Non-linear effects in the cyclotron resonance of a massless   quasi-particle in graphene
  We consider the classical motion of a massless quasi-particle in a magnetic field and under a weak electromagnetic radiation with the frequency $\omega$. Due to the non-parabolic, linear energy dispersion, the particle responds not only at the frequency $\omega$ but generates a broad frequency spectrum around it. The linewidth of the cyclotron resonance turns out to be very broad even in a perfectly pure material which allows one to explain recent experimental data in graphene. It is concluded that the linear response theory does not work in graphene in finite magnetic fields. 
S. A. Mikhailov