Optical Phase Control of Coherent Electron Dynamics in Metals

The control of electron distribution excited in a metal through optical phase of the excitation light is demonstrated. Two-photon photoemission from the Cu(111) surface is excited either by a pair of $\sim 15\mathrm{fs}$ laser pulses with a mutual delay fixed to an accuracy of $\pm 0.025\mathrm{fs}$, or by a single, frequency-chirped pulse. As a consequence of optical coherence in the two-photon excitation process, the photoemission spectra do not only depend on the frequency, as in conventional spectroscopy, but also on the phase of the excitation light. This may be a general phenomenon in multiphoton ionization.