We report studies of cyclotron resonance in monolayer graphene. Cyclotron resonance is detected using the photoconductive response of the sample for several different Landau level occupancies. The experiments measure an electron velocity at the K-(Dirac) point of c * K = 1.093 x 10 6 ms −1 which is substantially larger than in thicker graphitic systems. In addition we observe a significant asymmetry between the electron and hole bands, leading to a difference in the electron and hole velocities of 5% by energies of 125 meV away from the Dirac point. The observation of two dimensional electronic systems in monolayer graphene 1 , where the electrons behave as Dirac Fermions and show a variety of novel quantum Hall effects 2 , 3 , 4 , has led to an explosion of interest in this system. As well as new basic science, the exceptionally high electron velocities also mean that graphene has considerable potential for applications in high speed electronics 5 . The basis for this behaviour is the nearly linear dispersion of the energy bands close to the K point, where the dispersion relations cross with the form E = ±c * h k, where c * is the electron velocity. This has been predicted for over 50 years 6 , but has only been measured recently for bulk graphite 7 and ultrathin graphite layers 8 , while the first direct absorption measurements for monolayer graphene have just been reported 9 . We describe here a photoconuctance study of cyclotron resonance in a monolayer of graphene in which the application of a magnetic field leads to the formation of Landau levels given by 10where |N | is the Landau quantum index and B is the magnetic field. This allows us to make a precise measurement of the electron velocity and to examine deviations from exact linear behaviour which show that the electron and hole-like parts of the band structure have significantly different masses and that the velocity is significantly larger than for thicker graphitic material. The experiment studies the photoconductive response from a multiply contacted single monolayer sample of graphene, which was prepared using the techniques which have been described earlier 1,2 . The graphene films were deposited by micromechanical cleavage of graphite with multi-terminal devices produced by conventional microfabrication, with a typical sample displayed in figure 1 (a). Shubnikov-de Haas oscillations were first studied at 1.5K to establish the carrier densities as a function of gate voltage and to ensure that the film studied was a single monolayer of graphene, since bilayers and thicker films are known to have a completely different dispersion relation 11 , 12 , 13 . Cyclotron resonance was measured by detecting the modulation of the conductivity of the samples produced by chopped infrared radiation from a CO 2 laser operating between 9.2 and 10.8 µm. The sample was illuminated normally with unpolarized light parallel to the magnetic field in the Faraday geometry. Typical power densities were ∼3 x 10 4 Wm −2 , corresponding to a total power incident on the sampl...

We report far-infrared magnetoabsorption measurements of thin graphite samples exfoliated from highly ordered pyrolytic graphite showing transitions originating from the K and the H points. For the K point, both cyclotron resonance and interband transitions are measured which are not described well by the currently accepted values of the parameters in the Slonczewski-Weiss-McClure tight-binding model. We demonstrate that the observed data can be better described using an effective bilayer graphite model which has been modified to include an electron-hole asymmetry.

We report the observation of cross-polarized transitions of single-walled carbon nanotubes ͑SWCNTs͒ isolated with aromatic polymers. The use of photoluminescence excitation mapping allows the identification of the transitions of individual species of SWCNT. Comparing experimental observation with theory yields an estimate for the Coulomb interactions, which is dependent on the nanotubes' diameter and environment. Values for the single-particle energies are deduced and found to be in good agreement with the predictions of tight-binding models.

We have investigated the ultrafast far-infrared transmission of isolated single-walled carbon nanotubes using optical-pump THz-probe spectroscopy. The THz dielectric response is dominated by excitons with an initial, rapid decay due to Auger recombination followed by a slow decay of isolated single excitons. Frequencydependent analysis of the photoinduced dielectric function suggest an internal excitonic excitation at ∼11 meV with further low-frequency (∼0.6 and 1.4 THz) absorption features at high densities ascribed to exciton complexes. A featureless conductivity bleaching is attributed to an exciton-induced reduction in the mobility of free carriers caused by phase-space filling.

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