|M.Sc Student||Marderfeld Ilia|
|Subject||Determination of the Intrinsic Radiative Lifetime of|
Excitons Using Time Resolved Photoinduced
|Department||Department of Physics||Supervisor||Professor David Gershoni|
In intrinsic direct bandgap bulk semiconductors the interaction of three dimensional excitons with the electromagnetic radiation field leads to a stationary state called exciton-polariton. This state is a coherent state of the entire system and thus it does not decay radiatively. In geometrically restricted semiconductor quantum structures, the translational symmetry of the crystalline potential is violated in at least one direction. As a result, a low dimensional exciton with an in-plane crystal momentum, which is smaller than the momentum of the three dimensional photon of same energy, can decay radiatively within its intrinsic radiative life time of few tens picoseconds. We used visible-infrared dual beam time-resolved, photoinduced absorption excitation spectroscopy, together with photoluminescence excitation spectroscopy to directly measure the intrinsic radiative lifetime of two dimensional excitons. A set of photoluminescence and time resolved photoinduced absorption excitation spectra are obtained. Examination of this set shows that temporal evolution of the intersubband absorption excitation spectra occurs only at the heavy hole exciton resonance. While at early times the relative intensity of the resonance observed in the photoinduced measurements equals that observed in the photoluminesce excitation spectrum, at later times the photoinduced resonance rapidly loses intensity. The time resolved intersubband absorption spectrum is proportional to the number of electrons present in the first subband, at the measurement time. Thus, indeed, at early times the intersubband absorption spectrum equals the absorption spectrum. However, soon after, resonantly excited excitons can rapidly recombine radiatively before they reach thermal equilibrium, while non resonantly excited ones cannot. The temporal evolution of the photoinduced intersubband absorption excitation resonance can be described by simple exponential rise and decay model, with characteristic rise time of 5 ±1 picoseconds and decay time of 20±4 picoseconds.