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.
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.
New structures aiming at controlling ferromagnetic properties of Diluted Magnetic Semiconductors quantum wells are presented. The carrier density is monitored by applying voltage in p-i-n diode or adjusting a distance between quantum well and surface. Surface doping was successfully applied to obtain samples with CdMnTe quantum well with up to 9.3% Mn concentration.
The advantages of introduction of magnetic ions in semiconductor quantum structures are discussed and illustrated by relevant examples from recent research results, such as growth of spin superlattices and studies of resonant tunneling in double quantum well structures. Particular attention is paid to non-conventional methods of characterization of interfaces made possible by the presence of magnetic ions: spin tracing, useful for determination of the interface composition profile width, and detection of undulation of the quantum well surface by temperature dependent polarization measurements of the Mn ESR Raman cascade.
The principle of spin tracing is based on the exchange interaction between charge carriers and magnetic ions, which is known to produce giant magneto optical effects in Semimagnetic- or Diluted Magnetic Semiconductors. The idea of spin tracing is exploited in the Zeeman method of interface characterization to study spatial distribution of magnetic ions interacting with a carrier (exciton) confined in quantum structures made of ternary CdMnTe compounds. In this paper we give a concise description of the spin tracing method followed by a review of available experimental data related to the influence of interfaces on the Zeeman effect in structures with barriers containing magnetic ions. A critical review of strong and weak points of the method is given accompanied by a discussion of its practical applicability. Special attention is paid to studies of annealing processes of low dimensional structures using the Zeeman method. We analyze the results obtained so far on quantum wells and superlattices and discuss perspectives of further studies.