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

Abstract

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.

Optical studies of ballistic currents in semiconductors [Invited]

“…Additionally, it was used to stabilize the carrier-envelope phase of femtosecond lasers [53,54,75,76]. More recently, injection of pure charge currents by the QUIC technique in other materials and structures has been achieved, including GaAs quantum wells [56], silicon [70,71], germanium [70], CdSe and CdTe [69], carbon nanotubes [72,77], graphite [72,77], and graphene [78]. Other variations of the QUIC technique have also been demonstrated.…”

Evidence for interlayer electronic coupling in multilayer epitaxial graphene from polarization-dependent coherently controlled photocurrent generation

“…  in single-layer graphene, as calculated in previous theoretical work 21 . For graphite, this relationship has been measured to be η xyyx/ η xxxx =-0.19±0.03 for a fundamental beam ω at 1400 nm 18,22 . In this paper, we measure the X and Y components of the THz field generated by coherently controlled photocurrents in MEG as a function of  , in order to determine whether the third-order nonlinear tensor in MEG is consistent with a model incorporating only isolated graphene layers.…”

Dynamics of optically injected currents in carbon nanotubes

“…Realistic numerical parameters correspond to the radiation spectrum peaked at a fraction of a THz, in the range of experimental observation of Ref. [6].…”

“…By interference of singleand two-photon optical transitions induced by highly coherent laser beams, which can be controlled by the beams' phases, one can inject optically currents in semiconductors 1,2 and semiconductor nanostructures 3,4 . Recently, current injection using the same principle of interference was reported for a different class of systems such as graphene 5 and carbon nanotubes 6 . The understanding of the following dynamics of the injected currents can provide a valuable information both about the injection process and interactions in the system.…”