G. Ravindra Kumar

Studies of third-order optical nonlinearity and nonlinear absorption in tetra tolyl porphyrins using degenerate four wave mixing and Z-scan

We present our experimental results on the measurement of third-order optical nonlinearity in the ns and ps domain, in several Tetra Tolyl Porphyrin molecules (TTP), using degenerate four wave mixing (DFWM) and Z-Scan techniques. Our results indicate a very high value of nonlinearity for these molecules in the ns domain and reasonably high values in the ps domain. They are found to exhibit a strong nonlinear absorption at both 532 nm and 600 nm. The high value of nonlinearity for ns pulses is attributed to higher excited singlet and triplet states. Time-resolved studies indicate an ultra-fast temporal evolution of the nonlinearity in these molecules.

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 3.5nm) 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.

Direct observation of turbulent magnetic fields in hot, dense laser produced plasmas

Turbulence in fluids is a ubiquitous, fascinating, and complex natural phenomenon that is not yet fully understood. Unraveling turbulence in high density, high temperature plasmas is an even bigger challenge because of the importance of electromagnetic forces and the typically violent environments. Fascinating and novel behavior of hot dense matter has so far been only indirectly inferred because of the enormous difficulties of making observations on such matter. Here, we present direct evidence of turbulence in giant magnetic fields created in an overdense, hot plasma by relativistic intensity (1018W/cm2) femtosecond laser pulses. We have obtained magneto-optic polarigrams at femtosecond time intervals, simultaneously with micrometer spatial resolution. The spatial profiles of the magnetic field show randomness and their k spectra exhibit a power law along with certain well defined peaks at scales shorter than skin depth. Detailed two-dimensional particle-in-cell simulations delineate the underlying interaction between forward currents of relativistic energy “hot” electrons created by the laser pulse and “cold” return currents of thermal electrons induced in the target. Our results are not only fundamentally interesting but should also arouse interest on the role of magnetic turbulence induced resistivity in the context of fast ignition of laser fusion, and the possibility of experimentally simulating such structures with respect to the sun and other stellar environments.

Optical nonlinearity of monodispersed, capped ZnS quantum particles

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.

Nonlinear optical interactions in dye-doped solids

We present a brief review of non-linear optical investigations on dye-doped solids using low-power CW lasers. After a brief introduction to the photophysics of the dye molecules, we discuss specific nonlinear processes such as self-diffraction, optical phase conjugation, two-beam coupling and polarization gratings in these systems. The application potential of dye-doped solid devices is discussed.

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 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.

Laser damage studies of silicon surfaces using ultra-short laser pulses

Laser-induced damage morphology using femtosecond laser pulses on Si surfaces is reported. Damage morphology shows the ablation of material. A magnified view of the ablated portion shows a periodic surface structure in the form of ripples. The spacing of these ripples was between 0.5 and 2 μm and increased, on increasing the power density or number of pulses, and finally broke into parts, leaving well-ordered grains of approximate diameter 5 μm. Also for 100 or larger number of pulses, an amorphous ring in the periphery was formed. The diameter of this ring increased, on increasing either the laser fluence or the number of pulses. The formation of ripples has been explained with the help of the hypothesis of Boson condensation proposed by Van Vechten (Solid State Commun 39 (1981) 1285).

Sensitive measurement of absolute two-photon absorption cross sections

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 intensi...

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 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.

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

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.