Nonparabolicity effects and the spin-split electron dwell time in symmetric III-V double-barrier structures

We start from the fourth order nonparabolic and anisotropic conduction band bulk dispersion relation to obtain an one-band effective Hamiltonian which we apply to an AlGaSb symmetric double-barrier structure with resonant energies significantly (more than 200meV) above the well bottom. The spin-splitting is described by the k3 Dresselhaus spin-orbit coupling term modifying only the effective mass of the spin eigenstates in the investigated structure. Apart from the bulk-like resonant energy shift due to the band nonparabolicity, we obtain a substantial shift depending on the choice of boundary conditions for the envelope functions at interfaces between different materials. The shift of resonant energy levels leads to the change of spin-splitting and the magnitude of the dwell times. We attempt to explain the influence of both the nonparabolicity and boundary conditions choice by introducing various effective masses

Time delay in thin slabs with self-focusing Kerr-type nonlinearity

Time delays for an intense transverse electric (TE) wave propagating through a Kerr-type nonlinear slab are investigated. The relation between the bidirectional group delay and the dwell time is derived and it is shown that the difference between them can be separated into three terms. The first one is the familiar self interference time, due to the dispersion of the medium surrounding the slab. The other two terms are caused by the nonlinearity and oblique incidence of the TE wave. It is shown that the electric field distribution along the slab may be expressed in terms of Jacobi elliptic functions while the phase difference introduced by the slab is given in terms of incomplete elliptic integrals. The expressions for the field intensity dependent complex reflection and transmission coefficients are derived and the multivalued oscillatory behavior of the delay times for the case of a thin slab is demonstrated

Magnetotunnelling in resonant tunnelling structures with spin-orbit interaction

Magnetotunnelling spectroscopy of resonant tunnelling structures provides information on the nature of the two-dimensional electron gas in the well. We describe a model based on nonequilibrium Green's functions that allows for a comprehensive study of the density of states, tunnelling currents and current spin polarization. The investigated effects include the electron-phonon interaction, interface roughness scattering, Zeeman effect and the Rashba spin-orbit interaction. A qualitative agreement with experimental data is found regarding the satellite peaks. The spin polarization is predicted to be larger than ten percent for magnetic fields above 2 Tesla and having a structure even at the satellite peaks. The Rashba effect is confirmed to be observable as a beating pattern in the density of states but found to be too small to affect the tunnelling current.Comment: 31 pages, 11 figure

Confined Metamaterial Structures Based on Coordinate Transformations

We apply transformation optics to structures in which the electromagnetic field is confined by highly conducting coatings. The possibility of changing the field propagation direction without perturbation is demonstrated on the example of a waveguide bend. Using this approach it is also possible to reshape a confined structure in order to meet certain external requirements and to redistribute a field in order to obtain desired field distribution. The structure implementation implies replacing a part of given confined structure with a metamaterial designed using the technique of transformation optics. Simplification of structure realization based on using reduced set of material parameters is examined. Our theoretical considerations are confirmed by full wave finite element simulations of a waveguide bend

Holographic Fabrication of Periodic Microstructures in Dichromated Pullulan

Photonic crystal structures are fabricated in dichromated pullulan by the holographic technique. Relief structures of photonic lattices with rectangular, rhombic, and hexagonal arrays of peaks and holes are obtained. The structures have periodicities of the order 1.1μm and depth of about 50 nm

Time Delay in Thin Slabs with Kerr-Type Nonlinearity

In this paper we analyzed the following model: a thin slab with Kerr nonlinearity placed between two semi-infinite samples of linear and nonmagnetic materials. A general relation between the bidirectional group delay and the dwell time is derived for the thin slab. It is shown that the group delay is equal to the dwell time plus a self-interference delay. Particular attention is given to solving the Helmholtz equation for this case. Detailed and rigorous treatment revealed that the solutions of the Helmholtz equation are given via elliptic functions of the first kind. The boundary conditions at the interfaces are determined precisely. Finally, we provide an overall procedure for numerical calculation of the dwell times

Imperfect cloaking devices based on metamaterials

Cloaking devices designed using the coordinate transform approach were shown to be realizable, at least in principle, within the realm of electromagnetic metamaterials. In this paper we investigate the strictness of conditions imposed on the parameters of metamaterial cloaks by calculating the degree of wave scattering when those parameters have variations with respect to theoretically ideal values. A simple idea is used to obtain analytic results for the case of the nonideal two-dimensional cloaking cylinder. Also, results of realistic finite element simulations of the Helmholtz equation are presented and it is found that they are in excellent agreement with the analytic results

Spin Precession of Quasi-Bound States in Heterostructures with Spin-Orbit Interaction

We use a finite-difference model that is capable of describing the single state spin dynamics in a double-barrier AlGaAs heterostructure. The use of Green's functions enables a description of the double-barrier structure by a finite matrix while the interaction with contacts is described by appropriate self-energies. To account for interface roughness scattering, a self-energy $Σp_{IR}$(E, k) is derived within the random phase approximation. The dominant part is due to in-plane momentum relaxation while a smaller part describing spin-flip scattering is neglected. The former only decreases the state lifetime while the latter can also affect the spin precession frequency

Modelling the Variable Angle Reflection and Transmission from Metamaterial Slabs

We consider metamaterial slabs composed of perfectly conducting split-ring resonators. Assuming bianisotropic constitutive relations, we describe an S-parameter retrieval method that allows us to obtain the full material parameter tensors from numerically calculated S parameters. The retrieval has been done for various angles of incidence, ranging from 0° to 75°. It has been found that the parameters depend on the angle of incidence, but that this dependence is relatively small

Study of thiacyanine dye J-aggregates on single silver nanoparticle assemblies by surface enhanced Raman scattering and atomic force microscopy

Silver nanoparticles (AgNPs) capped with dye molecules and their J-aggregates often have remarkable optical properties that can lead to applications ranging from nanoscale sensing, light harvestering and bio-labelling to advanced composite materials for novel active and nonlinear optical devices. Here we investigate the J-aggregation of a thiacyanine dye (TC) on the surface of citrate capped AgNP nanoasseblies in which the average diameter of individual AgNPs is around 10 nm. Combining Raman mapping, surface-enhanced Raman spectroscopy (SERS) and atomic force microscopy (AFM), we study the influence of TC concentration on its J-aggregation on AgNPs surface. The TC – AgNPs assemblies were deposited onto freshly cleaved highly oriented pyrolytic graphite and mica surfaces. The spectral signature of citrate ions is identified by (i) the O-H band around 220 cm-1, (ii) the C-H band around 2960 cm-1 and (iii) pronounced blinking in the 1000-1800 cm-1 range. In contrast, dye molecules adsorbed on the nanoparticles are recognized by several stable Raman bands between 200 and 1600 cm-1. In situ AFM measurements indicate that the 'hot spots' are formed either on large single nanoparticles (diameter > 100 nm) or within aggregates of small nanoparticles (with diameters in the 10 - 50 nm range). However, only the latter are found to yield a citrate or TC dye SERS signal. Our study indicates that even in highly concentrated dye solutions, some citrate ions remain attached to the nanoparticles.2nd Optical Nanospectroscopy Conference : March 18-20, Dublin, Ireland, 2015.The second annual conference of COST Action MP130