(Invited) Recent Findings on the Continuous White Light Emission Induced By Infrared Laser Irradiation
Authors: González Federico , Khadka Rabindra , Lopez Rigoberto , Collins John , Di Bartolo Baldassare 
Published: February 2020

The generation of white-light (WL) in nanostructured materials undoped and doped with lanthanides using infrared lasers, has been a subject of different research studies. Despite all this effort, some important questions remain open, such as the origin of the WL; the effect of the crystallite size and the role played by some dopant ions on its generation, and if the phenomenon is intrinsic or not to nanostructured systems. In order to get more insight into these questions, we synthesize undoped, and Yb-doped powders and pellets based on the Y4Zr3O12 compound. In the case of the pellets, Pr-doped samples were also synthesized. Crystallite size was tuned by changing the annealing temperature (800 °C and 1100 °C for powders and 1550 ° for pellets), ranging from few to hundreds of nanometers. All the specimens investigated exhibit a cubic fluorite- like crystal structure. To produce the WL, the powders and pellets, at room temperature and most of them at a pressure of 0.5 Pa, were irradiated with a diode laser emitting at 975 nm. Only in Yb3+-doped compounds, WL generation is observed. Emission spectra of WL were recorded in the range from 425 nm to 880 nm, and corrected for the wavelength dependent response of the spectrometer/detector system. Emission spectra of the WL were successfully fitted by the Planck's law of blackbody radiation. This fact supports thermal radiation as the origin of the WL generation. The absolute temperatures resulting from fittings were plotted as a function of the laser’s power, and the observed behavior was explained in terms of the thermal conductivity and its changes associated with the microstructural modifications due to the exposition of the sample to the laser. Microstructural modifications were verified in powders annealed at 800 °C and 1100 °C by X-ray diffraction measurements. In accordance with previous reports, we detected a hysteresis of the emission intensity depending on whether the laser power is increasing or decreasing, and it seems also to be related to the microstructural modifications, because the effect diminishes in powder annealed at 1100 °C, and it is significantly reduced in the pellet. In the latter case, greater microstructural stability is expected when pellet is irradiated with the laser, since it was sintered at 1550 ° C. Regarding the Yb3+, it plays an important role in the WL generation, but in an intriguing way. In the low-power laser regime, the emission of Yb3+, ascribed to the transition 2F5/2 → 2F7/2 it is clearly seen, so that the ion is being excited by the laser. This result is logical because the laser matches the energy difference between the levels 2F7/2 and 2F5/2 of Yb3+. However, if a power laser threshold, which depends on each sample, is exceeded, WL starts to be generated. Then, an elusive mechanism is allowing the delocalization of the energy, originally residing in a localized way at Yb3+, which gives place to the continuous WL emission. In this regard, some preliminary results observed in Yb-Pr-co-doped pellets seem to provide a clue. When Pr3+ is incorporated into Y4Zr3O12, it tends to oxidize. Thus, the presence of Pr4+, introduces a ligand-to-metal charge transfer (LMCT) state associated with the strong O2--Pr4+ interaction, and clearly noticed by the change in the color of the pellets containing Pr. This state, having an intermediate energy of that of the band gap, inhibits the generation of WL. Therefore, we hypothesize, defects like that one associated with the LMCT act as a sink, preventing the generation of WL. In summary, our findings are important, for the following reasons: 1) we demonstrate that the microstructure seems to play an important role in favoring the white light generation; 2) as far as we now, this is the first report about the production of white light in bulk ceramic oxides under NIR laser irradiation, and 3) the introduction of additional electronic states, in this case associated with the Pr, inhibits the generation of white light.