Thickness optimization of metal films for the development of surface-plasmon-based sensors for nonabsorbing media

The surface-plasmon resonance (SPR) effect in metals is highly sensitive to fluctuations in the optical properties of the interface and has been frequently employed in the Kretschmann configuration for optical sensing. The operating conditions required for using the SPR effect for probing nonabsorbing media under maximum sensitivity are derived analytically under the Lorentzian approximation. It is found that the film thickness that maximizes sensitivity occurs when the radiation damping of the oscillation is half the intrinsic damping. Numerical results are presented for the spectral dependence of the optimum thickness as well as of the SPR parameters of gold, copper, silver, and aluminum films, useful for the design of optical sensors for both gaseous and aqueous environments.