Maria Vladimirova

Nonlinear optical spectroscopy of indirect excitons in coupled quantum wells

Indirect excitons in coupled quantum wells are long-living quasiparticles, explored in the studies of collective quantum states. We demonstrate that, despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons, and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons but also one of free electrons by two orders of magnitude.

Self-Organization of Silicon Dots Grown by Thermal Decomposition of HSiO3/2 Gels

Silicon crystallites embedded in thin silica films were prepared by the sol-gel route, using triethoxysilane as a precursor. The films of silicon crystallites were either obtained as free-standing films or were deposited on (001)-oriented silicon or glass substrates by a spin coating deposition of a liquid phase that was further heat-treated under static vacuum. It was found out that the processing temperature impacts both the silicon dot size and the density of dots. We also deposited such crystallites on silicon using the thermal decomposition of the dried gels under vacuum. In the latter case, two regimes of the silicon dot growth have been observed depending on the decomposition temperature. Specifically, at T≈800°C (regime I) the round-shaped silicon dots are virtually randomly distributed across the substrate surface, whereas the regime II at T≈1000°C is characterized by growth of the elongated silicon dots arranged in nanochains along the (1±10) axis.

Room-Temperature Transport of Indirect Excitons in (Al,Ga)N/GaN Quantum Wells

We report on the exciton propagation in polar (Al,Ga)N/GaN quantum wells over several micrometers and up to room temperature. The key ingredient to achieve this result is the crystalline quality of GaN quantum wells (QWs) grown on GaN template substrate. By comparing microphotoluminescence images of two identical QWs grown on sapphire and on GaN, we reveal the twofold role played by GaN substrate in the transport of excitons. First, the lower threading dislocation densities in such structures yield higher exciton radiative efficiency, thus limiting nonradiative losses of propagating excitons. Second, the absence of the dielectric mismatch between the substrate and the epilayer strongly limits the photon guiding effect in the plane of the structure,making exciton transport easier to distinguish from photon propagation. Our results pave the way towards room-temperature gate-controlled exciton transport in wide-bandgap polar heterostructures.

Transport of dipolar excitons in (Al,Ga)N/GaN quantum wells

We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al, Ga) N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density, measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons. We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 mu m is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the exciton emission pattern on the scale up to 100 mu m. The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

Transport of indirect excitons in ZnO quantum wells.

We report on spatially- and time-resolved emission measurements and observation of transport of indirect excitons in ZnO/MgZnO wide single quantum wells.

Nuclear spin warm up in bulk n-GaAs

We show that the spin-lattice relaxation in n-type insulating GaAs is dramatically accelerated at low magnetic fields. The origin of this effect, which cannot be explained in terms of well-known diffusion-limited hyperfine relaxation, is found in the quadrupole relaxation, induced by fluctuating donor charges. Therefore, quadrupole relaxation, which governs low field nuclear spin relaxation in semiconductor quantum dots, but was so far supposed to be harmless to bulk nuclei spins in the absence of optical pumping, can be studied and harnessed in the much simpler model environment of n-GaAs bulk crystal.

Significant photoinduced Kerr rotation achieved in semiconductor microcavities

Giant Kerr rotation and ellipticity are observed and investigated in an asymmetric planar microcavity with a quantum well in the active region. Rotation angle of the polarization plane as well as ellipticity were determined from time- and frequency-resolved measurements of the Stokes vector components of reflected light. It was found that in a small range of the cavity mode detunings the polarized pump pulse creates a large splitting of the lower polariton branch while leaving its linewidth almost the same. This fact gives a possibility to observe at such detunings the Kerr rotation angle and ellipticity, close to their extremes. A theoretical analysis shows that the decisive role in reaching extreme polarization rotation angles is played by the structure asymmetry. Comprehensive analysis of the polarization state of the light in this regime shows that both renormalization of the exciton energy and the saturation of the excitonic resonance contribute to the observed optical nonlinearities.

Exciton and polariton spin beats in a CdMnTe based microcavity

Exciton and polariton spin beats are observed in a CdMnTe quantum well embedded in a microcavity using time‐resolved Kerr rotation experiments under magnetic field. Photoinduced linear birefringence phenomenon allows to comprehend the polariton spin beats using linear polarized pumping in Faraday geometry. Exciton spin beats can be detected in the standard configuration with circularly polarized pump pulse and under in‐plane magnetic field.

Excitonic polarization grating in semiconductors induced by short light pulses

Abstract A scattering-state approach is proposed to study the propagation of extremely short optical pulses through a semiconductor. The formalism is applied to the propagation of exciton–polaritons in semiconductor films: Our simulated experiments predict the formation of an exciton-induced polarization grating when the light pulse is resonant with the excitonic transition, and suggest physical conditions for its experimental detection. Moreover, our analysis of the polariton transport in thick semiconductor layers reveals a decrease of the average polariton group velocity as a function of time, which we ascribe to a re-emission–re-absorption of light by excitons.

Resonant Faraday rotation effect in semimagnetic and nonmagnetic microcavities

Abstract Rotation of the polarization plane of a linearly polarized light (Faraday rotation, FR) at the quantum-well (QW) exciton resonance frequency is greatly amplified if the exciton is coupled to the cavity photon mode due to multiple round-trips of light inside the cavity. Simultaneously, a redistribution of the energy of the electromagnetic wave between linear and circular polarizations takes place. In the empty semimagnetic cavity this leads to the splitting of the cavity-mode dispersion curves in right- and left-circular polarizations. The purely electromagnetic theory of these effects has been constructed. Experimentally, resonant excitonic Faraday rotation achieves 3° in reflection in the microcavities studied.