Ellipsometer Equipped with Multiple Mirrors for Element-selective Soft X-ray Experiments

a The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan b Vacuum & Optical Instruments, 2-18-18 Shimomaruko, Ota-ku, Tokyo 146-0092, Japan c RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan d Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan e Sorbonne Universite, Institut de Mineralogie, de Physique des Materiaux et de Cosmochimie (IMPMC), F-75005 Paris, France f Sorbonne Universite, Institut des NanoSciences de Paris (INSP), F-75005 Paris, France g Ecole Normale Superieure de Paris, Paris, France † Corresponding author: m_araki@issp.u-tokyo.ac.jp


I. INTRODUCTION
A beam of soft X-rays has been a significant probe to examine electronic and chemical states of a material [1−10]. Furthermore, variations of polarization by light-matter interaction can provide magnetic information through measurements of, for example, X-ray magneto-optical Kerr effect (XMOKE) [5−10]. In the XMOKE experiments, it requires linearly polarized light and, thus, it matches with the beam generated at a facility of an X-ray free-electron laser (XFEL), such as SPring-8 Angstrom Compact free electron LAser (SACLA) [11−13]. Tuning the photon energy of the (soft) X-ray beam at absorption edges of a material, XMOKE signals can be element-specific. The analysis, so-called resonant MOKE, has successfully traced both stat-ic and dynamics behavior in magnetic multi-element compounds [5−10].
A measurement of XMOKE has been carried out by tuning photon energy to the absorption edges of a sample (the resonant effect) and by the ellipsometry technique to measure magneto-optical Kerr rotation angle with element-selectivity [5−10]. Figure 1(a) illustrates a schematic drawing of the ellipsometry measurement with soft X-ray. An optical beam is reflected on the sample surface, followed by reflection on a mirror (analyzer), and reaches a detector (microchannel plate, MCP). A set of a mirror and a detector rotates with angle, χ, with respect to the optical axis between the sample and the mirror. Functioning as a soft X-ray polarization analyzer, it is necessary to use a multilayer mirror that is customized for the light reflection of a specific photon energy, i.e., the absorption edges, demanded in the experiment. To guide the soft X-ray beam, all the optical components and the operation must be kept in vacuum. In the case of a sample that is composed of more than one element, it is necessary to change the multilayer mirror to conduct resonant experiments for different elements. This process has required breaking vacuum and subsequent pumping of the system, which has wasted much time during the valuable XFEL beamtime. In the present technical note, we report our development of an ellipsometer unit equipped with the multiple mirrors that enables the mirror-switching under the vacuum condition. The unit is compact enough to be mounted at any ConFlat flange (ICF70, which corresponds to CF 2-3/4) of a vacuum chamber and makes the precise rotation without disturbing the optical alignment. Moreover, our technique provides a cheap and easy tip for researchers who want to control a heavy vacuum unit.

II. SYSTEM
The ellipsometer unit was developed at the SACLA SXFEL beamline, as shown in Figure 1(b). The beamline, a measurement chamber, and the ellopsometer unit are composed of vacuum chambers, and they are connected with each other by vacuum tubes. The soft X-ray beam from the XFEL beamline is reflected at a sample and is guided to the unit, as indicated by red arrows in Figure 1 The χ-rotation of the heavy unit is made by the DPRF (differential pumping rotary flanges) with the support by role-stands, indicated in Figure 1(b). Figure 2 shows a photo of two roll-stands that support the unit chamber, while Figure 3 gives that of the individual ones. A roll-stand is commercially available as a pipe support that can hold a material that weighs up to 60−70 kg.
The χ-rotation keeps the vacuum condition in the unit by a differential pumping rotary flange (DPRF) B, shown in Figure 2. A rotary motion of the DPRF is motorized and its angular resolution is 0.0008°. Figure 4 illustrates connections to the DPRF B. We additionally introduce a pinhole spacer (inset in Figure 4) before the DRPF B to ease the optical alignment. The pinhole has a size of ⌀1 mm and the surrounding fluorescent agent enables one to check a position of the soft X-ray beam in vacuum through the attached viewport. Based on the beam position on a pinhole spacer, the optical beam can be easily optimized by adjusting an   X-Y stage or a sample manipulator, shown in Figure 1(b). Thanks to a combination of the DPRF and the two rollstands, the χ-rotation can be made precisely without disturbing the practical optical alignment of the ellipsometry measurement.
In resonant soft X-ray experiment of multi-element samples, it is necessary to change the photon energy to absorption edges of different elements in the material. Thus, it is required to change multilayer mirrors during experiment. To replace the mirrors without breaking vacuum, we developed the ellipsometer equipped with multiple mirrors. Figure 5 provides anatomical drawing of the ellipsometer unit with details of the composing parts. The whole unit is designed compact so that it can be additionally attached to an ICF 70 (CF 2-3/4) flange of a vacuum chamber. The system has the following features: i. An ICF114 (CF 4-1/2) flange is adopted for a mirror holder of two mirrors (10 mm × 10 mm each) to be installed. ii. A mirror holder can accommodate two mirrors on the front and back sides, and two mirrors can be switched by the rotary motion provided by converting the perpendicular rotary motion of the transfer rod through two gears. iii. As shown in an inset of Figure 5, a stopper is installed on the mirror stage for the two mirrors in order to secure the reflection angle at 45° with the accuracy better than 0.1°. iv. Several viewpoints are installed at flanges of ICF70 (CF 2-3/4) and ICF34 (CF 1-1/3) to ease an optical alignment. v. A reflected beam can be guided to the viewports by rotating the mirror stage by DRPF A. It is of note that more than two mirrors can be installed by the same system of rotational-switching. However, it requires a much larger UHV chamber and a new switching mechanism to secure the different mirror angles.

III. CONCLUSION
We developed an ellipsometer unit equipped with two mirrors that reversibly switched in vacuum. Adopting more than one multilayer mirror, it is useful for soft X-ray resonant experiment that requires different photon energies. For example, one can trace ultrafast spin dynamics of different magnetic elements in a material by XMOKE measurement at the SXFEL beamline [14]. It is of note that the system significantly saves time during the XFEL beamtime by eliminating the venting/pumping time that is generally required for replacing mirrors. Through our research, we also made usage of roll-stages for precisely rotating the heavy vacuum unit that is additionally attached to the chamber from the side.  : Anatomical illustration of the multiply equipped mirror system. Inset shows a heart of the system, mounted two mirrors (A and B) with the switching mechanism. Flanges of ICF114, ICF70, and ICF34 correspond to CF 4-1/2, CF 2-3/4, and CF 1-1/3, respectively.