Optofluidic refractive index sensor based on air-suspended SU‐8 grating couplers

This work presents the design, numerical simulation, fabrication and characterization of a label-free optofluidic refractive index sensor that is based on air-suspended SU‐8 grating couplers. By exploiting a polymer-onto-polymer lamination method for thin structured SU‐8 films, waveguide grating couplers can be fabricated in a film on top of a microfluidic channel system. A capillary force valve, integrated into the microchannels, precisely positions the employed test analytes, which are different DI water based sugar solutions, below the sensing grating coupler. By performing numerical simulations, the sensing grating coupler is optimized to a center wavelength of 1550 nm in the case that pure DI water (n = 1.33) is applied to the microfluidic channel. When a supported mode, guided in the waveguide, reaches the sensing grating region, it is exposed to the test solution resulting in a change of the effective refractive index of the mode. Similar to the simulation results, the experimental characterization of the sensor structure demonstrates a refractive index sensitivity of approximately 400 nm per refractive index unit (RIU) with respect to the wavelength shift of the grating coupler response, and 17 dB RIU‐1 with respect to the intensity decrease at the individual center wavelengths for refractive index variations between n = 1.33 and n = 1.36. Due to the combination of microfluidic channels and air-suspended grating couplers, analytes can directly be probed in-line in an integrated microfluidic channel making the presented principle suitable for low-cost, in-line polymer optofluidic and photonic sensing applications.