Intersubband Exciton Relaxation Dynamics in Single-Walled Carbon Nanotubes

We study exciton (EX) dynamics in single-walled carbon nanotubes (SWNTs) included in polymethylmethacrylate by two-color pump-probe experiments with unprecedented temporal resolution. In the semiconducting SWNTs, we resolve the intersubband energy relaxation from the EX2 to the EX1 transition and find time constants of about 40 fs. The observation of a photoinduced absorption band strictly correlated to the photobleaching of the EX1 transition supports the excitonic model for primary excitations in SWNTs. We also detect in the time domain coherent oscillations due to the radial breathing modes at $\approx 250{\mathrm{cm}}^{-1}$.

Figure 1

Linear optical absorption spectrum of the sample. The energy ranges for first (EX1), second (EX2), and third (EX3) excitonic transition of the semiconducting SWNTs, as well as for the metallic SWNTs, are indicated. Spectra of the different ultrashort pulses used for excitation are also shown.

Figure 2

(a) $\Delta T/T$ dynamics of SWNTs excited at 0.92 eV and probed at 0.95 eV (circles) together with fit (solid line) and pump pulse autocorrelation ($\mathrm{\text{squares}}+\mathrm{\text{line}}$); (b) same as (a) but at 2 eV probe wavelength and with pump-probe cross correlation. Inset shows the measurement on a longer time scale.

Figure 3

(a) $\Delta T/T$ dynamics of SWNTs excited in the visible and probed at 0.92 eV (circles) together with fit (solid line) and pump-probe cross correlation ($\mathrm{\text{squares}}+\mathrm{\text{line}}$); (b) same as (a) but at 2.15 eV probe energy and with pump pulse autocorrelation. Inset of Fig. 3b shows intensity dependence of the dynamics at 2.15 eV.

Figure 4

$\Delta T/T$ spectra of SWNTs excited by a sub-10 fs visible pulse at different pump-probe delays. The lower panel also shows a $\Delta T/T$ spectrum with IR excitation (0.92 eV).

Figure 5

(a) $\Delta T/T$ dynamics of SWNTs excited at 1.35 eV and probed at 1.35 eV (circles) together with fit (solid line) and pump pulse autocorrelation ($\mathrm{\text{squares}}+\mathrm{\text{line}}$); (b) $\Delta T/T$ dynamics of SWNTs excited at 2.75 eV and probed at 2.1 eV (circles) together with fit (solid line) and pump-probe cross correlation ($\mathrm{\text{squares}}+\mathrm{\text{line}}$).