  High-performing nanomaterials are paramount for advanced photonic
technologies, like sensing, lasing, imaging, data storage, processing, and
medical and biological applications. Metal nanoparticles play a key role,
because the localized surface plasmonic resonances enhance the linear and
nonlinear optical properties of hosting materials, thus increasing the range of
applications. The improved optical response results in an enriched and largely
unexplored phenomenology. In the present work, we show the formation and
interaction of two dispersive shock waves emerging by illuminating an aqueous
suspension of gold nanorods with a pulsed laser beam. By analyzing the
characteristic undular bores that regularize a shock front in a dispersive
material, we were able to distinguish dispersive shock waves of different
origins, i.e., electronic and thermal, first coexisting and then interacting to
form a double front of equal intensity. The observed scenario agrees with
existing literature reporting an electronic and thermal contribution to the
nonlinear refractive index of material containing gold nanoparticles. Although
a full comprehension of the reported results deserves an analytical
description, they already pave the way to using materials containing metal
nanoparticles as a platform for studying fundamental extreme nonlinear
phenomena and developing novel solutions for sensing and control.
