An ultrafast pump−probe spectroscopy was used to investigate the transient phenomena of the excitons induced by the electric field to gain an insight into the effect of electric field on the organic light emission diode. At high excitation density of 230μJ/cm2 the decay of the singlet excitons showed fast and slow components. The amplitude factor and the relaxation time of the fast component are field-dependent. The slow component is field-independent, and its relaxation time-constant is about 890ps. We also observed the longitudinal acoustic phonons with sound velocity of 17Å/ps generated by excitation pulse.

By femtosecond pump-probe absorption spectroscopy we demonstrated intersubband and phonon-assisted transitions in single-walled carbon nanotubes (SWNTs). After resonant excitations of specific tube types, electrons are promoted from the ground state to the second excitonic state (EX2) in PLV-prepared SWNTs or to vibrational excited levels of the first excitonic state (EX1) in HiPco-prepared SWNTs. At time-zero delay a [1 + 1] resonant cross-correlation due to short lifetimes of higher excited states or phonon modes can be observed. These results suggest that intersubband transitions between the excitonic states in SWNTs are allowed. The interpretation of the temporal profiles at pump/probe combinations implies that the G-band mode, which is known to have a high Raman intensity, plays an important role for the interpretation of both femtosecond cross-correlation measurements and photoluminescence spectra.

Two types of individualized single-walled nanotubes (SWNTs) in aqueous surfactant suspensions have been studied by femtosecond two-color absorption spectroscopy. A careful selection of pump and probe wavelengths allows for the determination of the “intrinsic” lifetimes of the lowest excited states which depend on the diameter of the SWNTs. Furthermore, a fast decay component in the ps to sub-ps regime was also observed and tentatively attributed to bundle relaxation. Likewise, this experimental approach can also assess spectral regions with decays faster than 1 ps, which is of great interest for all-optical switching devices near the optical telecommunication wavelength.

Femtosecond one- and two-colour pump-probe spectroscopy of single-walled carbon nanotubes (SWNTs) individual in aqueous surfactant suspensions has been used to assess the "intrinsic" lifetime of the lowest excited states. We demonstrate that such measurements can be perturbed by several competing photophysical processes thus making lifetime deconvolution difficult. Furthermore we show how these effects, arising primarily from sample heterogeneity, can be reduced. Measurements of induced transients in the near IR yield lifetimes of (35 +/- 10) ps and (56 +/- 10) ps, for nanotubes having mean diameters of 0.95 and 1.2 nm, respectively. Furthermore, a fast decay component in the ps to sub-ps regime is also observed. We tentatively attribute this to relaxation in SWNT bundle components.

The ultrafast photophysics of D2O/sodium dodecylbenzene sulfonate surfactant dispersions of single-walled carbon nanotubes enriched in individual tubes (versus tube bundles) were studied by femtosecond pump-probe spectroscopy in the near-IR (NIR) spectral range. Measurements at 920 nm excitation and variable probe wavelengths showed evidence of superimposed transient bleaching as well as induced absorption behaviour. Our results indicate that such nanotube samples manifest ultrafast pump-induced switching of probe transmission with switching times of less than 1 ps under appropriate conditions. Given their high photochemical and photophysical stability these materials may be suitable candidates for the development of ultrafast NIR optical switches and logic gates.