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 (τ>1 ns) transient bleach component for three semiconducting SWNT species.