OPTICAL PERFORMANCE OF RANDOM ANTIREFLECTION STRUCTURED SURFACES ON OPTICAL FLATS AND DIFFRACTIVE COMPONENTS
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Abstract
Random anti-reflection surface structures (rARSS) are non-periodic, densely packed, sub-wavelength structures, fabricated on an optical surface. Light incident to such a boundary propagates from superstrate to substrate through a gradient effective refractive index profile, where the gradual transition reduces the reflective losses and increases the transmission through the interface. The present dissertation mainly discusses anti-reflection (AR) treatments on optical components by the implementation of rARSS. Two fundamentally different rARSS fabrication techniques on planar substrates are featured, for applications as optical windows in the visible and infrared (IR) wavelengths. The rARSS were fabricated on fused silica (FS) flat optical windows, which were first masked by a discontinuous metal layer, then followed by etching the substrate in a reactive-ion (RIE) plasma process. The physical characteristics of the rARSS are presented, such as, the effective depth and lateral dimensions of the random structures, and the resulting spectral transmission performance. It was found that rARSS on FS act as a broad-band anti-reflective treatment. Following, a different technique to create rARSS on Cleartran Zinc Sulfide (ZnS) is presented, by irradiating the surface with a high-power, nanosecond-duration pulsed laser, resulting in localized sputtering and re-deposition in atmospheric conditions. The surface is characterized by measuring the structure’s height and lateral dimensions. The surface is then analyzed for presence of any contaminants, like zinc oxide (ZnO), due to the atmospheric conditions. The optical performance tests show that rARSS on Cleartran ZnS acts as an AR treatment in the IR wavelength region as well. Finally, the plasma-etching method, used previously on the planar optical silica windows, was transferred to transmissive silica binary diffraction gratings. Two pre-fabricated commercially available transmission gratings were used to investigate the rARSS effects on their surfaces. A comparison study of the performance of the original gratings (unprocessed) and the rARSS enhanced gratings is done, using a multi-wavelength He-Ne laser (594nm, 612nm and 633nm), to measure the propagating diffracted order angles and, individual reflection and transmission diffraction efficiencies of all non-evanescent orders. The diffracted beams profiles were measured, to quantify any effects after rARSS fabrication. Tests were also performed to measure the diffraction efficiency at variable angles of light incidence (AOI), from 0° to 70°, to determine the rARSS performance for AOI greater than 40° and, were compared to a single-layered AR coated grating simulation. It is shown that the fabrication of rARSS on pre-existing binary FS gratings was possible and produced the desired reduction in diffracted reflection efficiency, and enhancement of the total diffraction transmission efficiency, while maintaining the original diffractive properties of the pre-fabricated gratings. The work verifies that rARSS are applicable to optical windows, as well as, diffraction gratings, increasing their spectral transmittance, without any component performance degradation.