TEMPLATE SYNTHESIS OF INFRARED-TRANSPARENT METAL MICROCYLINDERS - COMPARISON OF OPTICAL-PROPERTIES WITH THE PREDICTIONS OF EFFECTIVE MEDIUM THEORY

Metal-insulator composites of varying metal volume fraction have been prepared by electrochemical deposition of gold into porous aluminum oxide membranes. The cylindrical pore array structure of the host oxide serves as a template for the formation of Au particles ca. 0.26 mum in diameter with lengths ranging from 0.3 to 3 mum depending on the deposition time. The composites display a significant transparency in the infrared spectrum between 2000 and 4000 cm-1. The Au volume fraction and effective medium theory screening parameter kappa were estimated from scanning electron microscopic analyses of cylinder dimensions and orientations in the composite membranes. Comparison of experimental spectra with those calculated using Maxwell-Garnett or Bruggeman theories indicates that neither approach is entirely satisfactory. The spectra are consistent, however, with retarded polarization effects due to nonnegligible Au particle separation distances in the composites.