In this work, we have designed, fabricated and characterized silicon nitride sub-wavelength gratings on glass substrate to enhance the fluorescence in the green-red wavelength range. Silicon nitride was chosen as the material to fabricate the gratings as it exhibits low absorption losses and negligible fluorescence at visible wavelengths. Due to lower refractive index contrast, the structures were designed such that the medium index contrast gratings still achieve good quality factor resonances by using higher duty cycles (~ 70%) which clearly distinguishes two-mode region from higher order diffraction regime. The designed structure (Duty cycle: ~70%, thickness: 290nm, pitch: 370nm) supports resonant modes at 542nm for TE and at 548nm and 568nm for TM polarization. Rhodamine B dye was attached to the grating through an intermediate polymer layer PAH (Polyallylamine hydrochloride) by dip coating method. Using a fluorescence microscope with suitable excitation (510-550nm) and emission (>590nm) filters, we observed fluorescence enhancement of 5.4x and 5.8x in TE and TM modes respectively.
Water quality monitoring has become important in today’s scenario due to severe chemical and bacterial contamination in urban and rural water bodies. However, current monitoring methods do not provide fast and reliable results. By using intrinsic fluorescence, microbial contamination and industrial pollutants in water can be monitored in real-time, continuously and at very low concentrations. Intrinsic fluorescence can be enhanced by using High Contrast Gratings (HCGs) spectrally tuned to the fluorescence signatures of pollutants. Compared to metallic gratings which suffer from higher losses especially at lower wavelengths and are easily prone to oxidation, an all dielectric approach can overcome these limitations. HCGs using silicon nitride as grating material on a glass substrate are optimized to detect the presence of tryptophan (a bio-chemical marker for bacterial contamination) and phenanthrene (chemical contaminant). Tryptophan and phenanthrene have a fluorescence emission wavelength of 410 nm and 420 nm respectively. HCGs are optimized to enhance fluorescence emission at both of these wavelengths, therefore the optimized grating parameters for tryptophan (period: 255 nm, duty cycle: 0.8 and thickness: 260 nm) and phenanthrene (period: 282 nm, duty cycle: 0.8 and thickness: 289 nm) resulted in Q factor of 683 and 709 respectively. The optimized HCGs show an electric field enhancement of eight times concentrated in the air region between the gratings which would result in enhanced fluorescence.