Mauro Falconieri

Thermo-optical effects in Z -scan measurements using high-repetition-rate lasers

In this paper the effects on Z-scan measurements of thermo-optical nonlinearities due to cumulative heating of the sample are investigated. A model calculation of the Z-scan signal induced by thermo-optical nonlinearities is presented which takes into account absorption processes involving an arbitrary number of photons as the sources of nonlinearity. It is quantitatively shown how weak linear or nonlinear absorption coefficients can give rise to large signals depending on the repetition rate of the laser and on the experimental conditions. Conversely, very weak nonlinear absorption coefficients can be measured exploiting the effect of cumulative heating when using a long train of laser pulses. The possibility of separating such thermal contributions from fast optical nonlinearities in Z-scan signals is discussed.

LARGE THIRD-ORDER OPTICAL NONLINEARITY OF NANOCLUSTER-DOPED GLASS FORMED BY ION IMPLANTATION OF COPPER AND NICKEL IN SILICA

Composite glasses containing different metal nanoclusters were obtained by implanting copper, nickel, or copper+nickel ions in SiO2 glass. The nonlinear refractive index of the composites was determined by the Z-scan method at a wavelength of 770 nm and with a laser pulse duration of 130 fs. Values of n2 up to 0.68 cm2 gW−1 were measured in the case of the Cu+Ni implanted sample.

Cobalt nanoclusters in silica glass: Nonlinear optical and magnetic properties

Fused silica plates were implanted with Co ions at room temperature at the energy of 50 keV and to the fluence of 4×1016 ions cm−2. The formation of metal nanoclusters was observed by transmission electron microscopy. The cluster size distribution is narrow with a mean-diameter value of about 3 nm. Atomic in-depth distribution was determined by Rutherford backscattering spectrometry, whereas the cobalt chemical state was characterized by electron spectroscopies. Nonlinear refractive index n2 is of the order of 0.2 cm2 G W−1, as determined by the Z-scan technique at a wavelength of 770 nm for 130 fs long pulses at a 76 MHz repetition rate. Zero-field-cooled and field-cooled magnetization curves at the liquid-helium temperature exhibit features of superparamagnetic behavior that are characteristic of assemblies of single-domain nanoparticles.

Optical and morphological characterization of Si nanocrystals/silica composites prepared by sol-gel processing

Abstract Linear (visible photoluminescence) and non-linear (third-order susceptibility) optical properties have been measured for laser-synthesized crystalline Si nanoparticles. The observed luminescence is compared to similar measurements on nanocomposite aerogels prepared integrating the Si-nanoparticles into a continuous silica phase by sol–gel processing. The results show that incorporation of the Si-nanoparticles into a silica matrix by sol–gel technology is not detrimental to the PL (photoluminescence) emission intensity. The effect on the nanocomposites of annealing in air (at temperatures in the range of 300–500°C) has been investigated by means of PL and Raman spectroscopy. The obtained results indicate that the oxidation treatment induces the bleaching of the smaller luminescent particles and is not effective in reducing the average size. Micro-Raman mapping and TEM analysis of the nanocomposites have shown the presence of agglomerated nanoparticles (in the sub-micrometer range), as confirmed by dynamic light scattering (DLS) measurements on colloidal suspensions of the as-synthesized powders.

Evidence of energy transfer in an aluminosilicate glass codoped with Si nanoaggregates and Er3+ ions

The enhancement of the Er3+ ions’ photoluminescence (PL) emission at 1.54μm in a Si and Er coimplanted aluminosilicate glass is investigated in detail. A postimplantation thermal treatment has been performed to recover the damage induced by the implantation process and to promote Si aggregation. It will be shown that 1h treatment in N2 atmosphere is not sufficient to induce Si precipitation for temperatures up to 500°C. Nevertheless, the most intense Er3+ PL emission at 1.54μm is achieved after a thermal treatment at 400°C. Such emission has been investigated by pumping in and out of resonance, showing a very efficient energy transfer process in the whole excitation wavelength range (360–515nm). These results suggest that good energy transfer mediators could be small Si aggregates and not only crystalline clusters. For the best performing sample, the effective Er excitation cross section has been measured to be higher than 10−17cm2 at 379 and 390nm and about 2×10−16cm2 at 476nm, that is, several orders of...

Fluorescence dynamics in an optically-excited Tm,Ho:YAG crystal

Abstract We present the results of measurements of the time evolution of the fluorescences at wavelengths between 400 and 2500 nm, emitted from an optically pumped Tm,Ho:YAG crystal in response to pulsed excitation at 785 nm. The measurements have been performed as a function of the pumping laser fluence. Comparison of the experimental results with the predictions of a rate equation model of this system has permitted to establish precise values for the rate parameters governing the relevant ion-ion interaction processes inside the crystal.

Effects of co-dopant concentrations and excitation conditions on the 2 ?m fluorescence dynamics in Tm, Ho:YLF crystals

Results of spectroscopic studies carried out on Tm, Ho:YLF crystals are reported. In particular, we have investigated the effects of the co-dopant concentrations on the time evolution of the fluorescence at 2 μm emitted in response to short-pulse laser excitation at 792 nm. The fluorescence intensity profiles are analyzed in terms of their temporal and amplitude characteristics over a wide range of excitation conditions.

Achievement of λ/4000 phase distortion sensitivity in the measurement of optical nonlinearities by using a modulated Z-scan technique

We report on a modulation detection system that efficiently improves the signal-to-noise ratio and the maximum achievable sensitivity of Z-scan measurements of purely optical nonlinearities. By using suitable signal processing electronics and combining an averaging procedure with the harmonic motion of the sample around the Z-scan lens focus position, we demonstrate an increase of the measurement sensitivity of more than one order of magnitude, which is in principle independent of the particular Z-scan set-up employed. The consequence is that nonlinear phase distortions of λ/4000 can be easily measured and improvements up to λ/105 are foreseen.

Microplastic pollution in the surface waters of Italian Subalpine Lakes

Plastic debris incidence in marine environment was already highlighted in the early 1970s. Over the last decade, microplastic pollution in the environment has received increasing attention and is now an emerging research area. Many studies have focused on quantifying microplastic abundance in the marine environment, while there are relatively few data on microplastic occurrence in freshwater environment. Recent studies have reported high concentrations of microplastics in lakes and rivers, although the understanding of several factors influencing source, transport and fate is still limited. This study compares different lakes and the common factors, which could influence the occurrence and distribution of microplastics. The three subalpine lakes monitored include Lake Maggiore, Iseo and Garda. The selected sampling transects reflect the hydrologic conditions, the morphometric characteristics of these lakes, and other factors influencing the release of plastics debris in lakes. Particles of microplastics (<5 mm) were found in all sampled surfaces. The particles collected were classified depending on their number, shape and composition. The shape distribution showed the dominating occurrence of fragments (73.7%). The chemical composition of all examined samples clearly shows dominating presence of polyethylene (45%), polystyrene (18%) and polypropylene (15%). The results provide significant relations among the different contribution of direct and diffuse sources to the quantity of microplastics, highlighting the importance of understanding the spatial distribution dynamics of microplastics within a lake system that acts as a sink and source of plastic particles.

Enhanced and shortened Mn 2+ emissions by Cu + co-doping in borosilicate glasses for W-LEDs

A novel pair of transition metal ions Cu+, Mn2+ is co-doped in borosilicate glasses. Both copper and manganese ions exist in lower valence states (Cu+, Mn2+) in the as-prepared glasses. Around 5-time enhanced Mn2+ emission under the UV excitation is observed, which, as demonstrated by excitation spectra and emission decay curves, is due to an energy transfer from Cu+ ions resulting in greatly increased absorption of Mn2+ ions in the UV region, and relaxation on doubly-forbidden transition of Mn2+ leading to the much shortened Mn2+ emission lifetime from millisecond to microsecond level. Besides, a composite white emission is generated by combining the blue-green part from Cu+ ions with the green-red part from Mn2+ ions and it can be effectively tuned from cold to warm by adjusting host glass composition and altering excitation wavelength. Relevant mechanisms are discussed.