S. Zaric

Interband recombination dynamics in resonantly excited single-walled carbon nanotubes.

Wavelength-dependent pump-probe spectroscopy of micelle-suspended single-walled carbon nanotubes reveals two-component dynamics. The slow component (5-20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and we attribute it to interband carrier recombination, whereas we interpret the always-present fast component (0.3-1.2 ps) as intraband carrier relaxation in nonresonantly excited nanotubes. The slow component decreases drastically with decreasing pH (or increasing H+ doping), especially in large-diameter tubes. This can be explained as a consequence of the disappearance of absorption peaks at high doping due to the entrance of the Fermi energy into the valence band, i.e., a 1D manifestation of the Burstein-Moss effect.

Excitons in carbon nanotubes with broken time-reversal symmetry.

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields (>55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or dark).

Stability of High-Density One-Dimensional Excitons in Carbon Nanotubes under High Laser Excitation

Through ultrafast pump-probe spectroscopy with intense pump pulses and a wide continuum probe, we show that interband exciton peaks in single-walled carbon nanotubes (SWNTs) are extremely stable under high laser excitations. Estimates of the initial densities of excitons from the excitation conditions, combined with recent theoretical calculations of exciton Bohr radii for SWNTs, suggest that their positions do not change at all even near the Mott density. In addition, we found that the presence of lowest-subband excitons broadens all absorption peaks, including those in the second-subband range, which provides a consistent explanation for the complex spectral dependence of pump-probe signals reported for SWNTs.

MAGNETO SPECTROSCOPY OF SINGLE-WALLED CARBON NANOTUBES

Carbon nano-tubes (CNTs) are attracting tremendous interest as the object of fundamental studies in condensed matter and molecular physics as well as possible functional units for future nano-devices. Some of the most striking features of CNTs are related to the symmetry breaking effect of high magnetic fields threading the tube axis. In the present paper we review the results of recent magneto-optical studies of single-walled CNTs in pulsed magnetic fields up to 71 T and at temperatures between 4.2 and 300 K. We present clear evidence for (a) the effect of the Aharonov-Bohm phase on the bandstructure of CNTs, (b) the existence of dark excitons in CNTs due to Coulomb mixing and (c) the effect of magnetic brightening, i.e. the increase of the quantum yield of CNTs under the influence of an external field. Experimental data was derived from large ensembles of individualized tubes in either aligned films or in liquid suspension. A brief discussion of the effect of dynamic magnetic alignment of suspended tube...

OPTICAL PROCESSES IN SINGLE-WALLED CARBON NANOTUBES THREADED BY A MAGNETIC FLUX

J. KONO,1,† S. ZARIC, J. SHAVER, X. WEI, S. A. CROOKER, O. PORTUGALL, G. L. J. A. RIKKEN, R. H. HAUGE, and R. E. SMALLEY Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, U.S.A. †E-mail: kono@rice.edu National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, U.S.A. National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A. Laboratoire National des Champs Magnetiques Pulses, 31432 Toulouse Cedex 04, France Department of Chemistry, Rice University, Houston, Texas 77005, U.S.A.

Ultrafast dynamics of unbundeled single-walled carbon nanotubes

We have used ultrafast pump-probe spectroscopy to explore carrier dynamics in micelle-suspended single-walled carbon nanotubes. We observe two distinct relaxation regimes and attribute them to intraband carrier relaxation towards the band edge and radiative interband recombination

Long-lived dilute photocarriers in individually-suspended single-walled carbon nanotubes

We have observed slow (> 1 ns) carrier decays in single-walled carbon nanotubes. This previously-unreported signal appears in a dilute limit and is accompanied by polarization memory, which persists as long as the photocarriers exist