UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS

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Conference: Bucharest University Faculty of Physics 2011 Meeting


Section: Optics, Spectroscopy, Plasma and Lasers


Title:
Studies on the optical properties of micro-droplets


Authors:
Mihai Boni, Viorel Nastasa, Ionut Relu Andrei, George Viorel Popescu, Mihai Lucian Pascu


Affiliation:
Laser Department, National Institute for Laser, Plasma and Radiation Physics, Bucharest-Magurele, Romania


E-mail
mihai.boni@inflpr.ro


Keywords:
microdroplets, Nd:YAG laser, laser dyes, optical emission, lasing


Abstract:
Our previous experiments shown that is better to irradiate substances in droplets instead in bulk. The changes that occur in the molecular structure of the substances are better observed in droplets. One of the reasons is that the beads are behaving as spherical optical cavity. In this paper we present lasing effect in pendant droplets having different volumes. The droplets were seeded with an organic dye (Rhodamine 6G – R6G) in ultrapure water at different concentrations and irradiated by pulsed laser beam emitted at 532 nm by a SHG Nd:YAG laser, (pulse time width 6ns, laser pulses repetition rate 10 pps). The droplets were generated using a computer controlled system Hamilton Microlab 500. The liquid volumes pumped were typically 12.5 µl. This produced droplets with diameter of 3mm. To observe the lasing emission we measured the laser induced fluorescence emitted by the droplets when excited at 532 nm. The fluorescence signal is collected by an optical fiber (1mm core), and analyzed with HR4000 Ocean Optics spectrometer (0.65nm resolution, 200-1100nm). We observed that the effect depends on several parameters, such as: the concentration of the Rh 6G in water; the droplet’s volume; the interaction angle of the pumping laser beam with the droplet’s surface. By varying the concentration of the R6G in water we obtained the typical fluorescence broad band and a narrow peak assigned to the lasing effect. The best results obtained on the measured samples were at the R6G 10-3M concentration in ultra pure water. In order to study how the volume changes by evaporation of the droplets during irradiation, we measured the temperature variations of the pendant droplets following their resonant interaction with laser radiation emitted at 532nm. The droplet temperature was measured with the thermo-camera ThermaCAM® E45. Depending of the droplet dye concentration and irradiation time, the increase of the temperature was maximum 3○ C.