  We investigate the optical response of graphene nanoribbons (GNRs) using the
broadband nonlinear generation and detection capabilities of nanoscale
junctions created at the LaAlO$_3$/SrTiO$_3$ interface. GNR nanoclusters
measured to be as small as 1-2 GNRs in size are deposited on the LaAlO$_3$
surface with an atomic force microscope tip. Time-resolved nonlinear optical
probes of GNR nanoclusters reveal a strong, gate-tunable second and third
harmonic response, as well as strong extinction of visible to near-infrared
(VIS-NIR) light at distinct wavelengths, similar to previous reports with
graphene.
