Resonant secondary light emission from plasmonic Au nanostructures at high electron temperatures created by pulsed-laser excitation

Significance Light emission from plasmonic nanostructures at wavelengths shorter than the wavelength of pulsed-laser excitation is typically described as two-photon absorption followed by fluorescence. In this work, we present an alternate description of the secondary light emission as a resonant electronic Raman scattering process. Experiments on aqueous suspensions of Au nanorods are quantitatively described by a two-temperature model. The results will facilitate the design of imaging experiments and understanding of background in surface-enhanced Raman scattering. Plasmonic nanostructures are of great current interest as chemical sensors, in vivo imaging agents, and for photothermal therapeutics. We study continuous-wave (cw) and pulsed-laser excitation of aqueous suspensions of Au nanorods as a model system for secondary light emission from plasmonic nanostructures. Resonant secondary emission contributes significantly to the background commonly observed in surface-enhanced Raman scattering and to the light emission generated by pulsed-laser excitation of metallic nanostructures that is often attributed to two-photon luminescence. Spectra collected using cw laser excitation at 488 nm show an enhancement of the broad spectrum of emission at the electromagnetic plasmon resonance of the nanorods. The intensity of anti-Stokes emission collected using cw laser excitation at 785 nm is described by a 300 K thermal distribution of excitations. Excitation by subpicosecond laser pulses at 785 nm broadens and increases the intensity of the anti-Stokes emission in a manner that is consistent with electronic Raman scattering by a high-temperature distribution of electronic excitations predicted by a two-temperature model. Broadening of the pulse duration using an etalon reduces the intensity of anti-Stokes emission in quantitative agreement with the model. Experiments using a pair of subpicosecond optical pulses separated by a variable delay show that the timescale of resonant secondary emission is comparable to the timescale for equilibration of electrons and phonons.