Nature Phys. 9, 159–163 (2013)

So far, no method exists that can observe the complex space–time structure of attosecond extreme-ultraviolet pulses. Now, Kyung Taec Kim and colleagues from the University of Ottawa and the National Research Council in Canada have demonstrated an all-optical method for measuring the space–time characteristics of an isolated attosecond pulse, without temporal and spatial averaging. The success of their approach relies on the fact that adding a single photon to an already highly nonlinear process only weakly perturbs the process, but can modify the spatial and spectral patterns of a beam. The team uses a fundamental laser pulse with a time-dependent polarization to produce an isolated attosecond pulse. They then use the second harmonic of the fundamental laser pulse to perturb the attosecond pulse generation process in both space and time. The perturbation modulates the spatial dependence of the phase, and hence the near- and far-field patterns. The researchers then measure the far-field pattern as a function of time delay, and reconstruct the amplitude and phase of the attosecond pulse in space and time by using a parametric fitting procedure. The approach will provide further insight into ultrashort pulse generation, and could potentially be used to finely control pulse characteristics.