Comparison of sensitivities of two label-free optical detection techniques: SPR vs. oblique-incidence reflectivity difference (OI-RD)

Surface plasmon resonance (SPR) and recently developed oblique- incidence optical reflectivity difference or OI-RD (a special form of nulling ellipsometry) are two label-free techniques for detecting biochemical reactions and other surface processes on a solid support. In SPR, one measures the change in surface plasmon resonance angle, $\Delta \theta _{SP}$, as a result of changes in thickness and dielectric response of a molecular adlayer on the solid support coated with gold. In OI- RD, one measures the difference in fractional reflectivity change between p-polarized and s-polarized light defined as (r$_{p}$-r$_{p0}) $/r$_{p0}$ - (r$_{s}$-r$_{s0})$/r$_{s0} \quad \equiv \quad \Delta _{p}- \Delta _{s}$, again in response to changes in thickness and dielectric response of a molecular adlayer on the solid support. I show that the two techniques measure the same physical quantities of the molecular adlayer, up to a constant that depends on the incidence angle in OI-RD. And since $\Delta _{p}-\Delta _{s}$ is enhanced near the Brewster angle, the OI- RD technique is at least as sensitive as and more versatile than the SPR technique for label-free detection of biochemical reactions and other processes on solid surfaces.