T. Cassano

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.

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.

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.

Investigation of luminescence in new blue-emitting poly(2′,5′-dioctyloxy-4,4′,4″-terphenylenevinylene)s

Abstract The dependence of the linear optical properties of some soluble poly(terphenylenevinylene)s on the relative amount of cis - and trans -vinylene units has been investigated. A regular blue-shift of the absorption spectra in solution has been observed upon the addition of cis double bonds. Luminescence spectra show a vibronic structure which accounts for the reduced inter-ring torsion in the excited state with respect to the ground state. Moreover, an increased electron–phonon coupling for cis -rich polymers has been observed.

Femtosecond relaxation dynamics in dialkoxy-substituted poly-(p-phenylenevinylene) derivatives

We investigated the sub-picosecond relaxation dynamics of two alkoxy-substitued poly-p-phenylenevinylenes that differ only for the structure of the alkoxy side chains. Photoinduced absorption, bleaching, and stimulated emission were observed in distinct spectral regions. Our results show that the temporal dynamics of stimulated emission strongly depends on excitation density. We ascribe such behavior to on-chain ultrafast nonlinear processes, which are almost independent on the substituents grafted onto the polymeric backbone.

Third-order nonlinear optical properties of copoly(2,3,5,6-tetrafluoro-1,4-phenylenevinylene-2,5-dialkoxy-1,4-phenylenevinylene)s: a novel class of push-pull substituted PPVs

A series of copoly(2,3,5,6-tetrafluoro-1,4-phenylenevinylene-2,5-dioctyl oxy-1,4-phenylenevinylene)s containing variable ratios of tetrafluorophenylenevinylene and dialkoxyphenylenevinylene repeating units has been synthesized via the Stille cross-coupling reaction in order to investigate the influence on the (chi) (3) of the simultaneous presence of electron-deficient and electron-rich aromatic monomers in the PPV backbone. Third-order nonlinear optical properties of the resulting materials have been studied in chloroform solutions at (lambda) =1064 nm using the picosecond Z-scan technique. The nonlinear refractive index has been found to be negative, strongly dependent on the copolymer composition, and as high as (-10+/- 2)x10-12cm2/W.

Nonlinear optical measurements in copoly(2,3,5,6-tetrafluoro-1,4-phenylenevinylene-2,5-dialkoxy-1,4-phenylenevinylene)s using the Z-scan technique

X(3) dependence on relative concentration of electron-rich and electron-deficient monomers has been investigated with Z-scan in copoly(2,3,5,6-tetrafluoro-1.4-phenylenevin ylene-2,5-dialkoxy-1,4-phenylenevinylene)s. The polymer obtained by reacting equimolar quantities of the two monomers showed X(3) as high as (6±2)x10-10 esu.