Compound diffraction grating and method of manufacture

A method comprising:

• forming a substrate of a low refractive index material in a master mold;
• removing the substrate from the master mold to form a template having a periodic side that is continuous and comprises areas of high relief and areas of low relief that periodically alternate in a first direction with a first periodicity;
• and
• forming a high refractive index diffraction grating in the template by adding high refractive index material to the template to form a continuous low relief surface, the high refractive index material filling the areas of low relief of the template and covering the areas of high relief of the template to form the high refractive index diffraction grating in the template, wherein the high refractive index diffraction grating comprises the high refractive index material configured to have a low relief side corresponding to the continuous low relief surface and configured by the template to have a periodic side comprising areas of high relief and areas of low relief, wherein the areas of high relief and the areas of low relief of the high refractive index material periodically alternate in the first direction with the first periodicity and are interconnected by the high refractive index material, wherein the method comprises forming a high refractive index diffraction grating in the template by adding high refractive index material to the template using atomic layer deposition.