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Practically all thin film systems for normal incidence can be realized using only two-layer materials. But for oblique incidence, polarization effects occur, designs may become complex, and polarization control is difficult or impossible to achieve. Here multi-index or gradient designs offer additional degrees of freedom, and can simplify or even enable challenging designs. Such gradient thin film stacks can be designed ab initio without any start or index profile approximations using a new design software developed by Carl Zeiss. With this software, a rugate omnidirectional AR coating was calculated and transferred to three different multi-index systems. All three examples were realized using ion beam sputter technology, and characterized at Laser Zentrum Hannover. Here we present comparative measurements of the optical performance together with femtosecond laser-induced damage threshold measurements.

Antireflective subwavelength structures (ARS) resembling nanostructures found on the cornea of night-active insects reduce the reflection of light by providing a gradual change in the refractive index at the interface. These artificial ARS have mainly been fabricated by a combination of conventional lithography and reactive ion etching, which constrains their application to planar substrates. We report on the fabrication of ARS using three different techniques including bottom-up and top-down methods as well as their combination on microlens arrays (MLAs) made of fused silica. The optical performance of the resulting ARS on the MLAs is as good as ARS fabricated on planar substrates with increased transmission of up to 96% at certain wavelengths., (© 2012 Optical Society of America)