Charged excitons in GaAs quantum wells

Il Nuovo Cimento D - We implement optical spectroscopy to study charged excitons (trions) in modulation-doped GaAs/AlGaAs quantum wells. We observe for the first time several new trions: the...     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.     

用B样条技术研究半导体微晶中激子的量子受限效应

在有效质量近似（EMA）下，采用B样条技术和变分方法，分别研究较大CdTe球量子点(25—35nm)和较小CdS球量子点(025—35nm)中激子的量子受限效应，计算出CdTe和CdS球量子点中受限激子的基态能和束缚能随参数的变化规律，比较两种计算结果得到：（1）较大量子点中受限激子的基态能和束缚能对量子点边界和量子点外部介质的介电常数不敏感，但较小量子点中受限激子的基态能和束缚能对量子点边界和量子点外部介质的介电常数比较敏感.（2）在较强受限区域，大量子点与小量子点的激子基态能和束缚能的变化规律完全不同.（3）B样条技术对于研究这种具有边界的束缚态系统是很精确的方法，这种方法特别适合用于多层结构量子点系统的精确计算. 关键词： B样条技术 量子受限效应 有效质量近似     

Rabi oscillations of excitons in single quantum dots

Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.     

Magnetic excitons in near-surface quantum wells: Experiment and theory

We have measured the photoluminescence spectra and photoluminescence excitation spectra of magnetic excitons in InGaAs/GaAs near-surface quantum wells in a magnetic field. We have quantitatively investigated the effect of dielectric enhancement of excitons in quantum wells brought about by decreasing the thickness of the barrier layer, both in a magnetic field and without. Fiz. Tverd. Tela (St. Petersburg) 40, 806–808 (May 1998)     

Spontaneous light emission from fractional-dimensional excitons

We apply the concept of fractional-dimensional excitons to study the process of light emission in quantum confined systems. We focus on a single parameter , known as the degree of dimensionality and which is related to the exciton coherence volume. We compute rates of light emission due to free excitons in GaAs/Al_xGa_1-xAs quantum wells as functions of and link them to experimental observations. The rates are compared with those of quantum well excitons embedded in a microcavity.     

Anomalous behavior of excitons at light holes in strained (In, Ga)As/GaAs heterostructures

Additional localization of holes due to Coulomb attraction to the electron located in a quantum well is important for light-hole excitons in the heterostructure (In, Ga)As/GaAs. The fine structure of the optical and magneto-optical spectra of these quantum wells is examined in detail with the formation of a “Coulomb well” and deformations taken into account. Fiz. Tverd. Tela (St. Petersburg) 40, 797–799 (May 1998)     

Quantum optical studies on individual acceptor bound excitons in a semiconductor

We demonstrate the generation of triggered single photons at a predetermined and well defined energy using the radiative recombination of single nitrogen-bound excitons in a semiconductor. The nitrogen atoms are embedded in a ZnSe quantum well structure and were excited by nonresonant optical pumping (82 MHz) at low temperature (4 K). We find resolution-limited photoluminescence lines (280 micro eV) which display photon antibunching under continuous optical pumping. Our results also suggest that single nitrogen-bound excitons are well suited for cavity quantum electrodynamics experiments.     

Relaxation of hot excitons in CdZnSe/ZnSe quantum wells and quantum dots

The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled by the interaction between excitons and acoustic phonons, causes a distinct linewidth broadening with increasing delay time in quantum wells. In contrast, this process is found to be strongly suppressed in quantum dots.     

Size quantization in planar graphene-based heterostructures: Pseudospin splitting,interface states,and excitons

A planar quantum-well device made of a gapless graphene nanoribbon with edges in contact with gapped graphene sheets is examined. The size-quantization spectrum of charge carriers in an asymmetric quantum well is shown to exhibit a pseudospin splitting. Interface states of a new type arise from the crossing of dispersion curves of gapless and gapped graphene materials. The exciton spectrum is calculated for a planar graphene quantum well. The effect of an external electric field on the exciton spectrum is analyzed.     

Collective properties of excitons in the presence of a two-dimensional electron gas

We have studied the collective properties of two-dimensional (2D) excitons immersed within a quantum well which contains 2D excitons and a two-dimensional electron gas (2DEG). We have also analyzed the excitations for a system of 2D dipole excitons with spatially separated electrons and holes in a pair of quantum wells (CQWs) when one of the wells contains a 2DEG. Calculations of the superfluid density and the Kosterlitz–Thouless (K–T) phase transition temperature for the 2DEG-exciton system in a quantum well have shown that the K–T transition temperature increase with increasing exciton density and that it might be possible to have fast long-range transport of excitons. The superfluid density and the K–T transition temperature for dipole excitons in CQWs in the presence of a 2DEG in one of the wells increases with increasing inter-well separation.     

Binding energies of negatively charged excitons in quantum disks

The Hamiltonian of a negatively charged exciton X⁻ (trion) in a quantum disk with parabolic confinement has been diagonalized to obtain the binding eigenenergy values of the L1 states as a function of the electron-to-hole effective mass ratio and the disk radius. It is found that a negatively charged exciton X⁻ in a quantum disk may have the second bound state with orbital angular momentum L=1 and the triplet state of the two bound electrons.     

Neutral and charged excitons in a quantum tube

The optical transition energies of neutral and charged excitons in a quantum tube are calculated as a function of the Aharonov-Bohm magnetic flux Φ. The oscillation amplitude of the ground state energy of the electron-hole relative motion is shown to be larger in a quantum tube than a quantum ring with strong confinement in the axis direction. We find a double maxima structure in the optical transition energy for a quantum tube with radius R = 0.5 in units of the effective Bohr radius because of the difference in the Φ dependencies between the single electron energy and the relative-motion energy of a charged exciton state.     

Scattering of excitons by excitons in semiconducting quantum well structures

We have theoretically investigated the scattering of excitons by excitons in a two-dimensional semiconducting quantum well system. The scattering cross sections have been calculated using the Born approximation for both the elastic and inelastic scattering of the excitons by excitons. The threshold for inelastic scattering is increased over the value in a bulk semiconductor because of the enhancement of the exciton binding energy by its confinement. The behavior of the scattering cross section as a function of the energy of relative motion of the excitons is different than in the bulk and the cross section is a more sensitive function of the ratio of the electron and hole masses than in the bulk.     

Polaron effects on excitons in parabolic quantum wells: fractional-dimension variational approach

Polaron effects on excitons in parabolic quantum wells are studied theoretically by using a variational approach with the so-called fractional dimension model. The numerical results for the exciton binding energies and longitudinal-optical phonon contributions in GaAs/Al_0.3Ga_0.7As parabolic quantum well structures are obtained as functions of the well width. It is shown that the exciton binding energies are obviously reduced by the electron (hole)-phonon interaction and the polaron effects are un-negligible. The results demonstrate that the fractional-dimension variational theory is effectual in the investigations of excitonic polaron problems in parabolic quantum wells.     

Detecting spatially localized excitons in InGaN quantum well structures with a micro-photoluminescence technique

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photoluminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 μm spatial resolution. A sharp PL line (linewidth of <0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions.     

Coherent spin precession of electrons and excitons in charge tunable InP quantum dots

Coherent spin precession of electrons and excitons is observed in charge tunable InP quantum dots under the transverse magnetic field by means of time-resolved Kerr rotation. In a quantum dot doped by one electron, spin precession of the doped electron in the quantum dot starts out of phase with spin precession of the doped electrons in a GaAs substrate just after a trion is formed and persists for more than 2 ns even after the trion recombines. Simultaneously spin precession of a trion (hole) starts. Observation of spin precession of both a doped electron and a trion (hole) confirms creating coherent superposition of an electron and a trion as the initialization process of spin of doped electrons in quantum dots. In a neutral quantum dot, the exciton spin precession starts out of phase with spin precession of the doped electrons in a GaAs substrate and the precession frequency does not converge to 0 at the zero field limit. It contains the electron–hole exchange interaction and corresponds to the splitting between bright and dark excitons under the transverse magnetic field.     

Optically induced phase transition of excitons in coupled quantum dots

The weak classical light excitations in many semiconductor quantum dots have been chosen as important solid- state quantum systems for processing quantum information and implementing quantum computing. For strong classical light we predict theoretically a novel phase transition as a function of magnitude of this classical light from the deformed to the normal phases in resonance ease, and the essential features of criticality such as the scaling behaviour, critical exponent and universality are also present in this paper.     

Photoluminescence quantum and surface states of excitons in ZnSe and CdS nanoclusters

The optical spectra of quantum dots (QDs) of CdS and ZnSe grown in borosilicate glass by the sol-gel method are obtained and analyzed. It is found that at concentrations of the two semiconductors x<0.06% the emission spectra are due to annihilation of free (internal) excitons in quantum states. The mean size of the quantum dots for a given concentration of ZnSe and CdS is calculated and found to be in good agreement with the X-ray data, and the exciton binding energy is calculated with allowance for the dielectric mismatch between the semiconductor and matrix. It is proposed that this mismatch may be the cause giving rise to the exciton percolation level that is observed in QD arrays for both systems at x>0.06%. The emission from the surface level of CdS QDs in the region ~2.7 eV, formed by the outer atoms with dangling bonds, is observed for the first time, as is the emission band from surface localized states. The relation between the position of the maximum of this band and the energy of the 1S state of the free exciton is established. It is shown that the properties of surface localized states are largely similar to the analogous properties of localized states of 3D (amorphous semiconductors, substitutional solid solutions of substitution) and 2D (quantum wells and superlattices) structures.