Analytic solution for electrons and holes in graphene under   electromagnetic radiation: gap appearance and non-linear current effects
  We find the exact solution of graphene s carriers under electromagnetic radiation. To obtain such solution, we combine Floquet theory with a trial solution. Then the energy spectrum is obtained without using any approximation. Using such results, we prove that the energy spectrum presents a gap opening which depends on de radiation frequency and electric wave intensity, whereas the current shows a strongly non-linear behaviour. 
Gerardo G. Naumis and Francisco J. Lopez-Rodriguez