Kai Dou

Optical limiting and nonlinear absorption of excited states in metalloporphyrin-doped sol gels

Optical limiting and upconverted luminescence have been studied in different chromophore-doped aluminosilicate gel materials and the effects of the dynamic processes from different central ions are discussed. Chromophores are metalloporphyrins of tetra-4-sulfonatophenylporphyrinato-copper(II) and tetra-4-sulfonatopfienylporphgrinato-zinc(II) (CuTPPS and ZnTPPS, respectively) which are embedded in aluminosilicate gel hosts. Photo-upconverted luminescence was observed and believed to result from the radiative recombination of the second excited singlet state. Saturated behavior of upconverted emission was found as a function of incident excitation intensity. An optical limiting phenomenon associated with reverse saturated absorption was studied. These effects are attributed to higher excited state, absorption. The observation of upconverted luminescence indicates that the upper excited state absorption occurs while nonlinear absorption is being observed. Dynamic analysis of excited state processes indicates that intersystem crossing and triplet-triplet (quartet-quartet) absorption dominates nonlinear processes and suggests that saturation absorption or reverse saturation absorption is dependent on the ratio of the effective absorption cross section for the excited states to that for the ground state. Absorption cross sections for the excited states are estimated by the data fitting to upconverted emission intensity and nonlinear absorption.

Optical limiting and upconverted luminescence in metalloporphyrin-doped sol-gels

In this paper, optical limiting and upconverted luminescence have been demonstrated in metalloporphyrin of ZnTPPS-doped aluminosilicate sol-gel hosts. The photo-upconverted radiative emission was observed and believed to result from the radiative recombination of the second excited singlet state. Intensity of the upconverted luminescence was found to be saturated as a function of excitation intensity. Optical limiting associated with reverse saturable absorption was observed. These effects are attributed to the higher excited state absorption. A method for determining absorption cross-section of excited states was developed by using dynamic calculation to fit the intensity dependence of upconverted intensity. The absorption cross sections of the excited singlet and triplet state were estimated.

Nonlinear absorption and optical limiting of fullerene complex C60[W(CO)3diphos] in toluene solutions and sol gel films

Abstract Wavelength-dependent optical limiting and nonlinearity in C60[W(CO)3diphos] toluene solutions and sol gel films were studied from 532 to 700 nm. Significant improvement of optical limiting performance in C60[W(CO)3diphos] compared to C60 was achieved in the visible spectral range. Optical nonlinearity increases with the increment of wavelength. The results of optical limiting were analyzed using dynamical rate equations to obtain absorption cross sections for excited states.

Surface microstructuring of aluminum alloy 2024 using femtosecond excimer laser irradiation

Surface microstructuring and modification of aluminum alloy 2024-T3 were investigated using a femtosecond pulse laser irradiation. The surface micrographs of the scanning electron microscopy were characterized as a function of incident laser fluence. The surface features ranging from submicrometer to micrometers were developed through the variation in laser fluence and laser pulse count. Results indicated that surface patterns were uniquely constructed in a controlled use of pulsed laser parameters for the ablation. Two ablation regimes were found in the logarithmic dependence of the ablated depth on laser fluence and two components were attributed to the optical and energy penetration. A thermal dynamic model was adopted to analyze the ablation processes, and the theoretical analysis agreed well with the experimental results.

Surface texturing of aluminum alloy 2024-T3 via femto- and nanosecond pulse excimer laser irradiation

Surface texturing effects of aluminum alloy 2024-T3 using femtosecond and nanosecond pulse laser irradiation were studied. The micrographs of the scanning electron microscopy (SEM) were characterized as a function of incident laser fluence. Results indicated that the surface features, ranging from nano to microdimension, can be developed through variation in laser fluence intensities. Two ablation regimes in the logarithmic fluence dependence of the ablated depth for the 500-fs-pulse irradiation were observed. The theoretical analysis for ablation processes is in good agreement with the experimental results.

OPTICAL LIMITING AND RELATED UPCONVERTED EMISSION IN FULLERENE COMPLEX

Optical limiting and related upconverted emission were reported in fullerene complex C60[W(CO)3diphos] in toluene solutions at various excitation wavelengths. Upconverted emission associated with the upper-lying excited states was found in both fullerene and its complex. The results indicated that C60[W(CO)3diphos] showed a good optical limiting performance in the visible spectral range.

Spectral study of colloidal photonic crystals

The spectroscopic and microscopic investigation in polystyrene photonic crystals was performed. Colloidal crystalline layers were deposited on glass and quartz by vertical evaporation deposition. Emission, excitation and up-converted emission were studied on both polystyrene powders and polystyrene photonic crystals. The results indicated that spectral features were strongly dependent on the particle properties of size, shape, and arrangement. Blue emission at 420 nm was also found by using a 532-nm excitation due to multiphoton process and a dynamic model was proposed to explain the mechanism of the up-converted emission.

Nonlinear absorption and emission of excited states in metalloporphyrin-doped sol gels

In this paper, we describe up-converted luminescence and reverse saturation absorption of metalloporphyrins of CuTPPS and ZnTPPS in aluminosilicate sol-gel materials. The photo- upconversion luminescence is believed to arise from the radiative recombination of the second excited singlet (singdoublet) state. These effects are attributed to the higher excited state absorption. The dynamic analysis for the excited state processes indicates that the saturation or reverse saturation absorption is dependent on the ratio of the average absorption cross-section of the excited states to that of the ground state. The absorption cross-sections of the excited states are estimated.

Laser-induced electrochemical characteristics of aluminum alloy 2024-T3

Laser-induced surface modification of aluminum alloy 2024-T3 was investigated as laser intensity dependence of surface composition and patterning related to electrochemical characteristics using techniques of environmental scanning electron microscopy (ESEM), potentiodynamic scan, and electrochemical impedance spectroscopy (EIS). Analysis of surface composition using ESEM and EIS indicated that laser fluences in a range of 1.1–1.3 J/cm2 were found to preferentially reduce the copper concentration in the alloy surface to approximately 0.1 wt % and grow the oxidized layer on the surface. Surface composition and patterning were accordingly correlated to electrochemical characteristics of corrosion resistance, capacitance, and potential, which were subsequently used to calculate the corresponding thickness of the formed oxide layer. An enhancement of corrosion resistance characteristics of the 2024-T3 aluminum alloy was attributed to the combination of laser-induced copper removal and oxide layer formation on...

Femtosecond pulse laser ablation and surface modification of aluminum alloy 2024

Laser processing of metallic surfaces using a fs pulse laser has bene shown to produce novel effects. We demonstrate nanoscaled surface texturing effects of aluminum alloy 2024 using femtosecond pulse laser irradiation. The laser source is a 248-nm excimer laser with a pulse width of 500fs delivering a laser output fluence in a range from 0.02 to 10 J/cm2. The micrographs of the scanning electron microscopy have been characterized as a function of incident laser fluence. Results indicate that the surface futures, ranging from nanoscale to microns, can be developed through variation in fluence intensities. Advantages of femtosecond pulse laser for precise metal microstructuring and physical mechanisms for ultrashort pulse laser ablation have been discussed.