Optical waveguide biosensor based on modal interference between surface plasmon modes

A detailed theoretical study on an optical waveguide biosensor utilizing the modal interference between surface plasmon modes is presented. It is examined that the interaction length i.e. longitudinal dimension of the metallic layer plays a very crucial role in determining the transmission characteristics of such sensors. And as expected, to get a dip in the transmission spectrum around a desired wavelength, the interaction length should be nearly an odd integer multiple of the coupling length corresponding to the surface plasmon modes. It is found that the minimum possible length (equal to one coupling length) of the sensor may not always give the highest figure of merit. Knowing this, we then show that the sensitivity of such a sensor can be further enhanced by optimizing the thicknesses of metal and intermediate layers.