Christian Eberl

Sub-5 nm hard x-ray point focusing by a combined Kirkpatrick-Baez mirror and multilayer zone plate

Compound optics such as lens systems can overcome the limitations concerning resolution, efficiency, or aberrations which fabrication constraints would impose on any single optical element. In this work we demonstrate unprecedented sub-5 nm point focusing of hard x-rays, based on the combination of a high gain Kirkpatrick-Baez (KB) mirror system and a high resolution W/Si multilayer zone plate (MZP) for ultra-short focal length f. The pre-focusing allows limiting the MZP radius to below 2 μm, compatible with the required 5 nm structure width and essentially unlimited aspect ratios, provided by enabling fabrication technology based on pulsed laser deposition (PLD) and focused ion beam (FIB).

Towards multi-order hard X-ray imaging with multilayer zone plates

Multilayer zone plates can be used for holographic imaging without an order-sorting aperture.

Two-dimensional sub-5-nm hard x-ray focusing with MZP

We present experiments carried out using a combined hard x-ray focusing set-up preserving the benefits of a large-aperture Kirckpatrick-Baez (KB) mirror system and a small focal length multilayer zone plane (MZP). The high gain KB mirrors produce a pre-focus of 400 nm × 200 nm; in their defocus, two MZP lenses of diameter of 1.6 μm and 3.7 μm have been placed, with focal lengths of 50 μm and 250 μm respectively. The lenses have been produced using pulsed laser deposition (PLD) and focused ion beam (FIB). Forward simulations including error models based on measured deviations, auto-correlation analysis and three-plane phase reconstruction support two-dimensional focus sizes of 4.3 nm × 4.7 nm (7:9 keV, W/Si)1 and 4.3 nm ×5.9 nm (13:8 keV, W/ZrO2), respectively.

Faster scanning and higher resolution: new setup for multilayer zone plate imaging

Hard x-ray imaging methods are routinely used in two and three spatial dimensions to tackle challenging scientific questions of the 21st century, e.g. catalytic processes in energy research and bio-physical experiments on the single-cell level [1–3]. Among the most important experimental techniques are scanning SAXS to probe the local orientation of filaments and fluorescence mapping to quantify the local composition. The routinely available spot size has been reduced to few tens of nanometres; but the real-space resolution of these techniques can degrade by (i) vibration or drift, and (ii) spreading of beam damage, especially for soft condensed matter on small length scales. We have recently developed new Multilayer Zone Plate (MZP) optics for focusing hard (14 keV) and very hard (60 keV to above 100 keV) x-rays down to spot sizes presumably on 5 or 10nm scale. Here we report on recent progress on a new MZP based sample scanner, and how to tackle beam damage spread. The Eiger detector synchronized to a piezo scanner enables to scan in a 2D continuous mode fields of view larger than 20μm squared, or for high resolution down to (virtual) pixel sizes of below 2nm, in about three minutes for 255×255 points (90 seconds after further improvements). Nano-SAXS measurements with more than one million real-space pixels, each containing a full diffraction image, can be carried out in less than one hour, as we have shown using a Siemens star test pattern.

Spectral control of elastic dynamics in metallic nano-cavities

We show how the elastic response of metallic nano-cavities can be tailored by tuning the interplay with an underlying phononic superlattice. In particular, we exploit ultrafast optical excitation in order to address a resonance mode in a tungsten thin film, grown on top of a periodic MgO/ZrO2 multilayer. Setting up a simple theoretical model, we can explain our findings by the coupling of the resonance in the tungsten to an evanescent surface mode of the superlattice. To demonstrate a second potential benefit of our findings besides characterization of elastic properties of multilayer samples, we show by micromagnetic simulation how a similar structure can be utilized for magneto-elastic excitation of exchange-dominated spin waves.

Ultra-high-aspect multilayer zone plates for even higher x-ray energies

Penetration lengths in the millimetre range make hard x-rays above 60 keV a well-suited tool for non-invasive probing of small specimens buried deep inside their surroundings, and enable studying individual components inside assembled, complex devices (solar cells, batteries etc.). The real-space resolution of typical imaging modalities like fluorescence mapping, scanning SAXS and WAXS depend on the available beam size. Although routine in the 5–25keV regime [1-4], spot sizes below 50nm are very challenging at x-ray energies above 50 keV: Compound refractive lenses lack in refractive power, the multilayer thickness of coated mirrors is bounded by interfacial diffusion, and lithographic Fresnel Zone Plates loose their efficiency in the two-digit keV regime. Multilayer Laue Lenses and Multilayer Zone Plates (MZP) are promising candidates for high-keV focusing to small spot sizes; compared to Fresnel Zone Plates, the aspect ratio comparing outermost layer width (~focal spot size) to optical thickness (efficiency) is virtually unlimited by the fabrication. Using Pulsed Laser Deposition on a rotating wire (several millimetre long), we have fabricated an MZP with 10nm outermost zone widths and optical thickness of 30 μm(optimum phase shift at 60 keV), yielding an unprecedented ultra-high aspect ratio of 1:3000 (outermost zone width compared to optical thickness). We present experimental results obtained at ESRF’s high energy beamline ID31, where for the first time scanning experiments with real-space resolutions below 50nm even at x-ray energies ranging from 60 keV to above 100 keV have been achieved.

Progress on multi-order hard x-ray imaging with multilayer zone plates

Hard x-ray focusing and imaging on the few nano metre scale has gained a lot of attraction in the last couple of years. Thanks to new developments in fabrication and inspection of high-N.A. optics, focusing of hard x-rays has caught up with the focusing performance for soft x-rays. Here we review the latest imaging experiments of the Göttinger Multilayer zone plate collaboration, summarising our route from 1D to 2D lenses for different hard x-ray energies, and recapitulate recent progress on a journey from focusing to imaging.

MZP design and fabrication for efficient hard x-ray nano-focusing and imaging

Efficient focusing optics are a key ingredient for high-resolution (few nanometer) hard x-ray imaging. In recent years, a combined optical scheme using prefocusing to match the coherent fraction of the synchrotron’s x-ray beam to a high-resolution multilayer zone plate (MZP) has been presented. This scheme allows sub-5 nm focusing of hard x-rays in two dimensions. Nevertheless, the first lenses prepared by pulsed laser deposition of alternating WW and Si layers were limited by a low deposition rate of W and the formation of lots of Si-droplets during film growth. Thus, the material combination has been changed to Ta2O5 and ZrO2, allowing a much faster and more accurate layer growth. Here we present latest developments achieved in both design and fabrication of high-resolution MMZPs: A MZP with a lens diameters of about 15 micrometers, sharp layer interfaces, 5 nm outermost zone widths and a focal length of 0.5 mm. Too increase the focusing efficiency even more, a tilted geometry using a pulled glass fibre was successfully implemented.