Picosecond dynamics of interminiband transitions in doped GaAs/AlGaAs superlattices

Semiconductor superlattices are an essential component of novel infrared devices such as detectors and quantum cascade lasers and their optical and transport properties have been investigated extensively during the past two decades. However, unlike for quantum well structures where considerable knowledge on the intersubband relaxation dynamics has been obtained, so far no experimental work has been published on the interminiband relaxation dynamics in superlattices.

In this work we have studied the transient transmission of a doped GaAs/Al0.3Ga0.7As superlattice in pump-probe experiments [1]. The superlattice with thickness of 9.0 nm and 2.5 nm of the wells and barriers, respectively, was n-doped in the center of the wells, resulting in a doping density of 1.51016 cm-3 averaged over one superlattice period. Picosecond infrared pulses with energies up to 100 nJ in the range from 4 µm to 22 µm were generated at 13 MHz repetition rate by the free-electron laser FELBE at the Forschungszentrum Rossendorf. In particular, the experiments were performed at the absorption maxima of the superlattice at 9.0 µm and 15.8 µm (compare Fig. 1). These wavelengths are the spectral positions of the van Hove singularities of the joint density of states in the center and at the edge of the mini-Brillouin zone, respectively.

The measured pump-probe signals shown in Fig 2 consist of a fast component due to the bleaching of the interminiband transition and subsequent relaxation and thermalization, and a slower component due to cooling of the heated electron system. The fast component decays typically around 2-2.5 ps, in reasonable agreement with published theoretical values [2]. The slower component due to cooling is positive for excitation at 9.0 µm and negative at 15.8 µm and shows a strong temperature and excitation density dependence with cooling times ranging from 5 to 50 ps. This behavior is consistent with the temperature dependence of the linear absorption spectrum, i.e. yielding higher or lower transmission for increasing electron temperature. The effect provides an internal thermometer for the miniband electrons on a picosecond timescale.

[1] D. Stehr et al., Appl. Phys. Lett. (in print)
[2] F. Compagnone, A. Di Carlo, and P. Lugli, Appl. Phys. Lett. 80, 920 (2002)