Refractive index of Ag nanocrystals composite films in the neighborhood of the surface plasmon resonance

Nanocomposite thin films formed by Ag nanocrystals embedded in an amorphous aluminum oxide $({\mathrm{Al}}_2{\mathrm{O}}_3)$ host were prepared by alternating-target pulsed laser deposition. Spectroscopic ellipsometry was used to determine the effective refractive index $({n}\mathrm{=n+ik).}$ When the Ag volume fraction is over 2%, the linear optical properties of the nanocomposite films differ from those of the pure dielectric host. The extinction coefficient shows a maximum around 435 nm that is related to the surface plasmon resonance. Near this wavelength, the real part of the refractive index undergoes anomalous dispersion, leading to a significant increase of the n value of the composite compared to that of the matrix.