Optical properties of carbon nanotubes under external electric fields

We study the optical properties of carbon nanotubes within the single-band tight-binding approximation. The great number of one-dimensional-like subbands of the nanotubes lead to typical van Hove singularities on the local density of states. When a tube is under the influence of a laser beam, optical transitions are allowed between Van Hove singularities and they can be observed experimentally in absorption spectra. External magnetic and electric fields modify the energy spectrum of carbon nanotubes inducing changes on the optical phenomena at low frequencies. Results for the inter-band absorption coefficient are shown for nanotubes under an additional external electrostatic potential for parallel light polarization. This is an important problem to understand since low energy laser beams are being used to identify the chirality and diameter of the tubes and their electronic character.