Using a free-electron laser for two-color spectroscopy of re-doped semiconductors
Van der Waals--Zeeman Institute, University of Amsterdam, 65 Valckenierstraat, NL-1018 XE Amsterdam, The Netherlands
Semiconductor matrices doped with Rare Earth (RE) ions feature an attractive combination of sharp atomic like emissions with relatively large cross section of band-to-band absorption. This makes these systems interesting for applications in solid-state light-emitting devices. The most investigated systems include InP:Yb, GaAs:Er and Si:Er. The latter one is currently recognized as a successful method for obtaining optical emission from silicon. Despite numerous studies, the knowledge of excitation and deactivation paths in these complex systems is mostly phenomenological. While the key role of the weakly bound states (excitons, shallow defect states) is generally accepted, details of their particular involvement remain not clear. For the best understood InP:Yb system a detailed energy transfer path has been proposed but urgently requires experimental confirmation. Here we present a spectroscopic approach to this problem by making use of a two-color experimental set-up with a tunable free-electron laser (FEL). Photoluminescence (PL) of RE ions is achieved by primary band-to-band excitation (second harmonics of a Nd:YAG pulsed laser correlated with the FEL). Intense mid-infrared (MIR) radiation from the FEL is used to access directly individual steps of the energy transfer processes. Individual shallow levels available in the material are selectively addressed by appropriate tuning of the FEL energy. In the presentation I will discuss a variety of effects revealed in Er-doped silicon by two-color optical spectroscopy with the FEL. These will include energy storage, manifested by luminescence "afterglow" and an optical memory effect, and optically-induced Auger process involving energy transfer from the RE ion core to free carriers in the bands. In addition, for the InP:Yb system, I will show the so-called energy "back transfer" effect of excitation reversal, selectively activated by the MIR beam.