Two important mechanisms in preparation of ultrathin films of magnetic oxides were systematically investigated in this work. First, influence of epitaxial strain on resulting magneto-optical properties of La2/3Sr1/3MnO3 (LSMO) ultrathin films was studied. The investigated films were grown by pulsed laser deposition on four different substrates, providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to the unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and off-diagonal elements of permittivity tensor. The off-diagonal elements confirmed presence of two previously reported electronic transitions in spectra of all films. Moreover, they revealed another electronic transition around 4.3 eV only in spectra of films grown under compressive strain. We proposed classification of this transition as crystal field paramagnetic Mn t2g → eg transition, which was further supported by ab initio calculations. A key role of strain in controlling electronic structure of ultrathin perovskite films was demonstrated. Dynamic application of...

The aim of this master thesis is to prepare and systematically characterize two groups of ultra-thin films of iron garnets with different compositions and deposition conditions. The first set consists of approximately 10 nm thick samples of gallium-doped yttrium-iron garnet on gadolinium gallium garnet (Gd3Ga5O12) (GGG) substrates with different crys- tallographic orientations and different annealing temperatures. The second group con- tains gallium-bismuth-doped neodymium-iron garnet on GGG substrates. These samples have different thicknesses, the concentration of individual elements and different annealing temperatures. All samples were prepared by the metal-organic decomposition method. The analysis of their physical properties was done by magneto-optical Kerr effect (MOKE) and optical experiments. Specifically, MOKE spectroscopy, Faraday hystere- sis loops measurements and spectroscopic ellipsometry. Obtained experimental results were further used to deduce the spectral dependence of complete permittivity tensor. Its spectral dependence was than discussed with relation to the electronic structure of in- vestigated materials which helped to select the best deposition conditions and substrate orientation. 1

Deux mécanismes importants reliant la préparation des couches ultraminces d’oxydes magnétiques à leurs propriétés physiques ont été étudiés dans ce travail. En premier lieu, l’influence de la contrainte épitaxiale sur les propriétés magnéto-optiques de la manganite La₂/₃Sr₁/₃MnO₃ (LSMO) a été étudiée. Les couches ultraminces ont été déposées par ablation laser pulsé sur quatre substrats différents, ce qui a fourni différentes valeurs statiques de la contrainte épitaxiale. Les propriétés magnétiques ont été révélées comme se détériorant avec l’augmentation de la contrainte, ce qui était prévisible à cause de la distorsion grandissante de la maille unitaire ainsi qu’à cause de l’effet de la couche magnétiquement inerte. La combinaison de l’ellipsométrie spectroscopique et de la spectroscopie Kerr magnéto-optique a été utilisée afin de déterminer les spectres des éléments diagonaux et non diagonaux du tenseur de permittivité. L’étude des éléments non-diagonaux a confirmé la présence déjà rapportée de deux transitions électroniques dans les spectres de toutes les couches. De plus, elle a révélé une autre transition électronique autour de l’énergie de 4.3 eV, mais seulement dans les spectres des couches déposées avec une contrainte compressive. Nous avons proposé la classification de cette transition comme une transition paramagnétique du champ cristallin Mn t2g → eg. Cette classification a été confortée par des calculs ab initio. Nous avons ainsi montré le rôle clé de la contrainte dans le contrôle des propriétés magnéto-optiques des couches pérovskites ultraminces. En revanche, l’application dynamique de la contrainte par l’utilisation d’une sous-couche piézoélectrique est restée peu concluante. Le transfert de la contrainte entre la sous-couche piézoélectrique et la couche LSMO nécessite des améliorations ultérieures. En second lieu, l’influence de la désorientation du substrat a été étudiée par rapport à la dynamique de l’aimantation dans l’oxyde SrRuO₃ (SRO). Comme attendu, nous avons trouvé qu’un grand angle de désorientation mène à la suppression de la croissance de plusieurs variants cristallographiques du SRO. Au moyen de la microscopie à force magnétique, nous avons montré que la présence de plusieurs variants de SRO mène à l’augmentation de la densité de défauts agissant comme points d’ancrage ou de nucléation pour les domaines magnétiques. Nous avons donc montré que l’emploi d’un substrat vicinal est important pour la fabrication des couches ultraminces de SRO de haute qualité, avec une faible densité de défauts cristallographiques et d’excellentes propriétés magnétiques. / Two important mechanisms in preparation of ultrathin films of magnetic oxides were systematically investigated in this work. First, influence of epitaxial strain on resulting magneto-optical properties of La₂/₃Sr₁/₃MnO₃ (LSMO) ultrathin films was studied. The investigated films were grown by pulsed laser deposition on four different substrates, providing a broad range of induced epitaxial strains. Magnetic properties were found to deteriorate with increasing value of the epitaxial strain, as expected due to the unit cell distortion increasingly deviating from the bulk and effect of the magnetically inert layer. A combination of spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy was used to determine spectra of the diagonal and off-diagonal elements of permittivity tensor. The off-diagonal elements confirmed presence of two previously reported electronic transitions in spectra of all films. Moreover, they revealed another electronic transition around 4.3 eV only in spectra of films grown under compressive strain. We proposed classification of this transition as crystal field paramagnetic Mn t2g → eg transition, which was further supported by ab initio calculations. A key role of strain in controlling electronic structure of ultrathin perovskite films was demonstrated. Dynamic application of strain via use of piezoelectric underlayer remained inconclusive, requiring further improvement of the strain transfer from the piezoelectric layer into the LSMO. Second, influence of substrate miscut on magnetization dynamics in SrRuO₃ (SRO) was studied. As expected we found that high miscut angle leads to suppression of multi-variant growth. By means of magnetic force microscopy we showed that presence of multiple SRO variants leads to higher density of defects acting as pinning or nucleation sites for the magnetic domains, which consequently results in deterioration of magnetic properties. We demonstrated that use of vicinal substrate with high miscut angle is important for fabrication of high quality SRO ultrathin films with low density of crystallographic defects and excellent magnetic properties.

This dissertation investigates two long standing issues in nonlinear optics: complete characterization of the ultrafast dynamics of simple molecules, and the potential of a two-photon laser using a bulk semiconductor gain medium. Within the Born-Oppenheimer approximation, nonlinear refraction in molecular liquids and gases can arise from both bound-electronic and nuclear origins. Knowledge of the magnitudes, temporal dynamics, polarization and spectral dependences of each of these mechanisms is important for many applications including filamentation, white-light continuum generation, all-optical switching, and nonlinear spectroscopy. In this work the nonlinear dynamics of molecules are investigated in both liquid and gas phase with the recently developed beam deflection technique which measures nonlinear refraction directly in the time domain. Thanks to the utility of the beam deflection technique we are able to completely determine the third-order response function of one of the most important molecular liquids in nonlinear optics, carbon disulfide. This allows the prediction of essentially any nonlinear refraction or two-photon absorption experiment on CS2. Measurements conducted on air (N2 and O2) and gaseous CS2 reveal coherent rotational revivals in the degree of alignment of the ensemble at a period that depends on its moment of inertia. This allows measurement of the rotational and centrifugal distortion constants of the isolated molecules. Additionally, the rotational contribution to the beam deflection measurement can be eliminated thanks to the particular polarization dependence of the mechanism. At a specific polarization, the dominant remaining contribution is due to the bound-electrons. Thus both the bound-electronic nonlinear refractive index of air, and second hyperpolarizability of isolated CS2 molecules, are measured directly. The later agrees well with liquid CS2 measurements, where local field effects are significant. The second major portion of this dissertation addresses the possibility of using bulk semiconductors as a two-photon gain medium. A two-photon laser has been a goal of nonlinear optics since shortly after the original laser*s development. In this case, two-photons are emitted from a single electronic transition rather than only one. This processes is known as two-photon gain (2PG). Semiconductors have large two-photon absorption coefficients, which are enhanced by ~2 orders of magnitude when using photons of very different energies, e.g., ћωa≈10ћωb. This enhancement should translate into large 2PG coefficients as well, given the inverse relationship between absorption and gain. Here, we experimentally demonstrate both degenerate and nondegenerate 2PG in optically excited bulk GaAs via pump-probe experiments. This constitutes, to my knowledge, the first report of nondegenerate two-photon gain. Competition between 2PG and competing processes, namely intervalence band and nondegenerate three-photon absorption (ND-3PA), in both cases are theoretically analyzed. Experimental measurements of ND-3PA agree with this analysis and show that it is enhanced much more than ND-2PG. It is found for both degenerate and nondegenerate photon pairs that the losses dominate the two-photon gain, preventing the possibility of a two-photon semiconductor laser.

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Το θέμα αυτής της διπλωματικής εργασίας είναι η παρασκευή με τη μέθοδο sputtering και ο χαρακτηρισμός μαγνητικών λεπτών υμενίων. Στo πρώτο κεφάλαιο, που αποτελεί και την εισαγωγή, γίνεται μια αναφορά στη τεχνολογία των λεπτών υμενίων και δίνεται το στοιχειώδες θεωρητικό υπόβαθρο των τεχνικών ανάπτυξης αυτών. Στο δεύτερο κεφάλαιο παρουσιάζεται η διάταξη sputtering που χρησιμοποιήθηκε για την ανάπτυξη των μαγνητικών υμενίων και δίνονται τα αποτελέσματα της βαθμονόμησης ενός μετρητικού πάχους υψηλής ακρίβειας. Το τρίτο κεφάλαιο αναφέρεται στην κατασκευή και πιστοποίηση μιας πλήρως αυτοματοποιημένης και χαμηλού κόστους διάταξης μέτρησης μαγνητικών βρόχων μέσω του φαινομένου Kerr, με μέγιστο πεδίο πόλωσης 2Τ. Το τέταρτο κεφάλαιο αναφέρεται στις πειραματικές λεπτομέρειες της παρασκευής των μαγνητικών λεπτών υμενίων Νικελίου και Κοβαλτίου και δίνονται τα αποτελέσματα του δομικού και μαγνητικού χαρακτηρισμού τους. Τέλος, στο πέμπτο κεφάλαιο, παρουσιάζονται τα συμπεράσματα και οι παρατηρήσεις που προέκυψαν κατά τη διάρκεια της ενασχόλησης με τη διπλωματική εργασία. Γίνονται, επιπλέον, και ορισμένες προτάσεις για τη μελλοντική εξέλιξη των συστημάτων sputtering και ΜΟΚΕ, ενώ τονίζεται και το τεχνολογικό ενδιαφέρον που παρουσιάζουν τα υμένια που παρασκευάσθηκαν. / The subject of this diploma thesis is the growth using the sputtering process and the characterization of thin magnetic films. The first chapter refers to thin films' technology and there is given the elementary theoretical background of the thin films’ growth. In the second chapter, there is presented the sputtering device, which is used for the growth of the magnetic films. Also, there is given the results of the calibration of a newly established thickness monitor, which is used for high accuracy thickness measurements. In the third chapter, there is given the description of the construction of a fully automatic and low cost magneto - optic Kerr effect magnetometer for the magnetic loops' measurement in a maximum magnetic field of 2T. There are, also, given the magnetic loops which are used for the construction’s certification. The fourth chapter refers to the experimental details about the preparation of the Nickel and Cobalt magnetic thin films. There are, also, given the results about the structural and magnetic characterization of the thin films. Finally, in the fifth chapter, there are presented the conclusions and the observations, which arose during the diploma thesis. There are, also, presented some proposals about the future progress of the sputtering and MOKE systems, whereas at the same, there is stressed the technical interest of the thin films, which are prepared.

Para o desenvolvimento da nanociência atual há forte demanda por equipamentos capazes de caracterizar sistemas em escalas da ordem nanométrica. Este contexto impulsionou o desenvolvimento de microscópios ópticos de varredura em campopróximo (Scanning Near-field Optical Microscope SNOM). Diferentemente da microscopia óptica tradicional, os SNOMs detectam a radiação eletromagnética evanescente e, conseqüentemente, a resolução não é limitada pelo critério de Rayleigh. No Laboratório de Materiais Magnéticos IFUSP desenvolvemos um SNOM sensível a efeitos Kerr magnetoópticos (MO-SNOM). Dessa maneira, associamos a alta resolução da técnica à alta sensibilidade dos efeitos magnetoópticos. Trata-se se uma área relativamente pouco explorada e carente de resultados sistemáticos na literatura. Utilizando o MO-SNOM, caracterizamos partículas microestruturadas de Co70.4Fe4.6Si15B10 amorfo com dimensões de 16x16x0.08 microm3 e 4x4x0.08 microm3. Os resultados compreendem dezenas de imagens de susceptibilidade magnetoóptica diferencial com resolução melhor que 200 nm e curvas de histerese local. Em primeira análise, a demonstração de resultados sistemáticos ajuda a estabelecer a técnica. O comportamento magnético das partículas, estudadas sob várias condições de campo aplicado, se mostrou determinado basicamente pela anisotropia de forma. As curvas de histerese local mostraram comportamentos intrinsecamente locais e motivaram uma interessante discussão sobre os parâmetros de caracterização magnética convencionais. As medidas realizadas indicam que o efeito Kerr magnetoótico transversal em campopróximo é similar ao campo-distante. Os resultados são fortemente sustentados por medidas de microscopia magnetoóptica de campo-distante, simulações micromagnéticas e medidas de microscopia de força magnética. Medidas complementares revelam o potencial do MO-SNOM para caracterizações de objetos extensos quanto a potenciais de pinning. Além disso, medidas em filmes finos de NiFe/FeMn acoplados por exchange-bias evidenciam a alta sensibilidade do MO-SNOM, estimada de DM ~ 2 x 10-12 emu. / To support nanosciences evolution, there is a strong demand for developing new instrumentation devoted to nano-scale characterization. In this context, the development of the Scanning Near-field Optical Microscope (SNOM) took place. In contrast to traditional optical microscopes, SNOM deals with evanescent electromagnetic radiation and, consequently, the resolution is no longer limited by the Rayleigh criterion. At Laboratório de Materiais Magnéticos (LMM) IFUSP a SNOM devoted to magneto-optical Kerr effect measurements (MO-SNOM) has been developed. The MOSNOM associates the high resolution of the near-field technique to the high sensibility of the magneto-optical Kerr effect. Near-field magneto-optical microscopy is not yet wellestablished and there is a lack of systematic results in the literature. Using the MO-SNOM, amorphous Co70.4Fe4.6Si15B10 particles with 16x16x0.08 microm3 and 4x4x0.08 microm3 dimensions were studied. With resolution better than 200 nm, several magneto-optical differential susceptibility images and local hysteresis loops were obtained. The systematic results uphold the establishment of this new technique. Under the different applied field conditions, the magnetic behavior of the particles was found to be determined by shape anisotropy. Local hysteresis loops presented shapes intrinsic of local field induced process. The unusual hystesesis loops motivated interesting discussion about the conventional magnetic parameters. The MO-SNOM measurements indicate that the near-field transverse magneto-optical Kerr effect is similar to the far-field case. The results are highly supported by far-field magneto-optical microscopy, micromagnetic simulations and magnetic force microscopy measurements. Complementary measurements indicate the MO-SNOM potential to extensive magnetic surface characterization related to pinning potential distribution. Furthermore, measurements on the exchange-bias coupled NiFe/FeMn thin films make evident the MO-SNOM high sensitivity, estimated to be DeltaM ~ 2 x 10-12 emu.

Dans le cadre des études sur l’endommagement laser liées au projet Mégajoule, nous analysons le couplage entre l’auto-focalisation induite par effet Kerr et la rétrodiffusion Brillouin stimulée pour des impulsions de durée nanoseconde se propageant dans des échantillons de silice. L’influence de la puissance d’entrée, des modulations de phase ou d’amplitude ainsi que la forme spatiale du faisceau sur la dynamique de filamentation est discutée. Nous montrons qu’une modulation d’amplitude appropriée divisant l’impulsion incidente en train d’impulsions de l’ordre de la dizaine de picosecondes supprime l’effet Brillouin pour toute puissance incidente mais réduit notablement la puissance laser disponible. A l’inverse, des impulsions modulées en phase avec une largeur spectrale comparable peuvent subir de la filamentation multiple et une auto-focalisation à distance plus courte causées par des instabilités modulationnelles. Nous démontrons cependant l’existence d’une largeur spectrale critique à partir de laquelle la rétrodiffusion peut être radicalement inhibée par une modulation de phase, même pour des fortes puissances. Cette observation reste valide pour des faisceaux de forme carrée avec des profils spatiaux plus larges, qui s’auto-focalisent beaucoup plus rapidement et se brisent en filaments multiples sur de courtes distances. L’inclusion de la génération de plasma pour limiter la croissance des ondes pompe et Stokes est finalement abordée. / As part of the studies on laser damage linked to the Megajoule project, we analyze the coupling between the Kerr induce self-focusing and the stimulated Brillouin backscattering pour nanosecond optical pulses propagating in silica samples. The influence of the incident power, phase or amplitude modulations as well as the spatial profile of the pulse of the filamentation dynamic is discussed. We show that an appropriate amplitude modulation dividing the incident pulse in pulse trains of picosecond durations suppresses the Brillouin effect for any incident power but noticeably reduces the available average laser power. On the contrary, phase modulated pulses with a comparable spectral width can undergo multiple filamentation and self-focusing at a shorter distance, caused by modulational instabilities. We demonstrate however the existence of a critical spectral bandwidth from which the backscattering can be radically inhibited by a phase modulation, even for high powers. This conclusion remains valid for spatially broader squared pulses, which self-focus earlier and break into multiple filaments at shorter distances. The inclusion of plasma generation to limit the growth of pump and Stokes waves is finally addressed.

Observamos o comportamento espectral do efeito Kerr transversal, para filmes amorfos de terra rara - cobalto na região do espectro visível e ultravioleta próximo (comprimento de onda, , entre 325 nm e 670 nm), onde o sinal magnetoóptico apresentou um máximo na região azul do espectro. Concluímos que, para esta região espectral, a variação relativa de refletividade é proporcional à magnetização do cobalto e a constante de proporcionalidade depende da composição da liga e do comprimento de onda da luz. Desenvolvemos uma técnica de modulação de fase, própria para medir o efeito Kerr transversal e a comparamos com a técnica de modulação de amplitude. Montamos um magnetômetro a efeito Kerr que opera com a modulação de fase e utiliza um laser de He-Cd ( = 325 nm e 442 nm) ou um diodo laser vermelho ( = 670 nm). Fazendo uma analogia com a figura de mérito para os efeitos Kerr longitudinal e polar (Fe), nós propusemos uma figura de mérito para o efeito Kerr transversal (FJ. Mostramos que sua dependência do ângulo de incidência é semelhante à da variação relativa de refletividade e, comparando medidas da figura de mérito do efeito Kerr longitudinal com a figura de mérito para o efeito transversal, comprovamos que a grandeza proposta é um bom parâmetro para medida de qualidade magnetoóptica de materiais. Observamos os efeitos do sperimagnetismo de ligas com anisotropia local fraca (Gd-Co) e forte (Ho-Co), através do comportamento térmico dos ciclos de histerese magnética e magnetoóptica. Dentre os fenômenos observados estão os efeitos da compensação, o crescimento hiperbólico do campo coercivo e os efeitos da reorientação de spin. Para a liga de Gd20C080a, transição observada foi uma inversão de spin, cujo o campo magnético aplicado, no qual ocorre a transição, cresce exponencialmente com o aumento da temperatura da amostra. / We have observed the spectral behavior of the transverse magnetooptical Kerr effect (TMOKE) in thin films of rare earth - cobalt amorphous alloys for the visible and the near-ultraviolet ranges of spectrurn (wavelength fiom 325 nm up to 670 nm). The relative change of reflectivity has shown a maximum at the blue wavelength. We have concluded that this magneto-optical signal is proportional to the cobalt magnetization and the proportionality constant depends on the alloy composition and on the wavelength. Besides developing an apparatus and a new phase modulation technique for the TMOKE, we have compared it to the more usual amplitude modulation TMOKE technique. This apparatus operates using either a He-Cd laser ( = 325 nm and 442 nm) for a red diode laser ( = 670nm). We have proposed the figure of merit for the transverse Kerr effect (F,) by analogy with the figure of merit for the longitudinal and polar Kerr effect (Fe). We have measured F, as a function of the angle of incidence and we have shown that F, has the same behavior of the magneto-optical signal. A comparison between F, and Fe has shown that F, is as good parameter as well as Fe for the characterization of the materials magneto-optical quality. We have studied the sperimagnetism of thin films of amorphous alloys with weak (Gd-Co) and strong (Ho-Co) random local anisotropy by the thermal behavior of the magnetic and magneto-optical hysteresis loops. The phenomena of compensation, coercivity and spin-reorientation phase transitions were observed. The comparison between magnetic and magneto-optical hysteresis loops for Gd20C~8h0as shown that the transition occurs from a colinear phase to an opposite collinear phase. In this case, the transition magnetic field grows exponentially when the temperature increases.