Pair Of Subbands Research Articles

Application of the WKB Theory to Investigate Electron Tunneling in Kek-Y Graphene

In this paper, we have constructed a WKB approximation for graphene having a Y-shaped Kekulé lattice distortion and a special folding of the K and K′ valleys, which leads to very specific linear energy dispersions with two non-equivalent pairs of subbands. These obtained semi-classical results, which include the action, electron momentum and wave functions, are utilized to analyze the dynamics of electron tunneling through non-square potential barriers. In particular, we explore resonant scattering of an electron by a potential barrier built on Kekulé-distorted graphene. Mathematically, a group of consecutive equations for a semi-classical action have been solved by following a perturbation approach under the condition of small strain-induced coupling parameter Δ0≪1 (a good fit to its actual value Δ0∽ 0.1). Specifically, we consider a generalized model for Kek-Y graphene with two arbitrary Fermi velocities. The dependence of the electron transmission amplitude on the potential profile V(x) and band parameters of Kekulé-patterned graphene has been explored and analyzed in detail.

Geophysical data fusion of ground-penetrating radar and magnetic datasets using 2D wavelet transform and singular value decomposition

This work addresses the problem of the lack of perceptibility that geophysical data may have. Data fusion allows us to combine datasets, providing an improved and more informative source of information about structures buried in the ground. After testing different approaches, a strategy was developed using ground-penetrating radar and magnetic datasets collected over the same area. Data collected at the Roman Villa of Pisões (Beja, Portugal), which is a place of easy application of geophysical methods, were used to test the method, but with problems caused by the properties of the soil. The approach was based on processing operations that allow the fusion of images obtained by different equipment widely used in medical imaging for tumor detection and image processing. The goal is to create an improved image with data fusion that has higher quality than the input images, allowing a better understanding of the object of the study. The approach is composed of two stages: pre-processing and data fusion. Pre-processing is applied to enhance the input data. It consists of removing background noise through singular value decomposition applied in the spectral domain. Then the calculation of the data entropy will highlight the differences corresponding to the spatial alignments compatible with buried structures. Then, both entropy maps of the two datasets are fused in the second processing step to produce the final image. This step involves applying the 2D wavelet transform to each entropy map, decomposing them into sub-bands. Algorithms to calculate multiresolution singular value decomposition and the image gradient are applied to the sub-bands. The processed sub-band pairs are then fused using specific fusion rules. The fused image is obtained by applying the inverse of the wavelet transform. Data fusion with the proposed approach allows us to obtain a detailed image that is sharper and of better quality than the input datasets. The increase in sharpness and quality can be quantified through the sharpness index and the BRISQUE quality index in several steps of the processing. The obtained values confirm the graphical results. Images produced by the proposed data fusion approach suggest that the perceptibility has increased, allowing us to provide conclusions about the existence of buried structures.

Multi-Chromatic Analysis of SAR Images for Coherent Target Detection

This work investigates the possibility of performing target analysis through the Multi-Chromatic Analysis (MCA), a technique that basically explores the information content of sub-band images obtained by processing portions of the range spectrum of a synthetic aperture radar (SAR) image. According to the behavior of the SAR signal at the different sub-bands, MCA allows target classification. Two strategies have been experimented by processing TerraSAR-X images acquired over the Venice Lagoon, Italy: one exploiting the phase of interferometric sub-band pairs, the other using the spectral coherence derived by computing the coherence between sub-band images of a single SAR acquisition. The first approach introduces the concept of frequency-persistent scatterers (FPS), which is complementary to that of the time-persistent scatterers (PS). FPS and PS populations have been derived and analyzed to evaluate the respective characteristics and the physical nature of the targets. Spectral coherence analysis has been applied to vessel detection, according to the property that, in presence of a random distribution of surface scatterers, as for open sea surfaces, spectral coherence is expected to be proportional to sub-band intersection, while in presence of manmade structures it is preserved anyhow. First results show that spectral coherence is well preserved even for very small vessels, and can be used as a complementary information channel to constrain vessel detection in addition to classical Constant False Alarm Rate techniques based on the sole intensity channel.

Spin-dependent electron transport in a Rashba quantum wire with rough edges

We investigate theoretically the spin-dependent electron transport in a Rashba quantum wire with rough edges. The charge and spin conductances are calculated as function of the electron energy or the wire length by adopting the spinresolved lattice Green function method. For a single disordered Rashba wire, it is found that the charge conductance quantization is destroyed by the edge disorder. However, a nonzero spin conductance can be generated and its amplitude can be manipulated by the wire length, which is attributed to the broken structure symmetries and the spin-dependent quantum interference induced by the rough boundaries. For a large ensemble of disordered Rashba wires, the average charge conductance decreases monotonically, however, the average spin conductance increases to a maximum value and then decreases, with increasing wire length. Further study shows that the influence of the rough edges on the charge and spin conductances can be eliminated by applying a perpendicular magnetic field to the wire. In addition, a very large magnitude of the spin conductance can be achieved when the electron energy lies between the two thresholds of each pair of subbands. These findings may not only benefit to further apprehend the transport properties of the Rashba low-dimensional systems but also provide some theoretical instructions to the application of spintronics devices.

Intersubband plasmons in a quasi-one-dimensional system

It has been shown that plasma oscillations in quasi-one-dimensional systems feature the presence of two branches of intersubband plasmons for each pair of subbands. The second (low-frequency) branch lies in the region of the frequency-momentum plane, where a plasma-wave-induced single-particle transition is allowed but there is no Landau damping.

Spin filtering in a nonuniform quantum wire with Rashba spin–orbit interaction

We investigate theoretically the spin-polarized electron transport for a wide-narrow-wide (WNW) quantum wire under the modulation of Rashba spin–orbit interaction (SOI). The influence of both the structure of the quantum wire and the interference between different pairs of subbands on the spin-polarized electron transport is taken into account simultaneously via the spin-resolved lattice Green function method. It is found that a very large vertical spin-polarized current can be generated by the SOI-induced effective magnetic field at the structure-induced Fano resonance even in the presence of strong disorder. Furthermore, the magnitude of the spin polarization can be tuned by the Rashba SOI strength and structural parameters. Those results may provide an effective way to design a spin filter device without containing any magnetic materials or applying a magnetic field.

Growth variation effects in SiGe-based quantum cascade lasers

Epitaxial growth of SiGe quantum cascade (QC) lasers has thus far proved difficult, and nonabrupt Ge profiles are known to exist. We model the resulting barrier degradation by simulating annealing in pairs of quantum wells (QWs). Using a semiclassical charge transport model, we calculate the changes in scattering rates and transition energy between the lowest pair of subbands. We compare results for each of the possible material configurations for SiGe QC lasers. The effects are most severe in n-type (001) Si-rich systems due to the large effective electron mass, and in p-type systems due to the coexistence of light holes and heavy holes. The lower effective mass and conduction band offset of (111) oriented systems minimizes the transition energy variation, and a large interdiffusion length (Ld = 1.49 nm) is tolerated with respect to the scattering rate. Ge-rich systems are shown to give the best tolerance with respect to subband separation (Ld = 3.31 nm), due also to their low effective mass.

Near-infrared two-color intersubband transitions in AlN/GaN coupled double quantum wells

A study on a four-energy-level system in asymmetric AlN/GaN coupled double quantum wells has been performed by solving Schrödinger and Poisson equations self-consistently. It is found that the transition selection rule is recovered when the first two subband pairs resonate in the four-energy-level system. The anticrossing gap between the second excited state (2odd) and the third excited state (2even) can be up to 135 meV when the Al composition of the central barrier is 0.80. The absorption coefficient of intersubband transition (ISBT) between the ground state (1odd) and the 2even subband is approximately equal to that between the first excited state (1even) and the 2odd subband. The wavelengths of the 1odd-2even and the 1even-2odd ISBTs are 1.31 and 1.55 μm, respectively. The results give possible application to the ultrafast two-color optoelectronic devices operating within optical communication wavelength range.

Strong excitonic mixing effect in asymmetric double quantum wells: On the optimization of electroabsorption modulators

AbstractWe investigate the mixing of excitons originating in different subband pairs in asymmetric double quantum wells (ADQWs) in a range of electric field where the two lowest exciton states anticross. This excitonic mixing is mainly attributed to the Coulomb interactions between subbands and the valence‐subband nonparabolicity. Results show that excluding the excitonic mixing effect results in significant error in both the energies and the oscillator strengths of the excitons in an ADQW with thick barrier (3 nm). Even in an ADQW with a fairly thin barrier (1.2 nm), the error in the oscillator strengths can be substantial, although the errors in the computed energies may be tolerable. We find that including the mixing of excitons is indispensable in optimizing the structures of the asymmetric double quantum well electroabsorption modulators. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electric-field-induced strong mixing betweene1−hh1ande1−hh2excitons in asymmetric double quantum wells

We analyze the electric-field-dependent strong mixing of the two near band-edge ground-state excitons in asymmetric double quantum wells (ADQWs). This excitonic mixing is mainly attributed to the Coulomb interactions between subbands and the valence-subband nonparabolicity. The effect of mixing on the energy levels and oscillator strengths is obtained by a comparison of results including and excluding the Coulomb interaction between different subband pairs which appears in the off-diagonal matrix elements of the Hamiltonian in the calculation of exciton states. We find that a substantial portion of the oscillator strength of the $e1\text{\ensuremath{-}}hh1$ ground-state exciton is due to the $e1\text{\ensuremath{-}}hh2$ subband pair in a bias range of the anticrossing region between pairs of valence subbands. Results also show that excluding the excitonic mixing effect results in significant error in both the energies and the oscillator strengths of the excitons in an ADQW with a thick barrier $(3\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. Even in an ADQW with a fairly thin barrier $(1.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$, the error in the oscillator strengths can be substantial, although the errors in the computed energies may be tolerable. Detailed analysis of ${k}_{\ensuremath{\Vert}}$-dependent Coulomb matrix elements and exciton expansion coefficients reveals that neglecting the off-diagonal elements in the exciton Hamiltonian diminishes the contribution of the ${k}_{\ensuremath{\Vert}}$-dependent subband envelopes around the anticrossing in ${k}_{\ensuremath{\Vert}}$ space to the corresponding excitons. Further, it is demonstrated that the application of almost-degenerate perturbation theory to the uncoupled excitons with the intention of accounting for this mixing effect is to be approached with caution.

Electrooptic Properties of InGaAsP Asymmetric Double Quantum Wells: Enhanced Slope Efficiency in Waveguide Electroabsorption Modulators

We present detailed theoretical estimates of the optical properties of InGaAsP quantum well (QW) electroabsorption modulators (EAMs) operating at ~1550 nm wavelength. Absorption coefficients of QWs are obtained from the linear optical susceptibility. Exciton states are calculated in momentum space, which includes valence-band mixing, mixing of excitons originating in different subband pairs, and exciton spin-related optical selection rules. Various line-broadening mechanisms relevant to InGaAsP-QWs are also included. Extending the study further to asymmetric double QWs (ADQWs) suggests that the small-signal modulation efficiency can be enhanced significantly at substantially lower operating bias voltage. Simple optimization of ADQW band structure results in a maximum slope efficiency ~3.8 times larger than that of SQW EAMs at a reduced operating bias field of 34 kV/cm compared with ~70 kV/cm for comparable SQWs.

Excitons and Many-Electron Effects in the Optical Response of Single-Walled Boron Nitride Nanotubes

We report first-principles calculations of the effects of quasiparticle self-energy and electron-hole interaction on the optical properties of single-walled boron nitride nanotubes. Excitonic effects are shown to be even more important in BN nanotubes than in carbon nanotubes. Electron-hole interactions give rise to complexes of bright (and dark) excitons, which qualitatively alter the optical response. Excitons with a binding energy larger than 2 eV are found in the BN nanotubes. Moreover, unlike the carbon nanotubes, theory predicts that these exciton states are comprised of coherent supposition of transitions from several different subband pairs, giving rise to novel behaviors.

The analytical solution and the complete electronic structure of quantum well

With a detailed analysis of the Bloch wave properties of one-dimensional infinite crystal, we find the exact analytical solution of the quantum confinement states in quantum well with infinite barrier. Based on the analytical solution, we calculate the complete electronic structure of quantum well with arbitrary geometries. The calculation results demonstrate that the transitions only occur between specific pairs of sub-bands. It also shows that the band edge state plays an insignificant role in the optical properties, although it is the most distinguishing feature of the quantum well.

Exciton levels and optical absorption in coupled double quantum well structures

We study exciton states in a coupled double quantum well (CDQW) semiconductor structure. Exciton levels and binding energies of direct and indirect excitons are calculated for a symmetric CDQW system with an applied electric field. The exciton states are obtained by solving the exciton effective-mass equation in the momentum space using the modified Gaussian quadrature method. Within this approach we perform realistic calculations of the exciton states by taking into account the coupling between different subband pairs and calculate optical-absorption coefficients. The calculated values of the exciton binding energy are in a good agreement with the experiment and the calculated absorption spectra qualitatively agree with the measured photoluminescence excitation spectra.

Transition from two-dimensional to three-dimensional electroabsorption in extremely shallow quantum wells

Calculations of electroabsorption in extremely shallow quantum wells are performed, accurately incorporating mixing of different subband pairs due to the Coulombic interaction. As the AlAs mole fraction is varied in the barriers of a 100 Å wide AlxGa1−xAs/GaAs square quantum well, a transition from red to blueshift of the absorption edge with applied electric field occurs at x ∼ 0.003. In a 20 Å wide square well, which more strongly confines the excitons, the redshift of the absorption edge is still observable at low electric field strength in the simulated absorption spectra, even for AlAs mole fractions as low as ∼0.001. However, on increasing the strength of the applied electric field the blueshift becomes apparent. It is demonstrated that inclusion of Coulombic coupling between different subband pairs, particularly between confined states and quasi-continuum states, is essential for a correct prediction of the absorption edge behaviour in extremely shallow quantum wells.

Characterization of nanographite and carbon nanotubes by polarization dependent optical spectroscopy

ABSTRACTThe optical absorption for ? electrons as a function of the electron wavevector k is investigated by first order perturbation theory in graphite and single wall carbon nanotubes (SWNTs). The matrix element connecting two states in the valence and conduction bands is found to be significantly anisotropic in k-space and polarization dependent. In the case of graphite, the absorption shows a node around the equi-energy contour, and in the case of SWNTs we obtain selection rules that allow only transitions between certain pairs of subbands. The strength of the optical absorption is not only diameter dependent but also chirality dependent. The implications of the optical absorption matrix element on the resonant conditions are discussed.

Electroabsorption in extremely shallow quantum wells: Comparison between theory and experiment

It is shown, via detailed comparison between measured electroabsorption spectra and electroabsorption spectra calculated by a full excitonic Green’s function method, a decoupled excitonic Green’s function method and a variational method, that accurate description of the Coulombic coupling between different subband pairs, including unbound quantum well states above the quantum well edge, is crucial for a correct prediction of electroabsorption in extremely shallow quantum well structures.

Electroabsorption in narrow coupled double quantum wells: Coulombic coupling effects

The effects of Coulombic coupling between different subband pairs on the electroabsorption spectra of narrow coupled double quantum wells (QWs) have been studied. It is shown via detailed comparison between electroabsorption spectra calculated using a full excitonic Green's function method, a decoupled excitonic Green's function method, a variational method, and experimental data that inclusion of the Coulombic coupling between different subband pairs is required for correct prediction of electroabsorption in the narrow well system. It is also shown that, due to Coulombic coupling, it is necessary to include unbound QW states, above the QW edge, in the simulation of electroabsorption and electrorefraction. These results are of particular significance for the accurate calculation of electrorefraction, by Kramers-Kronig transformation of QW electroabsorption spectra, in coupled QW structures containing narrow QWs.

Fano resonances in the optical spectra of semiconductor quantum structures.

The appearance of Fano resonances in the fine structure of the optical absorption spectra is predicted for all semiconductor quantum structures with one or two degrees of freedom. The resonances are explained by Coulomb coupling of the bound and scattering states of an exciton series belonging to different subband pairs but occuring at the same photon energy. To demonstrate this effect, two examples, the quantum well and the quantum-well wire, are treated numerically. The optical spectra show Fano resonances.

Excitons and fundamental absorption in quantum wells.

We present an accurate theory for the excitonic absorption in quantum-well structures which yields very good agreement with a wide range of experimental data. Our approach is based on an expansion of the exciton wave function in terms of multicomponent envelope functions of electron and hole states. It is rather general with respect to band-structure effects and potential profiles. In momentum space the two-particle Schr\odinger equation becomes a set of coupled integral equations for the expansion coefficients which we solve by means of a modified quadrature method. Our calculations reproduce all the essential details of a great variety of experimental spectra from high-quality GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well structures. We demonstrate that this requires one to take into account both band-structure effects (HH-LH coupling and nonparabolicity) and the coupling of different electron-hole subband pairs caused by the Coulomb interaction. These effects strongly modify the peaks in the exciton continuum which correspond to Fano resonances. Moreover, the Coulomb coupling of subband pairs is often so substantial that it is not possible to attribute excitons to a single subband pair. The free electron-hole absorption coefficient can have pronounced Van Hove singularities which disappear in the excitonic absorption spectrum. Thus, in contrast to bulk semiconductors, the Coulomb interaction can decrease the absorption coefficient at the fundamental absorption edge.