Hey -- I have 0 PHDs so take this with a grain of salt :)

I had thought for a while about a way to store data that makes use of an idea that I had for sub-diffraction limited imaging inspired by STED microscopy.

First an overview of STED. You have a "donut" shaped laser (or toroidal laser) that is fired on a sample. This laser has an inner hole that is below the diffraction limit. This laser is used to deplete the ability of the sample to fluoresce, and then immediately after a second laser is shone on the same spot. The parts of the sample depleted by the donut laser don't fluoresce and so you only see the donut hole fluoresce. This allows you to image below the diffraction limit.

My idea was to apply this along with a layer in the material that exhibits sum frequency generation (SFG). The idea is that you can shine the donut laser with frequency A and a gaussian laser with frequency B at the same spot. When they interact in the SFG material you get some third frequency C as a result of SFG. Then, below that material would be a material that doesn't transmit frequencies C and A.

Then what you'd be left with after the light shines through those two layers is some amount of light at frequency B. The brightness inside the hole and outside of the hole would depend on how much of the light from frequency B converts into frequency C. Sum frequency generation is a very inefficient process, with only some tiny portion of the light participating, but my thinking is that if laser B is significantly less bright than laser A, then what will happen is that most of the light from laser B will participate in sum frequency generation where it mixes with laser A, and that you'll be left with only a tiny bit of laser A outside of the hole, so that you get a nice contrast ratio for the light at frequency A between the hole and the surroundings that then allow you to image whatever is below these layers below the diffraction limit.

In my idea the final layer is some kind of optical storage medium that can be be read/written by the laser below the diffraction limit. Obviously aiming this would be hard :) My idea was that it would be some kind of spinning disk, but I never really got to that point.