Patrick Feneyrou

Excited state absorption: a key phenomenon for the improvement of biphotonic based optical limiting at telecommunication wavelengths

Spectroscopic properties, two-photon absorption (TPA) and excited state absorption (ESA), of two organic cyanine dyes and of a ruthenium based organometallic cyanine are compared in order to rationalize their similar ns-optical power limiting (OPL) efficiency in the telecommunication wavelength range. The TPA contribution to the ns-OPL behavior is higher for both organic cyanines, while the main process is a TPA-induced ESA in the case of the organometallic system, in which the ruthenium induces a broadening of the NIR-ESA band and resulting in a strong spectral overlap between TPA and ESA spectra.

Efficient hybrid materials for optical power limiting at telecommunication wavelengths

This work describes the preparation and characterization of the first efficient solid-state optical limiter operating in the NIR region (telecommunication wavelengths). Nonlinear absorbing chromophores (azabodipy) are incorporated into a sol–gel monolithic matrix using a specifically adapted process, which preserves the chemical integrity of the dyes in the solid. Efficient broadband OPL performances in the solid state could be observed for the first time in the NIR region between 1200 and 1600 nm, with a maximal efficiency around 1300 nm. The properties observed in the solid sol–gel matrix are even better than those in solution, which shows the great potential of this approach regarding protection of active imaging systems against self-dazzling and laser aggression.

Design of near-infrared dyes for nonlinear optics: toward optical limiting applications at telecommunication wavelengths

The rapid development of frequency-tunable pulsed lasers up to telecommunication wavelengths (1400-1600 nm) led to the design of new materials for nonlinear absorption in this spectral range. In this context, two families of near infra-red (NIR) chromophores, namely heptamethine cyanine and aza-borondipyrromethene (aza-bodipy) dyes were studied. In both cases, they show significant two-photon absorption (TPA) cross-sections in the 1400-1600 nm spectral range and display good optical power limiting (OPL) properties. OPL curves were interpreted on the basis of TPA followed by excited state absorption (ESA) phenomena. Finally these systems have several relevant properties like nonlinear absorption properties, gram scale synthesis and high solubility. In addition, they could be functionalized on several sites which open the way to numerous practical applications in biology, solid-state optical limiting and signal processing.

Near IR two photon absorption of cyanines dyes: application to optical power limiting at telecommunication wavelengths

The design and synthesis of symmetrical and unsymmetrical heptamethine cyanines is reported. These chromophores present significant two-photon cross section in the 1400-1600 nm spectral range. In addition, they display optical power limiting (OPL) properties. OPL curves were interpreted on the basis of two-photon absorption (2PA) followed by excited state absorption (ESA). Finally, these molecules present several relevant properties (nonlinear absorption properties, two-step gram scale synthesis, high solubility, good thermal stability), which could lead to numerous practical applications in material science (solid state optical limiting, signal processing) or in biology (imaging).

Biphenyl derivatives with enhanced nonlinear absorptivities for optical limiting applications

Novel conjugated chromophores were designed and investigated for optical power limitation based on multiphoton absorption processes. Their design is based on the push-push functionalization of a semi-rigid elongated system derived from the extension of biphenyl cores. Biphenyl moieties with tunable twist angle were examined. Phenylene-vinylene rods were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency. These derivatives combine wide linear transparency and enhanced nonlinear absorptivities in the visible range. Pump-probe Kerr ellipsometry indicates large excited-state absorption cross-sections (with typical σe values of 5 10-16 cm2) while nanosecond nonlinear transmission measurements and optical limitation experiments reveal very strong nonlinear absorption that can be fitted by a three-photon absorption process (leading to α3 values up to 18000 cm3 GW-2). Such behavior results from a sequential multiphoton process involving excited-state absorption subsequent to two-photon excitation (with typical σ2 values of 5 10-20 cm4 GW-1). Both the linear transparency, the photostability and the nonlinear absorption spectral characteristics of these derivatives can be tuned by playing on the biphenyl twist angle. As a result, chromophores combining good linear transparency and enhanced nonlinear absorptivities in the visible range have been obtained.

Optical limiting properties of organic nonlinear crystals

The optical limiting properties of 17 organic molecular crystals (natural faces) have been characterized with 250-ps pulses at &=532 nm. The best nonlinear absorption coefficients are f3=120 cm/OW and f3=400 cm/GW. Such high nonlinear absorptions are explained by the efficient ciystal packing of molecular two-photon absorptions. The ciystal phase allows high molecule concentration, efficient molecule orientation, and enhanced local field factor. Using the frequency-dependent molecular hyperpolarizability and the oriented-gas model one predicts that these high nonlinearities cover the whole visible spectrum.

New generalized model based on Onsager's transport equations for describing PDLC's morphology

Electro-optical properties of polymer dispersed liquid crystal (PDLC) or holographic polymer dispersed liquid crystal (H-PDLC) are very sensitive to the photoinduced phase separation process (PIPS). In order to improve initial mixture and recording setup, real time monitoring of diffraction efficiency is currently performed using a diffusion model based on the moderation Ficks law. Nevertheless, this model does not take into account neither change of affinity for liquid crystal molecules when the monomer polymerization occurs nor the droplets morphology observed by scanning electron microscopy. In this paper, a new model consistent with the general Onsager theory of transport is introduced. As an application, droplets growth and spatial response of H-PDLC films are described using dimensionless numbers and very general normalized parameters which open new method of improvement for electro-optical devices based on PDLCs or H-PDLCs.