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The effect of the refraction of a laser beam propagating through three different phase objects, i.e. a laser produced plasma and two different gas media, is investigated in this thesis. It is shown that these effects have useful applications. As an introduction to the work performed, a basic discussion of the theory of light is given. In the first experimental study, the accuracy of using the Refractive Fringe Diagnostic, as a tool to determine the electron density profiles of laser produced plasmas, is investigated [Buccellato et al. (1992)]. A comparative study is performed between an established method of determining the electron density profiles of laser produced plasmas, i.e. Nomarski interferometry, and the Refractive Fringe Diagnostic, by comparing experimental data obtained from the same laser shot. For the electron density profiles investigated, it is shown that the Refractive Fringe Diagnostic over-estimates the electron density by an order of magnitude. It is suggested that the electron density errors are due to the inherent assumptions of the Refractive Fringe Diagnostic. To verify this, a numerical simulation into the accuracy of the RFD is performed on a mathematically modelled plasma. The discrepancy in the numerical results are consistent with those of the experimental results and these can be attributed to the assumptions made by the Refractive Fringe Diagnostic. Laser light refracted by a gas medium, with a specific density profile, may produce a near diffraction limited focal spot. The remaining two experimental investigations deal with two novel gas lenses: the Pulsed Gas Lens and the Colliding Shock Lens. A radially expanding cylinder of gas produces a suitable density structure to focus laser light. A design of a gas lens, the Pulsed Gas Lens, using this principle is proposed as a final focusing lens for a laser fusion power station [Buccellato et al. (1993a)]. To establish the feasibility of such a lens a proof-of- principle design for the lens is given. A numerical simulation of this lens is performed by modelling the gas flow from the lens and raytracing through the determined density profiles inside the lens. It is found that this lens can be used as a focusing element. To establish certain practical aspects of the proof-of- principle design, a beam deflection device was constructed and tested. This beam deflection device models the lensing principle of the proposed lens. The laser beam deflection observed did not match the computed deflection. The opening mechanism for the proof-of-principle design did not produce an instantaneous opening of the chamber as was assumed in the simulation. The opening mechanism must be modified to decrease the opening time. Diverging spherical shock waves, produced by pairs of opposing electrodes evenly spaced on a circumference, produce a converging cylindrically symmetric shock wave. After convergence a suitable density structure exists for near diffraction li.mited focusing to occur. It is found that the Colliding Shock Lens is a varifocal lens: the focal length and lens diameter increase with time [Buccellato et al. (1993b)]. A numerical simulation is performed to model the operation of the Colliding Shock Lens. The numerical results compare favourably with the experimental results. From the simulation it is established that the lens diameter can be scaled up by increasing the physical size of the lens and the input energy to the lens. Potential applications of the colliding shock lens are discussed. To conclude this thesis, the results of the separate investigations are summarised. / Thesis (Ph.D.)-University of Natal, 1994.

A variation of the excite-and-probe technique is used to measure the picosecond evolution of laser-induced transient gratings that are produced in germanium by the direct absorption of 40 psec optical pulses at 1.06-μm. Grating lifetimes are determined for free carrier densities between 10¹⁸ cm⁻³ and 10²¹ cm⁻³ . For carrier densities less than 10¹⁹ cm⁻³ , a linear diffusion-recombination model for the grating provides a good fit to the experimental data and allows the extraction of the diffusion coefficient and an estimation of the linear recombination lifetime. Above carrier densities of approximately 10²⁰ cm⁻³ , the density dependence of the diffusion coefficient and nonlinear recombination processes must be considered. Numerical solutions to the resulting nonlinear partial differential equation are obtained that allow extraction of information concerning the high density diffusion coefficient and the nonlinear recombination rates.

The recent development of extreme ultraviolet (EUV) sources has increased the need for diagnostic tools, and has opened up a previously limited portion of the spectrum. With ultrafast laser systems and spectroscopy moving into shorter timescales and wavelengths, the need for nanosecond scale imaging of EUV is increasing. EUV’s high absorption has limited the number of imaging options due to the many atomic resonances in this spectrum. Currently EUV is imaged with photodiodes and X-ray CCDs. However photodiodes are limited in that they can only resolve intensity with respect to time and X-ray CCDs are limited to temporal resolution in the microsecond range. This work shows a novel approach to imaging EUV light over a nanosecond time scale, by using an EUV scintillator to convert EUV to visible light imaged by a conventional streak camera. A laser produced plasma, using a mass-limited tin based target, provided EUV light which was imaged by a grazing incidence flat field spectrometer onto a Ce:YAG scintillator. The EUV spectrum (5 nm-20 nm) provided by the spectrometer is filter by a zirconium filter and then converted by the scintillator to visible light (550 nm) which can then be imaged with conventional optics. Visible light was imaged by an electron image tube based streak camera. The streak camera converts the visible light image to an electron image using a photocathode, and sweeps the image across a recording medium. The streak camera also provides amplification and gating of the image by the means of a micro channel plate, within the image tube, to compensate for low EUV intensities. The system provides 42 ns streaked images of light with a iii temporal resolution of 440 ps at a repetition rate of 1 Hz. Upon calibration the EUV streak camera developed in this work will be used in future EUV development.