M. Oujja

Comparative SERS effectiveness of silver nanoparticles prepared by different methods: A study of the enhancement factor and the interfacial properties

Different Ag nanoparticles were prepared by four different methods (chemical reduction with trisodium citrate, chemical reduction with hydroxylamine hydrochloride, laser ablation and laser in situ photoreduction) to compare their applicability in surface-enhanced Raman scattering (SERS), their stability and other interfacial characteristics such as the pH, surface availability and the surface potential. This study was conducted by using the anthraquinone dye alizarin as a molecular probe since this molecule is able to be adsorbed onto the metal through three different forms, which relative proportions depend on the interfacial properties of the exposed metal surfaces.

Spectroscopic Analysis of Pigments and Binding Media of Polychromes by the Combination of Optical Laser-Based and Vibrational Techniques

Four spectroscopic techniques, Fourier transform Raman (FTR), Fourier transform infrared (FT-IR), laser-induced breakdown spectroscopy (LIBS), and laser-induced fluorescence (LIF), are employed to characterize the pigment and binding media composition of polychromes. Raman spectra allow the assignment of the main pigments; these are α-HgS (vermilion), 2PbCO3·Pb(OH)2 (lead white), As2S3 (orpiment), Pb3O4 (lead red), and amorphous Carbon. The IR spectra can readily identify the pigment 2PbCO3·Pb(OH)2, the binder CaSO4·2H2O (gypsum) and, in combination with the Raman results, the presence of organic constituents of the binding media such as aliphatic esters, free acids, carbohydrates, and amides. The elemental analysis provided by LIBS corresponds to the pigments identified by the previous techniques and shows in addition the presence of some Hg, Fe, and Pb containing components. The presence of a Hg derivative in some of the sampled areas is supported by the LIF analysis; the fluorescence spectrum is mainly assigned to the binding media with the spectral intensity and shape modified to some extent by the contribution of pigments. In some of the samples studied, a peak assigned to α-HgS is observed. The effect of low intensity KrF laser irradiation of the samples is examined; the combined observations point toward a laser cleaning effect except in the areas that contain the pigment 2PbCO3·Pb(OH)2.

The laser-induced discoloration of stonework; a comparative study on its origins and remedies

For understanding the phenomena associated with the discoloration observed in some cases of infrared laser cleaned stonework surfaces, a comparative study of three different types and morphologies of pollution encrustation and stone substrates was undertaken. Fragments originating from monuments with historic and/or artistic value, bearing homogeneous thin soiling on Pentelic marble (Athens, Greece), thick encrustation on Hontoria limestone (Burgos, Spain) and compact thin crust on gypsum decorations (Athens, Greece), have been studied on the basis of their composition and origin, together with the conditions that may induce yellowing effects upon their laser cleaning with IR wavelengths. While irradiation in the UV (i.e. at 355 nm) could not effectively remove the encrustations studied, irradiation at 1,064 nm was found efficient to remove all the studied pollution accumulations. Discoloration towards yellow was evident in all cases and at different levels, including the samples with intentional patination layer. To the limit of Raman detection no chemical alterations were detected on the irradiated areas while the presence of yellow polar compounds in all the pollution crusts studied supports the argument that the discoloration of the stone surfaces upon their IR irradiation may be due to the uncovering of existing yellow layers as result of the migration of these compounds inwards to the original stone surface. To correct and/or prevent such undesired coloration the use of IR and UV radiation both in sequential and synchronous mode was considered, with positive results.

Three dimensional microstructuring of biopolymers by femtosecond laser irradiation

A sequence of single femtosecond pulses is used to create a pattern of laser affected spots at increasing depths below the surface of transparent biopolymer samples. Materials with different water contents and mechanical strengths, gelatine, chitosan, synthetic polyvinyl pyrrolidone, and biopolymer-polymer blends, are irradiated near the edge of the sample with an amplified Titanium:Sapphire laser (800 nm) delivering 30 fs pulses through a 0.45 numerical aperture objective with energies of 100–3000 nJ. The micrometric modified region is observed by optical microscopy perpendicularly to irradiation. Self-focusing and optical aberration are major factors controlling morphology and size of the created spots.

UV laser removal of varnish on tempera paints with nanosecond and femtosecond pulses

Two laser cleaning approaches based on ablation by ultraviolet laser pulses of femtosecond (fs) and nanosecond (ns) durations for the removal of shellac varnish from egg-yolk based tempera paints are investigated. Laser irradiation effects, induced on the varnish layer and on the underlying temperas by multiple pulses in the fs domain at 398 and 265 nm and single pulses in the ns domain at 213 nm, were examined following a spectroanalytical approach. By using optical microscopy, colorimetry and laser induced fluorescence it was found that irradiation of the varnished temperas with fs pulses changes the texture of the varnish surface and results in degradation of the underlying coloured paint. In contrast, operating with pulses of 15 ns at the highly absorbed wavelength of 213 nm, controlled micrometric layer removal of the varnish is possible without noticeable modification of the coloured temperas. These results widen the choice of laser conditions for painting restoration.

Wavelength and pulse duration effects on laser induced changes on raw pigments used in paintings

In this study, the reaction of widely used artists pigments in raw form to pulsed laser radiation of different wavelengths and pulse duration was investigated. Vermilion, lead chromate and malachite (in the form of pellets) were irradiated using laser pulses of 500 fs at 248 nm, and pulses of 150 ps and 15 ns at 1064 and 213 nm. Optical microscopy, colorimetry, spectrofluorimetry, micro-Raman spectroscopy and X-ray photoelectron spectroscopy were employed to characterize the physicochemical changes induced to the pigments. Change of crystalline phase was identified for vermilion while reduction processes take place for lead chromate and malachite. It was found that these effects were minimized by application of ultraviolet ultrashort pulses (of femtosecond and picosecond duration) as compared with changes occurring by pulsed infrared irradiation (of both picosecond and nanosecond duration). The results presented are discussed in relation to previous research on painted mock-ups in order to elucidate the role and significance of the binding media in the laser induced discoloration of painted surfaces and thus to propose optimum laser cleaning practices.

Room temperature in-plane ⟨100⟩ magnetic easy axis for Fe3O4/SrTiO3(001):Nb grown by infrared pulsed laser deposition

We examine the magnetic easy-axis directions of stoichiometric magnetite films grown on SrTiO{sub 3}:Nb by infrared pulsed-laser deposition. Spin-polarized low-energy electron microscopy reveals that the individual magnetic domains are magnetized along the in-plane 〈100〉 film directions. Magneto-optical Kerr effect measurements show that the maxima of the remanence and coercivity are also along in-plane 〈100〉 film directions. This easy-axis orientation differs from bulk magnetite and films prepared by other techniques, establishing that the magnetic anisotropy can be tuned by film growth.We examine the magnetic easy-axis directions of stoichiometric magnetite films grown on SrTiO3:Nb by infrared pulsed-laser deposition. Spin-polarized low-energy electron microscopy reveals that the individual magnetic domains are magnetized along the in-plane film directions. Magneto-optical Kerr effect measurements show that the maxima of the remanence and coercivity are also along in-plane film directions. This easy-axis orientation differs from bulk magnetite and films prepared by other techniques, establishing that the magnetic anisotropy can be tuned by film growth.

Analytical Spectroscopic Investigation of Wavelength and Pulse Duration Effects on Laser-Induced Changes of Egg-Yolk-Based Tempera Paints:

The application of laser cleaning methodologies to light-sensitive substrates such as those encountered in artistic paintings is an extremely delicate issue. The cleaning of paintings and polychromes is an irreversibly invasive intervention; therefore, prior to the implementation of laser cleaning methodologies, a thorough characterization of the interaction between laser pulses and painting components is required. In this work, the modifications induced by irradiation with pulses of 150 picoseconds (at 1064 and 213 nm) and 15 nanoseconds (at 213 nm) on unvarnished aged model egg-yolk-based paints were examined following a spectroanalytical approach. Laser-induced chemical changes on samples of unpigmented and widely used artists pigment temperas were investigated by spectrofluorimetry and Fourier transform Raman spectroscopy, while color changes were quantified by colorimetry. Noticeable modifications of the Raman and fluorescence bands attributed to pigments are absent except for vermillion, for which the pigment bands tend to disappear upon irradiation at 1064 nm. Interestingly, no discoloration was observed on most of the pigments upon irradiation at 213 nm (150 ps), including the light-sensitive vermillion, while no indications of carbonization or charring of the paint layers, which could give rise to amorphous carbon bands, were obtained at any of the irradiation conditions explored. Comparison of the results using the two different pulse durations and wavelengths illustrates the participation of mechanisms of diverse origin according to the chemical composition of the pigment and highlights the importance of the optimization of the laser parameters, specifically fluence, pulse duration, and wavelength, in conservation treatments of paintings.

Harmonic generation in ablation plasmas of wide bandgap semiconductors

Third and fifth harmonic generation of an IR (1.064 μm) pulsed laser has been produced in ablation plasmas of the wide bandgap semiconductors CdS and ZnS. The study of the temporal behaviour of the harmonic emission has revealed the presence of distinct compositional populations in these complex plasmas. Species ranging from atoms to nanometre-sized particles have been identified as emitters, and their nonlinear optical properties can be studied separately due to strongly differing temporal behaviour. At short distances from the target (<1 mm), atomic species are mostly responsible for harmonic generation at early times (<500 ns), while clusters and nanoaggregates mostly contribute at longer times (>1 μs). Harmonic generation thus emerges as a powerful and universal technique for ablation plasma diagnosis and as a tool to determine the nonlinear optical susceptibility of ejected clusters or nanoparticles.

Generation of low-order harmonics in laser ablation plasmas

The third and fifth harmonics of a pulsed infrared laser (1064 nm) delivering pulses of nanosecond duration have been generated in the laser ablation plasmas of various materials including the metals Al and Fe and the highly ionic insulators MgF2 and NaCl. The harmonics were generated in a process triggered by laser ablation followed by frequency up-conversion of the fundamental laser beam that propagates parallel to the target surface. The spatial and temporal behaviour of the harmonics reveals the distinctive composition and dynamics of each plasma, gives insight into the species that act as nonlinear media and allows a comparison of different processes that control the generation efficiency. Low-order harmonic generation thus emerges as a powerful and universal technique for monitoring and diagnostics of ablation plasmas, and the results presented serve to guide the choice of solid materials for efficient high harmonic generation using ultrashort pulses in the femtosecond regime.