High-resolution Interferometric Diagnostics for Ultrashort Pulses

6.5 Spatial metrology of high-harmonic generationto be negligibly slow. Another proposed method uses the time-varying ponderomotive energy ofthe laser as the temporal phase modulation [407]. Finally, when the XUV pulses are longer thanan optical cycle so that the temporal phase modulation produces sidebands, a chirped laser willproduce sidebands with different separations. The interference of these sidebands produces aspectral shearing interferometry trace [403].Disadvantages of these methods include the requirement to generate two XUV pulses, theneed for precise calibration, the need for good photoelectron spectrometer resolution, which increasesthe acquisition time, and the relative phase ambiguity between separate spectral components,which is particularly relevant for the highly structured spectra generated by HHG.6.4.5 Generation of spectrally sheared replicasMotivated by the experimental complexity and long acquisition times of photoelectron spectrometers,there have been several efforts towards all-optical characterisation using SSI in which theshear is performed using the generation process itself. Specifically, two harmonic sources are generatedusing spectrally sheared laser pulses. Intuitively, one expects the spectral shear to be carriedover to the generated XUV pulses, and single-atom calculations for 30 fs drive pulses bear this out[403, 408]. There have been no systematic studies of the accuracy of this assumption for few-cyclepulses or with macroscopic effects included. There has been one experimental demonstration, inwhich collinear time-delayed spectrally sheared laser pulses are produced using an acousto-opticpulse shaper [409]. The disadvantage of this approach is that its temporal carrier imposes a highresolution requirement on the spectrometer, and the intensity of pulses must be low enough sothat ionization produced by the first pulse does not affect the second. An proposed alternative isa spatially encoded arrangement, in which the two laser pulses are brought to spatially separatefocii with nominally zero time delay [403, 408].6.5 Spatial metrology of high-harmonic generationAt least two methods of HHG wavefront characterisation have been reported. Point-diffractioninterferometry [410], described in section 2.4.1.2, involves a semitransparent membrane with a153