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Rubidium Detector Converts Infrared Images Directly to Visible Light

Chinese physicists demonstrate a practical system for converting infrared images directly into visible ones.

The technology that makes infrared cameras possible are sensors that generate a current when struck by infrared photons. An array of these sensors can then be used to recreate an image.

But there’s a problem. Infrared detectors are notoriously inefficient. At longer wavelengths, they are swamped by noise and so have to be cooled, often with liquid nitrogen. That makes them complex, delicate and expensive to run.

By contrast, visible light sensors are efficient, robust and cheap. So one obvious solution is to find a way to convert infrared photons into visible ones so that an image can be made using a conventional array of pixels.

That’s easier said than done. The process of upconverting infrared photons is not straightforward and relies on high power lasers and exotic non-linear crystals. Just how this could be turned into a practical device that would outperform today’s infrared cameras isn’t clear, if it is possible at all.

Today, Dong-Sheng Ding and pals at the University of Science and Technology of China in Hefei have perfected a different technique that could change this.

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.

It’s easy to see that when this infrared light is from some external scenery, the result is a visible light copy of the scene which can be picked up by a conventional pixel array.

Ding and co have tested their idea by making images of a set of numerals created by passing infrared light through a mask. In the image above, the upconverted images are on the bottom row.

The technique clearly has some limitations, not least of which is the drop in resolution that this process causes. That’s largely because of the motion of rubidium atoms in the gas, which must be heated to 140 degrees C. 

Nevertheless, the technique has obvious potential. It’s relatively simple to construct a cell of hot rubidium gas and the lasers they use are relatively easy to handle. “Our experimental setup is very simple,” they say.

Clearly, this will pique the interest of a number of different groups. “We believe our research results would be very useful  in research fields in astrophysics, night-vision  technology, chemical sensing, quantum communication and so on,” they say.

Of these, military stakeholders will have the deepest pockets. The only question is how quickly it can be studied in more detail. 

Ref: arxiv.org/abs/1203.6132: Experimental Upconversion of Images

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