X-ray diffraction from strongly bent crystals and spectroscopy of X-ray free-electron laser pulses

The use of strongly bent crystals in spectrometers for pulses of a hard X-ray free-electron laser is explored theoretically. Diffraction is calculated in both dynamical and kinematical theories. It is shown that diffraction can be treated kinematically when the bending radius is small compared with the critical radius given by the ratio of the Bragg-case extinction length for the actual reflection to the Darwin width of this reflection. As a result, the spectral resolution is limited by the crystal thickness, rather than the extinction length, and can become better than the resolution of a planar dynamically diffracting crystal. As an example, it is demonstrated that spectra of the 12 keV pulses can be resolved in the 440 reflection from a 20 µm-thick diamond crystal bent to a radius of 10 cm.

Быстрые приповерхностные изменения дефектной структуры в кристаллах тетрабората лития во внешнем электрическом поле

The results of study of the defect structure variation process in the near-surface layers of lithium tetraborate (Li2B4O7) single crystals under the influence of an external electric field applied along the [001] polar direction are presented. Using the X-ray diffractometry with 2 ms time resolution the dynamics of the 004 and 008 diffraction peak parameters (angular position and integral intensity) variation was determined. Two types of processes caused by redistribution of the charge localized at the surface of the polar dielectric and the migration of lithium ions were revealed with different velocity and response time to the external field switching. The measurements were carried out at voltages providing a reversible character of induced effects. The use of two orders of diffraction with different X-ray extinction length made it possible to visualize space charge layers near the anode and cathode by the intensity variation of the diffraction peaks. Estimation of the effective thickness of the charged surface layer gives a value of 25 μm for lithium ions at the cathode and about 45 μm for lithium vacancies at the anode.

The limit of application of the Scherrer equation

The Scherrer equation is a widely used tool to obtain crystallite size from polycrystalline samples. Its limit of applicability has been determined recently, using computer simulations, for a few structures and it was proposed that it is directly dependent on the linear absorption coefficient (μ0) and Bragg angle (θB). In this work, a systematic study of the Scherrer limit is presented, where it is shown that it is equal to approximately 11.9% of the extinction length. It is also shown that absorption imposes a maximum value on it and that this maximum is directly proportional to sin θB/μ0.

Electromagnetic-matter interactions and devices

This chapter explores electromagnetic-matter interactions from photon to extinction length scales, i.e., nanometer of X-ray and above. Starting with Casimir-Polder effect to understand interactions of metals and dielectrics at near-atomic distance scale, it stretches to larger wavelengths to explore optomechanics and its ability for energy exchange and signal transduction between PHz and GHz. This range is explored with near-quantum sensitivity limits. The chapter also develops the understanding phononic bandgaps, and for photons, it explores the use of energetic coupling for useful devices such as optical tweezers, confocal microscopes and atomic clocks. It also explores miniature accelerators as a frontier area in accelerator physics. Plasmonics—the electromagnetic interaction with electron charge cloud—is explored for propagating and confined conditions together with the approaches’ possible uses. Optoelectronic energy conversion is analyzed in organic and inorganic systems, with their underlying interaction physics through solar cells and its thermodynamic limit, and quantum cascade lasers.

Láser en odontología: fundamentos físicos y biológicos / Laser in Dentistry: Physical and Biological Foundations

RESUMEN. Antecedentes: El láser es una tecnología cada vez más utilizada en Odontología. Para tomar decisiones acertadas con respecto a las características y uso del láser es importante conocer sus bases físicas y biológicas en cuanto a su interacción con los tejidos. Objetivo: Analizar los fundamentos biológicos y físicos del láser en odontología. Métodos: Se realizó una revisión narrativa con base en literatura publicada entre 1990 y 2016 e incluida en el Medline. La muestra consistió en 30 artículos. Para el análisis se empleó un enfoque hermenéutico. Resultados: Los fundamentos físicos analizados incluyen luz, amplificación, emisión estimulada y radiación. En cuanto a los efectos biológicos se analizan el fototérmico, fotoquímico y fotoacústico. Asimismo, se describen las propiedades ópticas de los tejidos orales: absorción, penetración y longitud de extinción. Conclusiones: No todos los láseres actúan igual y una misma longitud de onda puede tener interacciones diferenciales en los tejidos. Existen varios estudios que evidencian la efectividad del láser en varias especialidades de la odontología y abren la posibilidad de varias líneas de investigación.ABSTRACT. Background: Laser technology usage is increasing in dentistry. In order to take adequate decisions about characteristics and use, it is important to know the physical and biological foundations of laser and its interaction with oral tissues. Objective: To analyze the physical and biological foundations of laser in dentistry. Methods: A narrative review of literature published between 1990 and 2016 and included in Medline was carried out. The sample consisted of 30 articles. Analysis of literature was performed through a hermeneutical approach Results: Physical foundations of laser analyzed include light, amplification, stimulated emission, and radiation. Biological effects studied are photothermal, photochemical, and photoacoustic. In addition, optical properties of oral tissues are described: absorption, penetration, and extinction length. Conclusion: Not all lasers act the same way and the same wave length can interact differently with tissues. Several studies show evidence of the effectiveness of laser in several dental specialties and open the possibility for several lines of research.

High diffraction efficiency in crystals curved by surface damage

Diffraction profiles of curved Si and GaAs crystals obtained by a controlled damage process on one side of planar crystals have been investigated at X-ray energiesE= 17, 59 and 120 keV. AtE= 17 and 59 keV in the condition of slight curvature, that is when the diffracting plane bending over the extinction length is lower than the Darwin width, the Laue diffraction profiles with lattice planes parallel or inclined with respect to the curvature radiusRshow an enhancement of integrated intensity proportional to 1/R, much larger than in the corresponding perfect bent crystals. AtE= 120 keV, in the condition of strong curvature, the crystals behave as bent perfect crystals with integrated intensity corresponding to that of a mosaic crystal. These crystals are proposed as optical elements for focusing hard X-ray beams.

High-energy X-ray diffuse scattering

Diffuse X-ray scattering has been an important tool for understanding the atomic structure of binary systems for more than 50 years. The majority of studies have used laboratory-based sources providing 8 keV photons or synchrotron radiation with similar energies. Diffuse scattering is weak, with the scattering volume determined by the X-ray absorption length. In the case of 8 keV photons, this is not significantly different from the typical extinction length for Bragg scattering. If, however, one goes to energies of the order of 100 keV the scattering volume for the diffuse scattering increases up to three orders of magnitude while the extinction length increases by only one order of magnitude. This leads to a gain of two orders of magnitude in the relative intensity of the diffuse scattering compared with the Bragg peaks. This gain, combined with the possibility of recording the intensity from an entire plane in reciprocal space using a two-dimensional X-ray detector, permits time-resolved diffuse scattering studies in many systems. On the other hand, diffraction features that are usually neglected, such as multiple scattering, come into play. Four types of multiple scattering phenomena are discussed, and the manner in which they appear in high-energy diffraction experiments is considered.

Energy-Filtering Techniques for Thick Samples

Imaging of sample regions with a thickness significantly larger than the extinction length and strong thickness variations introduces two major problems for transmission electron microscopy (TEM) : (i) inelastic scattering increases the energy width of the transmitted electrons and therefore the resolution decreases (ii) the contrast differences caused by thickness variations can be higher than the dynamic range of the detector system.Both problems can be solved by using energy filtering techniques. The advantage here is that for energy filtered imaging the resolution limit is not determined by the sample thickness but by the width of the energy selection aperture. Fig. 1 shows three envelope functions of the temporal coherence calculated for different values of the energy width. The functions were plotted for an acceleration voltage of 200 kV and a high voltage stability of 2 ppm.

X-ray diffraction in perfect t × l crystals. Rocking curves

A general formalism, based on the Takagi–Taupin equations, for calculating rocking curves in perfect t\times l crystals is presented. It includes nonsymmetrical scattering, refraction, and ordinary and anomalous absorption. t and l may be varied independently. In the limit of a semi-infinite crystal, the standard results from the fundamental theory are retrieved. For crystal dimensions less than the extinction length, the theory converges to the kinematical limit. Simulations for germanium and silicon show significant influence of crystal finiteness. When dynamical effects are prominent, the curves exhibit various degrees of asymmetry and the full width at half-maximum is generally larger than the corresponding Darwin width. This is attributed to combined Laue and Bragg contributions which are shifted with respect to each other owing to refraction.