Credit: Image courtesy of Hrvoje Petek
An interferogram showing the photoelectron energy vs. delay time between identical femtosecond pump and probe pulses, which excite coherent three-photon photoemission at a single crystal silver surface. The interferogram is taken from a movie of photoelectron energy vs. momentum with one frame corresponding to a 50-attosecond delay. The oscillations in the intensity of photoelectron signal for emission normal to the surface show how long light is trapped in the form of excitonic polarization during the coherent nonlinear interaction with the silver surface.