The onset and decay of photoconductivity in bulk GaAs has been measured with 200-fs temporal resolution using time-resolved THz spectroscopy. A low carrier density (Ͻ2ϫ10 16 cm Ϫ3) with less than 100-meV kinetic energy was generated via photoexcitation. The conductivity was monitored in a noncontact fashion through absorption of THz ͑far-infrared͒ pulses of several hundred femtosecond duration. The complex-valued conductivity rises nonmonotonically, and displays nearly Drude-like behavior within 3 ps. The electron mobilities obtained from fitting the data to a modified Drude model (6540 cm 2 V Ϫ1 s Ϫ1 at room temperature with Nϭ1.6ϫ10 16 cm Ϫ3 , and 13600 cm 2 V Ϫ1 s Ϫ1 at 70 K with Nϭ1.5ϫ10 16 cm Ϫ3) are in good agreement with literature values. There are, however, deviations from Drude-like behavior at the shortest delay times. It is shown that a scalar value for the conductivity will not suffice, and that it is necessary to determine the time-resolved, frequency-dependent conductivity. From 0 to 3 ps a shift to higher mobilities is observed as the electrons relax in the ⌫ valley due to LO-phonon-assisted intravalley absorption. At long delay times ͑5-900 ps͒, the carrier density decreases due to bulk and surface recombination. The time constant for the bulk recombination is 2.1 ns, and the surface recombination velocity is 8.5ϫ10 5 cm/s.

Terahertz spectroscopy emerged about 13 years ago with the demonstration that nearly single-cycle pulses of far-infrared radiation could be generated, propagated through free space, and subsequently detected in the time-domain. Since then, THz spectroscopy has found widespread applicability with studies ranging from condensed matter physics to gas-phase spectroscopy to biomedical imaging. In this article, the properties and applications of THz spectroscopy are described in the context of work being done in the Schmuttenmaer labs at Yale University. In particular, it is shown that an optical pump-THz probe configuration can elucidate phenomena such as the response of low-frequency collective solvent modes in liquids, and transient photoconductivity in a variety of semiconductor systems, such as bulk GaAs, low-temperature grown GaAs, nanocrystalline colloidal TiO 2 , and CdSe quantum dots. In addition, recent experiments measuring charge transfer in a very direct manner are discussed.

The transient photoconductivity of dye-sensitized titanium dioxide has been measured using time-resolved terahertz spectroscopy, a noncontact electrical probe with sub-picosecond temporal resolution. The photoconductivity deviates strongly from Drude behavior and is explained by disorder-induced carrier localization and/or backscattering of the photogenerated carriers. In addition, the carriers are found to thermally equilibrate with the lattice in roughly 300 fs. Ramifications for understanding the function of DSSCs and electrical transport in disordered media are discussed.

Estrogen-related receptor γ (ERRγ) regulates the perinatal switch to oxidative metabolism in the myocardium. We wanted to understand the significance of induction of ERRγ expression in skeletal muscle by exercise. Muscle-specific VP16ERRγ transgenic mice demonstrated an increase in exercise capacity, mitochondrial enzyme activity, and enlarged mitochondria despite lower muscle weights. Furthermore, peak oxidative capacity was higher in the transgenics as compared with control littermates. In contrast, mice lacking one copy of ERRγ exhibited decreased exercise capacity and muscle mitochondrial function. Interestingly, we observed that increased ERRγ in muscle generates a gene expression profile that closely overlays that of red oxidative fiber-type muscle. We further demonstrated that a small molecule agonist of ERRβ/γ can increase mitochondrial function in mouse myotubes. Our data indicate that ERRγ plays an important role in causing a shift toward slow twitch muscle type and, concomitantly, a greater capacity for endurance exercise. Thus, the activation of this nuclear receptor provides a potential node for therapeutic intervention for diseases such as obesity, which is associated with reduced oxidative metabolism and a lower type I fiber content in skeletal muscle.

It is shown that the laser induced ultrafast demagnetization of ferromagnetic films results in the emission of a terahertz electromagnetic pulse. This emission has been detected from Ni films using free-space electro-optic sampling. The radiated electric field E(t) is explained by Maxwell equations (radiation from a time dependent magnetic dipole), and is expected to be proportional to the second time derivative of the magnetization d2M/dt2, as measured in the far field. This technique opens appealing perspectives in the context of measuring and understanding the ultrafast spin dynamics as well as the interaction of electrons (both charge and spin) with electromagnetic fields.