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.