Condensed-Matter Physics & Materials Science Seminar

In condensed matter, the interplay between charge, spin and lattice degrees of freedom results in many of the fascinating properties of different systems. In this talk, I will describe experiments based on ultrafast lasers in which we independently create and probe these excitations in a time resolved manner. In cuprate superconductors, ultrafast laser pulses are used to break cooper pairs and inject quasiparticles (QP) into single crystal samples. Once injected, they cause a reflectivity change of the sample at the laser frequency, which allows time-resolved optical measurements of their density. Besides their recombination rate, QP diffusion coefficient is also measured by creating spatially non-uniform excitation density. To generate spin excitations, we make use of spin orbit coupling. In GaAs quantum wells, two coherent laser pulses with perpendicular polarization interfere to produce a sinusoidal variation of photon helicity. Because of spin-orbit coupling, the photon-helicity variation is transcribed into a periodic wave of electron spin polarization. The rates of spin transport and relaxation can be inferred from the evolution through time of the spin polarization grating. To probe lattice excitations, we use femtosecond laser pulses to generate ultrafast high energy electron pulses via photoelectric effect. Recording the diffraction pattern of these electron pulses at different times after photo-excitation of the sample provides a movie of laser induced structural changes with picosecond temporal resolution and sub-Angstrom spatial resolution.