In this work, we optimized a clean, versatile, compact source of soft X-ray radiation $(E_{\text{x}\text{-}\text{ray}}\sim 3~\text{keV})$ with an yield per shot up to $7\times 10^{11}~\text{photons}/\text{shot}$ in a plasma generated by the interaction of high-contrast femtosecond laser pulses of relativistic intensity $(I_{\text{las}}\sim 10^{18}{-}10^{19}~\text{W}/\text{cm}^{2})$ with supersonic argon gas jets. Using high-resolution X-ray spectroscopy approaches, the dependence of main characteristics (temperature, density and ionization composition) and the emission efficiency of the X-ray source on laser pulse parameters and properties of the gas medium was studied. The optimal conditions, when the X-ray photon yield reached a maximum value, have been found when the argon plasma has an electron temperature of $T_{\text{e}}\sim 185~\text{eV}$, an electron density of $N_{\text{e}}\sim 7\times 10^{20}~\text{cm}^{-3}$ and an average charge of $Z\sim 14$. In such a plasma, a coefficient of conversion to soft X-ray radiation with energies $E_{\text{x}\text{-}\text{ray}}\sim 3.1\;(\pm 0.2)~\text{keV}$ reaches $8.57\times 10^{-5}$, and no processes leading to the acceleration of electrons to MeV energies occur. It was found that the efficiency of the X-ray emission of this plasma source is mainly determined by the focusing geometry. We confirmed experimentally that the angular distribution of the X-ray radiation is isotropic, and its intensity linearly depends on the energy of the laser pulse, which was varied in the range of 50–280 mJ. We also found that the yield of X-ray photons can be notably increased by, for example, choosing the optimal laser pulse duration and the inlet pressure of the gas jet.

We theoretically investigate the delay-dependent attosecond transient absorption spectra in the helium atom dressed by an infrared laser pulse in the wavelength range of 800–2400 nm. By numerically solving the three-dimensional time-dependent Schr dinger equation, we find that the absorption spectrogram exhibits a multiple-fringe structure for using the mid-infrared dressing pulse. The quantitative calculation of the transition matrix between different Floquet states provides direct evidence on the origin of the multiple-fringe structure. Our result shows that the wavelength of the dressing pulse is an important parameter and the unique feature of attosecond transient absorption spectroscopy can be induced in the mid-infrared regime.

In this study, we investigate a new simple scheme using a planar undulator (PU) together with a properly dispersed electron beam ( beam) with a large energy spread ( ) to enhance the free-electron laser (FEL) gain. For a dispersed beam in a PU, the resonant condition is satisfied for the center electrons, while the frequency detuning increases for the off-center electrons, inhibiting the growth of the radiation. The PU can act as a filter for selecting the electrons near the beam center to achieve the radiation. Although only the center electrons contribute, the radiation can be enhanced significantly owing to the high-peak current of the beam. Theoretical analysis and simulation results indicate that this method can be used for the improvement of the radiation performance, which has great significance for short-wavelength FEL applications.

We demonstrate the suppression of soft X-ray high harmonics generated by two-color laser pulses interacting with Ne gas in a gas cell. We show that harmonic suppression can occur at the proper combination of the propagation distance and gas pressure. The physical mechanism behind is the phase mismatch between “short”-trajectory harmonics generated at the early and later times through the interplay of geometric phase, dispersion, and plasma effects. In addition, we demonstrate that the position and depth of harmonic suppression can be tuned by increasing the gas pressure. Furthermore, the suppression can be extended to other laser focusing configurations by properly scaling macroscopic parameters. Our investigation reveals a simple and novel experimental scheme purely relying on the phase mismatch for selectively controlling soft X-ray tabletop light sources without adopting the filters for applications.

A Schwarzschild microscope with a numerical aperture of 0.2 and a magnification of 130 in a 100 μm field of view (FOV) is designed and is working at 13.5 nm. Meanwhile, a CCD is used as a detector with a pixel size of 13 μm×13 μm and imaging area of 13 mm×13 mm. The imaging quality with tolerances of system and errors of mirrors are considered. We obtain that the best on-axes object resolution can be up to about 200 nm, the average value is 230 nm, and the resolution is about 360 nm at 80 μm FOV.

The vacuum-sealed miniature modulated x-ray source (VMMXS) with a hot cathode is fabricated via the single-step brazing process in a vacuum furnace. An experiment following the VMMXS is implemented to present its performances, including the influence of grid electrode potential on x-ray intensities. The modulation type of the grid electrode as a switch is proposed, and its feasibility is successfully demonstrated. It is noteworthy to discover a phenomenon for the first time, to the best of our knowledge, that the high repetition frequency grid pulse of the VMMXS has a significant effect on the x-ray intensity. The probable cause for this new finding is analyzed.

The Pd/B4C multilayer is a promising candidate for high reflectance mirrors operating in the 8–12 nm extreme ultraviolet wavelength region. To extend the working bandwidth beyond the L-edge of silcon, we theoretically design broadband Pd/B4C multilayers. We discuss the influence of the desired reflectance of the plateau, number of bilayers, and the real structural parameters, including the interface widths, layer density, and thickness deviation, on the reflectivity profile. Assuming the interface width to be 0.6 nm, we design aperiodic multilayers for broad wavebands of 9.0–10.0, 8.5–10.5, and 8.0–11.0 nm, with average reflectivities of 3.1%, 5.0%, and 9.5%, respectively.

Complementary analysis techniques are applied in this work to study the interface structure of Mo/Si multilayers. The samples are characterized by grazing incident x-ray reflectivity, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and extreme ultraviolet reflectivity. The results indicate that the layer thickness is controlled well with small diffusion on the interface by forming MoSi2. Considering MoSi2 as the interface composition, simulating the result of our four-layer model fits well with the measured reflectivity curve at 13.5 nm.

Near-field holography (NFH), with its virtues of precise critical dimensions and high throughput, has a great potential for the realization of soft x-ray diffraction gratings. We show that NFH with reflections reduced by the integration of antireflective coatings (ARCs) simplifies the NFH process relative to that of setups using refractive index liquids. Based on the proposed NFH with ARCs, gold-coated laminar gratings were fabricated using NFH and subsequent ion beam etching. The efficiency angular spectrum shows that the stray light of the gratings is reduced one level of magnitude by the suppression of interface reflections during NFH.