The generation and utilization of half-cycle cut-offs in high harmonic spectra

High-order harmonic spectra are composed of a coherent sum of half-cycle emissions, the cut-off energy of which depend sensitively on different sub-cycle portions of the driving laser field. By selecting the correct focal geometry the half-cycle cut-off emissions can be preferentially selected over the lower energy plateau emissions through phase matching, such that they form macroscopic half-cycle cut-off features in the far-field spectrum. The energy of these macroscopic half-cycle cut-offs can then be used to retrieve the waveform of the driving laser field. The processes through which these macroscopic half-cycle cut-offs are formed and their applications, both for measuring the laser waveform and the generation of wavelength tunable isolated attosecond pulses, are reviewed in detail. A wavelet transform of the simulated on-axis harmonic field generated by an atomic gas jet driven by an intense few-cycle laser pulse. The focal geometry has been selected to only phase match the half-cycle cut-offs. The energies of these half-cycle cut-offs form a "fingerprint" of the laser field, which can be used to determine important properties of the field waveform. (C) 2010 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim