  Optical absorption properties of magnetoexcitons in topological insulator
bilayers under a strong magnetic field are theoretically studied. A general
analytical formula of optical absorption selection rule is obtained in the
noninteracting as well as Coulomb intra-Landau-level interacting cases, which
remarkably helps to interpret the resonant peaks in absorption spectroscopy and
the corresponding formation of Dirac-type magnetoexcitons. We also discuss the
optical absorption spectroscopy of magnetoexcitons in the presence of
inter-Landau-level Coulomb interaction, which becomes more complex. Our results
could be detected in the magneto-optical experiments.
