Angular selective window coatings: theory and experiments

TLDR

Thin films with sub‑wavelength inclined absorbing regions exhibit angular selectivity, making them attractive for energy‑efficient window coatings in architecture and automotive applications. The optical behavior is modeled using rigorous dielectric bounds, effective‑medium theories for various microgeometries, and anisotropic thin‑film equations, allowing reconciliation of theory with experiment through plausible microstructural parameters. Experiments show evaporated Cr yields the greatest angular selectivity, reactively sputtered Al offers high luminous transmittance with moderate selectivity, Ti films are selective mainly in the infrared, and W films exhibit electrochromic angular selectivity, confirming that these behaviors can be explained by simple theoretical models.

Abstract

This review is devoted to the angular selectivity that can be obtained in thin films prepared under conditions such that they contain inclined absorbing regions of sizes much smaller than the wavelength of visible light. The films are of considerable interest as window coatings for energy-conscious architecture and, potentially, in the automotive sector. The theoretical basis for modelling the optical properties is presented, comprising rigorous bounds on the dielectric function, effective medium theories pertinent to different microgeometries and equations for treating the optics of anisotropic thin films. Experimental data are reported for films made by oblique-angle evaporation of Cr and for reactive and non-reactive oblique-angle sputtering of Cr, Al, Ti and W. The highest angular selectivity was obtained with evaporated Cr, whereas the highest luminous transmittance, combined with some angular selectivity, was found with reactively sputtered Al. Films made from Ti showed angular selectivity mainly in the infrared, whereas films made from W could display angular selective electrochromism. Samples of several types were subjected to elaborate theoretical analysis using effective-medium theories and it was seen that theory and experiment could be reconciled using plausible parameters to specify the microstructures of the films. Thus it appears that the angular, spectral and polarization dependences of obliquely deposited films can be understood, at least approximately, in terms of conceptually simple theoretical models.