Measurement of the complex dielectric constant of a single gold   nanoparticle
  A differential interference contrast microscopy technique, which employs a photonic crystal fiber as a white-light source, is used to measure both the real and imaginary parts of the complex dielectric constant of single 10 and 15 nm gold nanoparticles over a wavelength range of 480 to 610 nm. Noticeable deviations from bulk gold measurements are observed at short wavelengths and for individual particles even after taking into account finite-size surface damping effects. 
Patrick Stoller, Volker Jacobsen, Vahid Sandoghdar (Laboratory of   Physical Chemistry, ETH Zurich, Switzerland)