Intersubband optical transitions in one dimensional quantum wire structures

One dimensional (1D) quantum wire structures are emerging as the new generation of semiconductor nanostructures offering exciting physical properties which have significant potential for novel device applications. These structures have been the subject of intensive investigation recently including extensive theoretical and experimental studies of their interband optical properties. In this work we present the results of our study of the intersubband optical transitions in 1D semiconductor quantum wires. The crescent shaped quantum wire structures used for this research were grown on non-planar GaAs substrates. The intersubband transition energy spectra, the selection rules, and the two dimensional envelope wavefunctions were theoretically investigated by using our new LENS (local envelope states) expansion. We present recent experimental results on modulation doped V-groove quantum wires, including PL, PLE, TEM, CL, and infrared polarization resolved spectroscopy. We have observed a very unusual absorption lineshape at the far-infrared wavelengths that we assigned to phonon assisted Fano resonance in a modulation doped quantum wire structure.     

Spectroscopy of polarized luminescence of nanostructures

The effect of different distortions arising in nanostructures and of external fields on polarization of radiation has been studied. Occurrence of linear polarization of photoluminescence in a magnetic field applied in the plane of a structure containing a CdTe/CdMnTe quantum well has been considered. The influence of a magnetic field on the effects of optical orientation and optical alignment of excitons and trions in self-assembled CdSe/ZnSe quantum dots has been investigated. Analysis of the experimental data reveals low symmetry of the structures studied. It is shown that the distribution of the anisotropy axes in the plane of nanostructures is nearly random.     

Optical spin orientation of a single manganese atom in a quantum dot

The magnetic state of a single magnetic atom (Mn) embedded in an individual semiconductor quantum dot is optically probed using micro-spectroscopy. A high degree of spin polarization can be achieved for an individual Mn atom localized in a quantum dot using quasi-resonant or fully-resonant optical excitation at zero magnetic field. Optically created spin polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots. The Mn spin distribution prepared by optical pumping is fully conserved for a few microseconds. This opens the way to full optical control of the spin state of an individual magnetic atom in a solid state environment.     

Nuclear spin diffusion effects in optically pumped quantum wells

We studied the influence of the nuclear spin diffusion on the dynamical nuclear polarization of low dimensional nanostructures subject to optical pumping. Our analysis shows that the induced nuclear spin polarization in semiconductor nanostructures will develop both a time and position dependence due to a nonuniform hyperfine interaction as a result of the geometrical confinement provided by the system. In particular, for the case of semiconductor quantum wells, nuclear spin diffusion is responsible for a nonzero nuclear spin polarization in the quantum well barriers. As an example we considered a 57 Å GaAs square quantum well and a 1000 Å Al _x Ga_1?x As parabolic quantum well both within 500 Å Al_0.4Ga_0.6As barriers. We found that the average nuclear spin polarization in the quantum well barriers depends on the strength of the geometrical confinement provided by the structure and is characterized by a saturation time of the order of few hundred seconds. Depending on the value of the nuclear spin diffusion constant, the average nuclear spin polarization in the quantum well barriers can get as high as 70% for the square quantum well and 40% for the parabolic quantum well. These results should be relevant for both time resolved Faraday rotation and optical nuclear magnetic resonance experimental techniques.     

Circular-to-linear and linear-to-circular conversion of optical polarization by semiconductor quantum dots

We report circular-to-linear and linear-to-circular conversion of optical polarization by semiconductor quantum dots. The polarization conversion occurs under continuous wave excitation in the absence of any magnetic field. The effect originates from quantum interference of linearly and circularly polarized photon states, induced by the natural anisotropic shape of the self-assembled dots. The behavior can be qualitatively explained in terms of a pseudospin formalism.     

Electron–Nuclear Spin Dynamics in Semiconductor QDs

This work presents an overview of investigations of the nuclear spin dynamics in nanostructures with negatively charged InGaAs/GaAs quantum dots characterized by strong quadrupole splitting of nuclear spin sublevels. The main method of the investigations is the experimental measurements and the theoretical analysis of the photoluminescence polarization as a function of the transverse magnetic field (effect Hanle). The dependence of the Hanle curve profile on the temporal protocol of optical excitation is examined. Experimental data are analyzed using an original approach based on separate consideration of behavior of the longitudinal and transverse components of the nuclear polarization. The rise and decay times of each component of the nuclear polarization and their dependence on transverse magnetic field strength are determined. To study the role of the Knight field in the dynamic of nuclear polarization, a weak additional magnetic field parallel to the optical axis is used. We have found that, only taking into account the nuclear spin fluctuations, we can accurately describe the measured Hanle curves and evaluate the parameters of the electron–nuclear spin system in the studied quantum dots. A new effect of the resonant optical pumping of nuclear spin polarization in an ensemble of the singly charged (In,Ga)As/GaAs quantum dots subjected to a transverse magnetic field is discussed. Nuclear spin resonances for all isotopes in the quantum dots are detected in that way. In particular, transitions between the states split off from the ±1/2 doublets by the nuclear quadrupole interaction are identified.     

Tunable resonant radiation force exerted on semiconductor quantum well nanostructures: Nonlocal effects

Resonant radiation force exerted on a semiconductor quantum well nanostructure(QWNS) from intersubband transition of electrons is investigated by taking the nonlocal coupling between the polarizability of electrons and applied optical fields into account for two kinds of polarized states. The numerical results show the spatial nonlocality of optical response can induce the spectral peak position of the exerted force to have a blueshift, which is sensitively dependent on the polarized state and the QWNS width. It is also demonstrated that resonant radiation force is controllable by the polarization and incident directions of applied light waves. This work provides effective methods for controlling optical force and manipulating nano-objects, and observing radiation forces in experiment. This nonlocal interaction mechanism can also be used to probe and predominate internal quantum properties of nanostructures, and to manipulate collective behavior of nano-objects.     

Negative refraction in semiconductor metamaterials based on quantum cascade laser design for the mid-IR and THz spectral range

We have considered the realization of metamaterials based on semiconductor quantum nanostructures, in particular, with the structural arrangement as in quantum cascade laser (QCL) designed to achieve optical gain in the mid-infrared and terahertz part of the spectrum. The entire structure is placed in a strong external magnetic field, which facilitates the attainment of sufficient population inversion, necessary to manipulate the permittivity, and enable a left-handed regime.     

Exciton optical absorption coefficients and refractive index changes in a strained InAs/GaAs quantum wire: The effect of the magnetic field

Magnetic field induced exciton binding energy is investigated in a strained InAs/GaAs quantum wire within the framework of single band effective mass approximation. The strain contribution to the potential is determined through deformation potentials. The interband emission energy of strained InAs/GaAs wire is investigated in the influence of magnetic field with the various structural parameters. Magnetic field induced photoionization cross section of the exciton is studied. The total optical absorption and the refractive index changes as a function of normalized photon energy between the ground and the first excited state in the presence of magnetic field are analyzed. The optical absorption coefficients and the refractive index changes strongly depend on the incident optical intensity and the magnetic field. The occurred blueshift of the resonant peak due to the magnetic field will give the information about the variation of two energy levels in the quantum well wire. The optical absorption coefficients and the refractive index changes are strongly dependent on the incident optical intensity and the magnetic field.     

The anomalous Hanle effect in semimagnetic semiconductor quantum wells

The behavior of the optical-orientation signal under resonant optical excitation of localized excitons in quantum wells with semimagnetic layers was studied both experimentally and theoretically. The most remarkable experimental observation made in this study is the increase in the degree of polarization of the quantum well radiation in the conditions in which the Hanle effect is observed. The behavior of magnetically induced circular luminescence polarization in a slightly tilted field (quasi-Voigt geometry) also appears unusual. Possible specific contributions to the optical exciton orientation in semimagnetic nanostructures are discussed. A theoretical model is proposed which, while being based on the well-known concepts of collective spin dynamics of magnetic ions in the exchange field of a photoexcited hole, takes into account fluctuations of the local magnetization. The calculations agree quantitatively with experiment for reasonable values of the parameters.     

Spin-dependent transmission of holes through a semiconductor quantum wire with multiple stub

The spin transport of holes through a quantum wire made of many identical T-shaped diluted magnetic semiconductor/semiconductor units is investigated theoretically. The spin-down and spin-up transmission coefficients have been studied as a function of stub parameters. The spin-up transmission coefficient as a function of the stub length is extremely negligible, in the case of multiple-stub quantum wire, while the spin-down transmission coefficient shows a nearly periodic behaviour with regions of large transmission separated by forbidden bands. The spin polarization switches periodically between one and zero as the stub length is changed and shows a square-wave pattern.     

Ferromagnetism in diluted magnetic semiconductor quantum dot arrays embedded in semiconductors

We present an Anderson-type model Hamiltonian with exchange coupling between the localized spins and the confined holes in the quantum dots to study the ferromagnetism in diluted magnetic semiconductor (DMS) quantum dot arrays embedded in semiconductors. The hybridization between the quantum-confined holes in the quantum dots and the itinerant holes in the semiconductor valence band makes possible hole transfer between the DMS quantum dots, which can induce the long range ferromagnetic order of the localized spins. In addition, it makes the carrier spins both in the quantum dots and in the semiconductors polarized. The spontaneous magnetization of the localized spins and the spin polarization of the holes are calculated using both the Weiss mean field approximation and the self-consistent spin wave approximation, which are developed for the present model.Received: 17 Mars 2003, Published online: 30 January 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.30.Ds Spin waves - 75.50.Dd Nonmetallic ferromagnetic materials - 75.50.Pp Magnetic semiconductors     

Drag effect of one-dimensional electrons upon photoionization of D<Superscript>(−)</Superscript> centers in a longitudinal magnetic field

A theory is elaborated for the impurity photon drag effect in a semiconductor quantum wire exposed to a longitudinal magnetic field B directed along the axis of the quantum wire. The phonon drag effect is associated with the transfer of the longitudinal photon momentum to localized electrons in optical transitions from D^(?) states to hybrid-quantized states of the quantum wire, which is described by a confinement parabolic potential. An analytical expression for the drag current density is derived within the model of a zero-range potential in the effective mass approximation, and the spectral dependence of the drag current density is examined at different magnitudes of B and parameters of the quantum wire upon electron scattering by a system of impurities with short-range potentials. It is established that the spectral dependence of the drag current density exhibits a Zeeman doublet with a clear beak-shaped peak due to optical transitions of electrons from D^(?) states to states with the magnetic quantum number m=1. The possibility of using the photon drag effect in a longitudinal magnetic field for the development of laser radiation detectors is analyzed.     

Direct observation of the electron spin relaxation induced by nuclei in quantum dots

We have studied the electron spin relaxation in semiconductor InAs/GaAs quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of p-doped quantum dots decays down to 1/3 of its initial value with a characteristic time T(Delta) approximately 500 ps, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.     

Coherent nonlinear optical response of single quantum dots studied by ultrafast near-field spectroscopy

The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments. At negative time delays, transient reflectivity spectra show pronounced oscillatory structure around the quantum dot exciton line, providing the first evidence for a perturbed free induction decay of the excitonic polarization. Phase-disturbing Coulomb interactions between the excitonic polarization and continuum excitations dominate the optical nonlinearity on ultrafast time scales. A theoretical analysis based on the semiconductor Bloch equations accounts for this behavior.     

抛物量子线中弱耦合极化子的有效质量和光学声子平均数

讨论电子与体纵光学(LO)声子弱耦合时对抛物量子线中极化子性质的影响.采用Tokuda改进的线性组合算符法、Lagrange乘子和变分法,导出了抛物量子线中弱耦合极化子的有效质量和光学声子平均数随拉格朗日乘子变化的规律及极化子振动频率随量子线约束强度的变化规律.并以ZnS量子线为例进行了数值计算,结果表明:抛物量子线中弱耦合极化子的有效质量m^*和光学声子平均数N随着拉格朗日乘子u的增加而增大;该结论与体材料中结论基本一致,但量子线中的效应比体材料更明显,表明量子线对电子约束的增强,使极化子效应更明显.同时,极化子振动频率λ随约束强度ω₀的增强而增大.     

Spatial electric field effect in GaAs–AlAs quantum wires

The response of an electron to a three-dimensional electric field in an infinite quantum well wire of square cross-section is investigated within a variational scheme. The ionization energy and the polarization are calculated for different locations of impurity ion. It is found that the results for the spatial electric field differ from the previous results found for the electric field applied in the direction perpendicular to the wire axis. The ionization energy weakens rapidly with the axial component of the field as the polarization of the carrier distribution intensifies.     

Laser-induced reshaping of the density of impurity states in GaAs/AlGaAs nanowires

The binding energy of shallow-donor impurities in a cylindrical quantum well wire irradiated by an intense non-resonant laser field is calculated within the effective mass approximation by using a variational procedure. Accurate laser-dressing effects are considered for both the confinement potential of the wire and the Coulomb potential of the impurity. The computation of the ground state subband energy eigenfunctions for different laser field intensities is based on a bidimensional finite element method. Important changes of the electron probability density under intense laser field conditions are predicted. The study reveals that the laser field compete with the quantum confinement and breaks down the degeneracy of states for donors symmetrically positioned within the nanostructure. A proper analysis of the density of impurity states is found to be essential for controlling the optical emission related to shallow donors in semiconductor quantum wires.     

Spin Polarization and Andreev Conductance through a Diluted Magnetic Semiconductor Quantum Wire with Spin--Orbit Interaction

Spin-dependent Andreev reflection and spin polarization through a diluted magnetic semiconductor quantum wire coupled to normal metallic and superconductor electrodes are investigated using scattering theory. When the spin-orbit coupling is considered, more Andreev conductance steps appear at the same Fermi energy. Magnetic semiconductor quantum wire separates the spin-up and spin-down electrons. The Fermi energy, at which different- spin-state electrons begin to separate, becomes lower due to the effect of the spin-orbit interaction. The spin filter effect can be measured more easily by investigating the Andreev conductance than by investigating the normal conductance.