High numerical aperture waveguide absorption filter for fluorescence detection

Filtering strategies are a crucial aspect for signal detection in many fluorescence based systems such as chemical and/or biochemical sensors. The design, fabrication and characterization of a new waveguide absorption filter for the optimization of the fluorescence signal collection, thanks to its high numerical aperture, is here presented. The absorption filter is designed to work as an optical waveguide in order to increase the optical path and, consequently, the absorption of the excitation light. A comparison of the performances of the absorption filter and a conventional interference filter, with particular emphasis on the angular dependence of the spectral features, is also reported. We experimentally demonstrate, for what regards the attenuation capability of the excitation signal, the failure of the interference filter for incidence angles greater than 15° and the validity of the absorbing waveguide filter for large incidence angles. Finally, preliminary results performed in fluorescence on an IgG labelled/ anti-IgG assay show the improvement in detected fluorescence intensity collected by means of the proposed absorption filter compared to that measured with the interference filter. This suggests that the filtering strategy based on the waveguide absorption filter can greatly simplify fluorescence detection systems and find interesting applications in different areas of sensing, from Point of Care Testing (POCT) to environmental monitoring.