High-resolution Interferometric Diagnostics for Ultrashort Pulses

6. HIGH-HARMONIC GENERATIONtheir relative phase is φ q+2 − φ q − 2φ L . Either by scanning the phase of the laser or sampling itappropriately, the relative phase of adjacent harmonics may be determined by the usual interferometricmethods (section 2.3.3.2), yielding the temporal profile of the subpulses. This is the basisof the reconstruction of attosecond beating by interference of two-photon transitions (RABBIT)technique [320, 400].At the other extreme, the XUV field consists of a single pulse, shorter than half the laser period.If the XUV pulse is cotemporal with a peak of the laser field, it experiences a quadratic phasemodulation. A series of photoelectron spectra taken with different modulation amplitudes providethe necessary observations for temporal reconstructions based on a time-to-frequency mapping[342, 401, 402] or tomography [403].A more general method is frequency-resolved optical gating (FROG)-complete reconstructionof attosecond bursts (CRAB), in which the photoelectron spectrum is sampled across the full rangeof pulse overlap. Similarly to the case of the spectrogram, the two-dimensional dataset may be invertedusing the principal component generalized projections algorithm (PCGPA) [404, 405]. Inprinciple, arbitrarily complex pulses may be retrieved using this method, although to date themeasured pulses have been relatively simple, possessing smooth polynomial phases and littlesubstructure. In addition to the generic challenges of measuring complex pulses using spectographydiscussed in section 3.1, FROG-CRAB traces have long acquisition times placing stringentdemands on the stability of the system.6.4.4 Spectral shearing photoelectron interferometryIf two time-delayed XUV pulses are used to generate two photoelectron wavepackets, an interferencepattern is recorded on the photoelectron spectrometer. Spectral shearing interferometry maythen be performed by choosing the dressing laser field such that the photoelectrons experience aspectral shear. One approach, suitable for XUV pulses that are much shorter than an optical period,is to align one XUV pulse with a zero-crossing of the laser field, where it experiences a lineartemporal modulation which produces a spectral shear [406]. The other XUV pulse is aligned witha peak of the field, where the linear temporal phase is zero and the quadratic component assumed152