Rubidium Detector Converts Infrared Images Directly to Visible Light: These guys use a process known as four waving mixing, in which the interaction of three different wavelengths in certain media produces a fourth wavelength. This is usually done in non-linear crystals using high power lasers.
The trick these guys have pulled off is is to achieve this in a small container of rubidium gas using two ordinary diode lasers. The idea is that the lasers excite certain electronic states in the rubidium atoms. These states are chosen so that the atoms emit visible light when they relax.
But the system is set up so that the addition of a little extra infrared light triggers the emission of visible red light.