Evaporation of levitating microdroplets containing nanoparticles: luminescent upconverting Gd2O3:Er3+, illumination with 805 nm laser from below via optical fiber, collection with cooled-sensor spectrometer

Microdroplets of suspension were levitated in a linear quadrupole trap and studied similarly as described, e.g. in Y. Shopa et al., JQSRT 296 (2023), but illumination with an 805 nm diode laser (CW, ~1.5 W), which excited luminescence by up-conversion, was provided (vertically) from below via a 200 micrometer-core fiber for circularization. Its polarization was p. In some experiments, the droplet was additionally illuminated with a 515 nm diode laser with p-polarisation. The suspended phase consisted of nanoparticles of Gd2O3:Er3+ or Gd2O3:Nd3+ of several concentrations. The continuous phase consisted of diethylene glycol (2EG). The data is divided into the corresponding folders. In this series of experiments, a grating spectrometer with a cooled sensor was used (Simtrum MC/400-930), which enabled shorter integration times. Two groups of spectral lines were visible (green (stronger) and red). We followed the temporal evolution of several selected lines, both in luminescence and in scattering at right angle (in this series of experiments, usually 515, 523, 538, 550, 554, 560, 563, 630, 654, 661, 682, 805 nm): files *-Spectra.csv, where * (in hh_mm_ss format) always denotes the time of measurement file creation. Simultaneously, the shadow image of the droplet was recorded (non-coherent light, 630 nm LED) - in some cases, a movie: *_Shadow-*.avi is provided. The droplet radius was measured by fitting an ellipse to the shadow edge. The ellipse parameters' temporal evolution is stored in *-Shadow.csv files: time, mean diameter (mean of the ellipse axes), long axis rotation, X and Y of the centre, long and short axis, border length, X and Y of “the centre of mass” of the border, fitting error. The microdroplet's vertical position was stabilised with a DC field. The corresponding voltage variation and other stabilisation system parameters are stored in *.csv files: KeyStab: 0 – stabilisation off, 2 – on, k_PID – proportionality parameter of the software voltage regulator, freq – trapping AC field frequency – usually has no sense in this series of measurements, since it was provided from a manually controlled generator. Time stored in the corresponding files was synchronised. In-focus movies (Images_*.avi, see readme.txt for details) of the evaporating microdroplet were recorded for certain experiments to illustrate its evolution – no synchronisation is available. It must be kept in mind that the observed image is, for the most part laser speckle. The "Laser on/off" pertains to the 805 nm laser. Further details can be found in the readme.txt files in the corresponding folder. (No)_th_spectrum.csv represents the ith recorded spectrum from the corresponding experiment. The parameters of the Gd2O3:Er3+ samples can be found in the .ppt file.