Metal nanospheres under intense continuous wave illumination - a unique case of non-perturbative nonlinear nanophotonics

We show that the standard perturbative (i.e., cubic) description of the thermal nonlinear response of small metal nanospheres to intense continuous wave illumination is insufficient already beyond temperature rises of a few tens of degrees. In some cases, a cubic-quintic nonlinear response is sufficient to describe accurately the intensity dependence of the temperature, permittivity and field, while in other cases, a full non-perturbative description is required. We further analyze the relative importance of the various contributions to the thermal nonlinearity, identify a qualitative difference between Au and Ag, and show that the thermo-optical nonlinearity of the host typically plays a minor role, but its thermal conductivity is important.