Unconventional electromagnetic properties of the graphene quantum dots

Quantum dots based on the graphene stripes show unconventional optical properties in the THz frequency range. The graphene quantum dot (GQD) is made of electrically gated stripe with zigzag edges. Inside the active region (AR), which is enclosed between the source and drain electrodes, there are two sharp energy ($\pm$)-levels, whose separation $2\Delta$ is controlled with Stark effect by applying the lateral dc electric field. Such the edge states determine the unique nature of elementary excitations, chiral fermions, that are responsible for the non-linear optical responce revealing a potential for many applications. They are, e.g., the frequency multiplication and self-focusing of two dimensional solitons. Furthemore, when injection of the non-equilibrium electrons causes an inverse population of the levels localized in AR, the subsequent recombination of electrons and holes leads to a coherent emission of the THz waves.