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Laser Intensity (laser + intensity)
Selected AbstractsOxidative stress in glial cultures: Detection by DAF-2 fluorescence used as a tool to measure peroxynitrite rather than nitric oxideGLIA, Issue 2 2002Sanjoy Roychowdhury Abstract 4,5-diaminofluorescein diacetate (DAF-2DA) is widely used as a fluorescent probe to detect endogenously produced nitric oxide (NO). Recent reports that refer to the high sensitivity of DAF-2 toward NO prompted us to test its efficiency and specificity in a mixed murine primary glial culture model, in which the NO-synthesizing enzyme inducible nitric oxide synthase (iNOS) is expressed by stimulation with lipopolysaccharide (LPS) and interferon-, (IFN-,). Cultures were loaded with DAF-2DA and the fluorescence was measured using confocal microscopy. NO production in the cultures was determined using the ozone/chemiluminescence technique. Due to the extremely high photosensitivity of DAF-2, low laser intensities were used to avoid artifacts. No difference in DAF-2 fluorescence was observed in NO-producing cultures compared to control cultures, whereas the NO/peroxynitrite-sensitive dye 2,7-dihydrodichlorofluorescein (DCF) showed a significant fluorescence increase specifically in microglia cells. A detectable gain in fluorescence was seen when NO-containing buffer was added to the DAF-2DA,loaded cells with a minimum NO concentration at 7.7 ,M. An additional gain of DAF-2 fluorescence was obtained when the cells were depleted of glutathione (GSH) with L-buthionine S,R-sulfoximine (BSO). Hence, we monitored the change in DAF-2 fluorescence intensity in the presence of NO and O in a cell-free solution. The fluorescence due to NO was indeed larger when O was added, implying a higher sensitivity of DAF-2 for peroxynitrite. Nevertheless, our results also indicate that measurement of DCF fluorescence is a better tool for monitoring intracellular changes in the levels of NO and/or peroxynitrite than DAF-2. GLIA 38:103,114, 2002. © 2002 Wiley-Liss, Inc. [source] Photoinduced Luminescence Blinking and Bleaching in Individual Single-Walled Carbon NanotubesCHEMPHYSCHEM, Issue 10 2008Carsten Georgi Abstract The temporal evolution of photoluminescence in individual single-walled carbon nanotubes (SWNT) under strong laser irradiation is studied and pronounced blinking and bleaching is observed, caused by photoinduced oxidation that subsequently quenches mobile excitons. The nanotubes are isolated with sodium cholate and spun onto either a glass or mica surface. Their bleaching behavior is investigated for variable laser intensities in air and argon atmosphere. The decay rate for luminescence bleaching generally increases with higher laser intensity, however saturating on mica substrates, which is attributed to limited availability of oxygen in the vicinity of the nanotubes. Step-like events in the luminescence time traces corresponding to single oxidation events are analyzed regarding relative step height and suggest an exciton diffusion range of about 105 nm. [source] Spontaneous Formation of Complex Periodic Superstructures under High Interferential Illumination of Small-Molecule-Based Photochromic MaterialsADVANCED FUNCTIONAL MATERIALS, Issue 5 2009Eléna Ishow Abstract A series of push-pull azo compounds containing bulky substituents are synthesized, yielding fully amorphous materials with glass-transition temperatures above 200,°C. Thin films are subjected to holographic illumination and show superior bulk photomigration in terms of speed and efficiency compared to materials exhibiting similar electronic and photochromic properties in the solid state. The reported results give evidence that a microscopic consideration of the free volume rather than macroscopic parameters like the glass-transition temperature should to be adopted to interpret the matrix stiffness and its deformation ability. Irradiation performed at higher laser intensity produces periodic superstructures whose height is five to six times as high as the initial film thickness. The surface tension and instability effects are put forward to interpret the growth of such superstructures. [source] Up-Conversion Photoluminescence in Polyfluorene Doped with Metal(II),Octaethyl PorphyrinsADVANCED MATERIALS, Issue 24 2003P.E. Keivanidis The up-conversion photoluminescence (PL) in films of polyfluorene (PF) doped with metallated porphyrins is reported for the first time. The dependence of the up-conversion process on the pump laser intensity and wavelength, the central metal moiety of the dopants, and the temperature is presented. Up-conversion emission is observed at pump intensities as low as 2 kW,cm,2. Comparison of the PF integral PL intensities after laser excitation by 532, 543, and 405 nm enables the discussion of the energy-transfer mechanism and the efficiency of the process. [source] Numerical analysis of optical feedback phenomenon and intensity noise of fibre-grating semiconductor lasersINTERNATIONAL JOURNAL OF NUMERICAL MODELLING: ELECTRONIC NETWORKS, DEVICES AND FIELDS, Issue 3 2007Moustafa Ahmed Abstract This paper demonstrates numerical analysis of the dynamics and intensity noise of fibre-grating semiconductor lasers (FGSLs). The induced phenomenon of strong optical feedback (OFB) is analysed. The simulations are based on an improved time-delay rate equations model of a single-mode laser that takes into account the multiple round-trips of the lasing field in the fibre cavity. The analyses are performed in terms of the temporal trajectory of the laser intensity, bifurcation diagram and relative intensity noise (RIN). We explore influence of the fibre-cavity length on the dynamics and RIN. The results show that when the fibre cavity is short, the regime of strong OFB is characterized by either continuous-wave (CW) operation or periodic pulsation. The pulsation frequency is locked at the frequency separation of either the compound-cavity modes or the external fibre-cavity modes. The corresponding RIN level is close to or higher than the level of the solitary laser depending on pulse symmetry. When the fibre cavity is long, the laser exhibits unstable dynamics over wider range of OFB. Moreover, the strong-OFB pulsation becomes beating quasi-periodic at the relaxation oscillation frequency and the fibre-cavity mode-separation frequency. Copyright © 2007 John Wiley & Sons, Ltd. [source] Extreme multielectron ionization of elemental clusters in ultraintense laser fieldsISRAEL JOURNAL OF CHEMISTRY, Issue 2 2007Andreas Heidenreich In this paper we present computational and theoretical studies of extreme multielectron ionization in Xen clusters (n = 55-2171, initial cluster radii R0 = 8.7-31.0 Å) driven by ultraintense Gaussian infrared laser fields (peak intensity IM = 1015 -1020 W cm,2, temporal pulse length , = 10-100 fs, and frequency v = 0.35fs,1). The microscopic approach, which rests on three sequential-parallel processes of inner ionization, nanoplasma formation, and outer ionization, properly describes the high ionization levels (with the formation of {Xeq+}n with q = 5-36), the inner/outer cluster ionization mechanisms, and the nanoplasma response. The cluster size and laser intensity dependence of the inner ionization levels are determined by a complex superposition of laser-induced barrier suppression ionization (BSI), with the contributions of the inner field BSI manifesting ignition enhancement and screening retardation effects, together with electron impact ionization. The positively charged nanoplasma produced by inner ionization reveals intensity-dependent spatial inhomogeneity and spatial anisotropy, and can be either persistent (at lower intensities) or transient (at higher intensities). The nanoplasma is depleted by outer ionization that was semiquantitatively described by the cluster barrier suppression electrostatic model, which accounts for the cluster size, laser intensity, and pulse length dependence of the outer ionization yield. [source] Reduction in DNA Synthesis During Two-photon Microscopy of Intrinsic Reduced Nicotinamide Adenine Dinucleotide Fluorescence,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2005Michael G. Nichols ABSTRACT Two-photon laser scanning microscopy (TPLSM) of endogenous reduced nicotinamide adenine dinucleotide (NAD(P)H) provides important information regarding the cellular metabolic state. When imaging the punctate mitochondrial fluorescence originating from NAD(P)H in a rat basophilic leukemia (RBL) cell at low laser powers, no morphological changes are evident, and photobleaching is not observed when many images are taken. At higher powers, mitochondrial NAD(P)H fluorescence bleaches rapidly. To assess the limitations of this technique and to quantify the extent of photodamage, we have measured the effect of TPLSM on DNA synthesis. Although previous reports have indicated a threshold power for "safe" two-photon imaging, we find the laser power to be an insufficient indicator of photodamage. A more meaningful metric is a two-photon-absorbed dose that is proportional to the number of absorbed photon paris. A temporary reduction of DNA synthesis in RBL cells occurs whenever a threshold dose of approximately 2 × 1053 photon2 cm,4 s,1 is exceeded. This threshold is independent of laser intensity when imaging with average powers ranging from 5 to 17 mW at 740 nm. Beyond this threshold, the extent of the reduction is intensity dependent. DNA synthesis returns to control levels after a recovery period of several hours. [source] Laser-assisted electron emission from CVD nano-graphite filmsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 13 2006D. A. Lyashenko Abstract We demonstrate that irradiation with nano-second light pulses results in significant enhancement of the electron emission from nano-graphite films. The observed emission current density is as high as 10 A/cm2 at applied field of 2 V/µm. The duration of the emission pulse depends on the applied DC voltage and laser intensity. However, in our experimental conditions, the temporal profile of the electron pulse nearly reproduces that of the incident laser pulse. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Saturated absorption and reverse saturated absorption of Cu:SiO2 at , = 532 nmPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2004R. A. Ganeev Abstract Nonlinear absorption in copper-doped silicate glasses was investigated by the Z-scan technique using second harmonic radiation of a Nd:YAG laser (, = 532 nm, , = 55 ps). The simultaneous influence of saturated and reverse saturated nonlinear absorption processes was analyzed. The nonlinear absorption coefficient and the saturation intensity of copper-doped glasses at laser intensity of 5.4 × 109 W cm,2 were measured to be 6 × 10,6 cm W,1 and 4.3 × 108 W cm,2, respectively. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Radiation pressure induced splitting of resonant modes in a nanocrystal-coated microcavityPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2006M. Gerlach Abstract Melamine formaldehyde microspheres were coated with a soft polyelectrolyte multilayer (PAH/PSS) and one monolayer of CdTe nanocrystals. A micro-PL setup was used to trap the sample within the tightly focused laser beam. The radiation pressure was used to deform the spherical shape of the polyelectrolyte multilayer which changes the optical properties of the resonance modes (whispering gallery modes) in the microcavity. Optically induced shifting and splitting of the resonances were observed with changing characteristics based on varying particular parameters such as laser intensity and focus position. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effects of solvent, film thickness, and hydrogen bonding on surface-relief gratingsPOLYMER ENGINEERING & SCIENCE, Issue 5 2009Woo-Hyuk Jung This work focuses on the development of a new type of surface-relief grating (SRG) using more effective materials than a typical epoxy-based azopolymer for the recording layer of optical data storage. Thus, aniline-based azopolymers were synthesized by reaction of N,N -diglycidylaniline with aniline (An) followed by diazocoupling with 4-aminobenzonitrile or 4-nitroaniline. Such azopolymers when spin-cast from THF/dioxane showed better diffraction efficiency than those cast from THF alone due to residual dioxane creating a large free volume in the solid state. A second diazocoupling reaction of the diazopolymers initially obtained produced polymers with bis(diazobenzene) substituents which exhibited a higher saturation level of the diffraction efficiency for a thicker than for a thinner film. Azopolymers in which the diazo substituents incorporated hydroxyl groups ortho to the diazo unit were obtained via the reaction of the diglycidyl ether of bisphenol-A with aniline or 3-hydroxyaniline followed by coupling with diazotised hydroxynitroaminobenzene. These hydroxy groups, presumably as a result of their H-bonding to the diazo-N, effectively prevented the photoisomerization of the chromophores, so that the polymers showed no SRG at a normal laser intensity of 100 mW/cm2. Polyurethane-based azopolymers, synthesized with toluene-2,4-diisocyanate and disperse orange 17 containing no hydroxyl groups in the main chains, showed better diffraction efficiency than the other azopolymers with nitro group substituents. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers [source] GaAs converters for high power densities of laser illuminationPROGRESS IN PHOTOVOLTAICS: RESEARCH & APPLICATIONS, Issue 4 2008E. Oliva Abstract Photovoltaic power converters can be used to generate electricity directly from laser light. In this paper we report the development of GaAs PV power converters with improved conversion efficiency at high power densities. The incorporation of a lateral conduction layer (LCL) on top of the window layer resulted in a considerable gain in efficiency at high illumination levels. Additional performance improvements were obtained by using a metal electrode grid design and antireflection coating optimised for monochromatic and inhomogeneous laser light. Maximum monochromatic (810,nm) optical-to-electrical conversion efficiency of 54·9% at 36·5,W/cm2 has been achieved. The characteristics of laser power converters with p/n and n/p polarity are discussed in this paper. Moreover, different materials and doping levels were applied in the LCL. The performance of these different device structures at high laser intensity is presented and discussed. It is shown that the lateral series resistance of the cell has a major impact on the overall device performance. Copyright © 2008 John Wiley & Sons, Ltd. [source] Matrix-assisted laser desorption/ionisation mass spectrometry for the direct analysis of enzymatically digested kappa - iota - and hybrid iota/nu -carrageenans,RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 16 2005Aristotelis Antonopoulos Enzymatically digested oligosaccharides of kappa -, iota - and hybrid iota/nu -carrageenans were analysed using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry in the negative-ion mode. nor -Harmane was used as matrix. Depending on the stock concentration and the laser intensity applied, the oligosaccharides exhibited losses of sulphate units (neutralised by the Na+ ion, and thus non-stable), leaving the primary backbone structure in most cases with only the deprotonated sulphate groups (carrying the negative charge, stable). This meant that kappa - and iota -oligosaccharides could not be easily distinguished from one another since they share the same primary backbone structure. However, for the hybrid iota/nu -oligosaccharides the primary backbone structure could be identified since the nu -carrageenan repeating unit differs from that of the kappa/iota -carrageenan unit. For all types of oligosaccharides, the results indicated cleavage of an anhydrogalactose unit from the non-reducing end. Specifically, for the hybrid oligosaccharides of iota/nu -carrageenans, this type of fragmentation means that the nu -carrageenan unit is not positioned on the non-reducing end of the hybrid oligosaccharides. Dehydration reactions, and exchange reactions of Na+ with K+ and Ca2+, were also observed. Copyright © 2005 John Wiley & Sons, Ltd. [source] Photoinduced Luminescence Blinking and Bleaching in Individual Single-Walled Carbon NanotubesCHEMPHYSCHEM, Issue 10 2008Carsten Georgi Abstract The temporal evolution of photoluminescence in individual single-walled carbon nanotubes (SWNT) under strong laser irradiation is studied and pronounced blinking and bleaching is observed, caused by photoinduced oxidation that subsequently quenches mobile excitons. The nanotubes are isolated with sodium cholate and spun onto either a glass or mica surface. Their bleaching behavior is investigated for variable laser intensities in air and argon atmosphere. The decay rate for luminescence bleaching generally increases with higher laser intensity, however saturating on mica substrates, which is attributed to limited availability of oxygen in the vicinity of the nanotubes. Step-like events in the luminescence time traces corresponding to single oxidation events are analyzed regarding relative step height and suggest an exciton diffusion range of about 105 nm. [source] | ||||||||||||||||||||||