I would like a device to spread out/increase the angle of rays in a light source. In other words, I would like to reproduce the behaviour of light traveling from an area of higher to lower index of refraction. The problem with this simple method is that my light is coming from air, so there is no material I can find with a lower index of refraction. I investigated converging and diverging lenses but these bend on-axis rays to focus them. Instead, I want to leave on-axis rays untouched and spread everything else out, by a strong factor (multiplying the angle by at least a factor of 10).

My light source is 2.4GHz, nearly monochromatic, 5cm*5cm in spot size, and the rays are coming from all different directions.