Negative Group Velocity Research Articles

Gate-tunable polariton superlens in 2D/3D heterostructures

Polaritons in polar-dielectrics and van der Waals (vdW) materials provide a channel for strong photon confinement. Precise control of their propagation could lead to deep sub-wavelength photonic devices. Here, we report negative refraction of hybrid surface phonon-hyperbolic polaritons (SPh-HP) at the interface of two-dimensional (2D) van der Waals layers such as hexagonal boron nitride (h-BN) and 3D semiconductors such as germanium and silicon carbide. These hybrid polariton modes have naturally negative group velocity arising from the intrinsic Type-I hyperbolicity of h-BN resulting in negative refraction at interfaces with positive group velocity. Using this phenomenon, we demonstrate an in-plane superlensing effect in an ultrathin (~10 nm) slab with spatial confinement of long Infrared wavelengths to below 200 nm focal spots. We further demonstrate electrical tunability of the superlens by controlling the Fermi level of graphene, thereby offering potential for miniaturized infrared to THz modulators, photodetectors as well as logic switches.

Interplay between Raman self-frequency shift and cross-phase modulation in the vector-soliton of a birefringent fiber

Competition between cross-phase modulation (XPM) interaction and Raman-induced red-frequency shift is observed in the orthogonally polarized components (in the form of optical pulses) of a vector-soliton propagating in the fundamental mode of a birefringent fiber exhibiting negative third-order dispersion and group velocity birefringence. Both pulses experience opposite frequency shifts due to XPM, which reinforces the Raman self-frequency shift of one pulse while it hinders the self-frequency shift of the other. A large redshift in one of the pulses of the vector-soliton suggests transfer of a greater part of its energy to the dispersive radiation, leading to an asymmetry in the longitudinal evolution of the amplitudes as well as the frequencies of both the pulses of the vector-soliton. To support our analysis, an analytical approach is developed in order to understand the interplay between the Raman effect and XPM. The findings of the analytical approach are in good qualitative agreement with the numerical results, which is consistent with the results of Phys. Rev. E70, 016615 (2004)PLEEE81063-651X10.1103/PhysRevE.70.016615. The derived analytical relations accommodate additional nonlinear interactions originating from the XPM, as well as mutual Raman interaction, which makes our approach much superior to that used in Phys. Rev. E70, 016615 (2004)PLEEE81063-651X10.1103/PhysRevE.70.016615. It is also possible to understand the interaction between two frequency-shifted solitons in a true single-mode fiber or other waveguide system exhibiting negative third-order dispersion by a slight modification of our theoretical approach.

Abnormal wave propagation behaviors in two-dimensional mass–spring structures with nonlocal effect

This paper considers the propagation of elastic waves in periodic two-dimensional mass–spring structures with diagonal springs. The second-neighbor interactions in non-diagonal directions are included to account for the nonlocal effect. The influences of the spring stiffness in the diagonal directions and the nonlocal effect on the propagation characteristics of elastic waves are then scrutinized. Through the dispersion relation curve and the equi-frequency contours, it is seen that when the diagonal spring stiffness increases, the slope of the second curve in the –M direction will not always be positive, meaning that the negative group velocity occurs. Therefore, an incident wavevector with a chosen angle to the negative group velocity can lead to the negative refraction phenomenon in the two-dimensional mass–spring structure. Another interesting phenomenon called directional radiation of elastic waves can also be achieved by adjusting the nonlocal effect. Within a certain range, the stronger the nonlocal effect in a specific direction is, the more obviously the elastic waves propagate along this direction. In this paper, we theoretically analyze and numerically simulate the phenomena of negative refraction and directional wave propagation by choosing a proper set of parameters of the two-dimensional mass–spring structure.

Longitudinal and transverse elastic waves in 1D granular materials modeled as micromorphic continua

In this paper, the granular micromechanics approach proposed by Misra and Poorsolhjouy (2016) is used to study the dispersive behavior of granular materials in response to elastic deformation waves. This study is motivated by the typical lack of connection between the mathematical models, the parameters involved, and the physics of granular materials. Therefore, extensive parametric studies are carried out in order to understand how each intergranular stiffness coefficient contributes to the dispersive behavior of the material. Two cases of one dimensional wave propagation problems have been investigated. Case 1 focuses upon longitudinal wave propagation in a one dimensional continuum, while case 2 considers transverse wave propagation in a one dimensional continuum that has a two-dimensional micro-structure. Results predict the emergence of frequency band gaps and negative group velocities for certain values of the parameters involved. Such phenomena can be produced by starting from the micro-structure and producing materials for which the inter-granular stiffness parameters are the ones the granular micromechanics approach predict. This, however, is not a one to one mapping, and therefore, sets of solutions to achieve a particular behavior might exist. Therefore, granular micromechanics provides a systematic material design process, eliminating ad-hoc processes and potentially leading to large data libraries.

Type-I hyperbolic metasurfaces for highly-squeezed designer polaritons with negative group velocity

Hyperbolic polaritons in van der Waals materials and metamaterial heterostructures provide unprecedented control over light-matter interaction at extreme nanoscales. Here we propose a concept of type-I hyperbolic metasurface supporting highly-squeezed magnetic designer polaritons, which act as magnetic analogs of hyperbolic polaritons in the hexagonal boron nitride (h-BN) in the first Reststrahlen band. Compared with the natural h-BN, the size and spacing of the metasurface unit cell can be readily engineered, allowing for manipulating designer polaritons in frequency and space with greater flexibility. Microwave experimental measurements display a cone-like dispersion in momentum space, exhibiting an effective refractive index up to 60 and a group velocity down to c/400. Tailoring the metasurface, we demonstrate an ultra-compact integrated designer polariton circuit including high-transmission 90° sharp bending waveguides and waveguide splitters. The present metasurface could serve as a platform for polaritonics, and find applications in waveguiding, terahertz sensing, subdiffraction focusing/imaging, low-threshold terahertz Cherenkov radiation, and wireless energy transfer.

Material equations and Maxwell’s equations for isotropic media; waves with negative group velocity and negative values of ε (ω) and μ(ω)

The frequently used Maxwell’s equations that contain E, B, D, and H fields are only substantiated in the framework of linear material equations and for isotropic media alone. We have shown that accounting for the deviation of magnetic permittivity μ (ω) from unity in the usually employed dispersion equation implies a false precision. Therefore, if spatial dispersion is disregarded, transverse waves only exist in the energy region where ε(ω) > 0 and have a positive group velocity.

Parallel propagating electromagnetic waves in magnetized quantum electron plasmas

In this paper, we derive the quantum Vlasov equation as a differential equation of the Wigner function directly from the electromagnetic Schrodinger equation and apply it to the plasma waves propagating in the direction parallel to the ambient magnetic field. The upper branches of the L and R waves in the plot of (ω, k) space have dispersion relations similar to those of their respective classical waves, with only minor corrections. The lower R-wave branch also has a dispersion relation similar to that of the classical whistler wave for a small wavenumber k. However, the dispersion curve encounters a region of anomalous dispersion, exhibiting a negative group velocity, as k increases. Furthermore, the branch becomes a damping wave as k increases above a certain critical value and eventually the wave becomes ill-defined for larger k values.

The fundamentals of harnessing the magneto-optics of quantum wires for designing optical amplifiers: Formalism

Quantum wires occupy a unique status among the semiconducting nanostructures with reduced dimensionality, no other system seems to have engaged researchers with as many appealing features to pursue. This paper aims at a core issue related with the magnetoplasmon excitations in the quantum wires characterized by the confining harmonic potential and subjected to a longitudinal electric field and a perpendicular magnetic field in the symmetric gauge. Despite the substantive complexity, we obtain the exact analytical expressions for the eigenfunction and eigenenergy, using the scheme of ladder operators, which fundamentally characterize the quantal system. Crucial to this inquiry is an intersubband collective excitation that evolves into a magnetoroton — above a threshold value of magnetic field — which observes a negative group velocity between the maxon and the roton. The evidence of negative group velocity implies anomalous dispersion in a gain medium with the population inversion that forms the basis for the lasing action of lasers. Thus, the technological pathway that unfolds is the route to devices exploiting the magnetoroton features for designing the novel optical amplifiers at nanoscale and hence paving the way to a new generation of lasers.

Wave Propagation in Composites of Plasma and Metamaterials with Negative Permittivity and Permeability

Wave propagation is observed through a negative permeability metamaterial immersed in gaseous plasma. A 3D array of split ring resonators (SRR) is enveloped by an inductively heated argon plasma with a nominal plasma frequency of 2.65 GHz. Transmission spectra show electromagnetic waves traverse the composite medium from 1.3–1.7 GHz for which the permeability of the SRRs and the permittivity of the plasma are simultaneously negative. Only surface waves and evanescence are observed outside this frequency band. The edge of the transmission band also shows negative group velocity, albeit with high wave attenuation. The free electron density of the plasma is coupled to the inductive heating, allowing dynamic reconfiguration of the metamaterial’s frequency band and wave impedance.

Long-Range Retarded Elastic Metamaterials: Wave-Stopping, Negative, and Hypersonic or Superluminal Group Velocity

Metamaterials characterized by long-range forces introduce a breakthrough in the elastic dynamics. In this study, long-range interactions among particles in an elastic continuum are considered as an effect, for example, of the inclusion of electrical or electromagnetic forces, linking two different worlds: elasticity and electromagnetism. Three universal regimes are revealed and demonstrated in general, depending on the range, intensity and repulsion or attraction nature of the long-range force. Furthermore, the dynamics of populations, traffic models, and social behavior share interesting aspects related to interaction range, extending the reach of this work.

Generation of High-Power, Reversed-Cherenkov Wakefield Radiation in a Metamaterial Structure.

We present the first demonstration of high-power, reversed-Cherenkov wakefield radiation by electron bunches passing through a metamaterial structure. The structure supports a fundamental transverse magnetic mode with a negative group velocity leading to reversed-Cherenkov radiation, which was clearly verified in the experiments. Single 45nC electron bunches of 65MeV traversing the structure generated up to 25MW in 2ns pulses at 11.4GHz, in excellent agreement with theory. Two bunches of 85nC with appropriate temporal spacing generated up to 80MW by coherent wakefield superposition, the highest rf power that metamaterial structures ever experienced without damage. These results demonstrate the unique features of metamaterial structures that are very attractive for future high-gradient wakefield accelerators, including two-beam and collinear accelerators. Advantages include the high shunt impedance for high-power generation and high-gradient acceleration, the simple and rugged structure, and a large parameter space for optimization.

Eigenwave Spectra of an Anisotropic Cylindrical Solid-State Waveguide

Dispersion properties of eigenwaves of an anisotropic cylindrical solid-state waveguide without frequency dispersion in permittivity tensor components have been analyzed theoretically. The classification of waves has been performed. E type bulk–surface eigenwaves with negative group velocities have been found. The conditions for their existence have been determined. The existence in the waveguide of E-type surface eigenwaves and pseudo-surface eigenwaves of E- and H-types has been shown.

Материальные уравнения и уравнения Максвелла для изотропных сред; волны с отрицательной групповой скоростью и отрицательные значения ε(ω) и μ(ω)

Материальные уравнения и уравнения Максвелла для изотропных сред; волны с отрицательной групповой скоростью и отрицательные значения ε(ω) и μ(ω)

Поверхностные плазмон-поляритоны в тонкой пленке "графен-полупроводник-графен"

In this work for far-infrared range we investigated the dispersion properties of surface plasmon polaritons in a structure «graphene plate-thin semiconductor film graphene plate». It is shown the ability of control the propagation modes by changing the chemical potential on one or both plates. It were received the dispersion relations for waves with TE- and TM-polarization and the distribution of wave field and energy fluxes along the transverse axes relate to propagation wave. It is shown that there is the spectral range, which characterized by negative group velocity.

Спектры собственных волн анизотропного цилиндрического твердотельного волновода

AbstractDispersion properties of eigenwaves of an anisotropic cylindrical solid-state waveguide without frequency dispersion in permittivity tensor components have been analyzed theoretically. The classification of waves has been performed. E type bulk–surface eigenwaves with negative group velocities have been found. The conditions for their existence have been determined. The existence in the waveguide of E -type surface eigenwaves and pseudo-surface eigenwaves of E- and H -types has been shown.

Spoof Surface Plasmonic Graphene for Controlling the Transports and Emissions of Electromagnetic Waves

Inspired by the recent development of graphene, we propose and experimentally demonstrate a novel metasurface, namely, spoof surface plasmonic graphene, for controlling the transports and emissions of electromagnetic waves. Similar to graphene, at the corners of the Brillouin zone for our designed metasurface, two bands of spoof surface plasmon polaritons (SSPPs) linearly touch together, forming Dirac-like cones. In the experiments, we directly observed the Dirac cones in the momentum space by using near-field scanning. When the frequency is changed from the lower cone to the Dirac point and to the upper cone, the corresponding SSPPs’ transport on the metasurface shows negative group velocities, pseudodiffusion characteristics, and positive group velocities, respectively. The emission of the SSPPs to the air shows a transition of the wavefront from a concave to flat and finally to convex. The metasurface proposed here may find many potential applications in directional antennas, waveguide, diffusion-like transport, focusing and imaging devices, and integrated spoof plasmonic circuits.

Attosecond chirp compensation in water window by plasma dispersion.

Compensating attosecond chirp (atto-chirp) of broadband high-order harmonic pulses in the water window region (282 to 533 eV) is a major challenge, due to the lack of natural materials that exhibit negative group velocity dispersion and low loss. Analysis shows that the amount of dispersion of fully ionized hydrogen plasma with suitable density-length product is sufficient to compensate the chirp of attosecond pulses with center photon energy above 300 eV. This is confirmed by numerical simulations based on the Strong Field Approximation.

Quantum-spin-Hall topological insulator in a spring-mass system

It is proposed that a lattice, with constituent masses and spring constants, may be considered as a model system for topological matter. For instance, a relative variation of the inter- and intra-unit cell spring constants can be used to create, tune, and invert band structure. Such an aspect is obtained while preserving time reversal symmetry, and consequently emulates the quantum spin Hall effect. The modal displacement fields of the mass-spring lattice were superposed so to yield pseudospin fields, with positive or negative group velocity. Considering that harmonic oscillators are the basis of classical and quantum excitations over a range of physical systems, the spring-mass system yields further insight into the constituents and possible utility of topological material.

Heat transfer in rough nanofilms and nanowires using full band ab initio Monte Carlo simulation

The Boltzmann transport equation is one of the most relevant frameworks to study the heat transport at the nanoscale, beyond the diffusive regime and up to the micrometer-scale. In the general case of 3D devices, the particle Monte Carlo approach of phonon transport is particularly powerful and convenient, and requires reasonable computational resources.In this work, we propose an original and versatile particle Monte Carlo approach parametrized by using ab initio data. Both the phonon dispersion and the phonon–phonon scattering rates have been computed by DFT calculation in the entire 3D Brillouin zone. To treat the phonon transport at rough interfaces, a combination of specular and diffuse reflections has been implemented in phase space.Thermal transport has been investigated in nanowires and thin films made of cubic and hexagonal Silicon, including edge roughness, in terms of effective thermal conductivity, phonon band contributions and heat flux orientation. It is shown that the effective thermal conductivity in quasi-ballistic regime obtained from our Monte Carlo simulation cannot be accurately fitted by simple semi-analytical Matthiessen-like models and that spectral approaches are mandatory to get good results. Our full band approach shows that some phonon branches exhibiting a negative group velocity in some parts of the Brillouin zone may contribute negatively to the total thermal flux. Besides, the thermal flux clearly appears to be oriented along directions of high density of states. The resulting anisotropy of the heat flux is discussed together with the influence of rough interfaces.

New approach to detection of guided waves with negative group velocity: Modeling and experiment

A new method for detecting the forward and backward acoustic waves in the piezoelectric plates by using a system of the acoustically isolated interdigital transducers (IDT) with the different spatial period is developed. This method was tested on the piezoelectric acoustic modes A1 and SH1 propagating in Y-X LiNbO3 plate. Theoretical analysis has shown that the SH1 mode in the entire frequency range represented a forward wave. At that the dispersion dependence of the A1 mode near the cutoff frequency has a smooth transition from a forward wave to a backward one with decreasing frequency. In order to experimentally observe this transition 19 IDTs with different periods were deposited on a single lithium niobate plate. The measurement of the frequency dependence of the real part of the electrical impedance of these IDTs showed that for SH1 mode the resonance frequency decreased monotonically with a growth in the spatial period. This behavior corresponded to a forward wave. For A1 mode the resonance frequency initially decreased with the growth of the IDT period and then began to increase after reaching a value of ZGV frequency. This behavior is explained by the smooth transition from the region of the forward wave to the region of the backward wave. The calculation of the frequency dependences of the real part of the electrical impedance of each transducer for the waves considered, carried out by the finite element method, turned out to be in good agreement with the experiment.