Optical behavior of silver/dielectric and gold/dielectric bilayer thin films

The growth and optical properties of Ag/dielectric and Au/dielectric bilayer thin films are reported. The metal layer is deposited on top of the dielectric layer, which is either TiO2 or ZrO2. The dielectric films were prepared by the sol–gel method whereas the metal films (5–50 nm thick) were deposited on the dielectrics by ion beam sputter deposition. The surface morphology of the metal layer is strongly dependent on the dielectric layer over which it is deposited. The bilayer thin films show enhanced light absorption in the ultraviolet region and refractive index controlled optical absorption at near infrared wavelengths. In addition, they display the tunability in surface plasmon resonances exhibited normally by core–shell and bimetal nanostructures. In the presence of the metal layer, the absorption coefficient of the bilayer is enhanced, by almost 7 times, in the region of the band gap. The surface plasmon resonance of the metal exhibits a red-shift with increase in refractive index of the dielectric underlayer. The plasmon resonance for Ag is observed at a wavelength of 424 nm on ZrO2, which shifts to 434 nm on TiO2. Similarly in the case of Au films, the shift is from 630 nm on ZrO2 to 640 nm on TiO2. Interestingly, the optical absorption in the near IR region increases with increase in refractive index of the dielectric underlayer.