Dissertations / Theses on the topic 'Ultra-fast lasers'

Characterisation of the ultra-short optical pulses produced by infrared ffee-electron lasers (FELs) is an important task, not only for the further development of free electron lasers and their theory, but also for their operation as a research tool. The setting up and optimisation of the FEL requires effective and reliable diagnostics tools. This thesis presents techniques for the measurement of sub-picosecond optical and electron pulses. A range of techniques is developed that allows measurements of the electric field of both optical pulses and electron bunches to be made with an accuracy of better than 100 fs. These techniques have been used to obtain the first complete electricfield characterisation of ultra-short pulses from a far-infrared FEL; to study the formation of singlesided exponential optical pulses in two FELs; and to obtain the longitudinal profile of electron bunches, both by probing the near-field transition radiation and by directly sensing the Coulomb field of the electron bunches. Although the techniques described are not truly single-shot - requiring measurements averaged over a period of a few microseconds - ways in which they could be extended to provide single-shot capability are discussed.

Resonant injection of sub-picosecond optical pulses were explored as an ultra-fast analogue to injection locking, to influence the phase of the semiconductor laser on short time-scales. This allowed for efficiently altering the absolute phase of the lasing modes (without exciting new modes to lase) and produced mode interference beating; both demonstrated in vertical cavity surface emitting lasers (VCSELs). The ultra-fast optical sampling was extended to resolve polarization degrees of freedom, showing the nonlinear coupling between transverse modes through the charge carriers. Phase-sensitive double-pulse injection, commonly used for coherent control, was applied to VCSELs to introduce a field component into a specifically selected subset of the transverse lasing modes. The self-organization between transverse modes in VCSELs was investigated with resonant pulse injection and optical sampling. At different bias currents, mode-locking was observed to produce both a train of 2 ps pulses with an 11 ps repetition period, and self-organization of the laser mode resonances to tones within the Fibonacci sequence (with a 19 GHz fundamental tone). It was demonstrated that the nonlinear optical sampling technique could be used to measure the coherence length and type of fluctuations within the VCSEL. While the pulsed mode-locking showed the expected shot-noise fluctuations from spontaneous emission, the Fibonacci-type mode-locking was more stable and showed popcorn noise. The emergence of the Fibonacci-type mode-locking was explained by a spatio-temporal theory of the symmetry-breaking interaction between nearly degenerate modes and the carrier density. Edge-emitting semiconductor laser structures, with and without an optical grating, were also investigated. Resonant and non-resonant optical pulses were used to free laser light from a trapped defect state within an optical grating, and create carrier-heating relaxation oscillations. These effects were reproduced with a spatio-temporal model. Double pulse injection was used to measure the separate effects of group-velocity dispersion and gain curvature on pulse propagation within a Fabry-Pérot laser.

Dans plusieurs expériences, nous démontrons le potentiel du processus de génération d'harmoniques d'ordre élevé pour observer des dynamiques électroniques et nucléaires intra-moléculaires ultrarapides. La plus grande partie de cette thèse traite d'expériences où les molécules constituent le milieu de génération et le paquet d'ondes électronique recollisionnant joue le rôle d'une 'auto-sonde'. Les mesures de phase et amplitude de l'émission harmonique des molécules de CO2 et N2 alignées dans le référentiel du laboratoire nous permettent d'extraire l'élément de matrice du dipole de recombinaison. Ce dernier contient la signature d'une interférence quantique entre les parties libre et liée de la fonction d'onde électronique totale. L'utilisation de cette interférence quantique pour la mise-en-forme de l'émission XUV attoseconde (1as=10−18s) sera démontrée. De plus, nous étudions théoriquement la tomographie d'orbitales moléculaires à partir des éléments de matrice du dipole de recombinaison et nous démontrons sa faisabilité expérimentale. Ceci ouvre la perspective d'imager les distorsions ultra-rapides d'une orbitale frontière lors d'une réaction chimique. Dans une deuxième partie de cette thèse, nous utilisons la lumière XUV cohérente émise par des atomes d'argon pour photoioniser des molécules de N2 et mesurons comment une résonance auto-ionisante modifie la phase spectrale du paquet d'ondes de photoélectrons émis. Le dernier chapitre de ce manuscrit décrit des études de génération d'impulsions XUV attosecondes dans un milieu différent: des plasmas d'ablation. La première caractérisation temporelle d'une telle source démontre sa structure femtoseconde et attoseconde.

Les lasers de haute puissance permettent d'atteindre des intensités très importantes (jusqu'à 10²²W.cm⁻²). Parvenir à ce niveau d'intensité nécessite de concentrer une quantité modérée d'énergie (de l'ordre du joule) dans un temps très court (de l'ordre de la dizaine de femtosecondes) sur une surface réduite (de l'ordre du μm²). Ces faisceaux sont donc ultra-courts et focalisés à l'aide d'une optique à grande ouverture. Ces caractéristiques signifient que leur diamètre avant focalisation est grand et leur largeur spectrale est importante. Pour cette raison, ces faisceaux sont à même de présenter des distorsions spatio-spectrales (ou couplages spatio-temporels). Après focalisation, ces distorsions ont pour effet une diminution drastique de l'intensité pic. Ceci est d'autant plus vrai que le système laser est puissant et donc que son diamètre et sa largeur spectrale sont grands. En dépit de cet effet néfaste, les couplages spatio-temporels présentent aussi un intérêt lorsqu'ils sont maitrisés. On peut en effet introduire des couplages spatio-temporels de faible amplitude à des fins expérimentales. Dans les années 1990 et 2000, un effort important a été fourni pour permettre la caractérisation et l'optimisation du profil temporel des lasers femtoseconde. Dans le même temps, des solutions d'optique adaptative ont été développées pour contrôler le profil spatial des faisceaux ultra-intenses et obtenir la meilleure tache focale possible. Les systèmes laser de haute-puissance actuels sont maintenant caractérisés et optimisés indépendamment par ces deux types de diagnostics. Par essence, cette approche est aveugle aux couplages spatio-temporels. Seule une caractérisation spatio-temporelle permettrait de mesurer ces distorsions. Il existait déjà des méthodes de caractérisation spatio-temporelle avant le début de cette thèse. Aucun de ces dispositifs n'avait cependant été adapté à la mesure de faisceaux ultra-intenses. Lors de cette thèse, nous avons développé une nouvelle technique de caractérisation spatio-temporelle appelée TERMITES. Cette technique est basée sur un schéma de spectroscopie par transformée de Fourier auto-référencée. TERMITES nous a permis d'effectuer la première caractérisation spatio-temporelle totale d'un laser 100 TW (le laser UHI-100 du CEA Saclay). Les distorsions spatio-temporelles détectées à l'aide de ces mesures ont confirmé la nécessité d'une généralisation de la métrologie spatio-temporelle des lasers de haute puissance
High power laser make it possible to reach very high intensities (up to 10²²W.cm⁻²). In order to get to this level of intensity, a moderate quantity of energy (on the order of the Joule) is concentrated in a very short time (on the order of tens of femtoseconds) onto a small surface (on the order of 1 μm²). These beams are therefore ultra-short and focused with a high aperture optic. These features mean that their diameter prior to focus is large and their spectral width is big. As a result, these beams are subject to spatio-spectral distorsions (of spatio-temporal couplings). After focus, these distorsions induce a dramatic reduction of the peak intensity. This situation is all the more true when the laser is more intense and its diameter and spectral width are therefore bigger. Despite their detrimental effects, spatio-temporal couplings can be of great interest when controlled. One can indeed introduce weak spatio-temporal couplings for experimental purposes. In the 1990s and 2000s, a big effort was put in order to characterize dans optimize the temporal profile of femtosecond lasers. Meanwhile, adaptative optics solutions were developed to control the spatial profil of ultra intense laser beams and provide the best focal spot achievable. By nature, this approach is blind to spatio-temporal couplings. Measuring these distorsions requires a spatio-temporal characterization. Before the start of this Phd thesis, spatio-temporal characterization methods already existed. Although none of these devices were ever adapted to the measurement of ultra-intense laser beams. During this Phd Thesis, we developped a new spatio-temporal characterization technique which we called TERMITES. This technique is based on a self-referenced Fourier transform spectroscopy scheme. TERMITES made it possible for us to perform the first total spatio-temporal characterization of a 100 TW laser (UHI-100 at CEA Saclay, France). The detection of spatio-temporal distorsions with the help of these measurements confirmed the need for a generalization of spatio-temporal characterization of ultra-high power lasers

Short optical pulses and engineered nonlinear media is a powerful combination. Mode locked pulses exhibit high peak powers and short pulse duration and the engineered ferro-electric KTiOPO4 facilitates several different nonlinear processes. In this work we investigate the use of structured, second-order materials for generation, characterization and frequency conversion of short optical pulses. By cascading second harmonic generation and difference frequency generation the optical Kerr effect was emulated and two different Nd-based laser cavities were mode locked by the cascaded Kerr lensing effect. In one of the cavities 2.8 ps short pulses were generated and a strong pulse shortening took place through the interplay of the cavity design and the group velocity mismatch in the nonlinear crystal. The other laser had a hybrid mode locking scheme with active electro-optic modulation and passive cascaded Kerr lensing incorporated in a single partially poled KTP crystal. The long pulses from the active modulation were shortened when the passive mode locking started and 6.9 ps short pulses were generated. High-efficiency frequency conversion is not a trivial task in periodically poled materials for short pulses due to the large group velocity mismatch. Optimization of parameters such as the focussing condition and the crystal temperature allowed us to demonstrate 64% conversion efficiency by frequency doubling the fs pulses from a Yb:KYW laser in a single pass configuration. Quasi phase matching also offers new possibilities for nonlinear interactions. We demonstrated that it is possible to simultaneously utilize several phase matched second harmonic interactions, resulting in a dual-polarization second harmonic beam. Short pulse duration of the fundamental wave is a key parameter in the novel method that we demonstrated for characterization of the nonlinearity of periodically poled crystals. The method utilizes the group velocity mismatch between the two polarizations in a type II second harmonic generation configuration. The domain walls of PPKTP exhibit second order nonlinearities that are forbidden in the bulk material. This we used in a single shot frequency resolved optical gating arrangement. The spectral resolution came from Čerenkov phase matching, a non-collinear phase matching scheme that exhibits a substantial angular dispersion. The second harmonic light was imaged upon a CCD camera and with the spectral distribution on one axis and the temporal autocorrelation on the other. From this image we retrieved the full temporal profile of the fundamental pulse, as well as the phase. The spectral dispersion provided by the Čerenkov phase matching was large enough to characterize optical pulses as long as ~200 fs in a compact setup. The Čerenkov frequency resolved optical gating method samples a thin stripe of the beam, i.e. the area close to the domain wall. This provides the means for high spatial resolution measurements of the spectral-temporal characteristics of ultrafast optical fields.
QC 20100831

To our knowledge this is the first comprehensive study of laser-induced effects generated at intermediate distances using self-channeled femtosecond laser pulses. Studies performed were made both experimentally and theoretically with the use of novel modeling techniques. Peak laser pulse powers above 3 GW allow beam propagation without divergence for up to several kilometers. In this regime, experiments were performed at 30 meters from the laser system in a custom propagation and target range, utilizing the Laser Plasma Laboratory's Terawatt laser system. Experiments included investigations of laser ablation; electromagnetic pulsed (EMP) radiation generation over the 1-18 GHz region; shockwave formation in air and solid media; optical coupling of channeled pulses into transparent media; and, conservation of energy in these interactions. The use of bursts of femtosecond pulses was found to increase the ablation rate significantly over single-pulse ablation in both air and vacuum. EMP generation from near-field focused and distance-propagated pulses was investigated. Field strengths upwards of 400 V/m/[Lambda] for vacuum focusing and 25 V/m/[Lambda] for self-channeled pulses were observed. The total field strengths over 1-18 GHz measured at distance surpassed 12 kV/m. Shockwaves generated in transparent media at 30 meters were observed as a function of time. It was found that the interaction conditions control the formation and propagation of the shock fronts into the medium. Due to the processes involved in self-channeling, significant fractions of the laser pulse were coupled into the target materials, resulting in internal optical and exit-surface damage. Basic estimations on the conservation of energy in the interaction are presented. The results of the experiments are supported by hydrodynamic plasma physics code and acoustic modeling.
Ph.D.
School of Electrical Engineering and Computer Science
Engineering and Computer Science
Electrical Engineering PhD

Terahertz time-domain spectroscopy systems were developed and used for the anaylsis and characterization of various materials. By using ultra-fast Ti:Sapphire and Er-doped fiber lasers, terahertz time-domain spectrometers of different configurations were constructed and tested. To increase the accuracy and sensitivity of the measurements, the systems were optimized for spectroscopic analysis. MBE grown nominally undoped epitaxial GaAs samples were used for the spectroscopic measurements. These samples were first charactrized by electrical measurements in order to check the accuracy of the terahertz time-domain experiments. We have shown that the terahertz time-domin spectroscopic techniques provides a quick way of the determining the real ( ) and complex () components of the refractive index of material. In addition, we have investigated the photoexcitation dynamics of these GaAs samples. We have demonstrated that direct and photoexcited terahertz time-domain measurements give an estimate of the carrier densities and both the hole and electron mobility values with good precision. rnin An algorithm is developed to prevent the unwanted Fabry-Perot reflections which is commonly encountered in Terahertz Spectroscopy systems. We have performed terahertz time-domain transmission measurements on ZnTe <
110>
crystals of various thicknesses to test the applicability of this algorithm. We have shown that the algorithm developed provides a quick way of eliminating the &ldquo
etalon&rdquo
reflections from the data. In addition, it is also shown that these &ldquo
etalon&rdquo
effects can be used for the frequency calibration of terahertz time-domain spectrometers.

Atoms, molecules and clusters all constitute building blocks of macroscopic matter. Therefore, understanding the electronic and geometrical properties of such systems is the key to understanding the properties of solid state objects.

In this thesis, some atomic, molecular and cluster systems (clusters of O₂, CH₃Br, Ar/O₂, Ar/Xe and Ar/Kr; dimers of Na; Na and K atoms) have been investigated using synchrotron radiation, and in the two last instances, laser light. We have performed x-ray photoelectron spectroscopy (XPS) on all of these systems. We have also applied ultraviolet photoelectron spectroscopy (UPS), resonant Auger spectroscopy (RAS) and near-edge x-ray absorption spectroscopy (NEXAFS) to study many of the systems. Calculations using ab initio methods, namely density functional theory (DFT) and Møller-Plesset perturbation theory (MP), were employed for electronic structure calculations. The geometrical structure was studied using a combination of ab initio and molecular dynamics (MD) methods.

Results on the dissociation behavior of CH₃Br and O₂ molecules in clusters are presented. The dissociation of the Na₂ molecule has been characterized and the molecular field splitting of the Na 2p level in the dimer has been measured. The molecular field splitting of the CH₃Br 3d level has been measured and the structure of CH₃Br clusters has been determined to be similar to the structure of the bulk solid. The diffusion behavior of O₂, Kr and Xe on large Ar clusters, as a function of doping rate, has been investigated. The shake-down process has been observed from excited states of Na and K. Laser excited Na atoms have been shown to be magnetically aligned. The shake-down process was used to characterize the origin of various final states that can be observed in the spectrum of ground-state K.

Solid-state lasers are capable of providing versatile output characteristics with greater flexibility compared to other popular laser systems. Lasing action has been achieved in many hundreds of solid-state media, but Nd-ion doped gain media are widely used to reach high power levels with short pulses. In this work, commercially available Nd:KGW crystal served as a gain medium to achieve pulsed operation at 1067 nm. This laser crystal offers large stimulated emission crosssection and gain bandwidth which facilitates generation of high peak power pulses in the picosecond regime. The KGW crystal is monoclinic and biaxial in structure, and anisotropic in its optical and thermal properties. Due to poor thermal conductivity, this crystal can be operated within a limited power range before crystal fracture takes place. To reduce the amount of heat deposited in the gain media, we introduced a new pumping wavelength of 910 nm which reduces the quantum defect by more than 45%. Continuous-wave laser operation was optimized to operate in mode-locked regime. In order to achieve short light pulses from the continuous-wave laser, one of the end mirrors was replaced by a semiconductor saturable absorber mirror (SESAM) to generate 2.4 ps pulses at a repetition rate of 83.8 MHz. An average output power of 87 mW was obtained at lasing wavelength of 1067 nm and the beam was nearly diffraction limited with M^2 < 1.18. The peak power of the generated pulses was 427 W and energy of each pulse was >1 nJ. Pumping the crystal at longer wavelength (910 nm) reduced the thermal lensing of the crystal by half when compared to conventional pumping at shorter wavelength (808 nm). To the best of our knowledge, this is the first time passive mode-locking of a Nd:KGW laser was explored using the pump wavelength at 910 nm.
February 2017

La matrice de transmission permet de décrire les effets produit par un milieu multi-diffusant sur une onde monochromatique incidente. L'objectif des travaux présentés dans cette thèse est de développer le concept de matrice de transmission d'un milieu multi-diffusant au cas plus général d'une onde polychromatique impulsionnelle ultra-brève. Dans ce manuscrit nous présentons et mesurons la matrice de transmission multi-spectrale d'un milieu complexe. Cette nouvelle matrice nous donne l'information fondamentale sur le couplage spatio-temporel et spatio-spectral que le milieu engendre au passage d'une onde ultra-brève. Elle permet aussi de contrôler une source monochromatique et polychromatique, après avoir traversé un milieu complexe, de manière déterministe. Nous exploitons ainsi cette connaissance du milieu pour compenser les distorsions du champs en focalisant, façonnant et contrôlant spatialement, spectralement et temporellement un laser ultra-bref grâce à la seule mesure d'une matrice de transmission multi-spectrale. Cette méthode ouvre les portes de plusieurs applications d'imagerie à travers des milieux complexes, ainsi que pour l'interaction lumière-matière en milieux diffusants
The transmission matrix allows to describe the effects generated by a multiple scattering medium on an incident monochromatic wave. The aim of the work presented in this dissertation is to develop the concept of transmission matrix of a multiple scattering medium to the more general case of a polychromatic ultra-fast pulsed light. In this dissertation we present and measure the multi-spectral transmission matrix of a complex medium. This new matrix describes the spatio-temporal coupling and the spatio-spectral coupling induced by the medium on a polycrhomatic illumination passing through it. The measurement of the multi-spectral transmission matrix allows us to control a monochromatic as well as a polychromatic source, after being scattered by the medium, in a deterministic way. We exploit this knowledge about the medium to compensate the distortions of the optical field by focusing, shaping and controlling spatially, spectrally and temporally an ultra-fast laser, thanks to the knowledge of the multi-spectral transmission matrix. This method paves the way towards many applications in the domain of imaging and light-matter interaction of light through complex media

Philosophiae Doctor - PhD
Very few systems allow the study of the relationship between structural changes and physical properties in such a clear way as rare earth nickelate ReNi03 perovskites (Re (rare earth) = Pr, Nd, Sm and Gd). Synthesized for the first time by Demazeau et al [1] in 1971 and completely forgotten for almost twenty years, these compounds have regained interest since the discovery of high-temperature superconductivity and giant magnetoresistive effects in other perovskite-related systems. Due to its Metal-Insulator Transition (MIT) and thermochromic properties, the rare earth nickelate perovskite ReNi03 has received a great deal of attention for the past ten years in their thin films form [12]. Such unusual electronic and optical features are all the more interesting since the metal-insulator transition temperature (TMn) can be tuned by changing the Re cation: LaNi03 is metallic. No minimum of the metallic conductivity of Sm0 . ssNd 0.45Ni03, as observed by Gire et al [12] (entropic effect), was reported by Ambrosini and Hamet [11]. It has been suggested by Obradors et al. [13] that changing the rare earth cation in the ReNi03 system, acts as internal chemical pressure (increasing internal pressure by substituting the rare earth cation with another one of larger ionic radius) which can lead, as for the isostatic pressure experiment, to a tunability of the metal-insulator transition temperature [14, 15]. Obradors et al [13] reported on a decrease of T MIT upon increasing isostatic pressure but with remaining metallic properties of PrNi03 and NdNi03 (same magnitude and thermal dependence of the electrical resistivity)

The time-resolved electrical conductivity of a short-pulse generated plasma filament in air was studied. Close-coupled metal electrodes were used to discharge the stored energy of a high-voltage capacitor and the resulting microsecond-scale electrical discharge was measured using fast current sensors. Significant differences in the time dependence of the current were seen with the two electrode geometries used. Using sharp-tipped electrodes additional peaks in the time-resolved conductivity were seen, relative to the single peak seen with spherical electrodes. We attribute these additional features to secondary electron collisional ionization brought about by field enhancement at the tips. Additional discrepancies in the currents measured leaving the high-voltage electrode and that returning to ground were also observed. Implications for potential laser-induced discharge applications will be discussed.
M.S.
Other
Optics and Photonics
Optics

Ultra-fast laser heating of nano-films is investigated using 3-D Dual Phase Lag heat transport equation with laser heating at different locations on the metal film. The energy absorption rate, which is used to model femtosecond laser heating, is modified to accommodate for three-dimensional laser heating. A numerical solution based on an explicit finite-difference method is employed to solve the DPL equation. The stability criterion for selecting a time step size is obtained using von Neumann eigenmode analysis, and grid function convergence tests are performed. DPL results are compared with classical diffusion and hyperbolic heat conduction models and significant differences among these three approaches are demonstrated. We also develop an implicit finite-difference scheme of Crank-Nicolson type for solving 1-D and 3-D DPL equations. The proposed numerical technique solves one equation unlike other techniques available in the literature, which split the DPL equation into a system of two equations and then apply discretization. Stability analysis is performed using a von Neumann stability analysis. In 3-D, the discretized equation is solved using delta-form Douglas and Gunn time splitting. The performance of the proposed numerical technique is compared with the numerical techniques available in the literature.

Vanadium dioxide is an intensely studied material, since it goes through an insulator-metal transition at a critical temperature just above room temperature at 340~K. The dramatic change in conductivity and the easily accessible transition temperature makes it an attractive material for novel technologies. Thin films of VO2 have a reversible transition without any significant degradation in contrast, and depending on the microstructure of the films, the properties of the transition are tunable. In this work, I study the dynamics of the insulator-transition in thin films grown on different substrates using a pump-probe configuration. The energy needed to trigger the transition, as well as the time constants of the change in reflectivity are affected by the strain in the VO2 films. I also characterized the samples using Raman spectroscopy and XRD measurements in order to identify what underlies the differences in behavior. Finally, in collaboration with Dr. Yamaguchi's group at RPI, I show that it is possible to trigger the transition using a THz pulse that directly pumps energy into the lattice, and at lower energies than needed to pump films by photoinducing the electrons across the band gap.

En raison d’un désordre aléatoire à longue distance, un verre présente une symétrie d'inversion telle que la génération de seconde harmonique (GSH) est interdite. Cependant, par irradiation avec un laser femtoseconde (fs) très focalisé, il est possible de précipiter des cristaux optiquement non linéaires, et de rompre la symétrie d'inversion et donc d'induire une GSH. De plus, ceci peut être réalisé localement en trois dimensions. Pour la démonstration, on a appliqué, dans le système vitreux Li₂O-Nb₂O₅-SiO₂ le mode opératoire décrit ci-dessous qui permet la formation de cristaux de LiNbO₃, hautement optiquement non linéaire. La procédure est la suivante : 1) ajustement de la composition chimique du verre pour obtenir un verre suffisamment sensible au laser fs ; 2) contrôle des paramètres du laser (durée d'impulsion, fréquence de répétition des impulsions, vitesse de balayage du faisceau, énergie d'impulsion…) pour obtenir des nanocristaux avec répartition spatiale et taille correcte. En outre, la taille de la zone affectée doit être limitée ; 3) contrôle de l'orientation des nanocristaux. On montre qu'il est possible de satisfaire à cette condition, en contrôlant l'orientation de la polarisation du laser. Ceci a été montré par la méthode de rétrodiffusion d'électrons de diffraction (EBSD). En d'autres termes, ce processus peut être contrôlé directement avec la lumière. En outre, la spectroscopie par rayons X à dispersion d'énergie couplée à la microscopie à transmission électronique à balayage (STEM /EDS) et la microscopie électronique à transmission a révélé une microstructure orientable similaire à celle appelée nanoréseaux formée dans silice. L'originalité est que les nanocristaux optiques non linéaires texturées noyées dans un réseau de "murs" vitreux, sont alignés perpendiculairement à la direction de polarisation du laser. Il en résulte que la biréfringence et la propriété optique non linéaire peuvent être maîtrisées ensemble. Ceci est une percée dans ce travail de thèse. Ces résultats mettent en évidence des modifications spectaculaires de verre par rayonnement laser fs. Avec de nouvelles améliorations dans les techniques de fabrication, l'application de ce travail est de parvenir à réaliser un guide d'ondes biréfringent doubleur ou changeur de fréquences
Due to random disorder, a glass exhibits inversion symmetry such that second harmonic generation (SHG) is forbidden. However, by irradiation with a tightly focused femtosecond (fs) laser, it is possible to induce nonlinear optical crystal precipitation, in order to break the inversion symmetry and thus to induce SHG. Moreover, this can be achieved locally in three dimensions. For demonstration, we applied the procedure described below in the glass system Li₂O-Nb₂O₅-SiO₂ that allows the formation of LiNbO₃ crystal, a highly non linear optical one. The procedure is thus the following: 1) adjustment of the glass chemical composition for obtaining a glass sensitive enough to fs laser. 2) control of the laser parameters (pulse duration, pulse repetition rate, speed of beam scanning, pulse energy…) for obtaining nanocrystals with correct space distribution and size. In addition, the size of the affected zone has to be limited. 3) control of the orientation of the nanocrystals. We show that it is possible to fulfill this condition by controlling the laser polarization orientation. This has been achieved by electron backscatter diffraction method (EBSD). In other words, this process can be controlled with light directly. In addition, energy dispersive X-ray spectroscopy coupled to scanning transmission electron microscopy (STEM/EDS) and transmission electron microscopy revealed an orientable microstructure similar to the one called nanogratings form in silica. The originality here is a textured nonlinear optical nanocrystals embedded in a network of “walls” made of vitreous phase, aligned perpendicular to the laser polarization direction. It results that birefringence and nonlinear optical property can be mastered in the same time. This is a highly valuable aspect of the work. These findings highlight spectacular modifications of glass by fs laser radiation. With further improvements in the fabrication techniques, the application of this work is to achieve SHG waveguide and birefringence-based devices

Diese Arbeit wird einer Weiterentwicklung der Dichtematrixtheorie und ihrer Anwendung zum Studium ultraschneller laserpulsinduzierter Dynamik in Molekularsystemen in Wechselwirkung mit einem thermischen Bad gewidmet. Zwei grosse Themenkomplexe werden behandelt. Zuerst werden die sogenannten Gedächtniseffekte diskutiert. Diese folgen aus einer reduzierten Beschreibung des Molekularsystems, in der die Umgebungsfreiheitsgrade eliminiert werden. Im zweiten Teil wird die Laserpulssteuerung der dissipativen Molekulardynamik untersucht. Die theoretische Beschreibung von offenen Quantensystemen führt zu einer zeitlich nicht-lokalen Bewegungsgleichung: Die Zeitentwicklung des Molekularsystems hängt von seiner Vergangenheit ab. In dieser Arbeit wird eine numerische Methode zur Lösung der zeitlich nicht-lokalen Bewegungsgleichung entwickelt und mit einem minimalen Modell eines polyatomaren Moleküls unter dissipativem Einfluss der Umgebung getestet. Eine analytische Lösung der Bewegungsgleichung für den speziellen Fall einer sehr langen Gedächtniszeit wurde hergeleitet. Zur Identifizierung solcher Gedächtniseffekte vergleichen wir diese analytische Lösung mit numerischen Rechnungen inklusive Gedächtnis und mit approximativen Rechnungen, die die zeitliche Nicht-Lokalität vernachlässigen. Für eine Anregung mit einem Laserpuls, der kürzer als die Gedächtniszeit des Systems ist, zeigt das Molekularsystem eine erkennbar unterschiedliche Dynamik als ohne Gedächtniss. Die Gedächtniseffekte werden mit abfallender Laserpulslänge deutlich ausgeprägter. Der zweite Teil der Arbeit konzentriert sich auf die Anwendung der Theorie der Optimalen Kontrolle, um die molekulare Dynamik zu steuern. Aus der Theorie der Optimalen Kontrolle erhält man Laserpulse, die bestimmte Aufgaben erfüllen, z.B. die Besetzung gewünschter vibronischer Niveaus des Molekularsystems oder die Platzierung eines Wellenpakets auf einer vorgegebenen Position auf der molekularen Potentialfläche. Als erstes Beispiel haben wir die Kontrolle des dissipativen fotoinduzierten Elektronentransfers in einem Donator-Brückenmolekül-Akzeptor System betrachtet, wobei wir das Gedächtniss vernachlässigt haben. Die Steuerbarkeit des Elektronentransfers wird diskutiert und der Mechanismus, mit dem sie möglich wird, wird identifiziert. Wir haben festgestellt, dass die Steuerung der Elektronentransferreaktionen selbst unter dem Einfluss von Dissipation möglich ist, obwohl die Kontrollausbeute mit steigender Dissipation drastisch abfällt. In Anwesenheit von Dissipation verändert sich auch der Mechanismus der Steuerung. Die experimentelle Ausführbarkeit der Herstellung des aus der Theorie der Optimalen Kontrolle resultierenden Kontrollpulses wird diskutiert und Methoden werden präsentiert, die die Abschätzung der Effizienz ermöglichen, mit der ein Flussigkristall--Laserpulsformer, wie er heute in Experimenten verwendet wird, den gewünschten Puls erzeugen kann. Um zwischen verschiedenen Kontrollaufgaben zu unterscheiden, wird ein quantitatives Mass eingeführt, das die Komplexität der Kontrollaufgabe charakterisiert. Die Theorie der Optimalen Kontrolle wird auch für Molekularsysteme formuliert, die statische Unordnung zeigen, und wird auf ein Ensemble von Molekülen mit zufälligen Orientierungen angewendet. Zum Schluss wird die Bedeutung der Gedächtnisseffekte für die Steuerung der dissipativen Dynamik diskutiert und die Theorie der Optimalen Kontrolle neu formuliert um eine zeitliche Nicht-Lokalität in der Bewegungsgleichung des Molekularsystems zu berücksichtigen.
This work is dedicated to a further development of the density matrix theory and its application to the study of ultrafast laser pulse induced dynamics in molecular systems interacting with a thermal environment. Two topics are considered, first the so-called memory effects are analyzed which result from a reduced description of the molecular system excluding the environmental degrees of freedom. And secondly, the laser pulse control of dissipative molecular dynamics is examined. The theoretical description of open quantum systems results in a time non-local equation of motion so that the evolution of the molecular system depends on its past. In this work a numerical method to solve the time non-local equations of motion has been developed and tested for a minimal model of a polyatomic molecule subject to the dissipative influence of an environment. An analytical solution of the equation of motion for the special case of very long standing memory is also achieved. To identify signatures of such memory effects in general case we compare this analytical solution with numerical calculations involving memory and with approximative computations ignoring time non-locality. For the excitation by a laser pulse shorter than the duration of the memory the molecular systems exhibit noticeably different dynamics than for the absence of the memory. The effects become significantly more pronounced with decreasing laser pulse durations. The second part of the work concentrates on the application of the optimal control theory to guide molecular dynamics. Optimal control theory provides laser pulses which are designed in such a manner to fulfill certain control tasks, e.g. the population of a desired vibrational level of the molecular system or the placement of a wavepacket on a prescribed position on the molecular potential energy surface. As a first example the control of the dissipative photo-induced electron transfer in a donor--bridge--acceptor systems has been particularly considered ignoring the memory. The controllability of the electron transfer has been discussed and the mechanism by which it becomes possible has been identified. We have found the control of electron transfer reactions feasible even under the influence of dissipation although the yield of the control decreases drastically with increasing dissipation. In the presence of dissipation mechanism of the control has been found to change. The feasibility of the reproduction of the control pulses resulting for the optimal control theory in the experiment has been discussed and methods have been presented how to check the efficiency of the reproduction of optimal control pulses by liquid crystal pulse shapers, prevailingly used in modern control experiments. To distinguish different control tasks a quantitative measure has been introduced characterizing complexity of the control task. The optimal control theory has also been formulated for molecular systems showing static disorder and applied on an ensemble of molecules exhibiting random orientations. Finally, the importance of memory effects for the control of dissipative dynamics has been discussed and the optimal control theory has been formulated to account for a time non-locality in the equation of motion for molecular systems.

Dans cette thèse, nous démontrons qu’un rayonnement harmonique XUV polarisé linéairement peut être utilisé en spectroscopie d’absorption pour accéder à l’état de magnétisation de tout type d’échantillon, contrairement à toutes les techniques développées jusque-là. En effet, pour la première fois, des expériences résolues en temps ont été réalisées par l'effet Faraday magnéto-optique, que nous exploitons autour du seuil d'absorption magnétiquement dichroïque M2,3 du Cobalt à 60 eV. La technique pompe-sonde a été utilisée pour obtenir la réponse dynamique des échantillons magnétiques lors de l'excitation laser. Les changements dans l’aimantation de l'échantillon sont associés aux changements dans la polarisation du faisceau harmonique de sonde, i.e. à la fois à la rotation de l’axe de polarisation et à la variation de l'ellipticité. Les principaux résultats de cette thèse démontrent que la mesure de l’effet Faraday offre un moyen ultrasensible de caractériser l’aimantation de films très minces (seulement quelques nm de matériaux magnétique). De plus, l’effet Faraday ayant lieu sur une large plage spectrale il est possible de suivre la dynamique simultanée de différents matériaux et donc d’étudier des matériaux très complexes
In this thesis, we demonstrate that a linearly polarized XUV harmonic radiation can be employed in absorption spectroscopy to access the magnetization state of any type of sample, unlike all the techniques developed so far. Indeed, for the first time, time-resolved experiments were realized through the magneto-optical Faraday effect, which we exploit around the magnetically dichroic Co M2,3 absorption edge at 60 eV. The pump-probe technique was used to obtain the dynamic response of the magnetic samples upon laser excitation. The changes in the magnetization of the sample are associated to the changes in the polarization of the probe harmonic beam, i.e. the rotation of the polarization axis and the variation of the ellipticity. The main results of this thesis demonstrate that the measurement of the Faraday effect offers an ultra-sensitive way to characterize the magnetization of very thin films (only a few nm of magnetic materials). Moreover, since the Faraday effect takes place over a wide spectral range, it is possible to follow the simultaneous dynamics of different materials and thus to study very complex materials

Des années 50 à nos jours, la propulsion électrique n'a cessé d'évoluer afin de s'imposer dans le domaine de la propulsion spatiale. Les Propulseurs à effet Hall (PEH) sont principalement utilisés pour des missions de correction de trajectoire ou de maintien en orbite des satellites. Ils délivrent des faisceaux d'ions à forte densité de courant et à faible énergie, ce qui en font de bons candidats potentiels pour d'autres applications comme la microélectronique ou encore les traitements de surfaces. Le xénon est l'ergol le plus utilisé en raison de sa masse élevée et de son faible énergie d'ionisation. Cependant son coût élevé et la difficulté d’approvisionnement motivent la recherche d'alternatives pour le fonctionnement des MEH. C'est dans ce cadre que cette thèse s'est inscrite avec l'idée d'un développement d'une source de faible puissance fonctionnelle en argon. L'amorçage d'une telle décharge n'étant pas immédiat, une démarche progressive qui passe par des décharges de mélange de gaz a été adoptée. Les décharges Xe-Ar se sont révélées très intéressantes pour la compréhension des mécanismes physiques qui régissent les PEH. La caractérisation en vitesse des ions Xe II (par Fluorescence Induite par Laser) associée à l'analyse en énergie par RPA a permis de remonter à des informations utiles sur les zones d'ionisation et d'accélération. Une technique originale de résolution temporelle du RPA basée sur une interruption rapide de la décharge ou sur les oscillations naturelles du courant de décharge, a été développée et a permis l'identification et la quantification des différentes espèces présentes dans le jet d'ions. Grâce aux résultats de l'étude paramétrique des décharges de mélange Xe-Ar, une décharge d'argon pur a pu être amorcée et caractérisée pour la première fois dans un PEH de faible puissance
Since the 50s, electric propulsion has improved in order to establish itself on space propulsion field. The Hall Effect Thruster (HET) are mainly used for trajectory correction or satellites orbit maintaining. The HET provide high current densities and low energy ion beam that making it a good candidate for other applications such as microelectronics or surface treatments. Xenon propellant is most commonly used due to its high atomic mass and its low ionization energy. However, the high cost and difficult supply of xenon, leads to looking for alternative propellant for HET operation. In this context, this PhD thesis had as goal the development of a functional Argon low power source. Argon discharge ignition is not immediate, that why a progressive approach which involves gas mixture discharges was adopted. The Xe-Ar discharge gives very interesting results for the understanding of physical mechanisms governing HET. The characterization of Xe II ions velocity (Laser Induced Fluorescence) associated to the energy analysis by RPA have provided access to useful information on ionization and acceleration areas. An original time resolved RPA technique, based on an ultra-fast discharge interruption or on the discharge current oscillations, has been developed. This technique allows the identification and the quantification of different species present in the ion beam. Thanks to the discharge Xe-Ar study, a pure argon discharge could be initiated and characterized for the first time in a low power HET

Understanding material behavior under extreme conditions is an important area of research in physics and material science. One method to study the behavior of materials under these conditions is to drive a strong shock wave through a material and watch its response. In many cases the material response is complicated by phase transitions such as lattice restructuring (Barker 1975; Mabire and Hereil 2000; Swift, Tierney et al. 2005) and melting (Asay 1975; Elias, Chapron et al. 1988; Werdiger, Eliezer et al. 1999; Mabire and Hereil 2000; Swift, Tierney et al. 2005). To study these dynamics we are using lasers in high time resolution pump-probe experiments to develop a real time diagnostic on the phase of a shocked material. This technique enables probing of the entire phase history of a material as it shock compresses and releases. In addition to linear reflectivity and ultra-fast 2D displacement interferometry, we developed a melting diagnostics based on the non-linear optical technique of third harmonic generation (THG) using a circularly polarized laser pulse. This diagnostic resolves the less than 300 fs melting transition of laser excited Si and GaAs, and it also detects a response in shock compressed silicon. Our results show that Si remains crystalline during compression of an elastic 100 kbar shock wave. Results from Si shocked to higher pressures (> 300 kbar) indicate a decrease in THG, suggesting some level of disordering or unexplained phase change.
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碩士
國立清華大學
材料科學工程學系
101
Graphene, owing to its unique structural and outstanding properties, is regarded as one of the most important materials for future high performance devices and has been attracted lots of research groups all over the world. Until now, there are many ways to synthesize graphene for research purposes. However, there are no convinced and cost-efficient way for real industrial applications. In this regard, we successfully demonstrate a rapid fabrication of graphene on glass substrate by laser. The properties of low power consumption and fast growth that is highly compatible with real semiconductor manufacturing process. It can be also considered as a fast and dynamic growth process. The 808 nm near-infrared laser continuous wave laser is used to focus on glass substrate with a patterned nickel thin film (Ni TF) as metal catalyst. PMMA (or amorphous carbon) is deposited on top of the Ni TF as solid carbon source. As a result, few-layer graphene can be synthesized on both sides of Ni TF after the laser irradiation successfully. Microstructures were characterized by Raman spectroscopy and transmission electron microscopy (TEM). In addition, we propose a possible mechanism for this laser synthesis process on Ni. We believe this novel, fast and convenient way for graphene synthesis can have a significant contribution to future applications.

碩士
國立聯合大學
材料科學工程學系碩士班
105
Laser has the advantages of being capable of cutting irregular shape and finer edge quality with less maintain cost. However, the laser beam has effects on different materials to be processed and debris becomes a problem to be solved. In this study, ultra-fast pico green laser and pico IR laser have been applied to groove and drill Molding Compound and the material behaviors of Molding Compound after machined are then carefully investigated. Grooving on Molding Compound by two lasers are carried-out for various cutting conditions, such as line design, cutting pass and pulse energy, and the results, such as kerf width, depth of cut and cutting angle are fully observed by 3D Laser Scanning Microscope (Keyence VK-X200).Package-on-Package (PoP) using through mold via (TMV) technology is that a blind via (trench) was laser-drilled through Molding Compound of bottom package and the drilled via was then sputtered/plated on the sidewall, and filled with the conductive material. To develop TMV technology is the main objective in this research. Parametric studies consist of diameter of via (100, 200, 500, 1000 m) and with/without gas aided. Microscopy is applied to observe the post process of dust accumulation, spur and recast on surface as well as burnt and crack due to heat affected.The other process parameter is the focus of laser beam (z-axis position) in different vertical positions of Molding Compound specimen. Heat affected area has been carefully investigated for various positive focus in surface, middle and bottom of Molding Compound specimen. The shape, size and density of fillers in mold compound play an important role on edge surface quality. A series of comprehensive experiments has been conducted to determine the optimal process parameters and quality for ultra-fast laser machining, This research is cooperated with local industries and the results obtain in this research are capable to feed back to laser equipment industry and IC packaging industry.

碩士
國立中正大學
物理系
89
There are two main parts in this thesis: (Part one) Ultra-fast lasers have become more and more frequently used in research in the past years. The gain media used in the laser cavity has improved in quality and stability, thus this is an advantage for compressing laser pulses. Not only using the technique of mode-locking in order to gain ultra-fast pluses, but the quality of laser pulses should also be looked into. Personals involving experiments in time domain should look into the matter of both pulse width and stable laser output which are the keys to accurate experimental data readings. As mentioned above, the technique of laser output stabilization and feedback control are essential subjects. (Part two) It has become possible for the resolution of measuring the process of coherent coupling to reach femtosecond. We have demonstrated using Raman induced Kerr Effect Spectroscopy: RIKES to probe the response of liquid molecules. With the advantage of ultra-fast laser pulses, we are able to do measurements and analysis on fast motions of clear liquid molecules. We fit the experiment data according to the theory of impulse-forced oscillation. The response of liquid molecules due to ultra-fast laser pulses in the time domain are electron response and nuclear response. There are different motions among them, such as translation, libration, diffusive reorientation, and normal modes. We are able to aquire the distribution functions in the time domain of these motions through Fourier transform from the frequency domain which is a field well known. This is an opportunity to confirm on both sides.

A series of experiments have been performed to understand fast electron generation from ultra intense laser-solid interaction, and their transports through a cold material. Using Micro-Electro-Mechanical Systems (MEMS), we contrived various shape of cone and wedge targets. The first set of experiment was for investigating hot electron generations by measuring x-ray production in different energy ranges. K[alpha] and hard x-ray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though K[alpha] is maximized with wedge targets and parallel polarization. These results are explained with particle-in-cell (PIC) simulations utilizing PICLS and OOPIC codes. We also investigate hot electron transport in foil, wedge, and cone targets by observing the transition radiation emitted from the targets rear side along with bremsstrahlung x-ray measurement. Twodimensional images and spectra of 800 nm coherent transition radiation (CTR) along with ballistic electron transport analysis have revealed the spatial, temporal, and temperature characteristics of hot electron micro-pulses. Various patterns from different target-laser configurations suggest that hot electrons were guided by the strong static electromagnetic fields at the target boundary. Evidence about fast electron guiding in the cone is also observed. CTR at 400 nm showed that two distinct beams of MeV electrons are emitted from the target rear side at the same time. This measurement indicates that two different mechanisms, namely resonance absorption and j x B heating, create two populations of electrons at the targets front side and drive them to different directions, with distinct temperatures and temporal characteristics. This interpretation is consistent with the results from 3D-PIC code Virtual Laser Plasma Laboratory (VLPL).
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In donor-acceptor systems, ultrafast interfacial charge transfer (CT), charge separation (CS) and charge recombination (CR), are among the key factors in determining the overall efficiency of the optoelectronic devices. In this regime, precise knowledge of the mechanisms of these processes on the femtosecond scale is urgently required. In this dissertation, using femtosecond transient absorption and mid-Infrared spectroscopies along with steady-state absorption and emission measurements, we are not only able to address the fundamental understanding of these ultrafast dynamical processes, but also control them at various inter- and intramolecular electron donor-electron acceptor systems. In the photoinduced intermolecular charge transfer systems, where donor and acceptor are separated from each other, three systems have been investigated; cationic poly[(9,9-di(3,3′-N,N′-trimethylammonium) propyl fluorenyl-2,7-diyl)-alt-co-(9,9-dioctyl-fluorenyl-2,7-diyl)] diiodide salt (PFN) conjugated polymer donor with 1,4-dicyanobenzene (DCB) acceptor, negatively charged porphyrin (POS) donor with positively charged (PFN) acceptor, and finally, positively charged (PFN) donor with negatively charged graphene carboxylate (GC) acceptor. Based on studying these three systems, we were able to explore some important factors and deriving forces including chemical structure, electrostatic interactions, energy band alignment, hydrogen bonding and solvents with different polarities and capabilities for hydrogen bonding that influence the rate and efficiency of the charge transfer at the interfaces of these donor-acceptor systems. For instance, unlike the conventional understanding of the key role of hydrogen bonding in promoting the charge-transfer process, our results reveal that the hydrogen-bonding increases the spacing between the donor and acceptor units which significantly hinders the charge-transfer process. On the other hand, in the photoinduced intramolecular charge transfer systems, where donor and acceptor are chemically attached to each other, we investigate the effects of conjugation length on photoinduced charge transfer in π-conjugated oligomers naphthalene diimide (NDI) end-capped oligo(phenylene ethynylene)s (PEn-NDI), and poly-(phenylene ethynylene) (PPE) donor backbone with (NDI) acceptor end-caps (PPE-NDI-n) systems. The results of femtosecond transient absorption and mid-IR spectroscopies show that the charge separation occurs on the 1-10 ps time scale with the rates decreasing as oligomer length increases in PEn-NDI system. In addition, in PPE-NDI-n system, the fluorescence quenching measurements indicate very efficient photoinduced electron transfer from the PPE backbone to the NDI end-groups, and the transfer efficiency increases with decreasing the number of units. Finally, the new physical insights reported in this thesis provide an understanding of several key variable components involved, thus paving the way toward the exploitation of efficient charge transfer at donor-acceptor interfaces, which is the key element and urgently required for optimal optoelectronic-device performance.

Abstract: Understanding and controlling the ultrafast charge carrier and exciton dynamics at the interface of semiconductor nanocrystals (NCs) offer an excellent opportunity to improve the charge collection and the overall performance of many optoelectronic and energy-based devices. In this dissertation, we study how interfacial engineering of these materials can have a direct influence on controlling the charge transfer and the nonradiative losses in different donor-acceptor systems. The first introductory chapter provides an overview of all the fundamental photophysical processes controlling the interfacial phenomena. Then, the second chapter highlights all the chemicals and synthesis methods employed during this thesis. The subsequent two chapters discuss the detailed experimental studies and observations related to different materials and interfaces. First, it describes how we can dramatically tune the intersystem crossing (ISC) rate, the triplet state lifetime, turn on/off the electron injection at the CdTe-Prophyrin interface via tuning either the quantum dot size or the porphyrin molecular structure. Also, how the intermolecular distances, electronic coupling, and subsequently, the photoinduced charge transfer can be controlled by the interfacial electrostatic interactions at CdTe-Fullerene interfaces. Second, due to the promise that of perovskite NCs holds for improving many solar cell and optoelectronic applications, chapter 3 highlights the tremendous effect that the shape of perovskite nanocrystals has on the rate and the mechanism of charge transfer at the MAPbBr3- TCNE interface. Besides, it demonstrates how the confinement effect brought by changing the dimensionality influence the charge transfer dynamics at the MAPbBr3-BQ interface. Finally, it explains how the effective passivation of the surface defects and the subsequent suppression of the formation of surface nonradiative recombination centers in CsPbCl3 NCs controls the photoluminescence quantum yield and the photodetector performance.

碩士
國立交通大學
機械工程系所
92
This thesis investigates the applicability of the linear variation of the reflectivity with the electron temperature when the laser pulse duration is too short to measure the variation of electron temperature directly. A more exact relationship between optical reflectivity and electron temperature is derived, and is brought into an inverse analysis which is performed for simultaneous estimation of both thermal conductivity and the electron-phonon coupling factor for thin metal films subjected to ultra-fast laser heating. Results show that as the change range of the electron temperature is small the linear variation of the reflectivity with the electron temperature can be applied, while at small thickness the applicability of the linear relationship is questionable. Furthermore, the reflectance we get before electron temperature reaches balance between front surface and rear surface is too low at the front surface and is too high at the rear surface, the effect of electron temperature distribution on the reflectance should be addressed in order to obtain more accurate estimation of heat transfer characteristics. In the present study, the reflectivity distribution relates to the thickness of thin film, electron temperature, the photon energy and incident angle are also analyzed.