Development and Optimization of a Label-free Optical Biosensor Based on Bloch Surface Waves Sustained by One Dimensional Photonic Crystals

The Ph.D. thesis is focused on the development of a label-free optical biosensor based on the excitation of surface electromagnetic modes, Bloch Surface Waves (BSW), sustained by truncated One Dimensional Photonic Crystal (1DPC). BSWs are electromagnetic modes that can be coupled at the interface between a truncated dielectric 1DPC and a homogeneous external medium, for particular combinations of angle and wavelength of an incoming radiation. These modes are confined on the multilayer surface and propagate in the plane, while the electric field amplitude shows an exponential decay in the medium, perpendicularly to the stack. The surface waves can be resonantly excited by a light beam through the coupling with a glass prism in Kretschmann-Raether configuration. In the case of white light impinged on the 1DPC surface, BSW appears as a dip in reflected spectrum corresponding to those wavelengths coupled to BSW at specific incident angles. The BSW resonance is very sensitive to small variation of the refractive index of the medium surrounding the 1DPC, induced by surface and bulk perturbations yielding a shift of the spectral dip position. This can be exploited for sensing purpose, similarly to other standard sensing techniques such as Surface Plasmon Resonance (SPR). Different kinds of measurement modalities can be performed, such as fluorescence-based refractometric label free or scattering based detection. The aim of the project is to develop and optimize a sensitive sensor able to recognize target analytes exploiting an immunoassay scheme.