电子束和激光对物质作用的差异

The interaction of an intense coherent photon beam with free electrons is discussed. The photon beam is treated as a classical external electromagnetic field. The discussion is exact within the approximation of neglecting radiative corrections and the restriction to the case of a plane-wave field or arbitrary spectral composition and polarization properties. The scattering of a single photon out of a monochromatic beam by an isolated free electron is considered in detail. The cross sections corresponding to the scattering of the various harmonics of the incident beam are evaluated. These cross sections display a complicated dependence upon the intensity of the incident beam, at least for very intense beams. It is found that a mass change induced in the electron by the external field shifts the wavelength of the scattered photons by an amount depending on the intensity of the incident beam. Other processes involving free electrons in the final state are also considered briefly, and a discussion of the magnitude of the effects depending upon the intensity is given. Two Appendices are concerned with the electron Green's function and the vacuum-vacuum transformation function in the presence of a plane-wave field. In the course of the discussion of the latter, the problem of the correct definition of the vacuum current is encountered, and it is shown that a very careful procedure is necessary to obtain a covariant result.

No abstract

No abstract

No abstract

We propose a method of generation of ∼115 attosecond x-ray pulses in a free electron laser (FEL) by means of producing ultra-fast angular modulation of the electron trajectories prior to entering the FEL. For this modulation, we employ a few-cycle laser pulse in a higher-order Gaussian mode and with carrier-envelope phase stabilization.

No abstract

No abstract

The interaction of an ultraintense, 30-fs laser pulse with a preformed plasma was investigated as a method of producing a beam of high-energy electrons. We used thin foil targets that are exploded by the laser amplified spontaneous emission preceding the main pulse. Optical diagnostics show that the main pulse interacts with a plasma whose density is well below the critical density. By varying the foil thickness, we were able to obtain a substantial emission of electrons in a narrow cone along the laser direction with a typical energy well above the laser ponderomotive potential. These results are explained in terms of wake-field acceleration.

The production of high charge short bunches in a high brightness photoinjector requires laser pulses driving the cathode with GW range peak power on a mm spot size. The resulting transverse electric field experienced by the electron beam near the cathode is of the order of 200-500 MV/m, well in excess of a typical RF accelerating field of 50-100 MV/m. We present here a preliminary study of the resultant beam dynamics. Simulations including the electron beam-laser interaction have been performed with the code HOMDYN taking into account the superposition of incident and reflected laser pulses as well as space charge fields. Under this conditions the emittance degradation is negligible, as predicted by analytical methods, but a longitudinal charge modulation occurs on the scale of the laser wavelength, in case of oblique incidence, driven by the longitudinal component of the laser field. Charge modulation is transformed into energy modulation via the space charge field, which may produce enhanced microbunching effects when the beam is further compressed in a magnetic chicane.

We use simulations to investigate the interaction of ultra-intense laser pulses with a plasma. With an intensity greater than ${10}^{18}$ W/${\mathrm{cm}}^{2}$, these pulses have a pressure greater than ${10}^{3}$ M bar and drive the plasma relativistically. Hole boring by the light beam is a key feature of the interaction. We find substantial absorption into heated electrons with a characteristic temperature of order the pondermotive potential. Other effects include a dependence on the polarization of the incident light, strong magnetic field generation, and a period of intense instability generation in the underdense plasma.

Abstract In this present theoretical study, we investigate electron Bernstein wave (EBW) aided collisional nanocluster plasma heating by nonlinear interaction of two super-Gaussian laser beams. The interactions of laser beams electric field profiles with electronic clouds of nanoclusters cause the beat wave. The nonlinear ponderomotive force is generated through the beat wave. There may be good potential to excite the EBW aiding cluster plasma to lead electron heating via cyclotron damping of the Bernstein wave. An analytical scheme is proposed for the anomalous heating and evolution of electron temperature by using this mechanism. Graphical discussions were promised to achieve extreme heating rate via the spatial shape of super-Gaussian laser beams and the resonance condition of beat wave to surface plasmon frequency. The heating is controlled by tuning the laser beam width, mode index, collisional frequency, clustered radius, and density.

We propose a method based on the slice energy spread modulation to generate strong subpicosecond density bunching in high-intensity relativistic electron beams. A laser pulse with periodic intensity envelope is used to modulate the slice energy spread of the electron beam, which can then be converted into density modulation after a dispersive section. It is found that the double-horn slice energy distribution of the electron beam induced by the laser modulation is very effective to increase the density bunching. Since the modulation is performed on a relativistic electron beam, the process does not suffer from strong space charge force or coupling between phase spaces, so that it is straightforward to preserve the beam quality for terahertz (THz) radiation and other applications. We show in both theory and simulations that the tunable radiation from the beam can cover the frequency range of 1--10 THz with high power and narrow-band spectra.

No abstract

Orthorhombic black phosphorus (BP) and other layered materials, such as gallium telluride (GaTe) and tin selenide (SnSe), stand out among two-dimensional (2D) materials owing to their anisotropic in-plane structure. This anisotropy adds a new dimension to the properties of 2D materials and stimulates the development of angle-resolved photonics and electronics. However, understanding the effect of anisotropy has remained unsatisfactory to date, as shown by a number of inconsistencies in the recent literature. We use angle-resolved absorption and Raman spectroscopies to investigate the role of anisotropy on the electron-photon and electron-phonon interactions in BP. We highlight, both experimentally and theoretically, a nontrivial dependence between anisotropy and flake thickness and photon and phonon energies. We show that once understood, the anisotropic optical absorption appears to be a reliable and simple way to identify the crystalline orientation of BP, which cannot be determined from Raman spectroscopy without the explicit consideration of excitation wavelength and flake thickness, as commonly used previously.

Electron imaging in space and time is achieved in microscopy with timed (near relativistic) electron packets of picometer wavelength coincident with light pulses of femtosecond duration. The photons (with an energy of a few electronvolts) are used to impulsively heat or excite the specimen so that the evolution of structures from their nonequilibrium state can be followed in real time. As such, and at relatively low fluences, there is no interaction between the electrons and the photons; certainly that is the case in vacuum because energy–momentum conservation is not possible. In the presence of nanostructures and at higher fluences, energy–momentum conservation is possible and the electron packet can either gain or lose light quanta. Recently, it was reported that, when only electrons with gained energy are filtered, near-field imaging enables the visualization of nanoscale particles and interfaces with enhanced contrast (Barwick et al 2009 Nature 462 902). To explore a variety of applications, it is important to express, through analytical formulation, the key parameters involved in this photon-induced near-field electron microscopy (PINEM) and to predict the associated phenomena of, e.g., forty-photon absorption by the electron packet. In this paper, we give an account of the theoretical and experimental results of PINEM. In particular, the time-dependent quantum solution for ultrafast electron packets in the nanostructure scattered electromagnetic (near) field is solved in the high kinetic energy limit to obtain the evolution of the incident electron packet into a superposition of discrete momentum wavelets. The characteristic length and time scales of the halo of electron–photon coupling are discussed in the framework of Rayleigh and Mie scatterings, providing the dependence of the PINEM effect on size, polarization, material and spatiotemporal localization. We also provide a simple classical description that is based on features of plasmonics. A major part of this paper is devoted to the comparisons between the theoretical results and the recently obtained experimental findings about the imaging of materials and biological systems.

Ionizing radiation has become the most effective way to modify natural and synthetic polymers through crosslinking, degradation, and graft polymerization. This review will include an in-depth analysis of radiation chemistry mechanisms and the kinetics of the radiation-induced C-centered free radical, anion, and cation polymerization, and grafting. It also presents sections on radiation modifications of synthetic and natural polymers. For decades, low linear energy transfer (LLET) ionizing radiation, such as gamma rays, X-rays, and up to 10 MeV electron beams, has been the primary tool to produce many products through polymerization reactions. Photons and electrons interaction with polymers display various mechanisms. While the interactions of gamma ray and X-ray photons are mainly through the photoelectric effect, Compton scattering, and pair-production, the interactions of the high-energy electrons take place through coulombic interactions. Despite the type of radiation used on materials, photons or high energy electrons, in both cases ions and electrons are produced. The interactions between electrons and monomers takes place within less than a nanosecond. Depending on the dose rate (dose is defined as the absorbed radiation energy per unit mass), the kinetic chain length of the propagation can be controlled, hence allowing for some control over the degree of polymerization. When polymers are submitted to high-energy radiation in the bulk, contrasting behaviors are observed with a dominant effect of cross-linking or chain scission, depending on the chemical nature and physical characteristics of the material. Polymers in solution are subject to indirect effects resulting from the radiolysis of the medium. Likewise, for radiation-induced polymerization, depending on the dose rate, the free radicals generated on polymer chains can undergo various reactions, such as inter/intramolecular combination or inter/intramolecular disproportionation, b-scission. These reactions lead to structural or functional polymer modifications. In the presence of oxygen, playing on irradiation dose-rates, one can favor crosslinking reactions or promotes degradations through oxidations. The competition between the crosslinking reactions of C-centered free radicals and their reactions with oxygen is described through fundamental mechanism formalisms. The fundamentals of polymerization reactions are herein presented to meet industrial needs for various polymer materials produced or degraded by irradiation. Notably, the medical and industrial applications of polymers are endless and thus it is vital to investigate the effects of sterilization dose and dose rate on various polymers and copolymers with different molecular structures and morphologies. The presence or absence of various functional groups, degree of crystallinity, irradiation temperature, etc. all greatly affect the radiation chemistry of the irradiated polymers. Over the past decade, grafting new chemical functionalities on solid polymers by radiation-induced polymerization (also called RIG for Radiation-Induced Grafting) has been widely exploited to develop innovative materials in coherence with actual societal expectations. These novel materials respond not only to health emergencies but also to carbon-free energy needs (e.g., hydrogen fuel cells, piezoelectricity, etc.) and environmental concerns with the development of numerous specific adsorbents of chemical hazards and pollutants. The modification of polymers through RIG is durable as it covalently bonds the functional monomers. As radiation penetration depths can be varied, this technique can be used to modify polymer surface or bulk. The many parameters influencing RIG that control the yield of the grafting process are discussed in this review. These include monomer reactivity, irradiation dose, solvent, presence of inhibitor of homopolymerization, grafting temperature, etc. Today, the general knowledge of RIG can be applied to any solid polymer and may predict, to some extent, the grafting location. A special focus is on how ionizing radiation sources (ion and electron beams, UVs) may be chosen or mixed to combine both solid polymer nanostructuration and RIG. LLET ionizing radiation has also been extensively used to synthesize hydrogel and nanogel for drug delivery systems and other advanced applications. In particular, nanogels can either be produced by radiation-induced polymerization and simultaneous crosslinking of hydrophilic monomers in "nanocompartments", i.e., within the aqueous phase of inverse micelles, or by intramolecular crosslinking of suitable water-soluble polymers. The radiolytically produced oxidizing species from water, •OH radicals, can easily abstract H-atoms from the backbone of the dissolved polymers (or can add to the unsaturated bonds) leading to the formation of C-centered radicals. These C-centered free radicals can undergo two main competitive reactions; intramolecular and intermolecular crosslinking. When produced by electron beam irradiation, higher temperatures, dose rates within the pulse, and pulse repetition rates favour intramolecular crosslinking over intermolecular crosslinking, thus enabling a better control of particle size and size distribution. For other water-soluble biopolymers such as polysaccharides, proteins, DNA and RNA, the abstraction of H atoms or the addition to the unsaturation by •OH can lead to the direct scission of the backbone, double, or single strand breaks of these polymers.

We have designed two metal-organic frameworks (MOFs) to efficiently convert X-ray to visible-light luminescence. The MOFs are constructed from M6(μ3-O)4(μ3-OH)4(carboxylate)12 (M = Hf or Zr) secondary building units (SBUs) and anthracene-based dicarboxylate bridging ligands. The high atomic number of Zr and Hf in the SBUs serves as effective X-ray antenna by absorbing X-ray photons and converting them to fast electrons through the photoelectric effect. The generated electrons then excite multiple anthracene-based emitters in the MOF through inelastic scattering, leading to efficient generation of detectable photons in the visible spectrum. The MOF materials thus serve as efficient X-ray scintillators via synergistic X-ray absorption by the metal-cluster SBUs and optical emission by the bridging ligands.

1. An Introduction to Electron Energy-Loss Spectroscopy.- 1.1 Interaction of Fast Electrons with a Solid.- 1.2. The Electron Energy-Loss Spectrum.- 1.3. The Development of Experimental Techniques.- 1.4. Comparison of Analytical Methods.- 1.4.1. Ion-Beam Methods.- 1.4.2. Incident Photons.- 1.4.3. Electron-Beam Techniques.- 1.5. Further Reading.- 2. Instrumentation for Energy-Loss Spectroscopy.- 2.1. Energy-Analyzing and Energy-Selecting Systems.- 2.1.1. The Magnetic-Prism Spectrometer.- 2.1.2. Energy-Selecting Magnetic-Prism Devices.- 2.1.3. The Wien Filter.- 2.1.4. Cylindrical-Lens Analyzers.- 2.1.5. Retarding-Field Analyzers.- 2.1.6. Electron Monochromators.- 2.2. The Magnetic-Prism Spectrometer.- 2.2.1. First-Order Properties.- 2.2.2. Higher-Order Focusing.- 2.2.3. Design of an Aberration-Corrected Spectrometer.- 2.2.4. Practical Considerations.- 2.2.5. Alignment and Adjustment of the Spectrometer.- 2.3. The Use of Prespectrometer Lenses.- 2.3.1. Basic Principles.- 2.3.2. CTEM with Projector Lens On.- 2.3.3. CTEM with Projector Lens Off.- 2.3.4. Spectrometer-Specimen Coupling in a High-Resolution STEM.- 2.4. Recording the Energy-Loss Spectrum.- 2.4.1. Serial Acquisition.- 2.4.2. Electron Detectors for Serial Recording.- 2.4.3. Scanning the Energy-Loss Spectrum.- 2.4.4. Signal Processing and Storage.- 2.4.5. Noise Performance of a Serial Detector.- 2.4.6. Parallel-Recording Detectors.- 2.4.7. Direct Exposure of a Diode-Array Detector.- 2.4.8. Indirect Exposure of a Diode Array.- 2.4.9. Removal of Diode-Array Artifacts.- 2.5. Energy-Filtered Imaging.- 2.5.1. Elemental Mapping.- 2.5.2. Z-Contrast Imaging.- 3. Electron Scattering Theory.- 3.1. Elastic Scattering.- 3.1.1. General Formulas.- 3.1.2. Atomic Models.- 3.1.3. Diffraction Effects.- 3.1.4. Electron Channeling.- 3.1.5. Phonon Scattering.- 3.2. Inelastic Scattering.- 3.2.1. Atomic Models.- 3.2.2. Bethe Theory.- 3.2.3. Dielectric Formulation.- 3.2.4. Solid-State Effects.- 3.3. Excitation of Outer-Shell Electrons.- 3.3.1. Volume Plasmons.- 3.3.2. Single-Electron Excitation.- 3.3.3. Excitons.- 3.3.4. Radiation Losses.- 3.3.5. Surface Plasmons.- 3.3.6. Single, Plural, and Multiple Scattering.- 3.4. Inner-Shell Excitation.- 3.4.1. Generalized Oscillator Strength.- 3.4.2. Kinematics of Scattering.- 3.4.3. Ionization Cross Sections.- 3.5. The Spectral Background to Inner-Shell Edges.- 3.6. The Structure of Inner-Shell Edges.- 3.6.1. Basic Edge Shapes.- 3.6.2. Chemical Shifts in Threshold Energy.- 3.6.3. Near-Edge Fine Structure (ELNES).- 3.6.4. Extended Energy-Loss Fine Structure (EXELFS).- 4. Quantitative Analysis of the Energy-Loss Spectrum.- 4.1. Removal of Plural Scattering from the Low-Loss Region.- 4.1.1. Fourier-Log Deconvolution.- 4.1.2. Approximate Methods.- 4.1.3. Angular-Dependent Deconvolution.- 4.2. Kramers-Kronig Analysis.- 4.3. Removal of Plural Scattering from Inner-Shell Edges.- 4.3.1. Fourier-Log Deconvolution.- 4.3.2. Fourier-Ratio Method.- 4.3.3. Van Cittert'1. An Introduction to Electron Energy-Loss Spectroscopy.- 1.1 Interaction of Fast Electrons with a Solid.- 1.2. The Electron Energy-Loss Spectrum.- 1.3. The Development of Experimental Techniques.- 1.4. Comparison of Analytical Methods.- 1.4.1. Ion-Beam Methods.- 1.4.2. Incident Photons.- 1.4.3. Electron-Beam Techniques.- 1.5. Further Reading.- 2. Instrumentation for Energy-Loss Spectroscopy.- 2.1. Energy-Analyzing and Energy-Selecting Systems.- 2.1.1. The Magnetic-Prism Spectrometer.- 2.1.2. Energy-Selecting Magnetic-Prism Devices.- 2.1.3. The Wien Filter.- 2.1.4. Cylindrical-Lens Analyzers.- 2.1.5. Retarding-Field Analyzers.- 2.1.6. Electron Monochromators.- 2.2. The Magnetic-Prism Spectrometer.- 2.2.1. First-Order Properties.- 2.2.2. Higher-Order Focusing.- 2.2.3. Design of an Aberration-Corrected Spectrometer.- 2.2.4. Practical Considerations.- 2.2.5. Alignment and Adjustment of the Spectrometer.- 2.3. The Use of Prespectrometer Lenses.- 2.3.1. Basic Principles.- 2.3.2. CTEM with Projector Lens On.- 2.3.3. CTEM with Projector Lens Off.- 2.3.4. Spectrometer-Specimen Coupling in a High-Resolution STEM.- 2.4. Recording the Energy-Loss Spectrum.- 2.4.1. Serial Acquisition.- 2.4.2. Electron Detectors for Serial Recording.- 2.4.3. Scanning the Energy-Loss Spectrum.- 2.4.4. Signal Processing and Storage.- 2.4.5. Noise Performance of a Serial Detector.- 2.4.6. Parallel-Recording Detectors.- 2.4.7. Direct Exposure of a Diode-Array Detector.- 2.4.8. Indirect Exposure of a Diode Array.- 2.4.9. Removal of Diode-Array Artifacts.- 2.5. Energy-Filtered Imaging.- 2.5.1. Elemental Mapping.- 2.5.2. Z-Contrast Imaging.- 3. Electron Scattering Theory.- 3.1. Elastic Scattering.- 3.1.1. General Formulas.- 3.1.2. Atomic Models.- 3.1.3. Diffraction Effects.- 3.1.4. Electron Channeling.- 3.1.5. Phonon Scattering.- 3.2. Inelastic Scattering.- 3.2.1. Atomic Models.- 3.2.2. Bethe Theory.- 3.2.3. Dielectric Formulation.- 3.2.4. Solid-State Effects.- 3.3. Excitation of Outer-Shell Electrons.- 3.3.1. Volume Plasmons.- 3.3.2. Single-Electron Excitation.- 3.3.3. Excitons.- 3.3.4. Radiation Losses.- 3.3.5. Surface Plasmons.- 3.3.6. Single, Plural, and Multiple Scattering.- 3.4. Inner-Shell Excitation.- 3.4.1. Generalized Oscillator Strength.- 3.4.2. Kinematics of Scattering.- 3.4.3. Ionization Cross Sections.- 3.5. The Spectral Background to Inner-Shell Edges.- 3.6. The Structure of Inner-Shell Edges.- 3.6.1. Basic Edge Shapes.- 3.6.2. Chemical Shifts in Threshold Energy.- 3.6.3. Near-Edge Fine Structure (ELNES).- 3.6.4. Extended Energy-Loss Fine Structure (EXELFS).- 4. Quantitative Analysis of the Energy-Loss Spectrum.- 4.1. Removal of Plural Scattering from the Low-Loss Region.- 4.1.1. Fourier-Log Deconvolution.- 4.1.2. Approximate Methods.- 4.1.3. Angular-Dependent Deconvolution.- 4.2. Kramers-Kronig Analysis.- 4.3. Removal of Plural Scattering from Inner-Shell Edges.- 4.3.1. Fourier-Log Deconvolution.- 4.3.2. Fourier-Ratio Method.- 4.3.3. Van Cittert's Method.- 4.3.4. Effect of a Collection Aperture.- 4.4. Background Fitting to Ionization Edges.- 4.4.1. Energy Dependence of the Background.- 4.4.2. Background-Fitting Procedures.- 4.4.3. Background-Subtraction Errors.- 4.5. Elemental Analysis Using Inner-Shell Edges.- 4.5.1. Basic Formulas.- 4.5.2. Correction for Incident-Beam Convergence.- 4.5.3. Effect of Sample Orientation.- 4.5.4. Effect of Specimen Thickness.- 4.5.5. Choice of Collection Angle.- 4.5.6. Choice of Integration and Fitting Regions.- 4.5.7. Microanalysis Software.- 4.5.8. Calculation of Partial Cross Sections.- 4.6. Analysis of Extended Energy-Loss Fine Structure.- 4.6.1. Spectrum Acquisition.- 4.6.2. Fourier-Transform Method of Data Analysis.- 4.6.3. Curve-Fitting Procedure.- 5. Applications of Energy-Loss Spectroscopy.- 5.1. Measurement of Specimen Thickness.- 5.1.1. Measurement of Absolute Thickness.- 5.1.2. Sum-Rule Methods.- 5.2. Low-Loss Spectroscopy.- 5.2.1. Phase Identification.- 5.2.2. Measurement of Alloy Composition.- 5.2.3. Detection of Hydrogen and Helium.- 5.2.4. Zero-Loss Images.- 5.2.5. Z-contrast Images.- 5.2.6. Plasmon-Loss Images.- 5.3. Core-Loss Microanalysis.- 5.3.1. Choice of Specimen Thickness and Incident Energy.- 5.3.2. Specimen Preparation.- 5.3.3. Elemental Detection and Mapping.- 5.3.4. Quantitative Microanalysis.- 5.3.5. Measurement and Control of Radiation Damage.- 5.4. Spatial Resolution and Elemental Detection Limits.- 5.4.1. Electron-Optical Considerations.- 5.4.2. Loss of Resolution due to Electron Scattering.- 5.4.3. Statistical Limitations.- 5.4.4. Localization of Inelastic Scattering.- 5.5. Structural Information from EELS.- 5.5.1. Low-Loss Fine Structure.- 5.5.2. Orientation Dependence of Core-Loss Edges.- 5.5.3. Core-Loss Diffraction Patterns.- 5.5.4. Near-Edge Fine Structure.- 5.5.5. Extended Fine Structure.- 5.5.6. Electron-Compton Measurements.- Appendix A. Relativistic Bethe Theory.- Appendix B. FORTRAN Programs.- B.3. Incident-Convergence Correction.- B.4. Fourier-Log Deconvolution.- B.5. Kramers-Kronig Transformation.- Appendix C. Plasmon Energies of Some Elements and Compounds.- Appendix D. Inner-Shell Binding Energies and Edge Shapes.- Appendix E. Electron Wavelengths and Relativistic Factors Fundamental Constants.- References.

No abstract

Dose measurements using Fricke and ionization methods were compared for 60Co gamma rays, 4-25-MV photons, and 10-25-MeV electrons. Fricke derived doses based on a constant yield (epsilon mG) were in good agreement with ionization derived doses based on the American Association of Physicists in Medicine Task Group 21 protocol and the National Research Council of Canada (NRC) ND calibration or the NRC proposed Nx. These measurements also confirmed the validity of the double-voltage technique in the collection efficiency correction, even for swept electron beams. Assuming the correctness of the ionization derived doses, the radiation yield appeared to be 1% higher and to increase with photon energy when irradiation vessels were made of Pyrex but not with polystyrene cells. These glass wall effects could be due to the scattering perturbation of electrons between inhomogeneous materials and, in particular for photon beams, due to the mismatch in mass energy absorption ratios and mass collision stopping power ratios between the Fricke dosimeter and the wall materials.

The low-cycle fatigue properties of 30 mm Ti6Al4V titanium alloy joints welded by vacuum electron beam welding and laser wobble welding with filler wire were compared. The test results show that the low-cycle fatigue performance of the electron beam welding joint is close to that of the base meal, while the low-cycle fatigue performance of the laser wobble welding with a filler wire joint is clearly inferior to that of the electron beam welding joint and base meal. An examination of the microhardness of the base metal, heat-affected zones and weld area of the two joints found that the microhardness of the weld area of the electron beam welding joint is close to that of the base meal, but is lower than that of the heat-affected zones on both sides. The weld area of the laser wobble welding with the filler wire joint is significantly weakened.

A colored butterfly can be drawn in three dimensions by the precipitation and control of Au nanoparticles inside transparent materials (see picture). The precipitation involves two processes: photoreduction of Au ions induced by femtosecond laser irradiation, and precipitation of Au particles driven by heat treatment. The size of the nanoparticles and their spatial distribution can be controlled by the irradiation conditions and annealing temperature. Nanoparticles have a wide range of electrical and optical properties owing to the quantum-size effect, surface effect, and conjoint effect of nanostructures.1 Materials doped with noble-metal nanoparticles exhibit large third-order nonlinear susceptibility and ultrafast nonlinear responses.2 They are expected to be promising materials for ultrafast all-optical switches in the THz region. For the applications related to integrated optoelectronics, a well-defined assembly and spatial distribution of nanoparticles in materials are essential.3 Many studies have been carried out on the fabrication of nanoparticle-doped materials,4 but there are no effective methods to control the spatial distribution of nanoparticles in these materials. In addition, Zheng and Dickson reported the synthesis of photostable, water-soluble, silver nanodots by direct photoreduction of silver ions under ambient conditions.5 Photoactivated fluorescence has also been observed from individual silver nanoclusters.6 Herein, we report a method that can control the precipitation of Au nanoparticles in three dimensions inside transparent materials by using focused femtosecond laser irradiation. In brief, the precipitation involves two processes: the photoreduction of Au ions to atoms induced by multiphoton process, and the precipitation of Au particles driven by heat treatment. The size of nanoparticles and their spatial distribution can be controlled by the conditions of the laser irradiation. Interestingly, the precipitated nanoparticles obtained by this technique can be also space-selectively “dissolved” by the femtosecond laser irradiation, and reprecipitated by annealing. This implies that the laser can be used not only in practical applications, such as the 3D optical memory and the fabrication of integrated all-optical switches, but also in the study of the control of nucleation and crystal growth. Au2O3-doped (0.01 mol %) silicate glass samples were irradiated by using a focused Ti–sapphire mode-locked femtosecond laser beam (800 nm, 120 fs, 1 KHz) with an intensity of 3.5×1015 W cm−2 for 1/63 s (16 laser pulses) on each spot. Gray spots of about 40 μm in diameter were then observed in the focused area through an optical microscope after irradiation. No microcracks were observed in the samples. After the samples were annealed at 550 °C for 30 min, the gray spots became red. Using this technique, we first drew a gray owl with the laser beam, and then annealed the sample at 550 °C for 30 min, and as expected, the gray owl became red. After the sample cooled down to room temperature, we drew a gray butterfly in a different area of the sample. These images are shown in Figure 1. Photograph of images drawn inside the Au2O3-doped glass (0.01 mol %) by using the femtosecond laser irradiation: a) gray butterfly (without annealing); b) red owl (with annealing). Figure 2 shows extinction spectra of the Au2O3-doped glass sample before and after femtosecond laser irradiation. There is an apparent increase in extinction in the wavelength region from 300 to 800 nm in the irradiated area. The inset of the Figure 2 shows the difference extinction spectrum of the glass sample before and after the laser irradiation. The peaks at 245, 306, 430, and 620 nm can be assigned to E′ centers (E′=Si), which include an electron trapped in an sp3 orbital of silicon at the site of an oxygen vacancy, a hole trapped by an oxygen vacancy that neighbors alkali-metal ions, and nonbridging oxygen holes HC1 (a hole trapped by an SiO4 polyhedron that contain one bridging oxygen and three nonbridging oxygen atoms) and HC2 (a hole trapped by an SiO4 polyhedron that contain two nonbridging oxygen atoms).7 Extinction spectra of the Au2O3-doped glass (0.01 mol %). a) Before femtosecond laser irradiation; b) after femtosecond laser irradiation; c), d), e), and f) after femtosecond laser irradiation and subsequent annealing at 300, 450, 500, and 550 °C for 30 min, respectively. Inset of Figure 2. Difference extinction spectrum of the Au2O3-doped glass sample (0.01 mol %) before and after the femtosecond laser irradiation. The extinction spectra of the Au2O3-doped glasses, which were annealed at various temperatures for 30 min after irradiation, are also plotted in Figure 2. When the annealing temperature is below 300 °C, the extinction (300–800 nm) intensities induced by irradiation decrease as the annealing temperature increases, and completely disappear when the temperatures reaches 300 °C. One can see in Figure 2 that spectrum a and c are almost identical. The gray induced by the femtosecond laser irradiation disappears at 300 °C and the glass becomes colorless and transparent. Annealing at 450 °C results in the appearance of a new peak at 506 nm, and the laser-irradiated areas turn red. The extinction peak can be assigned to the surface plasmon resonance absorption of Au nanoparticles.2 The wavelength of the extinction peak increases from 506 to 526 to 548 nm with increasing annealing temperature, at the same time its intensity significantly increases. Based on the Mie theory, R∝λ/Δλ, in which R is the average radii of the metal nanoparticles, λp is the characteristic wavelength of surface plasmon resonance and Δλ is the full width at half maximum of the absorption band.8 The value of λ/Δλ increases from 1306 to 1774 to 2208 nm when the annealing temperature is increased from 450 to 500 to 550 °C. Therefore, the average size of the Au nanoparticles increases with increasing annealing temperature. Figure 3 is a TEM image showing the precipitation of nanoparticles in the laser-irradiated Au2O3-doped glass after annealing at 550 °C for 30 min. Composition analysis by using energy dispersive spectroscopy (EDS) in TEM confirms that these spherical nanoparticles are metallic Au. The size of the Au nanoparticles ranges from 6 to 8 nm. TEM image of Au nanoparticles (small white dots) in the laser-irradiated Au2O3-doped (0.01 mol %) glass after annealing at 550 °C for 30 min. The inset of Figure 4 shows the photograph of a Au2O3-doped glass sample, which is irradiated by using femtosecond laser beams of 6.5×1013, 2.3×1014, or 5.0×1016 W cm−2 in the different areas and then annealed at 550 °C for 1 hour. With increasing light intensity, the color of the laser-irradiated areas became violet, red, or yellow. Figure 4 shows the extinction spectra from these different colored areas. The extinction peak shifts from 568 to 534 to 422 nm with the increase of the light intensity. The peak with the wavelength longer than 500 nm observed at spectra a and b, can also be assigned to the surface plasmon resonance absorption of the Au nanoparticles. The apparent blue shift of the peak from 568 to 534 nm is due to the decrease in the average size of the Au nanoparticles. An extinction peak is observed at 420 nm (2.94 eV) in the spectrum c of the Figure 4. There are few reports on the observation of such peaks in glasses doped with Au nanoparticles. However, the peak position and shape are very similar to those of an undecagold compound with small Au clusters, for example, [Au11].9 The peak can be attributed to interband transitions from 5d to 6sp, that is, originating in the submerged and quasicontinum 5d band and terminating in the lowest unoccupied conduction band of the Au clusters. The average size of Au nanoparticles in area c (Figure 4 inset) is much smaller than those in areas a and b. Therefore, the average size of the Au nanoparticles decreases with an increase of the light intensity. This is probably because the high irradiation intensity produces a high concentration of reduced Au atoms per unit volume, and thus a high concentration of nucleation centers. As a result, under the same annealing process, the higher the light intensity, the smaller but denser the precipitated particles are. Further investigation is needed to verify the above hypothesis. Extinction spectra of Au2O3-doped glasses (0.1 mol %) irradiated by using different light intensities: a) 6.5×1013 W cm−2; b) 2.3×1014; c) 5.0×1016. All samples were annealed at 550 °C for 1 hour. Inset of Figure 4: Photograph of images drawn inside the Au2O3-doped (0.1 mol %) glass sample. The reduction of Au ions to atoms by femtosecond laser irradiation is the key process of this method. Au ions capture the “free” electrons created by multiphoton processes and are then reduced to atoms, which aggregate to form nanoparticles during annealing. A similar phenomena have also been observed with Ag+ ions that have been irradiated with X-rays.4 To test this mechanism, we studied the white emission observed during the femtosecond laser irradiation. If the light intensity was sufficiently high and the laser beam was not tightly focused, supercontinuum white light due to the self-phase modulation of the laser beam was observed during the laser irradiation. We observed that the gray area was induced in the area where supercontinuum white light was observed in the glass. We tightly focused the laser beam and confirmed that the gray area was generated in the area at which white emission was observed, even when no supercontinuum was detected. The white emission is due to plasma formation.10 It was also found that three areas, the white emission area, the gray area, and the nanoparticle-precipitated area were basically the same. If the diameter of the beam was kept the same (9 μm), the length of emission region was proportional to the light intensity, which increased from 1.2×1014 to 4.0×1015 W cm−2. In general, the light intensity, in order of 1014–1017 W cm−2, is high enough to generate multiphoton ionization in the glass matrix.10 Therefore, the active electrons and holes can be created in the glass through multiphoton ionization, Joule heating, and collisional ionization,10 and form plasma, which yield white emission. Electrons are driven out of the valence states by multiphoton absorption of the incident photon. Some of the Au ions capture free electrons to form Au atoms. At temperatures below 300 °C, only some trapped electrons and holes are excited by thermal energy and recombine with each other. When annealing at temperatures above 400 °C, Au atoms get sufficient energy to overcome the interaction between the Au atoms and the glass network structure and start to move. The formation of the Au nanoparticles is due to the aggregation of Au atoms. It is also confirmed that no change occurs in the extinction spectrum of the nanoparticle-precipitated glass sample at room temperature, even over a period of six months. This indicates that the precipitated nanoparticles are stable at room temperature. Additionally, the precipitation of Au nanoparticles was not seen in the glass sample without laser irradiation, even after the sample had been annealed at 600 °C for more than 2 h. Therefore, the reduction of an Au ion to an atom by femtosecond laser irradiation is essential in forming Au nanoparticles, and the Au atom acts as a crystal nucleus for crystal growth. Figure 5 shows the changes of the Au nanoparticle-precipitated Au2O3-doped glass sample after further laser irradiation. The glass sample was first irradiated by the focused laser with a light intensity of 5.8×1014 W cm−2 and a scanning rate of 1000 μm s−1, and then it was annealed at 550 °C for 30 min. The laser-irradiated area became red as shown in Figure 5 a, as discussed above. Then the laser beam was focused on the center of the region where the nanoparticles had been precipitated and lines were drawn with the laser that were slightly longer than the nanoparticle region (Figure 5 b). The light intensity and scanning rate were 3.9×1014 W cm−2 and 1000 μm s−1, respectively. One can see that there is a slight change between Figure 5 a and b5 due to the formation of colored centers. After the annealing process at 300 °C for 30 min, the second femtosecond laser-irradiated part became transparent, which is shown in Figure 5 c. Interestingly, the transparent part in the center became red after further annealing of the sample at 550 °C for 30 min. Figure 6 shows the extinction difference between the sample before (Figure 5 a) and after (Figure 5 b) the second laser irradiation. It is clear that the extinction due to the surface plasmon resonance absorption decreases while the absorption due to the nonbridging oxygen hole centers HC1(430 nm) and HC2 (620 nm) increases after further laser irradiation. Therefore, we suggest that some of the nanoparticles are broken into small-size particles or atoms owing to the strong interaction between the Au nanoparticles and ultrashort laser pulses such as dramatic heating of nanoparticles due to the linear and nonlinear absorption of laser energy during the further femtosecond laser irradiation. Photographs of images drawn inside the Au2O3-doped glass (0.01 mol %): a) by using femtosecond laser irradiation and annealing at 550 °C for 30 min; b) further irradiation at the center part of the image in (a) by focused femtosecond laser by using a 20× objective lens; c) then the glass was annealed at 300 °C for 300 min. Spectrum showing the difference in extinction between nanoparticle sample before (Figure 5 a) and after (Figure 5 b) the second laser irradiation. Many experiments have confirmed that it is possible to control the diameter and longitudinal spread of the structurally changed area from several hundred nanometers to several millimeters by selecting the appropriate irradiation conditions, such as light intensity and diameter of the laser beam.11 Our results further prove that it is also possible to precipitate Au nanoparticles in such microscopic dimensions inside materials by using the focused nonresonant femtosecond pulsed laser and successive annealing. The 3D gray images created by the laser can be erased after annealing at a lower temperature, and can be turned into various colors by annealing at higher temperatures. Therefore, this present technique will be useful in the fabrication of 3D multicolored industrial art objects, optical memory with ultrahigh storage density and ultrahigh recording speed, and integrated waveguide all-optical switches with ultrafast nonlinear responses. By using an Ag+ ion-doped photosensitive glass, we have space-selectively precipitated silicate crystals inside the glass.12 We believe it will be possible to spatially control the growth of other functional crystals in glasses. Recently, we have also succeeded in the fabrication of a grating of 400 nm in width by precipitating Au nanoparticles. Owing to the extremely short energy-deposition time and elimination of the thermal effect and nonlinear processes enabled by highly localizing laser photons in both time and spatial domains, the size of the laser-induced microstructures may be less than the diffraction limit. 13 In glasses with a high concentration of Au ions, we expect to able to produce 3D Au nanocircuits. The silicate glasses used in this study were 70 SiO2⋅10 CaO⋅20 Na2O (mol %) doped with different concentration of Au2O3. Reagent grade SiO2, CaCO3, Na2CO3, and AuCl3⋅HCl⋅4 H2O were used as starting materials. Approximately 40 g batches were mixed and melted in platinum crucibles in an electronic furnace at 1550 °C for 1 hour under the ambient atmosphere. The melts were then quenched to room temperature to obtain transparent and colorless glasses. The glass samples were cut and polished to sizes of 3×9×9 mm3 or 4×10×10 mm3, then were used in our experiments. A Ti sapphire laser system with an oscillator (Tsunami pumped by a solid-state laser Millennia, both from Spectra Physics Co. Ltd.) and an amplifier (Spitfire pumped by Merlin both from Spectra Physics Co. Ltd.) was used in this study. The system emits 800 nm, 120 fs laser pulses at a 1 kHz repetition rate. To write an image inside the glass sample, the laser beam was focused by a 10× objective lens with an aperture of 0.30 onto the interior of the glass, about 1 mm beneath the surface. The glass sample was put on a computer-controlled XYZ stage. The diameter of the laser beam was 9 μm. Extinction spectra were acquired with a spectrophotometer (JASCO V-570). The size and composition of precipitated nanoparticles were examined in a JEOL-2010FEF transmission electron microscope (TEM) equipped with energy dispersive X-ray spectrometer (EDS) operating at an accelerating voltage of 200 kV.

No abstract

No abstract

Intense ultrashort light pulses comprising merely a few wave cycles became routinely available by the turn of the millennium. The technologies underlying their production and measurement as well as relevant theoretical modeling have been reviewed in the pages of Reviews of Modern Physics (Brabec and Krausz, 2000). Since then, measurement and control of the subcycle field evolution of few-cycle light have opened the door to a radically new approach to exploring and controlling processes of the microcosm. The hyperfast-varying electric field of visible light permitted manipulation and tracking of the atomic-scale motion of electrons. Striking implications include controlled generation and measurement of single attosecond pulses of extreme ultraviolet light as well as trains of them, and real-time observation of atomic-scale electron dynamics. The tools and techniques for steering and tracing electronic motion in atoms, molecules, and nanostructures are now becoming available, marking the birth of attosecond physics. In this article these advances are reviewed and some of the expected implications are addressed.

By developing a comprehensive computer code for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</tex> -beam excited XeCl lasers, we studied mainly the effect of Ar and Ne diluents on the performance characteristics of XeCl lasers. According to the analysis of the XeCl* formation process, the XeCl* relaxation process, and the 308 nm absorption process, it is found that the XeCl* formation efficiency is determined mainly by the rate of the charge transfer process (from Ar+ and Ne+ diluent ions to Xe+); in other words, by the difference between ionic potentials of Xe and the diluent gas used. The extraction efficiency is found to be decided mainly by the quenching rate of a three-body reaction for a short-pulse (55 ns) and a high-excitation-rate (∼ 3 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) pumping, and by the absorption process for a long-pulse (500 ns) and a low-excitation-rate (∼ 0.2 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) pumping. However, note that no appreciable difference in the intrinsic efficiency is found between the Ar/Xe/HCl and Ne/Xe/HCl mixtures. We also analyzed the dependence of the intrinsic XeCl laser efficiency on the pumping pulse width and excitation rate for Ar/Xe/HCl and Ne/Xe/HCl mixtures. As a result, the same intrinsic efficiencies are obtainable for both Ar- and Ne-based mixtures although the optimum operating conditions are slightly different. The maximum intrinsic efficiency of 5 percent is obtainable both for the Ar/Xe/HCl mixture at 3 atm and with 1.5 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , 200 ns (FWHM) pumping and for the Ne/Xe/HCl mixture at 4 atm and with 2 MW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , 200 ns (FWHM) pumping.

We employ the lowest-order radially polarized axicon fields of a Gaussian laser beam to demonstrate that electrons may be accelerated from rest in vacuum to a few GeV. Petawatt power laser beams focused onto micron-size focal spots result in multi-TeV/m electron energy gradients.

This article gives a comparison between laser beam welding and electron beam welding. In a first step, the basic principles and properties of both methods and/or the resulting weld joints are specified, afterwards the research results from laser beam welding in vacuum are presented. Thanks to modern laser systems (fibre laser and/or disk laser) and their excellent beam quality, laser beam welding in vacuum allows a direct comparison of the process behaviour, the development of the keyhole and the respective welding results from both beam welding methods. The welding possibilities of this process variation are demonstrated, the advantages with regard to electron beam welding and/or the disadvantages compared to laser beam welding in atmosphere are discussed and the need for future research are specified.

The mechanisms of the impact of the laser assisting effect on the dispersion kinetics and on the structure of the deposited layers in electron beam dispersion of a polymer target were analyzed. The proposed model and analytical expressions adequately describe the kinetic dependence of the polymer materials dispersion rate in a vacuum on the intensity of laser processing of their dispersion zone.

A technological process of solid carbide cutting tool surface treatment by selective laser ablation of wear-resistant coating is described. Discontinuous coating is achieved by laser ablation of pre-deposited complex coating, formed by NbHfTi electron beam alloying with subsequent vacuum arc (TiAl)N coating. It is shown that this treatment helps to slow down the wearing process of cutting inserts on their rake and flank surfaces, thus increasing the tool lifetime.

No abstract

A theoretical kinetic model for an electron-beam-excited F2 laser (157 nm) was successfully developed to evaluate the performance characteristics in terms of electron-beam excitation rate, pumping pulse width, and total operating pressure. As a result, it is made clear that a high excitation rate (above 0.2 MW/cm3 atm) is essential to obtain efficient laser operation. An intrinsic laser efficiency of 4.3% is obtainable when a 6.5-atm mixture of He/F2=1000/1 pumped at an excitation rate of 0.5 MW/cm3 atm, giving a laser output of over 4 J/l. It is also found that a higher laser output is obtainable with increasing total operating pressure, while the intrinsic laser efficiency slowly decreases.

A unique experiment carried out at TWI a few years ago compared the welding performance of a number of fibre-delivered lasers with beam qualities ranging between 4 and 23mm.mrad, under identical processing conditions, and demonstrated the effect of laser beam brightness, together with beam quality and spot diameter, on the welding performance on both steel and aluminium. This paper describes a continuation of this earlier study, carrying out welding under identical processing conditions and comparing, against these initial results, the welding performances of laser beams with even higher beam qualities (higher brightness) and of an in-vacuum electron beam. This investigation demonstrates that the welding performance of a high-brightness laser set-up is highly dependent on the conditions of the metal vapour column forming between the processing point and the focusing lens. The effective removal of this metal vapour column, which scatters and/or absorbs some of the incident laser power, is essential in maximising the welding performance that is achievable with high-brightness lasers. By using an argon side-jet shielding and a series of argon cross-jets along the beam path between the focusing lens and the processing point, it was shown that the welding performance of high-brightness lasers could be improved considerably, and matching, or possibly surpassing, that of an equivalent beam quality in-vacuum electron beam used under similar conditions of power, spot size and welding speed.

We report on measurements of quantum electrodynamic processes in an intense electromagnetic wave, where nonlinear effects (both multiphoton and vacuum polarization) are prominent. Nonlinear Compton scattering and electron-positron pair production have been observed in collisions of 46.6 GeV and 49.1 GeV electrons of the Final Focus Test Beam at SLAC with terawatt pulses of 1053 nm and 527 nm wavelengths from a Nd:glass laser. Peak laser intensities of $\ensuremath{\approx}0.5\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ have been achieved, corresponding to a value of $\ensuremath{\approx}0.4$ for the parameter $\ensuremath{\eta}{=eE}_{\mathrm{rms}}/m{\ensuremath{\omega}}_{0}c$ and to a value of $\ensuremath{\approx}0.25$ for the parameter ${\ensuremath{\Upsilon}}_{e}{=E}_{\mathrm{rms}}^{\ensuremath{\star}}{/E}_{\mathrm{crit}}{=eE}_{\mathrm{rms}}^{\ensuremath{\star}}\ensuremath{\Elzxh}{/m}^{2}{c}^{3},$ where ${E}_{\mathrm{rms}}^{\ensuremath{\star}}$ is the rms electric field strength of the laser in the electron rest frame. We present data on the scattered electron spectra arising from nonlinear Compton scattering with up to four photons absorbed from the field. A convolved spectrum of the forward high energy photons is also given. The observed positron production rate depends on the fifth power of the laser intensity, as expected for a process where five photons are absorbed from the field. The positrons are interpreted as arising from the collision of a high-energy Compton scattered photon with the laser beam. The results are found to be in agreement with theoretical predictions.

The electron motions in the beat wave composed of two strong Gaussian laser beams in vacuum are numerically studied with special attention paid to the finite-diameter effects. It is shown that electrons can be strongly accelerated, if they are appropriately injected into the beat wave. The acceleration gradients are obtained for the various values of the laser amplitude. These results are discussed on the basis of the plane wave theory.

No abstract

Employing laser wigglers and accelerators provides the potential to dramatically cut the size and cost of X-ray light sources. Owing to recent technological developments in the production of high-brilliance electron beams and high-power laser pulses, it is now conceivable to make steps toward the practical realisation of laser-pumped X-ray free-electron lasers (FELs). In this regard, here the head-on collision of a relativistic dense electron beam with a linearly polarized laser pulse as a wiggler is studied, in which the laser wiggler can be realised using a conventional quantum laser. In addition, an external guide magnetic field is employed to confine the electron beam against self-fields, therefore improving the FEL operation. Conditions allowing such an operating regime are presented and its relevant validity checked using a set of general scaling formulae. Rigorous analytical solutions of the dynamic equations are provided. These solutions are verified by performing calculations using the derived solutions and well known Runge-Kutta procedure to simulate the electron trajectories. The effects of self-fields on the FEL gain in this configuration are estimated. Numerical calculations indicate that in the presence of self-fields the sensitivity of the gain increases in the vicinity of resonance regions. Besides, diamagnetic and paramagnetic effects of the wiggler-induced self-magnetic field cause gain decrement and enhancement for different electron orbits, while these diamagnetic and paramagnetic effects increase with increasing beam density. The results are compared with findings of planar magnetostatic wiggler FELs.

The penetration depth is one of the fundamental process parameters in laser beam welding. On-line monitoring and control of the penetration depth is a significant contribution to meet todays quality requirements and enhance the integration of laser technology into manufacturing. An experimental set-up to monitor the penetration depth by detecting the optical emission of the metal vapour is presented. For a wide range of welding parameters the radiation emitted coaxially with respect to the laser beam axis depends linearly on the penetration depth. The advantage of the chosen coaxial set-up compared with an off-axis arrangement is demonstrated. Spatially integrated spectroscopic measurements of the density and temperature of the metal vapour/plasma are carried out. Coaxial and off-axis observing positions of the spectrometer arrangement are compared. For a wide range of the welding parameters the coaxial alignment leads to elevated values for the vapour density and almost unchanged temperatures compared with the off-axis set-up. Using a CCD-camera the radiation signal can be monitored with a lateral resolution of 63.64 pixels. Taking into account the linear relation between the optical radiation signal and the penetration depth the geometrical shape of the keyhole is deduced from the CCD-frames/1 /. An additional well known monitoring signal is the acoustic emission of the welding process. The features with respect to the penetration depth of both signals are compared.

An examination of available solutions for the penetration depth for electron beam welding revealed that they have been based on mathematical models which are not truly representative of the physical phenomena. The deposition of electron beam energy has been represented by a steady line source. Experimental evidence, however, shows that due to an oscillating flow of the molten material, the electron beam energy is deposited on a surface which varies from close to horizontal to the sides of a cone-shaped cavity. To account for this a cylindrical constant temperature boundary condition was proposed. Results based on this model were found to agree within 20 percent with available depth of penetration data.

The conditions of energy and material flow during beam welding are investigated theoretically to determine the factors which govern the shape of the vapor cavity and of the molten zone. Flow conditions in the horizontal plane determine the dimensions of the weld. Material is moved around the advancing vapor cavity mainly by liquid flow, but there is some vapor flow across the cavity, providing the pressure which drives the liquid. The pressure inside the vapor cavity and its variation with depth is governed by surface tension, by the hydrostatic pressure in the liquid, and by the viscous forces acting on the vapor stream. These factors govern the radius of the cavity as a function of depth. The penetration is limited by the beam power or by the absorption of the beam. For the laser beam, absorption is decreased in the hot center, and beam penetration increases with power, but is insensitive to collimation. For electrons, absorption occurs mainly near the walls of the cavity, and the beam penetration depends on collimation and power. The balance between beam power and power dissipated by conduction, melting, and vaporization is discussed, and a self-consistent description is given of cavity formation and beam penetration.

With utilization of penetration mechanism in electron-beam welding, an equaption which estimates the penetration depth in bead by calculation is induced from thermal conduction theory in this report.Moreover, to examine the valiuity of the equation, each penetration depth which is estimated by the equation is compared and discussed with the actual depth in numerous beads under the same welding conditions.As a result of this discussion, it becomes apparent that the induced equation for calculation of the penetration depth gives successfully the penetration depth in actual bead.Therefore, if the welding condition and physical constants for the used material are known, the penetration depth in actual bead can be easily estimated by simple calculation using the above equation.

Laser and electron-beam welds were passed across selenium-doped zones in 21-6-9 stainless steel. The depth/width (d/w) ratio of a defocused laser weld with a weld pool shape similar to a GTA weld increased by over 200% in a zone where 66 ppm selenium had been added. Smaller increases were observed in selenium-doped zones for a moderately defocused electron beam weld with a higher d/w ratio in undoped base metal. When laser or electron beam weld penetration was by a keyhole mechanism, no change in d/w ratio occurred in selenium-doped zones. The results confirm the surface-tension-driven fluid-flow model for the effect of minor elements on GTA weld pool shape. Other experimental evidence bearing on the effect of minor elements on GTA weld penetration is summarized.

In laser (or electron-beam) welding a high-intensity beam is directed on to a metal surface causing melting and evaporation. If the rate of evaporation is sufficiently high, then the laser will drill a 'keyhole' into the molten metal, thereby depositing power deep into the material. This drilling process will be opposed by the flow of molten metal into the keyhole, and in the steady state the two effects balance each other over the entire surface of the hole. Steady-state hole profiles are obtained for a vertical beam including the effects of gravity and surface tension. It is shown that surface tension reduces the depth of penetration typically by a factor of about three.

The authors have developed a new chamber for laser welding under the low vacuum conditions achieved by using rotary pumps. High-power disk laser bead-on-plate welding was performed on Type 304 stainless steel or A5052 aluminium alloy plate at the powers of 10, 16 and 26 kW at various welding speeds under low vacuum. The sound welds of more than 50 and 70 mm in penetration depth could be produced in Type 304 at the pressure of 0.1 kPa, the speed of 0.3 m/min and the power of 16 kW and 26 kW, respectively. Similar penetration was achieved in A 5052 aluminum alloy. Welding phenomena under low vacuum were also understood by observing the behavior of a keyhole inlet, a molten pool, melt flows and a plume ejected from a keyhole through high speed video cameras. Low interaction between a laser beam and a plume under low vacuum was confirmed by using probe laser beam method.

Calculations of mass energy-transfer and mass energy-absorption coefficients for photon energies from 1 keV to 100 MeV have been developed, based on a re-examination of the processes involved after the initial photon interaction. The probabilities for the initial interaction are from the current photon interaction cross-section database at the National Institute of Standards and Technology. The calculations then take into account (1) electron binding effects on the Compton-scattered photon distribution; (2) the complete cascade of fluorescence emission after ionization events in any atomic subshell, including those associated with incoherent scattering and triplet production; and (3) the radiative energy losses of the secondary electrons and positrons slowing down in the medium, including the emission of bremsstrahlung, characteristic X rays from impact ionization, and positron in-flight as well as at-rest annihilation quanta. Consideration of the processes in (3) goes beyond the continuous-slowing-down approximation and includes the effects of energy-loss straggling. Results for the mass energy-absorption coefficient are compared with those from recent tabulations.

The dissociation of benzylamine ions following (i) electron impact (EI) ionization, (ii) multiphoton ionization (MPI) at 266 nm, and (iii) infrared multiple photon absorption (IRMPA) at 9.26 μm is reported. In the EI and MPI experiments, three competitive dissociation pathways are observed. In the IRMPA experiments, benzylamine ions prepared by MPI at low fluences are fragmented very efficiently following irradiation with the focused output from a pulsed CO2 laser. However, in contrast to the EI and MPI results, the IRMPD experiments reveal only a single, lowest energy, dissociation pathway and the fragmentation pattern is consistent with a sequential mechanism in which daughter ions continue to absorb the IR radiation and dissociate. The differences are explained by the different natures of the excitation processes: in IRMPA, the relatively slow up-pumping rate and the long rise time of the CO2 laser pulse restrict the levels of excitation in the dissociating parent ions and favor sequential processes along the lowest energy decomposition pathways.

The photoelectron spectrum of 2-furanmethanol (furfuryl alcohol) has been measured for ionization energies between 8 and 11.2 eV and the first three ionization bands assigned to π3, π2, and no ionizations in order of increasing binding energy. The photoabsorption spectrum has been recorded in the gas phase using both a synchrotron radiation source (5–9.91 eV, 248–125 nm) and electron energy-loss spectroscopy under electric-dipole conditions (5–10.9 eV, 248–90 nm). The (UV) absorption spectrum has also been recorded in solution (4.2–6.36 eV, 292–195 nm). The electronic excitation spectrum appears to be dominated by transitions between π and π* orbitals in the aromatic ring, leading to the conclusion that the frontier molecular orbitals of furan are affected only slightly on replacement of a H atom by the –CH2OH group. Additional experiments investigating electron impact at near-threshold energies have revealed two low-lying triplet states and at least one electron/molecule shape resonance. Dissociative electron attachment also shows to be widespread in furfuryl alcohol.

Transparent solids may show strong absorption when irradiated by a high-intensity laser pulse. Such laser induced breakdown is due to the formation of a free-electron gas. We investigate theoretically the role of ionization processes in a defect-free crystal, including in our model two competing processes: strong-electric-field ionization and electron impact ionization. Free-electron heating is described in terms of electron-phonon-photon collisions. Relaxation of the free electron gas occurs through electron-electron collisions and electron-phonon collisions. The latter are also responsible for energy transfer from the free-electron gas to the phonon gas. We solve numerically a system of time dependent Boltzmann equations, where each considered process is included by its corresponding collision integral. Our results show formation, excitation, and relaxation of the electron gas in the conduction band. We find that strong-electric-field ionization is mainly responsible for free-electron generation. No avalanche occurs at femtosecond laser irradiation. The electron density and the internal energies of the subsystems are calculated. Critical fluences obtained using various criteria for damage threshold are in good agreement with recent experiments.

We present an experimental and numerical study of the damage and ablation thresholds at the surface of a dielectric material, e.g., fused silica, using short pulses ranging from 7 to 300 fs. The relevant numerical criteria of damage and ablation thresholds are proposed consistently with experimental observations of the laser irradiated zone. These criteria are based on lattice thermal melting and electronic cohesion temperature, respectively. The importance of the three major absorption channels (multi-photon absorption, tunnel effect, and impact ionization) is investigated as a function of pulse duration (7--300 fs). Although the relative importance of the impact ionization process increases with the pulse duration, our results show that it plays a role even at short pulse duration ($&lt;$50 fs). For few optical cycle pulses (7 fs), it is also shown that both damage and ablation fluence thresholds tend to coincide due to the sharp increase of the free electron density. This electron-driven ablation regime is of primary interest for thermal-free laser-matter interaction and therefore for the development of high quality micromachining processes.

No abstract

No abstract

We have theoretically investigated nonlinear free-carrier absorption of terahertz (THz) radiation in InAs/AlSb heterojunctions. By considering multiple photon process and conduction-valence interband impact ionization (II), we have determined the field and frequency dependent absorption rate. It is shown that (i). electron-disorder scatterings are important at low to intermediate field, and (ii). most importantly, the high field absorption is dominated by II processes. Our theory can satisfactorily explain a long-standing experimental result on the nonlinear absorption in the THz regime.

Electron-impact excitation has been observed at incident electron energies of 10.1 and 20.1 eV to the first five excited electronic states of formaldehyde lying at and below the 1B2 state at 7.10 eV. These excitations include two new transitions in the energy-loss range 5.6–6.2 eV and 6.7–7.0 eV which have been detected for the first time, either through electron-impact excitation or photon absorption. The differential cross sections of these new excitations relative to that of the optically-allowed 1B2 ← X̃ 1A1 transition are given at scattering angles between 15° and 135°. These cross-section ratios peak at large scattering angles—a characteristic of triplet ← singlet excitations. From a comparison of the observed and calculated vertical transition energies, the transitions are assigned as 3A1 ← X̃ (5.6–6.2 eV) and 3B2 ← X̃ (6.7–7.0 eV). The design and performance of the electron-impact spectrometer used in the above observations is outlined and discussed.

In this article we present a detailed and unified theoretical treatment of secondary electron cascades that follow the absorption of x-ray photons. A Monte Carlo model has been constructed that treats in detail the evolution of electron cascades induced by photoelectrons and by Auger electrons following inner shell ionizations. Detailed calculations are presented for cascades initiated by electron energies between 0.1 and 10keV. The present article expands our earlier work [B. Ziaja, D. van der Spoel, A. Szöke, and J. Hajdu, Phys. Rev. B 64, 214104 (2001), Phys. Rev. B 66, 024116 (2002)] by extending the primary energy range, by improving the treatment of secondary electrons, especially at low electron energies, by including ionization by holes, and by taking into account their coupling to the crystal lattice. The calculations describe the three-dimensional evolution of the electron cloud, and monitor the equivalent instantaneous temperature of the free electron gas as the system cools. The dissipation of the impact energy proceeds predominantly through the production of secondary electrons whose energies are comparable to the binding energies of the valence (40–50eV) and of the core electrons (300eV). The electron cloud generated by a 10keV electron is strongly anisotropic in the early phases of the cascade (t⩽1fs). At later times, the sample is dominated by low energy electrons, and these are scattered more isotropically by atoms in the sample. Our results for the total number of secondary electrons agree with available experimental data, and show that the emission of secondary electrons approaches saturation within about 100fs following the primary impact.

X-ray two-photon absorption (TPA) spectrum of metallic copper is measured using a free-electron laser (XFEL). The spectrum differs from that measured by the conventional one-photon absorption (OPA), and characterized by a peak below the Fermi level, which is assigned to the transition to the 3d state. The impact of the XFEL pulse on the OPA spectrum is discussed by analyzing the pulse-energy dependence, which indicates that the intrinsic TPA spectrum is measured.

No abstract

No abstract

Material removal produced by the interaction of high-power-density electron beam with metal was studied theoretically and experimentally. A mechanism of molten metal removal due to the generation of bubbles inside the target metal is proposed, based on the observed phenomenon and the calculation, which takes into account heat-conduction theory and the penetration of the electron beam into the target metal. Further, as an extension of this mechanism, a drilling mechanism is proposed. These proposed mechanisms could successfully explain the results obtained experimentally.

We have studied in detail the physical phenomena involved in the interaction of high-powered nanosecond excimer-laser pulses with bulk targets resulting in evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating these laser-plasma--solid interactions has been developed. In this model, the laser-generated plasma is treated as an ideal gas at high pressure and temperature, which is initially confined in small dimensions, and is suddenly allowed to expand in vacuum. The three-dimensional expansion of this plasma gives rise to the characteristic spatial thickness and compositional variations observed in laser-deposited thin films of multicomponent systems. The forward-directed nature of the laser evaporation process has been found to result from anisotropic expansion velocities of the atomic species which are controlled by the dimensions of the expanding plasma.Based on the nature of interaction of the laser beam with the target and the evaporated material, the pulsed-laser evaporation (PLE) process can be classified into three separate regimes: (i) interaction of the laser beam with the bulk target, (ii) plasma formation, heating, and initial three-dimensional isothermal expansion, and (iii) adiabatic expansion and deposition of thin films. The first two processes occur during the time interval of the laser pulse, while the last process initiates after the laser pulse terminates. Under PLE conditions, the evaporation of the target is assumed to be thermal in nature, while the plasma expansion dynamics is nonthermal as a result of interaction of the laser beam with the evaporated material. The equations of compressible gas dynamics are set up to simulate the expansion of the plasma in the last two regimes. The solution of the gas-dynamics equations shows that the expansion velocities of the plasma are related to its initial dimensions and temperature, and the atomic weight of the species. Detailed simulations analyzing the salient features of the laser-deposition process have been carried out. The effects of various beam and substrate parameters including pulse energy density, substrate-target distance, irradiated spot size, and atomic mass of the species have been theoretically analyzed. This model predicts most of the characteristic experimental features of the laser evaporation and deposition of thin films. These characteristic features include (a) the effect of pulse energy density on atomic velocities, (b) the forward-directed nature of the deposit and its dependence on energy density, (c) spatial compositional variations in multicomponent thin films as a function of energy density, (d) dependence of the atomic velocities with atomic weights of various species in multicomponent films, (e) athermal non-Maxwellian-type velocity distribution of the atomic and molecular species, and (f) thickness and compositional variations as a function of substrate-target distance and irradiated spot size.

No abstract

No abstract

Generation of a high amplitude shock wave by laser plasma in a water confinement regime has been investigated for an incident 25–30 ns/40 J/λ=1.064 μm pulsed laser beam. Experimental measurements of temporal and spatial profiles of induced shock waves for this regime of laser shock processing of materials were performed using a velocimetry interferometer system for any reflector system. Above a 10 GW/cm2 laser intensity threshold, a saturation of the peak pressure is shown to occur while the pressure pulse duration is reduced by parasitic plasma occurring in the confining water. The observation of the interaction zone with a fast camera system shows that this breakdown plasma, which mainly occurs at the very surface of the water rather than within the water volume, limits the efficiency of the process. This plasma absorbs the incident laser energy, and the power density reaching the target gradually decreases with increasing power densities while the shock-wave duration is correspondingly reduced. Both pressure measurements and plasma observations allow explaining the current limit of high amplitude shock-waves generation by laser plasma in the water-confinement mode and open new research areas for the understanding of breakdown plasma effects at the surface of the confining water.

While great interest has been focused on low temperature plasma for material processing, the concept of using pulsed high speed and high temperature plasma beam for material surface treatment was put forward. Thus a novel instrument based on Thetatron-Pinch was built up and the plasma beam with a temperature of several hundred eVs and a density up to 1016 cm−3 was created uniformly over a large cross section. The plasma beam was then accelerated by a sample bias of 0–2.0 kV under a high vacuum circumstance. The energy density exerted on the sample surface is 1–20 J/cm2 with a duration time of less than 10 μs, which is comparable to that of laser treatment. Preliminary experiments indicate that this method has the combining feature of laser treatment and ion implantation techniques when it is used for surface modification; moreover, a transient high temperature and high pressure environment exists on the treated surface in this technique and the plasma contains a high concentration of radical particles. Both facts are advantageous to chemical vapor deposition applications.

Abstract Organic–inorganic hybrid perovskite solar cells with mixed cations and mixed halides have achieved impressive power conversion efficiency of up to 22.1%. Phase segregation due to the mixed compositions has attracted wide concerns, and their nature and origin are still unclear. Some very useful analytical techniques are controversial in microstructural and chemical analyses due to electron beam‐induced damage to the “soft” hybrid perovskite materials. In this study photoluminescence, cathodoluminescence, and transmission electron microscopy are used to study charge carrier recombination and retrieve crystallographic and compositional information for all‐inorganic CsPbIBr 2 films on the nanoscale. It is found that under light and electron beam illumination, “iodide‐rich” CsPbI (1+ x ) Br (2− x ) phases form at grain boundaries as well as segregate as clusters inside the film. Phase segregation generates a high density of mobile ions moving along grain boundaries as ion migration “highways.” Finally, these mobile ions can pile up at the perovskite/TiO 2 interface resulting in formation of larger injection barriers, hampering electron extraction and leading to strong current density–voltage hysteresis in the polycrystalline perovskite solar cells. This explains why the planar CsPbIBr 2 solar cells exhibit significant hysteresis in efficiency measurements, showing an efficiency of up to 8.02% in the reverse scan and a reduced efficiency of 4.02% in the forward scan, and giving a stabilized efficiency of 6.07%.

The current ITER design employs beryllium, carbon fiber reinforced composite and tungsten as plasma facing materials. Since these materials are exposed to high heat fluxes during the operation, it is essential to perform high heat flux tests for R&D of ITER components. Static heat loads corresponding to cycling loads during normal operation, are estimated to be up to 20 MW/m2 in the divertor targets and around 0.5 MW/m2 at the first wall in ITER. For the static high heat flux testing, tests in electron beam facilities, particle beam facilities, IR heater and in-pile tests have been performed. Another type, more critical heat loads, which have high power densities and short durations, corresponding to transient events, i.e. plasma disruption, vertical displacement events (VDEs) and edge localized modes (ELMs) deliver considerable heat flux onto the plasma facing materials. For this purpose, tests in electron beam (short pulses), plasma gun and high power laser facilities have been carried out. The present work summarizes the features of these facilities and recent experimental results as well as the current selection of ITER plasma facing components.