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.)
No, that doesn't exist. Visible light has a wavelength much larger than your particles, so the waves will go right around them unimpeded
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.