Study on X-ray Emission Using Ultrashort Pulsed Lasers in Materials Processing

The interaction of ultrashort pulsed laser radiation with intensities of 10¹³ W cm⁻² and above with materials often results in an unexpected high X-ray photon flux. It has been shown so far, on the one hand, that X-ray photon emissions increase proportionally with higher laser power and the accumulated X-ray dose rates can cause serious health risks for the laser operators. On the other hand, there is clear evidence that little variations of the operational conditions can considerably affect the spectral X-ray photon flux and X-ray emissions dose. In order to enhance the knowledge in this field, four ultrashort pulse laser systems for providing different complementary beam characteristics were employed in this study on laser-induced X-ray emissions, including peak intensities between 8 × 10¹² W∙cm⁻² < I₀ < 5.2 × 10¹⁶ W∙cm⁻², up to 72.2 W average laser power as well as burst/bi-burst processing mode. By the example of AISI 304 stainless steel, it was verified that X-ray emission dose rates as high as H˙′(0.07) > 45 mSv h⁻¹ can be produced when low-intensity ultrashort pulses irradiate at a small 1 µm intra-line pulse distance during laser beam scanning and megahertz pulse repetition frequencies. For burst and bi-burst pulses, the second intra-burst pulse was found to significantly enhance the X-ray emission potentially induced by laser pulse and plasma interaction.     

Influence of Pulse Duration on X-ray Emission during Industrial Ultrafast Laser Processing

Soft X-ray emissions during the processing of industrial materials with ultrafast lasers are of major interest, especially against the background of legal regulations. Potentially hazardous soft X-rays, with photon energies of >5 keV, originate from the fraction of hot electrons in plasma, the temperature of which depends on laser irradiance. The interaction of a laser with the plasma intensifies with growing plasma expansion during the laser pulse, and the fraction of hot electrons is therefore enhanced with increasing pulse duration. Hence, pulse duration is one of the dominant laser parameters that determines the soft X-ray emission. An existing analytical model, in which the fraction of hot electrons was treated as a constant, was therefore extended to include the influence of the duration of laser pulses on the fraction of hot electrons in the generated plasma. This extended model was validated with measurements of H (0.07) dose rates as a function of the pulse duration for a constant irradiance of about 3.5 × 10¹⁴ W/cm², a laser wavelength of 800 nm, and a pulse repetition rate of 1 kHz, as well as for varying irradiance at the laser wavelength of 1030 nm and pulse repetition rates of 50 kHz and 200 kHz. The experimental data clearly verified the predictions of the model and confirmed that significantly decreased dose rates are generated with a decreasing pulse duration when the irradiance is kept constant.     

X-ray Emission Hazards from Ultrashort Pulsed Laser Material Processing in an Industrial Setting

Interactions between ultrashort laser pulses with intensities larger than 10¹³ W/cm² and solids during material processing can lead to the emission of X-rays with photon energies above 5 keV, causing radiation hazards to operators. A framework for inspecting X-ray emission hazards during laser material processing has yet to be developed. One requirement for conducting radiation protection inspections is using a reference scenario, i.e., laser settings and process parameters that will lead to an almost constant and high level of X-ray emissions. To study the feasibility of setting up a reference scenario in practice, ambient dose rates and photon energies were measured using traceable measurement equipment in an industrial setting at SCHOTT AG. Ultrashort pulsed (USP) lasers with a maximum average power of 220 W provided the opportunity to measure X-ray emissions at laser peak intensities of up to 3.3 × 10¹⁵ W/cm² at pulse durations of ~1 ps. The results indicate that increasing the laser peak intensity is insufficient to generate high dose rates. The investigations were affected by various constraints which prevented measuring high ambient dose rates. In this work, a list of issues which may be encountered when performing measurements at USP-laser machines in industrial settings is identified.     

X-ray Dose Rate and Spectral Measurements during Ultrafast Laser Machining Using a Calibrated (High-Sensitivity) Novel X-ray Detector

Ultrashort pulse laser machining is subject to increase the processing speeds by scaling average power and pulse repetition rate, accompanied with higher dose rates of X-ray emission generated during laser–matter interaction. In particular, the X-ray energy range below 10 keV is rarely studied in a quantitative approach. We present measurements with a novel calibrated X-ray detector in the detection range of 2–20 keV and show the dependence of X-ray radiation dose rates and the spectral emissions for different laser parameters from frequently used metals, alloys, and ceramics for ultrafast laser machining. Our investigations include the dose rate dependence on various laser parameters available in ultrafast laser laboratories as well as on industrial laser systems. The measured X-ray dose rates for high repetition rate lasers with different materials definitely exceed the legal limitations in the absence of radiation shielding.     

Ultrashort Pulsed Laser Drilling of Printed Circuit Board Materials

We report on a comprehensive study of laser percussion microvia drilling of FR-4 printed circuit board material using ultrashort pulse lasers with emission in the green spectral region. Laser pulse durations in the pico- and femtosecond regime, laser pulse repetition rates up to 400 kHz and laser fluences up to 11.5 J/cm2 are applied to optimize the quality of microvias, as being evaluated by the generated taper, the extension of glass fiber protrusions and damage of inner lying copper layers using materialography. The results are discussed in terms of the ablation threshold for FR-4 and copper, heat accumulation and pulse shielding effects as a result of pulse to pulse interactions. As a specific result, using a laser pulse duration of 2 ps appears beneficial, resulting in small glass fiber protrusions and high precision in the stopping process at inner copper layer. If laser pulse repetition rates larger than 100 kHz are applied, we find that the processing quality can be increased by heat accumulation effects.     

Enhanced Hydrofobicity of Polymers for Personal Protective Equipment Achieved by Chemical and Physical Modification

The article presents significant results in research on creating superhydrophobic properties of materials which can be used as an interesting material for use in self-cleaning polymer protective gloves and similar applications where the superhydrophobicity plays a significant role. In this work the influence of laser surface modification of MVQ silicone rubber was investigated. The research was conducted using a nanosecond-pulsed laser at 1060 nm wavelength. After a process of laser ablation, the surface condition was examined using a SEM microscope and infrared spectroscopy. During the tests, the contact angle was checked both before and after the laser modification of samples pre-geometrised in the process of their production. The test results presented in the paper indicate that the chemical and physical modifications contribute to the change in the MVQ silicone rubber contact angle. A significant increase (by more than 30°) in the contact angle to 138° was observed. It was confirmed that surface geometrisation is not the only factor contributing to an increase in the contact angle of the analyzed material; other factors include a change in laser texturing parameters, such as mean beam power, pulse duration, scanning speed and pulse repetition frequency.     

Sub-Threshold Fabrication of Laser-Induced Periodic Surface Structures on Diamond-like Nanocomposite Films with IR Femtosecond Pulses

In the paper, we study the formation of laser-induced periodic surface structures (LIPSS) on diamond-like nanocomposite (DLN) a-C:H:Si:O films during nanoscale ablation processing at low fluences—below the single-pulse graphitization and spallation thresholds—using an IR fs-laser (wavelength 1030 nm, pulse duration 320 fs, pulse repetition rate 100 kHz, scanning beam velocity 0.04–0.08 m/s). The studies are focused on microscopic analysis of the nanostructured DLN film surface at different stages of LIPSS formation and numerical modeling of surface plasmon polaritons in a thin graphitized surface layer. Important findings are concerned with (i) sub-threshold fabrication of high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL) under negligible surface graphitization of hard DLN films, (ii) transition from the HSFL (periods of 140 ± 30 and 230 ± 40 nm) to LSFL (period of 830–900 nm) within a narrow fluence range of 0.21–0.32 J/cm², (iii) visualization of equi-field lines by ablated nanoparticles at an initial stage of the LIPSS formation, providing proof of larger electric fields in the valleys and weaker fields at the ridges of a growing surface grating, (iv) influence of the thickness of a laser-excited glassy carbon (GC) layer on the period of surface plasmon polaritons excited in a three-layer system “air/GC layer/DLN film”.     

Interaction of Long Time Pulses of an Nd3+:YAG Laser Beam with the Heusler AlloyNi45Co5Mn35.5In14.5

In this paper, the laser processing of the surface of bulk and layered samples (of thickness 75 nm) of Ni₄₅Co₅Mn_35.5In_14.5 alloy (NC5MI) was investigated using microsecond laser pulses. A Q-switched pulsed Nd³⁺:YAG laser, operating in the 1st harmonic (which had a wavelength of 1064 nm) with a pulse duration of 250 µs, was used. NC5MI is a metal resistant to thermal laser processing because its reflection coefficient is close to unity for long wavelengths. The aim of this paper was to learn the forms of laser processing (heating, microprocessing, ablation) for which the above-specified type of laser is useful. The samples were irradiated with various fluences in the interval of 5–32 J·cm⁻². The effect of the laser interaction with the surface was explored by SEM microscopy. The threshold fluences for the bulk sample were determined as: the visible damage threshold (F_th^d = 2 ± 0.2 J·cm⁻²), the melting threshold (F_th^m = 10 ± 0.5 J·cm⁻²), and the deep melting threshold (F_th^dm = 32 J·cm⁻²). Unexpectedly, these values wereincreased for the layer sample due to its silicon substrate. We have concluded that this type of laser radiation is advantageous for the annealing and melting of, or drilling holes in, the alloy, but disadvantageousto the ablation of the alloy.     

Effect of Laser Processing Parameters on Microstructure,Hardness and Tribology of NiCrCoFeCBSi/WC Coatings

In the present study, pulsed laser post-processing was applied to improve the properties of the thermally sprayed NiCrCoFeCBSi/40 wt.% WC coatings. The powder mix was deposited onto a mild steel substrate by flame spray method and then the as-sprayed coatings were processed by Nd:YAG laser. The peak power density applied was between 4.00 × 10⁶ and 5.71 × 10⁶ W/cm², and the laser operating speed ranged between 100 and 400 mm/min, providing processing in a melting mode. Scanning electron microscopy, energy dispersive spectroscopy, Knop hardness measurements, and “ball-on-disc” dry friction tests were applied to study the effect of the processing parameters on the geometry of laser pass and microstructure, hardness, and tribology of the processed layers. The results obtained revealed that pulsed laser processing provides a monolithic remelted coating layer with the microstructure of ultrafine, W-rich dendrites in Ni-based matrix, where size and distribution of W-rich dendrites periodically vary across remelted layer depth. The composition of W-rich dendrites can be attributed to a carbide of type (W, Cr, Ni, Fe)C. The cracks sensitivity of coatings was visibly reduced with the reduction of power density applied. The hardness of coatings was between ~1070 and ~1140 HK0.2 and correlated with microstructure size, being dependent on the processing parameters. The friction coefficient and wear rate of coatings during dry sliding were reduced by up to ~30% and up to ~2.4 times, respectively, after laser processing.     

Study of Micro/Nano Structuring and Mechanical Properties of KrF Excimer Laser Irradiated Al for Aerospace Industry and Surface Engineering Applications

Micro/nano structuring of KrF Excimer laser-irradiated Aluminum (Al) has been correlated with laser-produced structural and mechanical changes. The effect of non-reactive Argon (Ar) and reactive Oxygen (O₂) environments on the surface, structural and mechanical characteristics of nano-second pulsed laser-ablated Aluminum (Al) has been revealed. KrF Excimer laser with pulse duration 20 ns, central wavelength of 248 nm and repetition rate of was utilized for this purpose. Exposure of targets has been carried out for 0.86, 1, 1.13 and 1.27 J^·cm⁻² laser fluences in non-reactive (Ar) and reactive (O₂) ambient environments at a pressure of 100 torr. A variety of characteristics of the irradiated targets like the morphology of the surface, chemical composition, crystallinity and nano hardness were investigated by using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffractometer (XRD), Raman spectroscopy and Nanohardness tester techniques, respectively. The nature (reactive or non-reactive) and pressure of gas played an important role in modification of materials. In this study, a strong correlation is observed between the surface structuring, chemical composition, residual stress variation and the variation in hardness of Al surface after ablation in both ambient (Ar, O₂). In the case of reactive environment (O₂), the interplay among the deposition of laser energy and species of plasma of ambient gas enhances chemical reactivity, which causes the formation of oxides of aluminum (AlO, Al₂O₃) with high mechanical strength. That makes it useful in the field of process and aerospace industry as well as in surface engineering.     

Monitoring Dynamic Recrystallisation in Bioresorbable Alloy Mg-1Zn-0.2Ca by Means of an In Situ Acoustic Emission Technique

The tensile behaviour of the biocompatible alloy Mg-1Zn-0.2Ca (in wt.%) in the fine-grained state, obtained by severe plastic deformation via multiaxial isothermal forging, has been investigated in a wide range of temperatures (20 ÷ 300) °C and strain rates (5 × 10⁻⁴ ÷ 2 × 10⁻²) s⁻¹ with the measurements of acoustic emission (AE). The dependences of mechanical properties, including the yield stress, ultimate strength, ductility, and the strain-hardening rate, on the test temperature and strain rate, were obtained and discussed. It is shown for the first time that an acoustic emission method is an effective tool for in situ monitoring of the dynamic recrystallisation (DRX) process. The specific behaviour of the acoustic emission spectral density reflected by its median frequency as a function of strain at various temperatures can serve as an indicator of the DRX process’s completeness.     

Hole Morphology and Keyhole Evolution during Single Pulse Laser Drilling on Polyether-Ether-Ketone (PEEK)

Polyether-ether-ketone (PEEK), with its superior mechanical, chemical, and thermal properties, as well as high biocompatibility, has been used in aerospace, electronics, and biomedical applications. In this paper, a large number of experiments of single-pulse laser drilling on PEEK were performed to analyze the hole morphology and keyhole evolution, which were characterized by an optical microscope, charge-coupled device (CCD), and high-speed camera. A novel method is proposed to observe and measure the dimension of the processed hole rapidly right after laser drilling for special polymer materials with wear-resistance and non-conductivity. Morphological characteristics of holes are presented to illustrate the effect of pulse width and peak power on hole depth, hole diameter, and aspect-ratio. The obtained maximum drilling depth was 7.06 mm, and the maximum aspect-ratio was 23. In situ observations of the dynamic process of laser drilling, including the keyhole evolution together with ejection and vaporization behavior, were also carried out. The keyhole evolution process can be divided into three stages: rapid increment stage (0–2 ms) at a rate of 2.1 m/s, slow increment stage (2–4 ms) at a rate of 0.3 m/s, and stable stage (>4 ms). Moreover, the variation of dimensionless laser power density with the increase in pulse width was calculated. The calculated maximum drilling depth based on energy balance was compared with the experimental depth. It is proven that the laser–PEEK interaction is mainly influenced by a photothermal effect. Ejection is the dominant material-removal mechanism and contributes to over 60% of the depth increment during the rapid increment stage, while vaporization is dominant and contributes to about 80% of the depth increment during the slow increment stage. The results reveal the material removal mechanism for single-pulse laser drilling on PEEK, which is helpful to understand the dynamic process of keyhole evolution. This not only provides a processing window for future laser drilling of PEEK but also gives a guide for the manufacturing of other polymers.     

Hysteresis in Lanthanide Zirconium Oxides Observed Using a Pulse CV Technique and including the Effect of High Temperature Annealing

A powerful characterization technique, pulse capacitance-voltage (CV) technique, was used to investigate oxide traps before and after annealing for lanthanide zirconium oxide thin films deposited on n-type Si (111) substrates at 300 °C by liquid injection Atomic Layer Deposition (ALD). The results indicated that: (1) more traps were observed compared to the conventional capacitance-voltage characterization method in LaZrO_x; (2) the time-dependent trapping/de-trapping was influenced by the edge time, width and peak-to-peak voltage of a gate voltage pulse. Post deposition annealing was performed at 700 °C, 800 °C and 900 °C in N₂ ambient for 15 s to the samples with 200 ALD cycles. The effect of the high temperature annealing on oxide traps and leakage current were subsequently explored. It showed that more traps were generated after annealing with the trap density increasing from 1.41 × 10¹² cm⁻² for as-deposited sample to 4.55 × 10¹² cm⁻² for the 800 °C annealed one. In addition, the leakage current density increase from about 10⁻⁶ A/cm² at V_g = +0.5 V for the as-deposited sample to 10⁻³ A/cm² at V_g = +0.5 V for the 900 °C annealed one.     

Visualization of carrier dynamics in p(n)-type GaAs by scanning ultrafast electron microscopy

Four-dimensional scanning ultrafast electron microscopy is used to investigate doping- and carrier-concentration-dependent ultrafast carrier dynamics of the in situ cleaved single-crystalline GaAs(110) substrates. We observed marked changes in the measured time-resolved secondary electrons depending on the induced alterations in the electronic structure. The enhancement of secondary electrons at positive times, when the electron pulse follows the optical pulse, is primarily due to an energy gain involving the photoexcited charge carriers that are transiently populated in the conduction band and further promoted by the electron pulse, consistent with a band structure that is dependent on chemical doping and carrier concentration. When electrons undergo sufficient energy loss on their journey to the surface, dark contrast becomes dominant in the image. At negative times, however, when the electron pulse precedes the optical pulse (electron impact), the dynamical behavior of carriers manifests itself in a dark contrast which indicates the suppression of secondary electrons upon the arrival of the optical pulse. In this case, the loss of energy of material’s electrons is by collisions with the excited carriers. These results for carrier dynamics in GaAs(110) suggest strong carrier–carrier scatterings which are mirrored in the energy of material’s secondary electrons during their migration to the surface. The approach presented here provides a fundamental understanding of materials probed by four-dimensional scanning ultrafast electron microscopy, and offers possibilities for use of this imaging technique in the study of ultrafast charge carrier dynamics in heterogeneously patterned micro- and nanostructured material surfaces and interfaces.Recent advances in four-dimensional (4D) ultrafast electron microscopy (UEM) have made it possible to investigate nonequilibrium electronic and structural dynamics with atomic-scale spatial resolution and femtosecond temporal resolution (1). Unlike UEM, which operates in the transmission mode, scanning UEM techniques exploit the time evolution of secondary electrons (SEs) produced in the specimen, and provide additional marked advantages over the transmission mode. These include a relatively facile sample preparation requirement, an efficient heat dissipation, a lower radiation damage, and an accessibility to low-voltage environmental study (2, 3). Since its development this technique has been used to study carrier excitation dynamics in several prototypical semiconducting materials surfaces. In these studies, image contrast was monitored as a function of time, and it was found that Si exhibits a bright contrast in the image at positive times without appreciable dynamics at negative times, whereas CdSe displays bright contrast at positive times and dark contrast at negative times (2). However, the correlation between the measured time-dependent SE intensity and electronic structure of the material of interest remains elusive. Chemical doping is a widely used method to control the electronic properties of semiconducting materials by incorporating charge donating or accepting dopant atoms. It is a key element in developments involving modern semiconductor-based solid-state electronics.Here, we present a systematic study for the doping- and carrier-concentration-dependent carrier dynamics in the in situ cleaved GaAs(110) surface observed in the images obtained using scanning UEM. We show that the enhancement of the SE signal at time 0 is associated with the energy gained by the optical excitation, which increases SE production from the probing pulse, and this process mirrors the electronic doping characteristics of the semiconducting material. In contrast, the persistent dark contrast at both positive and negative times for carrier dynamics in GaAs(110) suggests an energy loss mechanism that involves strong suppression of SEs through carrier–carrier scatterings. Our simulations of the transient behavior further support this conclusion.A schematic representation of the experimental setup is given in Fig. 1. Electron pulses generated from a field-emission gun using femtosecond laser pulse irradiation are scanned across the specimen surface, which is illuminated with the optical pulse. The electrons emitted from the material surface are used to construct time-resolved images at various time delays between the optical pulse and the electron pulse. The detailed account of the experimental setup was described in previous publications from this laboratory (2–4), and thus here we briefly describe the imaging setup: the laser used in our experiments is an ytterbium-doped fiber laser system that generates ultrashort pulses at a central wavelength of 1,030 nm (measured pulse width of ∼400 fs). The second harmonic (photon energy of 2.4 eV) of the laser beam was directed to the sample at room temperature, whereas the quadrupled harmonic (photon energy of 4.8 eV) was used for the pulsed electron generation from the field-emission gun in SEM. For the series of experiments presented here, the pump laser fluence, repetition rate, and the data acquisition methodology were kept the same for comparison of samples with different doping characteristics. The pump laser fluence was deduced to be 69 μJ/cm², which is more than three orders of magnitude lower than that reported for the laser-induced damage threshold of a crystalline GaAs (∼0.1 J/cm² at a photon energy of 1.9 eV) (5). The emitted electrons from the material were measured using a positively biased Everhart-Thornley detector.Open in a separate windowFig. 1.Schematic representation of the scanning UEM at California Institute of Technology. Pulsed electrons are scanned over a specimen that is illuminated with an optical pulse, and SEs emitted from the material surface are detected to construct time-resolved images at various time delays between the optical and the electron pulse. In the case of a semiconducting material, at time 0, the optical pulse promotes electrons from the valence band to the conduction band, and immediately after that the electron pulse excites transiently populated conduction electrons above the vacuum level, resulting in an enhanced (bright contrast) SE emission. If SEs experience a material-dependent energy loss through the various channels of scattering processes involved while migrating toward the surface, then a decreased emission will result (dark contrast). Here, E_c, E_v, and E_vac are the energies of the bottom of the conduction band, the top of the valence band, and the vacuum level, respectively. Scale bars in the time-resolved images correspond to 50 μm.All scanning UEM images were acquired at a dwell time of 1 μs and were integrated 64 times to improve the signal-to-noise ratio. All experiments were conducted at a repetition rate of 4.2 MHz to ensure a full recovery of the material’s dynamical response before the arrival of a next pump pulse. Single crystals of GaAs (a direct band gap of 1.43 eV at room temperature) were all grown via Vertical Gradient Freeze method (purchased from MTI); the method is known to produce fewer defects during the growth, compared with those grown via the liquid encapsulated Czochralski method (6). The crystals were in situ cleaved in high vacuum (<1.5 × 10⁻⁶ Torr) to reduce the effects of contamination and formation of surface defects or adsorbates for the systematic study presented here. A clean (110) crystallographic orientation of GaAs does not possess any surface states within the band gap and thus a bulk-like band structure is expected at the surface without band-bending effects (7, 8). Cleavage along a direction perpendicular to the (001) orientation of GaAs exposes a fresh (110) plane. The cleaved surface was positioned at a working distance of 10 mm and perpendicular to the propagation direction of the pulsed primary electron beam with its energy of 30 keV.     

Generation of bright isolated attosecond soft X-ray pulses driven by multicycle midinfrared lasers

High harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, to date the shortest subfemtosecond (attosecond, 10⁻¹⁸ s) pulses have been produced only in the extreme UV region of the spectrum below 100 eV, which limits the range of materials and molecular systems that can be explored. Here we experimentally demonstrate a remarkable convergence of physics: when midinfrared lasers are used to drive high harmonic generation, the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of isolated attosecond pulses. The temporal window over which phase matching occurs shrinks rapidly with increasing driving laser wavelength, to the extent that bright isolated attosecond pulses are the norm for 2-µm driving lasers. Harnessing this realization, we experimentally demonstrate the generation of isolated soft X-ray attosecond pulses at photon energies up to 180 eV for the first time, to our knowledge, with a transform limit of 35 attoseconds (as), and a predicted linear chirp of 300 as. Most surprisingly, advanced theory shows that in contrast with as pulse generation in the extreme UV, long-duration, 10-cycle, driving laser pulses are required to generate isolated soft X-ray bursts efficiently, to mitigate group velocity walk-off between the laser and the X-ray fields that otherwise limit the conversion efficiency. Our work demonstrates a clear and straightforward approach for robustly generating bright isolated attosecond pulses of electromagnetic radiation throughout the soft X-ray region of the spectrum.High-order harmonic generation (HHG) is the most extreme nonlinear optical process in nature, making it possible to coherently upconvert intense femtosecond laser light to much shorter wavelengths (1, 2). High harmonics are radiated as a result of a coherent electron recollision process that occurs each half-cycle of the driving laser field while an atom is undergoing strong-field ionization. The short pulse duration of HHG (which must be shorter than the driving laser pulse) has made it possible to directly access the fastest timescales relevant to electron dynamics in atoms, molecules, and materials. The unique properties of attosecond HHG in the extreme UV (EUV) have uncovered new understanding of fundamental processes in atoms, molecules, plasmas, and materials, including the timescales on which electrons are emitted from atoms (3), the timescale for spin–spin and electron–electron interactions (4, 5), the timescale that determines molecular dissociation and electron localization (6–9), the timescale and mechanisms for spin and energy transport in nanosystems (10–12), as well as new capabilities to implement EUV microscopes with wavelength-limited spatial resolution (13).The temporal structure of HHG is related to the number of times a high-energy electron undergoes a coherent recollision process, as well as the time window over which bright harmonics emerge. Using multicycle 0.8-µm driving lasers, HHG generally emerges as a train of attosecond (as) pulses (14, 15) corresponding to a series of harmonic peaks in frequency space. This emission can narrow to a single isolated as burst when the driving laser field is a few optical cycles (∼5 fs) in duration (16, 17), with an associated broad continuous spectrum. Other techniques can isolate a single burst using a combination of multicolor fields and polarization control (18–26) or spatial lighthouse gating of the driving laser pulses (27, 28). Phase matching can also result in bright isolated as pulse generation for short driving laser pulses (29, 30). To obtain bright, phase-matched, high harmonic beams, the laser and HHG fields must both propagate at the speed of light c so that emission from many atoms interferes constructively. Above a critical ionization level, the phase velocity of the laser exceeds c, which terminates the HHG temporal emission. The chirp present on attosecond bursts can be compensated by using thin materials, gases, or chirped mirrors (31–33). To date, however, most schemes for creating isolated attosecond pulses require either very short-duration few-cycle 0.8-µm driving laser pulses that are difficult to reliably generate, or complex polarization modulation schemes. In addition, the carrier envelope phase (CEP) of the driving laser pulse must be stabilized.A more general understanding of how to efficiently sculpt the temporal, spatial, and spectral characteristics of HHG emission over an extremely broad photon energy range (from the EUV to the keV and higher) has emerged in recent years (34–39). This understanding is critical both for a fundamental understanding of strong-field quantum physics, as well as for applications which have fundamentally different needs in terms of the HHG pulse duration, spectral bandwidth, and flux. By considering both the microscopic single-atom response as well as the macroscopic coherent buildup of HHG, efficient phase-matched HHG can now be implemented from the EUV to >keV photon energies, simply by driving HHG with midinfrared (mid-IR) femtosecond driving lasers. This advance represents, to our knowledge, the first general-purpose, tabletop, coherent soft X-ray light source (39). Furthermore, theory suggested that bright isolated attosecond X-ray bursts would be achievable using multicycle mid-IR driving lasers in a phase-matched geometry (35). However, the low repetition rate of the driving lasers precluded experimental testing of these predictions. Moreover, formidable computation requirements meant that advanced simulations could not be fully extended into the mid-IR region at 2–4 µm.In this paper, we experimentally demonstrate a beautiful convergence of physics for mid-IR (2-µm) driving lasers by showing that the conditions for optimal bright, soft X-ray generation naturally coincide with the generation of bright isolated attosecond soft X-ray bursts. We combine advanced theory with a novel experimental method equivalent to high-resolution Fourier transform spectroscopy to measure bright, attosecond soft X-ray pulses for the first time, to our knowledge. Specifically, we measure a field autocorrelation pulse width of 70 as, corresponding to a transform-limited 35-as pulse, that is supported by a coherent supercontinuum spectrum extending to photon energies around 180 eV. We also validate experimentally, for the first time, to our knowledge, the most intuitive dynamic picture of phase matching of HHG in the time domain by clearly demonstrating that the temporal window during which phase matching occurs shrinks rapidly with increasing driving laser wavelength. Finally, we show through advanced theory that the isolated attosecond pulse is chirped to 300 as. Most surprisingly, we find that bright attosecond pulse generation in the soft X-ray region requires the use of longer-duration, multicycle, mid-IR driving lasers to mitigate group velocity walk-off issues that would otherwise reduce the conversion efficiency. By harnessing the beautiful physics of phase matching, this work represents the simplest and most robust scheme for attosecond soft X-ray pulse generation, and will make attosecond science and technology accessible to a broader community.     

Dual Laser Beam Processing of Semiconducting Thin Films by Excited State Absorption

We present a unique dual laser beam processing approach based on excited state absorption by structuring 200 nm thin zinc oxide films sputtered on fused silica substrates. The combination of two pulsed nanosecond-laser beams with different photon energies—one below and one above the zinc oxide band gap energy—allows for a precise, efficient, and homogeneous ablation of the films without substrate damage. Based on structuring experiments in dependence on laser wavelength, pulse fluence, and pulse delay of both laser beams, a detailed concept of energy transfer and excitation processes during irradiation was developed. It provides a comprehensive understanding of the thermal and electronic processes during ablation. To quantify the efficiency improvements of the dual-beam process compared to single-beam ablation, a simple efficiency model was developed.     

Carbon Nanotube Electron Emitter for X-ray Imaging

The carbon nanotube field emitter array was grown on silicon substrate through a resist-assisted patterning (RAP) process. The shape of the carbon nanotube array is elliptical with 2.0 × 0.5 mm² for an isotropic focal spot size at anode target. The field emission properties with triode electrodes show a gate turn-on field of 3 V/µm at an anode emission current of 0.1 mA. The author demonstrated the X-ray source with triode electrode structure utilizing the carbon nanotube emitter, and the transmitted X-ray image was of high resolution.     

Study on Characteristics of the Light-Initiated High Explosive-Based Pulse Laser Initiation

The silver acetylene silver nitrate loading technology of the light initiated high explosive, as one of important means to simulate the structural response of powerful pulsed X-ray, adopts the pulse laser initiation. It has advantages of improvement of practical control, heterogenous loading realization and simultaneous loading timeliness. In this paper, the physical and mathematical models of hot spot initiation and photochemical initiation of energetic materials under the action of laser are firstly established, and then the laser initiation mechanism of the light initiated high explosive is specifically analyzed, and the laser initiation experiment is conducted based on the optical adsorption property of the light initiated high explosive. From this study, the laser initiation thresholds of 193 nm, 266 nm, 532 nm, 1064 nm wavelengths are given, and they are 5.07 mJ/mm², 6.77 mJ/mm², 7.21 mJ/mm² and 10.61 mJ/mm², respectively, and the complete detonation process is verified by detonation velocity. This work technically supports the study of pulse laser initiation process, mechanism and explosion loading rule as well as the loading technology of the light initiated high explosive to simulate the structural response of X ray.     

Room temperature femtosecond X-ray diffraction of photosystem II microcrystals

Most of the dioxygen on earth is generated by the oxidation of water by photosystem II (PS II) using light from the sun. This light-driven, four-photon reaction is catalyzed by the Mn₄CaO₅ cluster located at the lumenal side of PS II. Various X-ray studies have been carried out at cryogenic temperatures to understand the intermediate steps involved in the water oxidation mechanism. However, the necessity for collecting data at room temperature, especially for studying the transient steps during the O–O bond formation, requires the development of new methodologies. In this paper we report room temperature X-ray diffraction data of PS II microcrystals obtained using ultrashort (< 50 fs) 9 keV X-ray pulses from a hard X-ray free electron laser, namely the Linac Coherent Light Source. The results presented here demonstrate that the ”probe before destroy” approach using an X-ray free electron laser works even for the highly-sensitive Mn₄CaO₅ cluster in PS II at room temperature. We show that these data are comparable to those obtained in synchrotron radiation studies as seen by the similarities in the overall structure of the helices, the protein subunits and the location of the various cofactors. This work is, therefore, an important step toward future studies for resolving the structure of the Mn₄CaO₅ cluster without any damage at room temperature, and of the reaction intermediates of PS II during O–O bond formation.     

High Temperature Behaviors of a Casting Nickel-Based Superalloy Used for 815 °C

The hot deformation behaviors of the SJTU-1 alloy, the high-throughput scanned casting Nickel-based superalloy, was investigated by compression test in the temperature range of 900 to 1200 °C and strain rate range of 0.1–0.001 s⁻¹. The hot processing map has been constructed with the instability zone. At the beginning of hot deformation, the flow stress moves rapidly to the peak value with the increased strain rates. Meanwhile, the peak stress is decreased with the increased temperature at the same strain rates. However, the peak stress shows the same tendency with the strain rates at the same temperature. The optimum hot deformation condition was determined in the temperature range of 1000–1075 °C, and the strain rate range of 0.005–0.1 s⁻¹. The microstructure investigation indicates the strain rate significantly affects the characteristics of the microstructure. The deformation constitutive equation has also been discussed as well.