Femtosecond mid-infrared spectroscopy of low-energy excitations in solids.

Recent progress in the generation and spectroscopic application of femtosecond pulses in the wavelength range between 3 and 25 μm has led to new insight into low-energy excitations of solids, in particular semiconductors, semiconductor nanostructures, and high-T_C superconductors. The ultrafast nonequilibrium dynamics of such excitations has been observed in real-time and the underlying microscopic interactions have been analyzed. This article gives a brief overview of this exciting field of ultrafast science with the main emphasis on pulse generation and applications in semiconductor research where femtosecond intersubband Rabi flopping in quantum wells and quantum coherent electron transport in quantum cascade devices have been observed. [ABSTRACT FROM AUTHOR]