In Situ Control and Nanofocusing of Extreme Ultraviolet Solid-State High Harmonics

Extreme ultraviolet (XUV), electromagnetic radiation with a wavelength of ~100 nm, is one of the key technologies of ultrafast science. A lot of progress has been made in developing high-brightness and high-energy coherent XUV sources, including the ones based on solid state materials. However, efficient manipulation and focusing of XUV photons to their ultimate diffraction limit remains a formidable challenge because of the precision of the focusing by curved optical surfaces.

We employ an alternative approach, by integrating a coherent short-wavelength high-order harmonics source, an MgO crystal, with a nanostructure etched in its surface, to achieve an in situ control of the emitted radiation. Shaping the surface into a high numerical aperture focusing element we can focus XUV light, 7^th harmonic of an 800 nm laser, down to a waist radius of 150 nm, as demonstrated by the knife-edge measurements. The numerical aperture of the element of 0.35 competes with the best ex situ focusing elements for XUV radiation, such as ellipsoidal mirrors, Schwartschild objectives and Fresnel zone plates.

We expect the unique combination of short wavelength, small focus and high intensity to enable many applications, such as direct laser nanostructuring and nonlinear imaging with chemical specificity, an extension to the element-specific imaging pioneered at higher photon energies, and photoelectron spectro-nanoscopy. Our approach will pave way to the XUV coherent sources on a chip.