I'm looking for an extremely sensitive photosensor. I have a setup where a beam of light of some sort (weak laser, or something else) will be shining into the photosensor. Every now and then, a molecule with a diameter of about 1.0 - 1.3 nanometers will pass between the light source and the photosensor, and the photosensor must be able to detect the difference. Does such a photosensor exist, and if so, where could I get one? (The cheaper, the better.)
Some very sensitive PIN diodes can be used as Geiger counters. The signal from the PIN diodes must be amplified a lot to detect radiation particles. Even with a weak light it should be able to detect dust particles flying by, but that is nowhere near the molucules that you want to detect.