Description
The focusing of extreme ultraviolet (XUV) radiation is essential for attosecond spectroscopy and microscopy, angle-resolved photoemission spectroscopy (ARPES) and has important applications in metrology and lithography. Designing transmissive optical elements in this wavelength regime is challenging due to strong material absorption, and reflective optics require expensive fabrication methods and complicated alignment procedures associated the required grazing incidence geometry. Recently, metaoptics have been employed for efficient focusing of 50 nm wavelength in a transmission geometry [1]. Diffractive optics, particularly Fresnel zone plates with dielectric, metallic, or hybrid structures are widely used for focusing radiation in the hard and soft X-ray spectral regions [2].
Here, we report a transmissive-phase based off-axis Fresnel Zone plate (FZP) to focus an XUV beam specifically designed for 60 eV photon energy (~20 nm wavelength), where many metals have L-edges that reveal their magnetization via magnetic circular dichroism. The FZP is fabricated from 190 nm thick dielectric poly methyl methacrylate (PMMA) on a 100 nm thick silicon membrane, patterned by electron beam lithography, which shifts the phase of transmitted light by π. The PMMA-based zone plate offers better efficiency than metal-based zone plates at this wavelength and involves fewer fabrication steps, however, may suffer degradation under prolonged exposure [3]. The fabricated FZP has a numerical aperture (NA) of 0.05 and diameter of 100 μm with an outer zone width of around 250-260 nm.
The focusing performance of the designed FZP was investigated using finite difference time domain simulations and its focusing performance will now be experimentally characterized using attosecond pulses generated via high-harmonic generation and the knife edge method.
Reference:
[1] M. Ossiander et al., Science 380, 59–63 (2023).
[2] X. Tong et al., J. Synchrotron Rad. 30, 319–326 (2023).
[3] K. Keskinbora et al., Adv. Opt. Mater. 3, 792–800 (2015).
