Coherent X-Ray Diffraction Imaging with Nanofocused Illumination

Coherent x-ray diffraction imaging is an x-ray microscopy technique with the potential of reaching spatial resolutions well beyond the diffraction limits of x-ray microscopes based on optics. However, the available coherent dose at modern x-ray sources is limited, setting practical bounds on the spatial resolution of the technique. By focusing the available coherent flux onto the sample, the spatial resolution can be improved for radiation-hard specimens. A small gold particle (size $<100\mathrm{nm}$) was illuminated with a hard x-ray nanobeam ($E=15.25\mathrm{keV}$, beam dimensions $\approx 100\times 100{\mathrm{nm}}^2$) and is reconstructed from its coherent diffraction pattern. A resolution of about 5 nm is achieved in 600 s exposure time.

Figure 1

(a) Schematic sketch of the coherent diffraction imaging setup with nanofocused illumination. (b) Scanning electron micrograph of gold particles (diameter $\approx 100\mathrm{nm}$) deposited on a ${\mathrm{Si}}_3{\mathrm{N}}_4$ membrane. (c) Diffraction pattern (logarithmic scale) recorded of the single gold particle pointed to by the arrow in (b) and illuminated by a hard x-ray beam with lateral dimensions of about $100\times 100{\mathrm{nm}}^2$. The maximal momentum transfer, both in horizontal and vertical direction, is $q=1.65{\mathrm{nm}}^{-1}$.

Figure 2

(a) Two individual reconstructions of the gold particle using the HIO algorithm, a left- and a right-handed one. To obtain the average particle shape from a series of reconstructions with random initial phases, the right-handed reconstructions were inverted and averaged together with the left-handed ones. (b) Reconstructed projected electron density of the gold nanoparticle shown in Fig. 1b after averaging the series of reconstructions. (c) Horizontal section through the center of the particle shown in (b). The error bars indicate rms variations in the density for the series of independent reconstructions.

Figure 3

Beam size $b_{\mathrm{ampl}}$ of the x-ray amplitude and lateral coherence length $l_t$ as a function of the geometric image size $b_{\mathrm{geo}}$ of the source. All quantities are in units of the diffraction limit $d_t$ (size of Airy disc).