Retrieving the complex polarizability of single plasmonic nanoresonators

The complex optical polarizability of a nano-oscillator determines the way it couples to light and to other nano-objects in its environment. Hence, its experimental evaluation at the single-particle level represents a crucial task in nano-optics. In this work we demonstrate that both phase and amplitude of a nanoresonator polarizability, which are embedded in its near-field response, can be decoupled by combining near-field and confocal far-field extinction imagings. The interpretation of our measurements based on a simple analytical model is further confirmed by finite-difference time-domain calculations.