X-ray emission from layered media irradiated by an x-ray free-electron laser

Abstract

This article presents a computational study of the x-ray fluorescence induced by the irradiation of thin layered media by intense, short x-ray pulses. The treatment is based on a numerical solution of the Helmholtz wave-equation both for the pump and for the fluorescence signal. Consistently with a possible heating of the medium during the x-ray pulse, complex refractive indices are calculated at each time step from the results of an underlying treatment of atomic physics. In the context of an important core-hole production as a result of photoionization, we discuss the peculiarities of the resulting amplified fluorescence grazing emission and of the Bragg diffraction which can be realized at some angles inside a multilayer material or even in a perfect crystal.

Maxwell-Bloch modeling of an x-ray pulse amplification in a 1D photonic crystal

“…As a test case, we consider here a sample already considered in a context of synchrotron irradiation [26]. It consists in a stack of 30 bilayers (Mg/Co) of thicknesses e 1 = 5.45 nm and e 2 = 2.55 nm, respectively so that the whole stack is denoted ones calculated for the same sample but by solving the Helmholtz wave equation for a plane wave incident in the sample [14], which is just a check of the optical reciprocity theorem stated as a point source at A will produce at B the same effect as a point of equal intensity placed at B will produce at A [29,30]. In Fig.…”

“…Here an oscillation feedback can be provided by Bragg reflection [12,13,33] so that large electric field enhancements can be obtained [14]. Note that, for a multilayer and in the linear response regime, a small deviation to the previous Bragg law exists [34,35].…”

“…Besides this, within the context of XFEL excitation and to extract energy stored in an inverted medium, the idea of using the phenomenon of collective spontaneous decay or superradiance (named also superfluorescence) has been discussed and explored [11] but in the visible range. Independently, it has been suggested that a laser action in the x-ray range can be provided by Bragg reflection inside a natural crystal or inside an artificial multilayer material [12][13][14]. Note that in the first case, Bragg condition is constrained by the crystal periodicity.…”

“…The goal of this paper is to study numerically x-ray feedback under Bragg conditions as well as pulse propagation in 1D photonic crystals in which a population inversion has been initiated by some external source. Here we go beyond a description where the multilayer is simply described by the complex refractive index of each layer [14]. Even when the complex part of the refractive index is negative (i.e.…”