Hole Exciton Research Articles

Spin-based all-optical quantum computation with quantum dots: Understanding and suppressing decoherence

We present an all-optical implementation of quantum computation using semiconductor quantum dots. Quantum memory is represented by the spin of an excess electron stored in each dot. Two-qubit gates are realized by switching on trion-trion interactions between different dots. State selectivity is achieved via conditional laser excitation exploiting Pauli exclusion principle. Read-out is performed via a quantum-jump technique. We analyze the effect on our scheme's performance of the main imperfections present in real quantum dots: exciton decay, hole mixing and phonon decoherence. We introduce an adiabatic gate procedure that allows one to circumvent these effects, and evaluate quantitatively its fidelity.

Many-particle effects in excitonic transitions in type-II Ge/Si quantum dots

The effect of quantum dot charging on the interband excitonic transitions has been studied in type-II Ge/Si heterostructures containing pyramidal Ge nanocrystals using electron-filling modulation absorption spectroscopy. The ground state absorption is found to be blueshifted when exciton–hole and exciton–exciton complexes are formed. For a positively charged dot, we argue that this is the consequence of dominance of the hole–hole interaction compared to the electron–hole interaction due to the spatial separation of the electron and hole. The results are explained by effects of the electron and hole localization and by electron wave function leakage in the dots. The electronic structure of spatially indirect excitons is calculated self-consistently in the effective-mass approximation for pyramidal-shaped Ge/Si quantum dots. The calculations show that the electron of an indirect exciton resides in the Si near the Ge pyramid apex due to maximum strain in this region, while the hole is confined close to the pyramid base. When two excitons are excited in the dot, the electrons are found to be spatially separated and have different single-particle quantization energies. We argue that this is the reason why the biexciton absorption is blueshifted as compared to a single exciton. A satisfying agreement is found between theoretical and experimental data.

Compressive strain induced heavy hole and light hole splitting of Zn 1− xCd xSe epilayers grown by molecular beam epitaxy

Compressive strain induced heavy hole and light hole splitting of the Zn 1− x Cd x Se epilayers grown by molecular beam epitaxy was studied by the reflectivity spectra. Heavy hole exciton (HHX) and light hole exciton (LHX) splittings for the ZnSe, Zn 0.999Cd 0.001Se, Zn 0.987Cd 0.013Se, and Zn 0.974Cd 0.026Se epilayers are 12.6, 14.0, 17.2 and 20.4 meV, respectively. HHX and LHX energy splitting depends linearly on the Cd composition. No strain relaxation was observed in these thin (about 50 nm) Zn 1− x Cd x Se epilayers. A simplified dielectric model was used to fit the reflectivity spectra. At 10 K, the obtained oscillator strength and broadening parameter are about 2.7×10 −2 eV 2 and 2.6 meV for HHX and 1.2×10 −2 eV 2 and 2.6 meV for LHX. Temperature dependence of HHX and LHX transition energy was fitted by Varshni’s and O’Donnell’s model. No clear temperature dependence was found for the HHX and LHX splitting.

Optical investigation of high-mobility dilute two-dimensional hole gases in GaAs (3 1 1)A quantum structures

We have investigated the photoluminescence (PL) and inelastic light-scattering spectra of dilute two-dimensional (2D) hole gases of ultra-high mobility. The samples were GaAs–Al x Ga 1− x As (3 1 1) A quantum structures, modulation-doped with Si. We find these samples not to deplete fully on illumination. The PL displays both excitonic and 2D hole gas transitions. We observe inelastic light-scattering, resonating on these intersubband transitions. Our measured intersubband excitation energies are in agreement with current calculations. These resonance enhancements are consistent with an electron-like dispersion of the first-excited hole subband at zero magnetic field.

Donor and Donor Bound Exciton Spectroscopy in Wurtzite GaN Heterostructures

Neutral donor bound excitons (I 2 ) and donor related electronic Raman scattering (ERS) in wurtzite GaN epilayers deposited onto Si, 6H-SiC, Al 2 O 3 and GaN substrates are studied at low temperature by high-resolution selective photoluminescence spectroscopy (SPL). Due to the presence of a large strain distribution in the heterostructures, the observation of the I 2 excited states under selective laser excitation is possible from tensile to compressive strain along the [0001] direction. It is shown that the I 2 spectra strongly depend on which valence band (A or B) the bound exciton hole belongs to. Resonant ERS and resonantly excited two-electron spectra reveal that two main different residual donor species are present both in MBE and MOCVD epitaxial samples. The donor binding energies are shown to vary noticeably with the biaxial strain field.

Donor and Donor Bound Exciton Spectroscopy in Wurtzite GaN Heterostructures

Neutral donor bound excitons (I2) and donor related electronic Raman scattering (ERS) in wurtzite GaN epilayers deposited onto Si, 6H-SiC, Al2O3 and GaN substrates are studied at low temperature by high-resolution selective photoluminescence spectroscopy (SPL). Due to the presence of a large strain distribution in the heterostructures, the observation of the I2 excited states under selective laser excitation is possible from tensile to compressive strain along the [0001] direction. It is shown that the I2 spectra strongly depend on which valence band (A or B) the bound exciton hole belongs to. Resonant ERS and resonantly excited two-electron spectra reveal that two main different residual donor species are present both in MBE and MOCVD epitaxial samples. The donor binding energies are shown to vary noticeably with the biaxial strain field.

An Alternative Model for Photochromism of Glasses: Reversible Injection of Carriers from a Microcrystal and Its Surface States into Point Defects of Glass

The radiation-induced absorption spectra of photochromic homogeneous silicate glasses and heterogeneous borosilicate glasses containing AgCl, AgBr, or CuCl microcrystals are analyzed. The inference is made that these spectra contain the bands of traditional radiation-induced color centers (RICCs) in glasses and the bands of halide defects in microcrystals. The band model of the photochromic process in the “microcrystal–surface states–glass” system is proposed on the basis of the performed analysis with due regard for burning of the exciton hole in the induced spectrum of CuCl-containing glass. This model makes it possible to explain a large number of experimental data.

Spin relaxation inGaAs/AlxGa1−xAsquantum wells

Spin dynamics of photoexcited carriers in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells have been investigated in a wafer containing twelve different single quantum wells, allowing full investigation of well-width and temperature dependences with minimal accidental variations due to growth conditions. The behavior at low temperatures is dominated by excitonic effects, confirming results described in detail by others. Between 50 and 90 K there is a transition from excitonic to free-carrier-dominated behavior; both the temperature and time scale of the transition are in excellent agreement with a theoretical model for exciton dissociation. Above 90 K we find two-component spin decays consisting of an unresolved component (faster than 2 ps) associated with exciton dissociation and hole spin-relaxation and a longer-lived component that yields the electron spin-relaxation time. In the free-carrier regime, the electron spin-relaxation rate in wide wells follows that for bulk GaAs, which varies approximately as ${T}^{2}.$ For narrow wells the rate is approximately independent of temperature and varies quadratically with confinement energy. This behavior is consistent with dominance of the D'yakonov-Perel mechanism of electron-spin relaxation and the expected behavior of the electron mobility. The data show evidence of the influence of electron scattering by interface roughness.

Theoretical interpretation of the experimental electronic structure of lens-shaped self-assembled InAs/GaAs quantum dots

We adopt an atomistic pseudopotential description of the electronic structure of self-assembled, lens shaped InAs quantum dots within the ``linear combination of bulk bands'' method. We present a detailed comparison with experiment, including quantites such as the single particle electron and hole energy level spacings, the excitonic band gap, the electron-electron, hole-hole and electron hole Coulomb energies and the optical polarization anisotropy. We find a generally good agreement, which is improved even further for a dot composition where some Ga has diffused into the dots.

Spin coherence and formation dynamics of charged excitons inCdTe/Cd1−x−yMgxZnyTequantum wells

We report on a study of the spin coherence and on the formation dynamics of charged excitons in a p-doped ${\mathrm{C}\mathrm{d}\mathrm{T}\mathrm{e}/\mathrm{C}\mathrm{d}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Mg}}_{x}{\mathrm{Zn}}_{y}\mathrm{Te}$ quantum well by time-resolved photoluminescence under strictly resonant excitation of either the neutral or the charged exciton transition. The analysis of the decay of the charged exciton photoluminescence polarization and of the oscillation of this polarization when a transverse magnetic field is applied allows us to conclude that the formation of a charged exciton via an exciton state does not affect either the electron spin orientation or its coherence. The radiative lifetime of the charged exciton is directly measured and is found to be \ensuremath{\sim}60 ps. Its formation time is determined from the experiments via a detailed model and is found to be \ensuremath{\sim}65 ps. We also obtain information on the excitonic spin-flip time (12 ps) and the single-carrier spin-flip times within the exciton (electron, 60 ps and hole, 20 ps).

High-efficiency transparent organic light-emitting devices

We demonstrate organic light-emitting devices (OLEDs) employing highly transparent cathodes comprised of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline (BCP) and an ultrathin film of Li capped with radio-frequency magnetron-sputtered indium–tin–oxide. The cathodes are incorporated onto a conventional bilayer small-molecule OLED. The operating voltages and the total device external quantum efficiencies emitted from the top and substrate surfaces (1.0±0.05)% are comparable to the best conventional undoped OLEDs employing thick metallic cathodes. The device characteristics are independent of the position of Li within the compound cathode, suggesting that Li readily diffuses through BCP to enhance electron injection. An increase of a factor ∼3.5 in the external quantum efficiency is observed compared to devices containing no Li. These results suggest that Li donates electrons to the BCP, increasing its conductivity to the point that band bending occurs to aid in the injection of charge.

Stimulated scattering of excitons into microcavity polaritons

We have observed the bosonic enhancement of the elastic exciton-exciton scattering rate in a semiconductor microcavity. The bosonic signature is in the enhancement of the scattering rate that is proportional to the final state occupation number. This gain process for exciton-polaritons is distinctly different from that in a conventional photon laser, because of the fermionic nature of the exciton's constituents, the electron and hole. This distinction becomes prominent at large final state occupation numbers, where we observe a saturation and reduction in the gain. We discuss the origins of this saturation behavior.

Spectrum of indirect magnetoexcitons in coupled quantum wells

The indirect Mott exciton (spatially-separated electron and hole) in coupled quantum wells in crossed electric and magnetic fields is discussed. The exciton spectrum is calculated for the case where the distance between the quantum wells of the electron and hole is larger than the exciton Bohr radius. The magnetoexciton creation probability is calculated and its dependence on the electric field is found. The absorption of electromagnetic radiation between the indirect magnetoexciton levels in coupled quantum wells is discussed.

Intermediately bound excitons

A possibility of intermediately bound excitons in semiconductors doped with transition metal impurities is considered. These sorts of exciton can appear in hexagonal (wurzite-type) semiconductors due to strong hybridization of the conduction band states and the impurity d states. In tetrahedral (zinc blende) semiconductors this hybridization is strongly suppressed due to a symmetry consideration. It is shown that the exciton hole in ZnS:Ni (zinc blende type) is bound by the Coulomb field of the exciton electron and may be considered within the framework of the hydrogen-like model. As for CdS:Ni (wurzite type) the orthogonalization central cell pseudopotential appears to be the leading attracting potential for the hole. These differences in the binding mechanisms account for striking differences measured experimentally in the structure of the exciton spectra and values of the g-factors of these two presumably similar systems.

Magnetic Circular Dichroic Spectra of CuCl Thin Films

Magnetic circular dichroic (MCD) spectra of 80-A to 1700-A thick CuCl films were measured around the Z 3 exciton region up to 6 tesla (T) at 10 K. The MCD spectra with quantized states of the translational motion of the Z 3 exciton are well explained by Zeeman splitting. The g values of these quantized states are deduced from the best fitting between the MCD spectra and the derivative of transmission spectra. CuCl films from 80 to 290 A thick showed no change in the g value with respect to the film thickness and the quantum number n . The g values of 700- and 1700-A thick CuCl films, which were expected to exhibit bulk characteristics, were found to be identical to those of thinner films. These results indicate that the quantum size effect does not contribute to the relative motion of electrons and holes in excitons for a confinement size exceeding 80 A.

Shake-up intersubband transitions observed in GaAs/AlGaAs quantum wells

Shake-up transitions involving QW hole subbands have been observed as satellites in selective photoluminescence spectra of undoped GaAs/AlGaAs QWs. These shake-up transitions are explained in terms of an interaction between localized exciton and valence-band hole states attached to the QW subbands, in which holes are shaken up from the n=1 heavy hole subband to higher subbands, either the n=1 light hole subband or the n=2 heavy hole subband. The required localization is due to the interface roughness; thus these new transitions are of intrinsic origin. From the observation of the intersubband shake-up processes we derive direct information about the hole inter-subband energies. Furthermore, the satellite intensity is strikingly enhanced in the presence of a magnetic field due to an increasing exciton localization related to the compression of its wave function in the field. The exciton wave function compression continues until its radius in the plane of the well is comparable with the radius of the "flat island" characterized by constant QW width. Accordingly, from the magnetic field dependence of the shake-up satellite intensity we can roughly estimate the size of the "flat islands" and consequently probe the interface roughness.

Hubbard model for intermediate-dimensional excitons.

A Hubbard model is solved exactly to characterize confined, intermediate-dimensional excitons for the full range of electron and hole hopping and interaction strengths. Finite systems with periodic boundary conditions model unconfined excitons. Finite systems with terminated ends model confined excitons. Exciton energies, oscillator strengths, and electron and hole distributions are determined. Oscillator strengths and electron-hole distributions of confined intermediate-dimensional excitons exhibit anomalous, nonmonotonic, nonuniversal dependences on the electron-hole interaction strength and hopping that are counter to the conventional behavior for quantum-confined excitons and free excitons. Perturbation theory is used to clarify the weak and large interaction limits. In the large-interaction limit, an exciton dead layer occurs near the boundary of the system because electron-hole correlation is suppressed near the boundary. Second-order perturbation theory determines the surface potential barrier caused by the suppression of pair correlation near the surface and determines the hopping rate for tunneling into this barrier. In the weak-interaction limit, on-site correlation of a confined electron-hole pair is suppressed by asymmetry in the electron and hole hopping. For large asymmetry in the electron and hole hopping, the oscillation strength of the confined intermediate-dimensional exciton can be less than the oscillator strength of an uncorrelated, noninteracting pair.

Optical Studies of free and Acceptor-Bound Excitons in GaAs/AlGaAs Symmetric Coupled Double Quantum Well Structures

Discrete peaks, due to the excited 2S state of the n=l symmetric heavy hole (hh) exciton, are observed in the photoluminescence excitation spectra of GaAs/AlGaAs coupled double quantum well (CDQW) samples. The observations of the 2S state allow a precise determination of 1S-2S splitting, and the experimental results show a good agreement with a simple calculation which is based on a model of fractional-dimension of the exciton state. The obtained values for the 1S-2S splitting are significantly reduced compared with the corresponding single (or multiple) quantum wells (QW). Central acceptors confined in CDQWs have also been studied. Due to built in electric fields, interwell and intrawell excitons bound to neutral acceptors are simultaneously observed in weakly coupled doped double QWs. The diamagnetic shift and binding energy of the bound excitons are similar to results of the single QW’s, while the diamagnetic shift of the free exciton differs from single QW’s.

Spectral Sensitization and Supersensitization

AbstractA theory of spectral sensitization and supersensitization is presented. According to this photoaggregation theory, over the toe and linear segments of the characteristic curve, dye and supersensitizer excitations decay by Auger processes at Ag2, Ag20, Ag2S or (Ag,Au)S sensitizing donor centres, the energy released by combination of the donor electron and the exciton hole is carried away as kinetic energy by the exciton electron in a state of the conduction band. Back-electron transfer is prevented by the rapid dissociation of the remaining Ag2+, Ag20+, AgS+ or (Ag,Au)S+ centre with passage of an /4g+ or Au+ ion into an interstitial position. Over the wavelength range between 400 nm (3.1 eV) and 1,200 nm (1 eV), the latent image is formed by the photoaggregation of Ag and Au atoms equivalent to sensitizing donor molecules to give Agf, AuAgf, Att2Agf or larger acceptor centres. The silver halide crystal does not undergo photolysis but acts as a carrier for the sensitizing molecules and as a transpor...

Near-band-gap photoluminescence from pseudomorphic Si1−xGex single layers on silicon

The systematic study of band-edge luminescence in pseudomorphic Si/Si1−xGex/Si double-heterostructure layers is reported for a wide composition range, 0.12<x<0.24, for the first time. An analytical expression for the exciton energy gap at 4.2 K valid for x<0.24 is derived from the no-phonon line energies: ESX(x) = 1.155−0.874x+0.376x2 eV. Addition of an expression for the exciton binding energy provides an approximation for the energy difference between the alloy valence band and the lowest conduction-band edge at low temperature. An exciton upshift of 16.9 meV due to quantum confinement is observed in a 6.3-nm Si0.83Ge0.17 alloy well. This is consistent with either type-I or type-II band alignment for which the conduction-band offset has a magnitude ‖ΔEc‖ ≤ 10 meV. The excitonic hole is closely confined in the alloy but the spectra suggest that the electron density in the silicon barriers is increased for the thin layer.