Electrodynamics of correlated electron materials

We review studies of the electromagnetic response of various classes of\ncorrelated electron materials including transition metal oxides, organic and\nmolecular conductors, intermetallic compounds with $d$- and $f$-electrons as\nwell as magnetic semiconductors. Optical inquiry into correlations in all these\ndiverse systems is enabled by experimental access to the fundamental\ncharacteristics of an ensemble of electrons including their self-energy and\nkinetic energy. Steady-state spectroscopy carried out over a broad range of\nfrequencies from microwaves to UV light and fast optics time-resolved\ntechniques provide complimentary prospectives on correlations. Because the\ntheoretical understanding of strong correlations is still evolving, the review\nis focused on the analysis of the universal trends that are emerging out of a\nlarge body of experimental data augmented where possible with insights from\nnumerical studies.\n