High-resolution Interferometric Diagnostics for Ultrashort Pulses

2. BACKGROUNDthin disc of light. By contrast, a flash of lightning, a commonly encountered optical “pulse”, has atypical duration of 200 ms, during which light travels 60,000 km, or approximately one and a halftimes the circumference of the earth [1].The restriction property of the Fourier transform requires an ultrashort pulse to span a range offrequencies inversely proportional to its duration, whilst electromagnetism prohibits a propagatingpulse from having a zero-frequency (DC) component. Together, these constraints imply thatthe minimum duration of an electromagnetic pulse is the inverse of its central frequency. Durationsbelow one picosecond therefore require frequencies above one terahertz. An ever-expandingarray of source technologies allows ultrashort pulses to be produced at a variety of frequenciesranging from terahertz up to the soft x-ray regime. Furthermore, many sources are capable ofproducing few-cycle pulses, which contain only a few oscillations of the electromagnetic field andapproach the aforementioned limit.Ultrashort pulses occupy a unique place in science and technology because of several relatedcharacteristics. They offer the highest possible temporal resolution for the study of ultrafast processes.They correspondingly possess great bandwidth — for example, few-cycle pulses in the visiblerange span such a broad spectrum of frequencies that they appear white. This holds great potentialfor optical communications, and enables ultrashort pulses to simultaneously couple manydifferent energy levels of an atomic or molecular system. Ultrashort pulses also offer high spatialresolution. Their longitudinal extent is given by their duration multiplied by the speed of light,whilst a high degree of spatial coherence enables focusing down to a diffraction-limited transversearea. Finally, ultrashort pulses offer the potential for high intensities at moderate pulse energiesand average powers. These high intensities enable access to optical nonlinearities, in which theanharmonic motion of the driven electrons reveals properties of matter which are inaccessible atlower intensities. The delivery of spatially and temporally concentrated energy also enables matterto be modified in unique ways. The next section elaborates one some of the applications ofultrashort pulses.6