rapid communications This section was established to reduce the lead time for the publication of Letters containing new, significant material in rapidly advancing areas of optics judged com­pelling in their timeliness. The author of such a Letter should have his manuscript reviewed by an OSA Fellow who has similar technical interests and is not a member of the author's institution. The Letter should then be sub­mitted to the Editor, accompanied by a LETTER OF EN-

Aluminum mirrors AI2O3-protected, with high reflectance at normal but greatly decreased reflectance at higher angles of incidence in the 8-12-µm region

J. Thomas Cox and Georg Hass U.S. Army Electronics Command, Night Vision & Elec­tro-Optics Laboratories, Fort Belvoir, Virginia 22060. Received 19 November 1977. Sponsored by W. R. Hunter, U.S. Naval Research Labo­ratory. 0003-6935/78/0201-0333$0.50/0. © Optical Society of America.

In a recent paper1 it was reported that aluminum mirrors protected with thin layers of silicon oxides (SiO, SiOx, and Si02, t ~ 1500-2000 Å) have in the 8-12-µm region practically the same high reflectance as unprotected Al at close to normal incidence but much lower reflectance at angles greater than 40°. This makes this type of mirror unsuitable for devices using 45° or scanning reflecting optics. This effect was found to be most severe when the optical constants of the protective layer had values of n less than 1 and k between 0.2 and 0.8 (N = n – ik). Further analysis which has been carried out and experimental coatings which have since been made are in complete agreement and corroborate the theoretical predic­tions. In addition, it has been found that the effect is not peculiar to aluminum but is essentially the same for any highly reflecting metal and depends almost entirely on the optical constants of the protective layer.

In searching for a protective material which would not re­duce the reflectance in the 8-12-µm region at high angles of incidence, A12O3 appeared to be promising. First, it not only forms hard and adherent layers on aluminum2-3 but it is also one of the few protective coatings which adheres strongly to silver.4,5 Second, the optical constants of A12O3 films reported in the literature6 in the 8-12-µm region (n = 1.4-1.7, k = 0.0-0.7) should not cause any considerable decrease in re­flectance for Al2O3-overcoated mirrors at higher angles of incidence in this wavelength region.

DORSEMENT FROM THE OSA FELLOW (who in effect has served as the referee and whose sponsorship will be indicated in the published Letter), A COMMITMENT FROM THE AUTHOR'S INSTITUTION TO PAY THE PUBLICATION CHARGES, and the signed COPYRIGHT TRANSFER AGREEMENT. The Letter will be published without further refereeing. The latest Di­rectory of OSA Members, including Fellows, was published in the Spring 76 issue of Optics News.

However, there were two other experimental results which cast some doubt on the validity of the above reported optical constants. One was the measurement of total hemispherical and normal emittance of aluminum coated with thin layers of Al2O3.

7 These measurements showed that, for an Al2O3 thickness of 2500 Å, the hemispherical emittance is three times as high as the normal emittance. This implies reflec­tances at high angles of incidence which must be lower than what would be predicted from the values of n and k of alu­minum oxide reported in the literature.

The other measurement was that of the reflectance of sapphire plates in the 8-12-µm region. Between 9.5 µm and 10 µm the measured reflectance was nearly zero. This could only occur if the value of n were close to 1 and k remained very small. At λ > 10 µm, n should decrease to values of less than 1 since sapphire has a reststrahlen reflectance maximum at about 14 µm. The values in Ref. 6 are in contradiction to this.

To decide finally if A12O3 would be a useful protective layer for front surface mirrors at high angles of incidence in the 8-12-µm region, aluminum films were coated with A12O3 layers of three different thicknesses (1000 Å, 1750 Å, and 2300 Å) by electron gun evaporation in high vacuum.3 The reflectance of the samples was measured with unpolarized radiation in the 7-15-µm region at close to 0°, 40°, and 60° angles of inci­dence. The reflectance measured at 0° angle of incidence was essentially the same as that of uncoated aluminum. Table I gives the reflectance at 0°, 40°, and 60° for unprotected Al and for Al coated with three thicknesses of A12O3. The re­flectance of unprotected Al was calculated from optical con­stants given in the literature,8 and the reflectance of Al coated with A12O3 was measured at the various angles of incidence with a specially designed accessory for a double-beam spec­trophotometer. Note that the reflectance of the uncoated Al remains high through the entire region 7-15 µm at all three angles of incidence; while in the case of Al + A12O3, even for 40°, the reflectance is seriously reduced, and the reduction is considerably greater at 60°. The reflectance has its lowest value between 10.6 µm and 10.8 µm. For the previously re­ported mirrors coated with silicon oxides the minimum re­flectances occurred at shorter wavelengths (λ = 8.1 µm for

1 February 1978 / Vol. 17, No. 3 / APPLIED OPTICS 333

Table I. Reflectance of Ala and Al + Al2O3 at 40° and 60° Angle of Incidence in the 7-15-µm Wavelength Region

a Calculated reflectance from optical constants given in Ref. 8.

SiO2 and 8.8 µm for SiO) since their reststrahlen reflectance maxima are located at shorter wavelengths. Therefore, the region where the n values of silicon oxides are less than 1 occur also at shorter wavelengths. Another important difference is that the low reflecting region is much broader for A12O3

mirrors than it is for the silicon oxide ones. For example, a mirror of Al + SiO used at 60°, for which the SiO is 1500 Å thick, has a reflectance less than 90% from about 8.3 µm to 9.7 µm with a minimum value of 76.5% at 8.8 µm. A similar coating of Al + 1750 Å of A12O3 has at 60° angle of incidence a reflectance less than 90% from 9.5 µm to almost 14 µm with a minimum reflectance of 67% at 10.7 µm.

It should also be mentioned that in the regions where Al2O3

and silicon oxide protected mirrors show low reflectance at large angles of incidence, the reflected ir radiation is highly polarized since the reflectance decrease is entirely caused by the decrease of the Rp component, while Rs slightly increases with increasing angles of incidence (see tables in Ref. 1).

It is, therefore, concluded that thin Al2O3 protection coat­ings are not suitable for mirrors requiring high reflectance or low polarization at high angles of incidence in the ir from 8 µm to 12 µm.

References 1. J. T. Cox, G. Hass, and W. R. Hunter, Appl. Opt. 14, 1247

(1975). 2. G. Hass, J. Opt. Soc. Am. 39, 532 (1949). 3. J. T. Cox, G. Hass, and J. B. Ramsey, Jr., J. Phys. (Paris) 25, 250

(1964). 4. G. Hass, J. B. Heaney, and J. J. Triolo, Opt. Commun. 8, 183

(1973). 5. G. Hass, J. B. Heaney, H. Herzig, J. F. Osantowski, and J. J. Triolo,

Appl. Opt. 14, 2639 (1975). 6. L. Harris, J. Opt. Soc. Am. 45, 27 (1955). 7. G. Hass, J. B. Ramsey, Jr., J. J. Triolo, and H. T. Albright, in

Progress in Astronautics and Aeronautics, G. B. Heller, Ed. (Academic Press, New York, 1966), Vol. 18, pp. 47-60.

8. D. E. Gray, Ed., American Institute of Physics Handbook (McGraw-Hill, New York, 1972), pp. 6-124 and 6-125.

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