  We theoretically study current dynamics of graphene nanoribbons subject to
bias dc and ac driven fields. We showed that graphene nanoribbons exhibit
negative high-harmonic differential conductivity. Negative differential
conductivity appears when bias electric filed is in the neighborhood of applied
ac filed amplitude. We also observe both even and odd high-harmonic negative
differential conductivity at wave mixing of two commensurate frequencies. The
even harmonics are more pronounced than the odd harmonics. A possible use of
the present method for generating terahertz frequencies at even harmonics in
graphene is suggested.
