Plasmonic Properties of Electrolytes Beyond Classical Nanophotonics — A Two-Fluid, Hydrodynamic Approach to Nonlocal Soft Plasmonics

Electrolytes consist of positively and negatively charged ions even in equilibrium. Hence, plasmonic behavior can be observed in ionic systems and interaction effects between the charge carriers may play a sizable role as compared to the quantum effects observed for metal nanoparticles beyond a classical Maxwellian description. I study ionic plasmon effects, i.e., collective charge oscillations, in electrolytes in the scope of a nonlocal, two-fluid model using the hydrodynamic theory of charges. Notably, nonlocal quenching is observed for particle sizes spanning orders of magnitude, tunable via ion concentration and their mass and charge through choice of material. A plasmonic theory for ions in solution can bridge hard and soft matter theory and allow studying interaction effects from a photonic perspective in full analogy to solid metal particles. The semi-classical approach presented here can be fully integrated into standard nano-optic simulation frameworks and is considered to be of great interest for plasmonic photo-catalysis introducing nonlocal aspects into electrolyte-electrode interactions.