Sensitivity enhancement of silver-based SPR sensors using ultrathin gold film and graphene overlay.

Most of the fiber optic-based surface plasmon resonance (SPR) sensors use gold or silver metal as part of the sensing region. It is known from the literature that silver presents a better performance in terms of sensitivity, although is less stable and strongly affected by oxidation, which in turns reduces its lifetime. To overcome this effect, other materials, such as gold, are proposed as alternatives in combined use with silver, forming bimetallic layers, leading to a higher sensor performance. Furthermore, adding graphene layers over the aforementioned sensors is prominent due to graphene electrical and optical properties. In this work, we propose a sensitivity enhancement of silver-based SPR sensors by adding an ultrathin layer of gold over the silver metal in order to protect against oxidation and also by adding graphene layers over the bimetallic SPR sensor, to increase the evanescent electric field. We also simulate, for comparison, a gold-based SPR sensor with graphene overlay, and it is observed, it is observed a sensitivity increase of over 90% when 13 graphene layers are added. For the silver-based sensor, an increase in sensitivity of approximately 50% (from 3,710 nm/RIU to 5,601.1 nm/RIU) is also achieved, when silver is covered with 13 graphene layers. For the silver and gold bimetallic layer sensor, sensitivity increases from 3,360 nm/RIU to 5,321.1 nm/RIU, using 19 graphene layers, which corresponds to an increase of almost 60%. FWHM and FOM parameters are also improved for all the analyzed configurations, in comparison with the conventional silver-gold bimetallic sensor.