Transmissive extreme ultraviolet metagrating
Extreme ultraviolet (EUV) radiation is a key tool for attosecond physics and lithography. However, strong material absorption limits the availability of transmissive optical elements at these wavelengths. Metaoptics exploit geometry to control the wavefront of transmitted light on the nanoscale and, due to their minimal thickness, promise to fill this gap. Here, we demonstrate the first EUV metaoptics for broadband applications: we design, fabricate, and experimentally investigate a blazed transmissive EUV metagrating and compare it with a focused-ion-beam-milled sawtooth-blazed grating serving as an in-situ reference. The metagrating achieves an angular dispersion of 0.04°/nm with a directionality (the ratio of the +1st and -1st diffraction order efficiency) of up to 5.8. The device shows phase-based operation up to 50 eV photon energy (down to 25 nm vacuum wavelength) and an octave-spanning bandwidth of 25 eV, doubling the previous spectral window addressable by metasurfaces. Comparing both gratings' performance reveals that, when accounting for fabrication constraints, EUV metasurfaces are competitive with free-form optics while offering scalability to large apertures and arbitrary phase profiles. Broadband transmissive operation removes the need for grazing incidence optics, defeating a major source of aberrations, and allows polarization-insensitive spectral analysis, enabling energy-resolved ultrafast spectroscopy in compact experimental configurations.