On photo-induced phenomena in complex materials: Probing quasiparticle dynamics using infrared and far-infrared pulses

It is now possible to routinely generate and detect subpicosecond pulses in the mid and far-infrared portions of the electromagnetic spectrum enabling novel time-resolved spectroscopic investigations. In the context of complex materials, spectral selectivity from approximately 0.001-1.0 eV is especially important since many relevant quasiparticle excitations lie in this range. This includes, as examples, gapped excitations related to superconductivity, charge ordering, and hybridization phenomena, phonon and polaron dynamics, and the coherent Drude response so intimately related to metal-insulator transitions. Temporally resolving spectral changes associated with such excitations in pump-probe-like experiments is proving to be a powerful tool in materials where multiple degrees of freedom (charge, lattice, spin, and orbital) conspire to determine functionality. Quite generally, important insights into ground state properties, coupling parameters, degrees of freedom influencing quasiparticle transport, the nature of phase transitions, and nonequilibrium dynamics can be obtained. Following a brief review of illustrative examples highlighting such possibilities we will present, in some detail, recent experiments on two different ferromagnetic materials. First, we describe terahertz emission experiments on single crystal iron thin films. Following photoexcitation, a burst of coherent THz radiation is emitted which is shown to be consistent with partial demagnetization of the Fe film occurring on a picosecond timescale. As a second example, we describe recent optical-pump infrared-probe measurements on the low carrier density magnetoresistive pyrochlore Tl2Mn2O7. In the ferromagnetic and paramagnetic phases, the recombination of photoexcited carriers is strongly influenced by spin fluctuations.