Cyclotron Resonance Gain for FIR and THz Radiation in Graphene

A cyclotron resonance maser source using low-effective-mass conduction electrons in graphene, if successful, would allow for generation of Far Infrared (FIR) and Terahertz (THz) radiation without requiring magnetic fields running into the tens of Tesla. In order to investigate this possibility, we consider a situation in which electrons are effectively injected via pumping from the valence band to the conduction band using an infrared (IR) laser source, subsequently gyrate in a magnetic field applied perpendicular to the plane of the graphene, and give rise to gain for a FIR/THz wave crossing the plane of the graphene. The treatment is classical, and includes on equal footing the electron interation with the radiation field and the decay in electron energy due to collisional processes. A set of integral expressions is derived by assuming that the non-radiative energy loss processes of the electrons can be adequately represented by a damping force proportional and antiparallel to their momentum. Gain is found even though there is no inversion of the energy distribution function. Gain can occur for electron damping times as short as hundreds of femtoseconds.