The opticd limiting action of CSU in toluene solution is mainly due to revem saturable absorption (RSA). It is shown that the formalism describing nonlinear optical response due to instantaneous two-photon absorption can be used in the case of sequential two-photon absorption, yielding effective values of the relevant parameters of optical nonlinearity due to RSA. The effective twc-photon absorption parameter and the effective nonlinear refractive index parameter y&, which are related respectively to the imaginary and real parts of the effective third-order susceptibility $2. were measured by the L-scan technique as a function of fullerene concentration and of incident laser intensity and wavelength over the 42W nm region. The concentration dependence of these parameters indicates that the solution is optically lhin as far as the ground state of Cm is concerned, whereas he wavelength dependence confirms the applicability of the formalism used in the sequential two-photon absorption model. Comparisons are made with other I-scan results on Cm.

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In this work we report on the preparation of some aqueous graphene oxide (GO) dispersions and the investigation of their nonlinear optical response under visible (532 nm) and infrared (1064 nm), picosecond and nanosecond laser excitation. The GO colloids were prepared under specific and well-defined conditions resulting in finely dispersed heavily oxidized large GO sheets. In all cases, GO colloids were found to present large nonlinear absorption and negligible nonlinear refraction. The physical mechanisms responsible for their nonlinear optical response are discussed. In addition, the so-prepared GO dispersions were found to exhibit large broadband optical power limiting action for both pulse durations, comparable to that of C 60 for visible laser pulses and much superior for infrared ones.

The synthesis of O‐doped polyaromatic hydro‐ carbons in which two polycyclic aromatic hydrocarbon sub units are bridged through one or two O atoms has been achieved. This includes high‐yield ring‐closure key steps that, depending on the reaction conditions, result in the formation of furanyl or pyranopyranyl linkages through intramolecular C−O bond formation. Comprehensive photophysical measurements in solution showed that these compounds have exceptionally high emission yields and tunable absorption properties throughout the UV/Vis spectral region. Electrochemical investigations showed that in all cases O annulation increases the electron‐donor capabilities by raising the HOMO energy level, whereas the LUMO energy level is less affected. Moreover, third‐order nonlinear optical (NLO) measurements on solutions or thin films containing the dyes showed very good values of the second hyperpolarizability. Importantly, poly(methyl methacrylate) films containing the pyranopyranyl derivatives exhibited weak linear absorption and NLO absorption compared to the nonlinearity and NLO refraction, respectively, and thus revealed them to be exceptional organic materials for photonic devices.

Laser-induced breakdown spectroscopy (LIBS) was employed for the in situ analysis of pigments used in painting. LIBS spectra were collected from a wide variety of pigments in powder form and in oil color test samples. Appropriate emission lines for the identification of the metallic elements in the pigments examined are proposed. Under optimal experimental parameters, the technique is minimally destructive; two pulses from a laser beam focused on the sample surface result in the formation of a small crater with typical diameter around 40 μm and depth of no more than 10 μm. Furthermore, recording LIBS spectra from successive laser pulses on the same spot of a model oil painting resulted in information regarding the pigment composition of several paint layers, showing the capability of the technique in performing depth profile analysis. Finally, a test case is presented in which an 18th century oil painting, subjected to partial restoration, was examined by LIBS, and the different pigments used in the original and in the restored part of the work were clearly identified. The results of our studies demonstrate the applicability of LIBS in the rapid, in situ, and practically nondestructive determination of pigments in painted artworks.

Laser-induced breakdown spectroscopy has been applied to polymer samples in order to investigate the possibility of using this method for the identification of different materials. The plasma emission spectra of high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polypropylene (PP) have been studied. Spectral features have been measured—for example, the 725.7 nm chlorine line, the 486.13 mm Hβ line, and the 247.86 nm carbon line—whose evaluation with neural networks permits identification accuracies between 90 and 100%, depending on polymer type.

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