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Recent results from numerical studies suggest that transversely distributed structures can be used to design nanoscale, binary phase, pseudo-randomly distributed structured surfaces (PDnS) that enhance transmission through dielectric optical windows. The PDnS are designed using deterministic rules, which allows minimumfeature dimensional control, repeatable uniformity, and some selection rules for transmitted intensity scatter profiles. Although the redistributed features within the PDnS unit cells are subwavelength in scale, numerical results indicate that the unit cells are not required to be subwavelength in size. This allows for customized surface correlated structures, with nearly zero root-mean-square surface (height) roughness. PDnS are in direct contrast to periodic subwavelength binary grating structures, which have constant periods, a single-phase transition within their unit cell, and are at least deep enough to result in π-phase shifted emerging wavefront segments. We chose a series of PDnS patterns to realize optical transmission enhancement above Fresnel limits, within a limited 2 μm wavelength bandwidth centered at 4 μm. To ease fabrication requirements, the designs used were restricted to a binary phase depth close to quarter-wave, and unit cell dimensions ranging from 4 µm to 6 µm. PDnS patterns were prototyped using two-photon-absorption direct laser-writing in a photosensitive polymer film supported by a silicon substrate. To investigate fidelity and tolerance of the candidate design, the PDnS patterns were characterized using a UV-laser confocal microscope. Unpolarized spectral transmission of the structure depth was measured using a spectrophotometer. The experimental results were compared to numerical predictions using rigorous coupled-wave analysis simulations.
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