Dynamics of coherent and incoherent spin polarizations in ensembles of quantum dots

The temperature dependence of spin coherence in InGaAs quantum dots is obtained from quantum beats observed in polarization-resolved pump-probe experiments. Within the same sample we clearly distinguish between coherent spin dynamics leading to quantum beats and incoherent long-lived spin-memory effects. Analysis of the coherent data using a theoretical model reveals approximately 10 times greater stability of the spin coherence at high temperature compared to that found previously for exciton states in four-wave-mixing experiments by Borri et al. [Phys. Rev. Lett. 87, 157401 (2001)]]. The data on incoherent polarization reveal a new form of spin memory based on charged quantum dots.     

THERMODYNAMIC AND PARTICLE-DYNAMIC STUDIES ON SYNTHESIS OF SILICA NANOPARTICLES USING MICROWAVE-INDUCED PLASMA CVD

1. Introduction1.1 Silica nanoparticles and synthesis methods Silica (SiO2) nanoparticles are widely used in industry asan active filler for polymer reinforcement, a rheologicaladditive in fluids, a free flow agent in powders, and anagent for chemical mechanical polishing during IC (inte-grated circuit) fabrication (Sniegowski & de Boer, 2000).Silica powder is also used for producing silicon carbide(Koc & Cattamanchi, 1998) or opaque silica aerosols (Leeet al., 1995). Many methods can …     

Reconstruction of the shape of echo-sounded objects

The paper considers the problem of reconstructing the shape of echo-sounded objects under conditions when the well-known methods, e.g., the Boyarskii method, are inapplicable or inefficient. A variant of the method is proposed for reconstructing the shape of an object using the function from the Gaussian curvature of the surface. As well, it is shown that the main information on this function is carried by the initial region of the echo signal and that the results of reconstructing the shape depend on the correlation duration of probing signals. When ultra-wideband probing signals are applied, this method is fundamentally suitable for reconstructing the shape of silt-covered objects. Formally, the method is applicable only for convex surfaces, but it can be expanded to more complex surfaces under certain conditions.     

Laser location and manipulation of a single quantum tunneling channel in an InAs quantum dot

We use a femtowatt focused laser beam to locate and manipulate a single quantum tunneling channel associated with an individual InAs quantum dot within an ensemble of dots. The intensity of the directed laser beam tunes the tunneling current through the targeted dot with an effective optical gain of 10(7) and modifies the curvature of the dot's confining potential and the spatial extent of its ground state electron eigenfunction. These observations are explained by the effect of photocreated hole charges which become bound close to the targeted dot, thus acting as an optically induced gate electrode.     

Nonlinear compression of optical solitons

In this paper, we consider nonlinear Schrödinger (NLS) equations, both in the anomalous and normal dispersive regimes, which govern the propagation of a single field in a fiber medium with phase modulation and fibre gain (or loss). The integrability conditions are arrived from linear eigen value problem. The variable transformations which connect the integrable form of modified NLS equations are presented. We succeed in Hirota bilinearzing the equations and on solving, exact bright and dark soliton solutions are obtained. From the results, we show that the soliton is alive, i.e. pulse area can be conserved by the inclusion of gain (or loss) and phase modulation effects.     

Symplectic supercuspidal representations and related problems

In this paper, we summarize the basic structures and properties of irreducible symplectic supercuspidal representations of GLn(F) over a p-adic local field F with characteristic zero, and explore possible topics for further investigation.     

Interference effects in the emission spectra of quantum dots in high-quality cavities

We have investigated theoretically and experimentally the emission of (quantum dot-cavity) systems for different coupling strength and a wide range of exciton-photon mode detunings controlled by temperature variation in the range 10–45 K. Under close to resonance conditions, the radiation spectrum from the cavity emission becomes essentially dependent on the primary excitation path, which can be either via resonant quantum-dot exciton or via cavity mode. Particularly, in the case of nonresonant cavity mode excitation, the emission line becomes split into two asymmetric lines already in the weak coupling regime. The text was submitted by the authors in English.     

Energy relaxation of excitonlike polaritons in semiconductor microcavities: Effect on the parametric scattering of polaritons

Polariton emission in GaAs-based microcavities has been studied under variable conditions, which made it possible to excite (a) polaritons from the upper polariton branch and hot free polaritons and electrons, (b) polaritons from the lower polariton branch (LPB) and localized excitons, and (c) the mixed system. Variation of the excitation conditions leads to substantial differences in the energy distributions of polaritons and in the temperature dependences of polariton emission. It is established that the energy relaxation of resonantly excited LPB polaritons via polariton and localized exciton states at liquid helium temperatures is ineffective. Instead, the relaxation bottleneck effect is suppressed with increasing temperature by means of exciton delocalization (due to thermal excitation by phonons). The most effective mechanism of relaxation to the LPB bottom is via scattering of delocalized excitons on hot free carriers. It is found that the slow energy relaxation of polaritons excited below the free exciton energy can be significantly accelerated at low temperatures by means of additional weak generation of hot excitons and, especially, hot electrons. This acceleration of the energy relaxation of polaritons by means of additional overbarrier photoexcitation sharply decreases the barrier for stimulated parametric scattering of polaritons excited at an LPB inflection point. Therefore, additional illumination can be used to control the polariton-polariton scattering.     

Stimulated parametric scattering of excitonic polaritons in planar GaAs microcavities: Distinctive feature of QW electric field

Instabilities in the electric field at the QW in the active region of a GaAs based microcavity have been investigated under the resonance excitation near the inflection point of the low polariton (LP) dispersion curve with the use of four wave mixing technique. The electric field jump due to an LP bistability has been found to precede the development of a stimulated parametric scattering of LPs. The latter has been found to develop with a delay of a few hundreds ps from the beginning of the excitation pulse. These results are in qualitative agreement with hard regime of excitation of the stimulated parametric LP scattering predicted recently.