High performance multilayer surface plasmon sensors

Though high performance SPR sensors are readily available, it remains desirable to fabricate sensors with enhanced sensitivity, resolution and evanescent fields for numerous applications. Since SPR characteristics of bimetallic (Ag/Au) and bimetallic waveguide coupled (Bi-WC) sensors are known to be better than those of single metal sensors,\footnote{K. S. Lee, T. S. Lee, I. Kim, and W. M. Kim, J Phys D Appl Phys 46 (12) (2013).} we have undertaken investigations of the performance of multilayer structures.\footnote{K. Tiwari, S. C. Sharma and N. Hozhabri, submitted (2014)} We employ the transfer matrix method (TMM) for calculating SPR characteristics of such structures as functions of Ag/Au and Si$_{3}$N$_{4}$ waveguide thickness. Several quartz/Ag/Si$_{3}$N$_{4}$/Au structures were deposited in a \textit{class-100} clean room facility. The thicknesses of Ag and Au were fixed at 35 and 28 nm respectively. However, the thickness of the intermediate Si$_{3}$N$_{4}$ waveguide layer was varied from 50 - 150 nm. The SPR curves were measured for all these structures by using the Kretschmann configuration system. We observe excellent agreement between the experimental SPR data and computational results. For an optimized 150 nm thickness of Si$_{3}$N$_{4}$ waveguide, we observe high sensitivity to changes in the refractive index (S$_{\mathrm{n}}\approx $ 52$^{0}$/RIU), extremely narrow SPR curves (FWHM$\le $0.28, yielding high figure-of-merit of 60-180) and increased decay length of evanescent fields ($\delta \approx $ 258 nm).