Silicon-nitride-based medium-contrast gratings for resonant fluorescence enhancement in the visible wavelength range

Abstract

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.