High Damage Threshold Gratings using Coated Silicon Substrates.

We are currently investigating the manufacture of high damage threshold gratings using a novel idea. We are placing high reflectivity coatings on holographically patterned Silicon grating blanks. This approach provides four principle advantages compared to current gratings made by placing a thin metallic layer over holographically patterned photoresist on glass substrates. First, groove shapes can be optimized because lithographic and etching techniques for Si can make almost any arbitrary profile while retaining holographic registry across large surfaces. Second, photoresist which damages at 100-200° C is replaced with metal-silicide or silicon with much higher damage thresholds. Third, Si efficiently conducts heat away from the surface whereas the glass substrates prevent cooling. Fourth, enhanced metallic and dielectric coatings, which can be made nearly 99% reflecting, can be used when the groove profile is blazed with flat surfaces (ie., triangular groove). These four advantages make us believe damage thresholds comparable to those of bulk metals (~ 300 mJ/cm2) are obtainable.