Electronic and Structural Dynamics of Complex Materials

The time-resolved determination of dynamics in complex materials is an important goal for understanding and controlling material properties, especially on ultrafast timescales. This thesis reports on the development of broadband spectroscopic techniques for characterising electronic and structural dynamics in such materials and their application to the quasi-two- dimensional charge-density-wave Mott insulator 1 T -TaS 2 . The experiments take two main approaches. First, optical pump-probe spectroscopy from the terahertz to the visible regions of the electromagnetic spectrum is used to investigate the collective response. Secondly, time- and angle-resolved photoemission spectroscopy monitors single-electron dynamics on the earliest timescales across the Brillouin zone. In the photoinduced phase of TaS 2 , terahertz spectroscopy demonstrates an increase in conductivity as the phase transition occurs and reveals the evolution of three phonon modes, which undergo transient Fano reshaping due to electron-lattice interactions. Pump-probe measurements at higher energy highlight coherent excitation of the charge density wave am- plitude mode and the emergence of a broad resonance feature associated with polaron for- mation in the new phase. The photoemission experiments show that photoexcitation causes prompt collapse of the Mott gap and leads to partial unfolding of the Brillouin zone on longer timescales, as structural distortions relax. The emerging picture of this transient phase is one in which the Mott gap is melted but the low-temperature symmetry is retained. Meanwhile, transport is dominated by polaronic conductivity. This unique phase is only accessible by photo-doping. Finally, a new method of ultrafast control, in which light is coupled to vibrational modes of the system rather than electronic excitations, is introduced and demonstrated in the man- ganites. An exploration of this technique using FELs promises to reveal the role of different types of distortion in driving ultrafast processes, while phase stabilisation of excitation pulses opens up new paths to coherent control