Optical characteristics of ultra-thin metallic films excited at visible range

• Ag and Ag/Au thin films were prepared by thermal evaporation.
• Roughness and thickness of single and double layer films were measured by AFM.
• A visible range of sources (lasers) have been used to excite surface plasmons.
• Optical properties of metal thin films were analyzed.
• Wavelength of 632 nm is found to be stable and appropriate to measure optical thickness of ultra-thin films.

An extensive research has been carried out in synthesizing and characterizing single layer Ag films and double layer Ag/Au films of different thicknesses including ultrathin films. AFM measurements confirmed the thickness and smoothness of the prepared films. Moreover, the refractive indices of the metal films and surface plasmon resonance (SPR) parameters like height, full width at half maximum (FWHM) of SPR dip, average inclinations of the slopes have been determined successfully. SPR field enhancement of the film samples has been calculated as well. It has been revealed that the refractive index of Ag single layer decreases with increasing film thickness at different exciting wavelengths. While in case of double layer, the refractive index of Au ultra-thin layer (metal clusters) increases with increasing thickness (5–9 nm). The optical measurements divulged that the light of wavelength 633 nm is the best for measuring SPR thickness below 10 nm metal thin films.