Highly nonlinear pump fluence dependence was observed in the ultrafast one-color pump-probe responses excited by 38 fs pulses resonant with the E(22) transition in a room-temperature solution of (6,5) carbon nanotubes. The differential probe transmission (ΔT/T) at the peak of the pump-probe response (τ = 20 fs) was measured for pump fluences from ∼10(13) to 10(17) photons/pulse cm(2). The onset of saturation is observed at ∼2 × 10(15) photons/pulse cm(2) (∼8 × 10(5) excitons/cm). At pump fluences >4 × 10(16) photons/pulse cm(2) (∼1.6 × 10(6) excitons/cm), ΔT/T decreases as the pump fluence increases. Analogous signal saturation behavior was observed for all measured probe delays. Despite the high exciton density at saturation, no change in the E(22) population decay rate was observed at short times (<300 fs). The pump probe signal was modeled by a third-order perturbation theory treatment that includes the effects of inhomogeneous broadening. The observed ΔT/T signal is well-fit by a pump-fluence-dependent dephasing rate linearly dependent on the number of excitons created by the pump pulse. Therefore, the observed nonlinear pump intensity dependence is attributed to the effects of quasi-elastic exciton-exciton interactions on the dephasing rates of single carbon nanotubes. The low fluence total dephasing time is 36 fs, corresponding to a homogeneous width of 36 meV (290 cm(-1)), and the derived E(22) inhomogeneous width is 68 meV (545 cm(-1)). These results are contrasted with photon-echo-derived parameters for the E(11) transition.

The exciton dynamics for an ensemble of individual, suspended (6,5), single-walled carbon nanotubes revealed by single color E(22) resonant pump-probe spectroscopy for a wide range of pump fluences are reported. The optically excited initial exciton population ranges from approximately 5 to 120 excitons per ∼725 nm nanotube. At the higher fluences of this range, the pump-probe signals are no longer linearly dependent on the pump intensity. A single, predictive model is described that fits all data for two decades of pump fluences and three decades of delay times. The model introduces population loss from the optically active zero momentum E(22) state to the rest of the E(22) subband, which is dark due to momentum selection rules. In the single exciton limit, the E(11) dynamics are well described by a stretched exponential, which is a direct consequence of diffusion quenching from an ensemble of nanotubes of different lengths. The observed change in population relaxation dynamics as a function of increasing pump intensity is attributed to exciton-exciton Auger de-excitation in the E(11) subband and, to a lesser extent, in the E(22) subband. From the fit to the model, an average defect density 1/ρ = 150 nm and diffusion constants D(11) = 4 cm(2)/s and D(22) = 0.2 cm(2)/s are determined.

Ultrafast two-color pump-probe measurements, time-resolved photoluminescence (TRPL), and photoluminescence excitation measurements were performed on Si-rich nitride (SRN) and Er doped SRN (Er:SRN) nanocrystals samples. Transient absorption data were compared with picosecond TRPL and excited state absorption cross (ESA) sections σ were measured at different wavelengths. Our data show that σ in Er:SRN, which is approximately 10−19cm2 at 1.54μm, does not scale with the ∼λ2 behavior predicted by simple free carrier absorption models. Finally, our data demonstrate that in Er:SRN efficient energy transfer to Er ions occurs on the nanosecond time scale with reduced ESA compared to Er-doped oxide-based systems.

Dispersed IR three-pulse photon echoes due to the antisymmetric (ν3) stretch mode of N2O dissolved in H2O and 1-octanol at room temperature are reported and analyzed. The experimentally determined transition frequency-frequency correlation function (FFCF) in these two solvents is explained in terms of inertial solvent contributions, hydrogen bond network fluctuations, and, for octanol, the motions of the alkyl chains. The H2O hydrogen bond fluctuations result in 1.5 ps FFCF decay, in agreement with relaxation rates determined from photon echo based measurements of other aqueous solutions including salt solutions. In octanol, hydrogen bond fluctuations decay on a slower time scale of 3.3 ps and alkyl chain motions result in an inhomogeneous broadening contribution to the ν3 absorption spectrum that decays on a 35 ps time scale. Rotational reorientation of N2O is nearly 3 times faster in octanol as compared to water. Although the vibrational ν3 N2O absorption line shapes in water and octanol are similar, the line widths result from different coherence loss mechanisms. A hot band contribution in the N2O in octanol solution is found to have a significant effect on the echo spectrum due to its correspondingly stronger transition moment than that of the fundamental transition. The dephasing dynamics of the N2O ν3 stretch mode is of interest as a probe in ultrafast studies of complex or nanoconfined systems with both hydrophobic and hydrophilic regions such as phospholipids, nucleic acids, and proteins. These results demonstrate the value of the N2O molecule to act as a reporter of equilibrium fluctuations in such complex systems particularly due to its solubility characteristics and long vibrational lifetime.

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UV photon echoes from a ∼120 nm GaN thin film exhibit a biexponential decay attributed to the coherence loss of slowly and rapidly decaying excitons corresponding to excitons with different degrees of localization or trapping in this material. Both exciton populations are strongly inhomogeneously broadened. For one exciton type, T2 is ∼300 fs at 10 K and ≤25 fs at 300 K; for the other, T2 is pulse-width limited (≤25 fs) at all temperatures. At low temperatures, coherence decay predominantly results from impurity and defect site interactions. At higher temperatures, strong exciton-LO phonon coupling dominates dephasing.

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