the quasi-BIC modes have high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> -factors. In this article, the relationship between the structural asymmetry of Fano resonance controlled by symmetry-protect BICs and the radiation <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> -factor is analyzed and demonstrated theoretically. At the same time, we use quasi-BIC resonances in asymmetric silicon nanoellipsoidal pairs of metasurfaces to achieve a huge Goos–Hänchen (GH) shift enhancement, which exceeds the third-order wavelength and a very large group index to achieve excellent slow-light effects. Our work is centered on the design of simulations for the BIC metasurface. Meanwhile, based on the simulation design results, we propose to advance the application of optical communication devices, BIC sensing, optical switches, and optical modulators.

Zero-GVD slow light of coupled topological edge states in a sandwiched photonic crystal waveguide

We propose a new scheme to realize topological photonic states with low group velocity (vg) and zero group velocity dispersion (zero-GVD) based on a sandwiched photonic crystal (PC) waveguide, which are composed of finite sized PCs with different topological phases. In our proposed sandwiched heterostructure, two coupled topological edge states (CTESs) can be found, one of which can be applied to slow light by modifying the radii of the dielectric rods. The slow light characteristics of CTES, including zero-GVD, large average group index and normalized delay-bandwidth product, are discussed in detail based on finite element method (FEM) simulation. Besides, the robustness of CTES with slow light is verified, when introducing random disorders. Time-domain simulation results demonstrate the dispersionless transport of CETS in zero-GVD region. Our findings pave a way of topological slow light, enrich the topological PC research, and have new application in optical buffers and optical delay lines.

Valley-Dependent Topological Photonic Crystals With Heterogeneous Bearded Interfaces for SOI-Based Integrated Optical Devices

Topologically protected photonic edge states allow backscattering-free and disorder-immune light propagations, offering a new scheme for the development of on-chip information systems. Valley degree of freedom is one of the most reliable way to achieve topologically non-trivial states. Here, we take advantage of heterogeneous topological interfaces to construct integrated optical devices in the valley-dependent photonic crystals. Localized Berry curvature around the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K′</i> point guarantees the existence of valley kink states at telecom wavelengths below the light cone, ensuring both optical robustness and light confinement. We harness the combination of two kinds of bearded interfaces to realize wavelength-selective topological routing of light in the micro-ring resonators. The proposed bearded interfaces can support edge modes with large group indices up to 90, showing a great potential to obtain remarkable slow light effects. This work paves the way to implement optical buffers and reconfigurable photonic devices with the adoption of photonic valley pseudospin.

Topological Cavity Based on Slow-Light Topological Edge Mode for Broadband Purcell Enhancement

Slow light in topological valley photonic crystal structures offers possibilities to enhance light-matter interaction. We report a topological cavity based on slow-light topological edge modes for broadband Purcell enhancement. The topological edge modes with large group indices over 100 can be realized with a bearded interface between two topologically distinct valley photonic crystals, featuring the greatly enhanced Purcell factor because of the increased local density of states. In the slow-light regime, the topological cavity supports much more cavity modes with a higher quality factor than that in the fast-light regime, which is both demonstrated theoretically and experimentally. We demonstrate the cavity enables the broadband Purcell enhancement together with substantial Purcell factor, benefiting from dense cavity modes with a high quality factor in a wide spectral range. It has great benefit for the realization of high-efficiency quantum-dot-based single-photon sources and entangled-photon sources with less restriction on spectral match. Such a topological cavity could serve as a significant building block toward the development of photonic integrated circuits with embedded quantum emitters.

Ultra-Compact Optical Switches Using Slow Light Bimodal Silicon Waveguides

Switches are essential components in several optical applications, in which reduced footprints are highly desirable for mass production of densely integrated circuits at low cost. However, most conventional solutions rely on making long switching structures, thus increasing the final device size. Here, we propose and experimentally demonstrate an ultra-compact 2x2 optical switch based on slow-light-enhanced bimodal interferometry in one-dimensional silicon photonic crystals. By properly designing the band structure, the device exhibits a large group index contrast between the fundamental even mode and a higher order odd mode for TE polarization. Thereby, highly dispersive and broadband bimodal regions for high-performance operation are engineered by exploiting the different symmetry of the modes. Two configurations are considered in the experiments to analyze the dimensions influence on the switching efficiency. As a result, a photonic switch based on a bimodal single-channel interferometer with a footprint of only 63 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , a power consumption of 19.5 mW and a crosstalk of 15 dB is demonstrated for thermo-optic tunability.

Enhanced tuneable rotatory power in a rotating plasma.

The gyrotropic properties of a rotating magnetized plasma are derived analytically. Mechanical rotation leads to a new cutoff for wave propagation along the magnetic field, and polarization rotation above this cutoff is the sum of the classical magneto-optical Faraday effect and the mechanico-optical polarization drag. Exploiting the very large effective group index near the cutoff, we expose here that polarization drag can be 10^{4} larger than Faraday rotation at GHz frequency. The rotation leads to weak absorption while allowing direct frequency control, demonstrating the unique potential of rotating plasmas for nonreciprocal elements. The very large rotation frequency of a dense non-neutral plasma could enable unprecedented gyrotropy in the THz regime.

A theoretical proposal for the tunable electromagnetically induced transparency with superior properties based on the solid-state plasma

Based on the solid-state plasma (SSP), a metamaterial analog of electromagnetically induced transparency (EIT-like) with superior properties is theoretically proposed, including tunability, a large group index, and delay-bandwidth product (DBP), polarization independence, and low-loss. By exciting different specific parts of the SSP, the EIT-like peak frequency switches from 8.518 GHz with 90.92% transmissivity in state I to 11.26 GHz with 93.76% transmissivity in state II, i.e. a 2.742 GHz shift. Meanwhile, the metamaterial has 0.679 ns or 1.314 ns group delay, 453 or 876.1 group index, and 0.422 or 1 DBP in state I or III. Additionally, the transmission curves are the same under the circular polarization waves or linear polarization waves at any angle. The low-loss feature is likewise demonstrated. Therefore, the EIT-like metamaterial has superior properties for related applications, especially in slow light, sensing, switching, and communication.

Slow light waveguides in topological valley photonic crystals.

Valley photonic crystals (VPhCs) are an attractive platform for the implementation of topologically protected optical waveguides in photonic integrated circuits (PICs). The realization of slow light modes in the topological waveguides may lead to further miniaturization and functionalization of the PICs. In this Letter, we report an approach to realize topological slow light waveguides in semiconductor-slab-based VPhCs. We show that a bearded interface of two topologically distinct VPhCs can support topological kink modes with large group indices over 100 within the topological bandgap. We numerically demonstrate robust light propagation in the topological slow light waveguide with large group indices of ∼60, even under the presence of sharp bends. Our work opens a novel route to implement topological slow light waveguides in a way compatible with current PIC technology.

Terahertz polarization-independent electromagnetically-induced transparency structures

Two simple polarization-independent electromagnetically-induced transparency (EIT) metamaterials are numerically and experimentally demonstrated at the terahertz region. The first structure is composed of two metal concentric rings on a substrate, while the second one is composed of one metal ring with a cross in it. The bright–bright coupling behavior appears in the two symmetric polarization-independent EIT structures while it is generally observed in asymmetrically structures. In addition, the large group index is extracted to verify the slow-light effect of the two EIT structures. These simple EIT structures may have potential applications in certain areas, including sensing, slow-light and filtering devices.

A novel metamaterial design for achieving a large group index via classical electromagnetically induced reflectance

In a numerical study, we propose a metamaterial structure mimicking electromagnetically induced reflectance at microwave frequencies. The structure contains imprinted metallic elements, including two long and two short wires on a dielectric substrate. In analyzing different loss mechanisms affecting the resonance behavior of the structure, we study effects of temperature and metallic thickness in detail, and see an enhanced resonance behavior as the temperature is reduced or the metallic thickness is increased. In applications, we first show that the proposed structure can be used as a sensitive microwave sensor. Furthermore, the group index of the structure is calculated to be 563.8 at 15.14 GHz, which can also be further increased by reducing temperature and increasing metallic thickness. Even this value is high compared to the values reported in the literature at microwave frequencies, and makes the structure a good candidate for slow light applications.

Electromagnetically Induced Transparency in Media with Rydberg Excitons 1: Slow Light.

In this paper, we show that Electromagnetically Induced Transparency (EIT) can be realized in mediums with Rydberg excitons. With realistic, reliable parameters which show good agreement with optical and electro-optical experiments, as well as the proper choice of Rydberg exciton states in the Cu2O crystal, we indicate how the EIT can be performed. The calculations show that, due to a large group index, one can expect the slowing down of a light pulse by a factor of about in this medium.

Coupling of a whispering gallery mode to a silicon chip with photonic crystal.

We demonstrate the efficient coupling (99.5%) of a silica whispering gallery mode microresonator directly with a silicon chip by using a silicon photonic crystal waveguide as a coupler. The efficient coupling is attributed to the small effective refractive index difference between the two devices. The large group index of the photonic crystal waveguide mode also contributes to the efficient coupling. A coupling Q of 2.68×106 is obtained, which allows us to achieve the critical coupling of a silica whispering gallery mode with an intrinsic Q of close to 107 with a Si chip.

Enhanced photon drag in closed-loop quantum systems

In recent years, photon drag has attracted enormous attention owing to both fundamental and practical interests. In this paper, we investigate dragging effect in quantum systems with closed-loop interaction. First, we demonstrate an enhanced photon drag in a moving atomic medium with a three-level Λ-type structure. By incorporating the interference of spontaneous emissions, the properties of the atomic medium can be easily controlled by the relative phase of applied fields so that a large group index along with a transparency or even a gain can be achieved. As the photon drag is proportional to the group index, the enhancement can be also found in the dragging of light. Such enhanced dragging effect can be applied for the efficient modulators of light, position control, and the detection of slow motion. We then investigate the Fresnel’s light drag effect in another closed-loop structure, i.e. chiral molecules with a cyclic three-level Δ-type configuration, and show that left- and right-handed chiral molecules exhibit different dragging effects. This provides a possible way to probe the molecular chirality, which may find applications in the sorting and separation of chiral molecules.

Demonstration of slow-light effect in silicon-wire waveguides combined with metamaterials.

We demonstrated a novel slow-light Si-wire waveguide combined with metamaterials, which can be easily integrated with other Si photonics devices. The slow-light effect can be produced simply by placing metamaterials at an appropriate position on a Si-wire waveguide. It was confirmed that the large group index of more than 40 could be obtained because of a steep and discontinuous change of dispersion relation near the resonance frequency of metamaterials.

Slow light performance enhancement of graphene-based photonic crystal waveguide

We propose a graphene-based photonic crystal (PC) slow light waveguide, which is realized by creating periodical air holes in a silicon layer to achieve spatially varying chemical potentials of graphene. The structure is optimized around 30 THz, and a large group index of 166.6 is obtained, with a very low propagation loss of −2.1 dB/um. The corresponding normalized delay-bandwidth product reaches as high as 4.00. Furthermore, the slow light performance can be dynamically tuned by changing a bias voltage. The center frequency of the slow light waveguide can be tuned between 19.1 THz and 27.4 THz. Our results suggest that graphene-based PC structures are very promising for slow light devices.