International audienceWe demonstrate experimentally and theoretically dielectric metamaterials exhibiting a tunable range of negative effective permeability in the terahertz spectral region (0.2–0.36 THz). Our structures consist of an array of intrinsically nonmagnetic rods made of an incipient ferroelectric SrTiO_3 which shows a high tunable permittivity. The magnetic response and its tuning are achieved by a temperature control of the permittivity of SrTiO_3, which defines the resonant confinement of the electromagnetic field within the rods

Time-resolved optical pump-terahertz (THz) probe experiments are currently used to obtain information about the ultrafast dynamics of photoexcited carriers in semiconductors and about the far-infrared nonlinear response during solvation in liquids. The THz dynamics in such photoexcited systems is fully characterized by a two-dimensional nonlinear susceptibility. We have developed a frequency-domain analytical method for the direct extraction of this susceptibility from the experimental data. Following effects are taken into account: dispersive propagation of radiation in a photoexcited medium, refraction on its surfaces, THz sensor responsivity, and spatio-temporal transformations of the THz pulses. Strategies for possible experiments are discussed.

We present the results of optical-pump-terahertz probe experiments applied to a set of thin-film microcrystalline silicon samples, with structures varying from amorphous to fully microcrystalline. The samples were excited at wavelengths 800 and 400 nm and studied at temperatures down to 20 K. The character of nanoscopic electrical transport properties markedly change on a subpicosecond time scale. The initial transient photoconductivity of the samples is dominated by hot free carriers with a mobility of ϳ70 cm 2 / Vs. These carriers are rapidly ͑within 0.6 ps͒ trapped into weakly localized hopping states. The hopping process dominates the terahertz spectra on the picosecond and subnanosecond time scales. The saturated high-frequency value of the hopping mobility is limited by the sample disorder in the amorphous sample and by electron-phonon interaction for microcrystalline samples.