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Ultrafast lasers of subpicosecond pulse duration have thus far been investigated for ablation, drilling, and cutting processes. Ultrafast lasers also have the potential for laser welding of small components of the order of microns and for laser shock peening to enhance the peening depth. In this paper, the potential for welding is investigated using the two-temperature model to analyze laser-matter interaction for ultrafast lasers. The model is implemented in a general-purpose commercial finite element method package, ABAQUS, tO enable broad based application of the two-temperature model in practical engineering problems. The implementation is validated by comparison with linear solutions obtained using separation of variables. It is then used to investigate the potential for microwelding using ultrafast laser pulse for both low and high fluence laser input. [DOI: 10.1115/1.3075869] Keywords: ultrashort laser pulses, femtosecond lasers, FEM, ABAQUS, two-temperature model laser microwelding

Remote filament-induced breakdown spectroscopy (R-FIBS) using ultrashort laser pulses was used to measure the carbon/clay ratios between three graphite composites of different hardness at a standoff distance of ~6 m. Measurements using R-FIBS and femtosecond laser-induced breakdown (fs-LIBS) reveal similar selectivity and ability to excite emission. Comparison of the two standoff techniques with optical microscopy and electron microprobe point detection confirmed the qualitative analysis capability of both femtosecond remote probing techniques. The R-FIBS technique produced more accurate results compared to fs-LIBS due to the intensity clamping nature of the filament ablation source. Measurement of the plasma temperatures for the metallic emission lines (~8500 K) and the [C.sub.2] Swan lines (~4500 K) suggest that the plasmas from different microdomains (clay and graphite) are not in equilibrium.

We investigate the interaction of a high-intensity ([~5.10.sup.18] W/[cm.sup.2]) and short (~65 fs) laser pulse with thin foils (from 0.08 to 105 pm) in a regime of ultrahigh contrast (> [10.sup.10]). This paper shows that for thicknesses less than about Ill pm, proton acceleration from both sides of the target presents quite symmetric features. Proton bunches emitted from each side show similar maximum energies and spatial characteristics. Moreover, we show that for ultrahigh-contrast pulses, the efficient acceleration mechanism is related to the Brunel effect and not to the ponderomotive force. Simulations performed with a 2-D particle-in-cell code are in close agreement with all experimental data. Index Terms--Acceleration. lasers, plasmas, proton beams.

In this paper, the propagation of a short intense laser pulse in a curved plasma channel is considered. The effects of the shape of the plasma density profile and feedback from the wakefield on the pulse envelope dynamics are studied, with particular attention being paid to the conditions for avoiding laser spot size and centroid oscillations. A possible application to laser wakefield acceleration in the nonlinear regime is discussed. Index Terms--Laser-plasma interaction, plasma-based acceleration methods, plasma waveguides.

Experiments performed with the high-contrast 0.7-J 35-fs Ti:sapphire laser at the Lund Laser Centre focused into a helium gas jet have produced quasi-monoenergetic electron beams with energies up to 150 MeV. The beam is produced as a result of controlled wavebreaking of a wakefield generated by the relativistically channeled laser beam. Under the best conditions of laser energy and plasma density, a beam with narrow-energy-spread component was produced on every shot with an energy stability of ~10% and a pointing stability of ~6 mrad. Furthermore, it can be deduced that the electron beam originates from a source with both transverse and longitudinal extents < 10 [micro]m, implying a pulselength of < 25 fs. These measurements mark an important improvement in the characterization, as well as repeatability, of laser-plasma-generated electron beams with a single driver laser beam. Index Terms--High-intensity ultrashort pulse laser, laser wakefield (LWF) accelerator, plasma wave.

Initial experiments on electron acceleration in self-modulated laser wake-field acceleration regime have been carried out at the Raja Ramanna Centre for Advanced Technology, Indore, India, using a 6-TW 45-fs Ti:sapphire laser. The laser beam was focused to an intensity >[10.sup.18] W/[cm.sup.2] on a supersonic helium gas jet. Total charge and spatial profile of the electron beam produced were measured using an integrating current transformer and a phosphor screen, respectively. For a plasma density of 5 x [10.sup.19] - [10.sup.20] [cm.sup.-3], electron beam was produced with a high charge of 2.5-2.7 nC/shot and a divergence of ~10-~100 mrad. Index Terms--Electron acceleration, self-modulated laser, ultrashort-pulse laser, wake-field acceleration.

We investigate the extension of self-injecting laser wakefield experiments to the regime that will be accessible with the next generation of petawatt-class ultrashort pulse laser systems. Using nonlinear scalings, current experimental trends, and numerical simulations, we determine the optimal laser and target parameters, i.e., focusing geometry, plasma density, and target length, that are required to increase the electron beam energy (to ~ 1 GeV) without the use of external guiding structures. Index Terms--High intensity laser, plasma accelerators.

Three-wave soliton with variable speed (trappon) is generated due to the parametric mixing of an ultrashort Zakharov-Manakov (ZM) soliton pulse with a quasi-continuous-wave (CW) signal in a weakly quadratic nonlinear and dispersive medium. Findings reveal that trappons provide a novel mechanism of cavityless ultrashort pulse train generation.

For the first time femtosecond-laser writing has inscribed low-loss optical waveguides in Schott BK7 glass, a commercially important type of borosilicate widely used in optical applications. The use of a variable repetition rate laser enabled the identification of a narrow processing window at 1 MHz repetition rate with optimal waveguides exhibiting propagation losses of 0.3 dB/cm and efficient mode matching to standard optical fibers at a 1550 nm wavelength. The waveguides were characterized by complementary phase contrast and optical transmission microscopy, identifying a micrometer-sized guiding region within a larger complex structure of both positive and negative refractive index variations. OCIS codes: 230.7370, 320.2250, 350.3390, 160.2750.

In this paper, spatiotemporal relaxation of a neon glow discharge plasma is theoretically investigated. The study concerns the positive column and the adjacent anode region. The relaxation is initiated by a short laser pulse that disturbs the column plasma by depopulating the metastable atoms near the cathode-sided boundary of the considered region. The resulting decrease of the ionization rate initiates a spatiotemporai relaxation of the plasma in the downstream region, which is similar to ionization waves. In accordance with the chosen discharge parameters, ionization waves of p-type are generated. After about 3-4 ms, relaxation is finished, and the undisturbed state is reached. The theoretical investigations are based on a self-consistent hybrid model comprising a fluid description of all relevant plasma components, the solution of the Poisson equation, and a spatially resolved kinetic treatment of the electrons. Index Terms--Self-consistent hybrid method, spatially inhomogeneous Boltzmann equation.

We have synthesized [Bi.sub.3] [Fe.sub.4][Ga.sub.1][O.sub.12] (BIGG) films on [Gd.sub.3][Ga.sub.3][O.sub.12] and [Gd.sub.3] [(ScGa).sub.5] [O.sub.12] substrates by a pulsed laser deposition technique. X-ray diffraction and rocking curve analyses confirm the epitaxial quality of the films with a lattice constant close to 12.61 [+ or -] 0.01 A. Compared with that of a bismuth iron garnet [Bi.sub.3][Fe.sub.5] [O.sub.12] (BIG) film, the bandwidth of the BIGG transmittance (T) spectrum is extended and its left edge shifted down in wavelength by approximately 50 nm. This enables its Faraday rotation [[theta].sub.F] and switching speed to be tested at a wavelength of 532 nm. While BIGG film has a Faraday rotation angle slightly less than that of BIG film, its magnetooptical figure of merit, 2|[[theta].sub.F] d|/-ln (T%), is greatly improved to a typical value ~1.8 times larger than that of the BIG film. Further, the high-speed pulse response time of the BIGG film reaches 2.4 ns with a 1 mm aperture size at the wavelength of 532 nm. Index Terms--Faraday effect, garnet films, pulsed laser deposition, ultrafast switching.

Optical properties of Sn-doped [In.sub.2][O.sub.3] (ITO) have been studied in the optical range of 0.4-12 [micro]m. A deposition has been made on BK7 glass, magnesium fluoride, sapphire, and zinc sulfide substrates. The layers have been characterized by their optical properties, DC electrical sheet resistivity, and Hall mobility. Sheet resistivity lies in the range of 6.8-318 [OMEGA]/sq for thicknesses between 16 and 280 nm. The best carrier mobility is obtained on BK7 and sapphire substrates, up to ~50 [cm.sub.2]/V s. The material shows good infrared transparency in the 3-5 [micro]m range on magnesium fluoride and 0.4-4 [micro]m on sapphire, and it is usable for practical applications up to 12 [micro]m on zinc sulfide. Simulations have been carried out for optical indices and spectra calculations. The Drude model has been used to exploit the results in either direction: from electrical measured data to the simulation of optical spectra and indices, and from measured optical spectra to simulated optical indices and electrical parameters (mobility, carrier density). Hall mobility is considered a worthy and convenient material quality criteria for materials aimed at optics. OCIS codes: 310.6860, 310.6870, 260.3060, 260.3090, 160.6000.

Circular arrays of plasma filaments induced by femtosecond laser pulses in atmospheric air are shown to support guided modes of electromagnetic radiation in the centimeter and millimeter wavelength range. With the refractive index of laser-induced filaments being lower than the refractive index of nonionized air, arrays of such filaments can serve as a structured waveguide cladding, providing an index guiding of radar signals in a nonionized gas region. In spite of attenuation of radar radiation induced by plasma absorption, filament-array waveguides are shown to enhance radar signal transmission relative to freely propagating radar beams.

The parameters for an effective laser-induced forward-transfer (LIFT) process of aluminum thin films using a femtosecond laser are studied. Deposited feature size as a function of laser fluence, donor film thickness, quality of focus, and the pulse duration are varied, providing a metric of the most desirable conditions for femtosecond LIFT with thin aluminum films.

Phase-shaped femtosecond laser pulses and mass spectrometry were implemented as a tool for improving molecular identification. We demonstrate that the specific lines in the mass spectra of several chemical warfare simulants are sensitive to the phase characteristics of the incident laser field. The deviation in the relative yield of fragment ions observed upon pulse shaping (enhancement or suppression) adds a new dimension to mass spectrometry that improves molecular identification and can be used to quantitatively analyze mixtures of isomers. OCIS codes: 320.5540, 300.6350.

It is shown that 20-ns laser pulses can be used to heat up the top silicon layer to temperatures necessary for dopant activation without affecting the quality of the devices and metal interconnects at lower levels. The processes can be easily adapted to fabricate good quality transistors on the upper levels of a 3-D integrated circuit.

Tattooing is a widespread process of puncturing pigments into skin, whereas the resulting concentration inside the skin remains unknown. Many tattoo colorants are organic pigments, such as azo pigments, manufactured for other uses. To remove tattoos from skin, laser pulses at very high intensities are applied to the skin to destroy the tattoo pigments. Recent investigations have shown that several azo compounds are cleaved by laser light leading to potentially toxic or carcinogenic compounds. To assess the risk of tattooing and laser treatment of tattoos, the concentration of the pigments and their decomposition products in the skin must be determined. Therefore, an extraction method was established to determine the concentration of tattoo pigments and decomposition products quantitatively. The extraction of two widely used azo compounds, Pigment Red 22 and Pigment Red 9, and their laser-induced decomposition products, 2-methyl-5-nitroaniline, 4-nitrotoluene, 2,5-dichloraniline, and 1,4-dichlorobenzene, was accomplished using recovery experiments and HPLC-DAD technology. Despite the poor solubility of the pigments, a nearly complete recovery from aqueous suspension (>92%) or lysed skin (> 94%) was achieved. The decomposition products were extracted from aqueous suspension or skin showing a recovery of up to 100%, except for the very volatile 1,4-DCB.

A compact soft x-ray source using Thomson scattering was developed aiming at biological studies such as those using an x-ray microscope. Upon collision of the electron beam with the amplified laser beam 300 eV soft x-rays were generated by Thomson backscattering.

An investigation of the excited-state dynamics of SO2(H2O)(sub n) clusters following excitation by ultrafast laser pulses to 4.7 eV and 9.3 eV is presented. A lack of cluster size dependence in the measured time constants indicates surface solvation of SO2 rather than a cluster structure with the SO2 molecule fully encompassed by water molecules.

Experiments, which use coherence in the laser pulses for controlling the physical and chemical processes, are presented. Results indicate that the powerful tool for coherence control is interference and a wave packet can be sculpted and its evolution controlled using the phase of femtosecond pulses.

A study about the behavior of polyatomic molecules in the presence of strong off-resonance ultrashort laser fields is presented. The electric field can induce alignment and bending in polyatomic molecules and these effects can take place in the absence of ionization.

Here light propagation and radiation transfer of ultrafast laser pulses in heterogeneous biological tissues are simulated by use of the discrete-ordinates method (DOM). Formulations for solving the time-dependent radiation-transfer equation are deduced for three-dimensional geometries incorporating the Fresnel specularly reflecting boundary condition and characteristics of ultrafast laser pulses. The present method can treat both the incident laser intensity and the scattered radiation intensity from the walls of the targeted tissue as two components, i.e., a diffuse part and a specular part. Reflectivity at the tissue-air interface is calculated by use of Snell's law and the Fresnel equation. The high-order [S.sub.10] DOM method is found to be adequate for describing the propagation and transfer of ultrafast laser radiation in heterogeneous tissues. The time-dependent radiation field in the tissue as well as the temporal radiation intensity profiles at the boundaries can be obtained simultaneously. The absolute values of the logarithmic slope of the temporal reflectance and transmittance at various detector positions are found to converge to a constant value in a homogeneous tissue model. With the inclusion of a small inhomogeneity, such a value will change in line with the property of the embedded inhomogeneity. The orientation of heterogeneity of the tissues also substantially affects the radiation intensity at the boundaries. The effect of the Fresnel boundary in the modeling is pronounced. The simulated transmitted signals are broadened and amplified under specularly reflecting boundary condition as compared with those under diffusely reflecting boundary conditions. OCIS codes: 170.5280, 170.6920, 030.5620, 140.7090, 000.4430.

Exact numerical solutions of the time-dependent Schrodinger equation for a 1-D model of the one-electron isotopomer systems H2(super +), HD(super +), and HT(super +) is obtained in a two-color laser control scheme, omega + 2omega, of dissociation and dissociative ionization in the high-intensity, nonlinear, nonperturbative regime of laser-molecule interaction.

Laser therapy for cutaneous hypervascular malformations such as port-wine stain birthmarks is currently not feasible for dark-skinned individuals. We study the effects of pulse duration, radiant exposure, and cryogen spray cooling (CSC) on the thermal response of skin, using a Monte Carlo based optical-thermal model. Thermal injury to the epidermis decreases with increasing pulse duration during irradiation at a constant radiant exposure; however, maintaining vascular injury requires that the radiant exposure also increase. At short pulse durations, only a minimal increase in radiant exposure is necessary for a therapeutic effect to be achieved because thermal diffusion from the vessels is minimal. However, at longer pulse durations the radiant exposure must be greatly increased. There exists an optimum pulse duration at which minimal damage to the epidermis and significant injury within the targeted vasculature occur. For example, the model predicts optimum pulse durations of approximately 1.5, 6, and 20 ms for vessel diameters of 40, 80, and 120 [micro]m, respectively. Optimization of laser pulse duration and radiant exposure in combination with CSC may offer a means to treat cutaneous lesions in dark-skinned individuals. OCIS codes: 170.0170, 170.1610, 170.1870, 170.3660.

In this paper, we propose a hybrid pulse position modulation/ultrashort light pulse code-division multiple-access (PPM/ULP-CDMA) system for ultrafast optical communications. The proposed system employs spectral CDMA encoding/decoding and PPM with very short pulse separation. The statistical properties of the encoded ULP are investigated with a general pulse model, and the performance of the proposed PPM/ULP-CDMA system is investigated. It is shown that we can improve upon the performance of other ULP-CDMA systems, such as those using on-off keying, by employing PPM. It is also shown that we can improve the performance of the proposed system by increasing the effective number of chips, by increasing the number of PPM symbols, and by reducing the ULP duration. The performance analysis shows that the aggregate throughput of the proposed PPM/ULP-CDMA system could be over 1 Tb/s. Index Terms--optical code-division multiple-access (CDMA), pulse position modulation (PPM), ultrashort light pulse.

Ultrafast infrared frequency selected vibrational echo (FSVE) experiments are used to study temperature-dependent dynamic interactions in liquids, glasses, and proteins. The calculated decay shows substantial agreement with the experimentally measured decays. The FSVE experiment probes protein dynamics and provides an observable that can be used to test structural assignments for the Mb-Co conformational substates.

A test vehicle has been specially realized to demonstrate that different physical mechanisms are responsible for single-event upset phenomena within an elementary memory cell, depending on the impact location. Validation is performed using pulsed laser equipment. Index Terms--Laser mapping, SEU, SRAM.

Surface initiated solidification is evident on Ge films during picosecond laser pulse irradiation.

Normal boiling induced by laser-pulse or ion bombardment is considered.

The narrowing effects of scatterers on the lifetime and the spectral width of the laser-induced fluorescence of organic dyes hosted in poly(methyl methacrylate) polymer sheets were studied. The excitation source was a distributed-feedback dye laser emitting 0.5-ps pulses at 496 nm. Spectral and temporal features were recorded simultaneously on a spectrograph-streak-camera detection system. The results were then compared with those obtained from dye solutions in methanol that were recorded in previous studies. The effects of the different host environments on the fluorescence characteristics of the dye were thus investigated. These effects are currently studied when the dye is inserted into human tissue in an attempt to boost tumor detection and photodynamic-therapy efficiency. Some initial results are presented.

Research is presented describing the use of femtosecond laser pulses at 195 and 260mn to photonize ion beam desorbed amino acids. Results suggest that the amino acids are desorbed and ionized intact and then fragment via radical cleavage reactions.

Research was conducted to examine the feasibility of performing a uniform surface elemental analysis via high laser power to postionize sputtered neutrals. A common 6 ns Nd:YAG laser operating at 532 nm was applied to photoionization. A nonresonant multiphoton ionization model based on rate equation and perturbation theory was used to measure the ionization yields. Results indicate that the 6 ns laser reaches uniform ionization at a lower power density.

The acoustic and thermal properties of various important structures having dimensions less than approximately 200 microns were studied using recently introduced transient grating (TG) methods. The objective was to investigate the possibility of using TG calculations for noncontact metrology of thin on-fiber coatings and actuators. Results demonstrate that TG methods that use phase masks and imaging optics provide convenient tools for probing the acoustic and thermal characteristics of several three-dimensional structures on the order of and less than 200 microns.

We have investigated the photoionization of gas-phase and ion-beam desorbed dopamine using femtosecond laser pulses at wavelengths of 800, 400, 267, and 200 nm. Photoionization of gas-phase dopamine is found to produce the molecular ion, and three fragment ions at all four wavelengths, with the branching ratios strongly wavelength dependent. Photoionization at 400 and 267 nm yields the highest molecular ion signal, while that at 800 and 200 nm produces very little molecular ion signal. An excited-state lifetime of [approximately]10 ps following 267-nm excitation has been measured for dopamine using time-resolved pump-probe techniques. The short-lived excited state suggests that internal conversion, intersystem crossing, and/or dissociation is a concern when ionizing at this wavelength using longer laser pulses. Photoionization of ion-beam-desorbed dopamine exhibits a large degree of fragmentation at all four wavelengths, though 267-nm photoionization produces the highest yield of dopamine fragment ions. Power dependence studies show a high degree of internal excitation. A direct comparison of ion yields obtained for photoionization of ion-beam-desorbed dopamine at 267 nm to that for SIMS shows a 20-fold increase in signal.

A study was conducted to examine laser-induced back ablation of aluminum thin film targets using picosecond laser pulses. It was assumed that all substrate damage occurs on the substrate front surface. The aluminum targets were characterized by a BK-7 glass strip with an aluminum film on the back surface. The velocity control of metal vapor plumes generated by laser-induced back ablation under substrate breakdown conditions. Experimental results also indicated a transmitted fluence limit consistent with observed edge velocity limits.

Ultrashort picosecond or sub-picosecond laser pulses with three consecutive time domains are used in controlling isomerization in intramolecular hydrogen transfer. This method is better than the pump-dump method because it uses laser intensities that are below the Keldysh limit. This is done by driving the wave packet through and not over the barrier. It will be proven that this type of laser-driven tunneling for different isomers can be realized easily for hydrogen transfer reactions. As such, it has been called the 'hydrogen-subway' method.

Increasing the carrier drift length by reducing the effective trap density through pre-illumination has been shown. The technique results in an increase of over a magnitude in the sensitivity of the device by increasing the number of quantum wells that are screened per carrier. Pre-illumination, too much of which can decrease the efficiency of four-wave mixing diffraction, can also be applied in speeding up the grating erasure by increasing the probability of electron hole recombination.

A theoretical investigation of the backward second-harmonic generation (SHG) process using subpicosecond and nanosecond laser pulses in periodically poled lithium niobate crystals was conducted. The amounts of pulse broadening and conversion efficiencies were different for the two configurations despite the fact that the temporal behaviors of the second-harmonic radiation were determined by the same functions as those for the forward SHG. The saturation of the conversion efficiencies and the effective third-harmonic generation were also investigated.

Ultrahigh-intensity lasers have a place in nonlinear optics in the relativistic regime for future applications in time-resolved chemistry. Ability to amplify pulses to extreme levels makes possible fundamentally new applications in science and technology. Interactions of lasers with matter in the ultrahigh-intensity regime makes generation of energetic photons and particles in the giga-electron-volt and X-ray area possible. That can make it possible to investigate ultrafast phenomena in chemistry, biology and physics with compact, fairly inexpensive systems such as those at universities.

A study was conducted to determine the effects of a laser power on the temperature dependence of the amplitude of fast and slow signals of epitaxial YBa2Cu3O7-delta films with 0.5 mm thick LaAlO3 substrate. Simultaneous measurement of the DC resistance and the fast and slow signals yielded significant inputs regarding optical response of the heat temperature superconductor. Results revealed that the optical response in the transition region can be attributed to the increase in the temperature.

Researchers examined the effects of using an infrared short pulse laser as a multi-photon pump to create population inversion in a large band gap semiconductor. The laser field creates electron-hole plasma, but the power density of the laser has to be high to create coherent recombination at ultraviolet wavelength, and the method induces only short pulses of coherent radiation. However, possible applications would be the production of millimeter waves or the operation of fast switches.

Research was conducted to analyze the energy transfer from chlorophyll b (Chl b) to chlorophyll a (Chl a) in monomeric preparations of light-harvesting complex II (LHCII) from spinach at 77 K using pump-probe experiments. Excitation at 650 nm was achieved using a narrow band (6-7 nm FWHM) interference filter. Results indicate that Chl b to Chl a transfer occurs only within monomeric subunits of the trimers and not between different subunits.

We developed a dynamic model for evaluating the gain and amplified spontaneous emission of multipass ultrashort-pulse laser amplifiers. This model takes into account the time dependence of the population inversion that is due to the time-varying pump power and the evolution of the amplified spontaneous emission. For gain media of a short upper-state lifetime, a time-dependent analysis is essential for the model to extend beyond order-of-magnitude estimation. We verified the model with a femtosecond dye laser amplifier. The calculated output energy is accurate to within [+ or -]5% of the experimental data. We utilized this model to optimize the conversion efficiency of the dye laser amplifier under low-energy pumping. An efficiency of 2.3% is achieved with an optimal pump energy of only 4 [[micro]joule].

Study was conducted on the rapid solidification phenomena in amorphous germanium films on silicon substrates through real time reflectivity measurements performed during irradiation with nanosecond laser pulses. The influence of the thermal response of the film/substrate system has been investigated by comparing the behavior of films with thicknesses in the range of 30-180 nm. Results show two different solidification scenarios depending on the ratio between film thickness and the thermal diffusion length of amorphous germanium.

The speed of light in an optical fiber is measured using a simple, commercially available educational kit. The procedure consists of determining the time-delay for a modulated light pulse traveling through fibers of different lengths. The time-delay caused by the electronic circuits is then accounted for in a systematic way.

A model for nonlinear pulse propagation solves the nonlinear Schrodinger equation and a rate equation that describes pulse generation in water. It takes into account self-focusing (SF), group velocity dispersion, and laser-induced breakdown (LIB). The simulation establishes a threshold for LIB and shows that SF is plays a major role in understanding LIB in water for pico- and femto-second pulses. The cascade process provides the breakdown mechanism for longer pulses, while the SF and the multiphoton plasma generation collectively dominate the breakdown threshold for shorter pulses.