Short Laser Pulses Research Articles

Plasmonic nanoresonator distributions for uniform energy deposition in active targets

Active targets implanted with core-shell-composition (CS) and nanorod-shaped (NR) plasmonic nanoresonators and doped with dyes were designed to ensure uniform energy deposition during illumination by two-counter propagating short laser pulses. The near-field enhancement, optical responses, and cross-sections were mapped above the concentration-Epump parameter-plane to inspect two different regions (I and II) with the potential to improve light-matter interaction phenomena. The distribution of steady-state absorption, as well as of the power-loss and power-loss density integrated until the complete overlap of the two short pulses was determined. The uniform distribution was adjusted to constrain standard deviations of the integrated power-loss distributions in the order of ∼10%. Dye doping of target-I/II implanted with uniform CS (NR) nanoresonator distributions results in larger absorption with increased standard deviation, larger power-loss, and power-loss density with decreased (decreased / increased) standard deviation. The adjustment allows larger absorption in CS-II and larger power-loss and power-loss density in CS-implanted targets, smaller standard deviation in targets-I for absorption, and in all targets for power-loss and its density. Larger dye concentration makes it possible to achieve larger absorption (except in adjusted NR-II), larger power-loss and power-loss density in all CS and in adjusted NR distributions, with decreased standard deviation in CS-implanted targets for all quantities and in NR-implanted targets for absorption. CS implantation results in larger absorption with a larger standard deviation, moreover allows larger power-loss in adjusted distributions and smaller standard deviation in power-loss quantities for larger concentration in both distributions and the same standard deviation for smaller concentration in adjusted distribution. Based on these results, adjusted CS distributions in targets doped with a dye of higher concentration are proposed.

Метод функций Грина для расчета нестационарных спектров люминесценции неравновесных молекулярных систем

Interest in ultrafast photochemical processes is due to their role in wildlife and the possibility of using them in solar energy conversion devices. Significant part of experimental research in this area is carried out using different spectroscopic techniques, for example, based on recording the dynamic response of the system to optical excitation by a short laser pulse. This method is basic for photochemistry and is used for studying both small inorganic molecules in liquids or mixtures, and complex biological objects, such as photosynthetic reaction centers of bacteria and plants. Time-resolved luminescence/absorption spectra contain information about the processes of population of the electronic and vibrational states of the system, and therefore make it possible to study the mechanisms of photoreactions. At the same time, the question of the correct interpretation of experimental data in the case of ultrafast reactions at picosecond timescales is still relevant. The traditional approach is based on the decomposition of transient spectra into the dynamic components, associated with different electronic states. This state-associated analysis turns out to be much less accurate in ultrafast nonequilibrium reactions. The spectral dynamics of the system in this case depends not only on chemical transformations (that is, changes in state populations), but also on the evolution of the spectral response for each of the states. Particularly, relaxation of high-frequency intramolecular vibrations in ultrafast reactions was shown to cause not only a shift in the luminescence spectrum, but also significant changes in spectral profiles themselves. An assumption about the invariable shape of individual spectral components cannot be justified here. Relevant analysis of spectral dynamics in such systems thus requires taking into account the nonequilibrium state of intramolecular degrees of freedom and the environment. Development of theoretical models of nonequilibrium processes, as well as software tools for numerical simulation of spectral dynamics and the fitting techniques for experimental results data, allows us to improve the relevance of the analysis. This study is devoted to the development of a mathematical model of spectral dynamics of macromolecular systems, in which photoexcitation triggers a series of ultrafast electron transfer reactions involving several redox centers. The main attention is paid to accounting for nonequilibrium states of the medium and intramolecular degrees of freedom formed both at the stage of photoexcitation and in the course of a multistage reaction. This allows us to expand the range of phenomena under study and apply the model for interpretation of experimental data on multistage charge transfer in biological objects. The model is based on the semiclassical theory of multistage electron transfer in a multicomponent non-Debye solvent, as well as the Green’s function method for the classical (polarization) and quantum (intramolecular) coordinates of the system.

Comparison of the Sensitivity of Spectral Methods for Multi-Element Analysis of Atmospheric Aerosol Using Short and Ultrashort Laser Pulses

Comparison of the Sensitivity of Spectral Methods for Multi-Element Analysis of Atmospheric Aerosol Using Short and Ultrashort Laser Pulses

Resistivity contribution to stopping power and plasma heating by laser-accelerated electrons

A model of plasma stopping power with respect to laser-accelerated electrons is generalized. The model includes both main mechanisms of fast electron energy loss caused by Coulomb collisions with plasma electrons and ions (“collisional” mechanism), and by the field of resistive plasma current (“Ohmic” mechanism). The results are used to develop an analytical model of plasma heating by fast electrons with energies corresponding to the wide range of energy flux density of laser pulse capable to provide an intense generation of these particles. It is shown that the contribution of the Ohmic stopping power component has a significant effect on the dynamics of plasma heating by high-current fast electron beam generated by short femtosecond and picosecond laser pulses of petawatt power.

Multi-species ion acceleration from sub-ps, PW interactions with ultra-thin foils

Multi-species ion acceleration from ultra-thin foils was studied experimentally, employing the Vulcan Petawatt laser at the Central Laser Facility, UK. Plastic (CH) foils with thicknesses in the range 10–340 nm were irradiated with intense, short (750 fs) laser pulses producing maximum energies of MeV and 25 MeV/nucleon obtained for H+ and C6+ ions, respectively. Ion spectra obtained from high resolution spectrometers suggest differences in the acceleration dynamics for the two species. Comparisons are made with two-dimensional Particle in Cell simulations, which identify, for an optimal thickness, two main mechanisms contributing to the ion acceleration process, namely multi-species target normal sheath acceleration (TNSA) and radiation pressure acceleration (RPA). Ion energies are further enhanced by the onset of relativistically induced transparency. A final stage in the acceleration is caused by the formation of electron jets (as the target undergoes transparency), which accelerate the ions off-axis. By analysing the spatial and temporal evolution of the accelerating fields, we are able to infer the effect of the different mechanisms on each species and how this translates to the experimental observations. The two main mechanisms, TNSA and RPA, are seen to each produce a distinct population of high energy protons whereas a single population of carbon is accelerated by a summation of these effects. This species-specific analysis sheds new light on the complex dynamics in a multi-species target expansion and on the contribution of different acceleration processes to the acceleration of the most energetic ions in the spectrum.

On-the-Fly Short-Pulse R2R Laser Patterning Processes for the Manufacturing of Fully Printed Semitransparent Organic Photovoltaics.

Ultrafast laser patterning is an essential technology for the low-cost and large area production of flexible Organic Electronic (OE) devices, such as Organic Photovoltaics (OPVs). In order to unleash the potential of ultrafast laser processing to perform the selective and high precision removal of complex multilayers from printed OPV stacks without affecting the underlying nanolayers, it is necessary to optimize its parameters for each nanolayer combination. In this work, we developed an efficient on-the-fly picosecond (ps) laser scribing process (P1, P2 and P3) using single wavelength and single step/pass for the precise and reliable in-line patterning of Roll-to-Roll (R2R) slot-die-coated nanolayers. We have investigated the effect of the key process parameters (pulse energy and overlap) on the patterning quality to obtain high selectivity on the ablation of each individual nanolayer. Finally, we present the implementation of the ultrafast laser patterning process in the manufacturing of fully R2R printed flexible semitransparent OPV modules with a 3.4% power conversion efficiency and 91% Geometric Fill Factor (GFF).

High-flux neutron generation by laser-accelerated ions from single- and double-layer targets

Contemporary ultraintense, short-pulse laser systems provide extremely compact setups for the production of high-flux neutron beams, such as those required for nondestructive probing of dense matter, research on neutron-induced damage in fusion devices or laboratory astrophysics studies. Here, by coupling particle-in-cell and Monte Carlo numerical simulations, we examine possible strategies to optimise neutron sources from ion-induced nuclear reactions using 1-PW, 20-fs-class laser systems. To improve the ion acceleration, the laser-irradiated targets are chosen to be ultrathin solid foils, either standing alone or preceded by a plasma layer of near-critical density to enhance the laser focusing. We compare the performance of these single- and double-layer targets, and determine their optimum parameters in terms of energy and angular spectra of the accelerated ions. These are then sent into a converter to generate neutrons via nuclear reactions on beryllium and lead nuclei. Overall, we identify configurations that result in neutron yields as high as sim 10^{10},{mathrm{n}},{mathrm{sr}}^{-1} in sim 1-cm-thick converters or instantaneous neutron fluxes above 10^{23},{mathrm{n}},{mathrm{cm}}^{-2},{mathrm{s}}^{-1} at the backside of lesssim 100-upmum-thick converters. Considering a realistic repetition rate of one laser shot per minute, the corresponding time-averaged neutron yields are predicted to reach values (gtrsim 10^7,{mathrm{n}} ,{mathrm{sr}}^{-1},{mathrm{s}}^{-1}) well above the current experimental record, and this even with a mere thin foil as a primary target. A further increase in the time-averaged yield up to above 10^8,{mathrm{sr}}^{-1},{mathrm{s}}^{-1} is foreseen using double-layer targets.

Ultraintense few-cycle infrared laser generation by fast-extending plasma grating

Ultraintense short-period infrared laser pulses play an important role in frontier scientific research, but their power is quite low when generated using current technology. This paper demonstrates a scheme for generating an ultraintense few-cycle infrared pulse by directly compressing a long infrared pulse. In this scheme, an infrared picosecond-to-nanosecond laser pulse counterpropagates with a rapidly extending plasma grating that is created by ionizing an undulated gas by a short laser pulse, and the infrared laser pulse is reflected by the rapidly extending plasma grating. Because of the high expansion velocity of the latter, the infrared laser pulse is compressed in the reflection process. One- and two-dimensional particle-in-cell simulations show that by this method, a pulse with a duration of tens of picoseconds in the mid- to far-infrared range can be compressed to a few cycles with an efficiency exceeding 60%, thereby making ultraintense few-cycle infrared pulses possible.

ANALYSIS OF HUMAN HAIR BY THE LASER-INDUCED BREAKDOWN SPECTROSCOPY

We experimentally studied the spectral characteristics of plasma generated on the surface of human hair under various focusing conditions (by the lenses with a focal length of 50, 100, 150 mm) by short (9 ns) laser pulses with an energy from 5 to 50 mJ in the regime of elemental composition analysis along the length of the samples. The minimal energetic levels of laser pulses were obtained. The optimal conditions for the registration of the emission lines of magnesium and calcium in the studied samples were determined.

Absorption and Transport Effects Induced in Plasmas by the Interaction of Electrons with Laser Speckles.

We show that the ponderomotive force associated with laser speckles can scatter electrons in a laser-produced plasma in a manner similar to Coulomb scattering. Analytic expressions for the effective collision rates are given. The electron-speckle collisions become important at high laser intensity or during filamentation, affecting both long- and short-pulse laser intensity regimes. As an example, we find that the effective collision rate in the laser-overlap region of hohlraums on the National Ignition Facility is expected to exceed the Coulomb collision rate by 1 order of magnitude, leading to a fundamental change to the electron transport properties. At the high intensities characteristic of short-pulse laser-plasma interactions (I≳10^{17} W cm^{-2}), the scattering is strong enough to cause the direct absorption of laser energy, generating hot electrons with energy scaling as E≈1.44(I/10^{18} W cm^{-2})^{1/2} MeV, close to experimentally observed results.

Reconstructing the Semiconductor Band Structure by Deep Learning

High-order harmonic generation (HHG), the nonlinear upconversion of coherent radiation resulting from the interaction of a strong and short laser pulse with atoms, molecules and solids, represents one of the most prominent examples of laser–matter interaction. In solid HHG, the characteristics of the generated coherent radiation are dominated by the band structure of the material, which configures one of the key properties of semiconductors and dielectrics. Here, we combine an all-optical method and deep learning to reconstruct the band structure of semiconductors. Our method builds up an artificial neural network based on the sensitivity of the HHG spectrum to the carrier-envelope phase (CEP) of a few-cycle pulse. We analyze the accuracy of the band structure reconstruction depending on the predicted parameters and propose a prelearning method to solve the problem of the low accuracy of some parameters. Once the network is trained with the mapping between the CEP-dependent HHG and the band structure, we can directly predict it from experimental HHG spectra. Our scheme provides an innovative way to study the structural properties of new materials.

Growth, spectroscopic, and 2.7-μm Q-switched laser properties of Cr,Er:YAP crystal by xenon-lamp pumping

We demonstrate a high-peak-power and narrow pulse mid-infrared (MIR) laser with xenon-lamp pumping and La3Ga5SiO14 (LGS) Q-switching on the Cr,Er:YAP crystal grown by Czochralski method. X-ray powder diffraction and x-ray rocking curves suggest the crystal has high crystalline quality. Segregation coefficients of the Cr3 + and Er3 + in the as-grown Cr,Er:YAP crystal are 4.96 and 1.07, respectively. The absorption and fluorescence spectra indicate that the Cr,Er:YAP crystal is a promising material for MIR laser by xenon-lamp pumping. The fluorescence lifetimes of the upper (I11/24) and lower (I13/24) laser levels are 0.86 and 3.57 ms, respectively. A maximum energy of 632 mJ is acquired at 5 Hz by xenon lamp pumping. By the LGS electro-optic Q-switching technology, single pulse energy of 148.6 mJ is achieved at 5 Hz with a pulse width of 35.4 ns and peak power of 4.2 MW. The results show that the LGS Q-switched Cr,Er:YAP crystal pumped by xenon lamp possesses potential application as a short-pulse and high-energy laser device.

Influence of coherent vibrational excitation on the high-order harmonic generation of diatomic molecules

The generation of high harmonics (HHG) in atomic systems by the highly non-linear response to an intense laser field is a prominent pathway to the synthesis of ever shorter laser pulses at increasingly higher photon energies. Extensions of this process to molecules add to the complexity but also offers novel opportunities as multi-center effects and the coupling to nuclear degrees of freedom can influence HHG. In this work we theoretically explore the impact of coherent vibronic excitations of diatomic molecules on the HHG spectrum within the framework of time-dependent density functional theory. We observe the appearance of novel interference structures in the HHG spectra controlled by resonances between the driving field and the vibronic wavepacket.

Ultra-compact x-ray spectrometer for high-repetition-rate laser-plasma experiments.

We present in this work the development of an ultra-compact, multi-channel x-ray spectrometer (UCXS). This diagnostic has been specially built and adapted to perform at high-repetition-rate (>1Hz) for high-intensity, short-pulse laser plasma experiments. X-ray filters of varying materials and thicknesses are chosen to provide spectral resolution up to ΔE ≈ 1keV over the x-ray energy range of 1-30keV. These filters are distributed over a total of 25 channels, where each x-ray filter is coupled to a single scintillator. The UCXS is designed to detect and resolve a large variety of laser-driven x-ray sources such as low energy bremsstrahlung emission, fluorescence, and betatron radiation (up to 30keV). Preliminary results from commissioning experiments at the ABL laser facility at Colorado State University are provided.

Ultrashort laser sintering of metal nanoparticles: A review

The unique shape and size dependant properties of nanoparticles (NPs) are responsible for their emerging, extensive applications in everyday industry. The application of NP inks for printed flexible electronics requires an additional post processing step since the presence of additives and stabilizers in the NP based inks often results in inferior electrical properties of the printed patterns. Laser sintering is a well-established technique for past number of years and is extensively used with continuous wave (CW), long, and short pulsed lasers. The literature is enriched with conventional sintering methods however the field of ultrashort laser sintering is relatively under-reported compared to CW laser and nanosecond laser-based techniques. This study concisely focuses on the potential use of ultrashort laser pulse processing of NPs and NP based thin films. The review provides the recent advances on ultrashort lasers sintering with a key focus on femtosecond laser pulses.

Thermal effects in short laser pulses: Suppression of wave collapse

An envelope model for short laser pulses is proposed. These pulses are short compared to fluid motion timescales and long compared to electron thermalization timescales. Numerical simulation of the thermal blooming of laser pulses at powers near the boundary of the self-focusing regime is presented in this model. The model includes terms representing the contributions of thermal blooming and the Kerr effect. In the context of the model, thermal blooming and the Kerr effect compete; Kerr driving and thermal blooming suppressing wave collapse. The dynamics of pulses whose energies are near the collapse threshold are numerically simulated.

Laser short-pulse effect on magneto-photo-elasto-thermodiffusion waves of fractional heat equation for non-local excited semiconductor

Laser short-pulse effect on magneto-photo-elasto-thermodiffusion waves of fractional heat equation for non-local excited semiconductor

Laser-Shock-Driven In Situ Evolution of Atomic Defect and Piezoelectricity in Graphitic Carbon Nitride for the Ionization in Mass Spectrometry.

Nanostructures─coupled with mass spectrometry─have been intensively investigated to improve the detection sensitivity and reproducibility of small biomolecules in laser desorption/ionization mass spectrometry (LDI-MS). However, the impact of laser-induced shock wave on the ionization of the nanostructures has rarely been reported. Herein, we systematically elucidate the laser shock wave effect on the ionization in terms of the in situ development of atomic defects and piezoelectricity in two-dimensional graphitic carbon nitride nanosheets (g-C3N4 NS) by short laser pulses. The mass analysis results of immunosuppressive drugs verify the enhanced LDI-MS performance, structurally originating from anisotropic lattice distortions in g-C3N4 NS, i.e., in-plane extension (contraction) and out-of-plane contraction (extension) that modulate the charge carrier motion. Along with the experimental investigations, density functional theory calculations on Mulliken charges and dipole moments demonstrate the contribution of defect and piezoelectricity to the ionization. The results of this study provide a mechanistic understanding of the underlying ionization processes, which is crucial for revealing the full potential of laser shock waves in LDI-MS.

Remote laser spot welding of AISI 430 sheets by fiber lasers—A phenomenal effect in refining weld microstructure with nanosecond pulses

In the present work, we have compared the results of microstructural and mechanical property characterization of AISI 430 ferritic stainless steel spot welds made remotely using a repetitive nanosecond pulsed fiber laser and a CW fiber laser. Optical/electron microscopy, microhardness test, and tensile shear tests were performed for evaluation of the welds. Welds made by the pulsed nanosecond laser were found to be superior due to the presence of the very narrow heat-affected zone along with finer grains in the fusion zone. The welds were also found to withstand more load before fracture and were more ductile. Use of short duration laser pulses with lower heat input were found to be responsible for the refinement of grains in the fusion zone and improvement of mechanical properties of nanosecond laser weld specimens.

A Review on Ultrafast-Laser Power Bed Fusion Technology

Additive manufacturing of metals by employing continuous wave and short pulse lasers completely changes the way of modern industrial production. But the ultrafast laser has the superiority to short pulse laser and continuous wave laser in additive manufacturing. It has higher peak power, small thermal effect, high machining accuracy and low damage threshold. It can effectively perform additive manufacturing for special materials and improve the mechanical properties of parts. This article reviews the mechanism of the interaction between ultrafast laser and metal materials to rule the manufacturing processes. The current application of ultrafast laser on forming and manufacturing special materials, including refractory metals, transparent materials, composite materials and high thermal conductivity materials are also discussed. Among the review, the shortcomings and challenges of the current experimental methods are discussed as well. Finally, suggestions are provided for the industrial application of ultrashort pulse laser in the field of additive manufacturing in the future.