Nonlinear-optical and terahertz investigations of complex oxides

Ultrafast pulsed-laser sources offer many benefits for experimental condensed-matter physics. This thesis describes a variety of ways in which their high field strength and extreme time resolution aid in the study of complex oxide materials. The design, construction and characterization of a visible-pump, terahertz-probe spectrometer is described. This instrument is applied to the study of photoexcited carriers in Sr2CuO2Cl2, an undoped, insulating cuprate. Ultrafast laser pulses are used to excite photocarriers in the antiferromagnetic insulating lattice. The low-frequency dynamical conductivity of the resulting nonequilibrium state is then measured with time-domain terahertz spectroscopy. Photoconductivity appears promptly, followed by a non-exponential decay on picosecond timescales. In this first direct measurement of the low-energy conductivity after photoexcitation, the estimated peak mobility is just 0.3 cm2/V·s, much lower than the Hall mobility found by other workers in chemically doped systems with similar carrier concentrations. Possible physical mechanisms behind this discrepancy are discussed. The spectrometer is then used to determine the photoexcited carrier relaxation time in a proton-damaged GaAs photoconductive antenna. A frequency-domain model for antenna behaviour is verified, demonstrating that the response of an antenna fabricated from an arbitrary substrate can be used directly for material characterization. Calculations and modelling are presented which show how a resonantly-enhanced optical frequency comb can efficiently generate terahertz pulses while preserving the stabilized comb structure. This could be a useful tool for high-precision comb spectroscopy or coherent excitation of structural modes. Several issues concerning the realization of such a source are described. Finally, the nonlinear-optical response of the pyrochlore oxide Cd2Re2O7 is used to characterize the structural phase transition it undergoes at 200 K, which has an unusual tensor character. Optical second harmonic generation with polarization sensitivity is able to resolve an ambiguity in the low-temperature crystal structure, assigning the I¯4m2 space group while verifying an auxiliary condition on the structure that is implied by the order parameter symmetry. The temperature-dependence of the order parameter is consistent with thermal occupation of a Goldstone mode that results from the Eu order parameter symmetry. This methodology may be applied more widely in characterizing ordered states in matter.