Effect of nanosilicon coating on the EUV/x-ray reflectance of silica polycapillary optic

Reflection-based lensing is used to focus or collimate extreme ultraviolet (EUV)/x rays (x-ray optics) propagating in grazing incidence small-angle x-ray scattering (GISAXS) inside poly-capillary glass. In the process, the refractive index of silica is slightly less than that of vacuum (1.0) for EUV/x rays, leading to total external reflection with a critical angle slightly less than 90°. Here, we examine the feasibility of widening the total external reflection window by coating the capillary interior surface with a silicon nanofilm that assumes the shape of the polycapillary, which may enhance the collection and collimation power. We use slow dynamic self-assembly via circulation of the nanoparticles through the capillary. Electromagnetic scattering computations are conducted using the model of a flat glass substrate decorated with closely packed Si nanoparticles. Optical, luminescent, and SEM imaging techniques, as well as spectroscopy, show that the polycapillary coating is uniform and luminescent under UV irradiation. The computations show that, for visible wavelengths, a 3 nm silicon nanoparticle film reduces the grazing reflectivity at the visible/UV band-to-band transitions E1 and E2. For EUV/x ray, there is a significant reflectivity enhancement of ∼10% due to a larger index dip below that of vacuum, affording a smaller critical angle. Widening the external reflection regime promises superior collection and focusing power, improving the intensity and resolution. Being luminescent under UV makes it useful for imaging, therapy, and other clinical procedures, while also enabling the development of more compact and versatile instruments.