Optically active zinc oxide-silica, inorganic-inorganic nanohybrid materials have been synthesized via a sol-gel process utilizing a room temperature scheme that does not require high temperature annealing. Random laser action is demonstrated from these ZnO-silica nanocomposites. The ZnO nanoparticles act as both the gain and the strong scattering medium, which leads to random optical feedback due to multiple elastic scattering, while the amorphous silica matrix offers a high degree of material stability. Optical pumping of the nanocomposites by ultraviolet laser pulses, of duration shorter than the ZnO photoluminescence lifetime, leads to a profound optical gain behavior and random laser action above a certain threshold value of the excitation energy density. In this context, the effects of laser and material parameters on the observed random lasing are investigated for a series of nanocomposites. The observed dependence of the emission wavelength on excitation energy and the excitation energy density value of the random laser threshold are interpreted on the basis of the formation and the inversion of an electron-hole plasma, respectively.

The ultraviolet (UV) laser irradiation (248 nm) of monocrystalline wurtzite ZnO with 450 fs pulses results in surface modification. A formation of two orthogonal ripple structures with a period of 400-500 nm was observed oriented parallel and perpendicular to the laser beam polarization. The UV exciton emission obtained on the irradiated domains is found greatly enhanced locally up to ~103 times. The photoluminescence band is redshifted by 2-3 nm and 40% narrower (full width at half-maximum), while at the same time the E2(439 cm−1) Raman band intensity increases up to ~50 times. The process is found irreversible with the threshold fluence of 11 mJ/cm2, which is considerably lower than the ablation threshold 115 mJ/cm2. Fine surface nanostructuring on the scale of ~10 nm may be responsible for the observed effect.

A convenient and straightforward way to fabricate random lasing films based on ZnO is proposed. Highly scattering films consisting of ZnO nanoparticles are fabricated following a two-stage process that involves a) spin-coating of aqueous ZnO nanoparticle dispersions on rigid (glass) or flexible (polyethylene terephthalate) substrates and b) subsequent surface modification by means of a controlled UV laser annealing processing. Upon excitation with 150 picosecond pulses at 355 nm, the films exhibit intense photo luminescence emission, which upon crossing a threshold pump energy density, develops laser-like characteristics, namely, gain amplification, spectral narrowing and, under optimum pump conditions and sample structure, distinct lasing modes.

The chemical synthesis and photoluminescence properties of either dispersed or supported carbogenic nanoparticles are described. In the first case, ionic nanoparticles are obtained by thermal oxidation of an appropriate citrate salt. In the second, carbogenic nanoparticles decorate the external surfaces of zeolite crystals and are obtained by thermal oxidation of a preexchanged zeolite host.

Thermal carbonization of bis(2-chloroethyl)amine hydrochloride at 260°C in air leads to a new, functional layered carbonaceous material that, although different in structure and composition, shares similar characteristics as the well-known graphite oxide. Specifically, the molecularly derived carbonaceous solid is layered with relatively small lateral dimensions, highly dispersible in water providing clear colloidal sols and possesses ion-exchange properties. The carbonaceous solid strongly fluoresces in the visible, when stimulated with a wide range of excitation wavelengths. Overall, the method presents an alternative synthesis towards molecularly derived layered carbonaceous materials with novel properties.

We report on the measurement of the longitudinal coherence of organic microcavity lasers based on a conjugated polymer. By using a modified Michelson interferometer configuration enabling single-shot measurements of the coherence length, the transition from spontaneous emission to lasing is investigated. The measured coherence length grows upon increasing the pumping fluence, saturating around 45 microm above threshold. At large fluences, possible thermal and photo-oxidation processes occurring in the gain medium limit the further increase of the coherence length. Our results are important for understanding lasing emission in organic microcavities and optimizing the device design and performances.

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Highly scattering ZnO-hybrid nanostructures are produced exhibiting random laser action upon optical excitation. Studies that investigate the influence of pump pulse duration on the random laser efficiency are presented along with coherence length measurements.

Graphite fluoride has been generally considered chemically inert against strong alkalis under ambient conditions. In the present study we demonstrate that treatment of graphite fluoride with eutectic NaOH-KOH mixture at 250 degrees C induces dramatic structural and textural changes in the solid as evidenced by XRD, FT-IR, Raman, UV-vis absorption and fluorescence and microscopy techniques (TEM, AFM). The reaction proceeds in the molten state leading to water-soluble, graphitized carbon particles which unlike graphite fluoride, adopt a variety of morphologies, like platy, tetragonal, triangular, discoid and spherical. The resulting carbon particles are dispersible in water and fluoresce under UV excitation. (c) 2008 Elsevier B.V. All rights reserved.

A series of studies that investigate the influence of pump pulse duration and sample temperature on the random laser efficiency are presented along with coherence length measurements (Papadakis et al. 2007) on selected samples. Nanocomposite fabrication issues and pumping conditions are discussed in the context of performance optimization and potential use of such materials in future light emission devices.

A new method, to our knowledge, is proposed that enables single-shot measurements of the temporal coherence properties of pulsed laser sources. By modification of a Michelson interferometer, two replicas of the source are formed. Owing to the symmetry of the configuration, the optical path difference and, consequently, the visibility of the resulting interference fringes are varied over one dimension. This effectively replaces the mechanical scanning performed in a typical interferometer and thereby provides a direct measurement of the temporal coherence of a single pulse. Our method is successfully applied to the study of the single-shot temporal coherence of a Rhodamine 6G-ZnO-poly(methyl methacrylate) random laser system. (c) 2006 Optical Society of America.

Surface functionalization of ZnO nanoparticles with a quaternary ammonium organosilane leads to ionically modified nanoparticles. Upon exchange with a poly(ethylene glycol)-tailed sulfonate anion (A−), strongly photoluminescent nanoparticles result, which exhibit liquidlike behavior in the absence of solvents.

The photoluminescence properties of zinc oxide (ZnO) ceramic thin films, prepared by spin coating of ZnO nanoparticle aqueous suspensions, were studied with emphasis on the influence of film structure and surface morphology on the observation of random laser action. Surface processing employing laser annealing transforms the particulate grain structure of the as-deposited films into a porous channel-like network. This modification was shown to be critical for achieving random laser action as it favors efficient coupling of the pump light into the film material.

Random laser action is demonstrated in organic-inorganic, disordered hybrid materials consisting of ZnO semiconductor nanoparticles dispersed in an optically inert polymer matrix. The ZnO particles provide both the gain and the strong scattering power that leads to light trapping due to multiple elastic scattering, whereas the polymer matrix offers ease of material fabrication and processability in view of potential applications. Excitation of the nanohybrids by a laser pulse with duration shorter than the ZnO photoluminescence lifetime leads to a dramatic increase in the emitted light intensity accompanied by a significant spectral and temporal narrowing above a certain threshold of the excitation energy density. Critical laser and material parameters that influence the observed laser-like emission behavior are investigated in a series of nanocomposites. (C) 2004 Optical Society of America.