IR diffractive structures produced using novel photodarkening in chalcogenide glasses.

We report on techniques used to manufacture IR diffractive elements in chalcogenide glasses and on measurements of the materials properties relevant to the performance of these elements. The characteristics of the elements produced are also presented and compared with theoretical predictions. The fabrication process used is based on photodarkening of amorphous As-S films. Low loss, refractive index modulated and surface relief structures have been made, both binary, and analogue.

The transmission of As-S films was found to be > 90 % over the range 2 - 12 mu m for films up to 12 mu m thick, the main loss mechanism being reflection. Theory suggests that for these As-S materials, green illumination (e.g. 514.5 nm) is the most efficient for producing the deep structures required for many of these IR elements. Surface relief structures can be produced by removing the unexposed material with an alkali etchant (e.g. NaOH).

A blazed surface relief grating was produced with near optimum depth and profile for operation at the thermal waveband around 10 mu m. For the surface relief grating, efficiencies of > 23% were observed for.first diffraction orders measured in air at 632.8 nm the main loss mechanisms being absorption and reflection, with some scatter. For the bulk transmission gratings produced, efficiencies of > 40 % were observed. Measurements at 1.5 mu m have given efficiencies >15 %.

The results of a theoretical analysis based on numerical solution of the appropriate coupled-wave equations and taking into account bulk losses with phase absorption modulation, are in good agreement with the observed diffraction efficiency data. Given the low material absorption in the IR, theoretical studies show that, with suitable coatings, > 95% efficiency should be possible for properly optimised bulk and blazed zone plates.