Influence of silver-nanoparticle layer in a chiral sculptured thin film for surface-multiplasmonic sensing of analytes in aqueous solution

Multiple surface-plasmon-polariton (SPP) waves at a single free-space wavelength can be guided by the interface of a metal and a chiral sculptured thin film (STF). Multilayers comprising a chiral STF of lanthanum fluoride deposited on an aluminum thin film deposited on a glass substrate were fabricated. In some chips, a 5-nm-thick layer of silver nanoparticles was deposited at one of two selected depths in the chiral STF. The chips were then deployed in a prism-coupled configuration in a custom-built machine for surface multiplasmonic resonance imaging (SMPRI), in order to observe the effects of the silver-nanoparticle layer on the multiple SPP-wave modes. The angular locations of the SPP-wave modes were found to be not greatly dependent on whether the silver-nanoparticle layer was deposited after the first or the second period of a three-periods-thick chiral STF. With aqueous solutions of sucrose as infiltrant fluids, the angular shifts of the SPP-wave modes were determined as the refractive index of the infiltrant fluid increased. The use of a charge-coupled devices camera and upgraded motion-control equipment for SMPRI was found to increase the sensitivity of the chip. The silver-nanoparticle layer was also found to enhance the sensitivity.