Search

Photoluminescence properties and optical limiting behavior of pure and Sm 3+ /Eu 3+ doped Tb 2 (MoO 4 ) 3 nanophosphors are investigated. The prepared nanophosphors exhibit excellent emission when excited by UV light. Color-tunable emissions in Tb 2−x Sm x (MoO 4 ) 3 and Tb 2−x Eu x (MoO 4 ) 3 are realized by employing different excitation wavelengths or by controlling the doping concentration of Sm 3+ and Eu 3+ . Luminescence quantum yield and CIE chromatic coordinates of the prepared phosphors were also presented. Optical limiting properties of the samples are investigated by open aperture Z-scan technique using 5 ns laser pulses at 532 nm. Numerical fitting of the measured Z-scan data to the relevant nonlinear transmission equations reveals that the nonlinear absorption is arising from strong excited state absorption, along with weak absorption saturation and it is found that the optical nonlinearity of Tb 2 (MoO 4 ) 3 increases with Sm 3+ /Eu 3+ doping. Parameters such as saturation fluence, excited state absorption cross section and ground state absorption cross section of the samples have been determined numerically, from which the figure of merit for nonlinear absorption is calculated. The excited state absorption cross-section of the samples is found to be one order of magnitude higher than that of the ground state absorption cross-section, indicating strong reverse saturable absorption. These results indicate that Sm 3+ /Eu 3+ doped Tb 2 (MoO 4 ) 3 nanophosphors are efficient media for UV/n-UV pumped LEDs, and are also potential candidates for designing efficient optical limiting devices for the protection of human eyes and sensitive optical detectors from harmful laser radiation.

Dye-doped nematic liquid crystals support random lasing under optical pumping, as well as reorientational optical spatial solitons acting as all-optical waveguides. By synergistically combining these two responses in a collinear pump-soliton geometry, the resulting soliton-enhanced random laser exhibits higher conversion efficiency and better directional properties. After a short account on random lasers and solitons in nematic liquid crystals - nematicons - we describe our experimental results on nematicon-molded random lasers. publishedVersion

We recently reported our experimental findings on the use of a passive Kerr-like nonlinear optical response, namely, reorientational self-focusing and spatial soliton generation, in conjunction with optically pumped random lasing in nematic liquid crystals (E7) doped with Pyrromethene dye (PM597).1 In Ref. 1, for the emission spectra at pump energies of 0:75 μJ/pulse (below threshold), we mistakenly used the wrong data and curves in Fig. 4(a). The amended figure (to replace Fig. 4 in Ref. 1) is presented here. This correction does not affect the discussion of the results and the conclusions presented in the article. (figure presented).

Random lasers are resonator-less light sources where the optical feedback for lasing arises from recurrent multiple scattering events in a disordered gain medium. During the past 20 years, random lasers have been the subject of intense theoretical and experimental investigations due to their novelty, simplicity and ruggedness as well as their great potential for applications ranging from imaging to biomedical diagnostics. Common random lasers, however, tend to emit light in a wide range of directions with poor beam quality. The lack of pre-determined directionality and its control along with poor spatial characteristics hinder the applications of random lasers in many fields.It has been shown that dye-doped nematic liquid crystals can random lase when optically pumped within the absorption region of the dye molecules. Moreover, through molecular reorientation they support the propagation of self-guided optical beams, known as nematicons, which act as real-time graded-index optical waveguides.In order to address the problems of conventional random lasers, this Thesis focuses on the demonstration of a configuration were an efficient random laser with directional output is realized in dye-doped nematic liquid crystals by combing optical gain, random scattering, and nematicons. A planar cell filled with a mixture of commercial E7 nematic liquid crystals (host) and PM 597 (guest) is used as the material. A near infrared continuous-wave beam is used to launch a nematicon, while a 6 ns pulsed beam at 532 nm is used to optically pump the material. The synergy of random lasing and nematicons in this geometry provides several breakthrough results.By adopting a pumping geometry collinear with the nematicon, we demonstrate random laser emission from the initial region of the nematicon. The nematicon waveguide provides transverse confinement to the emitted photons, resulting in directionality, high photon collection, and high slope efficiency. The nematicon also modulates the emission, allowing the realization of a random transistor laser, where a low-power continuous-wave input is able to lower the threshold for lasing.By taking advantage of the response of nematicons to external stimuli, we also demonstrate a random laser emitting in a voltage-controlled direction. Finally, we demonstrate in-plane random laser steering by external magnetic field. The latter approach allows us to steer the random laser beam over 14°.

We demonstrate a guided-wave random laser configuration by exploiting the coexistence of optical gain and light self-localization in a reorientational nonlinear medium. A spatial soliton launched by a near-infrared beam in dye-doped nematic liquid crystals enhances and confines stimulated emission of visible light in the optically-pumped gain-medium, yielding random lasing with enhanced features.

We report on novel features of random lasers assisted by near-infrared spatial solitons in nematic liquid crystals. Specifically, we study the role of light-induced reorientational waveguides (nematicons) on the spatial and spectral distributions of the laser modes. We show that the spatially spiky character of the laser emission propagating backwards with respect to the pump tends to disappear in the forward direction, due to the soliton confinement of the generated light. Moreover, the spectral features associated with various random laser resonances appear to merge upon guided-wave propagation along the nematicon, making the nematicon-aided random laser a bidirectional device with distinct emission properties at the two opposite outputs.

Spatial solitons can affect and enhance random lasing in optically-pumped dye-doped nematic liquid crystals. Upon launching two collinear beams in the sample, the first to pump the fluorescent guest molecules and the second to induce a reorientational soliton, strikingly the second beam not only guides the emitted photons in the soliton waveguide, but also enhances the lasing efficiency and modulates its spectral width. By altering the scattering paths of the emitted photons, the soliton also contributes to the selection of the lasing modes, as further confirmed by the observed kinks in the input/output characteristics. These experimental results demonstrate that random lasing can be efficiently controlled by a light beam which does not interact with the gain molecules, opening a route towards light-controlled random lasers. publishedVersion

Dye-doped nematic liquid crystals support random lasing under optical pumping, as well as reorientational optical spatial solitons acting as all-optical waveguides. By synergistically combining these two responses in a collinear pump-soliton geometry, the resulting soliton-enhanced random laser exhibits higher conversion efficiency and better directional properties. After a short account on random lasers and solitons in nematic liquid crystals–nematicons–we describe our experimental results on nematicon-molded random lasers.