Helen Tsitsishvili

Optical anisotropy is the dependence of the optical properties of crystals on the polarization state of the light field. A brief overview is given on the description of the anisotropic optical effects in bulk semiconductor crystals and in low-dimensional semiconductor structures.

Infrared transitions of excitons in Cu 2 O from the 1s to 2p levels have been investigated with a pump-probe experiment. 1s densities around 10 15 /cm 3 have been created by pumping with a cw-laser and the induced 1s M 2p absorption has been probed simultaneously. The main experimental finding is that there is a splitting of , 3.7 v meV in the 2p level scheme, which has not been reported so far. Analyzing the linewidth and shape of the transitions, we find that the effective mass of the 1s state might have been overestimated in earlier studies.

The temperature dependence of exciton peak energies in large CuI quantum dots embedded in a glass matrix has been measured. The gap shrinkage effect for both cubic and hexagonal CuI phases was observed. The temperature coefficients of the energy gaps and the effective phonon energies are deduced.

We study the phonon-induced flip of the exciton spin in single strongly confining quantum dots. The considered two-phonon process contributes to the exciton spin relaxation (longitudinal relaxation time T1) within the radiative doublet of the exciton ground state. The respective effective matrix element is driven by an interplay of the short-range exchange interaction and the lattice deformation induced by acoustic phonons. The two-phonon process involves the participation of the dipole-forbidden dark states. The here considered relaxation channel is of paramount importance for symmetrical dots and may be several orders of magnitude faster than the previously studied transition between bright states in asymmetrical quantum dots. The calculated relaxation rates depend on the dot composition and shape, and decrease very strongly upon reduction in the dot size. For various individual dots belonging to a large quantum-dot ensemble the respective relaxation times may differ by a several orders of magnitude. For a typical ensemble of InAs/GaAs self-organized quantum dots at low temperatures, the numerical estimates range from few hundreds of microseconds for the largest dots to few tens of nanoseconds for the smallest dots involved.

We study the phonon-assisted relaxation processes (longitudinal relaxation time T1) within the radiative doublet of the heavy-hole-exciton in asymmetrical quantum dots. Two different relaxation mechanisms are considered: the exciton spin–acoustic phonon coupling via the strain-dependent short-range exchange interaction and the second-order quasielastic interaction between charge carriers and LO phonons. For zero magnetic fields and low temperatures, the calculated relaxation times for typical QDs are very long compared to the exciton lifetime yet they are strongly reduced in high magnetic fields (of the order of a few Tesla) and high temperatures T ≥ 100 K.

In a differential absorption experiment the induced infrared transitions from the excitonic 1s to the 2p levels in Cu2O have been investigated. Intermediate densities of 1s excitons were created by cw-laser excitation while the interexcitonic 1s to 2p transitions were probed simultaneously, using Fourier spectroscopy. Our data give evidence for a surprisingly large splitting of the 2p level (≈3.7 meV) the origin of which is a matter of speculation. An analysis of lineshape and width of the transitions results in a ratio of the effective masses m1s/m2p which deviates from the literature value.

We present an analytic solution to the problem of the Rashba spin-orbit coupling in semiconductor quantum dots. We calculate the exact energy spectrum, wave-functions, and spin--flip relaxation times. We discuss various effects inaccessible via perturbation theory. In particular, we find that the effective gyromagnetic ratio is strongly suppressed by the spin-orbit coupling. The spin-flip relaxation rate has a maximum as a function of the spin-orbit coupling and is therefore suppressed in both the weak- and strong coupling limits. 

We present a quantitative interpretation of the anomalous temperature behavior—the so-called “S-shape dependence”—of the photoluminescence (PL) in quantum films containing nanoislands. Experimental data for CdSe/ZnSe samples are modeled using Monte Carlo simulations of the involved relaxation mechanisms and thus providing a realistic picture of the exciton kinetics. We are able to reproduce simultaneously the temperature dependence of the PL maximum and of the full width at half maximum of the PL with good accuracy. We deduce information about the distribution of the localization centers and identify hopping processes between spatially separated states within one island.

We study the relaxation of the exciton spin (longitudinal spin–relaxation time T1) in single asymmetrical quantum dots (QD) at low temperatures. The main relaxation mechanism is due to the exciton spin–acoustic phonon coupling via the strain–dependent exchange interaction; piezoelectric coupling is less efficient. For zero magnetic field, relaxation within the radiative exciton–doublet for typical QDs is very slow compared to the exciton lifetime. Relaxation to nonradiative states becomes important for QDs with large singlet–triplet splitting of a few meVs. The calculated relaxation times strongly decrease in high magnetic fields. 

Theoretical results are given for spin relaxation in semimagnetic semiconductor quantum wells due to longitudinal optical (LO) phonon-induced flips of exciton spins at zero temperature and modest magnetic fields. Relaxation in this scenario is due to spin-flip transitions within the heavy-hole exciton subbands which are mediated by the coupling of excitonic spin states via the electron-hole exchange interaction. Relaxation rates are found to depend strongly on a magnetic field, exciton momentum, and size of the quantum well. Results are illustrated by evaluations for the ZnSe-based semimagnetic quantum wells. In longitudinal magnetic fields (Faraday geometry) a maximum in the relaxation rate is found for zero-momentum excitons at a Zeeman splitting of ∼60meV. In transverse magnetic fields (Voigt geometry) the LO-induced spin relaxation is strongly suppressed.

We present theoretical results for the spin relaxation of exciton-bound electrons and holes in weakly confining quantum dots. The relaxation is driven by the spin-orbit interaction in the conduction band and the linear in the momentum term in the valence band, respectively. The relaxation occurs between the optically active (bright) and inactive (dark) exciton states due to acoustic-phonon-assisted spin flips. The exchange splitting between the bright and dark states acts as a constant external magnetic field. A sequential flip of the (exciton-bound) electron and hole spins results in the spin-flip transition between the bright exciton states (i.e., an exciton-spin relaxation). We find that the spin relaxation time for an exciton-bound electron is several orders of magnitude faster than for a single electron. The resulting exciton spin-relaxation time is also several orders of magnitude faster than the one in small dots which is driven by the electron hole exchange interaction. We obtain the dependence of the exciton-spin relaxation time on dot size and temperature.

  In weakly confining quantum structures such as interfacial islands or quantum disks the exciton-spin relaxation is governed by two independent electron and hole spin flip processes between the optically active and dark states. A microscopic theory for these transitions is presented which is based on second order spin–orbit and carrier–phonon interaction processes. We found that the sequential relaxation between bright and dark states leads to much faster exciton-spin relaxation than for strongly confining (“small”) quantum dots where the dominant process stems from electron–hole exchange interaction plus hole deformation potential coupling. In addition, the fast exciton spin relaxation implies that the (exciton-bound) electron spin flip time is also much shorter than for a single electron. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on a theoretical study of the fine structure of excited excitonic levels in semiconductor quantum disks. A particular attention is paid to the effect of electron–hole long-range exchange interaction. We demonstrate that even in the axisymmetric quantum disks, the exciton P-shell is split into three sublevels. The analytical results are obtained in the limiting cases of strong and weak confinement. A possibility of exciton spin relaxation due to the resonant LO-phonon-assisted coupling between the P and S shells is discussed.

Electron relaxation in a GaAs quantum dot is investigated to second order in electron-phonon interactions. Calculation of relaxation rate, as a function of level separation, indicates the significant contribution of LO±LA processes, which create a window of rapid (subnanosecond) relaxation around the longitudinal-optical phonon energy. This result may provide a possible solution to the problem of photoluminescence degradation in small quantum dots.

The nature and the relaxation properties of the localized electronic states responsible for the radiative recombination in the ensemble of self-organized CdSe/ZnSe quantum dots (QDs) are studied and discussed. It has been found that due to the complicated topological relief of the localizing potential in the lateral plane of the QDs, both ground and metastable states of dots contribute to the emission at helium temperatures. The metastable states are subjected to further energy relaxation, with the rate being dependent on the temperature. The temperature dependence of the relaxation rate is responsible for the anomalous temperature shift of the luminescence band maximum. By the study of polarization properties of the emission at resonant excitation by linearly or circularly polarized light we have shown that, in the samples studied, the photoluminescence (PL) of quantum dots arises due to the radiative recombination of both excitons and trions formed in the charged dots. The exciton emission comes mostly from the metastable states responsible for the high-energy part of the PL band, whereas the trion emission corresponds to the ground states of the quantum dots and forms the low-energy wing of the PL band. The main attention is given to the theoretical and experimental study of the spin-relaxation processes of the electronic states in QDs. We have demonstrated that the broadening of localized electronic levels caused by multi-phonon processes strongly influences the spin relaxation. Such broadening removes the limitations on the energy of phonons assisting the one-phonon transitions between spin sublevels. Taking into account the multi-phonon broadening of spin sublevels we were able to describe theoretically the temperature dependence of the polarization degree of PL spectra of CdSe/ZnSe quantum dots and to restore from experimental data the characteristic function, describing the interaction of localized electronic states with phonons.

We have calculated the characteristic time for the direct heating of the Mn spin system by spin-polarized photoexcited holes as well as the relaxation time for a single hole in semimagnetic quantum wells in an external longitudinal magnetic field. This spin-conserving heating process is due to the p−d exchange interaction-induced scattering of the holes with the Mn ions within the first heavy-hole subband. The relaxation time for a single hole determines the maximum relative change of the magnetization in the case of short (femtosecond) pulse excitation. Numerical calculations are given for the example of (Zn,Mn)Se-based quantum wells. Due to the strong spin-orbit and exchange interactions within the valence band, the photoexcited hole gas heats the Mn spin system on a nanosecond time scale. This is much more effective than for an electron gas under similar conditions, for which the corresponding characteristic time lies in microsecond range. For a single hole, the relaxation time is much smaller than the characteristic heating time and, for typical experimental conditions, lies in the picosecond range. In the frame of the developed theoretical model, we analyze the details of the dependence of the Mn heating on the hole concentration and temperature on the Mn content and on magnetic field.

We study the phonon-induced flip of the exciton spin in single strongly confining quantum dots. The considered two-phonon process contributes to the exciton spin relaxation (longitudinal relaxation time T1) within the radiative doublet of the exciton ground state. The respective effective matrix element is driven by an interplay of the short-range exchange interaction and the lattice deformation induced by acoustic phonons. The two-phonon process involves the participation of the dipole-forbidden dark states. The here considered relaxation channel is of paramount importance for symmetrical dots and may be several orders of magnitude faster than the previously studied transition between bright states in asymmetrical quantum dots. The calculated relaxation rates depend on the dot composition and shape, and decrease very strongly upon reduction in the dot size. For various individual dots belonging to a large quantum-dot ensemble the respective relaxation times may differ by a several orders of magnitude. For a typical ensemble of InAs/GaAs self-organized quantum dots at low temperatures, the numerical estimates range from few hundreds of microseconds for the largest dots to few tens of nanoseconds for the smallest dots involved.

We report measurements of dynamic (ac) electrical conductivity in borosilicate glasses doped with the semiconductors CdS x Se 1-x and AgI in a wide range of frequencies and temperatures. The concentrations of homogenously dissolved dopants are governed by the heat treatment conditions of the glass samples leading to a creation of CdSSe and AgI nanocrystals. The ac conductivity rises with increasing average size of the CdSSe nanocrystals, in contrast to the case of the metal halide doped borosilicate glasses.

We study the phonon-induced flip of the exciton spin in single strongly confining quantum dots. The considered two-phonon process contributes to the exciton spin relaxation (longitudinal relaxation time T1) within the radiative doublet of the exciton ground state. The respective effective matrix element is driven by an interplay of the short-range exchange interaction and the lattice deformation induced by acoustic phonons. The two-phonon process involves the participation of the dipole-forbidden dark states. The here considered relaxation channel is of paramount importance for symmetrical dots and may be several orders of magnitude faster than the previously studied transition between bright states in asymmetrical quantum dots. The calculated relaxation rates depend on the dot composition and shape, and decrease very strongly upon reduction in the dot size. For various individual dots belonging to a large quantum-dot ensemble the respective relaxation times may differ by a several orders of magnitude. For a typical ensemble of InAs/GaAs self-organized quantum dots at low temperatures, the numerical estimates range from few hundreds of microseconds for the largest dots to few tens of nanoseconds for the smallest dots involved.

We report measurements of dynamic (ac) electrical conductivity in borosilicate glasses doped with the semiconductors CdS x Se 1-x and AgI in a wide range of frequencies and temperatures. The concentrations of homogenously dissolved dopants are governed by the heat treatment conditions of the glass samples leading to a creation of CdSSe and AgI nanocrystals. The ac conductivity rises with increasing average size of the CdSSe nanocrystals, in contrast to the case of the metal halide doped borosilicate glasses.

We report measurements of dynamic (a.c.) electrical conductivity in borosilicate glasses doped with cadmium sulfoselenide and copper and silver iodides in a wide temperature range below the glass transition temperature Tg and at different frequencies. The concentration of the mobile dopant ions is governed by specific heat treatment conditions of the glass samples leading to a creation of the CdSSe, AgI and CuI semiconductor nanocrystals. Investigations include different cases from a full solution of the dopant ions coming from a dissociation of the dopants during the glass preparation to their almost complete incorporation into the nanocrystals in the glass matrix. At temperatures higher than 150 °C–200 °C the a.c. conductivity in all the examined glasses exhibits the Arrhenius behavior. In this temperature range the mixed mobile ion effect is detected: the doped glasses have the low values of the conductivity compared to the undoped ones. In the low temperature range only weak temperature dependence is detected for all the samples. The mixed mobile ion effect is still presented for the CdSSe-doped glasses, whereas the AgI- and CuI-doped glasses exhibit the classical MMIE which is essentially absent at low temperatures.

The phonon-induced flip of the exciton spin in single flat semiconductor quantum dots with a light-hole exciton ground state is studied. The corresponding quartet, split by the exchange interaction, consists of three bright states and a dark state located energetically below the bright exciton. The two in-plane polarized bright states contribute to single-phonon transitions to the dark state and also to the upper bright state polarized in the z growth direction of the dot. For these processes, the presented analytical results are calculated for the relaxation driven by the spin-orbit interaction in the conduction and the light-hole valence subbands. The estimated spin-relaxation times at low temperature are (at least) one order of magnitude lower than the bright exciton lifetime. Two other possible transitions, within the in-plane polarized doublet and between the z-polarized bright and dark states as well, proceed via intermediate states with a contribution from two acoustic phonons. These processes are strongly suppressed at low temperature, whereas they appear to be of the same intensity as single-phonon transitions at high enough temperatures.

We present analytical results which describe the properties of the exciton ground state in a single semiconductor quantum dot (QD). Calculations are performed within the Luttinger-Kohn and Bir-Pikus Hamiltonian theory. We show in an explicit form that an interplay of the exchange interaction and the heavy hole-light hole coupling, which is due to the in-plane asymmetries of the dot shape and the strain distribution, plays an essential role. For both the bright and dark exciton, this combined effect leads to a dependence of the fine structure splitting and polarizations on the main anisotropy axis direction relative to the dot orientation. Basing on the obtained analytical expressions, we discuss some special cases in details.