  We study theoretically the single-electron triangular zigzag graphene quantum
dot in three typical in-plane electric fields. The far-infrared absorption
spectra of the dot are calculated by the tight-binding method and then the
optical selection rules are identified by contrast with the corresponding
energy spectra. Our result shows that there exist the remarkable optical
selection rules due to the C3 symmetry of the dot. When the electric field
possesses also the C3 symmetry, there are only two absorption peaks in the
absorption spectra. As the C3 symmetry of the system is damaged by the electric
fields, both the intensity of the strongest peak and the number of the
forbidden transitions decrease gradually. Moreover, the polarization causes the
decrease of the peak intensities and even new forbidden transitions. Our
findings may be useful for the application of graphene quantum dots to
electronic and optoelectronic devices.
