Pattern formation in semiconductors.

In semiconductors, nonlinear generation and recombination processes of free carriers and nonlinear charge transport can give rise to non-equilibrium phase transitions, . At low temperatures, the basic nonlinearity is due to the autocatalytic generation of free carriers by impact ionization of shallow impurities. The electric field accelerates free electrons, causing an abrupt increase in free carrier density at a critical electric field. In static electric fields, this nonlinearity is known to yield complex filamentary current patterns bound to electric contacts.

Symmetry and spin dephasing in (110)-grown quantum wells.

Symmetry and spin dephasing in (110)-grown GaAs quantum wells (QWs) are investigated applying magnetic field induced photogalvanic effect and time-resolved Kerr rotation. We show that magnetic field induced photogalvanic effect provides a tool to probe the symmetry of (110)-grown quantum wells. The photocurrent is only observed for asymmetric structures but vanishes for symmetric QWs. Applying Kerr rotation we prove that in the latter case the spin relaxation time is maximal; therefore, these structures set the upper limit of spin dephasing in GaAs QWs. We also demonstrate that structure inversion asymmetry can be controllably tuned to zero by variation of delta-doping layer positions.

Tunneling processes induced by terahertz electric fields.

Tunneling processes induced by terahertz frequency electric fields havebeen investigated.A drastic enhancement of the tunneling probabilityhas been observed by increasing the frequency ω atωτ(e)≫ 1 whereτ(e) is the tunneling time.For a given constant tunneling rate an increase offrequency by a factor of seven leads to a drop of the requiredelectric field strengthby three orders of magnitude.It is shown that the enhancement of tunneling ionization at terahertz frequencies is due to the factthat electrons can absorb energy from the radiation field during tunnelingreducing the effective width of the tunneling barrier.

Far-infrared bandpass filters from perforated metal screens.

Precision nickel printing screens are shown to be very useful as far-infrared bandpass filters. For a range of such regularly perforated sheets, transmission peaks of ~ 0.9 have been observed to lie at frequencies between 60 and 140 cm(-1). We found that this value shows little degradation by cascading several screens, but the overall filter Q value may be increased from 2.7 to a value of ~ 6. These screens have considerable advantages over conventional far-infrared bandpass filters in terms of strength, optical characteristics, and cost.

Conversion of spin into directed electric current in quantum wells.

A nonequilibrium population of spin-up and spin-down states in quantum well structures has been achieved applying circularly polarized radiation. The spin polarization results in a directed motion of free carriers in the plane of a quantum well perpendicular to the direction of light propagation. Because of the spin selection rules the direction of the current is determined by the helicity of the light and can be reversed by switching the helicity from right to left handed. A microscopic model is presented which describes the origin of the photon helicity driven current. The model suggests that the system behaves as a battery which generates a spin polarized current.