Nanoporous alumina microtubes for metamaterial and plasmonic applications.

Double anodization of aluminium microwires in an acid bath yields cylindrical nanoporous alumina with nonbranching radially emanating pores. The obtained nanoporous alumina is a cylindrically anisotropic as well as radially inhomogeneous optical medium. Detailed structural characterization reveals that the nanopore diameter varies linearly with the radius of the aluminium microwire along the radial direction. Microcracks form on the alumina shell during the anodization when sufficient thickness is formed due to volume expansion and stress accumulation. The formation of the microcracks can be monitored by the anodization current which shows sudden jumps when the cracks are formed. After removing the remaining aluminium at the core of the anodised wire the anisotropic and inhomogeneous alumina microtube is obtained. Such nanoporous alumina microtubes form unique optical waveguides and are useful for microscale heat transfer applications.