Effect of Thin Cr and Cu Adhesion Layers on Surface Plasmon Resonance at Au/SiO2 Interfaces

We measure the effect on surface plasmon resonance of introducing Cr in a range of thicknesses between 0 and 6.4 nm at the interface between SiO₂ and Au using attenuated total reflectance at a free space wavelength of 658 nm. The measured curves were fit to the equations for reflectivity of a three-interface system to extract a thickness and effective ε_r for the Cr interface layer. For a 1.2 nm Cr interface layer, we extracted ε_r = 4.4 - j56, while for a 6.4 nm Cr layer, ε_r = -8.5 -j41. The real part of the adhesion layer's complex dielectric function is allowed to be positive during fitting. We consider the positive real part of the dielectric constants at low thicknesses for Cr to be an effective value that must be taken together with the Au overlayer to describe the overall system. Simulations of plasmonic propagation with these effective constants indicate that even at 1.2 nm in thickness, Cr will reduce the plasmon propagation distance in relevant structures to one-fifth the propagation distance of the interface without the presence of Cr. This behavior was compared to Cu, a metal with plasmonic properties comparable with Au, and it was found that a Cu interface layer has very little effect on plasmon propagation at the Au/SiO₂ interface, as expected.