2 research outputs found
Random Lasing at Localization Transition in a Colloidal Suspension (TiO<sub>2</sub>@Silica)
Anderson localization
of light and random lasing in this critical
regime is an open research frontier, which besides being a basic research
topic could also lead to important applications. This article investigates
the random laser action at the localization transition in a strongly
disordered scattering medium composed of a colloidal suspension of
coreāshell nanoparticles (TiO<sub>2</sub>@Silica) in ethanol
solution of Rhodamine 6G. The classical superfluorescence band of
the random laser was measured separately by collecting the emission
at the back of the samples, showing a linear dependence with pumping
fluence without gain depletion. However, frontal collection showed
saturation of the absorption and emission. Narrow peaks of approximately
equal intensity are observed on top of the classical superfluorescence
band, indicating suppression of the interaction between the peaks
modes. The linewidth of these peaks is lower than that of the passive
modes of the scattering medium. A method called fraction of absorbed
pumping allowed us to infer that this peakās mode (localized
modes) is confined to a shallow region near the input-pumping border
Tb<sup>3+</sup>āEu<sup>3+</sup> Energy Transfer in Mixed-Lanthanide-Organic Frameworks
In this work, we report a theoretical and experimental
investigation
of the energy transfer mechanism in two isotypical 2D coordination
polymers, <sub>ā</sub>[(Tb<sub>1ā<i>x</i></sub>Eu<sub><i>x</i></sub>)Ā(DPA)Ā(HDPA)], where H<sub>2</sub>DPA is pyridine 2,6-dicarboxylic acid and <i>x</i> = 0.05
or 0.50. Emission spectra of <sub>ā</sub>[(Tb<sub>0.95</sub>Eu<sub>0.05</sub>)Ā(DPA)Ā(HDPA)] and <sub>ā</sub>[(Tb<sub>0.5</sub>Eu<sub>0.5</sub>)Ā(DPA)Ā(HDPA)], <b>(1)</b> and <b>(2)</b>, show that the high quenching effect on Tb<sup>3+</sup> emission
caused by Eu<sup>3+</sup> ion indicates an efficient Tb<sup>3+</sup>āEu<sup>3+</sup> energy transfer (ET). The <i>k</i><sub>ET</sub> of Tb<sup>3+</sup><i>ā</i> Eu<sup>3+</sup> ET and rise rates (<i>k</i><sub>r</sub>) of Eu<sup>3+</sup> as a function of temperature for <b>(1)</b> are on
the same order of magnitude, indicating that the sensitization of
the Eu<sup>3+Ā 5</sup><i>D</i><sub>0</sub> level is highly
fed by ET from the <sup>5</sup><i>D</i><sub>4</sub> level
of Tb<sup>3+</sup> ion. The Ī·<sub>ET</sub> and <i>R</i><sub>0</sub> values vary in the 67ā79% and 7.15 to 7.93 Ć
ranges. Hence, Tb<sup>3+</sup> is enabled to transfer efficiently
to Eu<sup>3+</sup> that can occupy the possible sites at 6.32 and
6.75 Ć
. For <b>(2)</b>, the ET processes occur on average
with Ī·<sub>ET</sub> and <i>R</i><sub>0</sub> of 97%
and 31 Ć
, respectively. Consequently, Tb<sup>3+</sup> ion is
enabled to transfer energy to Eu<sup>3+</sup> localized at different
layers. The theoretical model developed by Malta was implemented aiming
to insert more insights about the dominant mechanisms involved in
the ET between lanthanides ions. Calculated single Tb<sup>3+</sup>ā Eu<sup>3+</sup> ETs are three orders of magnitude inferior
to those experimentally; however, it can be explained by the theoretical
model that does not consider the role of phonon assistance in the
Ln<sup>3+</sup><i>ā</i> Ln<sup>3+</sup> ET processes.
In addition, the Tb<sup>3+</sup>ā Eu<sup>3+</sup> ET processes
are predominantly governed by dipoleādipole (dād) and
dipoleāquadrupole (d<i>ā</i>q) mechanisms