Carbon Nanotube Polycarbonate Composites for Ultrafast Lasers

Single-wall carbon nanotubes (SWNTs) are at the center of nanotechnology research. Their nonlinear optical behavior makes them promising for applications in ultrafast photonics. SWNTs are indeed good saturable absorbers, i.e., they are transparent to high-intensity light, with ultrafast recovery time. Saturable absorbers must always keep a delicate balance between the modulation depth and nonsaturable losses, while ensuring a short recovery time. The use of SWNTs dispersed in a composite material has so far proven to be one of the most successful approaches to their application as saturable absorbers. In fact, the modulation depth, which is proportional to the SWNT concentration, can be controlled by tuning the SWNT concentration within the material, while the nonsaturable losses due to scattering from SWNT bundles can be minimized by making the bundles smaller than the device operation wavelength. The latter can be achieved when SWNTs are dispersed in a solvent. The presence of bundles is, however, useful to minimize the recovery time. The use of a polymer matrix offers the best compromise in terms of bundle-size control and ease of device integration. This can be optimized at the SWNT dispersion stage, and probed by photoluminescence (PL) spectroscopy, which allows for the detection of isolated SWNTsas well as small bundles. The ideal host polymer for SWNTs to be used as saturable absorbers must be highly transparent in the wavelength range of interest, reducing propagation losses and ensuring minimum refractive index mismatch with SWNTs to avoid scattering. The host should also be thermally and environmentally stable, as thermal damage or moisture adsorption can adversely affect its transparency. However, polyvinylalcohol (PVA), the polymer most widely used so far for this application, does not meet these requirements,