Angular selective window coatings: theory and experiments

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.

Anisotropic Bruggeman effective medium approaches for slanted columnar thin films

“…3) has been extensively used and applied to the analysis of experimentally acquired data of anisotropic thin films [11][12][13]18,20,22,[30][31][32][33] . This formalism will be called henceforth "traditional anisotropic Bruggeman effective medium approximation" (TAB-EMA).…”

Solar Energy Materials

“…[105,106] In this way, it is possible to take advantage of the fact that the indoors±outdoors contact across a window normally occurs along near-horizontal lines of sight, whereas solar irradiation comes from a small element of solid angle high up in the sky during most of the day. It is also possible to combine spectral and angular selectivity.…”

“…[105,106] Figure 12 illustrates this feature as computed for a five-layer stack of thin films, specifically with two 12 nm-thick Ag films embedded between three SiO 2 films with 120 or 170 nm thickness. Strong angular effects prevail at the larger of these thicknesses, and T sol is 23 % for normally incident light and as large as 58 % at an incidence angle of 60.…”

Nanostructured phothocatalytic TiO2 thin film fabricated by magnetron sputtering on glass

“…Reactive magnetron sputtering method is reliable to gain a large scale uniform and high adhesive coating on substrates. This technique is well known for its large degree of versatility 4,5 , as the microstructure can be nano-crystalline and columnar 6 . The fabricated layers have some problems that can be solved by adjusting the optimized parameters.…”