  We study theoretically the single-electron triangular zigzag graphene quantum
dot in uniform in-plane electric fields. The far-infrared absorption spectra of
the dot are calculated by the tight-binding method. The energy spectra and the
distribution of wave functions are also presented to analyse the far-infrared
spectra. The orthogonal zero-energy eigenstates are arranged along to the
direction of the external field. The remarkable result is that all intraband
transitions and some interband transitions are forbidden when the absorbed
light is polarized along the direction of the electric field. With x-direction
electric field, all intraband absorption is y polarized due to the
electric-field-direction-polarization selection rule. Moreover, with
y-direction electric field, all absorption is either x or y polarized due to
the parity selection rule as well as to the
electric-field-direction-polarization selection rule. Our calculation shows
that the formation of the FIR spectra is co-decided by the polarization
selection rules and the overlap between the eigenstates of the transition.
