Dr. Nuh Gedik
CalTech

Title: Ultrafast dynamics of quasiparticles in cuprates and spins in quantum wells

Femtosecond laser pulses are used to create and study quasiparticles (QP‚s) in cuprates and spin excitations in quantum wells. Once injected, they change the index of refraction at the laser frequency, which allows time-resolved optical measurements of their density. In cuprates, measurements of the decay of QP density after pulsed injection yields the QP recombination rate. Studying this rate as a function of doping reveals an abrupt transition in dynamics at optimal doping. To study QP propagation, a spatially periodic QP distribution is created by interfering two coherent laser pulses. Probing the evolution of this distribution by diffracting a time-delayed laser pulse yields the QP diffusion coefficient. In 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.