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Abstract
Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field–induced electron tunneling.
Watching electrons dart through silicon
The ultimate speed limit in electronic circuitry is set by the motion of the electrons themselves. Schultze et al. applied attosecond spectroscopy to glimpse this motion in a sample of silicon, the semiconducting building block of modern integrated circuits (see the Perspective by Spielmann). The technique distinguished the electron dynamics—which proceed faster than a quadrillionth of a second after laser excitation—from the comparatively slower lattice motion of the silicon atomic nuclei.
