Intense sub-micrometre focusing of soft X-ray free-electron laser beyond 10¹⁶ W cm⁻² with an ellipsoidal mirror

Abstract

Intense sub‐micrometre focusing of a soft X‐ray free‐electron laser (FEL) was achieved by using an ellipsoidal mirror with a high numerical aperture. A hybrid focusing system in combination with a Kirkpatrick–Baez mirror was applied for compensation of a small spatial acceptance of the ellipsoidal mirror. With this system, the soft X‐ray FEL pulses were focused down to 480 nm × 680 nm with an extremely high intensity of 8.8×1016 W cm−2 at a photon energy of 120 eV, which yielded saturable absorption at the L‐edge of Si (99.8 eV) with a drastic increase of transmittance from 8% to 48%.

Advanced fabrication technologies for ultraprecise replicated mirrors for x-ray telescopes

“…Because the thermal expansion coefficients of quartz glass and nickel are significantly different, these materials can be easily separated simply by dipping them in warm water. The mirrors we fabricated have been used for focusing soft X-rays in a beamline with high-order harmonics 15 , a synchrotron radiation facility 13,17 , and an X-ray free-electron laser facility 16 . Here we show an example of soft X-ray focusing at the SPring-8 synchrotron facility.…”

“…Although KB mirrors consist of vertical and horizontal focusing mirrors, an ellipsoidal mirror can focus X-rays in two dimensions with a single reflection, but improvement of the figuring accuracy is necessary for use in X-ray microscopy. We established an ellipsoidal mirror fabrication process employing mandrel fabrication and electroforming 13,14 , and successfully focused soft X-rays onto a spot several hundred nanometers wide [15][16][17] . A small Wolter-type mirror was also fabricated for imaging soft X-rays with a spatial resolution of 200 nm 18 .…”

Development of deposition system for the inner surface of Wolter mirror for x-ray telescopes

“…Therefore, the inner surface of Wolter-Ⅰ type mirrors should be highly accurate and smooth to conduct high-resolution observations. Our group has been developing a highprecision Ni electroforming method of full-shell grazing incidence mirrors for Wolter-Ⅰ type mirrors (Figure 2) [5][6][7][8][9] . A Ni layers with a thickness of less than 1 mm are formed on the super polished master mandrels by electroforming and the mirror is separated from the mandrel utilizing the difference of coefficients of thermal expansion between Ni and the fused silica.…”

Development of an inner surface profile measurement system for precise electroformed Wolter mirrors for x-ray telescopes

“…One of the key factors determining the imaging performance of X-ray telescopes is the figure accuracy of the mirrors. Our group has been developing a highly precise surface replication process for fabricating tube-shaped X-ray mirrors [4][5][6][7] . The fused silica mandrel having an aspheric shape is polished with a signal-nanometer surface accuracy, and its surface is replicated using a nickel electroforming process (Figure 2).…”