Ultrafast electron diffraction is a novel pump-probe technique which aims to determine transient structures during photoinduced chemical reactions and other structural transitions. This technique provides structural information at the atomic level of inspection by using an electron pulse as a diffractive probe. The atomic motions of interest happen on the 100 fs = 10^(−13) s time scale. To observe these atomic motions, a probe which matches this time scale is required. In this thesis, I describe the development of an electron diffractometer which is capable of 200 fs temporal resolution while maintaining high signal level per electron pulse. This was made possible by the construction of an ultra-compact photoactivated 60 keV femtosecond electron gun. Traditional electron pulse characterization methods are unsuitable for high number density femtosecond electron pulses such as the pulses produced by this electron gun. I developed two techniques based on the laser ponderomotive force to reliably determine the duration of femtosecond electron pulses into the sub-100 fs range. These techniques produce a direct cross-correlation trace between the electron pulse and a laser pulse. The results of these measurements confirmed the temporal resolution of the newly developed femtosecond electron diffractometer. This cross-correlation technique was also used to calibrate a method for the determination of the temporal overlap of electron and laser pulses. These techniques provide the pulse diagnostics necessary to utilize the temporal resolution provided by femtosecond electron pulses. Owing to their high charge-to-mass ratio, electrons are a sensitive probe for electric fields. I used femtosecond electron pulses in an electron deflectometry experiment to directly observe the transient charge distributions produced during femtosecond laser ablation of a silicon (100) surface. We found an electric field strength of 3.5 × 10^6 V/m produced by the emission of 5.3 × 10^11 electrons/cm^2 just 3 ps after an excitation pulse of 5.6 J/cm^2 . This observation allowed us to rule out Coulomb explosion as the mechanism for ablation under the conditions present in this experiment.

Ultrafast electron diffraction is a novel pump-probe technique which aims to determine transient structures during photoinduced chemical reactions and other structural transitions. This technique provides structural information at the atomic level of inspection by using an electron pulse as a diffractive probe. The atomic motions of interest happen on the 100 fs = 10^(−13) s time scale. To observe these atomic motions, a probe which matches this time scale is required. In this thesis, I describe the development of an electron diffractometer which is capable of 200 fs temporal resolution while maintaining high signal level per electron pulse. This was made possible by the construction of an ultra-compact photoactivated 60 keV femtosecond electron gun. Traditional electron pulse characterization methods are unsuitable for high number density femtosecond electron pulses such as the pulses produced by this electron gun. I developed two techniques based on the laser ponderomotive force to reliably determine the duration of femtosecond electron pulses into the sub-100 fs range. These techniques produce a direct cross-correlation trace between the electron pulse and a laser pulse. The results of these measurements confirmed the temporal resolution of the newly developed femtosecond electron diffractometer. This cross-correlation technique was also used to calibrate a method for the determination of the temporal overlap of electron and laser pulses. These techniques provide the pulse diagnostics necessary to utilize the temporal resolution provided by femtosecond electron pulses. Owing to their high charge-to-mass ratio, electrons are a sensitive probe for electric fields. I used femtosecond electron pulses in an electron deflectometry experiment to directly observe the transient charge distributions produced during femtosecond laser ablation of a silicon (100) surface. We found an electric field strength of 3.5 × 10^6 V/m produced by the emission of 5.3 × 10^11 electrons/cm^2 just 3 ps after an excitation pulse of 5.6 J/cm^2 . This observation allowed us to rule out Coulomb explosion as the mechanism for ablation under the conditions present in this experiment.

Thesis (MSc (Physics))--University of Stellenbosch, 2008. / Various autocorrelation techniques are employed to characterize ultrashort laser pulses in both the temporal and spectral domain. These techniques are; interference autocorrelation (IAC), modified spectrum autointerferometric correlation (MOSAIC), background-free autocorrelation (BFA) and frequency resolved optical gating (FROG). All of these techniques are based on the interaction of a pulse with a time delayed copy of itself within a Â(2) medium. Experimental setups for BFA and FROG experiments are developed, which exploit the phenomenon of second harmonic generation (SHG). An existing IAC setup is used for temporal pulse characterization. MOSAIC results are obtained through applying a specific Fourier filter to the IAC data. IAC and MOSAIC measurements performed on a commercially available femtosecond laser, indicate that the emitted pulse has a pulse duration less than 150 fs and possesses positive linear chirp. BFA and FROG measurements carried out on the same laser system mirror these results. Pulses emitted by a 20 Hz chirped pulse amplifier are characterized through BFA and FROG. BFA results suggest that the pulse from the amplifier is actually a double pulse. FROG results indicate that the pulse is highly chirped. The experiments and physical interpretations presented in this work demonstrate the preferred methods of optical pulse characterization for ultrashort laser pulses.

Ultralow threshold, compact and highly efficient femtosecond lasers based on Yb³⁺ -doped potassium yttrium tungstate (Yb:KYW) and Yb³⁺ -doped vanadium yttrium oxide (Yb:YVO 4 ) have been demonstrated within this PhD-research project. For a continuous wave unmode-locked Yb:KYW laser a threshold as low as 101 mW was obtained with a slope efficiency of 74 %. By employing a single prism for dispersion control, the laser was tunable between 1012 nm to 1069 nm. When operated in the mode-locked regime, this laser produced transform-limited pulses having durations of 210 fs at a central wavelength of 1044 nm. Stable mode locking was observed for an optimised incident pulse fluence on the SESAM between 140 μJ/cm² to 160 μJ/cm² which was 2-3 times higher than the designed energy pulse fluence of the SESAM (70 μJ/cm² ). The employment of several combinations of chirped mirror designs for control of intracavity group velocity dispersion led to excellent results. The threshold for mode locking was satisfied for a pump power of 255 mW where the slope efficiency was measured to be 62 %. This is the most efficient SESAM-assisted femtosecond laser yet reported and the highest optical-to-optical efficiency of 37 % is exceptional. Transform- limited pulses with durations as short as 90 fs were produced in a spectral region centred on 1052 nm. The success of this research thus represents a good foundation on which to design and build more compact configurations that will incorporate just one chirped mirror for dispersion compensation. A relatively high nonlinear refractive index, n₂ , of 15 x 10⁻¹⁶ cm² /W was measured in Yb:YVO 4 and this affords particular potential for this candidate material in Kerr-lens mode locking. In fact, for operation in the femtosecond domain, the threshold power was 190 mW with a slope efficiency of 26 % and near-transform-limited pulses as short as 61 fs were generated at a centre wavelength of 1050 nm. The main objectives in developing this type of laser relate to a demonstration of high peak power operation in thin disc laser configurations. The deployment of a diode-pumped Yb:KYW femtosecond laser as a pump source for frequency doubling in a periodically-poled LiTaO₃ crystal was realised. The maximum realized output power of 150 mW corresponded to an impressive second harmonic conversion efficiency of 43 %. 225-fs duration green pulses (centred at 525 nm) were generated under the condition of strong focusing in the nonlinear crystal.

The nexus of engineering and molecular biology has given birth to high-throughput technologies that allow biologists and medical scientists to produce previously unattainable amounts of data to better understand the molecular basis of many biological phenomena. Here, we describe the development of an enabling biotechnology, commonly known as microfluidics in the fabrication of high-throughput systems to study nerve degeneration and regeneration in the well-defined model nematode, Caenorhabditis elegans (C. elegans). Our lab previously demonstrated how femtosecond (fs) laser pulses could precisely cut nerve axons in C. elegans, and we observed axonal regeneration in vivo in single worms that were immobilized on anesthetic treated agar pads. We then developed a microfluidic device capable of immobilizing one worm at a time with a deformable membrane to perform these experiments without agar pads or anesthetics. Here, we describe the development of improved microfluidic devices that can trap and immobilize up to 24 individual worms in parallel chambers for high-throughput axotomy and subsequent imaging of nerve regeneration in a single platform. We tested different micro-channel designs and geometries to optimize specific parameters: (1) the initial trapping of a single worm in each immobilization chamber, simultaneously, (2) immobilization of single worms for imaging and fs-laser axotomy, and (3) long term storage of worms on-chip for imaging of regeneration at different time points after the initial axon cut. / text

Expérience réalisée a l'aide d'un laser a colorant en anneau a blocage de mode passif, et dont la dispersion de vitesse de groupe peut être contrôlée. On montre que le fonctionnement de ce type de laser est base sur des phénomènes solitons en analysant un régime particulier du laser qui émet alors des impulsions semblables à des solitons d'ordre 3. Description d'une chaine amplificatrice ayant un gain d'environ 10**(6). On présente un nouveau système de compensation de la dispersion de la chaine amplificatrice et une nouvelle méthode de mesure monocoup de la réponse temporelle de l'effet Kerr optique.

I present photonics applications of PbS quantum-dot-doped (QD-doped) glasses. The dissertation consists of two major parts: bulk material applications (Cr:forsterite laser modelocking, bleaching dynamics, optical gain, and photoluminescence) and the fabrication of QD-doped ion-exchanged waveguides.When this work began, these PbS QD-doped glasses were the state-of-the-art in QD glasses due to their narrow size distribution. Modelocking of a Cr:forsterite laser using this glass as a saturable absorber had been demonstrated, with little understanding of the dynamics. This work began by studying the dynamics of the saturable absorber to explain the ps-pulse width.In the bulk measurements, I functioned as secondary researcher. In the laser modelocking and bleaching measurements, my contribution was laser cavity alignment, sample preparation, collecting autocorrelation traces, and aiding in the setup and data collection for the bleaching measurements. On this work, I coauthored one refereed journal article in Applied Physics Letters [1] and one refereed conference paper [2], for which I am third and second author, respectively.For the gain measurements, I aided in the setup and data collection, whereas I set-up and took most of the luminescence data. The gain measurements resulted in one second-author refereed journal article in Applied Physics Letters [3] and I presented the luminescence results at CLEO2000 [4].I took the lead role in the waveguide fabrication and characterization and authored refereed journal articles in Applied Physics Letters [5], Journal of Applied Physics [6], and Journal of the Optical Society of America B [7]. I also presented an invited talk at Photonics West [8] and presented at CLEO2004 [9]. Additionally, I have been a coauthor of presentations at the Nanotechnology Symposium (2006), American Ceramic Society [10], and Photonics Europe (2006) [11]. A book chapter in The Photonics Handbook, 2nd edition [12] also discusses this work.The next step is to focus on reducing the waveguide losses. This requires new, circular wafers with better surface quality and glass homogeneity. I suggest using silver-film ion exchange followed by a field-assisted burial to eliminate the surface interaction.

This thesis focuses on research I have done on ytterbium-doped femtosecond fiber lasers. These lasers operate in the near infrared region, lasing at 1030 nm. This wavelength is particularly important in biomedical applications, which includes but is not limited to confocal microscopy and ablation for surgical incisions. Furthermore, fiber lasers are advantageous compared to solid state lasers in terms of their cost, form factor, and ease of use. Solid state lasers still dominate the market due to their comparatively high energy pulses. High energy pulse generation in fiber lasers is hindered by either optical wave breaking or by multipulsing. One of the main challenges for fiber lasers is to overcome these limitations to achieve high energy pulses. The motivation for the work done in this thesis is increasing the output pulse peak power and energy. The main idea of the work is that decreasing the nonlinearity that acts on the pulse inside the cavity will prevent optical wave breaking, and thus will generate higher energy pulses. By increasing the output energy, ytterbium-doped femtosecond fiber lasers can be competitive with solid state lasers which are used commonly in research. Although fiber lasers tend to lack the wavelength tuning ability of solid state lasers, many biomedical applications take advantage of the 1030 µm central wavelength of ytterbium-doped fiber lasers, so the major limiting factor of fiber lasers in this field is simply the output power. By increasing the output energy without resorting to external amplification, the cavity is optimized and cost can remain low and economical. During verification of the main idea, the cavity was examined for possible back-reflections and for components with narrow spectral bandwidths which may have contributed to the presence of multipulsing. Distinct cases of multipulsing, bound pulse and harmonic mode-locking, were observed and recorded as they may be of more interest in the future. The third-order dispersion contribution from the diffraction gratings inside the laser cavity was studied, as it was also considered to be an energy-limiting factor. No significant effect was found as a result of third-order dispersion; however, a region of operation was observed where two different pulse regimes were found at the same values of net cavity group velocity dispersion. Results verify the main idea and indicate that a long length of low-doped gain fiber is preferable to a shorter, more highly doped one. The low-doped fiber in an otherwise equivalent cavity allows the nonlinear phase shift to grow at a slower rate, which results in the pulse achieving a higher peak power before reaching the nonlinear phase shift threshold at which optical wave breaking occurs. For a range of net cavity group velocity dispersion values, the final result is that the low doped fiber generates pulses of approximately twice the value of energy of the highly-doped gain fiber. Two techniques of mode-locking cavities were investigated to achieve this result. The first cavity used NPE mode-locking which masked the results, and the second used a SESAM for mode-locking which gave clear results supporting the hypothesis.

Quantum mechanical motion of electrons in atoms and molecules is at the heart of many photophysical and photochemical processes. As the natural timescale of electron dynamics is in the range of femtoseconds or shorter, ultrashort pulses are required to study such phenomena. The ultrashort pulse light-matter interaction at high intensity regime can however dramatically alter the atomic and molecular structures. Our current understanding of such transient electronic modification is far from complete, especially when complicated light-induced couplings are involved. In this dissertation, we investigated how a femtosecond strong-field pulse can control or modify the evolution of atomic or molecular polarization, representing electric dipole excitation in various systems. Extreme ultraviolet (XUV) attosecond pulse trains are used to coherently prepare superposition of excited states in various atomic and molecular systems. A subsequent phase-locked infrared (IR) femtosecond pulse is applied to perturb the dipoles, and transient changes in the transmitted XUV spectra are measured. This scheme is termed as XUV attosecond transient absorption spectroscopy. In the first study, we applied this technique to study the modification of Rydberg states in dilute helium gas. We observed several transient changes to the atomic structure, including the ac Stark shift, laser-induced quantum phase, laser-induced continuum structure, and quantum path interference. When the experiments were extended to the study of a dense helium gas sample, new spectral features in the absorption spectra emerged which cannot be explained by linear optical response models. We found that these absorption features arise from the interplay between the XUV resonant pulse propagation and the IR-imposed phase shift. A unified physical model was also developed to account for various scenarios. Extending our work to argon atoms, we studied how an external infrared field can be used to impulsively control different photo-excitation pathways and the transient absorption lineshape of an otherwise isolated autoionizing state. It is found that by controlling the field polarization of the IR pulse, we can modify the transient absorption line shape from Fano-like to Lorentzian-like profiles. Unlike atoms, in our study of autoionizing states of the oxygen molecule, we observed both positive and negative optical density changes for states with different electronic symmetries. The predictions of two distinct and simplified dipole perturbation models were compared against both the experimental results and a full theoretical calculation in order to understand the origin of the sign of absorption change. We relate this symmetry-dependent sign change to the Fano parameters of static photoabsorption. The same approach was applied to study molecular nitrogen, in which we observed the decay dynamics of IR perturbed doubly-excited Rydberg states with many vibrational progressions. In addition, we also conducted experiments to investigate Rydberg state dynamics of other molecular systems such as carbon dioxide. In summary, we experimentally explored the ephemeral light-induced phenomena associated with excited states of atoms and molecules. These studies provide real-time information on ultrafast electronic processes and provide strategies for direct time-domain control of the light-matter interaction.

Master of Science / Department of Physics / Carlos Trallero / Over the past four decades, the single-electron ionization of atoms has been a subject of great interest within the ultra-fast community. While contemporary atomic ionization models tend to agree well with experiment across a wide range of intensities (10[superscript]13-10[superscript]15 W/cm[superscript]2), analogous models for the ionization of molecules are currently lacking in accuracy. The deficiencies present in molecular ionization models constitute a formidable barrier for experimentalists, who wish to model the single-electron ionization dynamics of molecules in intense laser fields. The primary motivation for the work presented in this thesis is to provide a comprehensive data set which can be used to improve existing models for the strong-field ionization of molecules. Our approach is to simultaneously measure the singly-charged ion yield of a diatomic molecule paired with a noble gas atom, both having commensurate ionization potentials. These measurements are taken as a function of the laser intensity, typically spanning two orders of magnitude (10[superscript]13-10[superscript]15 W/cm[superscript]2). By taking the ratio of the molecular to atomic yields as a function of laser intensity, it is possible to "cancel out" systematic errors which are common to both species, e.g. from laser instability, or temperature fluctuations. This technique is very powerful in our ionization studies, as it alludes to the distinct mechanisms leading to the ionization of both molecular and atomic species at the same intensity which are not a function of the experimental conditions. By using the accurate treatments of atomic ionization in tandem with existing molecular ionization models as a benchmark, we can use our experimental ratios to modify existing molecular ionization theories. We hope that the data procured in this thesis will be used in the development of more accurate treatments describing the strong-field ionization of molecules.