Non Linear Optical Properties of Singlewall Carbon Nanotubes for Optical Limiting
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The physical mechanisms involved are discussed. In order to gain a deeper insight into the origin of optical limiting properties in these materials, we carried out a pump-probe experiment at 1064nm [6]. The dual-beam (pulsed-pump, cw-probe) technique allows, simultaneous measurements of both fast and slow nonlinear mechanisms contributing to nonlinear scattering by recording cw-probe transmission. MATERIALS Singlewall carbon nanotubes were synthesised by the electric arc discharge technique [7], using a set-up similar to that used for the synthesis of fullerenes. As-prepared samples contain impurities such as amorphous carbon, graphite and residual catalysts. The SWNT samples were purified in a three-step procedure [8]. After such a purification procedure, the amount of SWNT is close to 90 vol.% (as estimated from scanning electron microscopy) in the samples. The diameter of the tubes lies between 1.3 and 1.5 nm [9] and their length is about several micrometers. For optical limiting and Z-scan, suspensions are stocked in 5-mm thick fused silica cells and for pump-probe experiment in 2-mm cells. The linear transmittances at 532 and 1064 nm, for the different materials, are about 70%.
257 Mat. Res. Soc. Symp. Proc. Vol. 597 0 2000 Materials Research Society
OPTICAL LIMITING BEHAVIOUR The experimental set-up for nonlinear transmittance measurements is based on a classical f/5 focusing geometry [4]. The source is a Q-switched Nd:YAG laser, generating 7ns pulses at 1064nm or 532nm. We carried out comparative optical limiting experiments for different carboneous materials (multiwall carbon nanotubes (MWNT and carbon black suspension (CBS)). A comparison between the different samples at 532 nm and 1064 nm is given in figure 1. ...... . . .
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Figure1 Comparisonof optical limiting behavioursof the suspensions of SWNTs, MWNTs and CBS in water/surfactantat 532 nm (a) and 1064 nm (b). The CBS and the nanotubes show similar responses at 532 nm, both for the optical limiting threshold and the minimum transmittance. By contrast, at 1064 nm, the minimum transmittance at 5 mJ is found to be twice smaller for nanotubes as compared to CBS. Nanotubes are therefore promising candidates for optical limiting application over a broad spectral range. Z-SCAN EXPERIMENT For a better understanding of the origin of optical limiting in SWNT [10], Z-scan experiments have been carried out (figures 2 and 3). The Z-scan experiment (open and closed aperture) is realized at 532 nm and 1064 nm using a f/30 focusing geometry. At low incident energies and with the fully open aperture, one gets the expected dip in the curves whose depth is nearly proportional to the incident energy at low energy. From these measurements, one can extrapolate [10] an effective nonlinear coefficient f3 : 5x10]
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