Ultrafast Ground-State Recovery of Single-Walled Carbon Nanotubes

The band-edge relaxation dynamics in isolated and bundled single-walled carbon nanotubes (SWNTs) were studied with ultrafast transient absorption spectroscopy. The loss of the excited state population was unexpectedly fast, with $>90\%$ of the transient absorption signal recovering in a few hundred fs. For isolated SWNTs in resonance with a SWNT electronic transition, a novel relaxation pathway was observed, characterized by a relatively slow signal recovery ($>100\mathrm{p}\mathrm{s}$). We ascribe this relaxation pathway to the decay of excited electrons from the band-edge.

Figure 1

Normalized one-color transient absorption of B-H-SWNTs (triangles) and Mic-H-SWNTs (circles) with the autocorrelation of the laser pulse (dashed line) with excitation at 1350 nm (nonresonant). The solid lines are curve fits to a bi-exponential function with time constants of 230 fs and 1 ps for the bundle sample and 330 fs and 2 ps for the isolated sample. Inset: Long time response ($>1\mathrm{p}\mathrm{s}$) of B-H-SWNTs (open circles) and Mic-H-SWNTs (solid circles) with pump and probe at 1250 nm. The solid line is a bi-exponential fit with time constants of 700 fs and 120 ps and 300 fs and 2 ps for Mic-H-SWNTs and B-H-SWNTs, respectively.

Figure 2

(a) Subpicosecond decay constant of Mic-H-SWNTs (open squares), B-H-SWNTs (solid squares), Mic-L-SWNTs (open circles), B-L-SWNTs (solid circles) and Mic-A-SWNTs (open diamonds) as function of pump wavelength in one-color experiments. (b) Normalized transient absorption signals for SWNTs pumped and probed at 1500 nm. The circles, squares, and triangles correspond to Mic-L-SWNTs, Mic-A-SWNTs, and Mic-H-SWNTs, respectively.

Figure 3

Two-color transient absorption spectra for Mic-H-SWNTs with $\mathrm{\text{pump wavelength}}=1250\mathrm{n}\mathrm{m}$.

Figure 4

Absorption spectrum (dashed line) and slow decay constant of Mic-H-SWNTs as a function of probe wavelength (solid line). The circles and squares correspond to pump at 1170 nm and 1250 nm, respectively.