Ions of the rare earth chemical elements are prominent and efficient photon emitters used in a crystalline matrix as an active medium for laser physics. Due to the long lifetime of their excited states in the micro- and milli-second range, they are also often used as a medium for quantum storage. This lifetime is largely determined by the local field surrounding the ions. In macroscopic crystals, it is constant, so there is hardly any scope for application-specific adjustment of the lifetime. This situation changes drastically when the size of the crystals is reduced to a few nanometres. Because then the field of the surrounding medium also influences the behavior of the ions.
A team of physicists from the Institute of Quantum Optics at the University of Hannover and chemists from the Fraunhofer Center for Applied Nanotechnology has now succeeded for the first time in reducing the size of praseodymium-doped LiYF4 crystals to 10 nm. They have now published their results in the scientific journal Nanoscale Advances.
These crystals are well studied in macroscopic form as a laser medium and show several emission lines in the visible spectral range. A comparison with the spectral behavior of the macroscopic crystals showed that their nanoscale counterparts have an intense, additional emission line. The scientists were able to show that this light is emitted from a low-energy excitation state that is efficiently populated by multiphonon processes. These processes occur mainly in the edge region 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 was over 100µs. These nanocrystals could therefore 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
