arXiv subjects

Godfrey Gumbs

Publications and source records attributed to Godfrey Gumbs.

At least 109 records · Page 6Linked to original sources

arXiv2014

Electromagnetic Wave Transmission Through a Subwavelength Nano-hole in a Two-dimensional Plasmonic Layer

An integral equation is formulated to describe electromagnetic wave transmission through a sub-wavelength nano-hole in a thin plasmonic sheet in terms of the dyadic Green's function for the associated Helmholtz problem. Taking the subwavelength radius of the nano-hole to be the smallest length of the system, we have obtained an exact solution of the integral equation for the dyadic Green's function analytically and in closed form. This dyadic Green's function is then employed in the numerical analysis of electromagnetic wave transmission through the nano-hole for normal incidence of the incoming wave train. The electromagnetic transmission involves two distinct contributions, one emanating from the nano-hole and the other is directly transmitted through the thin plasmonic layer itself (which would not occur in the case of a perfect metal screen). The transmitted radiation exhibits interference fringes in the vicinity of the nano-hole, and they tend to flatten as a function of increasing lateral separation from the hole, reaching the uniform value of transmission through the sheet alone at large separations.

physics.plasm-ph↗

arXiv2014

Surface Plasmon Instability Leading to Emission of Radiation

We propose a new energy conversion approach from a dc electric field to a terahertz wave based on hybrid semiconductors by combining two-dimensional (2D) crystalline layers and a thick conducting material with possible applications as a source of coherent radiation. The hybrid nano-structure may consist of a single or pair of sheets of graphene, silicene or a 2D electron gas as would occur at a semiconductor hetero-interface. When an electric current is passed through a layer, we discover that the low-frequency plasmons may become unstable beyond a critical wave vector $q_c$. However, there is no instability for a single driven layer far from the conductor and the instability of an isolated pair of 2D layers occurs only at ultra long wavelengths. To bring in frequency agility for this spontaneous radiation, we manipulate the surface-plasmon induced instability, which leads to the emission of radiation (spiler), to occur at shorter wavelengths by choosing the conductor electron density, layer separation, distances of layers from the conductor surface and the driving-current strength. Applications of terahertz radiation from spiler for chemical analysis, security scanning, medical imaging and telecommunications are expected.

cond-mat.mes-hall↗

arXiv2014

Engineering semiconductor hybrids for sensing

The effect of screening of the coulomb interaction between two layers of two-dimensional electrons, such as in graphene, by a highly doped semiconducting substrate is investigated. We employ the random-phase approximation to calculate the dispersion equation of this hybrid structure in order to determine the plasmon excitation spectrum. When an electric current is passed through a layer, the low-frequency plasmons in the layer may bifurcate into separate streams due to the current-driving effect. At a critical wave vector, determined by the separation between layers and their distance from the surface, their phase velocities may be in opposite directions and a surface plasmon instability leads to the emission of radiation. Applications to detectors and other electromagnetic devices exploiting nano-plasmonics are discussed.

cond-mat.mes-hall↗

arXiv2014

Role Played by Surface Plasmons on Plasma Instability in Composite Layered Structures

We demonstrate the engineering of a source of radiation from growing surface plasmons (charge density oscillations) in a composite nano-system. The considered hybrid nano-structure consists of a thick layer of a conducting substrate on whose surface a plasmon mode is activated conjoining a single or pair of thin sheets of either monolayer graphene, silicene or a two-dimensional electron gas as would occur at a hetero-interface. When an electric current is passed through either a layer or within the substrate, the low-frequency plasmons in the layer may bifurcate into separate streams due to the driving current. At a critical wave vector, determined by the separation between layers (if there are two) and their distance from the surface, their phase velocities may be in opposite directions and a surface plasmon instability leads to the emission of radiation (spiler). Spiler takes advantage of the flexibility of choosing its constituents to produce sources of radiation. The role of the substrate is to screen the Coulomb interaction between two layers or between a layer and the surface. The range of wave vectors where the instability is achieved may be adjusted by varying layer separation and type of material. Applications to detectors and other electromagnetic devices exploiting nano-plasmonics are discussed.

cond-mat.mtrl-sci↗

arXiv2014

Novel properties of graphene in the presence of energy gap: optics, transport and mobility studies

We review the transmission of Dirac electrons through a potential barrier in the presence of circularly polarized light. A different type of transmission is demonstrated and explained. Perfect transmission for nearly head-on collision in inffnite graphene is suppressed in gapped dressed states of electrons. We also present our results on enhanced mobility of hot Dirac electrons in nanoribbons and magnetoplasmons in graphene in the presence of the energy gap. The calculated carrier mobility for a graphene nanoribbon as a function of the bias field possesses a high threshold for entering the nonlinear transport regime. This threshold is a function of both extrinsic and intrinsic properties, such as lattice temperature, linear density, impurity scattering strength, ribbon width, and correlation length for the line-edge roughness. Analysis of non-equilibrium carrier distribution function confirms that the difference between linear and nonlinear transport is due to sweeping electrons from the right to left Fermi one through elastic scattering as well as moving electrons from low to high-energy ones through field-induced heating. The plasmons, as well as the electron-hole continuum are determined by both energy gap and the magnetic field, showing very specific features, which have been studied and discussed in details.

cond-mat.mes-hall↗

arXiv2014

Controlling quantum-dot light absorption and emission by a surface-plasmon field

The possibility for controlling the probe-field optical gain and absorption switching and photon conversion by a surface-plasmon-polariton near field is explored for a quantum dot above the surface of a metal. In contrast to the linear response in the weak-coupling regime, the calculated spectra show an induced optical gain and a triply-split spontaneous emission peak resulting from the interference between the surface-plasmon field and the probe or self-emitted light field in such a strongly-coupled nonlinear system. Our result on the control of the mediated photon-photon interaction, very similar to the `gate' control in an optical transistor, may be experimentally observable and applied to ultra-fast intrachip/interchip optical interconnects, improvement in the performance of fiber-optic communication networks and developments of optical digital computers and quantum communications.

cond-mat.mes-hall↗

arXiv2014

Resonant Scattering of Surface Plasmon Polaritons by Dressed Quantum Dots

The resonant scattering of surface plasmon-polariton waves by embedded semiconductor quantum dots above the dielectric/metal interface is explored in the strong-coupling regime. In contrast to non-resonant scattering by a localized dielectric surface defect, a strong resonant peak in the scattering field spectrum is predicted and accompanied by two side valleys. The peak height depends nonlinearly on the amplitude of surface plasmon-polariton waves, reflecting the feedback dynamics from a photon-dressed electron-hole plasma inside the quantum dots. This unique behavior in the scattering field peak strength is correlated with the occurrence of a resonant dip in the absorption spectrum of surface plasmon-polariton waves due to interband photon-dressing effect. Our result on the scattering of surface plasmon-polariton waves may be experimentally observable and applied to spatially selective illumination and imaging of individual molecules.

cond-mat.mes-hall↗

arXiv2014

Plasmons in a Superlattice of Fullerenes or Metallic Shells

A theory for the collective plasma excitations in a linear periodic array of spherical two-dimensional electron gases (S2DEGs) is presented. This is a simple model for an ultra thin and narrow microribbon of fullerenes or metallic shells. Coulomb coupling between electrons located on the same sphere and on different spheres is included in the random-phase approximation (RPA). Electron hopping between spheres is neglected in these calculations. The resulting plasmon-dispersion equation is solved numerically. Results are presented for a superlattice of single-wall S2DEGs as a function of the wave vector. The plasmon dispersions are obtained for different spherical separations. We show that the one-dimensional translational symmetry of the lattice is maintained in the plasmon spectrum. Additionally, we compare the plasmon dispersion when the superlatice direction is parallel or perpendicular to the axis of quantization. However, because of anisotropy in the Coulomb matrix elements, there is anticrossing in the plasmon dispersion only in the direction perpendicular to the quantization axis. The S2DEG may serve as a simple model for fullerenes, when their energy bands are far apart.

cond-mat.mes-hall↗

arXiv2014

Combined Effect of Stacking and Magnetic Field on Plasmon Excitations in Bilayer Graphene

The electronic excitations of bilayer graphene (BLG) under a magnetic field are investigated with the use of the Peierls tight-binding model in conjunction with random-phase approximation (RPA). The interlayer atomic interactions, interlayer Coulomb interactions, and magnetic field effects are simultaneously included in the dielectric-function matrix. That enables us to derive the magneto-Coulomb-excitation spectrum of different stacking structures. The two typical arrangements of BLGs, AB and AA, are considered in this article. AB-BLG exhibits many discrete energy-loss peaks, which correspond to the quantization of electron energies. On the other hand, the AA-BLG spectra possess a unique and pronounced peak at low frequency. This peak represents the collective excitation of the entire low-frequency Landau states. The dependence of the energy-loss peaks on the momentum transfer and the magnetic field strength is presented. Accordingly, two kinds of plasmon modes produced by the layer stacking are clearly distinguished.

cond-mat.mes-hall↗

arXiv2014

Revealing Hofstadter Spectrum for Graphene in a Periodic Potential

We calculate the energy bands for graphene monolayers when electrons move through a periodic electrostatic potential in the presence of a uniform perpendicular magnetic field. We clearly demonstrate the quantum fractal nature of the energy bands at reasonably low magnetic fields. We present results for the energy bands as functions of both wave number and magnetic flux through the unit cells of the resulting moire superlattice. The effects due to pseudo-spin coupling and Landau orbit mixing by a strong scattering potential have been exhibited. At low magnetic fields when the Landau orbits are much larger than the period of the modulation, the Landau levels are only slightly broadened. This feature is also observed at extremely high magnetic fields. The density of states has been calculated and shows a remarkable self-similarity like the energy bands. We estimate that for modulation period of 10nm the region where the Hofstadter butterfly is revealed at B < 2 T.

cond-mat.mes-hall↗

arXiv2014

Modeling Anisotropic Plasmon Excitations in Self-Assembled Fullerenes

The plasmon excitations in Coulomb-coupled spherical two-dimensional electron gases (S2DEGs) reveal an interesting dependence on the displacement vector between the centers of the spheres with respect to the axis of quantization for the angular momentum quantum number L. Specifically, plasmon modes for a bundle of three S2DEGs have been obtained within the random-phase approximation (RPA). The inter-sphere Coulomb interaction matrix elements and their symmetry properties were also investigated in detail. The case of a bundle gives an adequate picture of the way in which the Coulomb interaction depends on the orbital angular momentum quantum number L and its projection M. We concluded that the interaction between the S2DEGs aligned at an angle of 45o with the axis of quantization is negligible compared to the interaction along and perpendicular to the quantization axis, which are themselves unequal to each other. Consequently, the plasmon excitation frequencies reveal an interesting orientational anisotropic coupling to an external electromagnetic field probing the charge density oscillations. Our result on the spatial correlation may be experimentally observable. In this connection, there have already been some experimental reports pointing to a similar effect in nanoparticles

cond-mat.mes-hall↗

arXiv2014

Anisotropic plasmon-coupling dimerization of a pair of spherical electron gases

We have discovered a novel feature in the plasmon excitations for a pair of Coulomb-coupled non- concentric spherical two-dimensional electron gases (S2DEGs). Our results show that the plasmon excitations for such pairs depend on the orientation with respect to the external electromagnetic probe field. The origin of this anisotropy of the inter-sphere Coulomb interaction is due to the directional asymmetry of the electrostatic coupling of electrons in excited states which depend on both the angular momentum quantum number L and its projection M on the axis of quantization taken as the probe E-field direction. We demonstrate the anisotropic inter-sphere Coulomb coupling in space and present semi-analytic results in the random-phase approximation both perpendicular and parallel to the axis of quantization. For the incidence of light with a finite orbital or spin angular momentum, the magnetic field generated from an induced oscillating electric dipole on one sphere can couple to an induced magnetic dipole on another sphere in a way depending on the direction parallel or perpendicular to the probe E field. Such an effect from the plasmon spatial correlation is expected to be experimentally observable by employing circularly-polarized light or a helical light beam for incidence. The S2DEG serves as a simple model for fullerenes as well as metallic dimers, when the energy bands are far apart.

cond-mat.mes-hall↗

arXiv2013

Second Harmonic Generation in Gapped Graphene

The second-order nonlinear optical susceptibility $Π^{(2)}$ for second harmonic generation is calculated for gapped graphene. The linear and second-order nonlinear plasmon excitations are investigated in context of second harmonic generation (SHG). We report a red shift and an order of magnitude enhancement of the SHG resonance with growing gap, or alternatively, reduced electro-chemical potential.

cond-mat.mes-hall↗

arXiv2013

Influence of Dielectric Environment on Role of Spin-Orbit Interaction for Image Potentials

We present a formalism for calculating the image potential for a two-dimensional electron gas (2DEG) with Rashba spin-orbit interaction (SOI) as well as for a 2D topological insulator (TI). The formalism is further generalized for including the Coulomb coupled multiple layers. Roles of broken inversion symmetry near the surface and the dielectric environment are investigated by using a surface-response function. The insignificant role of SOI in 2DEG is dramatically enhanced in TI by selecting a small relative permittivity $ε_b$ for the dielectric environment. Manipulating $ε_b$ is proven to provide an efficient way to drive electrons with opposite spins into two different integral quantum Hall states. The prediction made in this paper is expected to be experimentally observable for a 2DTI system, such as Bi$_2$Se$_3$, with a helical spin behavior and a dominant linear Rashba SOI-like term in the energy dispersion.

cond-mat.mes-hall↗

arXiv2013

Effects of periodic scattering potential on Landau quantization and ballistic transport of electrons in graphene

A two-dimensional periodic array of scatterers has been introduced to a single layer of graphene in the presence of an external magnetic field perpendicular to the graphene layer. The eigenvalue equation for such a system has been solved numerically to display the structure of split Landau subbands as functions of both wave number and magnetic flux. The effects of pseudo-spin coupling and Landau subbands mixing by a strong scattering potential have been demonstrated. Additionally, we investigated the square barrier tunneling problem when magnetic field is present, as well as demonstrate the crucial difference in the modulated band structure between graphene and the twodimensional electron gas. The low-magnetic field regime is particularly interesting for Dirac fermions and has been discussed. Tunneling of Dirac electrons through a magnetic potential barrier has been investigated to complement the reported results on electrostatic potential scattering in the presence of an ambient magnetic field.

cond-mat.mes-hall↗

arXiv2013

Theory for entanglement of electrons dressed with circularly polarized light in Graphene and three-dimensional topological insulators

We have formulated a theory for investigating the conditions which are required to achieve entangled states of electrons on graphene and three-dimensional (3D) topological insulators (TIs). We consider the quantum entanglement of spins by calculating the exchange energy. A gap is opened up at the Fermi level between the valence and conduction bands in the absence of doping when graphene as well as 3D TIs are irradiated with circularly-polarized light. This energy band gap is dependent on the intensity and frequency of the applied electromagnetic field. The electron-photon coupling also gives rise to a unique energy dispersion of the dressed states which is different from either graphene or the conventional two-dimensional electron gas (2DEG). In our calculations, we obtained the dynamical polarization function for imaginary frequencies which is then employed to determine the exchange energy. The polarization function is obtained with the use of both the energy eigenstates and the overlap of pseudo-spin wave functions. We have concluded that while doping has a significant influence on the exchange energy and consequently on the entanglement, the gap of the energy dispersions affects the exchange slightly, which could be used as a good technique to tune and control entanglement for quantum information purposes.

cond-mat.mes-hall↗

arXiv2013

Dynamic and static control of the optical phase of guided p-polarized light for near-field focusing at large angles of incidence

Both dynamic and static approaches are proposed and investigated for controlling the optical phase of a p-polarized light wave that is guided through a surface-patterned metallic structure with subwavelength features. For dynamic control, field-induced transparency (FIT) from photo-excited electrons in a slit-embedded atomic system show up within a narrow frequency window for modulating the intensity of focused transmitted light in the near-field region. Based on the electromagnetic coupling, this is facilitated by surface plasmons between the two FIT-atom embedded slits. For static control, the role of surface curvature is obtained for focused transmitted light passing through a Gaussian-shaped metallic microlens embedded with a linear array of slits, in addition to a negative light-refraction pattern, which is associated with higher-diffraction modes of light, under a large angle of incidence in the near-field region. Most interesting, however, this anomalous negative light-refraction pattern becomes suppressible with leaked higher-order waveguide modes of light passing through a very thin film. At the same time, it is also suppressible with a reinforced reflection at the left foothill of a Gaussian-shaped slit array for the forward-propagating surface-plasmon wave at large angles of incidence. A prediction is given for near-field focusing of light with its sharpness dynamically controlled by the frequency of the light in a very narrow window. Moreover, a different scheme based on Green's second integral identity is proposed for overcoming a difficulty in calculating the near-field distribution very close to a surface by means of a finite-difference-time-domain method.

physics.optics↗

arXiv2013

Electronic properties of closed cage nanometer-size spherical graphitic particles

We investigate the localization of charged particles by the image potential of spherical shells, such as fullerene buckyballs. These "spherical image states" exist within surface potentials formed by the competition between the attractive image potential and the repulsive centripetal force arising from the angular motion. The image potential has a power law rather than a logarithmic behavior for a nanotube, leading to fundamental differences in the forms for the effective potential for the two geometries. The sphere has localized stable states close to its surface. At low temperatures, this results in long lifetimes for the image states. We predict the possibility of creating image states with binding energies of a few meV around metallic/non-metallic spherical shells by photoionization. Applications and related phenomena are discussed.

cond-mat.mtrl-sci↗