**4.1 Optical waveguide biosensors**

Various modalities have been developed for optical waveguide biosensor applications. They include grating-coupled waveguide sensors, interferometric waveguide sensors, photonic crystal waveguide sensors and resonant optical microcavity sensors, among others. **Figure 3** below provides examples for each of the optical waveguide options mentioned.

*Nanomaterial-Enhanced Receptor Technology for Silicon On-Chip Biosensing Application DOI: http://dx.doi.org/10.5772/intechopen.94249*

#### **Figure 4.**

*Schematic representation of an optical biosensor based on evanescent wave interaction. Due to interaction of the light with the target molecules (analytes) within the cavity in the interaction area, the optical wave undergoes a change in its propagation constant due to the variation in the effective refractive index.*

#### **4.2 Receptor layer in waveguide and evanescent field**

The difference in refractive index between the core and cladding of an optical waveguide exist. Thus, light is guided via the core with a higher refractive index on account of total internal reflection, which generates an evanescent optical field that decays exponentially from the sensor surface, as in **Figure 4**. Biomolecular binding events modulate the refractive index contrast and thus attenuate the propagation of light through the waveguide. By monitoring the coupling and/or propagation properties of light through an appropriately modified waveguide, it is possible to construct sensors responsive to the target biomolecular analytes of interest, as shown in **Figure 4**.
