Ultrafast photoresponse of metallic and semiconducting single-wall carbon nanotubes

Utilizing a transient absorption (TA) technique based on a chirp-corrected broadband probe, we have studied the ultrafast photoresponse of dispersed HiPco single-wall carbon nanotubes (SWNTs) over the range of 440–1050 nm for excitation in the range of 430–1700 nm. While both metallic and semiconducting SWNTs show transient bleaching at their $M_{11}$ and $S_{11}$ energies for excitation above these energies, metallic SWNTs uniquely exhibit a photoresponse to sub-$M_{11}$ excitation. We observe a TA spectral response for metallic SWNTs which is consistent with thermal broadening of the $M_{11}$ transition bands. In contrast to metallic SWNTs, specific semiconducting SWNTs exhibit transparency for sub-$S_{11}$ excitation, in accord with the expected zero density of states below the first interband transition. We report the observation of a long-lived $(\tau >1\mathrm{ns})$ transient bleach component for three semiconducting SWNT species.