Si3N4/SiO2/Si waveguide grating for fluorescent biosensors

The simulation of the coupling efficiency of fluorescence sources immobilized in the evanescent field of a waveguide mode has allowed the dimensioning of a sensing waveguide platform based on a silicon substrate exhibiting maximum fluorescence efficiency. The incoupling of the excitation light and the outcoupling of the fluorescence signal are performed by means of a corrugation grating. The presence of the highly reflective silicon surface under the waveguide grating causes interferences in the fluorescence output signal which depend on the wavelength and on the radiation angle. A theoretical modeling of the grating coupling under these conditions has led to the achievement of high incoupling efficiency at the excitation wavelength, and simultaneously to high outcoupling efficiency at the wavelength of the fluorescence peak. An experiment was performed with fluorescein at the surface of a 160 nm thick silicon nitride waveguide film on a silicon substrate with a 2100 nm thick buffer layer of silica. The measured outcoupled spectra fit well with the calculated spectra taking into account the output grating coupling, the fluorescence spectrum, and fluorescence coupling. These waveguide grating structures are applied to the detection of immunological reactions.