Best Surface Plasmon Quality Money can('t) buy
Alas, what we want and what we get are often two very different things!
Surface plasmon resonance is probably one of the biggest industries you’ve never heard about.
This chapter visually explores the optimisation of the SPR response on planar plasmonic films
by quantifying the resonance quality over an arbitrary n, k landscape, i.e, through assuming
the “metal” optical constants may be freely chosen. The resulting perspective is instructive
as it puts the SPR sensitivity of current metal films such as gold and silver quantitatively in
context against what is ultimately achievable, assuming future advancements can be made
with artificially engineered materials. It is frequently commented upon in the plasmonics
community how “loss” is impeding progress - the maps here show simply that a combination
of lower n with tailored k is equally required, in SPR sensing applications
Recall once again the simplified, real dispersion relation:
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##\begin k = {\frac \sqrt { \frac { \varepsilon \varepsilon } { \varepsilon + \varepsilon } } = k n \sin{ \theta }} \label \end
where we shall now specify the metal-dielectric constant $\varepsilon\textsubscript$ over an arbitrary \textit and \textit space (using plasmonically relevant values). As k\textsubscript is larger than that of free space light, light shining on metal from air, for example, cannot directly excite a surface plasmon\footnote{The effect would be common-knowledge if it could be directly excited!}. The incident light has increased momentum when passing through a prism. The "boost" required (the square root term in Equation \ref) to get from k\textsubscript to k\textsubscript is briefly worth noting. Figure \ref illustrates the effect that modifying the metal \textit or \textit values (equivalently expressed now in place of $\varepsilon\textsubscript$) has on this square root term. This is essentially the mismatch factor between k\textsubscript and k\textsubscript. There is at least one clear message coming from this figure \textemdash films with higher \textit values require less boosting. In practical terms, this would translate to less-shallow angles of incidence of light in the prism, and as we shall see, much sharper resonances.
