High-resolution Interferometric Diagnostics for Ultrashort Pulses

6 High-harmonic generationWhen the electric field of a laser becomes sufficiently strong, it may ionize an atom or moleculein a single nonresonant step. The laser field initially accelerates the electron away from the ion,before switching direction and driving the electron back towards the ion. The resulting collisionhas a number of effects — the electron may be scattered, the ion may be left in an excited state ordoubly ionized and radiation is generated from the recombination of the electron and the ion.High-harmonic generation (HHG) is the third of these processes, and has been the subject ofseveral decades worth of extensive research for several reasons. The emitted radiation is in theextended ultraviolet (XUV) and soft x-ray region and under certain conditions has subfemtosecondduration. High-harmonic generation is therefore the source of choice for attosecond science,and holds potential as a compact XUV source for imaging [301]. The emitted radiation also encodescertain properties of the target atom or molecule with subfemtosecond temporal resolutionand sub-Ångström spatial resolution. Therefore, HHG serves as a probe of ultrafast dynamics ofatomic and molecular structure.In this dissertation, I present two new diagnostics for HHG. The first achieves spectrally resolvedwavefront sensing for HHG, enabling the spatial amplitude and phase profiles to be determinedat many frequencies simultaneously. I present a comprehensive set of measurements usingthis technique, and demonstrate good agreement with theory. The second diagnostic involves alteringthe generation process itself so that the contributions from different quantum trajectories— each corresponding to a particular electron ionization and return time — can be observed independently.One purpose of this chapter is to explain enough of the physics of HHG to justifywhy such measurements are useful, and also to interpret the results which I present. This chapteralso outlines the current state of HHG metrology, in order to justify the development of newtechniques.The structure of this chapter is as follows: section 6.1 traces the historical development of HHGup to its present role as a useful source and diagnostic. Section 6.2 introduces the physics of HHG,starting with response of a single atom or molecule to the applied laser field. Section 6.3 discusseshow the single-atom responses sum coherently to produce the observed field. Section 6.4 outlines125