Time-resolved transmissivity and reflectivity of exfoliated graphene and thin graphite films on a 295 K SiO(2)/Si substrate are measured at 1300 nm following excitation by 150 fs, 800 nm pump pulses. From the extracted transient optical conductivity we identify a fast recovery time constant which increases from approximately 200 to 300 fs and a longer one which increases from 2.5 to 5 ps as the number of atomic layers increases from 1 to approximately 260. We attribute the temporal recovery to carrier cooling and recombination with the layer dependence related to substrate coupling. Results are compared with related measurements for epitaxial, multilayer graphene.

Ballistic electrical currents are optically injected into aligned single-walled carbon nanotubes and bulk graphite at 300 K via quantum interference between single and two photon absorption of phase-related 700 and 1400 nm, 150 fs pulses. The transient currents are detected via the emitted terahertz radiation. Optical phase and power dependence are consistent with the quantum interference optical process. Under similar excitation conditions, the peak current for a forest of nanotubes, with a diameter distribution of approximately 2.5 +/- 1.5 nm, is 9 +/- 1 times larger than that in graphite. At peak focused intensities of 10 GW cm(-2) (1400 nm) and 0.15 GW cm(-2) (700 nm), the peak current is approximately 1 nA per nanotube. The peak current for pump light polarized along the tubes is approximately 3.5 times higher than that for light polarized perpendicular to the tubes.

We report results from a comprehensive set of experiments to study coherently controlled electrical current injection in single-walled carbon nanotubes (SWNTs) and graphite. Photocurrents were injected at room temperature through the quantum interference of single-and two-photon absorption pathways induced by 150-fs optical pulses with 660-980 and 1320-1960-nm central wavelengths, respectively, and with maximum intensities of 10 and 0.15 GW cm −2 , respectively. Detection of the photocurrents was achieved via the emitted terahertz radiation. For bulk graphite samples and collinearly polarized 750-and 1500-nm pulses incident along the c axis, injected current densities up to 12 kA cm −2 have been observed just under the surface, independent of crystal azimuthal orientation and comparable to those generated in InP or GaAs. Current densities are ∼5 times smaller for cross-polarized pulses. A vertically aligned forest of carbon nanotubes (tube diameters ∼2.5 ± 1.5 nm) illuminated with 700-and 1400-nm pulses collinearly polarized along the alignment direction yields a maximum current of 8 nA per tube (current density of 35 kA cm −2). Terahertz emission drops by only 3.5 times after 90 • sample rotation about the normal, which is explained in terms of an imperfect alignment distribution (angular spread ∼19.5 •) and sample birefringence. Unaligned arc discharge and HiPco SWNTs with diameters of 1.44 ± 0.15 and 0.96 ± 0.15 nm, respectively, were sorted into semiconducting and metallic tubes. Photocurrents injected with collinearly polarized 750-and 1500-nm pulses in such semiconducting SWNTs showed peak current magnitudes similar to those in the aligned nanotubes, while metallic tubes yielded currents at least ten times smaller. Semiconducting SWNT currents showed spectral features as the second-harmonic wavelength varied from 660 to 980 nm, which were more consistent with current injection based on band-band transitions than on excitonic absorption effects.

We discuss the optical injection of magnetization into a nonmagnetic semiconductor by the absorption of circularly polarized light. A microscopic approach, which is based on Fermi's golden rule and k · p band models, is used to quantify the magnetization-injection rate in GaAs. We find that under conditions typical in optical orientation experiments, the magnetization-injection rate of holes is approximately 20 times larger than it is for electrons, reflecting the large hole magnetic moment. We then turn to the ultrafast excitation regime and explore the possibility that the injected magnetization can radiate a detectable terahertz field. By using a phenomenological approach for the magnetization relaxation dynamics, we predict that the terahertz field from magnetic injection is at the limit of current terahertz detection technology. We provide initial experimental measurements in search of this terahertz radiation.

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