Chemical diffusion treated volume holograms and methods for making the same

A method, comprising:

• making an optical grating having a non-uniform refractive index profile along a direction substantially perpendicular to a plane defined by the optical grating, including placing a layer of photopolymerization material having a first surface and a second surface that is opposite to the first surface between a first material transfer layer and a second material transfer layer so that the first surface of the layer of photopolymerization material is in contact with the first material transfer layer and the second surface of the layer of photopolymerization material is in contact with the second material transfer layer to allow material transfer between the layer of photopolymerization material and the first and second material transfer layers by diffusion;
• removing the first material transfer layer and the second material transfer layer from the layer of photopolymerization material;
• and
• exposing the layer of photopolymerization material to two beams after removing the first material transfer layer and the second material transfer layer from the layer of photopolymerization material.