On the possibility of a distributed feedback X-ray laser

Spontaneous soft x-ray fluorescence from a superlattice under Kossel diffraction conditions

“…Concerning the last point, let us emphasize that soft-x-ray stimulated fluorescence has been recently observed from silicon solid target with pumping by an XFEL [29]. It is reasonable to guess that such a stimulated fluorescence radiation pumped by XFEL could undergo Kossel diffraction in a way similar to the spontaneous one as reported in this paper, opening the way to the realization of an XDFL as proposed by several scientists in the seventies [27,30]. We have calculated [31] that a single XFEL bunch at 57.4 eV delivered by a facility such as FERMI is appropriate to stimulate the Mg L 2,3 emission (corresponding to the 3s-3d -> 2p 1/2,3/2 transition) at 49 eV in a Mg/SiC multilayer superlattice forming the distributed laser cavity.…”

Feasibility considerations of a soft-x-ray distributed feedback laser pumped by an x-ray free electron laser

“…Let us note that there exist now (or are under construction) some X-FEL sources whose photon energy can be as high as 10 keV (0.12 nm). Under these conditions it would be possible to use a single crystal as the optical cavity, as previously envisaged [5,10,28], to obtain aDFL laser working in a harder x-ray range. For example, instead of making the Beye's experiment with the Si L 2,3 characteristic emission, it could be possible to consider to use the Si K emission (2p -1s transition).…”

“…Moreover, owing to the periodicity of the structure, close to the Bragg conditions, the anti-nodes of the electric field are located within the SiC layers, thus limiting the energy deposit by the stimulated beam. To minimize the energy deposited by the incident beam, the sample can be oriented so that the standing wave taking place within the stack is also so that the anti-nodes of the electric field are located within the SiC layers.Moreover, from thermodynamics considerations it has been shown that the duration of the optical properties of the various layers and thus the periodicity of the stack can exceed the fluorescence lifetime [10]. So, even if some damage should occur,its time scale would be longer than the one of the x-ray pulse [16][17][18].…”

“…It is now possible to fabricate efficient multilayers with periodic lattice spacing d suited to satisfy the Bragg condition in the soft-x-ray range. The multilayer optical cavity has many advantages: it provides distributed feedback reducing the required pumping power [4][5][6]; it gives a shorter transit time than a standard cavity with external mirrors and eliminates the problem of alignment of multiple elements [9,10]. The spontaneous emission produced within a multilayer and diffracted under the Bragg condition has been recently observed using synchrotron radiation [11]: the synchrotron radiation from the BEAR beamline (ELETTRA) excited the Mg Kα and Co Lα lines in Mg/Co multilayers, which were Bragg-diffracted by the lattice.…”