Nanotube-based systems for broadband optical limiting: towards an operational system
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Nanotube-based systems for broadband optical limiting : towards an operational system N. Izard1,2, D. Riehl1, E. Anglaret2 1-Centre Technique d'Arcueil, DGA, Arcueil, France 2-Groupe de Dynamique des Phases Condensées, UMR CNRS 5581, Université Montpellier 2, Montpellier, France ABSTRACT Nanotube-based systems are good candidates for optical limiting against broadband laser pulses. We explore new routes to improve their limiting performances. We show that the diameter of the nanotubes is a key factor to control the performances. On the other hand, we demonstrate that chemically modified nanotubes can be mixed with organic chromophores, leading to high performance composite limiting systems which are particularly efficient in the nanosecond regime due to the cumulative effects of nonlinear scattering and multiphoton absorption. INTRODUCTION Carbon nanotubes are fascinating molecular systems which display original electronic and optical properties. When homogeneously dispersed in organic solvents, nanotubes display optical limiting properties [1]. The main limiting phenomenon is a strong nonlinear scattering due to heating of the nanotubes and the concomitant growth of solvent bubbles at low fluences and carbon vapor bubbles at larger fluences [1]. However, the performances of nanotubes still don't reach those required for effective, auto-activated, protection of eyes against laser beams. In this paper, we explore two routes for optimizing the optical limiting performances of nanotube-based systems. First, we study the relation between nanotube structure and limiting performances. Second, we develop composite systems by mixing nanotubes with organic chromophores, and we show that the association of nonlinear scattering from nanotubes and two-photon absorption from chromophores is an effective way to prepare high performance optical limiting systems. EXPERIMENTAL We studied both single wall (SWNT) and multiwall carbon nanotubes (MWNT). SWNT were prepared by the electric arc technique (from Nanoledge, Inc). MWNT were prepared by CVD techniques using different physico-chemical parameters in order to control both the length and diameter of the nanotubes (from Nanolab, Inc). For preparing composites, high performance two-photon absorbers (TPA) were synthesized in the group of Chantal Andraud at Ecole Normale Supérieure de Lyon. The results we present below are for a fluorene pentamer (FMP). Aqueous suspensions of SWNT and MWNT were prepared using SDS, an ionic surfactant. When the suspensions are sonicated at low powers for a few hours, homogeneous and stable suspensions are obtained. In the case of SWNT, the nanotubes remain aggregated into bundles, as stated by electron microscopy and Raman measurements (not shown). By contrast, high power sonication (500 W during 15 minutes) leads to exfoliation of the bundles [2]. After an ultracentrifugation step (120000 g during 4 hours), aqueous suspensions of individual SWNT are formed, as stated by electron microscopy, photoluminescence [3] and photon correlation spectro
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