Ce travail explore l'utilisation de l'effet magnéto-optique pour étudier la dynamique de spin des électrons de conduction dans les semi-conducteurs non magnétiques lorsqu'ils sont pompés avec des photons polarisés circulairement. En général, les moments magnétiques hors-équilibre induits optiquement dans les semi-conducteurs non magnétiques sont plus petits que ceux des matériaux magnétiques. L'effet magnéto-optique en principe offre une sensibilité suffisante pour détecter ces faibles moments magnétiques via une mesure de rotation Faraday dans la limite de bruit de photons. Nous avons comparés trois méthodes de détection: les polariseurs partiellement croisés, l’interféromètre de Sagnac et le pont optique. L'interféromètre de Sagnac se révèle fonctionnellement équivalent aux polariseurs partiellement croisés, avec une sensibilité diminuée par la perte de photons à chacun des séparateurs de faisceaux nécessairement présents dans cette configuration expérimentale. Par contre, il a été démontré précédemment que les interféromètres de Sagnac permettent de faire la distinction entre les rotations dites réciproques et non réciproques, et cette thèse propose de nouvelles géométries de Sagnac pour distinguer les rotations en fonction de leurs symétries en temps et en parité. La technique du pont optique présente les meilleures performances. Elle permet une mesure de l'angle de rotation de Faraday limitée par le bruit de photons, même avec des puissances lumineuses importantes reçues par les détecteurs, ce qui permet d'obtenir la meilleure figure de mérite possible. Dans les expériences conduites sur des matériaux magnétiques, un bruit de quelques nrad/√Hz a été mesuré pour une puissance de sonde de 10 mW. Une série de mesures de rotation Faraday pompe-sonde à température ambiante a été réalisée sur GaAs pompé optiquement. Les plus grands signaux sont obtenus lorsque le moment magnétique généré et détecté est maximisé en focalisant fortement les faisceaux pompe et sonde et en choisissant une longueur d'onde de la sonde accordée à une résonance optique dans la structure électronique. Les mesures en champ magnétique transversal montrent un champ Hanle de 0.43 T, à partir duquel on déduit la durée de vie de spin de 88 ps. / This work explores the use of the magneto-optical Kerr effect to study conduction electron spin dynamics in non-magnetic semiconductors when pumped with circularly polarized photons. Typically, non-equilibrium, optically-induced magnetic moments in non-magnetic semiconductors are orders of magnitude smaller than those of magnetized materials, including both magnetic and non-magnetic materials in an external magnetic field. The magneto-optical Kerr effect in principal offers sufficient sensitivity to detect such small magnetic moment via a measurement of the Faraday rotation angle of a probe beam in the photon shot noise limit. Three detection configurations have been experimentally compared: partially crossed polarizers, a Sagnac interferometer and an optical bridge. The Sagnac interferometer is shown to be functionally equivalent to partially crossed polarizers, although its sensitivity is compromised by lost photons at each of the obligatory beam splitters present in such a geometry. On the other hand, it has previously been shown that Sagnac interferometers can distinguish between so-called reciprocal and non-reciprocal rotations, and this thesis proposes novel Sagnac geometries to distinguish rotations according to their time and parity symmetries. The optical bridge technique allows for a photon-shot noise limited measurement of the Faraday rotation angle, even with large photon intensities on the detectors, thereby yielding the best possible figure-of-merit. In demonstrations on magnetic materials, a noise floor of a few nrad//√Hz was measured for a probe power of 10 mW. A series of room-temperature, pump-probe Faraday rotation measurements is performed on optically pumped GaAs to compare and contrast this method with standard polarized photo-luminescence techniques. The largest signals are found when the locally probed moment is maximized by strongly focusing the pump and probe beams, and by choosing a probe wavelength tuned to an optical resonance in the electronic structure. Measurements in transverse magnetic field show a Hanle field of 0.43 T, from which the spin lifetime of 88 ps is deduced.

We have investigated the Faraday effect of bismuth-doped rare-earth iron-garnets with varying doping levels of gallium from z = 1.0 to 1.35. We used lutetium to control the film's in-plane magnetic properties and found that gallium doping levels above the compensation point caused a loss of anisotropy control, a canted out-of-plane magnetization in the film, and an extremely weak but linear coercivity above 10 micro-Tesla fields. Using these results we focused on in-plane films to create 8 layer stacks of 500 um thick films to achieve a minimum detectable field of 50 pT at 1 kHz. Unlike previous Magneto-Optic (MO) studies that typically used thin films of approximately 1um thickness, we used approximately 400um thick films to allow experimentation with the final, robust, ideal form the MO sensor would take. We measured what most other MO studies with garnets neglected: the magnetic anisotropy axis or structure within the film. Knowledge of this structure is essential in improving the sensitivity of a stacked MO probe. Studying thick films proved to be key to understanding the magnetic anisotropy and domain properties that can degrade or enhance the sensitivity of the Faraday rotation in bismuth doped rare-earth iron-garnets to an applied magnetic field and to pointing the direction of future research to develop the conditions for rugged magnetometer sensors. / Physics

Title: Investigation of spin structure and dynamics in magnetically ordered thin films Author: Tomáš Janda Abstract: This thesis is devoted to the development of methods for imaging and ultrafast manipulation of magnetic textures, such as magnetic domains and domain walls (DWs), and to the investigation of the corresponding magnetiza- tion dynamics. We focused on techniques that are, besides ferromagnets (FMs), applicable also to antiferromagnets (AFs), in particular, AF thin films. The employed excitation techniques were mostly based on direct or indirect effects of ultrashort laser pulses. We studied the DW motion induced by a transfer of angular momentum from circularly polarized femtosecond laser pulses in a FM GaMnAsP and we found that the observed macroscopic DW displacement is only possible due to its inertia. In a distinct experiment, picosecond current pulses were generated by an absorption of the ultrashort laser pulses in a verti- cal PIN diode-based photoconductive switch and used to excite a magnetic film deposited directly on top of the diode. For imaging of AF domain patterns, we developed a simple table-top laser-scanning technique, which is based on a magneto-thermoelectric response of the AF textures in the presence of a local laser-induced heating. We first used the method to image the...

This thesis is devoted to numerical simulations of integrated optical isolators and circulators. The results contain: Polarization independent isolators Different magneto-optical configurations are required to produce large nonreciprocal phase shifters for orthogonally polarized modes. The polarization independent isolator can be realized by placing two different nonreciprocal phase shifters into the interferometer arms. The light interferes constructively or destructively at the end of the interferometer depending on the propagation direction. Another possibility is to find a magnetic configuration that yields equal nonreciprocal phase shift for transverse electric (TE) and transversemagnetic (TM) modes. Compared to the concept of polarization independent isolators with two different phase shifters in the interferometer arms, the concept with a polarization independent phase shifter has an advantage: the entire length of the device can be almost halved placing an additional nonreciprocal phase shifter into the second arm. Another advantage is that the power loss inside the nonreciprocal phaseshifter may differ from that in the rest of the structure. For the non-symmetrical setup it can lead to a reduction of the device performance. Utilization of multimode waveguides in magneto-optical devices The principle distinction of a Mach-Zehnder type isolator and an isolator based on multimode imaging is that in the latter case the input power is distributed between modes propagating in the same waveguide, whereas in the first case two separate waveguides are used. Nonreciprocal phaseshifters with different effects on guided modes are needed to produce a magneto-optic multi-mode imaging (MMI) isolator or circulator. Multimode imaging splitters with non zero phase difference between the output modes can be used in integrated optical isolators. If the essential phase difference is utilized by the splitter, the rest of the interferometer should be symmetrical.

Utilizou-se um Hamiltoniano KP (6x6) do tipo Kane para, se estudar a estrutura de bandas e níveis de Landau para heteroestruturas de semicondutores zincblende dos grupos III-V e II-VI. Os efeitos do acoplamento entre as bandas de condução e valência, da mistura dos estados da banda de valência, da não-parabolicidade dos níveis, da total degenerescência dos níveis, do warping e das descontinuidades das massas efetivas nas heterointerfaces são levados em conta. Mostrou-se que a interação entre as bandas de condução e valência não pode ser desprezada, mesmo para semicondutores de gap largo, como citado em trabalhos existentes na literatura. Para um estudo sistemático do modelo, utilizou-se um poço quântico de GaAs Ga(Al)As e então aplicou-se o modelo a um sistema de semicondutores semi-magnéticos (poço quântico de CdTe Cd(Mn)Te). / A Kane-like (6x6) KP Hamiltonian is used to study the subband structure and Landau levels for group III-V and group II-VI zincblende semiconductor heterostructures. The effects of conduction-valence band coupling, valence band states mixing, nonparabolicity of the levels, the full degeneracy of the levels, warping and effective masses discontinuities at the heterointerfaces are taken into account. It is shown that the interaction between conduction-valence bands cannot be neglected, even so the semicondutctor have wide gap, as claimed in previous work in the literature. GaAs-Ga(Al)As quantum well was used as a model for a systematic study of the effects of each effective KP parameters. Then, it was applied to the study the subband structure of semi-magnetic semiconductor system (a quantum well of CdTe-Cd(Mn)Te.

Ce travail de thèse est consacré à l’étude des propriétés magnéto-optiques de nanoparticules (NP) magnétiques de ferrite de cobalt (CoFe2O4) sous forme liquide et lorsqu’elles sont bloquées dans une matrice de silice produite par voie sol-gel. Cette dernière dispersion constitue un matériau composite à activité magnéto-optique obtenu par un procédé basse température qui le rend totalement compatible avec les technologies d’intégration. A plus long terme, ce matériau pourra contribuer à l’intégration de composants non-réciproques. L’objectif de ce travail est d’une part l’identification du type de NP qui maximise la rotation Faraday et le facteur de mérite (rapport de la rotation Faraday à l’absorption) dans le but d’améliorer la qualité magnéto-optique du matériau composite. Et d’autre part, il s’agit d’améliorer la compréhension des phénomènes physiques liés aux effets magnéto-optiques de ces nanoparticules et le lien avec leurs caractéristiques physiques. L’étude est menée sur des NP magnétiques synthétisées et dispersées en phase aqueuse au laboratoire PHENIX (UMR CNRS 8234). Les mesures optiques et magnéto-optiques réalisées au laboratoire Hubert Curien (UMR CNRS 5516) ont été complétées par des mesures magnétiques XMCD au synchrotron SOLEIL. L’étude des différentes nanoparticules magnétiques a révélé que l’utilisation d’une petite taille de NP permet de multiplier par deux le facteur de mérite du matériau pour une longueur d’onde de 1,5 µm, soit une division par deux des pertes pour les composants magnéto-optiques visés. L’analyse du comportement spectral de l’effet Faraday illustre l’influence de la distribution cationique des ions Co2+ et Fe3+ dans la structure cristalline. Couplée aux mesures XMCD, l’analyse montre le besoin d’une localisation de l’ion Co2+ en site tétraédrique dans la structure spinelle pour maximiser l’effet Faraday à 1,5µm, et obtenir une anisotropie uniaxe qui permette une pré-orientation aisée des NP lors de la gélification / This work is dedicated to the study of the magneto-optical properties of cobalt ferrite (CoFe2O4) nanoparticles (NP) dispersed in liquid as ferrofluid, or blocked in a solid silica matrix realized with a sol-gel method. This last dispersion is a magneto-optical composite material, obtained with a low temperature process which insures its compatibility with photonic integration technologies, to produce, in the future, integrated non-reciprocal devices. The aim of the study is, on one hand, to identify which kind of NP can improve the Faraday effect and the merit factor (ratio between the Faraday effect and the absorption) of the composite material. On the other hand, the aim is to give a better understanding of the link between the magneto-optical properties and the physical characteristics of the NP. The study has been led on NP synthetized and dispersed as ferrofluid in PHENIX laboratory (UMR CNRS 8234). Optical and magneto-optical measurements were made in Hubert Curien laboratory (UMR CNRS 5516) and completed by XMCD analysis in Synchroton SOLEIL. Results show that it is necessary to use a small size of NP (5 nm) to maximize the merit factor at a wavelength of 1,5 µm. The spectral analysis of the Faraday effect shows the influence of the cationic distribution of Co2+ et Fe3+ in the spinelle structure. Coupled to XMCD results, this analysis shows that it is necessary to maximize the quantity of Co2+ in tetraedric sites to maximize the Faraday effect at 1,5 µm and to obtain an uniaxial anisotropy which allows to orientate the NP during the gelification of the sol-gel matrix

i Abstract: This doctoral thesis is dedicated to the study of magnetization dynamics in ferromagnetic semiconductor (Ga,Mn)As using magneto-optical (MO) spectroscopy methods. The character of the magnetization dynamics after the impact of the laser pulse was investigated under different experimental conditions in an extensive set of optimized (Ga,Mn)As samples with Mn doping ranging from 1.5% to 14%. The thorough analysis of the measured MO signal enabled us to develop a new method that can be used to determine the laser pulse-induced real-space magnetization trajectory without any numerical modelling. Moreover, the investigation of the measured MO signals allowed us to determine the basic micromagnetic properties of (Ga,Mn)As, such as the magnetic anisotropy, the Gilbert damping or the spin stiffness. In addition to this, we found out that the light-induced magnetization precession can be caused by three distinct mechanisms - the sample heating due to the energy transfer from the laser pulses, the angular momentum transfer from the circularly polarized photons, and the influence of the non-equilibrium hole polarization induced by the relativistic spin-orbit interaction. The first of these mechanisms is rather well known but the two remaining ones, which are the optical analogues of the spin-transfer torque...

Utilizou-se um Hamiltoniano KP (6x6) do tipo Kane para, se estudar a estrutura de bandas e níveis de Landau para heteroestruturas de semicondutores zincblende dos grupos III-V e II-VI. Os efeitos do acoplamento entre as bandas de condução e valência, da mistura dos estados da banda de valência, da não-parabolicidade dos níveis, da total degenerescência dos níveis, do warping e das descontinuidades das massas efetivas nas heterointerfaces são levados em conta. Mostrou-se que a interação entre as bandas de condução e valência não pode ser desprezada, mesmo para semicondutores de gap largo, como citado em trabalhos existentes na literatura. Para um estudo sistemático do modelo, utilizou-se um poço quântico de GaAs Ga(Al)As e então aplicou-se o modelo a um sistema de semicondutores semi-magnéticos (poço quântico de CdTe Cd(Mn)Te). / A Kane-like (6x6) KP Hamiltonian is used to study the subband structure and Landau levels for group III-V and group II-VI zincblende semiconductor heterostructures. The effects of conduction-valence band coupling, valence band states mixing, nonparabolicity of the levels, the full degeneracy of the levels, warping and effective masses discontinuities at the heterointerfaces are taken into account. It is shown that the interaction between conduction-valence bands cannot be neglected, even so the semicondutctor have wide gap, as claimed in previous work in the literature. GaAs-Ga(Al)As quantum well was used as a model for a systematic study of the effects of each effective KP parameters. Then, it was applied to the study the subband structure of semi-magnetic semiconductor system (a quantum well of CdTe-Cd(Mn)Te.

Tato práce se zabývá charakterizací topologického izolátoru Bi2Te3, materiálu s nevodivými stavy v objemu, ale jedním vodivým pásem na povrchu. Tento materiál je zkoumán již přes 60 let, ale i přes to není jeho objemová pásová struktura úplně objasněna, obzvláště charakter zakázaného pásu je stále předmětem diskuze. V této práci jsou prezentovány výsledky infračervené spektroskopie na Landauových hladinách v magnetickém poli až do 34 T, doplněné elipsometrickým měřením mimo magnetické pole. Výsledky těchto měření by měli pomoci vyjasnit některé vlastnosti zakázaného pásu. Pozorovaná optická odezva odpovídá polovodiči s přímým zakázaným pásem, ve kterém se nosiče náboje chovají jako Diracovské hmotné fermiony. Šířka zakázaného pásu za nízkých teplot byla určena jako Eg = (175±5) meV a samotný zakázaný pás se nachází mimo trigonální osu, tím pádem se v první Brillouinově zoně vyskytuje 6 krát nebo 12 krát.

The main objective of the thesis was to construct a wide-field Kerr microscope to study all-optical helicity-dependent (AOHDS) switching in FePt nanograins. The wide- field Kerr microscope was successfully implemented into AOHDS experiments, was fully characterized and optimized for maximum image contrast. The real-time imaging and resolution of 2, 5µm enables the study of a wide range of magnetic materials and their dynamics. Moreover, a new light source, the High Lumen Density MODULE from CRY- TUR, spol. s r.o., was tested for future application in Kerr microscopy. The technical solution enabled to form a collimated beam with low divergence required for Kerr mi- croscopy. From the switching experiments on FePt nanograins, we observed a strong non-magnetic contribution to the magnetic signal, not reported in previous works. The experiments have also shown that the switching intensity depends on the laser spot size and total laser power, suggesting that the FePt grains are not entirely isolated. The grains' ensemble exhibits a more complex behavior than anticipated. 1