High-resolution Interferometric Diagnostics for Ultrashort Pulses

1. INTRODUCTIONprofile in space as well as time.This dissertation advances ultrashort pulse characterisation in several ways. Shearing interferometry,in which the phase is recovered by interfering a pulse with a displaced, or sheared, replicaof itself, plays a key role in two of the results. This principle underlies a broad class of ultrashortpulse characterisation devices. I develop a generalisation to shearing interferometry which allowsmany different shears to be combined, providing greater precision and recovering properties of apulse which are unobtainable using a single shear. I also develop a shearing interferometer capableof performing spatial and spectral shears simultaneously, enabling simple measurement ofthe spatio-temporal structure of optical pulses.High-harmonic generation is a process by which intense laser pulses at optical and infraredwavelengths may be converted to extended ultraviolet and soft x-ray radiation, and is the subjectof intensive study because of its potential as a short-wavelength light source of unprecedentedbrevity. However, adapting characterisation techniques designed for optical wavelengthsto high-harmonic radiation is not straightforward because of the limited range of optical componentsavailable at the shorter wavelengths. I describe an adaption of shearing interferometry forfrequency-resolved wavefront sensing of high-harmonic radiation. A comprehensive set of measurementsshow good agreement with theory.Besides its potential as a light source, high-harmonic generation is a rich physical processwhich exposes the properties of matter at atomic length scales and ultrashort timescales. Theelectric field of an intense laser pulse ionises a target atom or molecule and accelerates the electronaway, before reversing its direction and bringing the electron back with great velocity. In theresulting collision, the kinetic energy of the electron is released as a short-wavelength photon.Whilst this simple classical picture explains some of the properties of the generated radiation,there are in fact several possible trajectories that the electron could follow which lead to the sameemitted photon. Such is the peculiarity of quantum mechanics that in fact all of these trajectorieshappen simultaneously; the emitted radiation corresponds to their coherent superposition. I proposeand develop the theory of a technique for measuring the intensity and phase of the radiationcorresponding to each trajectory, thus decomposing this quantum superposition. A “numerical2