International audienceCarbon nanotubes are attractive materials with numerous possibilities of applications in various domains. Nevertheless, pertinent characterization studies are required in order to insure reproducibility in preparation, handling and incorporation in devices. Several spectroscopic tools have been developed so far, such as transmission electron microscopy and near-field electrical microscopy. Optical techniques have also been improved with the use of Raman Scattering, Surface Enhanced Raman Scattering (SERS) and near-field optical spectroscopy (SNOM). These non-destructive techniques have been used for nanotubes after synthesis and exploited to follow the modifications of their spectroscopic features when they are functionalized or embedded in host matrices. In this paper, we show that, by using SERS conditions, a new effect is observed: a "single-beam pumped" Coherent anti-Stokes Raman Scattering (CARS). We demonstrate that under a tight-focusing of the excitation light, this emission, resulting from a wave mixing process between the incident laser light (ωl) and Stokes Raman light (ωs), is generated by a Surface Enhanced Raman Scattering (SERS) mechanism. Since abnormal anti-Stokes/Stokes intensity ratios are also observed for carbon nanotubes in powders, we have investigated in details their behaviour as a function of several parameters which include the excitation wavelengths, the laser power, the sample support, as well as the presence of nanotubes in isolated or in bundled form. It appears that resonance phenomena explain the anti-Stokes/Stokes intensity ratios in the case of powders and that CARS is only observed when SERS conditions are used. In addition, we have investigated other materials in which anomalies are also observed. This is the case of poly(bithiophene) (PBTh) electrochemically polymerized on carbon nanotubes for which its main Raman line at 1450 cm−1 is enhanced, this enhancement being presumably generated by the plasmon excitation of metallic tubes. In the case of PEDOT, the analysis of the anti-Stokes branch provides additional information on the functionalization of carbon nanotubes with this polymer
