High-resolution Interferometric Diagnostics for Ultrashort Pulses

6. HIGH-HARMONIC GENERATIONcurrent methods of measuring the temporal and/or spectral profile of HHG, and section 6.5 doesthe same for the spatial profile. Section 6.6 then places the new results in context.6.1 Development and applicationsAdvances in laser technology throughout the 1990s provided focused intensities above 10 13 Wcm −2 .At these levels, irradiation of an atomic gas produces high-order harmonics in the XUV and softx-ray regions [302–310]. Early results include production of the 17th harmonic of a 248 nm KrFlaser in Neon [311], the 33rd harmonic of a 1064 nm laser in Ar [312], and the 135th harmonic ofa 1053 nm laser in Neon [313]. High-harmonic generation was immedately recognized as a possiblecompact short-wavelength source, notwithstanding its relative weakness, and was used as asource for atomic core level spectroscopy [314], as a probe for plasma dynamics [315] and in timeresolved x-ray fluorescence studies [316].6.1.1 Attosecond pulse generationThe high harmonic generation process consists of the ionization and acceleration of electronwavepackets before their eventual radiative recombination with the atom. This process happensonce in every half-cycle of the driving field, and the coherent superposition of these repetitionsshapes the spectrum into odd harmonics. It was quickly realized that the individual half-cycleemissions potentially had subfemtosecond [317, 318] duration i.e. high harmonic emission actuallyconsisted of a train of attosecond-duration pulses separated by half the laser period. ThusHHG seemed a promising embodiment of a scheme proposed by Hänsch [319] to produce pulsesshorter than an optical period through Fourier synthesis. Trains of pulses with durations as shortas 130 as [320, 321] have been observed.Whilst the attosecond pulse train produced by a many-cycle drive field is a useful tool forstudying ultrafast processes, a single attosecond pulse is required for general time-resolved measurements.An initial proposal was polarization gating, exploiting the fact that the electron onlyrecollides in a linearly polarized laser field [322–325]. Initial experiments [326–328] succeededin gating the harmonic emission to a duration of several optical cycles. More recent techniques126