Photo-excited Carrier Dynamics and Impact Excitation Cascade in Graphene
  Photo-excitation in solids can trigger a cascade in which multiple particle-hole excitations are generated. We analyze the carrier multiplication cascade of impact excitation processes in graphene and show that the number of pair excitations has a strong dependence on doping, which makes carrier multiplication gate-tunable. We also predict that the number of excited pairs as well as the characteristic time of the cascade scale linearly with photo-excitation energy. These dependences, as well as sharply peaked angular distribution of pair excitations, provide clear experimental signatures of carrier multiplication. 
Justin C. W. Song, Klaas J. Tielrooij, Frank H. L. Koppens, Leonid S.   Levitov
cond-mat.mes-hall