A series of structurally-related multipolar chromophores of different symmetry (dipolar, quadrupolar, octupolar, dendritic...), and shape (rod-like, Y-shaped...) propeller-shaped, were investigated for optical power limiting based on multiphoton absorption processes. Their design is based on the functionalization of nanoscale linear or branched conjugated backbones with electro-active (i.e. electron-releasing or electron-withdrawing) peripheral and core/node groups. Their two-photon absorption (TPA) spectra were determined by investigating their two-photon-excited fluorescence properties in the NIR region using pulsed excitation in the femtosecond regime. These studies provide evidence that the charge symmetry plays an important role, the quadrupolar chromophores leading to giant TPA cross-sections in the visible red. Furthermore, modulation of the nonlinear absorptivity/transparency/photostability trade-off can be achieved by playing on the nature of the electroactive groups and of the spacers. Interestingly, higher-order charge symmetries and branched structures provide an innovative route for TPA amplification and/or spectral broadening in the NIR region.

1,4-Bis(4-aminostyryl)benzene derivatives have been shown to exhibit large two-photon cross-sections at ca. 730-745 nm and are typically highly fluorescent in organic solvents. For biological imaging applications, we have been interested in identifying water-soluble dyes with high two-photon cross-sections and fluorescence quantum yields. Four new bis(aminostyryl)benzene chromophores have been synthesized; these chromophores are functionalized with phosphate, sulfonate, and sulfate hydrophilic groups. In some cases, this substitution pattern renders the chromophores water-soluble and, for the sulfonate and sulfate cases, moderate fluorescence quantum yields are retained in water.

We report here the nonlinear absorption of bifluorene derivatives induced by two-photon absorption (TPA), in order to study the effect of the dimension of the molecule on these properties. Measurements were performed in chloroform between 450 and 650 nm for nanosecond time duration pulses. The nonlinear absorption is attributed to a three-photon absorption process involving a first TPA step followed by an excited state absorption process. The 2D first generation organic dendrimer presents the highest values of the corresponding three-photon absorption coefficient α₃, while the 3D systems presents lower efficiencies close to that of the bifluorene itself.

We describe a general approach for the synthesis of 6,6'-(disubstituted)-3,3'-bipyridine based chromophores. This combinatorial type strategy is based on (i) the synthesis of a library of conjugated building blocks end-capped with electron donor or acceptor groups, and (ii) their homo- or cross-coupling. The compounds are either dipolar (push-pull molecules) or apolar (symmetric D-A-A-D) molecules. Depending on the building blocks, we are able to tune both the structural and NLO properties of the chromophores. For example, the D-A-A-D structure possesses ultrafast nonresonant nonlinearity around 1550 nm with excellent figures of merit, as well as TPA in the visible and NIR ranges.

Deoxyribonucleic acid (DNA), extracted from salmon sperm through an enzyme isolation process, is a by-product of Japan’s fishing industry. To make DNA a suitable material for nonlinear optic (NLO) applications, it is precipitated with a surfactant complex, hexadecyltrimethlammonium chloride (CTMA). Preliminary characterization studies suggest DNA-CTMA may be a suitable host material for guest-host NLO polymer based electro-optic (EO) waveguide devices. The optical and electromagnetic properties of DNA-CTMA, as well as the development and EO measurement of a disperse red 1 (DR1) guest / DNA/CTMA host NLO material, are reported. Comparisons to a DR1 guest / poly(methyl methacrylate) (PMMA) host NLO material are made.

We report on the fabrication of photo-driven polymer microsensors for viscosimetry, velocimetry and micropump applications. They are readily made with a low-cost polymerization technique based on two-photon absorption. Microsensors are free-floating in the liquid to be characterized. A linearly-polarized optical tweezers is used to trap one sensor at the laser focal point and to generate the optical torque needed for local hydrodynamic measurements. Viscosity and velocity microsensors have slab shapes that align in the polarization direction. The local viscosity is deduced from the maximum rotation frequency generated by the rotating linear polarization, while the fluid velocity is obtained by measuring the maximum angle that equilibrates the optical torque and drag torque. Experimental results are in good agreement with theoretical calculations. The micropump is based on a micron-size Archimedes screw that rotates around its long axis when it is trapped at the focal point. The laser-induced rotation is due to the optical torque that is transferred by the laser scattering on the screw.

The fluorescence of melanin in human skin tissue reflects the structural changes in the process of skin cancer progression towards malignant melanoma. A selectively excitation of melanin in skin tissue, however, is only possible by stepwise two-photon absorption (TPA) in the NIR spectral range (λ_exc ≈ 800 nm). Due to the very short lifetime of the energy level populated by the absorption of the first photon (< 5 ps) an effective TPA process can be only achieved with laser pulses on the femtosecond time scale. To use the two-photon excited fluorescence (TPF) of melanin for early diagnosis of malignant melanoma a small, mobile TPF spectrometer was developed. It consists (i) of a fiber laser the radiation of which was amplified in a short pulse nitrogen laser pumped dye cell, (ii) an articulated arm with special mirrors for nearly lossless direction of the fs laser beam to the skin position of investigation, (iii) a camera objective which include visualization and excitation as well as acquisition of fluorescence and (iv) a monochromator with a multichannel photomultiplier and a PC with special software for spectrometer control and data processing. First investigations of the equipment concerning the early diagnosis of malignant melanoma were carried out.

We propose a simple exactly solvable kinetic model of trans ↔ cis processes accompanying build-up of diffraction gratings in a polymer matrix doped with azobenzene chromophores, illuminated with spatially modulated and linearly polarized light in degenerate two-wave mixing experiments. This model mimics the essential features of a more realistic kinetic model studied recently using Monte Carlo simulations. A qualitative agreement between an experiment, Monte Carlo simulations and simple kinetic modelling implies that basic photoisomerisation processes during diffraction grating recording are of relatively simple nature. In particular, two exponential dependence of diffraction efficiency versus time measured in experiments and found in Monte Carlo simulations is easily described by the model.

A metallic nano-probe has locally induced coherent anti-Stokes Raman scattering (CARS) of adenine molecules in a nanometric DNA network structure. The excitation fields and CARS polarization are enhanced by the tip apex of the nano-probe through the excitation of local surface plasmons. Owing to the third-order nonlinearity, the excitation of the CARS polarization is extremely confined to the end of the tip apex, resulting in the spatial resolution far beyond the diffraction limit of light. Our CARS microscope using a silver-coated probe visualized the DNA network structure at a specific vibrational frequency (~1337 cm^-1) of adenine molecules with a spatial resolution of ~15 nm and sufficient sensitivity.

We report photophysical measurements and application of an effective three-photon absorption model that characterize the two-photon and excited state absorption in organic D-π-A chromophores. The key parameter is an effective three-photon absorption coefficient that depends on the intrinsic molecular two-photon absorption cross section and excited state photophysical properties. We measure all of these molecular parameters independently in a variety of experiments and then compare the model predictions with nanosecond nonlinear absorption measurements. We find excellent agreement with the data using only experimentally measured molecular quantities and no free parameters. We conclude that excited state absorption from both singlet and triplets states is the dominant contribution to the nonlinear transmittance loss in the nanosecond regime, and that the chief role of two-photon absorption in this regime is to populate the excited triplet state.

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.

Two-photon absorption (TPA) cross section of some previously reported compounds is reexamined by the femtosecond open-aperture Z-scan method and the comparison among cross-section values measured by different measurement techniques is discussed. Also a TPA spectrum of a diacetylene derivative measured by the Z-scan method is presented. Drastic enhancement has been observed in the visible wavelength region of the spectrum, which is interpreted in terms of resonance enhancement of simultaneous TPA process. Contribution of stepwise TPA process to the observed enhancement is estimated by simulation for the dynamics.

Absorption, photoluminescence and photoinduced absorption spectra of polycarbazolyldiacetylenes (polyCzDAs) carrying selected acyl (polyDPCHD) or alkyl (polyDCHD-HS) groups are reported. The absorption spectrum of polyDCHD-HS films, which is almost independent of the temperature, shows an excitonic absorption followed by an inhomogeneously broadened vibronic progression. Very different is instead the case of polyDPCHD whose thin films show a very sharp excitonic transition and a vibronic progression with a lower electron-phonon (el-ph) coupling with respect to that of polyDCHD-HS. Unlike other polydiacetylenes (PDAs), the polyCzDAs which exhibit narrow spectral features and reduced el-ph coupling, such as polyDPCHD films and polyDCHD-HS in benzene solutions, are luminescent thus suggesting a different ordering of the dipole forbidden and allowed states. This interpretation is confirmed by two-photon spectroscopic data. The ordering of the A_g and B_u excited states also affects the generation of triplet excitons, which are the main long-living excited states of PDAs. The rise and decay of the triplet signal for polyDCHD-HS are accounted for through a dynamical model, based on a monomolecular decay regime including saturation effects. A more complex kinetics is instead observed for polyDPCHD. From the study of the kinetics of the PIA spectra both the triplet generation efficiency and the density of traps are evaluated.