The pump-probe technique was used to study the reflectance of CdTe based quantum wells. The energy of a pump beam
was scanned across excitonic resonances. The observed dynamics for both the energetic position and oscillator strength
of excitonic lines is caused by the changes of the population of charge carriers as well as by the spin dependent
interactions. The polarization resolved resonance technique was applied to the system of single CdTe - based quantum
dots. Observation of linear to circular polarization conversion for pair of the adjacent, anisotropic dots allowed
determining the time of excitation transfer between them.
The talk summarizes results of our recent optical studies related to spin states in II-VI and III-V semiconductor quantum dot (QD) systems. First the influence of in-plane anisotropy on the QD excitonic spin states is recalled. Then various ways of circumventing, compensating, or exploiting this influence are discussed. Short lifetime of neutral excitons (governed by inter-dot tunneling) allowed us to transfer their spin polarization to another QD before its destruction by the anisotropic exchange interaction. This spin polarization, as well as single carrier spin memory effects in quantum dots are demonstrated using trion states, negligibly perturbed by the anisotropy. Modification of the anisotropy by external perturbations (electric and magnetic field) is shown. In particular, full compensation of the anisotropy by in-plane electric field is demonstrated using optical orientation of neutral excitons. Finally, the influence of the anisotropy will be exploited to achieve circular-to-linear and linear-to-circular polarization conversion in single QDs and in coupled QD pairs.
The paper presents a practical study of several methods of image analysis applied to polarimetric images of regular and
malignant human tissues. The images of physiological and pathologically changed tissues from body and cervix of
uterus, intestine, kidneys and breast were recorded in transmitted light of different polarization state. The set up of the
conventional optical microscope with CCD camera and rotating polarizer's were used for analysis of the polarization
state of the light transmitted through the tissue slice for each pixel of the camera image. The set of images corresponding
to the different coefficients of the Stockes vectors, a 3×3 subset of the Mueller matrix as well as the maps of the
magnitude and in-plane direction of the birefringent components in the sample were calculated. Then, the statistical
analysis and the Fourier transform as well as the autocorrelation methods were used to analyze spatial distribution of
birefringent elements in the tissue samples. For better recognition of tissue state we proposed a novel method that takes
advantage of multiscale image data decomposition The results were used for selection of the optical characteristics with
significantly different values for regular and malignant tissues.
This work is devoted to correlation spectroscopy of individual II-VI CdTe/ZnTe QDs in view to determine non-resonant
excitation mechanisms and provide information on spin relaxation of QD states. Second order photon autocorrelations
and cross-correlations were measured in a Hanbury-Brown and Twiss setup for neutral and charged exciton and
biexciton transitions, excited by pulses of a frequency-doubled femtosecond Ti:Sapphire laser. Some of the
measurements were circular- or linear polarization resolved and performed in magnetic field. Besides, measurements of
photoluminescence excited by pairs of laser pulses revealed fast excitation phenomena in the range of tens of ps. The
results of measurements without polarization resolution were interpreted using a simple rate equation model and allowed
us to establish the dominant role of single carrier capture in the non-resonant excitation of the QD. Polarization-dependent
correlation measurements were used to study the magnetic field controlled transition between anisotropic QD
exciton eigenstates active in linear polarization and those active in circular polarization. The same measurements
provided information on spin relaxation of the carriers left in the dot after charged exciton recombination.
We studied the influence of the populations of neutral and positively charged excitons (trions) on optical absorption of modulation p-doped CdTe-based quantum wells. The density of 2D hole gas in the quantum well was controlled by an additional cw illumination in the range from 1010 cm-2 to 1011 cm-2. Time-resolved absorption was measured following a picosecond, circularly polarized, resonant pump pulse, which created significant exciton population. A spectrally broad femtosecond probe pulse was used to detect the absorption over the excitonic region, including exciton, trion and biexciton transition energies. Besides, we used a small magnetic field (below 1T) to create a steady-state spin polarization of the hole gas. By exploiting polarization-dependent selection rules, we were able to identify exciton, trion and biexciton absorption lines without ambiguity. We studied the evolution of these absorption lines under influence of photo-created populations of excitons and trions. The results are interpreted in terms of spin-dependent exciton-exciton and exciton-carrier interaction, the latter being dominant, in contrast with results obtained on GaAs-based quantum wells. We propose a new explanation of the oscillator strength stealing phenomena observed in doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blocked. Experimental evidence is presented for creation of a transient spin polarization in the system by a circularly polarized pump pulse.