A proposal for development of a class of high-transmission and antireflection optical filters is derived from the observation that the eyes of moths reflect almost no light, regardless of the wavelength or the angle of incidence of illumination. The low-reflection property of moth eyes is attributable to dense arrays of microscopic pillars that exhibit little or no diffraction or scattering. This is 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 density from open space to a bulk solid material, so that an abrupt density change, which would generate reflections, is not present.

Long-wavelength-pass filters based on this principle could be used in spectrometers and possibly other optical instruments. For example, one could build a matrix of arrays of microscopic pillars over a matrix of miniature thermoelectric devices, thermopiles, or other detectors. The pillars in each array would have a size and pitch corresponding to a specific cutoff wavelength. Each array would transmit (almost totally) only light at wavelengths greater than its cutoff wavelength to the underlying detectors. The multiplicity of cutoff wavelengths associated with the arrays in the matrix would define increments of a spectrum; thus, in effect, one would have a filter/sensor spectrometer unit with no moving parts.

Large surfaces textured with moth-eye textures have been fabricated by use of holography. To fabricate arrays of microscopic pillars with specific sizes and shapes (or to fabricate matrices of such arrays with different pitches for different cutoff wavelengths), it would be necessary to use x-ray lithography. For example, one would expose a negative x-ray resist to x-rays through a density mask containing nanometer-scale features created by electron-beam milling. The pillars would then be formed as the material left after etching of the exposed resist.

It has also been proposed to make variable-cutoff long-wavelength-pass filters. Such a filter would consist of an array of suitably shaped, sized, and pitched micropillars on a transparent piezoelectric substrate. The cutoff wavelength would be varied by applying a voltage to expand or contract the substrate.

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


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Optical Filters Based on Dense Arrays of Microscopic

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This article first appeared in the May, 2000 issue of Photonics Tech Briefs Magazine.

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