High-resolution Interferometric Diagnostics for Ultrashort Pulses

6.4 Temporal metrology of high-harmonic generationFocusingopticGas jetsTwo-partmirrorτSlitωe −2DdetectorTOFFigure 6.11: Typical setup for two-colourphotoelectron spectroscopy.CylindricalgratingFigure 6.12: Flat-field XUV spectrometer.can be used to retard or accelerate the electrons for a desired spectral range and resolution.6.4.2 Nonlinear autocorrelationsIntensity autocorrelations have been implemented using two-photon absorption [398, 399]. Twoseparate harmonic sources are produced, and they are focused into an atomic gas. The spectrumof the resulting photoelectrons is monitored. In addition to the usual single-photon absorption,a two-photon absorption signal is observed, proportional to the intensity overlap of the pulses.Scanning their relative delay whilst recording the area of the two-photon signal yields an intensityautocorrelation trace. However, the signal is extremely weak and therefore requires long integrationtimes. This, along with the fact that the intensity autocorrelation does not provide a completecharacterization, has prevented these methods from becoming widely used.6.4.3 Two-colour photoelectron spectroscopyA broad class of techniques is based on the general principle of measuring the laser-dressed photoelectronspectrum as a function of laser-XUV delay. The methods differ in terms of their interpretationand the relative durations of the laser and XUV pulses.If the XUV consists of a train of near-identical pulses, and the streaking laser is that used to producethe harmonics, then the period of the train is exactly half the period of the laser, and in thefrequency domain the train is a series of discrete harmonics separated by 2ω L . In this situation,the phase modulation produces sidebands separated by ±ω L , above and below each harmonic.The upper sideband of harmonic q thus interferes with the lower sideband of harmonic q +2, and151