Towards Tabletop X‐Ray Lasers

Abstract

Short pulses in the X-ray regime offer unique opportunities for the study of ultrafast processes in biology, chemistry and physics. The absorption of X-rays can occur with high spectral selectivity for direct chemical or physical analysis, the very low diffraction limit of X-rays is useful for imaging and physical analysis, and the short wavelength supports extremely short pulses in the attosecond (1 as D 10 18 s) and even zeptosecond (1 zs D 10 21 s) range [1,2]. Over the past few years, X-ray sources have reached higher and higher intensities by either reducing the pulse duration and the size of the focal spot, or by increasing the energy available in the generating process.The availability of X-ray pulses with ultrahigh intensity has opened new avenues for nonlinear physics and ultrafast imaging (see Chapter 17). A major concern in the field of ultrafast imaging is the danger of sample decomposition due to ionization by X-rays before the diffraction image (2D or 3D) is acquired. A theoretical study by Neutze et al. proposed that the ideal light source for this type of imaging should have about 10 12 photons in a fully coherent pulse with 5 fs duration or less, and with 12 keV photon energy [3]. For soft X-ray imaging (typically 30 eV to 1 keV photon energy), the parameters differ significantly. This energy range corresponds to wavelengths from 30 nm down to 1 nm, allowing experiments with diffraction-limited resolution of a few nanometers. Unlike the case discussed by Neutze, diffraction will not occur on molecular electrons but on nanoscale structures or sample structural variation. The bulk matter structure does not evolve as fast as molecular electrons, which often acquire relativistic speed. The relevant time scales are therefore defined by the speed of sound, typically 10 4 ms 1 for the density and temperature under consideration. To reach a target spatial resolution of 1 nm, the soft X-ray pulse duration should be on the order of 100 fs to prevent blurring during image exposure at the irradiation levels considered.Soft X-rays are often absorbed by the sample, reducing the signal flux at the detector. Hence the experiments require a high energy per pulse. To find an esAttosecond and XUV Physics, First Edition. Edited by Thomas Schultz and Marc Vrakking.