Nonlinear Optical Properties of GaAs/(AlGa)As Multiple Quantum Well Structures under Quasistationary High Excitation Conditions

Abstract

The optical properties are investigated of undoped and n-modulation doped multiple quantum well structures (MQWS) under quasistationary high excitation conditions. The characteristic nonlinear optical behaviour is due to an electron-hole plasma. Its density, temperature, and the renormalized bandgap are determined via photoluminescence and pump and probe beam measurements. A model is developed for the line shape analysis of the plasma luminescence taking into account the broadening of the single particle density of states with results deviating from those in literature, but very good agreement is achieved with the experimental findings from pump and probe beam spectroscopy. The study of the renormalization of the higher, unpopulated subbands reveals that the pure intersubband screening is weak with respect to the shift of the fundamental bandgap. The exciton resonances of the higher subbands are found to be stable against high plasma densities in the fundamental subbands and are mainly bleached by direct population. The idea of a rigid shift of the whole subband structure must be clearly rejected.Die optischen Eigenschaften von undotierten und n-modulationsdotierten Vielfachquantentrogstrukturen unter quasistationaren Hochanregungsbedingungen werden untersucht. Die Art der beobachteten optischen Nichtlinearitaten wird durch die Zahl der erzeugten Ladungstriiger bestimmt und uber ein Elektron-Loch-Plasma beschrieben. Dessen Dichte und Temperatur sowie die reduzierte Bandlucke werden iiber Photolumineszenz-und Pump-und Probestrahl-Messungen bestimmt. Ein zur Auswertung der Lumineszenzlinienform entwickeltes Model1 liefert zur Literatur kontrare Ergebnisse, die allerdings mit denen der Pump-und-Probe-Experimente iibereinstimmen. Die Untersuchung der Renormierung hoherer, unbesetzter Subbander zeigt, daR die reine Intersubband-Abschirmung schwach ist im Vergleich zur Verschiebung der fundamentalen Bandlucke. Die Exzitonen der hoheren Subbander sind stabil gegenuber einer Ladungstragerbesetzung in den fundamentalen Subbandern und bleichen nur durch direkte Besetzung aus. Das Konzept einer starren Subbandverschiebung muO fallengelassen werden.

Absorption and Optical Gain Spectra and Band Gap Renormalization of Highly Excited Quantum Well Systems

“…The shift of the second subband gap (dashed line) shows a small shoulder at a density of about 2 x lo1' cm-', because at this density the second subbands start to get populated. The theory yields in the low-density regime a stronger shift compared with corresponding measurements on GaAs quantum well structures [12]. The deviations for the experimental results for the second subband are probably due to the use of perfect particle in a box wave functions for the averaging of the Coulomb potential.…”