Femtosecond UV Laser Research Articles

Ultrafast pre-breakdown dynamics in Al₂O₃SiO₂ reflector by femtosecond UV laser spectroscopy.

Ultrafast carrier dynamics in Al2O3/SiO2 high reflectors has been investigated by UV femtosecond laser. It is identified by laser spectroscopy that, the carrier dynamics contributed from the front few layers of Al2O3 play a dominating role in the initial laser-induced damage of the UV reflector. Time-resolved reflection decrease after the UV excitation is observed, and conduction electrons is found to relaxed to a mid-gap defect state locating about one photon below the conduction band . To interpret the laser induced carrier dynamics further, a theoretical model including electrons relaxation to a mid-gap state is built, and agrees very well with the experimental results.. To the best of our knowledge, this is the first study on the pre-damage dynamics in UV high reflector induced by femtosecond UV laser.

Control of thermal effects in fast-switching femtosecond UV laser system

Femtosecond laser systems are becoming essential tools in various areas of material processing, medicine and scientific research. In order to tailor the wavelength of radiation to the absorptivity of materials being processed, UV harmonic generators are often employed. At high average powers and high repetition rates, the long warm-up time of harmonic generators, caused by UV absorption in nonlinear crystals, can be a serious obstacle for efficient and fast processing. To increase the speed and reduce the cost of technological processes, new methods are required to speed up the settling of the output power when the harmonic generators are switched on after longer idle period. We have investigated a fourth-harmonic generator of a high-repetition rate femtosecond laser and studied the properties of the output radiation in order to optimize its fast-switching capabilities. Theoretical modeling of thermal effects in the nonlinear crystal allowed us to explain temporal dependencies of temperature and output power after switching the laser on. Based on these results, we were able to optimize the trajectory of nonlinear crystal rotation following the system start-up and reduce the switching-on time from tens of seconds to 50 ms without any negative effect on the output power.

Nonequilibrium plasma channel in gaseous media formed by powerful UV laser as a waveguide for transportation and amplification of short microwave pulses

The evolution of strongly nonequilibrium plasma in a channel created in gaseous media by powerful UV femtosecond laser pulse is studied. It is demonstrated that such a plasma channel can be used as a waveguide for both transportation and amplification of the microwave radiation. The specific features of such a plasma waveguide are studied on the basis of the self-consistent solution of the kinetic Boltzmann equation for the electron energy distribution function in different spatial points of the gas media and the wave equation in paraxial approximation for the microwave radiation guided and amplified in the channel. The results of modeling for rare gases (xenon) and air are compared. The amplification factor in xenon plasma in dependence on channel radius, intensity and frequency of the input RF radiation is analyzed.

Determination of relative rare earth element distributions in very small quantities of uranium ore concentrates using femtosecond UV laser ablation – SF-ICP-MS coupling

An alternative analytical method has been developed to determine lanthanide distribution using hundreds of nanograms of uranium ore concentrates by femtosecond ultra-violet laser ablation coupled to inductively coupled plasma sector-field mass spectrometry.

A new high-precision method for determining stable chlorine isotopes in halite and igneous rock samples using UV-femtosecond laser ablation multiple Faraday collector inductively coupled plasma mass spectrometry

A new analytical protocol for measurement of stable chlorine isotopes was proposed for LA-MFC-ICPMS.

High resolution scanning microanalysis on material surfaces using UV femtosecond laser induced breakdown spectroscopy

Abstract Femtosecond lasers together with high resolution optics have given us the ability to achieve submicron ablation spots which can play an important role in specific micromachining applications. Light emitted from the plasma at the sample surface created by a focused femtosecond laser pulse can also be used in laser induced breakdown spectroscopy (LIBS) and allows us to characterize the chemical composition of the target surface with micron-level lateral resolution. The spatial resolution using LIBS has often been defined by measuring the FWHM of the crater size. In this report, we study the application of femtosecond 266 nm laser pulses with very low energies of 10׳s of nanojoules. We have investigated spatial resolution using the detection of thin strips of chromium on silicon substrates and compared the actual width of the chromium versus the experimentally obtained width using LIBS detection. The variation of signal levels for low pulse energies is investigated on chromium surfaces. A spatial resolution of ~1 μm was obtained for detection of chromium from the emission.

Surface nanopatterning of Al/Ti multilayer thin films and Al single layer by a low-fluence UV femtosecond laser beam

The effects of UV femtosecond laser beam with 76MHz repetition rate on two types of thin films on Si substrate – the Al single layer thin film, and the multilayered thin film consisted of five Al/Ti bilayers (total thickness 130nm) – were studied. The surface modification of the target was done by low fluences and different irradiation times, not exceeding ∼300s. Nanopatterns in the form of femtosecond-laser induced periodic surface structures (fs-LIPSS) with periodicity of <315nm and height of ∼45nm were registered upon irradiation of the thin films. It was shown that: (i) the fs-LIPSS evolve from ruffles similar to high spatial frequency LIPSS (HSFL) into a low spatial frequency LIPSS (LSFL) if a certain threshold of the fluence is met, (ii) the number of LSFL increases with the exposition time and (iii) the LSFL remain stable even after long exposure times. We achieved high-quality highly-controllable fabrication of periodic structures on the surface of nanosized multilayer films with high-repetition-rate low-fluence femtosecond laser pulses. Compared to the Al single layer, the presence of the Ti underlayer in the Al/Ti multilayer thin film enabled more efficient heat transmittance through the Al/Ti interface away from the interaction zone which caused the reduction of the ablation effects leading to the formation of more regular LIPSS. The different outcomes of interactions with multi and single layer thin films lead to the conclusion that the behavior of the LIPSS is due to thin film structure.

Micro/Nano Surface Texturing in Si Using UV Femtosecond Laser Pulses

A fast laser texturing technique has been utilized to produce micro/nanosurface textures in Silicon by means of UV femtosecond laser. We have prepared good absorber surface for photovoltaic cells. The textured Silicon surface absorbs the incident light greater than the non-textured surface. The results show a photovoltaic current increase about 21.3% for photovoltaic cell with two-dimensional pattern as compared to the same cell without texturing.

Inscription of strong Bragg gratings in pure silica photonic crystal fibers using UV femtosecond laser pulses

We demonstrate our experimental results on inscription of strong first-order Bragg gratings in hydrogen-free and hydrogen-loaded pure silica photonic crystal fibers (PCFs) using high-intensity 160 femtosecond (fs) UV laser pulses at 266nm and a two-beam Talbot interferometer. Bragg reflectivities up to 37dB for the H2-loaded fiber as well as Bragg wavelength tuning over the telecom C-band using a single phase mask were achieved. The in-situ nonlinear filamentation of UV fs laser pulses, scattered by the fiber’s micro-structured cladding, is demonstrated by direct refractive index profile measurement across the cross section of a written fiber. The results of thermal annealing measurements for a H2-loaded PCF are also presented.

Single-shot ablation threshold of chromium using UV femtosecond laser pulses

Single-shot ablation threshold for thin chromium film was studied using 266 nm, femtosecond laser pulses. Chromium is a useful material in the nanotechnology industry and information on ablation threshold using UV femtosecond pulses would help in precise micromachining of the material. The ablation threshold was determined by measuring the ablation crater diameters as a function of incident laser pulse energy. Absorption of 266 nm light on the chromium film was also measured under our experimental conditions, and the absorbed energy single-shot ablation threshold fluence was $$46 \pm 5$$ mJ/cm2. The experimental ablation threshold fluence value was compared to time-dependent heat flow calculations based on the two temperature model for ultrafast laser pulses. The model predicts a value of 31.6 mJ/cm2 which is qualitatively consistent with the experimentally obtained value, given the simplicity of the model.

Photophysics and Photochemistry of a DNA–Protein Cross-Linking Model: A Synergistic Approach Combining Experiments and Theory

The photophysical and photochemical properties of 5-benzyluracil and 5,6-benzyluracil, the latter produced by photocyclization of the former through irradiation with femtosecond UV laser pulses, are investigated both experimentally and theoretically. The absorption spectra of the two molecules are compared, and the principal electronic transitions involved are discussed, with particular emphasis on the perturbation induced on the two chromophore species (uracil and benzene) by their proximity. The photoproduct formation for different irradiation times was verified with high-performance liquid chromatography and nuclear magnetic resonance measurements. The steady-state fluorescence demonstrates that the fluorescence is a distinctive physical observable for detection and selective identification of 5- and 5,6-benzyluracil. The principal electronic decay paths of the two molecules, obtained through TDDFT calculations, explain the features observed in the emission spectra and the photoreactivity of 5-benzyluracil. The order of magnitude of the lifetime of the excited states is derived with steady-state fluorescence anisotropy measurements and rationalized with the help of the computational findings. Finally, the spectroscopic data collected are used to derive the photocyclization and fluorescence quantum yields. In obtaining a global picture of the photophysical and photochemical properties of the two molecules, our findings demonstrates that the use of 5-benzyluracil as a model system to study the proximity relations of a DNA base with a close-lying aromatic amino acid is valid at a local level since the main characteristics of the decay processes from the excited states of the uracil/thymine molecules remain almost unchanged in 5-benzyluracil, the main perturbation arising from the presence of the close-lying aromatic group.

Fiber Bragg grating inscription based on optical filamentation of UV femtosecond laser pulses

We present the fabrication and characterization of strong first-order Bragg gratings operating around 1550nm in standard telecom fibers (SMF-28) by use of 160fs UV laser pulses at 266nm and a Talbot interferometer. Bragg reflectivities exceeding 50dB are achieved in both hydrogen-free and hydrogen-loaded fibers. In addition, Bragg wavelength tuning over the telecom C-band using a single phase mask is demonstrated. We also show that the formation of Bragg gratings, relying on a two-photon absorption process, is intimately accompanied by the optical filamentation process of UV fs laser pulses during nonlinear propagation in the fiber.

Nano- and femtosecond UV laser pulses to immobilize biomolecules onto surfaces with preferential orientation

By relying on the photonic immobilization technique of antibodies onto surfaces, we realized portable biosensors for light molecules based on the use of quartz crystal microbalances, given the linear dependence of the method on the laser pulse intensity. Here, we compare the quality of the anchoring method when using nanosecond (260 nm, 25 mJ/pulse, 5 ns, 10 Hz rep. rate) and femtosecond (258 nm, 25 μJ/pulse, 150 fs, 10 kHz rep. rate) laser source, delivering the same energy to the sample with the same average power. As a reference, we also tethered untreated antibodies by means of the passive adsorption. The results are striking: When the antibodies are irradiated with the femtosecond pulses, the deposition on the gold plate is much more ordered than in the other two cases. The effects of UV pulses irradiation onto the antibodies are also analyzed by measuring absorption and fluorescence and suggest the occurrence of remarkable degradation when nanosecond pulses are used likely induced by a larger thermal coupling. In view of the high average power required to activate the antibodies for the achievement of the photonic immobilization technique, we conclude that femtosecond rather than nanosecond laser pulses have to be used.

Photovoltaic enhancement of Si micro- and nanostructure solar cells via ultrafast laser texturing

The purpose of the present study was to achieve laser-microtextured Si structures, the material used as silicon wafers, with a thickness of 330 mu m for a fast laser texturing technique, in order to produce micro-/nanosurface textures by means of a UV femtosecond laser. A textured silicon surface was prepared that has the capability to trap light, thereby greatly reducing the visible light reflection for photovoltaic cells. The textured surface was measured by scanning electron microscope. The results showed a photocurrent increase of about 25% in the laser-textured zones.

Nonresonant EUV-UV two-color two-photon ionization of He studied by single-shot photoelectron spectroscopy

Single-shot photoelectron spectroscopy of nonresonant two-color two-photon ionization of He has been carried out using an intense free-electron laser (FEL) at 59.7 nm and a femtosecond UV laser at 268 nm. By shot-by-shot analysis of photoelectron spectra, the cross section for nonresonant two-color ionization of He is determined to be ${\ensuremath{\sigma}}^{(2)}=4.1(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}52}$ cm${}^{4}$ s. The cross-correlation trace derived from the nonresonant signals shows that the temporal resolution of the system is 0.36(3) ps, which is mainly governed by the timing jitter between the FEL and UV laser pulses.

Iron Isotope Composition of Particles Produced by UV-Femtosecond Laser Ablation of Natural Oxides, Sulfides, and Carbonates

The need for femtosecond laser ablation (fs-LA) systems coupled to MC-ICP-MS to accurately perform in situ stable isotope analyses remains an open question, because of the lack of knowledge concerning ablation-related isotopic fractionation in this regime. We report the first iron isotope analysis of size-resolved, laser-induced particles of natural magnetite, siderite, pyrrhotite, and pyrite, collected through cascade impaction, followed by analysis by solution nebulization MC-ICP-MS, as well as imaging using electron microscopy. Iron mass distributions are independent of mineralogy, and particle morphology includes both spheres and agglomerates for all ablated phases. X-ray spectroscopy shows elemental fractionation in siderite (C-rich agglomerates) and pyrrhotite/pyrite (S-rich spheres). We find an increase in (56)Fe/(54)Fe ratios of +2‰, +1.2‰, and +0.8‰ with increasing particle size for magnetite, siderite, and pyrrhotite, respectively. Fe isotope differences in size-sorted aerosols from pyrite ablation are not analytically resolvable. Experimental data are discussed using models of particles generation by Hergenröder and elemental/isotopic fractionation by Richter. We interpret the isotopic fractionation to be related to the iron condensation time scale, dependent on its saturation in the gas phase, as a function of mineral composition. Despite the isotopic variations across aerosol size fractions, total aerosol composition, as calculated from mass balance, confirms that fs-LA produces a stoichiometric sampling in terms of isotopic composition. Specifically, both elemental and isotopic fractionation are produced by particle generation processes and not by femtosecond laser-matter interactions. These results provide critical insights into the analytical requirements for laser-ablation-based stable isotope measurements of high-precision and accuracy in geological samples, including the importance of quantitative aerosol transport to the ICP.

In-situ Fe isotope ratio determination in Fe–Ti oxides and sulfides from drilled gabbros and basalt from the IODP Hole 1256D in the eastern equatorial Pacific

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe–Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (>300°C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower δ56Fe values by 0.2‰–0.5‰ as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals.We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of <0.1‰ (2σ level) at micrometer-scale. We have found significant variations of the δ56FeIRMM-014 values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of δ56Femagnetite in 1256D rocks ranges from −0.12 to +0.64‰, and of δ56Feilmenite from −0.77 to +0.01‰. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive δ56Fe values between +0.29 and +0.56‰, whereas pyrite in the other samples has generally negative δ56Fe values from −1.10 to −0.59‰.One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite–ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including β-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the δ56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite–ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures.Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.

197 nm femtosecond laser-pulse duration: comparison of autocorrelation measurements

An easy and reliable way is presented to measure the duration of UV femtosecond laser pulses of λ < 200 nm. The used autocorrelation techniques are based on two-photon absorption (TPA) in different TPA media, especially calcium fluoride (CaF2). For 197 nm, the laser-pulse energy transmission and the laser-induced fluorescence of self-trapped excitons at 278 nm are applied. Both methods yield nearly the same second-order autocorrelation functions allowing to analyze the investigated laser pulse and obtain its duration of \((350\pm10)\) fs.

Synthesis and Photostructuring of Hybrid Photopolymers with Cinnamate and Anil Moieties by using UV Light and Femtosecond Laser Pulses

Two photosensitive methacrylates containing urea and two cinnamates (N,N–di(cinnamoyloxyethyl) N'-methacryloyloxyethyl urea, DC-MA), or urethane and saliciliden-aniline (anil) groups (methacryloyloxyethyl-2-carbamoyloxy(m-methyl, o-hydroxybenziliden)-aniline, UAn-MA) were synthesized and used besides N,N'-(methacryloxyethyldietoxy-methylsilylpropyl)-urea (UH-MA) in the preparation and photostructuring of hybrid photopolymers. Polymerization of the monomers was investigated by differential scanning photo calorimetry (photo-DSC), Fourier transform infrared (FTIR) and near-infrared (NIR) spectroscopies. The rate of photopolymerization increased upon adding UH-MA in formulation (DC-MA/UH-MA, Rp = 9.1 × 10−2 s−1; UAn-MA/UH-MA, Rp = 2.32 × 10−2 s−1), the degree of conversion being between 33 and 76%. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical microscopy confirmed the formation of bidimensional hybrid microstructures by using two-photon polymerization (2PP).

Intra-cluster dynamics induced in molecular clusters by femtosecond UV radiation

Dynamics of intra-cluster processes induced by femtosecond UV laser radiation in molecular clusters of CF3 I and ICF2 COF molecules is studied. The measured time constants of the observed reactions are within the range from 1 ps to dozens of picoseconds.