Journal Article STXM 3D elemental distinguishing Research Support, Non-U.S. Gov't S-track SSRF X-ray microscopy bidirectional scanning methods dual-energy focal stacks imaging method electrical phase transition energy resolution energy-resolving power fast polarization switching free-electron lasers high-speed beam intensity monitoring mechanical vibration mixed‐phases molecular ferroelectric noise removal post-processing ray tracing second‐harmonic generation polarimetry soft X-ray magnetic circular dichroism beamline soft X-rays source positional jitter spectrometers tandem twin elliptically polarized undulators top-up mode variable-line-spacing grating variable-line-spacing gratings