Band Gap Photobleaching in Isolated Single-Walled Carbon Nanotubes

Using pump-probe characterization, band gap photobleaching has been observed from semiconducting single-walled carbon nanotubes (SWNTs) isolated in aqueous suspensions in an effort to evaluate SWNTs as a near-infrared optical material. The measured photobleaching was optimized for co-polarization of pump and probe lasers in resonance with the second and first van Hove transitions, respectively. A saturation of photobleaching intensity was observed for large pump intensities greater than ∼1 kW cm^-2 and for probe intensities approaching ∼500 W cm^-2. The observed photobleaching is attributed to the relaxation of excited electron-hole pairs to the band edges, resulting in decreased absorption and increased stimulated emission.