Fabrication and properties of chalcogenide IR diffractive elements

The authors report on techniques used to manufacture IR diffractive elements in chalcogenide glasses and on measurements of the material 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 the photodissolution of Ag into amorphous As-S films. Both surface relief and volume phase modulated transmission elements have been made. The transmission of Ag photodoped and undoped As-S films was found to be >80% over the range 2-12 micrometers for films up to 2 micrometers thick, the main loss mechanism being reflection. The difference in refractive index between Ag photodoped and undoped As-S over the range 0.5-12 micrometers was 0.5 for the most heavily doped material, so that high modulations are achievable for phase gratings. 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 undoped material with an alkali etchant (e.g., NaOH). For transmission gratings, any remaining metallic Ag must be removed, to avoid high losses: the most successful Ag etchant was found to be Fe(NO3)3 in water. For the bulk holographic transmission gratings produced, efficiencies of >33% were observed for first diffraction orders measured in air at 632.8 nm, the main loss mechanisms being absorption and reflection, with some scatter. Measurements at 1.5 micrometers have given efficiencies of >30%, stability requirements during holographic recording currently being the main limitation to higher efficiencies at these and longer wavelengths. The results of a theoretical analysis based on numerical solution of the appropriate coupled-wave equations and taking into account bulk losses with phase and 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 optimized bulk gratings and blazed zone plates.