  Graphene is a nonlinear material which can be used as a saturable absorber,
frequency mixer and frequency multiplier. We investigate the third harmonic
generation from graphene lying on different substrates, consisting of a
dielectric (dispersionless or polar), metalized or non-metalized on the back
side. We show that the third harmonic intensity emitted from graphene lying on
a substrate, can be increased by orders of magnitude as compared to the
isolated graphene, due the LO-phonon resonances in a polar dielectric or due to
the interference effects in the substrates metalized on the back side. In some
frequency intervals, the presence of the polar dielectric substrate compensates
the strongly decreasing with $\omega$ frequency dependence of the third-order
conductivity of graphene making the response almost frequency independent.

Mesoscale and Nanoscale Physics;Optics