  We present a theoretical description of the nonlinear response induced by an
elliptically-polarized terahertz beam normally-incident on intrinsic and
extrinsic metallic armchair graphene nanorib- bons. Our results show that using
a straightforward experimental setup, it should be possible to observe novel
polarization-dependent nonlinearities at low excitation field strengths of the
or- der of 10 4 V/m. At low temperatures the Kerr nonlinearities in extrinsic
nanoribbons persist to significantly higher excitation frequencies than they do
for linear polarizations, and at room tem- peratures, the third-harmonic
nonlinearities are enhanced by 2-3 orders of magnitude. Finally, the
Fermi-level and temperature dependence of the nonlinear response is
characterized.
