The first demonstrations of upconversion were reported in fluorescence studies in the 1960s. Previously it had been largely understood that fluorescence light emitters should follow Stokes law, and that emitted photons should have lower energy than the incident photons as energy was lost in the process. As Francois Auzel described in his review in 2004, when he was working as a retired researcher at CNRS in France, this principle is, in a sense, an indirect statement that efficiency cannot be larger than one. However he added cannily, "This principle is valid, of course, only when one excited ion system is considered." In his review Auzel, one of the pioneers in the field of upconversion research, describes how coupled lanthanide, uranide and transition-metal ions embedded in solids, quite readily produce anti-Stokes upconversion emissions.

Since then upconversion processes have attracted a great deal of research activity not only for bulk but also for nanocrystalline systems. Localised control of optical properties could have uses in in data storage, anti-counterfeiting protection, novel bio-sensor sand reversible optical switching or irreversible optical latching.

Artur Bednarkiewicz and colleagues at the Polish Academy of Sciences, Wroclaw Research Centre and Wroclaw University of Technology recently demonstrated irreversible spatially confined infrared-laser-induced annealing of 8 nm NaYF4 nanocrystals, achieving a complete recrystallization to different phases and giant enhancement in luminescence yield. In their report they describe their results as "highly encouraging", concluding that "the upconverting ultra-small lanthanide-doped nanophosphors are particularly promising for direct laser writing applications." Yuansheng Wang and colleagues at the Chinese Academy of Sciences also studied some size effects noting some abnormal size-dependent upconversion emissions and multi-colour in lanthanide nanocrystals, which they expect will deepen understanding of these systems.

Applications of upconversion

Upconverting infrared radiation with fluorescent nanoparticles could be hugely useful in medical imaging as infrared can penetrate body tissues with minimal scattering and absorption. Sounderya Nagarajan and Yong Zhang from the National University of Singapore reported a study to determine the depths to which imaging can still be achieved using upconversion nanoparticles, demonstrating successful imaging as deep as 2 mm.

Other work has suggested that upconversion nanoparticles could be used in anti-counterfeiting procedures by printing QR codes that can only be read under incident infrared radiation. Jon Keller and colleagues at the South Dakota School of Mines and Technology and the University of South Dakota demonstrated an ink using lanthanide doped β-NaYF4 to print QR codes that could be read with the infrared laser on a mobile phone.

Photovoltaic and photocatalytic applications

The photovoltaic and photocatalytic applications of upconversion materials tackle a particularly topical technological demand as limited fossil fuel reserves and the need to harness alternative forms of energy continue to hit headlines. As Liu and colleagues point out in their review, "The energy conversion efficiency of current photovoltaic cells is far from satisfactory as they only respond to a relatively small fraction of the solar photons with energy higher than the threshold bandgap of the system." Their review highlights the important contribution lanthanides can make to tap into more of the sun’s resources outside this small fraction of the electromagnetic spectrum. The discussion includes some of the recent progress in technologies using upconversion nanomaterials integrated with photovoltaic and photocatalytic devices.

While the review provides some detailed descriptions of approaches to synthesis and fabrication of optoelectronic devices based on lanthanide-doped upconversion materials, Liu and colleagues also point out some crucial directions for future research that hark back to some of the fundamental processes studied since the 1960s. As the saying goes, nothing worth having comes easy, and the authors conclude, "The fundamental question of how the transfer of energy occurs between the upconversion nanocrystals and the sensitizers remains unresolved. Finding answers to this question certainly requires considerable, multidisciplinary efforts."