Revestimentos superficiais são aplicados à muitas ferramentas de usinagem na indústria metalúrgica com o intuito de melhorar a eficiência de corte e aumentar sua vida útil. Neste trabalho foram realizados testes para remoção do recobrimento de nitreto de titânio alumínio (TiAlN) em pastilhas de carboneto de tungstênio (WC-Co), utilizando um feixe laser de pulsos ultracurtos. Após a determinação dos limiares de dano do filme e do substrato foram ablacionados na superfície do recobrimento, traços utilizando duas condições de ablação. Inicialmente operou-se no regime de baixa fluência do filme, e posteriormente no regime de baixa fluência do substrato, muito abaixo do limiar do filme, aplicando-se alta sobreposição de pulsos. Um sistema de espectroscopia de emissão atômica induzida por laser (LIBS) foi montado para monitoramento dos materiais presentes no plasma gerado pelo laser, porém o sistema não apresentou sensibilidade suficiente para leitura da baixa intensidade do plasma proveniente do processo e não foi utilizado. Após a análise dos traços por microscopia eletrônica, perfilometria óptica e espectroscopia por fluorescência de Raios-X, não foi possível determinar um processo seguro para realizar a remoção seletiva do filme em questão, porém, devido aos dados obtidos e observações dos resultados em alguns traços, novas possibilidades foram levantadas, abrindo a discussão para a realização de trabalhos futuros. / Surface coatings are applied to many cutting tools in the metallurgical industry in order to improve cutting efficiency and extend its useful life. In this work, tests were performed to remove the coating of titanium aluminum nitride (TiAlN) on tungsten carbide (WC-Co) pellets, using an ultrashort laser pulses beam. After determination of the damage thresholds of the film and the substrate, were ablated on the surface of the coating lines using two ablation conditions, it was initially operated on the low fluence regime for the film, and later on the low fluence regime of the substrate, far below the threshold of the film, applying high overlapping pulses. A laser induced breakdown spectroscopy (LIBS) system was set up to monitor the materials present in the plasma generated by the laser, but the system did not present sufficient sensitivity to read the low intensity of the plasma generated in the process and was not used. After the analysis of the traces by electron microscopy, optical profilometer and X-ray fluorescence spectroscopy, it was not possible to determine a safe process to carry out the selective removal of the film in question, however, due to the data obtained and observations of the results in some traces, new possibilities were raised, opening the discussion for future work.

This dissertation presents experimental and theoretical studies of transient degenerate four wave mixing (DFWM) in organic dyes. Chapter 1 is an introduction to DFWM. Chapter 2 describes DFWM experiments that were performed in the gain medium of a dye laser. Chapter 3 presents the theory of DFWM of short pulses in three level saturable media. Chapter 4 presents DFWM experiments of femtosecond pulses in the saturable absorber of a passively modelocked ring dye laser. Chapter 5 presents the theory of DFWM of ultrashort pulses in resonant media.

La propagation des impulsions laser ultra brèves dans l’air se fait sous la forme de structures d’une centaine de micromètres de diamètre appelées filaments, qui ont entre autres les propriétés d’être autoguidées, de se propager sur plusieurs centaines de mètres, de générer un continuum de lumière blanche, etc. Ces propriétés originales trouvent de nombreuses applications dans le domaine de la télédétection des polluants par mesures lidar, le déclenchement et le guidage de la foudre par laser, le LIBS à distance, etc.Au cours de mon travail de thèse, nous avons mené de nombreuses expériences de laboratoire et sur terrain dans le cadre du projet Tera mobile. Nous avons en particulier étudié la géométrie de la filamentation, sa robustesse dans une région de turbulence étendue, la propagation verticale d’un faisceau d’impulsions ultra brèves dans un régime multi joules, et des applications atmosphériques de la filamentation. Nous avons par exemple caractérisé la distribution angulaire de l’émission conique dans le visible et dans l’ultraviolet. Nous avons également prouvé que la turbulence atmosphérique n’est pas un facteur limitant de la propagation des filaments qui arrivent même à garder leurs propriétés spectrales nécessaires aux applications atmosphériques. Enfin nous avons illustré une méthode de déclenchement et de guidage de foudre par laser et réalisé une expérience de condensation de gouttelettes d’eau assistée par laser en laboratoire ainsi que dans une atmosphère réelle. / Ultrashort laser pulses propagate in the air in the form of structures of one hundredmicrons of diameter called “filaments”, which have the properties of self-guiding, propagatingfor hundreds of meters, white light generation, etc. These original properties find severalapplications in the domain of remote sensing of pollutants by non-linear Lidar measurements,lightning control, remote LIBS, etc.During my PhD work we have performed several laboratory experiments and field campaignwithin the context of Teramobile project. In particular we have studied the geometry offilamentation, its robustness in an extended region of turbulent air, the propagation ofultrashort pulses beam in multijoules regime, and atmospheric applications of filamentation.For example, we have characterized the angular distribution of the conical emission in thevisible and ultraviolet spectral bands. In another series of experiments, we have proved thatatmospheric turbulence is not a limiting factor of filaments propagation, which also keep theirspectral properties useful for atmospheric applications. Finally, we have illustrated a methodof laser triggering and guiding of lightning and realized laser induced condensation of waterdroplets in laboratory as well as in a reel atmosphere.

Femtosecond time-resolved photoelectron spectroscopy (fsTRPES) experiments have been used to study the photoelectron energy spectra of simple molecules since the 1980’s. Analysis of these spectra provides information about the ultrafast internal conversion dynamics of the parent ions. However, ultraviolet pulses must be used for these pump-probe experiments in order to ionize the molecules. Since current solid state lasers, such as the Ti:sapphire laser, typically produce pulses centered at 800nm, it is necessary to generate UV pulses with nonlinear frequency mixing techniques. I therefore constructed an optical setup to generate the 3rd and 4th harmonics, at 266.7nm and 200nm, respectively, of a Ti:sapphire (Ti:sa) chirped-pulse amplified (CPA) laser system that produces 35fs pulses centered at 800nm. Thin Beta-Barium Borate (β-BaB2O4 or BBO) crystals were chosen to achieve a compromise between short pulse durations and reasonable conversion efficiencies, since ultrashort pulses are quite susceptible to broadening from group velocity dispersion (GVD). Output energies of around 11μJ and 230nJ were measured for the 266.7nm and 200nm pulses, respectively. The transform limits of the 3rd and 4th harmonic pulse lengths were calculated from their measured spectral widths. We found that the 266.7nm bandwidth was large enough to support sub-30fs pulses, and due to cutting at the lower-wavelength end of the 200nm spectrum, we calculated an upper limit of 38fs. The pulses were compressed with pairs of CaF2 prisms to compensate for dispersion introduced by transmissive optics. Two-photon absorption (TPA) intensity autocorrelations revealed fully compressed pulse lengths of 36 ± 2 fs and 42 ± 4 fs for the 3rd and 4th harmonics, respectively.

No description available.

Chirped Bragg gratings (CBGs) recorded in photo-thermo-refractive (PTR) glass provide a very efficient and robust way to stretch and compress ultra-short laser pulses. These gratings offer the ability to stretch pulses from hundreds of femtoseconds, to the order of 1 ns and then recompress them. However, in order to achieve pulse stretching of this magnitude, 100 mm thick CBGs are needed. Using these CBGs to both stretch, and re-compress the pulse thus requires propagation through 200 mm of optical glass. This therefore demands perfect control of the glass homogeneity, as well as the holographic recording process of the CBG. In this thesis, we present a study of the CBG parameters that lead to distortions in the quality of diffracted beams. We first present the challenges associated with measuring the quality of these beams and we show that such measurements are not easily achieved using commercial systems that rely on the ISO standard M2 method. Thus, we introduce a new metric of beam quality, which we have coined S2 , that is a combination of both the M2 and power in the bucket metrics. Subsequently, we investigate the influence of the CBG parameters on the quality of diffracted beams. In particular, we examine the impact of small optical heterogeneities known as striae, as well as the impact of the optically and thermally induced distortions in the grating. We then use this data to improve the fabrication and characterization of 100 mm long CBGs. Finally, we characterize the performance of CBGs recorded in PTR for stretching and compression of femtosecond pulses using a custom autocorrelation system. We present data on high quality 100 mm long CBGs and an analysis on the correlation between beam quality and the final pulse duration after stretching and re-compressing the pulse.

In this thesis, advanced spectroscopy methods are discussed and applied to gain understanding of the physical properties of organic conjugated molecules, II-VI thin film semiconductors, and vertical cavity surface emitting lasers (VCSEL). Experiments include single photon and two-photon excitation with lasers, with subsequent measurements of the absorption and photoluminescence, as well as photocurrent measurements using tungsten and xenon lamps, measuring the direct current and the alternating current of the devices. The materials are investigated in dissolved form (conjugated polymers), thin films (polymers, II-VI semiconductors), and complex layer structures (hybrid device, VCSEL). The experiments are analyzed and interpreted by newly developed or applied theories for two-photon saturation processes in semiconductors, bandgap shrinkage due to optically induced electron hole pairs, and the principle of detailed balance to describe the photoluminescence in thin film cadmium sulfide. / Ph. D.

We have investigated processes of ionization, energy absorption and subsequent explosion of atomic and molecular clusters under intense laser illumination using numerical as well as analytical methods. In particular, we focused on the response of composite clusters, those consisting of different atomic elements, to intense light pulses. Another major theme is the effect of the molecular structure of clusters on their Coulomb explosion. The action of intense laser pulses on clusters leads to fundamental, irreversible changes: they turn almost instantaneously into nanoplasmas and subsequently disintegrate into separate ions and electrons. Due to this radical transformation, remarkable new features arise. Transient cluster nanoplasmas are capable of absorbing enormous amounts of laser energy. In some cases more than 90 % of incident laser energy is absorbed by a gas of clusters with a density much smaller than that of a solid. After the efficient absorption, the energy is transformed into production of energetic ions, electrons, photons, and even neutrons. Composite clusters show especially interesting behavior when they interact with intense laser pulses. Nanoplasmas formed in composite clusters may absorb even more laser energy, than those formed in homogeneous clusters, as we demonstrate in this work. One of the most important results of this thesis is the identification of a novel type of plasma resonance. This resonance is enabled by an unusual ellipsoidal shape of the nanoplasma created during the ionization process in a helium droplet doped with just a few xenon atoms. In contrast to the conventional plasma resonance, which requires significant ion motion, here, the resonant energy absorption occurs at a remarkably fast rate, within a few laser cycles. Therefore, this resonance is not only the most efficient (like the conventional resonance), but also, perhaps, the fastest way to transfer laser energy to clusters. Recently, dedicated experimental studies of this effect were performed at the Max Planck Institute in Heidelberg. Their preliminary results confirm our prediction of a strong, avalanche-like ionization of the helium droplet with a small xenon cluster inside. A conventional plasma resonance, which relies on the cluster explosion, also exhibits interesting new properties when it occurs in a composite xenon-helium cluster with a core-shell geometry. We have revealed an intriguing double plasma resonance in this system. This was the first theoretical study of the influence of the helium embedding on the laser- driven nanoplasma dynamics. Our results demonstrate the important role of the interaction between xenon and helium parts of the cluster. Understanding this interaction is necessary in order to correctly interpret the experimental results. We have elucidated several important properties of Coulomb explosion in atomic and molecular clusters. Specifically, it was found that the kinetic energy distribution of ions after the Coulomb explosion of an atomic cluster is quite similar to the initial potential energy distribution of ions and is only weakly influenced by ion overtake effects, as was believed before. For the case of molecular hydrogen clusters, we have shown that the alignment of molecules inside the cluster affects its Coulomb explosion. Investigation of the dynamical processes in composite and molecular clusters induced by intense laser pulses is a step towards understanding them in more complex nano-objects, such as biomolecules or viruses. This is of great interest in the context of x-ray diffractive imaging of biomolecules with atomic resolution, which is one of the main goals of new x-ray free electron laser facilities.

We have investigated processes of ionization, energy absorption and subsequent explosion of atomic and molecular clusters under intense laser illumination using numerical as well as analytical methods. In particular, we focused on the response of composite clusters, those consisting of different atomic elements, to intense light pulses. Another major theme is the effect of the molecular structure of clusters on their Coulomb explosion. The action of intense laser pulses on clusters leads to fundamental, irreversible changes: they turn almost instantaneously into nanoplasmas and subsequently disintegrate into separate ions and electrons. Due to this radical transformation, remarkable new features arise. Transient cluster nanoplasmas are capable of absorbing enormous amounts of laser energy. In some cases more than 90 % of incident laser energy is absorbed by a gas of clusters with a density much smaller than that of a solid. After the efficient absorption, the energy is transformed into production of energetic ions, electrons, photons, and even neutrons. Composite clusters show especially interesting behavior when they interact with intense laser pulses. Nanoplasmas formed in composite clusters may absorb even more laser energy, than those formed in homogeneous clusters, as we demonstrate in this work. One of the most important results of this thesis is the identification of a novel type of plasma resonance. This resonance is enabled by an unusual ellipsoidal shape of the nanoplasma created during the ionization process in a helium droplet doped with just a few xenon atoms. In contrast to the conventional plasma resonance, which requires significant ion motion, here, the resonant energy absorption occurs at a remarkably fast rate, within a few laser cycles. Therefore, this resonance is not only the most efficient (like the conventional resonance), but also, perhaps, the fastest way to transfer laser energy to clusters. Recently, dedicated experimental studies of this effect were performed at the Max Planck Institute in Heidelberg. Their preliminary results confirm our prediction of a strong, avalanche-like ionization of the helium droplet with a small xenon cluster inside. A conventional plasma resonance, which relies on the cluster explosion, also exhibits interesting new properties when it occurs in a composite xenon-helium cluster with a core-shell geometry. We have revealed an intriguing double plasma resonance in this system. This was the first theoretical study of the influence of the helium embedding on the laser- driven nanoplasma dynamics. Our results demonstrate the important role of the interaction between xenon and helium parts of the cluster. Understanding this interaction is necessary in order to correctly interpret the experimental results. We have elucidated several important properties of Coulomb explosion in atomic and molecular clusters. Specifically, it was found that the kinetic energy distribution of ions after the Coulomb explosion of an atomic cluster is quite similar to the initial potential energy distribution of ions and is only weakly influenced by ion overtake effects, as was believed before. For the case of molecular hydrogen clusters, we have shown that the alignment of molecules inside the cluster affects its Coulomb explosion. Investigation of the dynamical processes in composite and molecular clusters induced by intense laser pulses is a step towards understanding them in more complex nano-objects, such as biomolecules or viruses. This is of great interest in the context of x-ray diffractive imaging of biomolecules with atomic resolution, which is one of the main goals of new x-ray free electron laser facilities.:1. Introduction 1 2. Interaction of clusters with intense laser pulses 5 2.1. Cluster formation and structure . . . . . . . . . . . . . . . . . . 5 2.1.1. Cluster formation . . . . . . . . . . . . . . . . . . . . . . 5 2.1.2. Cluster structure . . . . . . . . . . . . . . . . . . . . . . 6 2.1.3. Composite clusters . . . . . . . . . . . . . . . . . . . . . 7 2.2. Matter in intense light fields . . . . . . . . . . . . . . . . . . . . 9 2.2.1. Laser sources . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2.2. Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3. Clusters under intense laser pulses . . . . . . . . . . . . . . . . . 11 2.3.1. Three stages of intense laser-cluster interaction . . . . . 12 2.3.2. Pathways of cluster ionization and energy absorption . . 13 2.3.3. Composite clusters in intense laser fields . . . . . . . . . 14 2.4. Scenarios of cluster explosion . . . . . . . . . . . . . . . . . . . 15 2.4.1. Coulomb explosion vs. quasi-neutral expansion . . . . . 15 2.4.2. Anisotropic explosion . . . . . . . . . . . . . . . . . . . . 17 2.5. Comparison between experiment and theory . . . . . . . . . . . 18 3. Theoretical methods for intense laser-cluster interaction 21 3.1. The Hamiltonian . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2. Survey of simulation methods . . . . . . . . . . . . . . . . . . . 22 3.2.1. Quantum methods . . . . . . . . . . . . . . . . . . . . . 22 3.2.2. Classical methods . . . . . . . . . . . . . . . . . . . . . . 23 3.3. Our method: classical microscopic molecular dynamics . . . . . 24 3.3.1. Initial configuration . . . . . . . . . . . . . . . . . . . . . 24 3.3.2. Integrating the equations of motion . . . . . . . . . . . . 26 3.3.3. Observables . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.4. The role of quantum effects . . . . . . . . . . . . . . . . . . . . 31 4. Cluster nanoplasma: a statistical approach 33 4.1. Vlasov-Poisson formalism . . . . . . . . . . . . . . . . . . . . . . 33 4.2. Nanoplasma electrons at quasi-equilibrium . . . . . . . . . . . . 34 4.2.1. Self-consistent potential and electron density . . . . . . . 34 4.2.2. Energy distribution of nanoplasma electrons . . . . . . . 36 4.3. Harmonic oscillator model . . . . . . . . . . . . . . . . . . . . . 41 4.3.1. Derivation from kinetic equations . . . . . . . . . . . . . 42 4.3.2. Comparison with the molecular dynamics results . . . . 44 4.4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 5. Ionization and energy absorption in helium droplets doped with xenon clusters 47 5.1. Local ignition and anisotropic nanoplasma growth . . . . . . . . 48 5.1.1. Cluster size dependence . . . . . . . . . . . . . . . . . . 50 5.1.2. Nanoplasma resonance during its anisotropic growth . . 51 5.1.3. Range of laser frequencies and intensities . . . . . . . . . 55 5.1.4. Plasma resonance for circular polarization . . . . . . . . 56 5.1.5. Summary and future work . . . . . . . . . . . . . . . . . 57 5.2. Electron migration and its influence on the cluster expansion . . 59 5.2.1. Charging dynamics . . . . . . . . . . . . . . . . . . . . . 59 5.2.2. Explosion dynamics . . . . . . . . . . . . . . . . . . . . . 61 5.3. Interplay between nanoplasma expansion and its electronic response 63 5.3.1. Single pulse: time-dependence . . . . . . . . . . . . . . . 64 5.3.2. Two pulses: a pump-probe study . . . . . . . . . . . . . 67 5.4. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . 71 6. Coulomb explosions of atomic and molecular clusters 75 6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 6.2. Analytical treatment of the Coulomb explosion . . . . . . . . . . 76 6.2.1. Steplike density profile . . . . . . . . . . . . . . . . . . . 76 6.2.2. Kinetic approach . . . . . . . . . . . . . . . . . . . . . . 79 6.2.3. Gradually decreasing initial density . . . . . . . . . . . . 83 6.3. Coulomb explosions of atomic and molecular hydrogen clusters: a molecular dynamics study . . . . . . . . . . . . . . . . . . . . 84 6.3.1. Kinetic energy distributions of ions (KEDI) . . . . . . . 85 6.3.2. Information loss during the explosion . . . . . . . . . . . 87 6.3.3. Ion overtake processes . . . . . . . . . . . . . . . . . . . 90 6.3.4. Non-radial motion of ions . . . . . . . . . . . . . . . . . 91 6.3.5. Three-body effects in Coulomb explosion . . . . . . . . . 93 6.4. Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . 96 7. Conclusions and outlook 97 7.1. Physical conclusions . . . . . . . . . . . . . . . . . . . . . . . . 97 7.2. Methodological conclusions . . . . . . . . . . . . . . . . . . . . . 99 7.3. Research perspectives . . . . . . . . . . . . . . . . . . . . . . . . 100 A. Suppression of the cluster barrier 101 B. Structure determination for Xen@Hem clusters 103 C. Calculation of the time-dependent phase shift 107 D. Potential of a uniformly charged spheroid 109 E. On the possibility of molecular alignment inside hydrogen clusters 111 Bibliography

Thesis (PhD)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Low-dimensional materials are known to undergo phase transitions to differently or- dered states, when cooled to lower temperatures. These phases often show a periodic modulation of the charge density (called a charge density wave – CDW) coupled with a periodic perturbation of the crystal lattice (called a periodic lattice distortion – PLD). Although many experiments have been performed and much has been learnt about CDW phases in low-dimensional materials, the reasons for their existence are still not fully understood yet. Many processes, involving either strong electron–electron or electron–lattice coupling, have been observed which all might play a role in explaining the formation of different phases under different conditions. With the availability of femtosecond lasers it has become possible to study materials under highly nonequilibrium conditions. By suddenly introducing a known amount of energy into the system, the equilibrium state is disturbed and the subsequent relax- ation processes are then observed on timescales of structural and electronic responses. These experiments can deliver valuable information about the complex interactions between the different constituents of condensed matter, which would be inaccessible under equilibrium conditions. We use time resolved electron diffraction to investigate the behaviour of a CDW system perturbed by a short laser pulse. From the observed changes in the diffraction patterns we can directly deduce changes in the lattice structure of our sample. A femtosecond electron diffraction setup was developed at the Laser Research In- stitute in Stellenbosch, South Africa. Short laser pulses produce photo electrons which are accelerated to an energy of 30 keV. Despite space charge broadening effects, elec- tron pulses shorter than 500 fs at sample position can be achieved. Technical details of this system and its characterisation as well as sample preparation techniques and analysis methods are described in detail in this work. Measurements on two members of the quasi-two-dimensional transition metal di- chalcogenides, namely 4Hb-TaSe2 and 1T-TaS2, are shown and discussed. Both show fast (subpicosecond) changes due to the suppression of the PLD and a rapid heating of the lattice. When the induced temperature rise heats the sample above a phase tran- sition temperature, a complete transformation into the new phase was observed. For 4Hb-TaSe2 we found that the recovery to the original state is significantly slower if the PLD was completely suppressed compared to only disturbing it. On 1T-TaS2 we could not only study the suppression of the original phase but also the formation of the higher energetic CDW phase. Long (100 ps) time constants were found for the tran- sition between the two phases. These suggest the presence of an energy barrier which has to be overcome in order to change the CDW phase. Pinning of the CDW by de- fects in the crystal structure result in such an energy barrier and consequently lead to a phase of domain growth which is considerably slower than pure electron or lattice dynamics. / AFRIKAANSE OPSOMMING: Dit is bekend dat lae-dimensionele materie fase oorgange ondergaan na anders ge- ori¨enteerde toestande wanneer afgekoel word tot laer temperature. Hierdie fases toon dikwels ’n periodiese modulasie van die elektron digtheid (genoem ’n “charge density wave” – CDW), tesame met ’n periodiese effek op die kristalrooster (genoem ’n “peri- odic lattice distortion” – PLD). Alhoewel baie eksperimente al uitgevoer is en al baie geleer is oor hierdie CDW fase, is die redes vir hul bestaan nog steeds nie ten volle verstaan nie. Baie prosesse, wat of sterk elektron–elektron of elektron–fonon interaksie toon, is al waargeneem en kan ’n rol speel in die verduideliking van die vorming van die verskillende fases onder verskillende omstandighede. Met die beskikbaarheid van femtosekonde lasers is dit nou moontlik om materie onder hoogs nie-ewewig voorwaardes te bestudeer. Deur skielik ’n bekende hoeveel- heid energie in die stelsel in te voer, word die ewewigstaat versteur en word die daar- opvolgende ontspanning prosesse waargeneem op die tydskaal van atomies struktu- rele en elektroniese bewiging. Hierdie eksperimente kan waardevolle inligting lewer oor die komplekse interaksies tussen die verskillende atomiese komponente van ge- kondenseerde materie, wat ontoeganklik sou wees onder ewewig voorwaardes. Ons gebruik elektrondiffraksie met tyd resolusie van onder ’n pikosekonde om die gedrag van ’n CDW stelsel te ondersoek nadat dit versteur is deur ’n kort laser puls. Van die waargenome veranderinge in die diffraksie patrone kan ons direk aflei watse veranderinge die kristalstruktuur van ons monster ondergaan. ’n Femtosekonde elektronendiffraksie opstelling is ontwikkel by die Lasernavors- ingsinstituut in Stellenbosch, Suid-Afrika. Kort laser pulse produseer foto-elektrone wat dan na ’n energie van 30 keV versnel word. Ten spyte van Coulomb afstoting ef- fekte, kan elektron pulse korter as 500 fs by die monster posisie bereik word. Tegniese besonderhede van hierdie opstelling, tegnieke van die voorbereiding van monsters asook analise metodes word volledig in hierdie tesis beskryf. Metings op twee voorbeelde van kwasi-tweedimensionele semi-metale, naamlik 4Hb-TaSe2 en 1T-TaS2, word gewys en bespreek. Beide wys ’n vinnige (subpikosekon- de) verandering as gevolg van die versteuring van die PLD en ’n vinnige verhitting van die kristalrooster. Wanneer die ge¨ınduseerde temperatuur bo die fase oorgang tempe- ratuur styg, is ’n volledige transformasie na die nuwe fase waargeneem. Vir 4Hb-TaSe2 het ons gevind dat die herstelling na die oorspronklike toestand aansienlik stadiger is as die PLD heeltemal viernietig is in vergelyking met as die PLD net versteur is. Met 1T-TaS2 kon ons nie net alleenlik die vernietiging van die oorspronklike fase sien nie, maar ook die vorming van ’n ho¨er energie CDW fase. Lang (100 ps) tydkonstante is gevind vir die oorgang tussen die twee fases. Hierdie dui op die teenwoordigheid van ’n energie-versperring wat eers oorkom moet word om die CDW fase voledig te ver- ander. Vaspenning van die CDW deur defekte in die kristalstruktuur veroorsaak so’n energie versperring en gevolglik lei dit tot ’n fase van groeiende CDW gebiede wat heelwat stadiger as pure elektron of kritalrooster dinamika is.