PLASMONIC MODE PROPERTIES PROBED ON THE BI-METALLIC STRUCTURE Ag-Au

Abstract

Optical surface plasmon resonance sensors have been known for a long time. In the Kretschmann configuration, a long interface between a metal and a dielectric, the optical coupling of an incident light to collective oscillation is governed by the metallic thickness. In this work, we discuss the use of a bi-metallic structure to generate surface plasmon-polaritons (SPPs). Firstly, we study the influence of the thickness of silver and gold on the surface plasmon-polariton resonance separately. Secondly, we present the analytical results of the reflected light (versus the incidence angle) for two dispositions Ag-Au and Au-Ag excited with an electromagnetic wave in the visible range (λ=427 nm – 514 nm). In particular, for the metals, we have taken a frequency-dependence in their dielectric permittivities εAg(λ) and εAu(λ) as tabulated in the reference of Johnson and Christhy. Finally, we found the basic characteristics for the resonances of the surface plasmon-polaritons. We underline an important result to produce coupling phenomenon where the surface plasmon resonance becomes independent of the metallic thickness. The plasmonic resonance is just shifted towards total reflection. However, we explain this effect that SPPs on bi-metallic structure can be trapped with an optimal rate in the reflected light. The plasmonic trap obtained by the disposition Au-Ag in contact with a dielectric (ambient air) which is irradiated at a wavelength λ=514 nm, imposes a layer dAu+dAg = 47 nm. For different values of dAu fixed in the range 20 nm – 46.9 nm, the dip in reflectivity is not related to the thickness dAu beyond the threshold value of 15 nm. An identical characteristic is verified with the wavelength of λ=427 nm where the dip is narrow and the angular resonance is not strongly shifted. When the wavelength is decreased, the control of the plasmonic trap the threshold of dAu is increased.