Spin polarization dependent optical transition rates observed in the integer quantum Hall region

We report on a field-dependent photoluminescence (PL) emission rate for the transitions between band states in modulation-doped CdTe/Cd_1−xMg_xTe single quantum wells in the integer quantum Hall region. The recombination time observed for the magneto-PL spectra varies in concomitance with the integer quantum Hall plateaus. Furthermore, different PL decay times were observed for the two circular polarizations, i.e. for the transitions between the Zeeman split subbands of the Landau levels. We analyzed the data in comparison with the experimentally determined spin polarization of the conduction electrons and the Zeeman splitting of the valence band. Furthermore, we discuss the relevance of the spin polarization of the conduction electrons, the electron–hole exchange interaction and the spin-flip processes of the hole states for the PL decay time.     

Polarization memory in single quantum dots

We measured the polarization memory of excitonic and biexcitonic optical transitions from single quantum dots at either positive, negative or neutral charge states. Positive, negative and no circular or linear polarization memory was observed for various spectral lines, under the same quasi-resonant excitation below the wetting layer bandgap. We developed a model which explains both qualitatively and quantitatively the experimentally measured polarization spectrum for all these optical transitions. We consider quite generally the loss of spin orientation of the photogenerated electron–hole pair during their relaxation towards the many-carrier ground states. Our analysis unambiguously demonstrates that while electrons maintain their initial spin polarization to a large degree, holes completely dephase.     

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.     

Pump–probe analysis of polaron decay in InAs/GaAs self-assembled quantum dots

We report on polaron decay in InAs/GaAs self-assembled quantum dots. The polarons are probed by pump–probe spectroscopy through their optical intersublevel absorption around 62 meV (20 μm wavelength). A T₁ polaron lifetime of the order of tens of picosecond is deduced from the low-temperature pump–probe measurements. We show that a long-lived component can be additionally observed on the pump–probe measurements. The spectral dependence of this long-lived component is, however, not correlated to the polaron absorption. It is thus not a signature of polaron relaxation quenching. The origin of this long-lived component is attributed to the two-phonon absorption of the bulk GaAs substrate.     

Demonstration of electrical spin injection into a semiconductor using a semimagnetic spin aligner

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn_{1 − x − y}Be_xMn_ySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.     

Single-electron charging spectra: from natural to artificial atoms

We present a theoretical study of the charging spectra in natural and artificial atoms. We apply a model electrostatic potential created by a homogenously charged sphere. This model potential allows for a continuous passage from the Coulomb potential of the nucleus to parabolic confinement potential of quantum dots. We consider electron systems with N=1,…,10 electrons with the use of the Hartree–Fock method. We discuss the qualitative similarities and differences between the chemical potential spectrum of electron systems bound to nucleus and confined in quantum dots.     

Antiresonance of electron tunneling through a quantum dot array

Electronic transport through a one-dimensional quantum dot array is theoretically studied. In such a system both electron reservoirs of continuum states couple with the individual component quantum dots of the array arbitrarily. When there are some dangling quantum dots in the array outside the dot(s) contacting the leads, the electron tunneling through the quantum dot array is wholly forbidden if the electron energy is just equal to the molecular energy levels of the dangling quantum dots, which is called as antiresonance of electron tunneling. Accordingly, when the chemical potential of the reservoir electrons is aligned with the electron levels of all quantum dots, the linear conductance at zero temperature vanishes if there are odd number dangling quantum dots; Otherwise, it is equal to 2e²/h due to resonant tunneling if the total number of quantum dots in the array is odd. This odd–even parity is independent of the interdot and the lead–dot coupling strength.     

TE- and TM-polarization-resolved spectroscopy on quantum wells under normal incidence

We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane (p_x and p_y) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p_z components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture (NA=0.9). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p_z components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p_z components on and off easily.Experimental evidence of these features has been achieved by exploiting the selection rules for e–hh and e–lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption.     

THz spectroscopy of diluted magnetic semiconductors and semiconductor quantum structures in ultrahigh magnetic fields

Magneto-absorption spectra in ferromagnetic semiconductor In_1−xMn_xAs films and self-organized PbSe/PbEuTe quantum dot superlattices have been studied in the terahertz range at very high magnetic fields up to 500 T. Both heavy hole (HH) and light hole (LH) cyclotron resonance (CR) have been observed in bulk In_1−xMn_xAs thin films with different Mn concentrations. The detailed Landau level calculation in terms of the effective mass approximation well explained the CR peak positions, line shapes and the dependence of the circular polarization of the incident light on the CR spectra. In InMnAs/GaSb heterostructures that have higher ferromagnetic transition temperature (T_c) than the bulk samples, the observed HH and LH cyclotron masses are larger than that in the bulk thin films. We found that the CR peak position and its line shape suddenly change in the vicinity of the ferromagnetic transition temperature, suggesting the change in the electronic structure due to the ferromagnetic transition. Electron CR in PbSe/PbEuTe quantum dots has been observed and it was found that the effective mass of the electrons is considerably modified by the quantum confinement potential and the lattice strain around the dots. A large wavelength dependence of the absorption intensity was observed due to the interference effect of the radiation inside the sample.     

Ultrafast carrier dynamics of resonantly excited InAs/GaAs self-assembled quantum dots

We study theoretically the time development of electronic relaxation in quantum dots. We consider the process of relaxation of the state with an electron prepared at the beginning of relaxation in the electronic ground state. We obtain a fast (in picoseconds) increase of electronic population in the excited state. Also, we consider the process of relaxation of an electron from an excited state in the dot. Here we obtain an incomplete depopulation of the electron from the excited state. We compare these results to experiments in which a fast decrease of luminescence is reported during the first period of relaxation after resonant excitation of the ground state. We estimate numerically the role of electron–LO–phonon (Fröhlich's coupling) mechanism in these processes. We show that this effect may be attributed to the influence of multiple scattering of quantum dot electrons on LO phonons. A single-electron two-energy-level quantum dot model is used to demonstrate this effect in an isolated semiconductor quantum dot.     

Study on the coupled multiple nanocrystal quantum-dot system

We have studied excess electron filling rule in the coupled multiple nanocrystal quantum-dot systems, i.e. quantum chain and quantum pattern, by the unrestricted Hartree–Fock–Roothaan method. Assuming each quantum dot of quantum pattern to be confined in a three-dimensional spherical potential well of finite depth, we have studied the intradot and interdot electron Coulomb and exchange interactions. By varying the center distance d between the coupled quantum dots, the transition from the strong- to weak-coupling situation is realized. For the systems in question, our results show that, with the filling of excess electrons into the quantum pattern, the corresponding chemical potentials form quasi-band structure, which is similar to the energy-band structure of crystal material. In each chemical-potential band of quantum pattern, the number of chemical-potential curves is equal to the number of quantum dots, and the distributions of them depend strongly on the quantum-dot arrangement structure of quantum pattern.     

Intraband population inversion and amplification of IR radiation through charge-carrier injection into quantum wells and quantum dots

A mechanism is proposed for obtaining intraband population inversion of electrons in size-quantization levels through the injection of electron-hole pairs into the i region of a heterostructure with quantum wells or quantum dots. Key elements of the mechanism are the simultaneous generation of interband (hv≈E _g ) near-IR radiation and the presence of a “metastable” level. In quantum wells such a level can be produced by making use of the weak overlap of the wave functions of electrons in the levels of a quantum well of complicated configuration and exploiting the characteristic features of the interaction of electrons with optical phonons in polar semiconductors. In quantum dots such a level forms as a result of the phonon bottleneck effect. Estimates are made of the gain for mid-IR radiation in intraband optical transitions of electrons. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 392–399 (10 September 1998)     

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.     

Electron-related optical responses in triple δ-doped quantum wells

A detailed theoretical study on the electron-related optical responses in triple δ-doped GaAs quantum wells in the presence of non-resonant, monochromatic intense laser field is presented. For this purpose, we first obtained the bound subband energy levels and their corresponding envelope wave functions of the structure for different central doping concentrations within the effective-mass approximation. Then, we calculate the effect of the non-resonant intense laser field on the optical properties of this structure using the compact-density-matrix approach via the iterative method. We found that the optical absorption coefficients and refractive index changes in the triple δ-doped GaAs quantum well can be modulated by changing the central doping concentration and the intensity of the non-resonant, monochromatic laser field. In addition, it is shown that a sufficiently intense laser field suppresses the multiple quantum well configuration towards a single potential well one and the optical response becomes practically independent of the δ-doping concentration.     

Theoretical analysis of the electronic structure of truncated-pyramidal GaN/AlN quantum dots

We present a theoretical analysis of the electronic structure of GaN/AlN quantum dots (QD) with a hexagonal, truncated-pyramidal shape. We use a Fourier-transform technique that we had previously developed to calculate the 3D strain and built-in electric fields due to the QD structure. The electron and hole energy levels and wavefunctions are then calculated in the framework of an 8-band k·P model (with zero spin–orbit splitting), using an efficient plane-wave expansion method. We show that because of the large built-in piezoelectric and spontaneous polarization fields, the calculated transition energy is sensitive to variations in the wetting layer width, pyramid top diameter and also to the values chosen for the piezo-electric constants and spontaneous polarization values of bulk GaN and AlN. Numerical results are presented for a set of GaN/AlN QD structures that have been studied experimentally and described in the literature. We find that the calculated value of the ground-state optical transition energy for these structures is in good agreement with experiment.     

Acoustically induced dynamic potential dots

Mobile potential dots (dynamic dots, DDs) formed by surface acoustic waves (SAWs) are used to transport photogenerated electrons and holes in GaAs quantum wells (QWs). We investigate the interaction between the transported carriers and microscopic trap centers in the QW plane using spatially and time-resolved photoluminescence (PL) spectroscopy. The carriers recombine at the trap site emitting short (width0.6 ns) light pulses at a repetition rate corresponding to the SAW frequency. The dependence of the PL intensity from the traps on the number of carriers transported per DD n exhibits a well-defined, distinct plateau for n in the range from 5–20, which is attributed to the emission of a well-defined number of photons.     

Optically detected electron spin polarization and Hanle effect in AlGaAs/GaAs heterostructures

The spin polarization of optically created conduction electrons in p-type AlGaAs/GaAs heterostructures was observed via the degree of circular polarization of the photoluminescence. Application of a magnetic field perpendicular to the propagation of the light allows one to determine the spin relaxation time T₁ and the electron lifetime τ in the conduction band. By tilting the magnetic field with respect to an estimate of the effective nuclear field acting on the electrons can be obtained.     

Spin-dependent electron dynamics and recombination in GaAs<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys at room temperature

Conduction-electron spin polarization dynamics achieved by pulsed optical pumping at room temperature in GaAs_1−x N _x alloys with a small nitrogen content (x = 2.1, 2.7, and 3.4%) is studied both experimentally and theoretically. It is found that the photoluminescence circular polarization determined by the mean spin of free electrons reaches 40–45% and this giant value persists within 2 ns. Simultaneously, the total free-electron spin decays rapidly with the characteristic time ≈ 150 ps. The results are explained by spin-dependent capture of free conduction electrons on deep paramagnetic centers resulting in the dynamical polarization of bound electrons. A nonlinear theory of spin dynamics in the coupled system of spin-polarized free and localized carriers has been developed which describes the experimental dependencies, in particular, the electron spin quantum beats observed in a transverse magnetic field. The text was submitted by the authors in English.     

Studies of the third-order nonlinear optical susceptibility for InxGa1−xN/GaN cylinder quantum dots

The resonant third-order susceptibilities at various directions (both parallel and vertical to Z-axis) in self-assembled quantum dots (QDs) have been investigated. The nonlinear susceptibilities associated with the intraband transition in the conduction band are theoretically calculated for wurtzite In_xGa_1−xN/GaN-strained cylinder QDs. The confined wave functions and energies of electrons in the dots have been calculated in the effective-mass approximation by solving the 3D Schrödinger equation, in which a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization has been taken into account. Furthermore, it is shown that the magnitude and the resonant position of the nonlinear susceptibility χ⁽³⁾(3ω) strongly depend on the dots’ size as well as size distribution.