Plasmonic bipyramids for fluorescence enhancement and protection against photobleaching.

A great number of studies focus their interest on the photophysical properties of fluorescent hybrid gold nanoparticles for potential applications in biotechnologies such as imaging and/or treatment. Spherical gold nanoparticles are known to quench a chromophore fluorescent signal, when moieties are located in their close vicinity. The use of a polymer spacer on such a system allowed only partial recovery of the dye emission by controlling the surface to dye distance. Gold-based anisotropic sharp nanostructures appear to exhibit more interesting features due to the strong electric field generated at their edges and tips. In this paper, a complete study of hybrid fluorescent bipyramidal-like gold nanostructures is presented. We describe the chemical synthesis of gold bipyramids functionalized with fluorescent water-soluble polymers and their photophysics both in solution and on a single object. We show that the use of a bipyramidal shape instead of a spherical one leads to total recovery of the fluorescence and even to an enhancement of the emission of the dyes by a factor of 1.4.

Novel ruthenium(II) and zinc(II) complexes for two-photon absorption related applications.

Two new fluorene derivatized 1,10-phenanthroline ligands and related tris-chelate Ru(II) or Zn(II) coordination complexes have been synthesised. The linear and nonlinear (two-photon induced fluorescence) photophysical measurements have contributed to highlight the possibility to tune the absorption spectral range and excited lifetime, depending on ligand substitution and nature of the metal. More significantly, the observation of two-photon absorption (TPA) associated with long-lived metal-to-ligand charge-transfer (MLCT) excited states in the Ru(II)-based chromophores, opens a wide range of applications in the near infrared.

Diarylethene microcrystals make directional jumps upon ultraviolet irradiation.

Microcrystals of a diarylethene {1,2-bis[5'-methyl-2'-(2"-pyridyl)thiazolyl]perfluorocyclo-pentene} undergo jumps upon photoirradiation. These photochromic crystals present molecular structural changes upon irradiation with ultraviolet light because of reversible photocyclization reactions. When the energy absorbed by crystals reaches about 10 microJ, the uniaxial stress induced in the crystal lattice relaxes through directional jumps. If one prevents crystals from jumping, then parallel, equidistant cracks appear on crystal surfaces. These photomechanical effects could result from a Grinfeld surface instability.

Synthesis, characterization and optical properties of pi-conjugated systems incorporating closo-dodecaborate clusters: new potential candidates for two-photon absorption processes.

Non-centrosymmetric pi-conjugated systems incorporating closo-dodecaborate clusters, [NC-C6H4-C(H=N(H)-B12H11]-(2), [NC-C6H4-C(H)=C(H)-C(6)H(4)-C(H)=N(H)-B12H11]-(3), and [NC-C6H4-C(H)=C(H)-C6H4-C(H)=C(H)-C6H4-C(H)=N(H)-B12H11]-(4) have been synthesized by reaction of the monoamino derivative of B12, [B12H11NH3]-(1), with various arylaldehydes, R-C6H4-CHO. These Schiff base-like compounds were fully characterized by multinuclear NMR spectroscopy and mass spectrometry. In order to evaluate these boron rich pi-systems as potential materials for two-photon absorption (TPA) processes, UV linear absorption curves were recorded for 3 and 4, and comparatively studied with those of the boron-free pi-systems NC-C6H4-C(H)=N-CH3(5) and NC-C6H4-C(H)=C(H)-C6H4-C(H)=N-CH3(6). The donor effect of the boron cluster was evidenced by a shift to the lower energy of the absorption band in the spectra of systems incorporating B12. The two photon absorption (TPA) spectrum of compound , obtained by the up-conversion method, shows a resonance at 720 nm with a cross-section sigma(TPA) of 35 x 10(-50) cm(4) s photon(-1) molecule(-1). This value suggests the potential of B12 clusters to be used as new donor groups for the synthesis of non-linear materials.

Characterization of the two-photon absorption resonance that is due to the internal charge-transfer transition of a push pull molecule, 4-(diethylamino)-beta-nitrostyrene.

The two-photon absorption resonance that is due to the internal charge-transfer transition of an organic push-pull molecule has been characterized. A nonlinear absorption spectrum of the 4-(diethylamino)- beta-nitrostyrene molecule was measured in a tetrahydrofuran solution by optical Kerr ellipsometry. The shape and the amplitude of the two-photon absorption spectrum are well described by a model that uses the relevant one-photon absorption spectrum related to the internal charge-transfer transition.

Optical wave breaking and pulse compression due to cross-phase modulation in optical fibers.

When a probe pulse copropagates with a pump pulse inside an optical fiber, the two can interact through cross-phase modulation. It is shown that an interplay between the effects of group-velocity dispersion and cross-phase modulation can lead to optical wave breaking that manifests as rapid oscillations near the leading or the trailing side of the probe pulse. Qualitative features of this new kind of optical wave breaking are discussed, as well as the conditions under which it can be observed experimentally. The probe pulse can be compressed significantly by optimizing the initial delay between the pump and probe pulses even when the two pulses experience normal dispersion in the fiber.

15-THz pulse generation arising from modulation instability oscillation in a colliding-pulse mode-locking dye laser.

15-THz trains of a few-tens-of-femtoseconds optical pulses were generated by modulation instability oscillation in a colliding-pulse mode-locked dye laser. A repeatable and stable modulation instability regime was obtained with the laser operating in the anomalous-dispersion regime and with a low saturable-absorber concentration. Autocorrelation measurements indicate that subpulses were completely separated in real time. Variations of the modulation with the pump power and the amount of dispersion in the cavity are in good agreement with modulation instability theory.

Self-focusing, self-phase modulation, and diffraction in bulk homogeneous material.

The simultaneous effects of self-focusing, self-phase modulation, and diffraction on the propagation of an ultrafast pulse in a homogeneous Kerr medium are described. The competing effects of self-focusing and diffraction are shown to modify the shape and magnitude of the pulse amplitude and phase. These modifications are shown to affect the spectral distribution of the supercontinuum from that predicted by the conventional self-phase-modulation theory.

Observation of self-focusing in optical fibers with picosecond pulses.

Self-focusing was observed at Raman frequencies, using picosecond pulses propagating in a large-core optical fiber of 100-microm diameter. For intense input pulses, a continuum of Stokes frequencies was generated in a small ring-waveguide structure. The ring diameter of 11 microm was about 10 times smaller than the beam diameter at low intensities. The ring structure was attributed to an induced-gradient-index profile arising from the nonlinear index of refraction.