Mapping heat origin in plasmonic structures.

We investigate the physics of photoinduced heat generation in plasmonic structures by using a novel thermal microscopy technique based on molecular fluorescence polarization anisotropy. This technique enables us to image the heat source distribution in light-absorbing systems such as plasmonic nanostructures. While the temperature distribution in plasmonic nanostructures is always fairly uniform because of the fast thermal diffusion in metals, we show that the heat source density is much more contrasted. Unexpectedly the heat origin (thermal hot spots) usually does not correspond to the optical hot spots of the plasmon mode. Numerical simulations based on the Green dyadic method confirm our observations and enable us to derive the general physical rules governing heat generation in plasmonic structures.