Attosecond metrology comes of age

Atoms exposed to a few oscillation cycles of intense visible or near-infrared light are able to emit a single x-ray burst of sub-femtosecond duration (1 fs = 10^-15 s). Precise temporal control of this energetic photon emission can be achieved by full control of the hyperfast field oscillations in the laser pulses driving the emission process. Sub-femtosecond x-ray pulses along with intense, synchronized, waveform-controlled few-cycle laser pulses led to the development of a new measuring apparatus, which has been dubbed a light-field-controlled streak camera. It measures the time-momentum distribution of electrons ejected from atoms following an impulsive excitation by a sub-femtosecond x-ray pulse. From the time-momentum distribution of ejected primary (photo) and secondary (Auger) electrons the excitation dynamics (i.e. characteristics of the exciting x-ray pulse) and the subsequent relaxation of the electron shell of the excited atom, respectively, can be inferred, currently with a resolution of ∼100 attoseconds (1 as = 10^-18 s). The techniques reviewed in this paper offer the potential for advancing time-domain metrology towards the atomic unit of time (24 as).