Large Group Velocity Research Articles

Analysis and design of hybrid ARROW-B plasmonic waveguides

A hybrid antiresonant reflecting waveguide, type B (ARROW-B) plasmonic waveguide based on the resonant coupling between a guided dielectric mode and surface plasmon polariton wave is proposed. Employing the finite element method, hybrid modes including two bound supermodes are obtained at visible frequencies by varying the environmental refractive index. We investigate the propagation characteristics of hybrid modes, where the significant change of modal power by the symmetric bound mode is observed in plasmonic waveguide coupling suitable for highly sensitive detection of bulk refractive index change. Further, anomalous dispersion is shown by the antisymmetric bound mode which leads to large group velocity dispersion of -3.165×10(4) ps/km nm and, thus, makes this hybrid plasmonic waveguide ideal for observation of soliton generation.

Dispersion relation, propagation length and mode conversion of surface plasmon polaritons in silver double-nanowire systems

We study the surface plasmon modes in a silver double-nanowire system by employing the eigenmode analysis approach based on the finite element method. Calculated dispersion relations, surface charge distributions, field patterns and propagation lengths of ten lowest energy plasmon modes in the system are presented. These ten modes are categorized into three groups because they are found to originate from the monopole-monopole, dipole-dipole and quadrupole-quadrupole hybridizations between the two wires, respectively. Interestingly, in addition to the well studied gap mode (mode 1), the other mode from group 1 which is a symmetrically coupled charge mode (mode 2) is found to have a larger group velocity and a longer propagation length than mode 1, suggesting mode 2 to be another potential signal transporter for plasmonic circuits. Scenarios to efficiently excite (inject) group 1 modes in the two-wire system and also to convert mode 2 (mode 1) to mode 1 (mode 2) are demonstrated by numerical simulations.

Large normal group velocity dispersion of micro/nano optical fiber near 2-μm wavelength

Based on the exact solution of the Maxwell's equations, the group velocity dispersion characteristics of micro/nano optical fibers near 2-μm wavelength have been studied numerically. The results show that the micro/nano optical fiber has large normal dispersion near 2-μm wavelength by carefully choosing the core size. For air-clad micro/nano optical fiber with core size 900 nm, the group velocity dispersion can be up to 3318.78 ps 2 ∕km. By introducing a thin dielectric layer, the maximum dispersion can be adjusted by varying the thickness and refractive index of the dielectric layer. © 2013 Society of Photo-Optical

Using abrupt changes in magnetic susceptibility within type-II superconductors to explore global decoherence phenomena

A phenomenon of a periodic staircase of macroscopic jumps in the admitted magnetic field has been observed, as the magnitude of an externally applied magnetic field is smoothly increased or decreased upon a superconducting (SC) loop of type-II niobium–titanium wire that is coated with a non-SC layer of copper. Large temperature spikes were observed to occur simultaneously with the jumps, suggesting brief transitions to the normal state, caused by en masse motions of Abrikosov vortices. An experiment that exploits this phenomenon to explore the global decoherence of a large SC system will be discussed, and preliminary data will be presented. Although further experimentation is required to determine the actual decoherence rate across the SC system, multiple classical processes are ruled out, suggesting that jumps in magnetic flux are fully quantum mechanical processes that may correspond to large group velocities within the global Cooper pair wavefunction.

Silicon nanomembrane based photonic crystal waveguide array for wavelength-tunable true-time-delay lines

We demonstrate a four-channel on-chip true-time-delay module based on a photonic crystal waveguide array. Using the photonic crystal taper to minimize the coupling loss, the delay lines with 1–3 mm long photonic crystal waveguides can operate up to a group index ng ∼ 23 without significant loss. The large group velocity dispersion enables continuous and wavelength-tunable time delays. Measurements show a highly linear phase-frequency relation, highest time delay up to 216.7 ps, and large tuning ranges of 58.28 ps, 115.74 ps, and 194.16 ps for 1–3 mm delay lines. The chip-scale true-time-delay module occupies only 0.18 mm2 area and can provide ±44.38° steering for an X-band phased-array-antenna.

On the high performance of channel photonic crystal waveguide comprising different plasmonic active metals

We present a detailed design principle and the propagation characteristics of a channel photonic-crystal-surface-plasmon waveguide (PCSPW), comprised of silica- photonic crystal waveguide (PCW) and plasmonic waveguide based on different plasmonic active metals such as gold (Au), silver (Ag) and aluminium (Al). We found that the phase-matching/resonance wavelength can be substantially tuned over a broad spectral range by judiciously choosing different plasmonic metals. The Al-based channel PCSPW has been found to exhibit ultra large group-velocity dispersion (∼±2.2×105 ps/km-nm), an extremely narrow bandwidth (4.5 nm) and long propagation lengths (∼400 % longer) as compared to Au-based channel PCSPW. The oxidation problem of Al has been addressed by using a bimetallic combination of Al+Au. Also, such a waveguide can be used as a sensor with a sensitivity as high as ∼5000 nm/RIU, thereby opening a new window for lab on chip.

Magnon Lifetimes on the Fe(110) Surface: The Role of Spin-Orbit Coupling

We provide direct experimental evidence which demonstrates that, in the presence of a large spin-orbit coupling, the lifetime, amplitude, group, and phase velocity of the magnons propagating along two opposite (but crystallographically equivalent) directions perpendicular to the magnetization are different. A real time and space representation reveals that magnons with the same energy (eigenfrequency) propagate differently along two opposite directions. Our findings can inspire ideas for designing new spintronic devices.

Dual-frequency division de-multiplexer based on cascaded photonic crystal waveguides

A dual-frequency division de-multiplexing mechanism is demonstrated using cascaded photonic crystal waveguides with unequal waveguide widths. The de-multiplexing mechanism is based on the frequency shift of the waveguide bands for the unequal widths of the photonic crystal waveguides. The modulation in the waveguide bands is used for providing frequency selectivity to the system. The slow light regime of the waveguide bands is utilized for extracting the desired frequency bands from a wider photonic crystal waveguide that has a relatively larger group velocity than the main waveguide for the de-multiplexed frequencies. In other words, the wider spatial distribution of the electric fields in the transverse direction of the waveguide for slow light modes is utilized in order to achieve the dropping of the modes to the output channels. The spectral and spatial de-multiplexing features are numerically verified. It can be stated that the presented mechanism can be used to de-multiplex more than two frequency intervals by cascading new photonic crystal waveguides with properly selected widths.

Modeling the Dispersion of the Nonlinearity in Slow Mode Photonic Crystal Waveguides

Nanophotonic structures enabling the engineering of linear dispersion are often exploited for nonlinear processing. However, the dispersion of the nonlinear response has been generally overlooked, also because of the difficulty of accurately taking this effect into account. Here, we first show the necessity of considering the nonlinear dispersion and then propose a simplified approach through which the modeling can be performed. As an example, we consider waveguides in which a large group index and low group velocity dispersion are achieved at the same time by modifying a single design parameter, i.e., an asymmetric translation of the hole rows closest to the core. It is shown that the dependence on the wavelength of the normalized Bloch mode overlap integrals for self-phase modulation (SPM) can be interpolated by a four-parameter formula that is similar to the Morse potential. The parameters can be related to specific properties of the nonlinear SPM coefficient, which in the specific case depends on the translation parameter, by simple linear and exponential functions. Cross-phase modulation and four-wave mixing coefficients can then be derived from the self-phase one, through a geometric mean, slightly corrected to account for the interacting wave detuning, and the complete modeling of nonlinear dispersion is achieved.

Optical modulation effects on nonlinear electron transport in graphene in terahertz frequency range

We describe very fast electron dynamics for a graphene nanoribbon driven by a control electromagnetic field in the terahertz frequency regime. The mobility as a function of bias field has been found to possess a large threshold value when entering a nonlinear transport regime. This value depends on the lattice temperature, electron density, impurity scattering strength, nanoribbon width and correlation length for the line-edge roughness. An enhanced electron mobility beyond this threshold has been observed, which is related to the initially-heated electrons in high energy states with a larger group velocity. However, this mobility enhancement quickly reaches a maximum governed by the Fermi velocity in graphene and the dramatically increased phonon scattering. Super-linear and sub-linear temperature dependences of the mobility are seen in the linear and nonlinear transport regimes, which is attributed separately to the results of sweeping electrons from the right Fermi edge to the left one through elastic scattering and moving electrons from low-energy states to high-energy ones through field-induced electron heating. The threshold field is pushed up by a decreased correlation length in the high field regime, and is further accompanied by a reduced magnitude in the mobility enhancement. This implies an anomalous high-field increase of the line-edge roughness scattering with decreasing correlation length due to the occupation of high-energy states by field-induced electron heating. Additionally, a self-consistent device modeling has been proposed for graphene transistors under an optical modulation on its gate, which employs Boltzmann moment equations up to the third-order for describing fast carrier dynamics and full wave electromagnetics coupled to the Boltzmann equation for describing spatial-temporal dependence of the total field. Finally, a detailed comparison of the derived Maxwell–Boltzmann moment equations in this paper with the well known Vlasov–Maxwell equations is also included.

Wideband slow light with ultralow dispersion in a W1 photonic crystal waveguide

A dispersion tailoring scheme for obtaining slow light in a silicon-on-insulator W1-type photonic crystal waveguide, novel to our knowledge, is proposed in this paper. It is shown that, by simply shifting the first two rows of air holes adjacent to the waveguide to specific directions, slow light with large group-index, wideband, and low group-velocity dispersion can be realized. Defining a criterion of restricting the group-index variation within a ±0.8% range as a flattened region, we obtain the ultraflat slow light with bandwidths over 5.0, 4.0, 2.5, and 1.0 nm when keeping the group index at 38.0, 48.8, 65.2, and 100.4, respectively. Numerical simulations are performed utilizing the three-dimensional (3D) plane-wave expansion method and the 3D finite-difference time-domain method.

Soliton interaction in birefringent optical fibers: Effects of nonlinear gain devices

This paper presents the influences of polarization mode dispersion (PMD) on the performance of soliton transmission system in birefringent fibers. Dispersive waves generated in single mode fibers due to PMD degrade the soliton transmission system in two aspects. First, solitons continuously lose their energy, thus cause enhancement in pulse width. Second, the dispersive waves interact with neighboring pulses and cause distortion in a sequence of pulses. Both these effects reduce the effective bit-rate and degrade the performance of high-speed optical transmission systems. Optical fibers with large group velocity dispersion (GVD) have less dispersive waves and are relatively robust to pulse broadening, but it enhances the interaction between the adjacent pulses. In this paper, we analyzed these effects of PMD on soliton propagation in birefringent fibers and introduced nonlinear gain devices with perturbation terms proportional to second and fourth power of amplitudes to reduce these effects. We proposed Symmetric Split-Step Fourier Method to solve the coupled nonlinear Schrödinger equations (CNLSE); which yields better results over the existing Split-Step Fourier Method.

Field-enhanced electron mobility by nonlinear phonon scattering of Dirac electrons in semiconducting graphene nanoribbons

The calculated electron mobility for a graphene nanoribbon as a function of applied electric field has been found to have a large threshold field for entering a nonlinear transport regime. This field depends on the lattice temperature, electron density, impurity scattering strength, nanoribbon width and correlation length for the line-edge roughness. An enhanced electron mobility beyond this threshold has been observed, which is related to the initially-heated electrons in high energy states with a larger group velocity. However, this mobility enhancement quickly reaches a maximum due to the Fermi velocity in graphene and the dramatically increased phonon scattering. Super-linear and sub-linear temperature dependence of mobility seen in the linear and nonlinear transport regimes. By analyzing the calculated non-equilibrium electron distribution function, this difference is attributed separately to the results of sweeping electrons from the right Fermi edge to the left one through the elastic scattering and moving electrons from low-energy states to high-energy ones through field-induced electron heating. The threshold field is pushed up by a decreased correlation length in the high field regime, and is further accompanied by a reduced magnitude in the mobility enhancement. This implies an anomalous high-field increase of the line-edge roughness scattering with decreasing correlation length due to the occupation of high-energy states by field-induced electron heating.

Material loss influence on the complex band structure and group velocity in phononic crystals

The influence of material loss on the complex band structure of two-dimensional phononic crystals is investigated. A viscoelasticity model is added to the extended plane-wave expansion (EPWE) method, with viscosity proportional to the frequency. It is found that losses have a stronger influence on the real than on the imaginary part of Bloch waves, in contrast with propagation in homogeneous media. Flat bands, i.e., bands initially showing low group velocity without losses, acquire an enhanced damping as compared to bands with larger group velocities. Losses are also found to limit the appearance of large group slownesses, or conversely small group velocities.

Detailed structure of electromagnetic pulses passing through one-dimensional photonic crystal

The electromagnetic wave propagation in one-dimensional photonic cyrstals is studied by a finite-difference time domain method. We consider a conventional perfect photonic crystal structures consists of identical alternating layers of high and low refractive indices. When the central frequency is close to the edge of the photonic band gap, the electromagnetic pulse undergoes a large group-velocity dispersion. By using transfer matrix method we examined the transmittance spectrum of terahertz wave filter based on one-dimensional photonic crystals with line defect. We show that is identical structure can be sandwiched between different substrate. Simulation results demonstrate that the terahertz wave filter can achieve a narrow transmission band and high transmission centered at λ0, having a flat passband at the central frequency, and sharp edges.

Defect assisted coupled resonator optical waveguide: Weak perturbations

Coupled resonator optical waveguides (CROW) comprised of only a few resonators including one defective element are investigated analytically and numerically. The defect is introduced by perturbing weakly the resonator at the center of the chain, creating a ‘cavity’ in what is an analogue of a Fabry–Pérot resonator or a defect-assisted photonic crystal structure. The device is found to have a very narrow resonance within the original stopband of the unperturbed CROW, and an accompanying extremely large group velocity excursion, that extends over both the subluminal and the superluminal ranges. The properties of the resonance and the magnitude of the group velocity dispersion are found to be conveniently controllable by coupling strength and by the number of resonators used. The perturbation can be caused thermo-optically or electro-optically. The complex effects of waveguide loss are extensively discussed.

Three dimensional character of whistler turbulence

It is shown that the dominant nonlinear effect makes the evolution of whistler turbulence essentially three dimensional in character. Induced nonlinear scattering due to slow density perturbation resulting from ponderomotive force triggers energy flux toward lower frequency. Anisotropic wave vector spectrum is generated by large angle scatterings from thermal plasma particles, in which the wave propagation angle is substantially altered but the frequency spectrum changes a little. As a consequence, the wave vector spectrum does not indicate the trajectory of the energy flux. There can be conversion of quasielectrostatic waves into electromagnetic waves with large group velocity, enabling convection of energy away from the region. We use a two-dimensional electromagnetic particle-in-cell model with the ambient magnetic field out of the simulation plane to generate the essential three-dimensional nonlinear effects.

Ultrafast and Precise Interrogation of Fiber Bragg Grating Sensor Based on Wavelength-to-Time Mapping Incorporating Higher Order Dispersion

An interrogation scheme based on wavelength-to-time mapping to achieve ultrafast, high-precision, and large dynamic range interrogation of fiber Bragg grating (FBG) sensors is proposed and experimentally demonstrated. The wavelength-to-time mapping, also called temporal self-imaging effect, is realized in the optical domain, using a dispersive element that has a large group velocity dispersion. For a practical dispersive element, higher order dispersions exist, which makes the wavelength-to-time mapping nonlinear. Thus, an interrogation system based on wavelength-to-time mapping without considering the high-order dispersion would reduce the interrogation accuracy. In this paper, for the first time to the best of our knowledge, a mathematical model that incorporates higher order dispersion to achieve an accurate wavelength-to-time mapping is developed, which is then verified by a numerical simulation. An FBG-based strain sensor interrogated based on the developed wavelength-to-time mapping scheme is experimentally investigated. The system has a sampling speed of 48.6 MHz, a dynamic range as large as 20 nm, and a sensing accuracy as high as 0.87 ?? for a single-shot measurement.

Enhanced CRLH Transmission Line Performances Using a Lattice Network Unit Cell

A novel topology for the composite right/left-handed transmission line (CRLH TL) is presented. This type of CRLH TLs has the advantage of exhibiting an all-pass behavior and a frequency-independent characteristic impedance, in the so-called balanced case. Moreover, it presents a larger group velocity than its conventional counterpart, thereby allowing for wider bandwidth operation. This letter investigates the fundamental properties of this structure, and demonstrates them by full-wave simulation and measurement results on a practical implementation in parallel stripline technology.

Study of a slab waveguide loaded with dispersive anisotropic metamaterials

We have numerically studied the characteristics of a slab waveguide loaded with dispersive anisotropic metamaterials. For two typical-structured metamaterials, we have analyzed the tensor expressions of frequency-dependent permittivity and permeability and discussed influences of the resonant cell structures on waveguiding characteristics. Numerical results show that the slab waveguide loaded with anisotropic metamaterials may, to some extent, behave as the slab waveguides of isotropic left-handed metamaterials (LHMs) core and conventional right-handed materials core. To one’s interest, some unusual properties are revealed in this paper, such as the coexistence of the fundamental guided mode and negative group velocity, and extraordinarily large normal or anomalous group velocity dispersion. Our results may find some potential applications in both linear and nonlinear optical technology.