Extraordinarily high spectral sensitivity in refractive index sensors using multiple optical modes.

The extraordinary spectral sensitivity of surface plasmon resonance (SPR) sensors is commonly attributed to the modal overlap or unique dispersion of surface plasmons. In contrast to this belief, we show that such high sensitivity is due to the multi-mode nature of the sensing scheme. This concept of multi-mode sensing can be applied to dielectric systems as well in order to achieve similar extraordinary spectral sensitivity. We also show that there is a fundamental constraint between the spectral sensitivity and quality factor in such multi-mode sensing approach.