Present-day methods for determining the performance of third-order nonlinear optical materials include Z-scan, degenerate four-wave mixing and third-harmonic generation (THG). All these techniques possess severe drawbacks; for example, in THG, since all media (air and glass walls of the cell) present a third-order effect, eliminating these contributions requires careful, complex analysis or use of vacuum chambers. We have developed nonlinear scattering as a sensitive, straightforward technique for determining the second hyperpolarizability of samples in solution. Herein, we will for the first time show the applicability of the technique to measure organometallic Ru-complexes, optimized for high nonlinear responses. The investigated compounds showed a significant second hyperpolarizability |γ|, ranging from 1.1 for the least efficient to 2.8 ∙ 10-33 esu for the most efficient molecule, and comparable to fullerene C60 in thin films. It was deemed infeasible to extract hyperpolarizabilities using a high-frequency femtosecond laser source by a modified z-scan setup, which, in contrast to nonlinear scattering, could not account for the high degree of thermal lensing present in the investigated compounds.
Both nonlinear absorption and nonlinear refraction are effects that are potentially useful for a plethora of applications in
photonics, nanophotonics and biophotonics. Despite substantial attention given to these phenomena by researchers
studying the merits of disparate systems such as organic materials, hybrid materials, metal-containing molecules and
nanostructures, it is virtually impossible to compare the results obtained on different materials when varying parameters
of the light beams and different techniques are employed. We have attempted to address the problem by studying the
properties of various systems in a systematic way, within a wide range of wavelengths, and including the regions of onephoton,
two-photon and three-photon absorption.
The objects of our studies have been typical nonlinear chromophores, such as π-conjugated molecules, oligomers and
polymers, organometallics and coordination complexes containing transition metals, organometallic dendrimers, small
metal-containing clusters, and nanoparticles of various kinds, including semiconductor quantum dots, plasmonic
particles and rare-earth doped nanocrystals. We discuss herein procedures to quantify the nonlinear response of all of
these systems, by defining and comparing the merit factors relevant for various applications.
There is much interest in nonlinear absorbing chromophores for applications in photonics, nanophotonics and biophotonics. We have performed studies of dispersion of the nonlinear absorption cross sections and the refractive nonlinearities of organic and organometallic nonlinear chromophores using the technique of Z-scan, with a tunable amplified femtosecond laser system. Z-scan is less sensitive than the popular technique of two-photon induced fluorescence but has advantages of being suitable for non-fluorescent substances and providing information on both absorptive and refractive nonlinearities. We have analysed the experimental results in terms of simple models and using the Kramers-Kronig transformation as shown in this paper for Coumarine 307 and an organometallic dendrimer. The dispersion curves are often dominated by two-photon resonances but inclusion of other nonlinear mechanisms seems to be necessary for better understanding of their features.
We have been studying third-order NLO properties of ruthenium-containing organometallic dendrimers. These molecules offer large hyperpolarizabilities and the possibility to control both the refractive and absorptive parts of the nonlinear response by electrochemical switching of the oxidation state of the ruthenium centres. Time-resolved studies indicate that it is possible to switch the molecules between the form in which they are two-photon absorbers and the state where they are fast (~ 1 picosecond time scale) saturable absorbers. Measurements of the dispersion of third-order nonlinearity have been made by the Z-scan technique on a nitro decorated dendrimer. They indicate that competition between two-photon absorption and absorption saturation is present in some wavelength ranges. The dispersion of both the real and imaginary parts of the cubic polarizability could be modelled by considering leading terms of the dispersion relation.
Transition metal carbonyl clusters incorporating group 6 (molybdenum, tungsten) and iridium atoms in a tetrahedral or butterfly-shaped four-atom cluster core, together with carbonyl, cyclopentadienyl and alkyne ligands, have been synthesized and incorporated into oligourethanes, and their optical limiting properties assessed by open-aperture Z-scan (ns pulses, 523 nm) and time-resolved pump-probe studies (ps pulses, 527 nm). The Z-scan studies reveal that the tetrahedral [M2Ir2] cluster cores (M = Mo, W) displayed a greater effective nonlinear absorption coefficent β2 than the [MoIr3] cores; the tungsten example, W2Ir2(CO)10(η-C5H5)2, exhibited the highest response. Substitution at the cyclopentadienyl group (including incorporation into a polymer backbone) had little effect on the response measured. A time-resolved investigation of the alkyne-adduct Mo2Ir2(μ4-η2-MeC2Ph)(CO)8(η-C5H4Me)2 using picosecond pulses at 527 nm reveals optical-power-limiting behaviour that results from electronic processes [specifically, a fast nonlinear absorption process followed by reverse saturable absorption involving long-lived (>1000 ps) metastable excited states].
We have investigated a large number of organometallic structures possessing high second and third order optical nonlinearities. Most third-order NLO experiments were performed with 100 femtosecond light pulses at 800 nm. While investigating structure-property relationships we note an enhancement of the real and imaginary parts of the cubic hyperpolarizability in structures of trigonal symmetries, and, in particular, of dendritic geometries with multipolar charge distribution, due to the presence of charge acceptor groups. Two-photon absorption cross sections achievable in these structures are comparable with the best values reported in the literature. We also show that, for some of the ruthenium alkynyl complexes investigated in this study, it is possible to perform electrochemical switching between two forms of the compound. The nonlinear properties of the two forms were investigated by the technique of Z-scan carried out in situ in an electrochemical cell. The neutral form is essentially nonabsorbing in the infrared but shows two-photon absorption at 800 nm. Upon oxidation the complex becomes infrared absorbing and the oxidized form exhibits an absorption saturation effect at 800 nm. This electrochromic switching of both the linear absorption and the sign of the imaginary part of the third-order susceptibility is found to be reversible and is, therefore, of potential application interest.
Hyper-Rayleigh scattering is used to investigate the nonlinear optical properties of novel organometallic compounds. For a series of ruthenium and gold y-arylacetylides, including some bimetallic complexes, the influence of the organometallic donor group, chain length and type of acceptor on the quadratic hyperpolarizability is studied. The hyperpolañzabiities found are among the highest (1027 esu) known for organometallic compounds. Moreover, electric-field induced second-harmonic generation confirms these values and indicates that only one major tensor component is present.
Keywords: hyper-Rayleigh scattering, nonlinear optics, second-harmonic generation, organometallic compounds
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