How to make windows for mid-infrared light more transparent?

What is it about?

When light enters an interface between air and a solid medium such as glass, part of the light is reflected. This reflection becomes more severe the higher refractive index the solid medium has. When designing optical elements for transmission of mid-infrared light, high index materials such as chalcogenide glasses are often used, which makes it even more challenging to couple the light in and out of the elements. The losses due to interface reflection can however be minimized by mimicking nature, and adding so called moth-eye structures, which are tapered close packed protrusions, to the the interfaces. When made with a center-center distance below about half of the wavelength of the light considered, they have the effect of making the transition between refractive indices of air and the solid medium gradual, and thereby reducing reflections considerably. Here we demonstrate how such moth-eye structures can be imprinted directly on the surfaces of chalcogenide glass optical windows for transmission of mid-infrared light.

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Why is it important?

Direct imprinting of anti-reflective moth-eye structures in chalcogenide glass windows will pave the way for tailoring the transmission enhancement at specific spectral ranges for chalcogenide glass-based applications, such as chemical sensing, nonlinear optical devices, supercontinuum generation, and thermal imaging. Chalcogenide glass materials are perfectly suited for thermal imprinting by having relatively low glass transition temperatures, and feature fabrication of both atmospheric transparent windows at 3–5 μm and 7–14 μm as well as the molecular fingerprint region at 6–20 μm.

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