Internal motions of a quasiparticle governing its ultrafast nonlinear response

P.Gaal, , W. Kühn, K. Reimann, M. Wörner

Nature, 450 (2007), 1210 - 1213

The crystal strikes back

The dashing start of electrons in a crystal does not remain without consequences for their further fate. This is reported by the Berlin researchers Peter Gaal, Wilhelm Kuehn, Klaus Reimann, Michael Woerner, and Thomas Elsaesser of the Max-Born Institute and Rudolf Hey of the Paul Drude Institute in the latest issue of the magazine Nature (Vol. 450, Page 1210). They examined the ultrafast movement of electrons in a gallium arsenide crystal exposed for a short time to a very high electrical field. This conceptually new experiment shows for the first time a collective, oscillatory motion of the electrons with ultrahigh frequency, which arises additionally to the well-known drift motion of these particles. This newly discovered effect could play an important role in connection with the miniaturization of electronic devices.

Overview of the lab in which the experiments were carried out. On the left there is the laser that generates ultrashort red light pulses with extremely high peak powers. In the center is the vacuum chamber (partly opened), in which the red light pulses are converted into pulses of much longer wavelengths (mid and far infrared). These long-wavelength pulses then hit the sample. On the right one sees part of the electronics used.

View into the opened vacuum chamber with some optical elements needed for the experiment.

Gallium arsenide (GaAs) is one of the most important materials for semiconductor optoelectronics. A GaAs crystal consists of a regular lattice of gallium and arsenic atoms, in which the gallium atoms carry a small positive and the arsenic atoms a small negative electric charge. An electron moving slowly through the crystal causes in its neighbourhood a distortion of the crystal lattice. The negative electric charge of the electron repels negatively charged atoms and attracts positively charged atoms. This causes oscillations of the atoms around their rest position: Lattice vibrations, so called phonons, develop. “That is similar to a heavy ball rolling over a mattress”, describes Michael Wörner. “The metal springs of the mattress are squeezed together and relax again.” By the generation of lattice vibrations, the electrons lose energy and thus are slowed down. This deceleration is nothing else but the electrical resistance. The electrons drift with constant velocity through the lattice. This physical picture is the basis of the long-known law for the electrical resistance, Ohm’s law.

A completely new situation arises if the electrons experience a dashing start, i.e., if they are—by an extremely high electrical field—accelerated faster than the response time of the atoms in their neighbourhood. The Berlin researchers use for this strong acceleration an electrical field of 2 million Volts per meter, which is applied to the crystal for the extremely short duration of 0.3 picoseconds (1 picosecond is a millionth of a millionth of a second). The motion of the electrons caused by this high electric field is observed with ultrashort light pulses in the infrared spectral region. In contrast to the drift motion with constant velocity observed for small electrical fields, for high fields the velocity of the accelerated electrons changes periodically between high and low values. The frequency of these velocity oscillations corresponds exactly to the highest frequency with which the atoms can vibrate, the frequency of so-called longitudinal optical phonons. Theoretical computations confirmed quantitatively this experimentally found behaviour. MBI director Professor Thomas Elsaesser says, “the fact that strongly accelerated electrons can excite vibrations of the atoms and that in turn they are decelerated and accelerated by the vibrating atoms is of great importance for the charge transfer in nanostructures.” In such nanostructures, electrical fields of similar size can arise due to the small dimensions. Elsaesser adds: “Therefore our results are important for the optimization of transportation characteristics of semiconductor nanostructures.”