Non-perturbative time-dependent String Backgrounds and Axion-induced   Optical Activity
  Conformal invariance for bosonic strings in time-dependent backgrounds of graviton, dilaton and Kalb-Ramond field is obtained by imposing Weyl-beta functions to be homogeneous in time, to all orders in $\alpha^{'}$. This construction is possible in any target space dimension, as a result of the non-trivial background configurations. The electromagnetic effects of the antisymmetric tensor field, when coupled to an Abelian (electromagnetic) gauge field, are discussed in the framework of a four-dimensional Minkowski Universe, for concreteness. Non-trivial optical activity is demonstrated, which constitutes a way of detecting string-inspired axion/dilaton effects in such models. 
Jean Alexandre, Nick E. Mavromatos, Dylan Tanner