Femtosecond Spectroscopy of Hot Carriers in Low-Dimensional Semiconductors

Abstract

The rapid development of femtosecond light sources tunable in a wide spectral range allows the direct observation of elementary scattering processes in bulk or low-dimensional semiconductor structures. The non-equilibrium dynamics of carriers are commonly studied by femtosecond absorption and luminescence techniques via valence to conduction band transitions [1, 2]. Such experiments give information on the combined relaxation of electrons and holes. The interaction among them leads to excitonic effects and a complex scattering scenario, making the interpretation of the data with respect to the microscopic dynamics quite difficult. Experiments with doped semiconductors allow the separate investigation of either electrons or holes [3, 4]. Application of mid-infrared (MIR) excitation pulses in the range below the fundamental bandgap makes experiments possible in which the excitation process leads exclusively to a redistribution of carriers at constant total density. In this paper, we study the inter- and intra-subband relaxation of electrons in quasi-two-dimensional semiconductors structures by such a technique. The dynamics of electron redistribution after femtosecond excitation to a higher subband is monitored in spectrally and temporally resolved experiments, providing new information on the lifetimes of higher subbands and on electron thermalization. The paper is organized as follows. In the first part, we describe a novel laser system exclusively based on solid state components which allows the generation of femtosecond pulses in the MIR and is used in our experiments [5]. In the second part, we report recent results on the ultrafast relaxation of electrons in n-type modulation-doped GalnAs/AlInAs quantum wells.