Two-photon absorption in ZnSe and ZnSe/ZnS core/shell quantum structures

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 806 nm using Ti:sapphire 100 fs 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

Hot ion generation from nanostructured surfaces under intense, femtosecond irradiation

We present the effect of a nanostructured surface on the emission of ions and electrons from intense (5-36 Petwatt per sq.cm) femtosecond laser produced plasmas. Electrons from optically polished copper targets coated with copper nanoparticles (CuNP) are observed to be hotter than those from uncoated polished targets. A nearly two-fold enhancement is observed for ions in the range 14-74 keV, while ion yield decreases by a factor of 2 in the 74-2000 keV range. The total ion yields measured using a large area Faraday cup are more from CuNP targets than those from polished Cu targets, indicating increased ion beam divergence due to surface modulations.Comment: 14 pages, 4 figure

Filamentation without intensity clamping

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 1013 W/cm2 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

Enhanced optical limiting and nonlinear absorption properties of azoarene-appended phosphorus (V) tetratolylporphyrins

Optical limiting performance, third-order nonlinearity X(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

Contribution of two-photon and excited state absorption in 'axial-bonding' type hybrid porphyrin arrays under resonant electronic excitation

Two-photon absorption (2PA) and excited state absorption (ESA) properties of 'axial-bonding' type hybrid porphyrin arrays are investigated in the picosecond regime under single-photon resonant excitation condition. A crossover from reverse saturable absorption (RSA) to saturable absorption (SA) and back to RSA is observed with the increase of excitation intensity. In the corresponding intensity ranges, third- and fifth-order phase conjugate signals from degenerate four wave mixing are observed. These observations are explained using rate equations for population densities in a three-level energy scheme. At higher intensities, 2PA is found to be dominant contributor to nonlinear absorption compared to ESA

Nonlinear absorption properties of 'axial-bonding' type tin(IV) tetratolylporphyrin based hybrid porphyrin arrays

The nonlinear absorption properties of 'axial-bonding' type hybrid porphyrin arrays based on a tin(IV) tetratolylporphyrin (SnTTP) scaffold are studied with picosecond and nanosecond pulses. The effect of different central metal atoms substituted adjacent to the tin(IV) porphyrin in the oligomer structure is discussed. In the picosecond regime the lifetimes of the excited singlet states and two-photon absorption (TPA) processes dominate leading to interesting switching of nonlinear absorption behaviour. The TPA cross-section (σTPA) is found to be as high as 396 × 10-46 cm4 s photon-1 molecule-1, for an oligomer with Sn and Ni porphyrin macrocycles. However, in the nanosecond regime the optical limiting performance has increased considerably with increasing number of porphyrins in the array and excited state absorption is found to play a major role

Three-photon absorption in ZnSe and ZnSe/ZnS quantum dots

ZnSe and ZnSe/ZnS core/shell quantum dots (QDs) of two different sizes (4.5 and 3.5 nm) have been synthesized. The nonlinear absorption is measured at 1064 nm using a 35 ps 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 4.5 to 3.5 nm, due to strong confinement effect

Multiple filamentation and supercontinuum emission from tightly focused femtosecond laser pulses in air

Filamentation characteristics of tightly focused 45 fs laser pulses in air are investigated using transverse imaging in the power range of 20-200 GW. The effect of linear and circular polarized light pulses on the onset and spatial evolution of filamentary substructures and the associated supercontinuum emission is studied