Nanostructuring of alumina optical waveguides by hot water treatment for tuning sensor output

A study of the nanostructuring of alumina integrated optical waveguides by means of hot water treatment to tune their outputs, namely, total internal reflection and scattering, is presented. Homogeneous alumina thin films fabricated by atomic layer deposition were exposed to hot water to form surface nano-pillars of various heights and densities. The 135-, 232- and 307-nm thick alumina films were immersed in hot water at 50, 60, and 80 °C for 2, 5, 10, 15, 20 and 30 min. Topology measurements of the nanostructured integrated optical waveguides (n-IOWs) were made using an atomic force microscope and a scanning electron microscope. Optical transmission and coupling measurements for n-IOWs were taken. Average pillar height and density, controlled by adjusting the water temperature and exposure time, act to tune the scattered and guided fractions of the coupled light. Well controlled fabrication of nanopillars (up to 110-nm-tall) as well as fine tuning of the scattered component of sensor output is possible. Fabricated n-IOW sensors with dual outputs, total internal reflection and scattering, could be quite useful for the enhanced sensing of surface-adsorbed molecular species.