Laser based listening devices work by bouncing the beam off of a window. Sounds in the room cause minute vibrations in the window. These vibrations modulate the laser beam. The laser beam is then converted back into sound at the receiver.I think I know why my psycho-neighbor is so paranoid about infrared light now. Apparently invisible, near IR light is one of the types of light that works well for these things.
I doubt I would do it, but it's amusing enough that I'd almost be tempted to pick up a strong IR light and point it in the general direction of his windows.