Slow Light Properties Research Articles

Independently tunable electromagnetically induced transparency effect and dispersion in a multi-band terahertz metamaterial

In this article, we experimentally and numerically investigate a planar terahertz metamaterial (MM) geometry capable of exhibiting independently tunable multi-band electromagnetically induced transparency effect (EIT). The MM structure exhibits multi-band EIT effect due to the strong near field coupling between the bright mode of the cut-wire (CW) and dark modes of pair of asymmetric double C resonators (DCRs). The configuration allows us to independently tune the transparency windows which is challenging task in multiband EIT effect. The independent modulation is achieved by displacing one DCR with respect to the CW, while keeping the other asymmetric DCR fixed. We further examine steep dispersive behavior of the transmission spectra within the transparency windows and analyze slow light properties. A coupled harmonic oscillator based theoretical model is employed to elucidate as well as understand the experimental and numerical observations. The study can be highly significant in the development of multi-band slow light devices, buffers and modulators.

Low loss slow light propagation in silicon slot waveguide.

Silicon slot waveguide Bragg gratings have been designed, fabricated and the experimental data has been analyzed for its slow light properties. Slow light with a group index of 12.38 at a wavelength near 1555 nm and having a low propagation loss of 5.1 dB/mm has been determined for internally corrugated slot waveguide Bragg gratings on a silicon-on-insulator platform. The combination of slow light and low propagation loss make the internally corrugated slot waveguide Bragg gratings especially attractive as a phase shifter section for low drive voltage, high speed and compact electro-optic modulators.

Electromagnetically induced transparency in an integrated metasurface based on bright–dark–bright mode coupling

Based on a bright–dark–bright coupling model, a classical analog of an integrated electromagnetically induced transparency (EIT) metasurface at a microwave frequency band is numerically and experimentally demonstrated. The proposed EIT metasurface structure consists of three perpendicularly connected metallic elements: two split-ring resonators (SRRs) which couple directly with normally incident wave supporting the bright modes, and the I-shaped configuration treated as the dark mode. Due to the dual coupling excitation of the middle I-shaped configuration, two anti-parallel electric dipoles were formed in the upper and lower SRRs, resulting in the destructive interference. In addition, the wide-angle incidence and the different polarization-angle incidence of the proposed metasurface were calculated and studied. The group index is extracted and calculated to verify the slow light property within the transparent window. The research in this work reveals that the scheme based on multi-mode coupling is of referable value for constructing EIT-like metamaterials or high transmission bandpass filters.

Dual plasmon-induced transparency and slow light effect in monolayer graphene structure with rectangular defects

A novel monolayer graphene structure whose unit cell possesses two rectangular defects is proposed. A very obvious dual plasmon-induced transparency (PIT) effect can be successfully achieved by the destructive interference in the terahertz region. The dual PIT effect can be easily tuned by changing the Fermi energy of the monolayer graphene. Since the graphene of our structure exists in a continuous form, we can simply apply a bias voltage to achieve the tuning performance compared to those structures with discontinuous graphene patterns. We have deduced the expression of the theoretical transmittance and the numerical simulation results are very consistent with the theoretical data. Moreover, we find that this structure has a good slow light property and its group refractive index is as high as 545. Thus, the proposed structure and findings may provide a good guidance for the highly tunable optoelectronic devices and excellent slow light device.

Mid-Infrared Slow Light Engineering and Tuning in 1-D Grating Waveguide

The design, fabrication, and characterization of 1-D grating waveguide as slow light structure working in mid-infrared (MIR) region are demonstrated for the first time. The effects of various structural parameters on slow light properties are investigated through theoretical analysis, simulation and experimental verification, providing guidance on slow light engineering in 1-D grating waveguide. By adjusting structural parameters, average group indices of 9.4–15.5 with bandwidths of 23–66 nm and normalized delay-bandwidth products of 0.093–0.164 are obtained. Thermo-optic tuning is also demonstrated with π phase shift and group index tuning from 23 to 33 at the wavelength of 3.9024 μm by applying 1.4 mA current. The proposed tunable 1-D grating slow light waveguide provides a promising platform for various MIR applications such as on-chip absorption-based biochemical sensors, tunable optical buffers, and thermo-optic modulators.

Ultra-wideband slow light transmission with high normalized delay bandwidth product in W3 photonic crystal waveguide

By separating two photonic crystals with a distance of 23a, where a is the lattice constant, shifting the positions of some discriminatory air holes in the upper and lower edges, and inserting a defect central row, a specific waveguide is set up, W3 photonic crystal waveguide (W3 PCW). An equivalent transmission line model is given, giving the theoretical basis of for the proposed waveguide to have a wider transmission bandwidth and thus higher normalized delay bandwidth product (NDBP). Slow-light properties and performance are studied by using plane-wave expansion and finite-difference time-domain methods. We obtained ultra-wideband of 65.70 nm with low group-velocity dispersion and at the same time a high NDBP of 0.496 through optimization which is a worthy indicator for slow light device performance. Wider bandwidth can allow shorter duration of pulse waves, causing higher field intensity and stronger nonlinear effect.

Investigation of tunable plasmon-induced transparency and slow-light effect based on graphene bands

We have constructed a graphene-based planar metamaterial structure. This structure can achieve a clear plasmon-induced transparency phenomenon via destructive interference. We further find that this structure has an excellent tuning function. The tuning function can be realized only by adjusting the Fermi level of graphene, and the structure is a simple and continuous design. In addition, we find that the structure has excellent slow-light properties through an analysis of the dispersion relation, and the slow-light coefficient can approach 439. This lays a solid theoretical foundation for the construction and design of tunable devices and slow-light devices.

Slow light in a hyperbolic metamaterial waveguide cladded with arbitrary nonlinear dielectric materials

We have demonstrated a hyperbolic metamaterial (HMM) waveguide cladded with arbitrary nonlinear dielectric materials to explore the properties of slow light. The HMM proposed in this paper is assumed as a metal–dielectric stack which can lead to slow light phenomena while being cladded by dielectric materials. Both the asymmetric (i.e., linear–HMM–linear case and nonlinear–HMM–linear case) and symmetric (i.e., linear–HMM–linear case and nonlinear–HMM–nonlinear case) dielectric structures are considered to examine the properties of slow light in the paper. The dispersion relations which are required to investigate the properties of slow light are derived and presented in detail. The results show that the metal filling factor, the dielectric permittivities, and the arbitrary nonlinearity have significantly changed the characteristics of slow light. Parameter dependence of the effects is calculated and discussed.

Slow-light transmission with high group index and large normalized delay bandwidth product through successive defect rods on intrinsic photonic crystal waveguide

We proposed a strategy with successive cavities as energy reservoirs of electromagnetic energy and light-speed reducers introduced in the first and second rows of rods on the walls of an intrinsic photonic crystal waveguide (PCW) for slow-light transmission in the PCW concerning applications for optical communication, optical computation and optical signal processing. Subsequently, plane-wave expansion method (PWE) is used for studying slow-light properties and finite-difference time-domain (FDTD) method to demonstrate the slow-light propagating property of our proposed structure. We obtained group index as exceedingly large as 6123 with normalized delay bandwidth product (NDBP) as high as 0.48. We designed a facile but more generalized structure that may provide a vital theoretical basis for further enhancing the storage capacity properties of slow light with wideband and high NDBP.

Polarization-independent electromagnetically induced transparency-like metasurface

A classical electromagnetically induced transparency-like (EIT-like) metasurface is numerically simulated. This metasurface is composed of two identical and orthogonal double-end semitoroidals (DESTs) metal resonators. Under the excitation of the normal incidence waves, each of the two DESTs structure exhibits electromagnetic dipole responses at different frequencies, which leads to the polarization-independent EIT-like effect. The features of the EIT-like effect are qualitatively analyzed based on the surface current and magnetic field distribution. In addition, the large index is extracted to verify the slow-light property within the transmission window. The EIT-like metasurface structure with the above-mentioned characteristics may have potential applications in some areas, such as sensing, slow light, and filtering devices.

Dual plasmonically tunable slow light based on plasmon-induced transparency in planar graphene ribbon metamaterials.

We propose a simulated terahertz design based on planar graphene ribbons. With numerical simulation, we can achieve a very obvious dual plasmon-induced transparency phenomenon through the destructive interference in this structure. Moreover, due to the simple design of this structure and the complete continuous graphene ribbons, the Fermi level of graphene can be regulated by voltage. Thus, the dual plasmon-induced transparency phenomenon can be easily tuned in the numerical simulation. Further structural analysis shows that the two graphene chips on the side of the graphene ribbons play a crucial role in the dual plasmon-induced transparency phenomenon. As the length of the two chips is close, the dual plasmon-induced transparency phenomenon gradually becomes a single plasmon-induced transparency phenomenon. The theoretical analysis of this structure shows that this system has a very high group index, and its maximum value is 800, which is far greater than that of other types of slow light devices. This work may open up a new way for designing tunable terahertz graphene-based devices and slow light devices.

Analogue of ultra-broadband and polarization-independent electromagnetically induced transparency using planar metamaterial

In this paper, a classical analogue of electromagnetically induced transparency (EIT) metamaterial is numerically and experimentally demonstrated. The unit cell of our proposed structure is composed of two identical and orthogonal double-end fork (DEF) metallic resonators. Under the excitation of the normally incident waves, each of the two DEFs exhibits different frequency of electric dipole response, which leads to the ultra-broadband and polarization-independent EIT-like effect. The resonant feature of the EIT-like effect has been qualitatively analyzed from the surface current distributions and quantitatively by the “two-oscillator” coupling model. In addition, the large group index is extracted to verify the slow light property within the transmission window. The EIT metamaterial structure with the above-mentioned characteristics may have potential applications in some areas, such as sensing, slow light, and filtering devices.

Plasmon-Induced Transparency and High-Performance Slow Light in a Plasmonic Single-Mode and Two-Mode Resonators Coupled System

We investigate the plasmon-induced transparency (PIT) and slow-light properties in a plasmonic single-mode and two-mode resonators coupled system. The bare-state PIT effect originates from the destructive interference between a radiant standing-wave mode in the slot cavity and two subradiant travelling-wave modes in the ring resonator. A theoretical model is established to describe this PIT system, and the analytical results agree well with the numerical simulations. The difference between our structure and similar coupled resonators structures is shown. The transparency window can be tuned by adjusting the structural parameters, and two types of PIT lineshape are discussed in detail. The numerical results reveal that the PIT window with weakly resolved and equal doublet peaks exhibits high-performance slow light. A delay-bandwidth product larger than 1 and ultralow dispersion are obtained, and a delay of 1 bit with negligible pulse distortion is achieved in the proposed system.

Experimental Investigation of Top Cladding on Properties of Silicon Slotted Photonic Crystal Waveguides

In this paper, we present an experimental study, validated by two different approaches, of the influence of the top cladding refractive index on the transmission and slow light properties of dispersion engineered silicon slot photonic crystal waveguides. We demonstrate that even though the operation wavelength is very sensitive to refractive index changes, the structures remain, nevertheless, robust in terms of group index, group velocity dispersion properties, and bandwidth figures of merit.

Subwavelength Micro-Antenna for Achieving Slow Light at Microwave Wavelengths via Electromagnetically Induced Transparency in 2D Metamaterials

In this paper, a tunable slow light 2D metamaterial is presented and investigated. The metamaterial unit cell is composed of three metallic strips as radiative and non-radiative modes. Once introducing asymmetry, a transparency window induced by coupling between the dark and bright modes is observed. The transmission characteristics and the slow light properties of the metamaterial are verified by numerical simulation, which is in a good agreement with theoretical predictions. The impact of asymmetric parameter on transparency window is also investigated. Simulation results show the spectral properties and the group index of the proposed 2D metamaterial can be tunned by adjusting asymmetric structure parameter, temperature and also the metal used in the metamaterial. Furthermore, the electromagnetic field distributions, excited surface currents, induced electric dipole and quadruples, and slow light properties of the metamaterial are investigated in details as well as transmission spectral responses. The outstanding result is that, the 2D-metamaterial is in a high decrease of the group velocity and therefore slow light applications, because in the best state, the group velocity in our structure decreases by a factor of 221 at T=100 K using copper as metal in optimization asymmetric case.

Slow light in dual-periodic photonic crystals based slotted-waveguide coupled cavity

Considering the capacity of the nanoscale width area with the low-refractive index can confine light waves, the dual-periodic slotted photonic crystals, which is constructed by coupling low-refractive index's slotted-waveguide with high-refractive index's cavity is proposed in this paper. The best slow light properties and the optimal slotted-waveguide coupled cavity are achieved by adjusting the slotted-width and the period of cavity respectively. In this structure, the slow-light properties are simulated by Plane Wave Expansion (PWE), the result reveals that the group velocities are all three orders of magnitude smaller than the speed of light in vacuum, the slowest value is 7.96×10−4c when the slotted-width is 0.54a and the period of cavity is 0.95a. Moreover, the corresponding Normalized Delay-Bandwidth Product (NDBP) values are larger than 0.24. Besides, the slotted-waveguide coupled cavity can be reconfigured, which accordingly changes the corresponding slow-light property. At last, the numerical results provide a new thought and method for decreasing group velocity and potential application for optical buffer in photonic crystals field.

Tunable slow light in 1-D photonic crystal

We study the effect of different defect layer refractive indices and thicknesses on group velocity and dispersion properties of slow light in one-dimensional photonic crystal. It is found that the increase of the refractive index, number of the defects and defect layer thickness will induce the decrease of group velocity. We take advantage of these variations to introduce a novel technique to tune and control the slowing light in photonic crystals.

Photonic mesophases from cut rod rotators

The photonic band properties of random rotator mesophases are calculated using supercell methods applied to cut rods on a hexagonal lattice. Inspired by the thermodynamic mesophase for anisotropic building blocks, we vary the shape factor of cut fraction for the randomly oriented basis. We find large, stable bandgaps with high gap isotropy in the inverted and direct structures as a function of cut fraction, dielectric contrast, and filling fraction. Bandgap sizes up to 34.5% are maximized at high dielectric contrast for rods separated in a matrix. The bandgaps open at dielectric contrasts as low as 2.0 for the transverse magnetic polarization and 2.25 for the transverse electric polarization. Additionally, the type of scattering that promotes the bandgap is correlated with the effect of disorder on bandgap size. Slow light properties are investigated in waveguide geometry and slowdown factors up to 5 × 104 are found.

Slow light in reconfigurable two-dimensional nested ferrite magnetic fluid photonic crystal coupled-cavity waveguides

Photonic crystal waveguide (PCW), due to its large bandwidth, small size, and diversified design of the waveguide structures, represents a breakthrough in the field of optical buffer. Here, a nested structure is designed to improve delay-bandwidth product with ferrite magnetic fluid infiltrated photonic crystal coupled-cavity waveguides. Specifically, three types of coupled cavities, i.e., horizontal-, vertical-, and cross-defect cavity are studied with different concentrations of magnetic fluid. Overall, this aspect of the work is interesting from the perspective of coupled defect waveguides design, which is different from previous line-defect waveguides. Plane wave expansion simulations on the slow light property of the three PCWs reveal that the group velocities are all three orders of magnitude smaller than the speed of light in vacuum and that the corresponding normalized delay-bandwidth product (NDBP) values are all >0.35, with 0.41 being the highest value achieved in vertical-defect coupled cavity. Interestingly, the quality factor (Q), normalized bandwidth (Δω), and NDBP can be readily tuned by varying the concentrations of magnetic fluid. Moreover, all the three coupled-cavity waveguides can be reconfigured, which accordingly changes the corresponding slow light property. Compared to previous works, the results of this work have further improved the slow light property in slow light devices and realized the tunable slow light.

Tunable Slow Light with Large Bandwidth and Low-dispersion in Photonic Crystal Waveguide Infiltrated with Magnetic Fluids

Two kinds of magnetic fluids with different volume fractions are symmetrically filled into the W0.9 photonic crystal waveguide structure. The 2D plane-wave expansion method is used to investigate the slow light properties numerically. The constant group index criterion is employed to evaluate the slow light performance. The wavelength bandwidth <TEX>${\Delta}{\lambda}$</TEX> centering at <TEX>${\lambda}_0=1550nm$</TEX> varies from 32.4 to 44.2 nm when the magnetic field factor <TEX>${\alpha}_{\parallel}$</TEX> changes from 0 to 1. And the corresponding normalized delay bandwidth product can be tuned from 0.221 to 0.258. For comparison and optimization, two infiltration cases are investigated and the more advantageous infiltration scheme is found.