Precision-Machined Hollow-Core Waveguides for Extreme-Ultraviolet Wavelengths

While optical waveguides for visible and infrared radiation are widely used for many applications, including photonic integrated circuits, optical communications and resonators for lasers, guiding extreme ultraviolet (XUV) radiation below 100 nm is challenging. Since virtually no solid material is transparent at XUV wavelengths, vacuum is the only option for the core. A suitable choice of cladding material can confine the XUV radiation and allow it to propagate with low losses [1], [2]. In this study, we demonstrate multimode hollow-core waveguides formed in a solid substrate that can transport vacuum-ultraviolet radiation from 20 to 200 nm. Figure 1(a, b) shows an example structure of straight hollow-core waveguide and the numerical evaluation results of the fundamental TE mode at a wavelength of 120 nm. To fabricate this waveguide, a NiP plated stainless steel base plate was mechanically machined to form triangular-shaped micro grooves. An ultra-high precision five-axis machining center developed by Canon Inc was used for this process [3]. Another NiP plated substrate was bonded to the top to complete the hollow-core waveguides. The inner surfaces of the waveguides were coated with gold, which acted as a cladding for the waveguides. The longitudinal length of each waveguide was 20 mm and the side length of the triangular cross-section ranged from 4 to
$35\ \upmu\mathrm{m}$
. The rms roughness of the waveguide surface was less than 1 nm, which is not expected to affect the waveguide performance.