▎ 摘　　要

We study the low-lying excited spectra and optical transitions of a single Dirac electron in a graphene sheet that is subjected to a homogeneous magnetic field and an electrostatic potential produced by an applied top gate of disk shape. Numerical results based on the Dirac equation and the transfer matrix method show that in the regime of a small circle-inner magnetic flux, the variable potential induces switches between extended Landau-type bound states and localized quantum-dot-type ones. We indicate that the frequency of emitted or absorbed photons can be tuned almost linearly by the potential within specific ranges, which may have potential applications as a near-linearly-controlled photon frequency filter. These properties are robust to the sharpness of the potential boundary. (C) 2011 American Institute of Physics. [doi:10.1063/1.3583650]