Anisotropic saturable absorption of single-wall carbon nanotubes aligned in polyvinyl alcohol

We dispersed single-wall carbon nanotubes (SWNTs) in polyvinyl alcohol (PVA) film mostly with nanoscale uniformity and mechanically stretched it. Polarized absorption spectra showed strong anisotropy in the near-infrared band gap transition of semiconductor SWNT with a dichroic ratio of 7.4, indicating good alignment of the embedded SWNT. The saturable absorption property of the band gap transition measured using a polarized femtosecond laser showed strong dependence on the polarization angle. When the polarization of the light was parallel to the stretching direction, the saturation power was 2.6 times smaller than that of the unstretched film, which we ascribed to the increased transition probability.