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This thesis reports on the work performed on the manipulation of micro­-particles on substrate surfaces using short laser pulses. For particles with diameters on the order of microns, the binding forces to surfaces are significantly larger than gravitational forces. To overcome these binding forces and manipulate the particles the use of femtosecond laser pulses has been investigated. Individual micro-particles (poly-divinylbenzene, glass and silver materials) with diameters around 2 um were removed from substrate surfaces (dielectric, semiconductor and metal substrates) us­ing a Ti:Sapphire laser system. The pulses produced at 800 nm had pulse lengths around 140 fs and were tightly focussed onto the surface using 5x and 10x micro­scope objectives. The peak fluence thresholds for particle removal were determined and the surfaces examined after irradiation by a scanning electron microscope and atomic force microscope to check for damage. The experimental results indicate that ablation of the substrate below the micro-particles is most likely to be respon­sible for micro-particle removal from the substrate surface when using femtosecond pulses. Ablation pits were observed for the dielectric micro-spheres on semiconductor substrates. It is also believed that ablation is responsible for the removal of other types of micro-particles from various substrates. Unlike the dielectric micro-sphere on semiconductor substrate results, the other particle-substrate combinations show a close correspondence between the removal and substrate ablation thresholds. It is believed that these results indicate the occurrence of ablation leading to the removal of the micro-particles. Calculations of the local electromagnetic fields around the micro-particles have also been carried out and the distributions used to interpret the experimental results. / Thesis / Master of Applied Science (MASc)

In recent years, there has been an intense interest in understanding the microscopic mechanism of laser induced ultrafast magnetization dynamics in picosecond time scales. Magnetization switching on such a time scale has potential to be a significant boost for the data storage industry. It is expected that the writing process will become ~1000 times faster by this technology, compared to existing techniques. Understanding the microscopic mechanisms and controlling the magnetization in such a time scale is of paramount importance at present. In this thesis, laser induced ultrafast magnetization dynamics has been studied for Fe, Co, GdFe, CoMn and Heusler alloys. A multiscale approach has been used, i.e., first-principles density functional theory combined with atomistic spin dynamics utilizing the Landau –Lifshitz-Gilbert equation, along with a three-temperature phenomenological model to obtain the spin temperature. Special attention has been paid to the calculations of exchange interaction and Gilbert damping parameters. These parameters play a crucial role in determining the ultrafast magnetization dynamics under laser fluence of the considered materials. The role of longitudinal and transversal excitations was studied for elemental ferromagnets, such as Fe and Co. A variety of complex temporal behavior of the magnetic properties was observed, which can be understood from the interplay between electron, spin, and lattice subsystems. The very intricate structural and magnetic nature of amorphous Gd-Fe alloys for a wide range of Gd and Fe atomic concentrations at the nanoscale was studied. We have shown that the ultrafast thermal switching process can happen above the compensation temperature in GdFe alloys. It is demonstrated that the exchange frustration via Dzyaloshinskii-Moriya interaction between the atomic Gd moments, in Gd rich area of these alloys, leads to Gd demagnetization faster than the Fe sublattice. In addition, we show that Co is a perfect Heisenberg system. Both Co and CoMn alloys have been investigated with respect to ultrafast magnetization dynamics. Also, it is predicted that ultrafast switching process can happen in the Heulser alloys when they are doped with heavy elements. Finally, we studied multiferroic CoCr2O4 and Ca3CoMnO4 systems by using the multiscale approach to study magnetization dynamics. In summary, our approach is able to capture crucial details of ultrafast magnetization dynamics in technologically important materials.

Ultrafast spectroscopy allows us to probe and understand material properties. With it, we can measure phonon-polaritons (optical phonons coupled with light) and the resulting dispersion curve in lithium niobate. Customizing the excitation source in ultrafast measurements can excite phonon modes to large amplitudes, allowing the experimental exploration of the Potential Energy Surface in solids. However, stronger pump fluences and bigger signal isn't always the answer in ultrafast spectroscopy. When sample signals and their nonlinear and mechanisms cannot be distinguished with 1D measurements, simple 2D THz measurements are a great place to start searching for distinct factors as was the case in cadmium tungstate. 2D measurements when paired with modeling and first principles calculations can reveal cutting edge information about exciting materials.

The invention of chirped-pulse amplification (CPA) in 1985 led to a tremendous increase in obtainable laser pulse peak intensities. Since then, several table-top, Ti:sapphire-based CPA systems exceeding the 100 TW-level with more than 10 W average power have been developed and several systems are now commercially available. Over the last decade, the complementary technology of optical parametric chirped-pulse amplification (OPCPA) has improved in its performance to a competitive level. OPCPA allows direct amplification of an almost-octave spanning bandwidth supporting few-cycle pulse durations at center wavelengths ranging from the visible to the mid-IR. The current record in peak power from a table-top OPCPA is 16 TW and the current record average power is 22 W. High energy, few-cycle pulses with stabilized carrierenvelope phase (CEP) are desired for applications such as high-harmonic generation (HHG) enabling attoscience and the generation keV-photon bursts. This dissertation conceptually, numerically and experimentally describes essential aspects of few-cycle OPCPA, and the associated pump beam generation. The main part of the conducted research was directed towards the few-cycle OPCPA facility developed in the Laser Plasma Laboratory at CREOL (University of Central Florida, USA) termed HERACLES. This facility was designed to generate few-cycle pulses in the visible with mJ-level pulse energy, W-level average power and more than 100 GW peak power. Major parts of the implementation of the HERACLES facility are presented. The pump generation beam of the HERACLES system has been improved in terms of pulse energy, average power and stability over the last years. It is based on diode-pumped, solid-state amplifiers with picosecond duration and experimental investigations are presented in detail. A iii robust system has been implemented producing mJ-level pulse energies with ~100 ps pulse duration at kHz repetition rates. Scaling of this system to high power (>30 W) and high peak power (50-MW-level) as well as ultra-high pulse energy (>160 mJ) is presented. The latter investigation resulted in the design of an ultra-high energy system for OPCPA pumping. Following this, a new OPCPA facility was designed termed PhaSTHEUS, which is anticipated to reach ultra-high intensities. Another research effort was conducted at CELIA (Univeristé de Bordeaux 1, France) and aimed towards a previously unexplored operational regime of OPCPA with ultra-high repetition rates (10 MHz) and high average power. A supercontinuum seed beam generation has been established with an output ranging from 1.3 to 1.9 µm and few ps duration. The pump beam generation has been implemented based on rod-type fiber amplifiers producing more than 37 W average power and 370 kW peak power. The utility of this system as an OPCPA pump laser is presented along with the OPA design. The discussed systems operate in radically different regimes in terms of peak power, average power, and repetition rate. The anticipated OPCPA systems with few-cycle duration enable a wide range of novel experimental studies in attoscience, ultrafast materials processing, filamentation, LIBS and coherent control

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Thesis (MSc (Physics))--Stellenbosch University, 2009. / ENGLISH ABSTRACT: This thesis aims to comprehensively discuss ultrafast electron di raction and its role in temporally resolving ultrafast dynamics on the molecular level. Theory on electron pulses and electron pulse propagation will be covered, but the main focus will be on the method, equipment and experimental setup required to generate sub-picosecond electron pulses, which are needed to perform time resolved experiments. The design and construction of an electron gun needed to produce the electron pulses will be shown in detail, while preliminary pulse characterization experiments will also be illustrated. An introduction into the theory of electron diffraction patterns and how to interpret these diffraction patterns will conclude the thesis. / AFRIKAANSE OPSOMMING: Hierdie tesis het ten doel om ultravinnige elektrondi raksie deeglik te bespreek asook die rol wat dit speel om ultravinnige tyd-dinamika op 'n molekulêre vlak op te los. Die teorie van elektonpulse en die voortplanting van elektronpulse sal gedek word, maar die fokus sal op die metode, gereedskap en eksperimentele opstelling wees wat benodig is om sub-pikosekonde elektronpulse te genereer. Die ontwerp en konstruksie van 'n elektrongeweer, wat benodig word om elektronpulse te produseer, sal in detail bespreek word, terwyl aanvanklike pulskarakterisasie eksperimente ook illustreer sal word. 'n Inleiding tot die teorie van elektrondi raksie patrone en hoe om hulle te interpreteer sal die tesis afsluit.

We demonstrate experimentally that the multiphoton ionization rate in gallium arsenide depends on the alignment of the laser polarization with respect to the crystal axis. We show real-time modulation of 1900nm laser ionization rate, through viewing transmission, which mimics the symmetry of the semiconductor crystal. We propose that the modulation in the ionization rate arises because the varying reduced effective carrier mass, as predicted by Keldysh theory. We show direct comparison of the experimental transmission modulation depth with that predicted by Keldysh theory. This opens up a novel method for real-time non-invasive crystallography of crystalline materials.

In the ultracold regime, where the interactions between atoms become quantum mechanical in nature, we can investigate the fundamental properties of matter. A natural progression from the catalogue of pioneering experiments using ultracold atoms is to extend the size of our quantum system by producing ultracold molecules in prescribed low-energy internal states. Techniques for cold molecule production are split into two methods: direct and indirect cooling. While direct cooling methods have yet to realize ultracold temperatures, collisional relaxation in the molecules leads to low internal energy states. By contrast, indirect cooling — the association of molecules from pre-cooled atoms—has produced a range of molecules at ultracold temperatures; the challenge with this technique is to control the internal state. This thesis concentrates on a technique that is complementary to those already in existence: ultrafast photoassociation. Key to this technique is the formation of time non-stationary wavepackets in the excited-state in order to improve FranckCondon overlap of the excited state with deeply bound ground-state vibrational levels. A pump-probe experiment was designed and built to demonstrate the formation of bound excited-state dimers. In this work we show that the initial state from which the wavepacket originates is of critical importance to the evolution of excited-state population. We find that the internuclear separation of the wavepacket produced in a rubidium magneto-optical trap is too large to observe coherent oscillations in the excited state. The implications of this are discussed along with recommendations for future ultrafast photoassociation experiments. Consequently, a new ultracold atom apparatus was built utilizing magnetic and dipole-force trapping to increase the density of the atomic sample; this apparatus will enable future experiments combining the exciting fields of ultracold matter and ultrafast light.