High-resolution Interferometric Diagnostics for Ultrashort Pulses

1 IntroductionUltrafast optics is a burgeoning area of science and technology, driven by the unique abilityof ultrashort pulses to probe, control and modify matter on subpicosecond timescales. Throughtheir brevity, these pulses enable unprecedented temporal resolution in the study of dynamic processessuch as chemical reactions. The high intensity achievable by concentrating even a modestpulse energy into such a fleeting instant also provides access to optical nonlinearities, rangingfrom second harmonic generation all the way up to relativistic optics. Such nonlinearities revealnew properties of matter whilst dramatically increasing the capabilities of light-matter interactions.Ultrashort pulses also enable the highly space-time localised delivery of energy, enablingmatter — including biological tissue — to be altered in unique ways.The cost of the attractive properties of ultrashort pulses is that they are challenging to create,manipulate, and measure — three essential tasks in the development of successful ultrafastscience and technology. This dissertation deals with the third of these problems. The principaldifficulty in measuring the properties of ultrashort pulses is that both the shape of the pulse andthe electric field oscillations within it occur on a significantly shorter timescale than the responseof any electronic detector. In particular, detectors generally exhibit integrating square-law behaviour,and thus respond to the energy of incoming radiation, but not its phase. Ultrashort pulsecharacterisation is the large subfield of ultrashort optics which has arisen in search of practicalsolutions to this fundamental problem of performing a subpicosecond resolution measurementusing a detector with significantly slower temporal resolution.In parallel with the advances of the pulse sources themselves, great progress has been made inmetrology. For example, measurement of the temporal electric field (that is, ignoring, averaging orotherwise neglecting the spatial variations) of simple ultrashort pulses at optical and infrared frequencieswith durations ranging from a 10–100 fs is now routine, with commercial devices available.Close to the cutting edge, several groups have measured the temporal electric field of softx-ray pulses of durations around 100 as. The demands of complex ultrashort pulse sources andexperiments are driving metrology towards extremes of pulse brevity and complexity, applicabilityto different wavelengths, and the ability to gather more information about a pulse, such as its1