Femtosecond Nanoplasmonic Dephasing of Individual Silver Nanoparticles and Small Clusters.

We present experimental measurements of localized surface plasmon emission from individual silver nanoparticles and small clusters via accurately delayed femtosecond laser pulses. Fourier transform analysis of the nanoplasmonic coherence oscillations reveals different frequency components and dephasing rates for each nanoparticle. We find three different types of behavior: single exponential decay, beating between two frequencies, and beating among three or more frequencies. Our results provide insight into inhomogeneous and homogeneous broadening mechanisms in nanoplasmonic spectroscopy that depend on morphology and nearby neighbors. In addition, we find the optical response of certain pairs of nanoparticles to be at least an order of magnitude more intense than the response of single particles.