Low-cost Vertical Taper for high efficient light in-coupling in Bimodal Nanointerferometric Waveguide Biosensors

D. Grajales , A. Fernández Gavela, C. Domínguez, J. R. Sendra, L. M. Lechuga

J. Phys. Photonics, 1, 2019


Abstract: There is still a need of a compact and cost-effective solution for efficient light in-coupling in
the integrated waveguides employed in the photonic biosensors activity. Especially when these
waveguides are of submicron dimensions and operate at visible wavelengths. The employment
of a vertically stacked taper with a larger input area is proposed to solve this need. The design
of the taper is divided in two stages: in the first stage, light is guided downwards by two
vertically stacked tapers; in the second stage, an inverted taper directly confines the light inside
the waveguide. The design parameters are optimized using a commercial software, obtaining a
total theoretical light coupling efficiency of 72.25%. The taper is manufactured using SU-8
polymer as the main material, employing standard photolithography techniques at wafer level.
After characterization, the results show the practicality of the taper when coupling light from
macro sources to the nanometric waveguides, obtaining an experimental coupling efficiency of
55%. With this vertical taper, a compact, easy-to-couple and cost-effective solution is
accomplished for waveguide based biosensors operating at visible wavelengths, opening the
route for a truly portable point-of-care biosensor for low cost and label-free diagnostics.

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