Table of contents

Ultrashort laser pulses are finding increasing application in numerous branches of science and technology. This article is an attempt to provide a comprehensive review of the whole field of ultrashort pulse generation, covering both the experimental and the theoretical aspects of the subject. By far the commonest method of generating pulses in the picosecond and subpicosecond regime is the technique known as 'mode-locking' and it is with this that the review is therefore largely concerned. After introductory remarks and a simple treatment of some of the general principles, the central sections of the review deal with the main generic techniques, namely spontaneous mode-locking, active mode-locking (including mode-locking by synchronous pumping) and passive mode-locking. Within each section the material is organised into three main categories, namely (a) general background, (b) historical survey (mainly relating to experimental work and densely packed with references to published work) and (c) theoretical methods (presented with a strongly pedagogical flavour). A major section at the end deals with a mass of important (and mostly recent) work that does not fit naturally into the main framework of the review. This includes USP generation in semiconductor lasers, a number of special methods such as injection mode-locking (and others where two or more mode-locking techniques are combined), as well as chopping and switching techniques.

The central idea of diffusive shock acceleration is presented from microscopic and macroscopic viewpoints; applied to reactionless test particles in a steady plane shock the mechanism is shown to produce a power law spectrum in momentum with a slope which, to lowest order in the ratio of plasma to particle speed, depends only on the compression in the shock. The associated time scale is found (also by a macroscopic and a microscopic method) and the problems of spherical shocks, as exemplified by a point explosion and a stellar-wind terminator, are treated by singular perturbation theory. The effect of including the particle reaction is then studied. It is shown that if the scattering is due to resonant waves these can rapidly grow with unknown consequences. The possible steady modified shock structures are classified and generalised Rankine-Hugoniot conditions found. Modifications of the spectrum are discussed on the basis of an exact, if rather artificial, solution, a high-energy asymptotic expansion and a perturbation expansion due to Blandford. It is pointed out that no steady solution can exist for very strong shocks; the possible time dependence is briefly discussed.