International audienceDipolar and symmetrical heptamethine dyes featuring strong linear absorption in the near-infrared spectral range (700−900 nm) were prepared at a gram scale. X-ray crystallographic structure of the dipolar chromophore is described revealing the remarkable planarity of the polyenic chain due to the central ring of the molecule. These chromophores present promising nonlinear absorption properties at telecommunication wavelengths (1400−1600 nm):  (i) significant two-photon cross-sections were measured using the femtosecond Z-scan method, and (ii) their exceptional solubility allows us to carry out an optical-limiting experiment at 1500 nm in the nanosecond regime. In this regime, experimental data were interpreted on the basis of two-photon absorption (2PA) and 2PA-induced excited-state absorption (ESA). Finally, these systems, which present a lot of relevant properties (significant nonlinear absorption properties, two-step gram scale synthesis, high solubility, good thermal stability, and several sites of functionalization), open the way to numerous practical applications in biology, solid-state optical limiting, and signal processing

The nonlinear optical response of gold particles (with 2.5, 9, or 15 nm radius) prepared by γ-radiolysis in water solution and stabilized by poly(vinyl alcohol) is size-dependent. The 2.5 nm clusters do not limit light transmission even at very high fluence of nanosecond laser pulses, while the larger clusters strongly limit the laser beam transmission at 530 nm. The threshold of limitation and the amplitude depend on the size of the particles. The rise of the optical limiting effect is measured by picosecond laser spectroscopy. For the largest particles, it lasts 1 ns. The limitation effect is attributed to large light-scattering centers induced by the pulse around the initial particles.