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Neuartiger Mechanismus in Nanokristallen erlaubt Anregung einer langlebigen Emission im sichtbaren Spektralbereich

Novel mechanism in nanocrystals allows excitation of long-lived emission in the visible spectral range

Publication is cover article of current issue of Nanoscale Advances

Ions of the rare earth chemical elements are prominent and efficient photon emitters. Because of the long lifetime of their excited states in the micro- and milli-second range, they often serve as an active medium in laser physics. This lifetime is largely determined by the local field surrounding the ions. In macroscopic crystals, it is constant, so there is little room for application-specific adjustment of the lifetime. This situation changes drastically when the size of the crystals is reduced to a few nanometers, because the field of the surrounding medium also influences the behavior of the ions.

A team of physicists from chemists from the Fraunhofer Center for Applied Nanotechnology and our group Laser Components and Fibres at the Leibniz University Hannover has now succeeded in reducing the size of praseodymium-doped LiYF4 crystals to 10 nm for the first time. We have now published these results in the scientific journal Nanoscale Advances and were awarded to contribute a cover.

Praseodymium-doped LiYF4 bulk crystals are well-known as a laser medium and show several emission lines in the visible spectral range. A comparison with the spectral behavior of the bulk crystals showed that their nanoscale counterparts have an intense, additional emission line. We were able to show that this light is emitted from a subjacent excitation state that is efficiently populated by multiphonon processes. These processes occur mainly on the surface area of the nanocrystals, because here the energy is released particularly efficiently into the surrounding medium. An investigation of the lifetime of the thus populated state showed that the lifetime decreases exponentially with the number of ions. With a size of about 10nm and an average ion number of 13, the lifetime is exceeding 100µs.

Excitation states with long lifetimes are particularly advantageous for the development of lasers because irradiated pump light is efficiently stored by these states in the active medium. The newly discovered excitation mechanism could therefore be used to develop efficient composite lasers in the future. Furthermore, the long lifetime of these nanocrystals could be an interesting alternative to existing quantum storage systems based on rare earth ions.


Original publication:
Rajesh Komban, Simon Spelthann, Michael Steinke, Detlev Ristau, Axel Ruehl, Christoph Gimmler, Horst Weller
Bulk-like emission in the visible spectrum of colloidal LiYF4:Pr nanocrystals downsized to 10 nm
Nanoscale Adv., 2022, 4, 2973-2978
DOI: 10.1039/d2na00045h