For the sample with a high Au particle density (600 NPs/μm 2 ), a small value (0.05) of ζ indicates that collective heating dominates. ζ is estimated with R = 15 nm, p = 40 nm, and D = 700 nm, where p is estimated from the Au NP density. The FWHM of the beam, D, is obtained assuming a Gaussian beam profile and using experimental optical parameters (NA = 0.50).