We show that the optical responses of a graphene pn junction is dominated by nonlinear intraband and interband processes. At the experimentally relevant electric field intensity, nonlinear conductance is an order of magnitude larger than the linear conductance. Furthermore, the total conductance is negative in the terahertz to far infrared regime. The negative conductance provides a unique mechanism for photon generation in graphene and could be used for developing coherent terahertz radiation sources.
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