The Femtosecond Delay in Electron Collective Motion

The femtosecond delay in the advent of collective motion among electrons in a semiconductor has been observed, for the first time, by an experiment at the Technical University of Munich.

In general, multi-body interactions, whether at the electron level or planetary level, cause a change in properties different from that observed when only two bodies are present.

For example, electrons moving through a semiconductor crystal have very different solo and collective motions; each particle's effective mass and charge become modified by the changes it induces in the surrounding lattice, such as by causing subtle vibrations (phonons) to draw near. The electron is no longer just its former self but has become a corporate entity (particle plus collective motions), or quasiparticle. This alteration, sometimes referred to by the name of screening or dressing, does not happen instantly.

The Munich group, using ultrafast laser pulses, first excited a plasma of electrons and holes and then, with a secondary probe pulse bouncing off the collective motions of the quasiparticles, monitored the growth of the screening process on a femtosecond basis. They observed that it takes tens of femtoseconds for the screening to be complete. (Huber et al., Nature, 15 November 2001.)