AbstractsPhysics

Investigations of the energy states and dynamics of highly excited In0.2Ga0.8As/GaAs quantum wells in strong magnetic fields have been performed at the National High Magnetic Field Laboratory (NHMFL). Time-integrated and time-resolved magneto-spectroscopy reveal significant new information about the nature of electronic correlations, energy states, and dynamical relaxation processes of photo-induced magneto-excitonic and magneto-plasmonic states in this system. Time-integrated spectroscopy reveals much about the character of the electron-hole magneto-plasma at high densities. Time-resolved investigations are crucial because they reveal the mechanisms by which a macroscopic polarization can form in dense electron-hole magneto-plasmas. In addition, they allow us to map out the complex evolution of the relaxation dynamics of photo-excited carriers excited by ultrafast laser pulses in a highly quantized Landau level system. Using the unique Fast Optics facility that we have developed at the NHMFL in Tallahassee, we perform three distinct but interrelated investigations. A femtosecond chirped pulse amplified system (CPA) is used to generate carrier densities of 1013/cm2 and greater in the samples. First, by measuring the absorption spectra and emitted photoluminescence (PL) spectra and fitting the spectra to determine the energy states as a function of carrier density, we are able to clearly identify interband Landau level (LL) transitions and resolve the transformation from a steplike 2- dimensional density of states (DOS) at zero field to a delta-function like 0-dimensional DOS. Significantly, we find a novel excitonic metal-insulator transition (Mott transition) for the lowest lying LL induced by the presence of the magnetic field. Second, we experimentally characterize the inter- and intra-LL relaxation dynamics of the In0.2Ga0.8As/GaAs system using time-resolved transient absorption. The multi-level system consisting of many Landau levels results in complicated and non-exponential relaxation dynamics. At high magnetic fields, we observe the fast initial increase of transmittance in all LLs, however 100 ps after the initial excitation we observed the abrupt, non-exponential reduction of the transmittance signal. The abrupt dynamics result from rapid inter-LL electron-hole recombination. A third experiment probing the time-structure of the PL emissions reveal that rapid bursts of emission are correlated with the temporal signature of the absorption recovery. In addition, for high excitations multiple bursts are observed which result from re-loading of the LLs through intra-LL sub-band relaxation.