Subband Edge Research Articles

Distributed Multiband Synthetic Aperture Radar Image Fusion Based on Wavelet Transform in the Internet of Things Environment

Abstract In order to improve the detection and recognition capabilities of distributed multiband synthetic aperture radar (SAR) images in the Internet of Things environment, a distributed multiband SAR image fusion algorithm based on wavelet transform is proposed for the Internet of Things environment. The multispectral/hyperspectral imager is used to detect and process the distributed multiband SAR image. The feature extraction method of fast spatial geographic water target range radar signal source is used to extract and segment the distributed multiband SAR image. The wavelet multiscale transform method is used to segment the SAR image, and the linear filtering and nonlinear filtering methods are used to detect the edge contour features. Using the distributed multiband SAR image fusion technology based on the calculation of high-frequency subband edge function and the segmentation of regional gray contour curve, the splitting and broadening of the peak spectrum of the target image of the radar signal source in the fast spatial geographical waters, as well as the radar target positioning parameters, the noise filtering, and anti-jamming detection of the distributed multiband SAR image are realized, and the distributed multiband SAR image fusion is realized combined with wavelet transform. The test results show that the output peak signal-to-noise ratio of distributed multiband SAR image fusion using this method is high, and the performance of detection and recognition of SAR imaging targets and the ability of edge contour feature extraction are good.

Research on Channelization Techniques of Radio Astronomical Wideband Signal with Oversampled Polyphase Filter Banks

Digital channelization decomposes a wideband signal into multiple adjacent sub-bands using Parallel Technology. Channelization can effectively reduce the pressure on the radio astronomy digital backends system and make wideband signal processing possible. Aiming at the problems of signal attenuation at sub-band edge, spectral leakage and aliasing encountered in wideband signal channelization, algorithms to reduce the problems are studied. We design a Critically Sampled Polyphase Filter Bank (CS-PFB) based on the Finite Impulse Response digital filter with a Hamming Window and systematically analyze the frequency response characteristics of the CS-PFB. Based on the channelized structure of the CS-PFB, an OverSampled Polyphase Filter Bank (OS-PFB) is designed by data reuse, and the filtering frequency response characteristics of CS-PFB and OS-PFB are compared and analyzed. Using the wideband baseband data generated by the CASPSR (Collaboration for Astronomy Signal processing and electronics research Parkes Swinburne Recorder), we implement sub-band division and 16-band output of these data based on the 2× oversampling OS-PFB, and the problem of spectrum inversion in the sub-bands is corrected. After removing 25% of redundant data in the head and tail of each sub-band, we recombine the sub-bands into a wideband. The wideband signal is almost identical to the original observed signal. Therefore, the experimental results show that the OS-PFB can improve the channel response. For the 400 MHz baseband data of J0437-4715, we compare the pulse profile obtained from the original baseband data with the pulse profile obtained after the channelization and recombination. The phase and amplitude information of the pulse profiles are consistent, which verifies the correctness of our channelization algorithm.

Synthesis of titanium-doped CdIn2S4 thin film and its photoelectric properties for intermediate band solar cells

The series Titanium-doped CdIn2S4 (Ti-CIS) thin films, with a spinel structure, were successfully synthesized by a typical technology of magnetron sputtering. The prepared Ti-CIS thin films were characterized by X-ray diffraction patterns, Raman spectrometer, energy dispersive X-ray analysis, transient photocurrent response and UV–vis–NIR spectrophotometer. The experimental results demonstrated that the optical absorption spectra were widened in Ti-CIS films. Due to Ti doping, two additional sub-band edge responses were observed at about 760 nm (1.63 eV) and 1150 nm (1.07 eV), which indicated a partially filled intermediate band (IB) was formed in the forbidden band of Ti-CGS3 film by the spin-up t2 g states, and leaded to lower-energy photon absorption. Whereas, no additional sub-band edge responses were obviously observed in the optical absorption spectra of the Ti-CIS1 and Ti-CIS2 films, probably because of the Ti-CIS films inferior in quality or less doped-Ti content. The result analysis of the transient photocurrent response further verified that a larger number of carriers are generated in the Ti-CIS films, the surface recombination process rarely occurs. In general, the good photoelectric properties of Ti-CIS films prepared by magnetron sputtering expected to be a promising candidate IB material for the high-efficiency thin film solar cells.

Effect of Coulomb interaction on transport gap in ideal graphene nanoribbons

Quantum-mechanical calculations of electron transport in ideal graphene nanoribbons show that the transport gap that is predicted by noninteracting theories vanishes if the long-range Coulomb interaction between electrons is taken into account. This is a result of charge screening with the lowest subband edge being pinned to the chemical potential. However, the transport gap reappears if a ribbon is connected to wider leads, which is typically realized in an experimental setup that is based on lithographically patterned graphene ribbons. The gap is determined by scattering at the lead-to-ribbon interface, which can already be captured by the noninteracting theory.

Valley-resolved transport in zigzag graphene nanoribbon junctions

Applying the transfer matrix and Green’s function methods, we study the valley-resolved transport properties of zigzag graphene nanoribbon (ZGNR) junctions. The width of the left and right ZGNRs are N L and N R , and N L ≥ N R . The step/dip positions of the conductance G, the intravalley transmission coefficients (T KK and ), and the valley polarization efficiency correspond to the subband edges of the right/left ZGNR that are controlled by N R /N L . The intervalley transmission coefficients ( and ) exhibit peaks at most of the subband edge of the left and right ZGNRs. In the bulk gap of the right ZGNR, =0, and = ±1, the valley polarization is well preserved. As N R increases, the energy region for = ±1 decreases. When N L is fixed and N R decreases, G, T KK , and exhibit more and more dips, and the peaks of () become more and more high, especially when (N L − N R )/2 is odd. These characters are quite useful for manipulating the valley dependent transport properties of carriers in ZGNR junctions by modulating N L or N R , and our results are helpful to the design of valleytronics based on ZGNR junctions.

On the exciton-assisted radiative recombination via impurity trap levels in AlGaN deep ultraviolet light-emitting diodes

For decades, problems of parasitic emissions have been ubiquitously encountered in nearly all deep ultraviolet light-emitting diodes (DUV-LEDs). In this work, 450 nm parasitic peaks in 275 nm AlGaN DUV-LEDs have been studied in details. Upon careful comparisons and analyses on the electroluminescence and photoluminescence spectra at various injection levels and different temperatures, we have discovered a mechanism of exciton-assisted radiative recombination, namely, the reinforcement on radiative recombination via other impurity-trap levels (ITLs) by excitons that are generated in the midst of the band gap. For DUV-LED samples under investigation herein, a system of radiative ITLs within the band gap cannot be neglected. It includes two types of impurities located at two different energy levels, 3.80 eV and 2.75 eV, respectively. The former, establishing a sub-band edge, which behaves like an energy entrance to this system, contains a series of hydrogen-like excitons at a temperature lower than 100 K, which behaves like an energy entrance to this system. On the one hand, these excitons absorb carriers from band-edge and reduce the band-edge recombination. On the other hand they transfer the energy to lower impurity levels, enhancing the radiative recombination and giving rise to the 450 nm parasitic peak.

Large Zeeman Splitting in Out-of-Plane Magnetic Field in a Double-Layer Quantum Point Contact

In this study, we observe that the conductance of a quantum point contact on a GaAs/AlGaAs double quantum well depends significantly on the magnetic field perpendicular to the two-dimensional electron gas. In the presence of the magnetic field, the subband edge splitting due to the Zeeman energy reaches 0.09 meV at 0.16 T, thereby suggesting an enhanced g-factor. The estimated g-factor enhancement is 17.5 times that of the bare value. It is considered that a low electron density and high mobility makes it possible to reach a strong many-body interaction regime in which this type of strong enhancement in g-factor can be observed.

Consequence of Mn and Ni doping on structural, optical and magnetic characteristics of ZnO nanopowders: the Williamson–Hall method, the Kramers–Kronig approach and magnetic interactions

The crucial role of Ni and Mn dopants on the structural, optical, and magnetic characteristics of the zinc oxide (ZnO) nanopowders have been investigated in detail using XRD, diffuse reflectance spectroscopy (DRS), Kramers–Kronig (KK), and vibrating sample magnetometer (VSM). The structural analysis indicates that all patterns are mostly assigned to the hexagonal wurtzite structure. Also, XRD analysis shows that the Ni2+ and Mn2+ ions replace Zn2+ ions which causes lattice strain. Other than the Debye–Scherrer method, the Williamson–Hall method has also been applied to study the influence of strain in the calculation of the crystallite size. Based on DRS data, the band gap of the nanoparticles (NPs) is calculated by Tauc relation in two different ways. The band gap of the synthesized undoped ZnO NPs decreases from 3.25 to 3.23 and 3.15 eV through Ni and Mn doping, respectively. The band gap investigation shows a red-shift with Ni and Mn doping. The width of the localized states or sub-band gap absorption edge (often termed Urbach energy) of the samples was determined by the curves of Ln α versus photon energy hυ. The optical parameters such as complex refractive index, complex dielectric function, and absorption coefficient of the samples were evaluated by applying KK method. Investigating on magnetic properties showed a room temperature ferromagnetic (RTFM) behavior. Also, the interactions and reasons of the observed RTFM mechanisms are examined in detail.

Рассеяние электронов и дырок глубокими примесями в полупроводниковых гетероструктурах с квантовыми ямами

Electron and hole scattering by deep impurities in gallium arsenide heterostructures with two quantum wells under arbitrary doping profile was considered within the strongly localized potential approximation. The de-pendence of scattering rate on the carrier energy was shown to reproduce the step-like form of the density of states for size quantization subbands of the heterostructure accounting for the contribution of the overlap integral of the carrier wave functions. For hole subbands of negative effective mass the scattering rates at the subband edges have singularities common for one-dimensional systems.

Determination of g-factor in InAs two-dimensional electron system by capacitance spectroscopy

We determine the effective g-factor () of a two-dimensional electron gas (2DEG) using a new method based on capacitance spectroscopy. The capacitance-voltage profile of a 2DEG in an InAs/AlGaSb quantum well measured in an in-plane magnetic field shows a double-step feature that indicates the Zeeman splitting of the subband edge. The method allows for simultaneous and independent determination of and effective mass . Data suggests that the biaxial tensile strain in the InAs layer has considerable impact on both and . Our method provides a means to determine that is complementary to the commonly used coincidence technique.

Temperature Dependence of the Urbach Energy in Lead Iodide Perovskites.

To gain insight into the properties of photovoltaic and light-emitting materials, detailed information about their optical absorption spectra is essential. Here, we elucidate the temperature dependence of such spectra for methylammonium lead iodide (CH3NH3PbI3), with specific attention to its sub-band gap absorption edge (often termed Urbach energy). On the basis of these data, we first find clear further evidence for the universality of the correlation between the Urbach energy and open-circuit voltage losses of solar cells. Second, we find that for CH3NH3PbI3 the static, temperature-independent, contribution of the Urbach energy is 3.8 ± 0.7 meV, which is smaller than that of crystalline silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN), underlining the remarkable optoelectronic properties of perovskites.

Enhanced discrete wavelet packet sub-band frequency edge detection using Hilbert transform

The wavelet packet transform as a mathematical tool has found recent application in spectrum sensing. The result of this application has produced very promising results. Primarily, wavelets were designed for edge detection in images. Recently, cognitive radio literature have reported on wavelet application to detect sub-band frequency edges in wide band spectrum. In this paper, we present the combination of the Hilbert transform and the wavelet packet transform with the aim of enhancing the detection of the sub-band frequency edges of a wavelet-packet-decomposed signal. The simulation results show the effectiveness of this approach. The new scheme detected sub-band frequency edges of the wavelet-packet-decomposed signal much better than the wavelet packet transform without combination with the Hilbert transform.

Lattice relaxation and energy band modulation in twisted bilayer graphene

We theoretically study the lattice relaxation in the twisted bilayer graphene (TBG) and its effect on the electronic band structure. We develop an effective continuum theory to describe the lattice relaxation in general TBGs and obtain the optimized structure to minimize the total energy. At small rotation angles $< 2^{\circ}$, in particular, we find that the relaxed lattice drastically reduces the area of AA-stacking region, and form a triangular domain structure with alternating AB and BA stacking regions. We then investigate the effect of the domain formation on the electronic band structure. The most notable change from the non-relaxed model is that an energy gap up to 20meV opens at the superlattice subband edges on the electron and hole sides. We also find that the lattice relaxation significantly enhances the Fermi velocity, which was strongly suppressed in the non-relaxed model.

SAR image edge detection via sparse representation

In this paper, we propose a new synthetic aperture radar (SAR) image detection algorithm based on the de-noising algorithm via the sparse representation and a new morphology edge detector. Firstly, we apply the Shearlet transform to the SAR image to get the sparse representation of it. Then, morphological edge detector with direction is applied to directional sub-band coefficients of the Shearlet which are recovered by the iterative de-noising process. Finally, the completed SAR image edge is obtained by merging each sub-band edge using Dempster–Shafer evidence theory. By completely using the directional sub-bands of the Shearlet transform, the proposed algorithm overcomes the disadvantages of transform detection algorithms which are very unrobust to noise and can also generate inaccurate edges. The experimental results demonstrate the effectiveness and superiority of our proposed algorithm in terms of the edge positioning accuracy, integrity, and the number of false edge points.

Signatures of an annular Fermi sea

We report Shubnikov-de Haas oscillations measurements revealing experimental signatures of an annular Fermi sea that develops near the energy band edge of the excited subband of two-dimensional holes confined in a wide GaAs quantum well. As we increase the hole density, when the Fermi level reaches the excited subband edge, the low-field magnetoresistance traces show a sudden emergence of new oscillations at an unexpectedly large frequency whose value does $\textit{not}$ correspond to the (negligible) density of holes in the excited subband. There is also a sharp and significant increase in zero-field resistance near this onset of subband occupation. Guided by numerical energy dispersion calculations, we associate these observations with the unusual shape of the excited subband dispersion which results in a "ring of extrema" at finite wavevectors and an annular Fermi sea. Such a dispersion and Fermi sea have long been expected from energy band calculations in systems with strong spin-orbit interaction but their experimental signatures have been elusive.

Modulation of electrical mobility in Au ion irradiated titanium oxide with crystal field splitting

Electrical modulation of radio frequency (RF) sputtered TiO2−x films were investigated as a function of Au swift heavy ion irradiation dose at room temperature. The prepared TiO2−x films were irradiated with 130 MeV Au swift heavy ion in the range from 1 × 1011 to 5 × 1012 ions/cm2. As the Au ion irradiation dose increased up to 1 × 1012 ions/cm2, the electrical mobility of TiO2−x films were dramatically increased 3.07 × 102 cm2 V−1 s−1 without the change of carrier concentration. These changes in electrical properties of Au irradiated TiO2−x film, are related to the modification of electronic structure such as crystal field splitting of Ti 3d orbital hybridization and sub-band edge states below the conduction band as a function of Au swift heavy ion irradiation dose.

Rise and fall of shape resonances in thin films of BCS superconductors

The confinement of a superconductor in a thin film changes its Fermi-level density of states and is expected to change its critical temperature $T_c$. Previous calculations have reported large discontinuities of $T_c$ when the chemical potential coincides with a subband edge. By solving the BCS gap equation exactly, we show that such discontinuities are artifacts and that $T_c$ is a continuous function of the film thickness. We also find that $T_c$ is reduced in thin films compared with the bulk if the confinement potential is lower than a critical value, while for stronger confinement $T_c$ increases with decreasing film thickness, reaches a maximum, and eventually drops to zero. Our numerical results are supported by several exact solutions. We finally interpret experimental data for ultrathin lead thin films in terms of a thickness-dependent effective mass.

Effect of periodic potential on exciton states in semiconductor carbon nanotubes

We develop a theoretical background to treat exciton states in semiconductor single-walled carbon nanotubes (SWCNTs) in the presence of a periodic potential induced by a surface acoustic wave (SAW) propagating along SWCNT. The formalism accounts for the electronic band splitting into the Floquet sub-bands induced by the Bragg scattering on the SAW potential. Optical transitions between the Floquet states and correlated electron–hole pairs (excitons) are numerically examined. Formation of new van Hove singularities within the edges of Floquet sub-bands and associated transfer of the exciton oscillator strengths resulting in the photoluminescence quenching are predicted. The simulations demonstrate the exciton energy red Stark shift and reduction in the exciton binding energy. Comparison of our results with reported theoretical and experimental studies is provided.

A novel synthetic bandwidth method based on BP imaging for stepped-frequency SAR

ABSTRACTA novel imaging first and synthesizing later (IFSL) synthetic bandwidth method for stepped-frequency synthetic aperture radar is proposed in this article to compensate the space-variant range errors between the pulses in a cluster. The sub-band images are separately obtained through the time-domain back projection (BP) imaging algorithm. The space-variant range errors are corrected through concise multiplication and the continuity of spatial spectrum phase at the edges of adjacent sub-bands is well maintained due to the accurate imaging ability of BP algorithm. The derivation of analytical BP imaging formula is very difficult because of the complicated integrand. This difficulty is overcome through some rational approximations. By using this new method, the space-variant range error compensation is greatly simplified and the synthesized image quality is considerably enhanced compared with the conventional IFSL methods in which frequency-domain imaging algorithms are used. The simulation results validate the advantages.

Asymmetric valley-resolved beam splitting and incident modes in slanted graphene junctions

Electron injection into a graphene sheet through a slanted armchair graphene nanoribbon (AGNR) is investigated. An incident mode, or subband, in the AGNR is valley-unpolarized. Our attention is on the valley-resolved nature of the injected electron beams and its connection to the incident mode. It is known for a normal injection that an incident mode will split symmetrically into two valley-resolved beams of equal intensity. We show, in contrast, that slanted injections result in asymmetric valley-resolved beam splitting. The most asymmetric beam splitting cases, when one of the valley-resolved beams has basically disappeared, are found and the condition derived. This is shown not due to trigonal warping because it holds even in the low incident energy regime, as long as collimation allows. These most asymmetric beam splitting cases occur at energies within an energy interval near and include the subband edge of an incident mode. The physical picture is best illustrated by a projection of the slanted AGNR subband states onto that of the 2D graphene sheet. It follows that the disappearing of a valley-resolved beam coincides with the situation that the group velocities of the projected states in the corresponding valley are in backward directions.