Picosecond UV Laser Research Articles

Influence of organic solvent on optical and structural properties of ultra-small silicon dots synthesized by UV laser ablation in liquid

Ultra small silicon nanoparticles (Si-NPs) with narrow size distribution are prepared in a one step process by UV picosecond laser ablation of silicon bulk in liquid. Characterization by electron microscopy and absorption spectroscopy proves Si-NPs generation with an average size of 2 nm resulting from an in situ photofragmentation effect. In this context, the current work aims to explore the liquid medium (water and toluene) effect on the Si-NPs structure and on the optical properties of the colloidal solution. Si-NPs with high pressure structure (s.g. Fm3m) and diamond-like structure (s.g. Fd3m), in water, and SiC moissanite 3C phase (s.g. F4[combining macron]3m) in toluene are revealed by the means of High-Resolution TEM and HAADF-STEM measurements. Optical investigations show that water-synthesized Si-NPs have blue-green photoluminescence emission characterized by signal modulation at a frequency of 673 cm(-1) related to electron-phonon coupling. The synthesis in toluene leads to generation of Si-NPs embedded in the graphitic carbon-polymer composite which has intrinsic optical properties at the origin of the optical absorption and luminescence of the obtained colloidal solution.

Band-pass filters for THz spectral range fabricated by laser ablation

The terahertz resonant metal-mesh filters were fabricated using the laser direct writing technique. UV picosecond laser was employed to cut matrixes of cross-shaped holes in stainless steel foil and molybdenum layer deposited on polyimide substrate. Different laser processing strategies were developed: holes were cut through in the metal foil and the molybdenum film was removed from the polyimide by laser ablation. Band-pass filters with a different center frequency were designed and fabricated. The regular shape, smoothness of edges and sharpness of corners of the cross-shaped holes in the metal were the main attributes for quality assessment for the laser ablation process. Spectral characteristics of the filters, determined by the mesh period, cross-arm length, and its width, were investigated by terahertz time-domain spectroscopy and conventional space-domain Fourier transform spectroscopy. Experimental data were supported by three-dimensional finite-difference time-domain simulations.

Experimental Studies on Joining thin Silver Foils by Recrystallization using Ultra Short Laser Pulses

Theoretical and experimental studies on joining thin silver foils by UV nanosecond and picosecond laser radiation were carried out for an application-orientated process design. Simulations of energy impact in the material at varying laser pulse energies, pulse durations and repetition rates were used to examine the feasibility of a laser-based joint of the highly reflective and thin material. Experimental studies using different laser systems were performed to validate the possibilities of joining thin Ag-foils. As a result it was possible to heat up the material using a UV picosecond laser in order to achieve a recrystallization of the grain boundaries

Structure and characteristics of laser crystallized thin amorphous Si films

Abstract Pure amorphous Si thin films deposited on oxidized crystalline Si surface (111) were crystallized by picosecond UV laser pulses. The Raman scattering spectra show that pulse energy of 330 mJ/cm2 is enough to fully crystallize Si film and further increase of the energy does not improve crystallinity. A large grained polycrystalline Si was obtained as revealed by surface analysis. A significant increase in carrier mobility was observed after laser crystallization.

Picosecond laser structuration under high pressures: Observation of boron nitride nanorods

We report on picosecond UV-laser processing of hexagonal boron nitride (BN) at moderately high pressures above 500 bar. The main effect is specific to the ambient gas and laser pulse duration in the ablation regime: when samples are irradiated by 5 or 0.45 ps laser pulses in nitrogen gas environment, multiple nucleation of a new crystalline product-BN nanorods-takes place. This process is triggered on structural defects, which number density strongly decreases upon recrystallization. Nonlinear photon absorption by adsorbed nitrogen molecules is suggested to mediate the nucleation growth. High pressure is responsible for the confinement and strong backscattering of ablation products. A strong surface structuring also appears at longer 150 ps laser irradiation in similar experimental conditions. However, the transformed product in this case is amorphous strongly contaminated by boron suboxides BxOy.

Velocity distribution of CO desorbing from NiO(1 0 0)/Ni(1 0 0) after picosecond UV laser irradiation

The UV laser-induced desorption of CO from an epitaxial film of NiO(1 0 0) on Ni(1 0 0) yields a linear dependence of the desorption signal on the laser intensity with a cross-section of (8.3 ± 0.9) × 10 −18 cm 2. Rotational and vibrational temperatures of 870 K and 1340 K, respectively, are observed. State-resolved velocity distributions are observed, where one part can be described by a Maxwellian flux distribution with 〈 E kin〉/2 k ranging from 1000 to 1800 K. In addition a faster component is present which gains in importance for increasing J″. A distinct rotation–translational coupling is found.

Autofluorescence Lifetime Imaging of Cultivated Cells Using a UV Picosecond Laser Diode

Lifetime images of autofluorescence of cultivated endothelial cells were recorded using a novel picosecond laser diode in the near ultraviolet range (375 nm). In contrast to existing picosecond light sources this wavelength permits efficient excitation of the free and protein bound coenzyme NADH with fluorescence lifetimes of 0.4-0.5 ns and 2.0-2.5 ns, respectively. The effective fluorescence lifetime tau(eff) (depending on both lifetimes) was homogenously distributed over the cells with some shortening in the perinuclear region, possibly close to mitochondria. A slight decrease of tau(eff) was observed after inhibition of the mitochondrial respiratory chain, whereas a slight increase was observed after inhibition of the glycolytic pathway, thus indicating variations of the ratio of free and protein bound NADH. Although present applications are still limited by their low pulse energy (< or = 5 pJ), uv picosecond laser diodes have a large potential in high resolution fluorescence microscopy and fluorescence lifetime endoscopy.

Low‐density polyethylene films doped with europium(III) complex: their properties and applications

AbstractLight‐converting polyethylene film containing europium(III) complex with phenanthroline was manufactured under an extrusion process. The film was characterized first by means of time‐of‐flight secondary‐ion‐mass spectrometry (TOF‐SIMS), ultraviolet‐visible‐infrared (UV‐vis‐IR) spectroscopy, then a study of excitation and luminescence spectra, quantum yield and lifetime of luminescence. The quantum yield of luminescence was measured as 5±2%, and the lifetime of luminescence was determined about 470 μsec. TOF‐SIMS surface mapping showed the uniform distribution of europium(III) over the film. Visualization of strong picosecond UV laser beams through the use of the developed film has been demonstrated. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. II. Intracluster vibrational energy redistribution of the OH stretching vibration of hydrogen-bonded clusters.

A picosecond time-resolved IR-UV pump-probe spectroscopic study has been carried out for investigating the intracluster vibrational energy redistribution (IVR) and subsequent dissociation of hydrogen-bonded clusters of phenol (C6H5OH) and partially deuterated phenol (C6D5OH, phenol-d5) with various solvent molecules. The H-bonded OH stretching vibration was pumped by a picosecond IR pulse, and the transient S1-S0 UV spectra from the pumped level as well as the redistributed levels were observed with a picosecond UV laser. Two types of hydrogen-bonded clusters were investigated with respect to the effect of the H-bonding strength on the energy flow process: the first is of a strong "sigma-type H-bond" such as phenol-(dimethyl ether)(n=1) and phenol dimer, and the second is phenol-(ethylene)(n=1) having a weak "pi-type H-bond." It was found that the population of the IR-pumped OH level exhibits a single-exponential decay, whose rate increases with the H-bond strength. On the other hand, the transient UV spectrum due to the redistributed levels showed a different time evolutions at different monitoring UV frequency. From an analysis of the time profiles of the transient UV spectra, the following three-step scheme has been proposed for describing the energy flow starting from the IVR of the initially excited H-bonded OH stretching level to the dissociation of the H bond. (1) The intramolecular vibrational energy redistribution takes place within the phenolic site, preparing a hot phenol. (2) The energy flows from the hot phenol to the intermolecular vibrational modes of the cluster. (3) Finally, the hydrogen bond dissociates. Among the three steps, the rate constant of the first step was strongly dependent on the H-bond strength, while the rate constants of the other two steps were almost independent of the H-bond strength. For the dissociation of the hydrogen bond, the observed rate constants were compared with those calculated by the Rice, Ramsperger, Kassel, and Marcus model. The result suggests that dissociation of the hydrogen bond takes place much faster than complete energy randomization within the clusters.

Analysis of Electron Transport in Surface-Passivated Nanocrystalline Porous Silicon

The electron transport mechanism in a quantum-sized silicon system has been studied for a structure- and surface-controlled nanocrystalline porous silicon (nc-PS) sample by a time-of-flight (TOF) measurement using a picosecond UV laser pulse. From observed characteristic TOF signals, we have obtained the drift velocity of the electrons in nc-PS as a function of the electric field. Unlike in single-crystalline silicon (c-Si), the drift velocity in nc-PS shows no apparent saturation with increasing field strength. It reaches 2.2 ×108 cm/s at room temperature at an electric field of 29.1 kV/cm. This drift velocity is 22 times as large as that in c-Si. To obtain such a high drift velocity, the electrons should be accelerated ballistically over 1.6 µm. It appears that the probability of scattering to cause a large change of the electron momentum vector is significantly reduced in nc-PS. These results support the model that electrons can travel ballistically via a multiple-tunneling cascade through the interfacial barriers between interconnected silicon nanocrystallites.

Laser ablation and photo-dissociation of solid-nitrogen film by UV ps-laser irradiation

Nitrogen solid film deposited on a copper plate at 10 K was irradiated with a picosecond UV laser at 263 nm in vacuum. Photo-dissociation of nitrogen molecule in the solid film was confirmed by the optical emissions, which were ascribed to atomic nitrogen, during the laser irradiation at the fluence of 5 J cm −2 pulse −1. This photolysis was discussed by the comparison with laser-induced breakdown of nitrogen gas. At the fluence over ca. 10 J cm −2 pulse −1, the ablation of the frozen nitrogen film was observed. Employing the ablation plume including a reactive species such as nitrogen atoms, the surface reaction of a graphite (highly oriented pyrolytic graphite (HOPG)) plate and silicon wafer was studied. XPS analysis indicated that nitrides were formed on the surfaces by the treatment. The ps-laser ablation of nitrogen solid film provides a novel technique for surface modification of materials.

LUMINESCENCE AND ULTRAFAST DECAY PROFILES OF NANO-SIZE ZnS:Cu,Al IN SOL-GEL SILICA GLASSES

ZnS:Cu,Al nanocrystals are prepared in nanoporous xerogeland sol-gel silica glasses to investigate optical properties by the measurements of luminescence, time-resolved luminescence (TRS) and lifetimes in the temperature range 10-300 v; K. At room temperature, the doped-xerogel represents a green band at 19,000 v; cm m; 1 due to D(donor)-A(acceptor) pair recombinations. Under an excitation by a pico-second UV-laser light (300 v; nm), a combination of blue and green bands is observed in a delay time of 2 v; ns. Luminescence spectral features are associated with D-A pairs and self-activated (SA) centers in the presence of surface-modified defect centers.

Picosecond IR−UV Pump−Probe Spectroscopy. IVR of OH Stretching Vibration of Phenol and Phenol Dimer

Intramolecular and intracluster vibrational energy redistribution(IVR) from OH stretching vibrations of phenol and phenol dimer have been investigated for the first time by picosecond time-resolved IR−UV pump−probe spectroscopy. The OH stretching level was pumped by a picosecond IR laser pulse and the decay of the pumped level as well as appearance of the energy redistributed vibrational levels was observed using resonance enhanced multiphoton ionization scheme with a picosecond UV laser. For bare phenol, IVR of the OH stretching level was found to occur with a lifetime of 14 ps. For the dimer, a remarkable site-dependence was observed for the IVR lifetime of the OH stretching level; IVR of the proton-donating site occurs less than 5 ps, while that of the proton accepting site occurs with 14 ps, comparable to that of bare phenol. In the case of a cluster, IVR was followed by vibrational predissociation, which occurs with lifetimes of several tens of picoseconds.

Femtosecond and picosecond ultraviolet laser filaments in air: experiments and simulations

We report a study of the long-range self-guided propagation of intense femtosecond and picosecond UV laser pulses in air. Measurements of the laser beam profile along the propagation axis together with the ionization rate, the pulse temporal profile and power spectrum provide a comprehensive image of the UV filamentation process and its characteristics. Self-guiding is shown to subsist in both fs and ps cases, with a robust filament propagating over several meters in air. Stable filamentation is accompanied by an important restructuring of the pulse temporal profile. Complementary numerical simulations, using a three-dimension propagation code, are compared to the experimental results. The simulations reproduce most experimental observations, allowing for a better understanding of the phenomena involved in this unique propagation mode.

Highly stabilized femtosecond Ti:sapphire laser designed for beam interaction experiment

Present status of the laser research is described relating with the specified purposes dedicated for beam experiments. A compact and stable UV picosecond laser was developed for photo-cathode RF gun application. An experimental terawatt Ti:sapphire laser is now under development to demonstrate the required performance for the inverse Compton femtosecond X-ray generation at high repetition rate.

Selective vibronic excitation and bond breaking by picosecond UV and IR laser pulses: application to a two-dimensional model of HONO2

A new method to control a bond-selective dissociation is presented: first, an optimal UV laser pulse accomplishes vibrationally state-selective complete population transfer from the ground to the excited electronic state and, second, an optimal IR laser pulse induces an efficient bond-selective multiphoton dissociation. The method is demonstrated by means of computer simulation within the Schrödinger wavefunction formalism for a two-dimensional model of HONO2 wherein the OH and ON single-bond stretches are treated explicitly. Selective breaking of the ON single bond is achieved on a picosecond timescale, with the dissociation probability being almost 100%. The proposed control scheme is much less demanding of the field strength of the dissociating IR laser as compared to that required for breaking the ON single bond solely in the ground electronic state.

High-Intensity UV Laser Photolysis of DNA and Purine 2‘-Deoxyribonucleosides: Formation of 8-Oxopurine Damage and Oligonucleotide Strand Cleavage as Revealed by HPLC and Gel Electrophoresis Studies

Emphasis was placed in this work on the measurement of purine oxidation products generated upon nano- and picosecond UV laser biphotonic photolysis of 2‘-deoxyadenosine, 2‘-deoxyguanosine, calf thy...

Crosslinking of progesterone receptor to DNA using tuneable nanosecond, picosecond and femtosecond UV laser pulses.

UV laser crosslinking is a potentially powerful tool to investigate transient DNA-protein interactions and binding kinetics in intact cells. As the processes underlying UV laser crosslinking are not fully understood, we have performed a study of the influence of laser pulses with different physical parameters on crosslinking of the progesterone receptor to an oligonucleotide containing a hormone-responsive element. We also studied the influence of the various parameters on the amount of laser-irradiated DNA that can be correctly primer extended as an operational measurement of DNA integrity. A strong influence of pulse intensity and pulse length on the crosslink yield was found, likely due to a change in the 'two photon' processes responsible for crosslinking. The highest efficiency of protein crosslinking to DNA was achieved with femtosecond pulses and should be sufficient to enable use of this technique for in vivo studies.

Time-resolved spectroscopy of brain tissue with an all-solid-state tunable picosecond UV laser

Time-resolved spectroscopy of brain tissue with an all-solid-state tunable picosecond UV laser

Photoemission monitored cleaning of pure and implanted tungsten photocathodes by picosecond UV laser

New results about picosecond laser cleaning of pure and implanted tungsten photocathodes are reported. During the transport from the preparation cell to the experimentation cell, the exposure of photocathodes to air leads to the contamination and the formation of surface oxides. Before any photocurrent measurement, in situ cleaning of surfaces is required. The surface cleaning effects are investigated in high vacuum (5×10−8 Torr) by using a picosecond mode-locked Nd3+:YAG laser with fifth order harmonic generator (λ=213 nm, τ=16 ps, repetition rate 10 Hz) irradiation. This non-destructive processing allows real-time monitoring by measuring continuously the surface photocurrent. During cleaning, the photocurrent increases versus time and tends to reach a plateau, depending on the pressure in the vacuum cell. The influence of the laser energy density and the effects of surface recontamination by residual gas are investigated. A phenomenological model is developed to describe the laser cleaning. Fitting the experimental data of photoemitted charges with calculated values given by this model is able to estimate the contamination rate and the work function at the surface during the cleaning.