Anti-Stokes Emission from Hot Carriers in Gold Nanorods

The origin of light emission from plasmonic nanoparticles has been strongly debated lately. It is present as the background of surface-enhanced Raman scattering and, despite the low yield, has been used for novel sensing and imaging applications because of its photostability. Although the role of surface plasmons as an enhancing antenna is widely accepted, the main controversy regarding the mechanism of the emission is its assignment to either radiative recombination of hot carriers (photoluminescence) or electronic Raman scattering (inelastic light scattering). We have previously interpreted the Stokes-shifted emission from gold nanorods as the Purcell effect enhanced radiative recombination of hot carriers. Here we specifically focused on the anti-Stokes emission from single gold nanorods of varying aspect ratios with excitation wavelengths below and above the interband transition threshold while still employing continuous wave lasers. Analysis of the intensity ratios between Stokes and anti-Stokes emission yields temperatures that can only be interpreted as originating from the excited electron distribution and not a thermally equilibrated phonon population despite not using pulsed laser excitation. Consistent with this result as well as previous emission studies using ultrafast lasers, the power-dependence of the upconverted emission is nonlinear and gives the average number of participating photons as a function of emission wavelength. Our findings thus show that hot carriers and photoluminescence play a major role in the upconverted emission.