Surface plasmon propagation in novel multilayered metallic thin films

Multilayered Ag/Au/Ag/Au and Au/Ag/Au/Ag films with 200 nm of thickness (50 nm for each layer) were evaporated onto BK7 glass substrates. Sequences of slits (around 60-600 nm of width) were milled with a focused gallium ion beam in the films. We have undertaken a series of high-resolution measurements of the optical transmission through the slits. The transmission measurement setup consists of 488.0 nm (for the Ag/Au/Ag/Au film) and 632.8 nm (for the Au/Ag/Au/Ag sample) wavelength light beams from Ar ion and HeNe lasers, respectively, aligned to the optical axis of a microscope. The beam is focused onto the sample surface by a microscope objective in TM polarization (magnetic Hfield component parallel to the long axis of the slits). As well, theoretical estimates investigating the slits optical transmission were performed. The origin of the slits transmission is mainly attributed to plasmonic surface excitations. Based on the present results, it was possible to observe that (1) the transmission increases linearly with increasing slit width, and (2) the transmission of the multilayered structures is augmented in comparison with a single perforated metal film of equal thickness, for a fixed slit width. A very good correspondence between theory and experiment was observed.