Pump-power-dependence of a CsPbBr₃-in-Cs₄PbBr₆ quantum dot color converter

The detailed study of the pump-intensity dependent characteristics of an all-inorganic CsPbBr3-in-Cs4PbBr6 quantum dot (QD) color converter is reported. This is an attractive material to color convert UV/blue GaN optical pump sources for digital lighting applications, e.g. visible light communications (VLC). It demonstrates narrow spectral emission (522 nm peak emission and < 20 nm full-width-at-half-maximum), invariant with the pump power density (from 0 to 7.15 kW/cm2). The optical bandwidth increases, from 10 MHz at 300 μW/cm2, up to 22 MHz at 3 W/cm2, and 41 MHz at 7.15 kW/cm2. This acceleration of the emission is ascribed to both an increase of the radiative decay rate in the low-pump-density regime, and to the emergence of non-radiative pathways at higher pump density. The higher bandwidth at high-pump density enables a 30% increase in the data rate of a free space VLC link using this color converter

Slow relaxation, confinement, and solitons

Millisecond crystal relaxation has been used to explain anomalous decay in doped alkali halides. We attribute this slowness to Fermi-Pasta-Ulam solitons. Our model exhibits confinement of mechanical energy released by excitation. Extending the model to long times is justified by its relation to solitons, excitations previously proposed to occur in alkali halides. Soliton damping and observation are also discussed

Influence of Annealing on the Optical and Scintillation Properties of CaWO4_4 Single Crystals

We investigate the influence of oxygen annealing on the room temperature optical and scintillation properties of CaWO$_4$ single crystals that are being produced for direct Dark Matter search experiments. The applied annealing procedure reduces the absorption coefficient at the peak position of the scintillation spectrum ($\sim430$ nm) by a factor of $\sim6$ and leads to an even larger reduction of the scattering coefficient. Furthermore, the annealing has no significant influence on the \emph{intrinsic} light yield. An additional absorption occurring at $\sim400$ nm suggests the formation of O$^-$ hole centers. Light-yield measurements at room temperature where one crystal surface was mechanically roughened showed an increase of the \emph{measured} light yield by $\sim40$ and an improvement of the energy resolution at 59.5 keV by $\sim12$ for the annealed crystal. We ascribe this result to the reduction of the absorption coefficient while the surface roughening is needed to compensate for the also observed reduction of the scattering coefficient after annealing