fs/ns dual-pulse LIBS analytic survey for copper-based alloys

The quantitative analytic capability of a fs/ns dual-pulse Laser-Induced Breakdown Spectroscopy technique, based on the orthogonal reheating of a fs-laser ablation plume by a ns-laser pulse, is presented. In this work, it is shown how the effect played by the delay times between the two laser beams can vary the analytical response of this dual-pulse LIBS configuration. In order to address this task, the Sn, Pb and Zn calibration curves of five certified copper-based samples have been investigated. These calibration curves have been obtained, in air at atmospheric pressure, by integrating the emission data collected in two different inter-pulse delay zones, one in the delay interval of 1-41 μs, the other within the range of 46-196 μs. For drawing the species calibration curves, the emission intensities of the considered Pb(I), Sn(I) and Zn(I) electronic transitions have been normalized with a non-resonant Cu(I) emission line. The experimental results have shown that, by varying the inter-pulse delay between the two laser beams, complementary analytical results can be induced. By considering at once all data acquired within the inter-pulse delay time of 1-196 μs, this hypothesis has been strengthened. The calibration curves obtained in this way are characterized by excellent linear regression coefficients (0.988-0.999) despite of the large Sn, Pb and Zn compositional variation of the targets employed. The results presented reveal, for the first time, that, by taking into account the role played by the inter-pulse delay time between the two laser beams, the fs/ns dual-pulse LIBS configuration here used can be improved and provide very good opportunities for performing quantitative analysis of copper-based alloys.     

Optical emission spectroscopy investigation of an ultra-short laser induced titanium plasma reheated by a ns laser pulse

The dynamics of a titanium plasma species, induced in air by coupling a fs-ablating laser pulse with an orthogonal ns-reheating laser source placed at the fixed distance of 1.0 mm from the target surface, has been followed by temporally resolved emission spectroscopy. The temporal evolutions of plasma features such as excitation temperatures and electron densities have been evaluated by using two different laser energies of the first fs-ablating laser pulse (0.8 mJ and 3.0 mJ). Optimum inter-pulse delay times, experimentally determined, of 250 μs and 500 μs were used for the fs laser energy of 3.0 mJ and 0.8 mJ, respectively. By experimental inspections of the main plasma species electronic transitions so obtained, a strong enhancement was evaluated up to one and two orders of magnitude for Ti(I) and Ti(II), respectively. Independently from the fs laser energy employed, the plasma features showed the same temporal behaviour implying that the ns-reheating characteristics of this process belong to the reheating mechanism itself. The experimental results have been discussed and the excited species evolutions and elementary processes involved, as well as, the local thermodynamic equilibrium departures, have been outlined.     

Mg-based photocathodes prepared by ns, ps and fs PLD for the production of high brightness electron beams

Mg-based films have been prepared by pulsed laser deposition technique for photocathode applications. We have investigated the influence of pulse laser duration on morphology and photoemissive properties. Two laser sources have been used, generating pulses of 30 ns at 308 nm (XeCl excimer laser), 5 ps and 500 fs at 248 nm (KrF excimer laser) to grow Mg films onto Si and Cu substrates in high vacuum (∼10⁻⁷ Pa) and at room temperature. Morphological investigations carried out by scanning electron microscopy (SEM) have revealed that, in our experimental conditions, the number and the mean size of the droplets on the films surfaces decreases as the pulse laser duration shortens. The contamination level of Mg film surfaces have been studied by energy dispersive X-ray spectroscopy (EDX). The photoelectron performances in terms of quantum efficiency (QE) and emission stability have been tested in a UHV DC photodiode cell (10⁻⁷ Pa). Measures of the QE of the samples surfaces have revealed a decrease on the initial value for Mg-based photocathodes prepared by fs laser (from 7.8 × 10⁻⁴ to 6.6 × 10⁻⁴) PLD with respect to ps (from 6.2 × 10⁻⁴ to 7.4 × 10⁻⁴) and ns lasers (from 5.0 × 10⁻⁴ to 1.6 × 10⁻³). A comparison among Mg-based photocathodes prepared by ns, ps and fs PLD for the production of high brightness electron beams has been presented and discussed.     

Line patterning with large refractive index changes in the deep inside of glass by nanosecond pulsed YAG laser irradiation

Nanosecond (∼100 ns) pulsed (10 Hz) Nd:YAG laser operating at the wavelength (λ) of 1064 nm with pulse energies of 0.16-1.24 mJ/cm² has irradiated 10Sm₂O₃·40BaO·50B₂O₃ glass. It is demonstrated for the first time that the structural modification resulting the large decease (∼3.5%) in the refractive index is induced by the irradiation of YAG laser with λ=1064 nm. The lines with refractive index changes are written in the deep inside of 100-1000 μm depths by scanning laser. The line width is 1-13 μm, depending on laser pulse energy and focused beam position. It is proposed that the samarium atom heat processing is a novel technique for inducing structural modification (refractive index change) in the deep interior of glass.     

Pulsed inductive discharge CO2 laser

A pulsed inductive discharge CO₂ laser with a wavelength of 10.6 μm has been created for the first time. The excitation system of a cylindrical pulsed inductive discharge (pulsed inductively coupled plasma) in the gas mixture of CO₂:N₂:He was developed. The temporal and energy parameters of the laser radiation were investigated. The maximum inductive discharge CO₂ laser radiation energy of 104 mJ was achieved. An average power of 3.2 W was obtained at laser generation energy of 65 mJ and pulse repetition rate of 50 Hz. In the cross-section, the laser radiation had the ring shape with an external diameter of 34 mm and thickness of 4-5 mm. The measured divergence of laser radiation was 12 mrad.     

High repetition rate Q-switched microchip Nd:YVO4 laser with pulse duration as short as 1.1 ns

We have demonstrated a compact and an efficient passively Q-switched microchip Nd:YVO₄ laser by using a composite semiconductor absorber as well as an output coupler. The composite semiconductor absorber was composed of an LT (low-temperature grown) In_0.25Ga_0.75As absorber and a pure GaAs absorber. To our knowledge, it was the first demonstration of the special absorber for Q-switching operation of microchip lasers. Laser pulses with durations of 1.1 ns were generated with a 350 μm thick laser crystal and the repetition rate of the pulses was as high as 4.6 MHz. The average output power was 120 mW at the pump power of 700 mW. Pulse duration can be varied from 1.1 to 15.7 ns by changing the cavity length from 0.45 to 5 mm. Pulses with duration of 1.67 and 2.41 ns were also obtained with a 0.7 mm thick laser crystal and a 1 mm thick laser crystal, respectively.     

Drilling enhancement by nanosecond-nanosecond collinear dual-pulse laser ablation of titanium in vacuum

Laser ablation of titanium in vacuum was performed using single- and dual-pulse regime in order to study crater formation. Crater profiles were analyzed by optical microscopy. It was found that the repetition-rate plays an important role in a process of laser ablation. The drilling is most effective for the highest repetition-rate. For the same total number of laser pulses clear drilling enhancement was achieved by dual-pulse regime of ablation in comparison to single-pulse regime. The strongest ablation rate in dual-pulse regime was achieved for the delay time between the pulses τ = 370 ns. Results are discussed in terms of decreased ablation threshold due to continuous heating of the target during the experiment.     

Background gas collisional effects on expanding fs and ns laser ablation plumes

The collisional effects of a background gas on expanding ultrafast and short pulse laser ablation plumes were investigated by varying background pressure from vacuum to atmospheric pressure levels. For producing Cu ablation plumes, either 40 fs, 800 nm pulses from a Ti: Sapphire laser or 6 ns, 1,064 nm pulses from a Nd:YAG laser were used. The role of background pressure on plume hydrodynamics, spectral emission features, absolute line intensities, signal to background ratios and ablation craters was studied. Though the signal intensities were found to be maximum near to atmospheric pressure levels, the optimum signal to background ratios are observed ~20–50 Torr for both ns and fs laser ablation plumes. The differences in laser–target and laser–plasma couplings between ns and fs lasers were found to be more engraved in the crater morphologies and plasma hydrodynamic expansion features.     

Experimental and theoretical studies of polaron optical properties in KNbO3 perovskite

Time-resolved absorption and luminescence spectra have been measured in KNbO₃ perovskite crystals after pulsed band-gap excitation by 200 fs laser pulses and 10 ns electron pulses. Quantum chemical calculations using the large unit cell periodic model support the interpretation of the observed transient absorption bands at 0.8 and 1.1 eV as the self-trapped electron polarons and bound hole polarons, respectively. The activation energy for the 2.2 eV green luminescence quenching is 0.05 eV. We suggest that the short lifetime (<15 ns) of the luminescence at RT is caused by the radiative recombination of nearest electron and hole polarons.     

ns- and fs-LIBS of copper-based-alloys: A different approach

A self-calibrated analytical technique, based on plasmas induced by either 250 fs or 7 ns laser pulses, is presented. This approach is comparable to other calibration-free methods based on LTE assumption. In order to apply this method to very different laser pulse durations, the partial-local thermodynamic equilibrium (p-LTE) has been considered within the energy range of 30,000-50,000 cm⁻¹. In order to obtain the neutral species densities, the detected plasma species emission lines intensities have been treated together with the experimental evaluated background black-body Planck-like emission distribution. For validating the followed method, three certified copper-based-alloys standards were employed and their minor components (Ni, Pb and Sn) amounts were determined. As a result, it arises, that this standardless method, independently from the laser source pulse durations, provides good quantitative analysis, and, consequently, that the composition of the plasma plume emitting species induced is not affected by the laser pulse time width.     

Femtosecond laser induced photoluminescence in poly(methyl methacrylate) and three-dimensional optical storage

We report on a femtosecond-laser induced photoluminescence (PL) in poly(methyl methacrylate) and its potential application to three-dimensional optical storage. Irradiation with a focused 800 nm, 1 kHz, 100 fs pulsed laser induced a strong PL change in UV-visible region. Absorption spectra and Fourier-transform infrared spectra before and after laser irradiation indicate the PL may result from the emissive oxidized products of photo-degradation reaction of PMMA. This makes it possible to read out the stored data by detecting the PL change. The pulse energy threshold of the light-induced PL change of PMMA is found to be at ∼2 μJ/pulse and the optimal recording energy is ∼3 μJ/pulse. A ten-layer pattern inside the bulk sample recorded by tightly focusing a pulsed laser beam was read out by a reflection-type fluorescent confocal microscope, which detected the emission in visible range as the signal. High-contract fluorescent images with a much higher signal-to-noise ratio were obtained without crosstalk in comparison with the ordinary reflection mode.     

Pulsed infrared radiation transmission through chalcogenide glass fibers

Two different kinds of chalcogenide glass IR fibers were evaluated relative to transmission of pulsed IR radiation produced by several laser sources in the wavelength range from 1 to 10 μm. Fibers composed either from As-Se-Te or from As₂S₃ glass, of 250, 500, 750 and 1000 μm and 250, 750 and 1000 μm core diameters were studied, respectively. Attenuation measurements were obtained as a function of the laser energy input and as a function of curvature, wherever this was possible. The output beam quality was also studied using a beam profiler. The lasers used were a Q-switched Nd:YAG laser, emitting at 1.06 μm, a free-running or Q-switched Er:YAG laser emitting at 2.94 μm and a tunable pulsed CO₂ laser emitting in the range of 9.3-10.6 μm. The fibers exhibited better behavior when tested with the Er:YAG laser and they were found fragile in pulsed radiation from the Nd:YAG and the CO₂ laser. The output beam profiles generally showed a central multi-spiking energy distribution.     

Orthogonal fs/ns double-pulse libs for copper-based-alloy analysis

The analytical response of a fs/ns double-pulse laser induced breakdown spectroscopy technique based on the orthogonal reheating induced by a ns-laser pulse on a fs-laser ablation plume is presented. All investigations have been performed in air at atmospheric pressure and employing certified copper-based-alloy targets. The emission intensities of the considered electronic transitions of Pb(I), Sn(I) and Zn(I) have been normalised with a Cu(I) emission line intensity belonging to the same considered spectral range. Emission data, acquired with inter-pulse steps of 2 μs within the delay range of 1–200 μs, have shown that fractionation takes place. Nevertheless, excellent linear regression coefficients (0.998–0.999), despite the target’s large compositional variation and fractionation effects, have been obtained by integrating all emission intensity data along the whole inter-pulse delays used. Deviations from the theoretical ratio of the Zn(I)/Cu(I) emission intensities are shown and some hypotheses about the processes involved are formulated.     

Fast photovoltaic characteristic of silver nano-cluster doped ZnO thin films induced by 1.064 μm pulsed laser

Fast photovoltaic characteristic was found from silver nano-cluster doped ZnO thin films when it was irradiated by 1.064 μm infrared laser. Its arising time is about 1 ns with an open-circuit photovoltage of ∼2 ns full width at half-maximum. A transient photovoltaic signal of ∼80 mV occurred with a full width at half-maximum of about 2 ns was observed under the illumination of a 4 mJ pulsed laser in duration of 25 ps. This fast photovoltaic response perhaps relate to the non-uniform distribution of the silver clusters along direction perpendicular to ZnO surface.     

Submicron foaming in gelatine by nanosecond and femtosecond pulsed laser irradiation

We compare the foaming characteristics induced by irradiation with single ns and fs laser pulses of UV, VIS and IR wavelengths on gelatines differing in gel strength (bloom values 75 and 225) and in crosslinking degree. We have observed that while laser irradiation with nanoseconds leads to the formation of a microfoam layer at 266 nm, and melting and crater formation at longer wavelengths (532 and 1064 nm), fs pulse irradiation leads to submicron foaming at all wavelengths studied (266, 400 and 800 nm). These results show the possibility of controlling the submicrometric foam structure in this biomaterial and can shed light into the working mechanisms of fs laser nanoprocessing in biomaterials, where increase of temperature, thermoelastic stress generation, and stress-induced bubble formation are mediated by the generated plasma.     

Laser impulse coupling at 130 fs

We measured the momentum coupling coefficient C_m and laser-generated ion drift velocity and temperature in the femtosecond (fs) region, over a laser intensity range from ablation threshold to about one hundred times threshold. Targets were several pure metals and three organic compounds. The organic compounds were exothermic polymers specifically developed for the micro-laser plasma thruster, and two of these used “tuned absorbers” rather than carbon particles for laser absorption. The metals ranged from Li to W in atomic weight. We measured time of flight (TOF) profiles for ions. Specific impulse reached record values for this type of measurement and ablation efficiency was near 100%. These measurements extend the laser pulsewidth three orders of magnitude downward in pulsewidth relative to previous reports. Over this range, we found C_m to be essentially constant. Ion velocity ranged from 60 to 180 km/s.     

Fabrication of grating structures by simultaneous multi-spot fs laser writing

Two-photon polymerisation is an established technique for the fabrication of three-dimensional microstructures. To date structures have mostly been developed using single beam serial writing. A novel approach to simultaneous multi-spot two-photon polymerisation, that uses a SiO₂ on glass Fraunhofer diffractive optical element to generate an array of beamlets, is described. A Ti:sapphire laser, with wavelength 790 nm, 80 MHz repetition rate, 100 fs pulse duration and an average power of 25 mW, was used to initiate two-photon polymerisation. The DOE, in combination with a high power microscope objective, efficiently transforms the laser beam into a linear array of four spots of equal intensity. The fabrication of a periodic transmission grating, using parallel processing with these four spots, is shown. The grating was written in a Zr-loaded resin prepared on a glass substrate using dip coating deposition of a Zr/PMMA hybrid prepared by the sol-gel method. The operation of the diffractive element and the performance of the diffraction grating are also discussed.     

LD end pumped, actively mode locked and passively Q-switched Nd:YAP laser at 1341 nm

An end pumped Nd:YAP laser at 1341 nm is actively mode locked and passively Q-switched. Pumping was done with a pulsed high power laser diode with maximum power 425 W. V³⁺:YAG with 61% initial transmission served as saturable absorber, and an acousto-optic modulator is used for active mode locking. The output pulse train with 69 ns duration has a total energy of 3.2 mJ with ±4% shot-to-shot fluctuation. The peak output energy of a single mode locked pulse is 0.25 mJ. The pulse duration of a single mode locked pulse is less than 800 ps. The output laser beam is nearly diffraction limited with 1.6 mm diameter, and beam propagation factor M² about 1.3.     

Surface damage morphology investigations of silicon under millisecond laser irradiation

The surface damage morphologies of single crystal silicon induced by 1064 nm millisecond Nd:YAG laser are investigated. After irradiation, the damage morphologies of silicon are inspected by optical microscope (OM) and atomic force microscope (AFM). The plasma emission spectra of the damaged region are detected by the spectrometer. It is shown that surface oxidation and nitridation have occurred during the interaction of millisecond laser with silicon. In addition, the damage morphologies induced by 2 ms and 10 ns pulse width laser are compared. The damage morphology obtained by 2 ms laser is an evident crater. Three types of damage morphologies are formed at different laser energy densities. The circular concentric ripples are found surrounding the rim of the crater. The spacing of the ripples is 15 ± 5 μm. Two types of cracks are observed: linear crack and circular crack. The linear crack is observed in the center of the damaged region which propagates to the periphery of the damaged region. The circular crack is located at the rim of the crater. The damage morphology induced by 10 ns laser is surface layer damage. The periodic linear waves are generated due to the interference between the incident beam and the scattered beam. The spacing of the ripples is 1.54 μm which is close to the incident laser wavelength 1.064 μm. The linear crack is located at the center of the damaged region. Furthermore, for the same laser energy density, the dimension of the damaged region and the crater depth induced by 2 ms laser are greater than that of 10 ns laser. It indicates that the damage mechanism under millisecond pulse laser irradiation is strongly different from the case of nanosecond pulse laser.     

Construction and timing system of the EPOS beam system

The Forschungszentrum Dresden-Rossendorf provides an intense pulsed 40 MeV electron beam with high brilliance and low emittance (ELBE). The pulse has a length of 1-10 ps and a repetition time of 77 ns, or in slow mode 616 ns. The EPOS system (ELBE Positron Source) generates by pair production on a tungsten converter and a tungsten moderator an intense pulsed beam of mono-energetic positrons. To transport the positrons to the laboratory (12 m) we constructed a magnetic beam guidance system with a longitudinal magnetic field of 75 G. In the laboratory outside the cave, the positron beam is chopped and bunched according to the time structure, because the very sharp bunch structure of the electron pulses is broadened for the positron beam due to transport and moderation.