Raffaele Tommasi

Substituent-dependence of the optical nonlinearities in poly(2,5-dialkoxy-p-phenylenevinylene) polymers investigated by the Z-scan technique

Abstract We investigated the relationship existing between the third-order nonlinear optical (NLO) properties and the chemical structure of some poly(2,5-dialkoxy-1,4-phenylenevinylene) polymers. The latter have similar degrees of polymerization and polydispersity indexes and differ only for the structure of the side groups in the 2,5 positions of the phenylene rings. The two-photon absorption coefficient and the nonlinear refractive index have been measured using the picosecond Z -scan technique. Experimental results show that all the investigated organic molecules exhibit a small Im χ (3) and a large, negative Re χ (3) at λ =1064 nm. A considerable dependence of the third-order NLO properties on the steric effect of the substituents has been observed.

Photoinduced conductivity and nonlinear optical properties of [M(R,R′timdt)2] dithiolenes (M=Ni, Pd, Pt; R,R′timdt=monoreduced imidazolidine-2,4,5-trithione) as materials for optically driven switches and photodetectors

Abstract The unusual NIR-photoconducting properties of [M(Et,Pent-timdt)2] neutral dithiolenes (M=Ni, Pd, Pt; Et,Pent-timdt=monoreduced N-ethyl,N′-pentylimidazolidine-2,4,5-trithione) have been investigated with the aim of developing wavelength-selective air-stable photodetectors. In addition, the ps time-resolved dynamics of the absorption saturation has been studied on the Pd-complex by means of pump–probe experiments.

TiO2 nanorods/PMMA copolymer-based nanocomposites: highly homogeneous linear and nonlinear optical material

Original nanocomposites have been obtained by direct incorporation of pre-synthesized oleic acid capped TiO(2) nanorods into properly functionalized poly(methyl methacrylate) copolymers, carrying carboxylic acid groups on the repeating polymer unit. The presence of carboxylic groups on the alkyl chain of the host functionalized copolymer allows an highly homogeneous dispersion of the nanorods in the organic matrix. The prepared TiO(2)/PMMA-co-MA nanocomposites show high optical transparency in the visible region, even at high TiO(2) nanorod content, and tunable linear refractive index depending on the nanoparticle concentration. Finally measurements of nonlinear optical properties of TiO(2) polymer nanocomposites demonstrate a negligible two-photon absorption and a negative value of nonlinear refractive index, highlighting the potential of the nanocomposite for efficient optical devices operating in the visible region.

High third-order nonlinear optical susceptibility in new fluorinated poly(p-phenylenevinylene) copolymers measured with the Z-scan technique

The third-order nonlinear optical properties of a series of copoly(2, 3, 5, 6-tetrafluoro-1, 4-phenylenevinylene-2, 5-dioctyloxy-1, 4-phenylenevinylene) that contain variable ratios of two differently substituted monomers have been studied in chloroform solutions at l=1064 nm by the picosecond Z-scan technique. Nonlinear refractive index n(2) of the samples investigated has been found to be negative, and a strong dependence of its magnitude on the copolymers composition has been observed. The highest third-order nonlinear optical susceptibility, |x((3))|=(6 +/- 2)x 10(-10) esu, was measured for a copolymer obtained by reaction of equimolar quantitites of the parent monomers.

Spectroscopic study on imidazolium-based ionic liquids: effect of alkyl chain length and anion.

Room temperature ionic liquids are currently used as functional materials in several application and their optical investigation can provide a better understanding of their physical and chemical behavior. Absorption and emission properties of imidazolium-based ILs have been attributed to the imidazolium moiety and related to the presence of energetically different aggregates. Here, time-integrated and time-resolved investigation has been carried out on 1-alkyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids with different chain lengths in order to probe the occurrence of energy transfer processes, and hence to disclose the presence of various states with different energy. Such a study contributes to provide relevant insight on the effect of alkyl chain and anion type on the emission characteristics, and, hence, on the presence of associated structures.

High quality CdS nanocrystals: surface effects

Stable and highly luminescent CdS nanocrystals (NCs) have been synthesized by means of decomposition of suitable precursors (CdO and elemental sulphur) in octadecene (ODE) and oleic acid at high temperature. Absorption and steady state photoluminescence (PL) measurements have been carried out to study the intrinsic properties of nanocrystals and the effect of ligand and solvent on emission properties. Ageing of nanocrystals has been investigated, and an efficient mechanism of emission recovery has been identified, showing the active role of non coordinating solvent and oleic acid in modifying the surface states.

Investigation of the excited-state absorption of a Ru dioxolene complex by the Z-scan technique.

We investigated nonlinear absorption of picosecond laser pulses in a Ru complex [{Ru(bipy)2}2L][PF6] using the open-aperture Z-scan technique. Experiments were performed in a spectral region of moderate linear absorption (lambda=532 nm, sigmag=0.65 x 10(-16) cm(-2)). We observed a transition from reverse saturable absorption to saturable absorption when the excitation intensity was higher than 50 GW/cm2. To determine the population level kinetics, a model based on three- and four-level systems was considered, and the rate equations solved taking into account the propagation equation of the laser pulse inside the sample. Fitting of the experimental data allows extraction of the excited-states absorption cross sections and lifetimes.

Investigation of the nonlinear absorption of [M(Et2timdt)2 ] (M = Pd, Pt) in the pico- and nanosecond timescales using the Z-scan technique

The nonlinear optical absorption of two neutral metal dithiolenes [M(Et2timdt)2] (M = Pd and Pt) has been investigated at λ = 1064 nm by the open-aperture Z-scan technique using nanosecond and picosecond pulses. For picosecond photoexcitation, both dithiolenes mainly exhibit saturable absorption. Conversely, using nanosecond pulses, a switch from saturable absorption to reverse saturable absorption has been observed depending on the central metal. Based on experimental results, energy-level structures are suggested to explain the nonlinear absorption for both temporal regimes and, in particular, the sign-reversal of nonlinear absorption.

Picosecond absorption saturation dynamics in neutral [M(R,R′timdt)2] metal-dithiolenes

Picosecond nonlinear absorption dynamics of neutral metal-dithiolenes belonging to the general class [M(R,R′timdt)2] (M=Ni, Pd, Pt; R,R′ timdt=monoreduced form of disubstituted imidazolidine-2,4,5-trithione) has been investigated at λ=1064 nm using the pump–probe technique. Both central metal ion M and substituents R,R′ have been found to strongly influence the temporal evolution of the absorption saturation. In particular, it has been observed that the intersystem crossing rate always increases by a factor ∼1.5 when changing the central metal from Pd to Pt. The mechanisms leading to the measured dynamics have been explained using a multilevel rate equation analysis, based on the results of relativistic time-dependent density functional calculations.

A multidisciplinary study of the extracutaneous pigment system of European sea bass (Dicentrarchus labrax L.). A possible relationship between kidney disease and dopa oxidase activity level

Infectious diseases and breeding conditions can influence fish health status. Furthermore it is well known that human and animal health are strongly correlated. In lower vertebrates melano-macrophage centres, clusters of pigment-containing cells forming the extracutaneous pigment system, are widespread in the stroma of the haemopoietic tissue, mainly in kidney and spleen. In fishes, melano-macrophage centres play an important role in the immune response against antigenic stimulants and pathogens. Hence, they are employed as biomarker of fish health status. We have investigated this cell system in the European sea bass (Dicentrarchus labrax L.) following the enzyme activities involved in melanin biosynthesis. We have found a possible relationship between kidney disease of farmed fishes and dopa oxidase activity level, suggesting it as an indicator of kidney disease. Moreover variations of dopa oxidase activity in extracutaneous pigment system have been observed with respect to environmental temperature. At last, for the first time, using femtosecond transient absorption spectroscopy (Femto-TA), we pointed out that pigment-containing cells of fish kidney tissue present melanin pigments.