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Summary
The photoelectric effect, the emission of electrons from a metal surface after absorbing light, was explained by Einstein's model (1), where light particles (photons) must have a minimum energy (frequency) to ionize atoms (see the figure inset). The number of excited atoms is proportional to the intensity (the number of photons delivered). However, when the light is supplied by very intense, very fast pulses from lasers, the number of ionized atoms will depend on the electric field strength—the amplitude of the light seen as an electromagnetic wave. This change occurs because ionization occurs via quantum tunneling through the relevant energy barrier during a short time window near the maxima of the electric field (2). Isolated attosecond pulses (3) recently enabled studies of the dynamics of tunneling ionization of atoms in gases (4). On page 1348 of this issue, Schultze et al. (5) experimentally show that atoms in a solid are also excited via the tunneling process.
