ABSTRACT
This chapter demonstrates that sufficiently strong optical excitation may result in the population inversion in graphene, so that the real part of the ac conductivity can be negative in the terahertz range of frequencies. It shows that the ac conductivity can be negative in certain range of frequencies. The effect of negative ac conductivity can also occur in gated graphene structures in the structures with graphene separated from a highly conducting substrate playing the role of the gate. However, an increase in the equilibrium density of electrons might lead to a significant increase in the intraband contribution to the ac conductivity, which can prevail the contribution of the interband transitions. The self-excitation of plasma waves due to the negative ac conductivity in the terahertz range followed by their conversion into electro-magnetic radiation can also be used in sources of terahertz radiation.
