Lasing by Optically Pumped Pendant Droplets

Pendant droplets have optical properties that reproduce the characteristics of corresponding bulk materials. A particular case is the droplet hanging in air, made of a laser dye solution and optically side-pumped. Here are shown results about laser induced fluorescence (LIF) emitted by a single pendant droplet excited with 532 nm pumping pulsed laser beam when its content is solution of rhodamine 6G in water kept as such, or doped with TiO2. By changing the geometry of fluorescent medium from bulk to pendant droplet and its volume, significant LIF changes occur. The main difference is amplification of emitted fluorescence in pendant droplets, explained by the confinement of light via total internal reflection in droplet. A new technique is shown, developed to distinguish temporal changes of a droplet emission in air. It was observed that the presence in the droplet of rhodamine 6G solution in ultrapure water of TiO2 nanoparticles induces LIF spectra modifications depending on nanoparticles number density and laser beam pumping energy. A concentration of 1011 part/cm3 nanoparticles favours formation of two new emission bands shifted towards blue with respect to main band. The increase with one order of magnitude of TiO2 number density, produces the disappearance of these bands regardless the pumping energy. Data suggest that addition of TiO2 nanoparticles to rhodamine droplet solutions influences emission spectra which can be modulated by varying nanoparticles concentration and pumping beam energy.

Keywords: Fluorescence, Laser active medium, Laser dye, Laser induced fluorescence, LIF, Lasing, Microdroplet, Nd:YAG laser, Spherical cavity, Rhodamine 6G, Rh6G, TiO2 nanoparticles.