Pump-probe spectroscopy of interminiband relaxation and electron cooling in doped superlattices

The picosecond dynamics of electrons in a doped Ga As ∕ Al Ga As superlattice have been investigated by pump-probe experiments using an infrared free-electron laser. We observe a fast bleaching of the interminiband absorption followed by thermalization and a slower cooling component. The latter can lead to a positive or negative transmission change, resulting from the temperature dependence of the linear absorption spectrum at the respective wavelength. We show that the superlattice in contrast to quantum wells provides a unique picosecond thermometer for the electron temperature based on the dependence of the absorption on the electron distribution function.