HH1 Subbands Research Articles

Strain relaxation effects on TE-polarized light emission and in-plane polarization ratio in c-plane ultraviolet AlGaN/AlN quantum well structures

Abstract Strain relaxation effects on TE-polarized light emission and in-plane polarization ratio in c-plane ultraviolet (UV) Al x Ga 1 − x N/AlN quantum well (QW) structures were investigated as a function of Al content. In the case of the strain relaxation along y (or x )-direction , the x (or y )-polarized light emission is shown to be larger than the y (or x -)polarized light emission, which can be explained by the topmost subband characteristics change from the HH1 subband to the admixture of HH1 and LH1 subbands by an anisotropic strain. In the case of ϵ y y = 0 . 4 ϵ y y o , the polarization ratio has values from 0.4 to 0.7 in an investigated range of Al content. In particular, the QW structure with the strain relaxation along x or y direction shows that the peak intensity is much larger than that for the convention AlGaN/AlN QW structure in a range of x 0 . 6 .

Dual wavelength operation of the GaSb-based Y-branch distributed Bragg reflector lasers near 2.1 μm

The GaSb-based diode lasers operating simultaneously at two wavelengths near 2.1 μm have been designed and fabricated. The Y-branch devices used the 6th order distributed Bragg reflectors (DBR) to provide spectrally selective feedback. The laser active region contained two asymmetric quantum wells with allowed optical transitions between two lowest electron (e1 and e2) and top hole (hh1) subbands. The laser emission lines loosely matched the e1-hh1 and e2-hh1 optical transitions with separation corresponding to ∼3.1 THz. The 10 μm wide deeply etched straight ridge DBR lasers demonstrated stable single mode operation with more than 50 mW of output power.

Copy-Move Forgery Detection Using DyWT

In this paper, we proposed a passive method for copy-move region duplication detection using dyadic wavelet transform (DyWT). DyWT is better than discrete wavelet transform (DWT) for data analysis as it is shift invariant. Initially we decompose the input image into approximation (LL1) and detail (HH1) sub-bands. Then LL1 and HH1 sub-bands are divided into overlapping sub blocks and find the similarity between the blocks. In LL1 sub-band the copied and moved blocks have high similarity rate than the HH1 sub-band, this is just because, there is noise inconsistency in the moved blocks. Then we sort the LL1 sub-band blocks pair based on high similarity and in HH1 blocks are sorted based on high dissimilarity. Then we apply threshold to get the copied moved blocks. Here we also applied some post processing operations to check the robustness of our method and we get the satisfactory results to validate the copy move forgery detection.

IMAGE FORGERY DETECTION USING DYADIC WAVELET TRANSFORM

In this paper, we propose a blind copy move image forgery detection method using dyadic wavelet transform (DyWT). DyWT is shift invariant and therefore more suitable than discrete wavelet transform (DWT) for data analysis. First we decompose the input image into approximation (LL1) and detail (HH1) subbands. Then we divide LL1 and HH1 subbands into overlapping blocks and measure the similarity between blocks. The key idea is that the similarity between the copied and moved blocks from the LL1 subband should be high, while the one from the HH1 subband should be low due to noise inconsistency in the moved block. We sort pairs of blocks based on high similarity using the LL1 subband and high dissimilarity using the HH1 subband. Using thresholding, we obtain matched pairs from the sorted list as copied and moved blocks. Experimental results show the effectiveness of the proposed method over competitive methods using DWT and the LL1 or HH1 subbands only.

Manifestation of a semimetallic state in cyclotron resonance in low-symmetry HgTe-based quantum wells

Cyclotron-resonance measurements in 21-nm-thick HgTe/CdHgTe quantum wells of different crystallographic orientations have been performed. It has been found that, in contrast to the structures with the (001) orientation of the quantum-well plane, (013)-oriented quantum wells are semimetallic and their absorption spectra exhibit both electron and hole cyclotron-resonance lines. The simultaneous presence of the two types of charge carriers originates from an overlap between the upper heavy-hole quantum-confinement subbands hh1 and hh2. This overlap is caused by the strong interaction of these subbands with the Dyakonov-Khaetskii interface state. Calculations carried out using the eight-band kp-Hamiltonian indicate that, for known values of the band-structure parameters, the overlap between hh2 and hh1 subbands does not occur; this result is in agreement with the cyclotron-resonance data for (001)-oriented structures. The enhanced interaction between heavy-hole and interface states owing to the existence of steps at low-symmetry heterointerfaces may be the mechanism responsible for the appearance of an overlap between subbands in HgTe quantum wells with orientation different from (001).

Passive copy move image forgery detection using undecimated dyadic wavelet transform

In this paper, a blind copy move image forgery detection method using undecimated dyadic wavelet transform (DyWT) is proposed. DyWT is shift invariant and therefore more suitable than discrete wavelet transform (DWT) for data analysis. First, the input image is decomposed into approximation (LL1) and detail (HH1) subbands. Then the LL1 and HH1 subbands are divided into overlapping blocks and the similarity between blocks is calculated. The key idea is that the similarity between the copied and moved blocks from the LL1 subband should be high, while that from the HH1 subband should be low due to noise inconsistency in the moved block. Therefore, pairs of blocks are sorted based on high similarity using the LL1 subband and high dissimilarity using the HH1 subband. Using thresholding, matched pairs are obtained from the sorted list as copied and moved blocks. Experimental results show the effectiveness of the proposed method over competitive methods using DWT and the LL1 or HH1 subbands only.

Effect of the (101~0) crystal orientation on the optical gain of wurtzite GaN-AlGaN quantum-well lasers

We analyze the valence subband structures of (101~0)oriented wurtzite (WZ) GaN-AlGaN quantum wells (QWs) using the multiband effective-mass theory and calculate the optical gain taking into account the subband structure modification due to the crystal orientation effect and the pseudomorphic strain which is anisotropic in the QW plane. We show that, for the (101~0) GaN/AlGaN QW, the two topmost subbands, Y'1 and X'1, are more widely separated than the HH1 and LH1 subbands in the (0001) GaN-AlGaN QW. The in-plane energy dispersion of the (101~0) QW also becomes anisotropic, giving a reduced band-edge density-of-states in comparison with the (0001) QW. Moreover, states constituting the topmost valence subband at the /spl Gamma/ point and along k-/sub x/, are predominantly |Y'>-like. A combination of the reduced band-edge density-of-states and the existence of the preferred symmetry at the valence band maximum contributes to an improvement of the y'-polarized TE optical gain. A comparison of the QWs of both orientations reveals that the (101~0) QW is capable of achieving lower transparency current densities. Thus, the (101~0) QW could be useful in improving the threshold performance of WZ GaN-based QW lasers.

Uniaxial strain effect on the electronic and optical properties of wurtzite GaN-AlGaN quantum-well lasers

The valence subband structures of uniaxial-strained wurtzite (WZ) GaN-AlGaN quantum wells (QW's) are calculated using multiband effective-mass theory. The optical gain is investigated using a numerical approach in which we account for the subband structure modification and mixing due to the anisotropic strain in the QW plane. We show that the mixing of the HH and LH bases in the uniaxial-strained (0001) GaN-AlGaN QW decouples |X> and |Y> at the /spl Gamma/ point, giving two topmost subbands, Y1 and X1, which can be more widely separated than the HH1 and LH1 subbands in the biaxial-strained (0001) GaN-AlGaN QW. We resolve the states of the subband dispersion in terms of the |X>, |Y>, and |Z> bases, and show the compositional variation as a function of the in-plane wavevector. Under uniaxial strain, it is possible to exploit the existence of the preferred symmetry at the valence band maximum and the reduced band-edge density-of-states due to the anisotropic in-plane energy dispersion to achieve lower transparency carrier and current densities and higher differential gain in comparison with a pseudomorphic biaxial-strained QW. We show that, for a QW laser structure with the optical cavity along the x axis, uniaxial compressive strain in the y direction shows greater improvement than the uniaxial tensile strain in the x direction of the same magnitude. Thus, a suitable uniaxial strain could be used to improve the threshold performance of WZ GaN-based QW lasers.

Fine structures in the electroabsorption spectra of GaAs quantum wells.

We present theoretical calculations of detailed electroabsorption spectra for GaAs quantum wells, using a k-space sampling method. The valence-band mixing, excitonic effect, and Fano resonances due to the interaction of discrete exciton states with continua are all properly taken into account. Our calculation offers a direct comparison between theory and experiment. By including the coupling between states derived from the HH1 and LH1 subbands, we can account for the interesting anticrossing behavior between the 2p excited state of the HH1-CB1 exciton and ground state of the LH1-CB1 exciton, as observed by Vin\ifmmode \tilde{}\else \~{}\fi{}a et al. The agreement between experiment and our theory is quite satisfactory.

Strain effects on GaxIn1−xAs/InP single quantum wells grown by organometallic vapor-phase epitaxy with 0≤x≤1

Single-quantum-well structures were grown by atmospheric pressure organometallic vapor-phase epitaxy, with GaxIn1−xAs layers (0≤x≤1) coherently strained to match the lattice parameter of the InP barrier layers in the (100) growth plane. The strain effects on the band lineups were analyzed using the ‘‘model solid’’ theory of Van de Walle and Martin. The hydrostatic strain component for alloys with x≊1 is shown to be sufficient to marginally convert the type-II lineups for the unstrained case to type I. The band lineups remain type I for x≊0. Considering the effect of strain, the ‖ (3)/(2) , (1)/(2) 〉 valence subband becomes a slowly varying function of x. Band offsets are predicted over the entire alloy composition and compared with the reported data. The photoluminescence (10 K) peak energies for the 100-Å GaxIn1−xAs/InP single quantum wells compare quite favorably with the calculated strained band gap versus x. For nominal monolayer quantum wells, the peak energies are slightly above 1.1 eV over the entire alloy range. It is shown that for x>0.47, the LH1 and HH1 subbands cross at a smaller well width as x increases. The ground-state exciton is light hole-like for larger values of x and Lz, and heavy hole-like for smaller x and Lz.