Virtually every linear and nonlinear optical process relies on the interaction of light and matter. Especially under conditions of strongly pronounced light-matter coupling, rich physical phenomena with strong scientific and technological implications emerge. For example, plasmonic metal nanostructures may serve as a means to access highly nonlinear optical phenomena even at extremely low optical pulse energies. This includes processes such as harmonic generation, Raman sensing at a single molecule level, or field driven ionization to generate and control attosecond electronic wave packets at the nanoscale. Very recently, an unprecedentedly strong and qualitatively different regime of light-matter interaction (‘deep strong’ coupling) has been demonstrated in plasmonic nanoparticle supercrystals characterized by coupling frequencies that exceed the involved plasmonic frequencies. This is a result of the extreme field enhancement of large collective plasmonic modes that localize at the nanometric interparticle gaps. With the support of the PIER Seed funding we will harness such structures as electro-optical transducers to study collective electronic phenomena at ultrafast time scales upon optical excitation.