  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.
