Time-resolved infrared spectroscopy of magnetic semiconductors and superconductors

The photo-induced non-equilibrium dynamics of certain condensed matter systems have been studied with the pump-probe, time-resolved infrared spectroscopy. The broadband synchrotron radiation was used as a probe light source at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), which was synchronized with a near infrared pump laser.;We have studied a Nb0.5Ti0.5N thin film to explore the quasiparticle (QP) relaxation process in the BCS superconductivity. The temperature dependent linear optical transmittance supported the Tinkham's thin film transmission equation and BCS Mattis-Bardeen theory. The time-resolved experiments showed excellent agreement with the Rothwarf-Taylor theory and Gray's model, thus proved the phonon-trap effect and that the effective relaxation followed a simple exponential decay shape. The experiments showed that the excess QP effective lifetime was in the range of 1 ns time scale.;The temperature dependence measurements supported Kaplan's theoretical prediction in the middle to high temperature range. We did not observe the two-component relaxation processes as reported in other literature, and we believe it was due to the more efficiency of the heat transferring mechanics in our experiments.;The pump fluence and magnetic field dependence of the photo-induced experiments were studied with the broad band probing at a best of 300 ps time resolution. We found that the effective relaxation at high fluence behaved much different with the low fluence regime which could be explained qualitatively. The magnetic field dependence experiments showed that the effective lifetime did not decrease as the field enhanced.;The photo-induced gap shift effects were observed and directly measured by the time-resolved spectroscopy. The result supported the Owen-Scapapino model.;We also studied the InGa(Mn)As/InGaAs thin film to explore the electron-hole recombination process in the Dilute Magnetic Semiconductor system. From the broadband time-resolved experiments, we found a two-component decay with the fast one in the ns and the slow one in the tens of ns time scales. We proposed a model to explain this phenomenon. The time-resolved spectroscopy experiments gave the carrier density by fitting with the Drude model. We first observed Franz-Keldysh effect from the DMS system through a photoinduced transmission measurements. This effect is due to the internal electric field introduced by Mn doping. The internal field is in the order of tens of kV/cm.