Surfaces would be textured with dense arrays of pillars characterized by micron and submicron dimensions, according to a proposal, in order to impart desired optical properties to the surfaces. In an important class of potential applications, suitably shaped and dimensioned microscopic pillars would be etched into the surfaces of lenses or photodetectors to suppress reflections and thereby also increase the proportion of light utilized. In another important class of potential applications, surfaces would be so textured in order to obtain both absorption and low reradiation in a wavelength range of interest.

This proposal is an extension of the one reported in "Optical Filters Based on Dense Arrays of Microscopic Pillars" (NPO-20448), NASA Tech Briefs, Vol. 24, No. 5 (May 2000), page 27a. To recapitulate: It has been observed that the eyes of moths reflect almost no light. It has been conjectured that the low-reflection property of moth eyes is attributable to dense arrays of microscopic pillars that exhibit little or no diffraction or scattering because (1) the dimensions and pitches of the pillars are smaller than the shortest wavelength of incident light in the wavelength range of interest and (2) a dense array of pillars provides a gradual transition in the effective index of refraction from open space to a bulk solid material, so that an abrupt index change, which would generate reflections, is not present.

Microscopic Pillars Would Be Densely Arrayed over a surface and would be shaped and dimensioned to impart desired optical properties. Mushroom-shaped micropillars would be more effective in preventing undesired radiation from the surface.

Going beyond the previously reported proposal, the present one calls for exploitation of the fact that a dense array of micropillars at a given temperature can absorb electromagnetic radiation predominantly in one wavelength range while reradiating predominantly in another (usually longer) wavelength range. For example, a baffle in a visible-light telescope could be textured with pillars shaped and dimensioned to maximize absorption of visible light. At a typical operating temperature, the black-body radiation from such a baffle would occur predominantly at wavelengths in the infrared region - out of the pass band of the telescope.

For another example, germanium micropillars with a pitch of about 1.5 µm would absorb infrared light at wavelengths in the vicinity of 1.5 µm and would reradiate predominantly at wavelengths >6 µm - the wavelength range that contains the peaks of black-body spectra for temperatures in the cryogenic range. Thus, the micropillar-textured germanium surface would behave somewhat as a radiative diode. It could be used, for example, to absorb solar infrared radiation for heating during the day. It would also help retain the heat during the night because it would reradiate only slightly, even though it would likely be warm in relation to its environment.

Surfaces textured with pyramidal, conical, and rectangular parallelepiped micropillars have been fabricated by use of holography. However, in order to resemble true moth-eye structures more closely and thereby afford more of the benefits of moth-eye structures, micropillars would have to be shaped more like mushrooms (see figure). It would be necessary to use x-ray lithography to fabricate arrays of mushroom-shaped micropillars. The large depth of focus achievable in x-ray lithography would make it possible to generate arrays of precise micropillars on curved surfaces, including concave and convex lens surfaces.

This work was done by Frank Hartley of Caltech for NASA's Jet Propulsion Laboratory.


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Optical Surfaces Based on Arrays of Microscopic Pillars

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This article first appeared in the January, 2001 issue of Photonics Tech Briefs Magazine.

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