Optical waveguide sensor based on a porous anodic alumina/aluminum multilayer film

An optical waveguide sensor was fabricated by forming a multilayer film made by porous anodic alumina (PAA) and Al layers on a glass substrate. The fabricated sensor system was based on the monitoring of a waveguide coupling mode, which is sensitive to the change in the refractive index of the PAA layer caused by collection of target molecules into the pores of the PAA layer. The PAA/Al multilayer film was formed by partial anodization of an Al film deposited on the glass substrate, and the waveguide coupling mode was examined by measuring angular spectra (reflectivity dependence on the incident angle of monitoring light; green He-Ne laser, 534.5 nm). A deep and sharp waveguide coupling dip was obtained for the PAA/Al multilayer system where the thicknesses of the PAA and Al layers were 200 and 17 nm, respectively. The optical sensor response of the PAA/Al multilayer system was compared to the responses of a surface plasmon resonance (SPR) sensor made by a Au thin film on a SF10 glass substrate. It was inferred that the optical waveguide sensor made by the PAA/Al multilayer could detect a smaller change in the refractive index of a solution, and it provided higher resolution than the SPR sensor. The sensor response for a change in the complex refractive index of the PAA layer was examined next, and it was found that the optical waveguide sensor was sensitive to the change in the imaginary part of the complex refractive index rather than the change in the real part. This result indicated that the sensitivity of the optical waveguide sensor could be improved by using the light absorption of a target compound.