  The capability to dynamically control the nonlinear refractive index of
plasmonic suspensions may enable innovative nonlinear sensing and signaling
nanotechnologies. Here, we experimentally determine the effective nonlinear
refractive index for gold nanorods suspended in an index matching oil aligned
using electric fields, demonstrating an approach to modulate the nonlinear
optical properties of the suspension. The nonlinear optical experiments were
carried out using a Hartmann-Shack wavefront aberrometer in a collimated beam
configuration with a high repetition rate femtosecond laser. The suspensions
were probed at 800 nm, overlapping with the long-axis absorption peak of the
nanorods. We find that the effective nonlinear refractive index of the gold
nanorods suspension depends linearly on the orientational order parameter, S,
which can be understood by a thermally induced nonlinear response. We also show
the magnitude of the nonlinear response can be varied by ~60%.
