Petra Horodyska

Light-induced magnetization precession in GaMnAs

We report the dynamics of the transient polar Kerr rotation (KR) and of the transient reflectivity induced by femtosecond laser pulses in ferromagnetic (Ga,Mn)As with no external magnetic field applied. It is shown that the measured KR signal consists of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in (Ga,Mn)As. The origin of the light-induced magnetization precession is discussed and the magnetization precession damping (Gilbert damping) is found to be strongly influenced by annealing of the sample.

Systematic Study of Mn-Doping Trends in Optical Properties of (Ga,Mn)As

We report on a systematic study of optical properties of (Ga,Mn)As epilayers spanning the wide range of accessible Mn(Ga) dopings. The material synthesis was optimized for each nominal Mn doping in order to obtain films which are as close as possible to uniform uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the mid-infrared absorption spectra whose position exhibits a prevailing blueshift for increasing Mn doping. In the visible range, a peak in the magnetic circular dichroism also shifts with increasing Mn doping. The results are consistent with the description of ferromagnetic (Ga,Mn)As based on the microscopic valence band theory. They also imply that opposite trends seen previously in the optical data on a limited number of samples are not generic and cannot serve as an experimental basis for postulating the impurity band model of ferromagnetic (Ga,Mn)As.

Exciton spin dynamics in spherical CdS quantum dots

Exciton spin dynamics in quasi-spherical CdS quantum dots is studied in detail experimentally and theoretically. Exciton states are calculated using the 6-band k.p Hamiltonian. It is shown that for various sets of Luttinger parameters, when the wurtzite lattice crystal field splitting and Coulomb interaction between the electron-hole pair are taken into account exactly, both the electron and hole wavefunction in the lowest exciton state are of S-type. This rules out the spatial-symmetry-induced origin of the dark exciton in CdS quantum dots. The exciton bleaching dynamics is studied using time- and polarization-resolved transient absorption technique of ultrafast laser spectroscopy. Several samples with a different mean size of CdS quantum dots in different glass matrices were investigated. This enabled the separation of effects that are typical for one particular sample from those that are general for this type of material. The experimentally determined dependence of the electron spin relaxation rate on the radius of quantum dots agrees well with that computed theoretically.

Laser-Induced Precession of Magnetization in GaMnAs

We report on the photo-induced precession of the ferromagnetically coupled Mn spins in (Ga,Mn)As, which is observed even with no external magnetic field applied. We concentrate on various experimental aspects of the time-resolved magneto-optical Kerr effect (TR-MOKE) technique that can be used to clarify the origin of the detected signals. We show that the measured data typically consist of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in the sample.

Molecular beam epitaxy of LiMnAs

We report on the molecular beam epitaxy (MBE) growth of LiMnAs. Our epilayers are of high crystalline quality. The introduction of fluxes of a group-I alkali metal element Li comparable to fluxes of Mn and As has not caused any apparent damage to the MBE system after as many as 15 growth cycles performed on the system to date.

Recent achievements on ASPIICS, an externally occulted coronagraph for PROBA-3

This paper presents the current status of ASPIICS, a solar coronagraph that is the primary payload of ESA’s formation flying in-orbit demonstration mission PROBA-3. The “sonic region” of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as ~1.1 solar radii. The images will be tiled and compressed on board in an FPGA before being down-linked to ground for scientific analyses. ASPIICS is built by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent development status of the ASPIICS instrument as it is approaching CDR.

Development of ASPIICS: a coronagraph based on Proba-3 formation flying mission

This paper presents the recent achievements in the development of ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), a solar coronagraph that is the primary payload of ESA’s formation flying in-orbit demonstration mission PROBA-3. The PROBA-3 Coronagraph System is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the 1.4 meter wide external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as ~1.1 solar radii. Besides providing scientific data, ASPIICS is also equipped with sensors for providing relevant navigation data to the Formation Flying GNC system. This paper is reviewing the recent development status of the ASPIICS instrument as it passed CDR, following detailed design of all the sub-systems and testing of STM and various Breadboard models.

Influence of oxygen on the quality of the PZT thin films prepared by IBS

Pb(Zr,Ti)O3 (PZT) is a ferroelectric material interesting for its high dielectric constant and piezoelectric response. PZT thin films can be prepared by various methods, e.g. pulsed laser deposition, chemical vapor deposition, sol-gel and, most frequently, sputtering. Though the magnetron sputtering is used more frequently, PZT thin films can be prepared also by ion-beam sputtering (IBS). In this paper we study the deposition process of PZT thin films in our IBS system with a possibility of ion-beam assisted deposition (IBAD), which has the advantage that more energy can be added to the growing layer. We show how in our system the resulting layers, mainly their quality, the Pb content, which is important for the creation of the perovskite crystal structure, and the resulting crystal structure are influenced by the oxygen flux during the deposition for the samples grown on the silicon substrate with and without an intermediate Ti seeding layer.

Attempt to prepare perovskite PZT at low temperatures using IBAD

Lead zirconate titanate (Pb[ZrxTi1-x]O3 ) is well-known for his excellent ferroelectric, piezoelectric and electromechanical properties. These properties are closely related to the perovskite crystal structure of PZT. A common way to achieve thin film of perovskite PZT is to anneal the layer after deposition. The high annealing temperature (600 – 700°C) limits a set of usable substrates. To grow a thin layer of perovskite PZT at reduced temperature it is necessary to add crystallization energy to the system by another way. In this article are presented some results of using ion beam sputtering system (IBS) with ion beam assistance (IBAD) to growth perovskite PZT layer at reduced temperature. This process is very complicated and the resulting layer properties are strongly influenced by deposition parameters (ions energy, chemical composition of the atmosphere in the sputtering chamber etc.). We achieved partial success when pyrochlore crystal structure of PZT was grown at reduced substrate temperature (110°C) (at this temperatures are the PZT layers usually amorphous)

Control of chemical composition of PZT thin films produced by ion-beam deposition from a multicomponent target

Lead zirconate titanate (PZT) is widely used for its ferroelectric and piezoelectric properties, which are conditioned by perovskite structure. Crystallization into this desired phase is determined also by a proper stoichiometry, where the lead concentration is a crucial parameter. The crystallization process takes place during annealing under high temperatures, which is linked to heavy lead losses, so the lead has to be in excess. Therefore, this paper is devoted to the control of chemical composition of PZT thin films deposited via ion beam sputtering (IBS). A commonly used approach for IBS relies on employing a multicomponent target to obtain films with the same composition as that of the target. However, in the case of PZT it is favorable to have the ability to controllably change the chemical composition of thin films in order to acquire high perovskite content. Our study revealed that the determinative lead content in PZT layers prepared by simple and dual ion-beam deposition from a multicomponent target can be easily controlled by the power of primary ion source. At the same time, the composition is also dependent on the substrate temperature and the power of assistant ion source. Thin PZT films with more than 30 % lead excess were acquired from a stoichiometric multicomponent target (i.e. a target without any lead excess). We can therefore propose several possible sets of deposition parameters suitable for the PZT deposition via IBS to obtain high perovskite content.