Despite the fundamental importance of momentum transfer, the precise details of how light passes its impulse on to matter are still not fully understood. One reason is that the transferred impulse changes during an optical cycle on extremely fast, sub-femtosecond timescales. So far, studies revealed mainly information on time-averaged behaviour, missing time-dependent aspects of the linear-momentum transfer during photoionisation. This gap has now been filled by the group of Ursula Keller at the Institute for Quantum Electronics, as they report in a paper published today in Nature Communications.

They looked at the case of high laser intensities, where multiple photons are involved in the ionisation process, and investigated how much momentum is transferred in the direction of laser propagation. To achieve sufficient time resolution, they employed the so-called attoclock technique, which has been developed and refined in the Keller lab over the past decade. In this method, attosecond time resolution is achieved without having to produce attosecond laser pulses. Instead, information about the rotating laser-field vector in close to circular polarised light is used to measure time relative to the ionisation event with attosecond precision. Very similar to the hand of a clock -- just now this clock hand is rotating through a full circle within one optical cycle of 11.3-fs duration.

With this versatile tool at hand, the ETH physicists were able to determine how much linear momentum electrons gained depending on when the photoelectrons were 'born'. They found that the amount of momentum transferred in the propagation direction of the laser does indeed depend on when during the oscillation cycle of the laser the electron is 'freed' from the matter, in their case xenon atoms. This means that at least for the scenario they explored, the time-averaged radiation pressure picture is not applicable. Intriguingly, they can reproduce the observed behaviour almost fully within a classical model, whereas many scenarios of light-matter interaction, such as Compton scattering, can only be explained within a quantum mechanical model.