CiteULike: kas optics

Orientation dependence of the optical spectra in graphene at high frequencies

Chao Zhang — 2008-06-19T14:09:46-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 77, No. 24. (2008)

On the basis of the Kubo formula we evaluated the optical conductivity of a graphene sheet. The full behavior of frequency as well as temperature dependence of the optical conductivity is presented. We show that the anisotropy of conductivity can be significantly enhanced at high frequencies. The photon absorption depends on the field polarization direction. At the frequency comparable to the maximum separation of upper and lower bands the photon-induced conduction of electrons is strongly suppressed if the polarization of field is along the zigzag direction. The corresponding optical conductivity is several orders of magnitude weaker than that when the light is polarizing along the armchair direction. We propose that the property of orientation selection of absorption in the graphene can be used as a basis for a high-frequency partial polarizer.

Tuning of energy levels and optical properties of graphene quantum dots

ZZ Zhang — 2008-06-14T16:43:52-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 77, No. 23. (2008)

We investigate theoretically the magnetic levels and optical properties of zigzag- and armchair-edged hexagonal graphene quantum dots (GQDs) utilizing the tight-binding method. A bound edge state at zero energy appears for the zigzag GQDs in the absence of a magnetic field. The magnetic levels of GQDs exhibit a Hofstadter-butterfly spectrum and approach the Landau levels of two-dimensional graphene as the magnetic field increases. The optical properties are tuned by the size, the type of the edge, and the external magnetic field.

Photoconductivity of intrinsic graphene

FT Vasko — 2008-05-23T08:12:59-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 77, No. 19. (2008)

We examine the photoconductivity of an intrinsic graphene associated with far- and midinfrared irradiation at low temperatures. The model under consideration accounts for the excitation of the electron-hole pairs by incident radiation, the interband generation-recombination transitions due to thermal radiation, and the intraband energy relaxation due to acoustic phonon scattering. The momentum relaxation is assumed to be caused by elastic scattering. The pertinent collision integrals are adapted for the case of the massless energy spectrum of carriers that interact with the longitudinal acoustic mode and the thermal radiation. It is found that the photoconductivity is determined by interplay between weak energy relaxation and generation-recombination processes. Due to this, the threshold of nonlinear response is fairly low.

High-Energy Limit of Massless Dirac Fermions in Multilayer Graphene using Magneto-Optical Transmission Spectroscopy

P Plochocka — 2008-05-16T12:14:36-00:00

Physical Review Letters, Vol. 100, No. 8. (2008)

We have investigated the absorption spectrum of multilayer graphene in high magnetic fields. The low-energy part of the spectrum of electrons in graphene is well described by the relativistic Dirac equation with a linear dispersion relation. However, at higher energies (>500 meV) a deviation from the ideal behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation from linearity is 40 meV. This result is in good agreement with the theoretical model, which includes trigonal warping of the Fermi surface and higher-order band corrections. Polarization-resolved measurements show no observable electron-hole asymmetry.

Gate-Variable Optical Transitions in Graphene

Feng Wang — 2008-05-16T11:36:28-00:00

Science, Vol. 320, No. 5873. (11 April 2008), pp. 206-209.

Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy, we find that they also have strong interband transitions and that their optical transitions can be substantially modified through electrical gating, much like electrical transport in field-effect transistors. This gate dependence of interband transitions adds a valuable dimension for optically probing graphene band structure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, whereas in a bilayer, it reveals a dominating van Hove singularity arising from interlayer coupling. The strong and layer-dependent optical transitions of graphene and the tunability by simple electrical gating hold promise for new applications in infrared optics and optoelectronics. 10.1126/science.1152793