Antennas capture, upconvert weak IR light

Ashley N. Paddock, ashley.paddock@photonics.com

A new technique that uses special molecules as light antennas to harvest the energy from weak infrared light and amplify the process 3300 times could lead to improved medical imaging methods.

Materials scientists and chemists from the University of Groningen and from the FOM Foundation harvested infrared light – which has too little energy to release electrons in solar cells – more efficiently by modifying an organic dye that acts as light antennas to transmit the energy to the nanoparticles to which they are attached. These particles subsequently convert two weak captured photons into a single strong, energy-rich photon in a process called upconversion.

“There are inorganic materials made from rare-earth metals that can facilitate this upconversion process,” said Jan C. “Kees” Hummelen, a University of Groningen professor of organic chemistry and leader of the FOM focus group on next-generation organic photovoltaics. “However, these materials absorb very few infrared photons. We have therefore attached organic molecules to them [as antennas] that can capture these photons and transmit the energy to the upconversion material.”

Inspiration from nature: (left) a natural photosynthesis system with light-harvesting (LH) molecules and a reactive center (RC); (right) a schematic representation of the nanocrystal that realizes the upconversion (UC) with the attached antennas in green. Courtesy of University of Groningen.

Because of this, the entire infrared absorption process, upconversion and the emission of visible light is increased by a factor of 3300, Hummelen said.

Even with the antennas, his group can capture only a limited amount of infrared light. He predicts that an even better yield can be obtained, but because the upconversion process inside the nanocrystal is still inefficient, it is not yet possible to achieve.

“Two photons must come together in the material within a short space of time,” he said. “In practice, the efficiency of this process is still very low. However the harvest is already much better, so step one has been achieved.”

The upconversion system could be applicable for medical imaging techniques.

“Infrared light penetrates further into biological tissues than visible light,” he said. “If you allow compounds that carry out upconversion to bind to specific cells in tissues, then you can make images using infrared light.”

The scientists’ work also is applicable to solar cells, as about half of all the solar energy reaching Earth’s surface consists of infrared light.

The research was published online in Nature Photonics (doi: 10.1038/nphoton.2012.158).