From Molecules to Meta-Molecules: A nano-plasmonic revival of multipolar molecular engineering for NLO

Molecular engineering for NLO has experienced a first revival from the early nineties to this day, when its scope was enlarged from a more restricted earlier pool of dipolar entities, to encompass the broader pool of multipolar molecules, with octupoles as a new tensor symmetry guided template for quadratic effects (1,2). In the wake of the development of nanotechnologies and nanosciences, the field of nanoplasmonics appeared, some ten years later, as a worthwile and further enlarged playground for molecular engineering now in a broader sense, with the early proposition of octupolar associations of nanoparticles showing the way towards upscaled and enhanced nonlinear optical enteties, in both 2-D and 3-D arrangements (3). In more recent years, the concept of « meta-molecules » has emerged, based on associations of « meta-atomic » building blocks in the form of either metallic nano-cavities or nanoparticles at a typical 200nm scale. This second revival has been guided, like the previous one, by symmetry arguments, albeit applied now in the different physical frame of localized and propagative plasmons. We will review in this presentation the different steps of this ongoing story, as per today, subject to fast evolutions in this emerging field. Four steps in the fabrication technologies, design rules, experimental results and theoretical models for nonlinear meta-molecules will be discussed and shown to have helped shaped the domain at this still early and fast evolving stage.