Monolayer-protected Au, Ag, and Au:Ag alloy nanoclusters have been synthesized using octanethiol and octadecanethiol as capping agents. The particle-size distribution is narrow with an average core size of 3-4 nm. Optical nonlinearity induced by 35 ps pulses at 532 nm has been investigated in these samples using the Z-scan technique. It is found that in general, they behave either as saturable absorbers or reverse saturable absorbers depending on the intensity of excitation. Au and Ag clusters show nearly the same efficiency for optical limiting, but the alloy clusters are found to be less efficient in limiting and are less photostable. The observed effects are explained in terms of the electron dynamics of the excited-state species.

ZnSe and ZnSe∕ZnS core/shell quantum dots (QDs) of two different sizes (4.5 and 3.5nm) have been synthesized. The nonlinear absorption is measured at 1064nm using a 35ps laser with an open aperture Z-scan setup. Three-photon absorption (3PA) has been observed in ZnSe and ZnSe∕ZnS QDs. 3PA cross section is found to be about four orders of magnitude larger than bulk ZnSe, and three orders of magnitude higher than ZnS QDs. 3PA cross section is found to be increased in ZnSe and in ZnSe∕ZnS QDs with decreasing size from 4.5to3.5nm, due to strong confinement effect.

Optical limiting performance, third-order nonlinearity chi(3), and nonlinear absorption properties have been investigated in a new class of azoarene phosphorus (V) porphyrins with charge transfer (CT) states. The introduction of axial azoarene groups into the phosphorus porphyrin structure is found to reduce the limiting threshold by a factor of 2 and lead to a rise in the second hyperpolarizability by 1 order of magnitude in the picosecond time regime and by 2 orders of magnitude in the nanosecond regime. The experimental data show reverse saturation of absorption in the nanosecond time regime and a saturation of the nonlinear absorption above a fluence of 0.5 J/cm2 in the picosecond regime. The presence of the CT state reduces saturation of excited-state absorption (ESA) in the S1 --> Sn transition through the S1 --> CT transition. Faster CT --> T1 transition increases the ESA from T1 --> Tn states in the nanosecond regime. A self-consistent theoretical analysis based on rate equations is used to estimate the high-lying excited-state lifetimes and absorption cross sections from the experimental results.

We present measurements of the supercontinuum emission (SCE) from ultrashort Ti:Saph laser pulse filamentation in air in a tightly focused geometry. The spectral broadening of SCE indicates that peak intensities exceed the clamping value of a few 10(13) W/cm(2) obtained for filamentation in a loose focusing geometry by at least one order of magnitude. We provide an interpretation for this regime of filamenation without intensity clamping.

We develop a method for measuring absolute two-photon absorption cross sections (σ2) and employ it to determine the σ2 of Rhodamine-6G in methanol (16.2±2.4 GM at 806 nm). Our measurement calibrates the relative excitation spectrum previously reported for this chromophore. The method is based on our derivation of an analytical expression describing the transmission of Gaussian laser pulses through a two-photon absorbing medium. The expression is valid for arbitrary absorber thickness, at all distances from the focus. This generalizes the prevalent “z-scan” (translation of the sample along the beam direction) technique for measuring two-photon absorbance, removing the requirements of a “thin” (thickness ≪ Rayleigh range of the focused laser beam) sample and of placing the sample at the focus. This leads to an improvement of the sensitivity of the technique by over two orders of magnitude, enabling measurement of the two-photon absorption cross sections of even weakly absorbing specimens at moderate intensities. The results are significant for applications such as nonlinear microscopy, optical data storage and optical power limiting.

Articles you may be interested inLinear and nonlinear optical properties of ZnO/ZnS and ZnS/ZnO core shell quantum dots: Effects of shell thickness, impurity, and dielectric environment ZnS quantum dots are synthesized by a high-temperature chemical route with narrow size distribution at diameters of 1.4 and 1.8 nm. Significantly small size dispersion of 1.4-nm-sized ZnS quantum dots is vivid from the transmission electron microscopic measurements. The nonlinear absorption is measured at wavelengths 532 and 520 nm using a picosecond laser in an open aperture z-scan setup. The measured two-photon absorption coefficients are 0.08 and 0.2 cm/GW for smaller and larger nanoparticles. Two photon absorption cross sections for nanoparticles are about six orders of magnitude larger than bulk ZnS.

The third order nonlinear optical properties of two different sized ZnSe and ZnSe∕ZnS quantum dots (QDs) are investigated. The nonlinear absorption is measured at 806nm using Ti:sapphire 100fs laser pulses in an open aperture Z-scan setup. Two-photon absorption (2PA) is found to be dominant in core and core shell QDs. 2PA cross section is enhanced by three orders of magnitude compared bulk ZnSe. 2PA cross section is observed to increase with reduction in QD diameter, due to strong confinement effect. ZnSe∕ZnS QDs exhibit higher 2PA cross section compared with corresponding ZnSe QDs, indicating better passivation of the QD surface.

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