Femtosecond infrared spectroscopy of semiconductors and semiconductor nanostructures

Infrared spectroscopy on ultrafast time scales represents a powerful techni que to investigate the nonequilibrium dynamics of elementary excitations in bulk and nanostructured semiconductors. In this article, recent progress i n this field is reviewed. After a brief introduction into electronic excita tions below the fundamental bandgap and ultrafast processes in semiconducto rs, infrared pulse generation and the methodology of time-resolved infrared spectroscopy are reviewed. The main part of this paper is devoted to coher ent optical polarizations and nonequilibrium excitations of the electronic system in the spectral range below the fundamental band gap. The focus is o n the physics of single component plasmas, i.e. electrons or holes. In part icular, intraband, inter-valence and intersubband transitions are considere d. Processes of phase relaxation, carrier and energy redistribution are ana lyzed. The potential of ultrafast infrared technology and spectroscopy for future applications is discussed in the final part. (C) 1999 Elsevier Scien ce B.V. All rights reserved.