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Correlation between dielectric constants and enhancement of surface plasmon resonances for thin gold films



Optical properties of thin gold films evaporated on glass substrates were studied with the method of spectroscopic ellipsometry in both external and internal reflection configurations as well as with the UV–visible absorption spectroscopy. Here, we discussed how ellipsometric functions Psi and Delta in the spectral region of excitation of surface plasmon waves are related to the dielectric functions (ratio ϵ1m/ϵ2m) of the thin gold films for different thickness and adhesive layers. The obtained results showed a gradual shift of the plasmon peak from 580 nm for 55 nm thick films to more than 700 nm for thin (15 nm) gold films. This study also reports an effective method to enhance the surface plasmon resonance (SPR) on Au films by using a thin Al seed layer assisted deposition. Experimental results show that the Au/Al films exhibit an enhanced SPR over the Au/Cr and Au/Ti films with a narrower full width at half-maximum. It was found that Al acts as a roughness-diminishing growth layer for the Au film while at the same time maintaining and enhancing the plasmonic properties of the combined structures.