Ultrafast time-resolved optical absorption has been measured as a function of doping and temperature in Bi 2 Sr 2 Ca 1Ϫy Y y Cu 2 O 8 ͑0ϽyϽ1͒ and YBa 2 Cu 3 O 7Ϫ␦ ͑0.06 Ͻ␦ Ͻ1͒. A fast transient response is observed, which changes sign as a function of y in Bi 2 Sr 2 Ca 1Ϫy Y y Cu 2 O 8 and ␦ in YBa 2 Cu 3 O 7Ϫ␦ . Careful consideration of the possible mechanisms for the observed behavior leads us to suggest that two independent mechanisms are operating, whereby the two observed positive photoinduced absorption components are due to the electrodynamic response of photoinduced carriers, while the negative signal arises due to spectral hole burning. In the metallic samples a long-lived component is observed, which we suggest comes from photoinduced localized states intrinsic only to the superconducting species of the high-T c materials. A very significant increase in the scattering rate at temperatures just above T c signifies coupling to charge and possibly spin fluctuations just above T c . A large reduction in scattering rate is observed below T c , similar to that observed previously in

Fluctuation spectroscopy measurements of quasi-two-dimensional organic charge-transfer salts (BEDT-TTF) 2 X are reviewed. In the past decade, the method has served as a new approach for studying the low-frequency dynamics of strongly correlated charge carriers in these materials. We review some basic aspects of electronic fluctuations in solids, and give an overview of selected problems where the analysis of 1/ f-type fluctuations and the corresponding slow dynamics provide a better understanding of the underlying physics. These examples are related to (1) an inhomogeneous current distribution due to phase separation and/or a percolative transition; (2) slow dynamics due to a glassy freezing either of structural degrees of freedom coupling to the electronic properties or (3) of the electrons themselves, e.g., when residing on a highly-frustrated crystal lattice, where slow and heterogeneous dynamics are key experimental properties for the vitrification process of a supercooled charge-liquid. Another example is (4), the near divergence and critical slowing down of charge carrier fluctuations at the finite-temperature critical endpoint of the Mott metal-insulator transition. Here also indications for a glassy freezing and temporal and spatial correlated dynamics are found. Mapping out the region of ergodicity breaking and understanding the influence of disorder on the temporal and spatial correlated fluctuations will be an important realm of future studies, as well as the fluctuation properties deep in the Mott or charge-ordered insulating states providing a connection to relaxor or ordered ferroelectric states studied by dielectric spectroscopy.

We have measured the relaxation of photoexcited states in C60 using femtosecond excitation at energies corresponding to the h1u → t1u bandgap. Induced absorption with a fast 2 ps relaxation is observed, which has bimolecular kinetics indicative of exciton-exciton annihilation. The induced absorption is spectrally broad with peaks at 1.92 and 2.25 eV. These features are assigned to excited-state absorption from the t1u level to higher-lying hg excited states at ∼ 4.0 eV.

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