Recently, a joint research team from the High Power Laser Physics Joint Laboratory at the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences, collaborating with a team from Harbin Institute of Technology, has for the first time achieved anisotropic three-dimensional array beam splitting control at extremely short wavelengths. This breakthrough addresses the limitations of beam splitters for diffraction imaging and interferometric sensing in the short-wavelength regime. The related research findings were published in Optica under the title "Anisotropically multiplanar-focal photon-sieve splitter from extreme ultraviolet to soft X-ray."

    Since the discovery of X-rays in 1895, highly coherent short-wavelength light sources and high-performance short-wavelength focusing elements have been two major bottlenecks hindering the development of X-ray science. The emergence of photon sieves provided an alternative device option to zone plates for high-performance focusing at short wavelengths; however, their single-focus characteristic could not meet the technical demands of short-wavelength diffraction imaging and interferometric sensing. With the resolution of the highly coherent short-wavelength light source issue, the need for efficient beam splitters and combiners at extremely short wavelengths has become even more urgent.

    The joint team designed an anisotropic beam-splitting photon sieve by utilizing the ancient Greek ladder sequence combined with an optimization algorithm. In the experiment, a 46.9 nm laser irradiated the photon sieve, Polymethyl methacrylate (PMMA) was used to record the diffracted light field, an atomic force microscope (AFM) was employed to read the data, and anisotropic three-dimensional array nano-scale focal spots were successfully obtained through data inversion. The experimental results are consistent with theoretical calculations.

    The realization of anisotropic beam splitting control at extremely short wavelengths opens up new possibilities for structured light fields in the short-wavelength regime, imaging of living biological cells in the water window, X-ray interferometric diagnosis of laser plasmas, soft X-ray microscopy, coherent diffraction imaging, and more.

Fig. 1. Focusing schematic of MPFPSS

Fig. 4. Normalized intensities obtained from AFM data and the derived results. Normalized intensities of monofocal spot (a) and bifocal spots (b). FWHMs of derived spots as a function of defocus distances and normalized cross-sectional profiles of monofocal spot (c) and right one of bifocal spots (d).

    This research work received support from projects such as the National Natural Science Foundation of China and the Chinese Academy of Sciences Strategic Priority Research Program (Category A).