Physicists have typically assumed that a free charged particle introduced into a nonequilibrium Coulomb system instantaneously becomes a "dressed" particle, completely integrated into the system. However, researchers from Technische Universität München in Garching, Germany, have observed a short delay, on the scale of 10^-14 s.

They used a 10-fs pulse from a Ti:sapphire laser from Femtolasers Produktions GmbH in Vienna, Austria, to induce free electron-hole pairs in a GaAs film. A single-cycle 27-fs, 28-THz pulse generated via optical rectification of the Ti:sapphire pulse then probed the excited GaAs crystal. The changes induced in the probe's electric field amplitude revealed direct information about the many-body effects taking place to renormalize the system.

As reported in the Nov. 15 issue of Nature, the researchers discovered that the dressing occurs on a timescale comparable to the inverse plasma frequency. This information could benefit studies of superconductors, lattice dynamics in organic semiconductors and vibrational relaxation in large molecules.