The generation and utilisation 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. Controlling electron dynamics within atomic and molecular systems typically evolve attosecond timescales, so that their illumination requieres a probe that can be controlled with comparable temporal resolution, i.e. sub‐cycle pulses. 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. The energy of these macroscopic half‐cycle cut‐offs can be used to retrieve the waveform of the driving laser field.