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.
NPO-20448
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Optical Filters Based on Dense Arrays of Microscopic
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Overview
This document discusses the development of high-transmission and antireflection optical filters inspired by the unique properties of moth eyes. The eyes of nocturnal insects, such as moths, reflect almost no light due to their specialized structure, which consists of dense arrays of microscopic pillars. These pillars are designed such that their dimensions and spacing are smaller than the wavelengths of light they interact with, effectively minimizing diffraction and scattering. This results in a gradual transition in density from open space to solid material, eliminating abrupt changes that would typically cause reflections.
The primary goal outlined in the document is to create an efficient broadband infrared-visible (and even ultraviolet) spectrometer that operates without moving parts and has reduced temperature dependence, thus eliminating the need for cryogenics. The proposed solution involves using a matrix of 'moth eye' arrays with varying dimensions to selectively transmit light of specific wavelengths to an underlying matrix of miniature thermoelectric devices or thermopiles. Each array would function as a high transmission bandpass filter, allowing only light above a certain cutoff wavelength to pass through, thereby enabling the construction of a filter/sensor spectrometer unit without mechanical components.
The document details the fabrication techniques required to create these structures, including holography for large area 'moth eye' textures and X-ray lithography for producing specific shapes and sizes of the microscopic pillars. The use of negative working X-ray resist and etching processes is also described, which allows for the precise formation of these pillar arrays.
Additionally, the document proposes the potential for variable-cutoff long-wavelength-pass filters. These would consist of micro-pillar arrays on a transparent piezoelectric substrate, where the cutoff wavelength could be adjusted by applying voltage to change the substrate's dimensions.
The work is attributed to Frank Hartley of Caltech, conducted for NASA’s Jet Propulsion Laboratory, and emphasizes the innovative application of biological structures in advancing optical technologies. Overall, the document highlights the potential for these moth-eye-inspired filters to significantly enhance the performance of optical instruments, particularly in spectrometry.
