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

Hiroto Motoyama, Shigeki Owada, Gota Yamaguchi, Takehiro Kume, Satoru Egawa, Kensuke Tono, Yuichi Inubushi, Takahisa Koyama, Makina Yabashi, Haruhiko Ohashi, Hidekazu Mimura

X-ray free-electron lasers (XFELs) have produced femto­second X-ray pulses (Emma et al., 2010; Ishikawa et al., 2012; Shintake et al., 2008; Ackermann et al., 2007; Allaria et al., 2012; Kang et al., 2017; Milne et al., 2017; Tschentscher et al., 2017) with high intensity, which can be further enhanced by using a focusing optical system. For the hard X-ray region, total reflection optics in the Kirkpatrick–Baez (KB) configuration (Kirkpatrick & Baez, 1948), which consists of a pair of one-dimensional grazing-incidence elliptical mirrors, have been widely utilized due to their high damage threshold and high reflectivity in a wide range of photon energies (Boutet & Williams, 2010; Yumoto et al., 2012; Mimura et al., 2014; Kim et al., 2018; Matsuyama et al., 2018). By tightly focusing hard X-rays, ultra-intense X-ray light fields can be generated, promoting research into nonlinear X-ray optics, such as X-ray two-photon absorption (Tamasaku et al., 2014, 2018; Ghimire et al., 2016), saturable absorption of X-rays (Yoneda et al., 2014; Rackstraw et al., 2015) and lasing from an atomic inner shell (Rohringer et al., 2012; Yoneda et al., 2015).

For focusing of soft X-ray FELs, the KB configuration has been widely employed (Owada et al., 2018; Dziarzhytski et al., 2016; Schlotter et al., 2012). However, it is more difficult to achieve a small focus in the longer wavelength regime, because the surface profile becomes much steeper under a higher incident angle required for a higher numerical aperture (NA) condition. Typical focus sizes are currently limited from one to several micrometres. A multilayered normal-incidence mirror in a single-reflection geometry was developed for sub-micrometre focusing with an intensity of 1016 W cm−2 (Nelson et al., 2009), which enabled the observation of saturable absorption in solid aluminium (Nagler et al., 2009). The normal-incidence mirror has a large numerical aperture and large acceptance, but the multilayered structure imposes a limitation on the wavelength to be applicable.

Recently, we have developed a new scheme to produce high-quality grazing-incidence ellipsoidal mirrors, where a two-dimensional focus is generated with a single optical device. The single reflection geometry provides the advantage of short focal lengths in both the vertical and horizontal directions so as to produce a small focus. Furthermore, an alignment procedure becomes straightforward with a simpler mirror manipulator system. In order to fabricate high-NA ellipsoidal mirrors with a steep surface profile, a two-step fabrication process was developed. A precise quartz mandrel, which has an ellipsoidal surface, is first fabricated (Takei & Mimura, 2017), and the shape of the mandrel is replicated to a thick nickel layer by means of a room-temperature electroforming process (Kume et al., 2014). The figure error of the mandrel was ∼20 nm peak-to-valley, which is sufficiently low for reflecting soft X-rays with a wavelength around 10 nm. A nickel surface has a broadband reflectance for the wavelength ranging from 10 nm to 30 nm (Henke et al., 1993). With this process, we were able to suppress the wavefront error below λ/4, and produce a sub-micrometre focusing spot for broadband high-order harmonics in the extreme ultraviolet (EUV) region (λ = 10–20 nm; Motoyama et al., 2019) and for soft X-ray synchrotron radiation (λ = 4 nm; Mimura et al., 2018).

In this report, we describe a focusing system with an ellipsoidal mirror for sub-micrometre focusing of soft X-ray FEL pulses with high efficiency. Since the spatial acceptance of the ellipsoidal mirror fabricated with the present process is limited to a few millimetres, being smaller than the typical size of the unfocused soft X-ray FEL beam, we combined a KB mirror system as the prefocusing optics to match the incident beam size with the acceptance of the ellipsoidal mirror. We constructed this hybrid two-stage focusing system at the soft X-ray FEL beamline (BL1) of SACLA (Owada et al., 2018), and produced a 480 nm × 680 nm focus with an extremely high intensity of 8.8 × 1016 W cm−2 at a photon energy of 120 eV. Saturable absorption in Si3N4 thin film was successfully observed.


Journal of Synchrotron Radiation: