Real-time polarimetric optical sensor using macroporous alumina membranes

We report on the demonstration of real-time refractive index sensing within 60 μm thick free-standing macroporous alumina membranes with pore diameters of 200 nm. The free-standing macroporous alumina membranes allow the analytes to flow through the pores for targeted delivery, resulting in fast sensing responses. The polarimetric measurement platform exploits the optical anisotropy of the membranes in monitoring the refractive index variations of the analytes that fill the pores, providing highly sensitive and real-time measurements. The experimental characterization of the membranes' birefringence at wavelengths of 808, 980, and 1500 nm showed a decrease in birefringence for shorter wavelengths caused by the depolarization process that takes place when polarized light passes through a porous medium. Volumetric sensing experiments performed at the same wavelengths demonstrated detection limits of 8.1 × 10^-6, 5.2 × 10^-6, and 6 × 10^-6 refractive index units at wavelengths of 808, 980, and 1500 nm, respectively.