Interferometric control of spin-polarized electron populations at a metal surface observed by multi-photon photoemission

In an interferometric pump-probe experiment, we demonstrate the phase tuning of the spin polarization of photoelectrons emitted in a three-photon process from Cu(001). A phase shift of pi between delayed ultrafast circularly polarized light pulses can switch the spin polarization from +/- 20% to -/+ 40%. In the delay regime of overlapping pulses, we show the dominating role of optical interference effects in determining the spin polarization. For longer delays, we detect the influence of the coherent material response, manifested in both the final state electron population as well as the final state spin polarization.