Light-hole Excitons Research Articles

Energy spectrum of excitons in single and double narrow wells in a magnetic field

A theoretical investigation is presented of the properties of heavy- and light-hole excitons in multiple GaAs/Ga1−xAlxAs quantum wells in the presence of a magnetic field directed along the growth axis. The low-lying energy levels are calculated as functions of the barrier width, the well size and the magnetic field strength. Effects of interwell coupling on the energies of different exciton levels are consistently included in our calculations. The calculation is realized by means of the diagonalization method.

The binding energy of excitons in a cylindrical quantum wire under the influence of laser field intensity

We study the effects of laser field intensity over the ground state binding energy of light and heavy hole excitons confined in GaAs/Ga 1− x Al x As cylindrical quantum wire. We have applied the variational method using 1s-hydrogenic wavefunctions, in the framework of the single band effective mass approximation and computed the exciton binding energy as a function of the wire radius for different field of laser intensities. The valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions. The results show that (i) the binding energy is found to increase with decrease in the wire radius, and decrease with increase in the value of laser field amplitude, (ii) the heavy-hole exciton in a cylindrical quantum wire is more strongly bound than the light-hole exciton and (iii) the binding energy of the impurity for the narrow wire is more sensitive to the laser field amplitude.

Effect of Exciton-Longitudinal Optical Phonon Interaction on Exciton Binding Energies in CdxZn1-xS/ZnS Single Quantum Wells

We study the effect of the exciton-longitudinal optical (LO) phonon interaction on the exciton binding energies in CdxZn1-xS/ZnS single quantum wells (SQWs) for the Cd alloy content (x) range 0.1—0.3. The heavy- and light-hole exciton binding energies increase with the exciton-LO phonon interaction. The increase in the maximum heavy-hole (light-hole) exciton binding energy for x = 0.3 is 9.1 meV (4.9 meV). In narrow CdxZn1-xS/ZnS SQWs, the heavy-hole exciton binding energy calculated by taking into account the exciton-LO phonon interaction for values of x in the range 0.1—0.3 exceed the LO phonon energy of CdxZn1-xS. [DOI: 10.1380/ejssnt.2010.340]

Effect of Dielectric Mismatch on Exciton Binding Energy in ZnS/MgxZn1-xS Quantum Wells

We calculate the binding energies of heavy- and light-hole excitons in ZnS/MgxZn1-xS single quantum wells (SQWs) as functions of the well width by using a variational method. We take into account the effect of the mismatch between the dielectric constants of the well and the barriers (dielectric mismatch effect) in our calculation. The increases in the heavy- and light-hole exciton binding energies in ZnS/MgxZn1-xS SQWs for a Mg content (x) of 1.0 are 58.9 meV and 60.2 meV, respectively. In the case of narrow wells, the heavy- and light-hole exciton binding energies calculated by considering the dielectric mismatch effect exceed the longitudinal optical phonon energy of ZnS when x ≥ 0.3. [DOI: 10.1380/ejssnt.2010.145]

Effect of Image Charge on Exciton Binding Energy in ZnS/BexZn1-xS Quantum Wells

We calculate the binding energies of heavy- and light-hole excitons in ZnS/BexZn1-xS single quantum wells (SQWs) as functions of well width using a variational method by considering the effect of the mismatch between the dielectric constants of the well and barriers (effect of image charge). The increases in the heavy- and light-hole exciton binding energies in ZnS/BexZn1-xS SQWs at a Be content (x) equal to 0.76 are estimated to be 29.9 and 28.2 meV, respectively. For narrow wells, the heavy- and light-hole exciton binding energies calculated by considering the effect of image charge exceed the longitudinal optical phonon energy of ZnS for x≥0.2.

Transient optical response of quantum well excitons to intense narrowband terahertz pulses

Intense narrowband terahertz pulses are used to modify excitonic transitions in semiconductor quantum wells and to study the dephasing properties of the optically dark 2p states. Time-resolved terahertz-pump and optical-probe measurements exhibit strong nonlinear optical transients of the 1s heavy-hole and light-hole exciton resonances when the terahertz radiation is tuned near the 1s to 2p intraexciton transition. A microscopic theory attributes the observed nonlinearities to Rabi sidebands showing that the 2p-dephasing time is three times that of the 1s-state.

Fine Structure of the Localized Emission from GaInNAs Layers Studied by Micro-Photoluminescence

R. Kudrawieca,∗, M. Latkowska, G. Sek, J. Misiewicz, J. Ibanez, M. Henini and M. Hopkinson Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland Institut Jaume Almera, Consell Superior d’Investigacions Cientifiques, 08028 Barcelona, Spain School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, United Kingdom Department of Electronic and Electrical Engineering, University of Sheffield, S3 3JD Sheffield, United Kingdom GaInNAs bulk-like layers (≈ 20% In and ≈ 3% N) grown on GaAs substrate with various crystallographic orientations have been studied by micro-photoluminescence at low temperatures for a broad range of excitation conditions. In addition to photoluminescence peaks, which are associated with heavyand light-hole free exciton recombination, a band of sharp lines was observed below the fundamental free exciton transition at low excitation. It shows that the localized emission which is typical of this alloy at low temperatures is composed of individual narrow photoluminescence lines which are associated with the recombination of single excitons. They can be localized on various local potential minima including those originating from the alloy content fluctuations and/or deep acceptor(donor)-like complexes.

Photoluminescence and Reflectance Spectra of CdTe films on GaAs(100) Substrates

Photoluminescence (PL) and reflectance spectra were measured on CdTe films grown on GaAs(100) substrates by metal organic chemical vapor deposition (MOCVD). By comparing the PL spectra with the reflectance spectra in a certain exciton energy range, we identified heavy- and light-hole exciton transitions in the CdTe films on the GaAs(100) substrates. The energy separation between the heavy- and light-hole exciton transitions was estimated to be 3.1 meV. This energy separation was caused by a compressive strain due to a difference in thermal expansion coefficient between the grown CdTe films and GaAs substrates. Bound exciton emissions and the 1.42 eV band in the PL spectra were also studied on the basis of their temperature dependence.

Optical measurement of electron spin coherence in a semiconductor quantum well

Coherent electron spin dynamics is observed by tomographic Kerr rotation measurement in a GaAs/AlGaAs quantum well. Coherent measurement of the electron spin state prepared via the light-hole exciton was reported previously. In this study, we confirm that the electron spin state prepared via the heavy-hole exciton is also observable coherently and directly. Additionally, the magneto-optical spectrum is measured around the energy of the light-hole exciton under an in-plane magnetic field. These results lead us to better understanding of the tomographic Kerr rotation measurement.

Exciton Binding Energies in Cd0.11Zn0.89S/Mg0.22Zn0.78S Quantum Wells Lattice-Matched to GaP Substrates

The exciton binding energy in Cd0.11Zn0.89S/Mg0.22Zn0.78S single quantum wells (SQWs) is calculated to study their exciton properties in detail. The heavy-hole exciton binding energy is larger than the light-hole exciton binding energy in narrow wells, because the degree of confinement of heavy-hole excitons is larger than that of light-hole excitons in these SQWs. The heavy- and light-hole exciton binding energies are found to increase owing to the image-charge effect when the well width is less than 5 nm.

Effects of Dielectric Confinement on Exciton Binding Energies in ZnSe/MgxBeyZn1-x-ySe Quantum Wells

We study the effects of dielectric confinement on exciton binding energies in ZnSe/MgxBeyZn1-x-ySe single quantum wells. Heavy- and light-hole exciton binding energies are found to increase owing to dielectric confinement when the well width is less than 10 nm.

All-Optical Spin Manipulation of a Single Manganese Atom in a Quantum Dot

For a CdTe quantum dot doped with a single Mn atom we analyze the dynamics of the Mn spin when the dot is excited by ultrashort laser pulses. Because of the exchange interaction with the Mn atom, electron and hole spins can flip and induce a change of the Mn spin. Including both heavy and light-hole excitons and using suitable pulse sequences, angular momentum can be transferred from the light to the Mn system while the exciton system returns to its ground state. We show that by a series of ultrashort laser pulses the Mn spin can be selectively driven into each of its six possible orientations on a picosecond timescale. By applying a magnetic field the total switching time and the required number of pulses can be strongly reduced.

Spin control by ultra short laser pulses in a Mn doped quantum dot

AbstractOptically induced spin flip processes in quantum dots doped with a single Mn atom are discussed. On the femtosecond time scale the exciton can be controlled by ultra short laser pulses. Via the exchange interaction the Mn spin can flip on a picosecond time scale by transferring its angular momentum to the exciton. For a heavy hole system this flipping process can be described in terms of exchange‐induced Rabi oscillations between the bright and dark exciton state accompanied by a flip of the Mn spin. Because of the energy splitting between bright and dark excitons the Rabi oscillations are off‐resonant and a series of laser pulses is required to achieve a complete flip. When a light hole exciton is resonantly excited in addition to the heavy hole exciton we show that a flip of the Mn spin by two can be achieved by a series of light pulses while the exciton system returns to its initial state. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lattice‐mismatch‐strain effects on excitons in GaAs1–xNx /GaAs heterostructures

AbstractWe have investigated the lattice‐mismatch‐strain effect on excitons in GaAs1‐x Nx (500 nm)/GaAs heterostructures with x = 0.4, 0.8, and 1.5% grown on a GaAs substrate using photoreflectance spectroscopy that is sensitive to excitonic transitions. It is found that the heavy‐hole (HH) and light‐hole (LH) excitons of the GaAs1‐x Nx layer, which are degenerate in a bulk crystal, are split and that the energy of the LH exciton is lower than that of the HH exciton. In this case, a quantum‐size effect is negligible because the GaAs1‐x Nx layer thickness is sufficiently thick. The HH‐LH splitting energy increases with an increase of nitrogen concentration (x) in addition to a drastic decrease of the energies of the HH and LH excitons. We have quantitatively analyzed the nitrogen‐concentration dependence of the energies of the HH and LH excitons, taking account of the lattice‐mismatch‐strain effect on the band structure and nitrogen‐induced giant bowing of the band‐gap energy. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Changes of the Light-Hole Exciton Line in CdMnTe/CdMgTe Quantum Wells Under Resonant Excitation of the Heavy-Hole Exciton

Changes of the Light-Hole Exciton Line in CdMnTe/CdMgTe Quantum Wells Under Resonant Excitation of the Heavy-Hole Exciton

Excitonic band edges and optical anisotropy of InAs∕InP quantum dot structures

Optical transitions in InAs∕InP quantum dot structures are studied and interpreted in terms of heavy- (hh) and light-hole (lh) excitons originating in InAs valence band. Appearance of a step-like absorption and two activation energies in photoluminescence spectra of quantum dots under temperature elevation is described as a transition from the excitonic states to the two-dimensional-like states realized in the InAs∕InP quantum dots. A mixture of hh and lh states determines the anisotropy of the transmission spectra and may result in rotation of the polarization plane for light transmitted through the InAs∕InP quantum dot system.

Polaronic shift of the Wannier-Mott exciton binding energy in a quantum wire with allowance for the phonon confinement effect

Within the framework of the Li-Low-Pines model the interaction of a Wannier-Mott exciton with polar optical phonons in a cylindrical semiconductor wire is studied, taking into account the phonon confinement effect. An analytical expression for the exciton binding energy with allowance for the polaronic effect is obtained. Numerical calculations of the binding energy are carried out for AlAs/GaAs/AlAs and ZnSe/CdSe/ZnSe wires with a various degree of polarity of quantum wire materials. The polaronic shift of the binding energy of light and heavy hole excitons is calculated.

Correlation coefficient for dephasing of light-hole excitons and heavy-hole excitons in GaAs quantum wells

Dephasing of light-hole (LH) and heavy-hole (HH) excitons and of the non-radiative LH–HH exciton Raman coherence in weakly-disordered GaAs quantum wells is probed using three-beam transient four-wave mixing (TFWM). By varying the two inter-pulse delays independently, the radiative inter-band and non-radiative intra-band coherences can be probed independently. A theoretical model based on the density matrix and including inhomogeneous broadening and inhomogeneous dephasing rates is fit to the TFWM data to extract the dephasing rates. The dephasing rate of the LH–HH Raman coherence can be expressed as a function of the dephasing rates of the LH and HH excitons and a correlation coefficient, R p h , allowing for the calculation of R p h . We find R p h < 0 , suggesting that anti-correlated scattering dominates the exciton dephasing.

Enhancement of terahertz radiation from coherent optical phonons via impulsive interference of excitons in GaAs/AlAs multiple quantum wells

We have investigated the terahertz (THz) radiation from the coherent GaAs-like longitudinal optical (LO) phonons at room temperature in GaAs/AlAs multiple quantum wells utilizing an optical gating method with an ultrashort pulse laser. It has been found that the intensity of the THz radiation with a long dephasing time is markedly enhanced under the condition that the energy spacing between the fundamental heavy-hole and light-hole excitons is almost equal to the LO-phonon energy. This fact demonstrates that the impulsive excitonic interference, which produces no beating oscillation at room temperature owing to a very fast dephasing time, acts as a driving force for the coherent LO phonon emitting the intense THz radiation.

Excitonic quantum beat at the mini-Brillouin-zone boundary in a GaAs/AlAs superlattice

We have investigated excitonic quantum beats (QBs) at the mini-Brillouin-zone boundary (π-point) in a GaAs (6.8 nm)/AlAs (1.1 nm) superlattice by using a femtosecond reflection-type electro-optic sampling technique. Photoreflectance spectroscopy was adopted to characterize the optical transition energies of the heavy-hole (HH) and light-hole (LH) excitons at the mini-Brillouin-zone center (Γ-point) and the π-point. By systematically changing the pump-pulse energy in the energy region from the Γ-point exciton to the π-point exciton, we have succeeded to observe the QB of the HH and LH excitons at the π-point in addition to the ordinary QB at the Γ-point. The observed frequencies agree with the values estimated from the energy difference between the relevant two excitons. The decay time of the QB signal at the π-point is much shorter than that at the Γ-point. This behavior reflects a very fast relaxation process of the π-point excitons in the miniband dispersion.