Vertically coupled polymer microresonators for optofluidic label-free biosensors

In this paper we report on the design and fabrication of polymeric microracetracks optical resonators for optofluidic label-free biosensing. In the domain of optical integrated devices, polymer materials offer the advantages of low cost, easy fabrication, low scattering loss on waveguide sidewalls, and high coupling efficiency to optical fibres and waveguides. Moreover, for biochemical sensing, polymer surfaces can be easily modified to immobilize a wide choice of target molecules. Polymers are also well compatible with microfluidic circuits, favoring the insertion of photonic circuits into optofluidic cells. The vertical coupling configuration, in which resonators are vertically coupled to the buried bus waveguide, presents several advantages in comparison with the lateral coupling configuration, particularly in the context of optofluidic biosensors. Polymeric microracetracks were fabricated using the SU-8 negative photoresist and the CYTOP fluorinated polymer, using a combination of a simple near UV lithography and reactive ion etching technology. Vertically coupled microracetracks immersed in deionized water display high Q-factors (> 35000) and finesse up to 25. Surface sensing experiments performed with these microresonators using TAMRA-cadaverine as a test molecule, which can be quantified through fluorescence analysis, demonstrated a very low detection limit of 0.22 attogram.