Linear dynamic polarizability and absorption spectrum of an exciton in a   quantum ring in a magnetic field
  The problem of an electron-hole system interacting through a contact potential and moving in a one-dimensional quantum ring threaded by an Aharonov-Bohm flux is considered, both with respect to the system's energetics as well as its optical properties. An exact analytical expression for the energy spectrum is derived using a straightforward method based on boundary conditions for wavefunctions and their derivatives along the ring. The optical properties of this exciton system, namely the linear dynamic polarizability and absorption spectrum are investigated demonstrating certain unusual features. It is shown, for example, that for special values of the magnetic flux there are energies in the spectrum that correspond to the dark excitonic states. 
A. V. Ghazaryan, A. P. Djotyan, K. Moulopoulos, A. A. Kirakosyan
cond-mat.mes-hall quant-ph