I thought there might be a way to use constructive and destructive interference of a large number of sources to build an open air 3d hologram out of pure light. If light didn't have to travel in a straight line, and we could control the index of refraction at any point in space, this would probably work.
Standing sound waves could achieve this. Albeit, the change in refractive index is small.
Another possibility would be using molecules having a high refractive index when exited. A laser tuned to that would probably work. But that's not exactly in air.
