Converting Infrared radiation into visible light

Jeppe Seidelin Dam and colleagues at the Technical University of Denmark in Roskilde are developing a device that can convert infrared radiation into visible light. Attached to a digital camera fitted with an infrared flash, it could detect tumours by recording the telltale pattern of infrared light they reflect.

"This would allow a surgeon to quickly determine if the entire tumour has been removed before finishing an operation," he says.

At the heart of the system is a multilayered crystal of potassium titanium oxide phosphate in which the infrared photons from the object to be imaged interfere with photons from an infrared laser, also fired into the crystal. The interaction shifts the wavelength into the visible spectrum while preserving the image information, allowing it to be captured by a normal camera.

Mirror amplifiers

The idea was first explored in the 1970s, but improvements to methods for growing crystals since then have improved the resolution of the device 300-fold. By placing a pair of mirrors on either side of the crystal so that the laser light reflects back and forth, the team increased the odds of its photons interfering with infrared photons from the object.

"We pass the same photons through the crystal up to 100 times," says Dam. The crystal was able to capture an infrared panorama with a resolution of 200 by 1000 pixels, the team says.

The device could be placed in front of a digital camera lens like a filter, and be used to take thermal photographs or video. Shrinking it down to a size suitable for everyday use should not be difficult, says Dam. "These are basically the same components that are in green laser pointers."

While current infrared colour imagers need to run at -200°C and cost around $100,000, Dam says that an upconversion imager would run at room temperature and cost about $10,000.

Stefano Bonora of the University of Padua, Italy, calls the upconversion technique "really interesting" for its potential to generate infrared images at room temperature. Such detectors are lacking at the moment, he says.

Journal reference: Optics Letters, DOI: 10.1364/OL.35.003796