Absolute and Relative Methods for Fluorescence Quantum Yield Evaluation of Quantum Dots.

Optical spectroscopy techniques are crucial for the evaluation and use of quantum dots (QDs) in life and materials science. In that context, the fluorescence quantum yield (Φf) is an essential parameter in the assessment of the luminescent features of QDs. The fluorescence quantum yield can be defined as the ratio of the number of emitted photons to the number of absorbed photons by a luminescent material. In this chapter, we describe absolute and relative methods to measure the fluorescence quantum yield of QDs in solution phase. The advantages and limitations of the techniques are reviewed.

UV random laser emission from flexible ZnO-Ag-enriched electrospun cellulose acetate fiber matrix.

We report an alternative random laser (RL) architecture based on a flexible and ZnO-enriched cellulose acetate (CA) fiber matrix prepared by electrospinning. The electrospun fibers, mechanically reinforced by polyethylene oxide and impregnated with zinc oxide powder, were applied as an adsorbent surface to incorporate plasmonic centers (silver nanoprisms). The resulting structures - prepared in the absence (CA-ZnO) and in the presence of silver nanoparticles (CA-ZnO-Ag) - were developed to support light excitation, guiding and scattering prototypes of a RL. Both materials were excited by a pulsed (5 Hz, 5 ns) source at 355 nm and their fluorescence emission monitored at 387 nm. The results suggest that the addition of silver nanoprisms to the ZnO- enriched fiber matrix allows large improvement of the RL performance due to the plasmon resonance of the silver nanoprisms, with ~80% reduction in threshold energy. Besides the intensity and spectral analysis, the RL characterization included its spectral and intensity angular dependences. Bending the flexible RL did not affect the spectral characteristics of the device. No degradation was observed in the random laser emission for more than 10,000 shots of the pump laser.

Direct three-photon excitation of upconversion random laser emission in a weakly scattering organic colloidal system.

We report the operation and characterization of an upconversion random laser emitting at 560 nm, when directly pumped by three photon excitation at the near IR wavelength of 1350 nm in a colloidal dye solution in the weakly scattering regime. Using a special dye with a high three-photon cross-section and TiO(2) nanoparticles (250 nm diameter), optimized upconverted emission was obtained for particle densities of ~2 x 10(9)/cm3. A strong dependence on the nanoparticle concentration and the pumping area was verified. The presence of spikes with linewidths ~0.4 nm in the emitted spectrum is the signature of coherent emission from this three-photon pumped random laser.