The physics of attosecond light pulses

The word 'attosecond' (1 as = 10−18 s) officially entered the vocabulary of physics when sub-femtosecond pulses of UV/XUV light produced either by nonlinear frequency conversion of a ultra-short infrared pump pulse or Fourier synthesis of broad bandwidth radiation were established. The physics of these pulses is based on nonlinear, nonperturbative laser–atom interaction: stimulated Raman scattering or high harmonic generation (HHG) is used to generate the necessary bandwidth, which naturally encompasses the visible and UV/XUV spectral range. However, the crucial element for attosecond pulse generation is the control of the spectral phase. New methods of temporal characterization at frequencies lying in the UV/XUV had to be elaborated. These methods rely on the energy/momentum analysis of photoelectrons produced by XUV attosecond flashes in the presence of an intense infrared field whose optical cycle itself becomes the basic clock. Single 650 as pulses have been produced and applied to trace the dynamics of electrons inside atoms following the creation of an inner-shell hole. Periodic combs of 250 as pulses have been synthesized by superposing just four harmonics and applying to the attosecond timing of the electron motion in HHG. Although it is easy to increase the bandwidth by coupling more harmonics, a fundamental limit to the duration of the light bursts produced has been discovered. It is imposed by the lack of synchronization of the different harmonic orders. The current limit is estimated to be 130 as. The latest advances include a direct autocorrelation of an attosecond pulse train and the production of a single 250 as soft x-ray pulse. This paper offers a snapshot of the state-of-the-art in the production and characterization of attosecond light pulses, with a glimpse at the first steps in attophysics.