Attosecond spectroscopy of band-gap dynamics

The ability to transfer electrons from valence to conduction band states in a semiconductor is the basis of modern electronics. Here, attosecond spectroscopy is used to resolve this process in real-time. The excitation of electrons across the band-gap of silicon by few-cycle laser pulses is found to induce lasting modifications of the XUV absorbance spectrum in steps synchronized with the laser electric field oscillations, indicative of light-field induced electron tunneling. In contrast, in SiO₂, a dielectric with 9 eV band-gap, similar excitation pulses are found to cause a transient field-induced polarizability without lasting population transfer.