CERAC Coating Materials News - Supplement

Breakthrough in Thin Film Coating?

There has been great interest in the recent announcement of an omnidirectional reflector in Science with the belief that something unknown and completely new in thin film theory and materials has been discovered [3]. What the 8 MIT researchers reported was the design and construction of a reflector that maintains very high reflectance over a very wide range of incidence angles. Applications would include heat retention in a living space and consequent energy savings.

 

In reality these solid-state physicists merely reinvented what thin film engineers have known, namely that the combination of quarter-wave layers of alternating high and low index produces such an angle-independent reflector. The larger the index contrast is, the higher is the reflectance for the layer count. Fink, et al. used tellurium (n = 5) and polystyrene (n = 1.5) layers. Figure 2 demonstrates how just such an omnidirectional reflector is constructed using 9 layers of PbTe (n = 5.5) and IRB™ (n = 1.45), materials used in IR filters and other coatings. Reflectance is found to be greater than 99% over a large reflection zone for angles of <30° to >80°.

 

(click on image to enlarge)

 

Figure 2: Omnidirectional reflector consisting of nine QW layers of PbTe and IRB™. Reflectance remains above 99% over 5 um bandwidth for all incidence angles.

 

Prof. Angus Macleod has explained very well how such a “discovery” can arise in one discipline, but is well known and commonplace in another [4]. The terminology used is different between solid-state and optical disciplines, but the physics of multi-layers is the same. (Where was the reviewer?).

References

1. Samuel F. Pellicori and Herbert L. Hettich, Appl. Opt. V27, No. 15, 3061 (1988).
2. S. F. Pellicori and E. Colton, Thin Solid Films, 209, 109 (1992).
3. Fink, Yoel, Winn, Fan, Chen, Michel, Joannopolous, Thomas, Science, 282, 1679 (1998).
4. Angus Macleod, Macleod Medium, V7, No. 1 (1999).

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Dr. Mitchell C. Colton, Editor
CERAC, inc.
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Samuel Pellicori, Principal Contributor
Pellicori Optical Consulting
P.O. Box 60723 | Santa Barbara, CA 93160
Phone/FAX: 805-682-1922

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