Photoinduced transient mid-infrared absorption in single-walled carbon nanotubes

We have performed optical pump - mid-infrared (MIR) probe spectroscopy on single-walled carbon nanotubes (SWNTs). The second excitonic absorption band ($E_{22})$ of (6,5) SWNTs was resonantly excited and the resulting photoinduced absorption was monitored in the MIR range (3.5 -- 5.5 $\mu $m) in a time range up to several hundred ps. Carrageenan films containing individualized CoMoCAT SWNTs formed on sapphire substrates were used for the measurement. This sample is optically transparent in the $\sim $3.5 -- 6 $\mu $m region, where the transition of $E_{11}$ excitons from the lowest dark state (1$g)$ to the second bright state (2$u)$ is expected to be observed. Our preliminary data shows the existence of photoinduced absorption in the investigated range. The origin of the observed transient absorption will be discussed.