Coherent injection and control of ballistic charge currents in single-walled carbon nanotubes and graphite

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.