The opticd limiting action of CSU in toluene solution is mainly due to revem saturable absorption (RSA). It is shown that the formalism describing nonlinear optical response due to instantaneous two-photon absorption can be used in the case of sequential two-photon absorption, yielding effective values of the relevant parameters of optical nonlinearity due to RSA. The effective twc-photon absorption parameter and the effective nonlinear refractive index parameter y&, which are related respectively to the imaginary and real parts of the effective third-order susceptibility $2. were measured by the L-scan technique as a function of fullerene concentration and of incident laser intensity and wavelength over the 42W nm region. The concentration dependence of these parameters indicates that the solution is optically lhin as far as the ground state of Cm is concerned, whereas he wavelength dependence confirms the applicability of the formalism used in the sequential two-photon absorption model. Comparisons are made with other I-scan results on Cm.

We present a systematic study of the effects of post-fabrication annealing on the performance of photovoltaic devices that incorporate a photoactive donor–acceptor dispersed heterojunction system. Solar cells have been fabricated based on poly(3-octylthiophene) as donor (D) and single-wall carbon nanotubes as the electron acceptor (A) with a PEDOT:PSS hole transport layer. A post-fabrication annealing treatment was carried out at a temperature range of 40–200 °C. The best results were obtained at 120 °C; at this temperature the cell shows a short circuit current, Isc = 0.5 mA cm−2, an open circuit voltage, Voc = 0.75 V and a fill factor, FF = 0.6, resulting in a power conversion efficiency of η = 0.22% under AM 1.5 (100 mW cm−2) white light illumination. The annealing treatment led to a doubling of the power conversion efficiency. This was attributed to a better charge carrier transport in the polymer matrix and more effective charge separation and collection.

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We have introduced functionalized single-walled carbon nanotubes (SWNTs) in bulk heterojunction polymer–fullerene photovoltaic devices. As compared with the pristine device with only fullerenes, the addition of SWNTs resulted in an improvement in both the short-circuit current density and the fill factor (FF). The best results were obtained with a P3HT/PCBM (1 : 1) mixture with 0.5 wt% of SWNTs under an open-circuit voltage (Voc) of 0.55 V, a short-circuit current density (Jsc) of 4.95 mA cm−2 and a FF of 0.52 which led to a power conversion efficiency of 1.4% at illumination at 100 mW cm−2 AM1.5. This device takes advantage of the electron accepting feature of fullerenes and the high electron transport capability of SWNTs. The results indicate that the addition of nanotubes enhances the performance of polymer photovoltaic cells.

The interaction of a laser with potassium atoms is investigated by monitoring the radiation of the 5P–4S transition under the two-photon 4S1/2 –6S1/2 excitation. For low number density and laser intensity the observed radiation is parametric axial or conical, depending on the laser detuning from the two-photon resonance, the number density and the laser intensity. With increasing laser intensities and number densities, an additional delayed component is observed corresponding to a two-step four-wave mixing with the participation of internally generated photons. A two-photon quantum interference effect is observed for the total (axial and conical) parametric four-wave mixing emission. A deviation from the proposed model under certain experimental parameters is attributed to a two-step four-wave mixing and the phase mismatch of the parametric processes.