High-resolution diffraction microscopy using the plane-wave field of a nearly diffraction limited focused x-ray beam

X-ray waves in the center of the beam waist of nearly diffraction limited focused x-ray beams can be considered to have amplitude and phase that are both almost uniform, i.e., they are x-ray plane waves. Here we report the results of an experimental demonstration of high-resolution diffraction microscopy using the x-ray plane wave of the synchrotron x-ray beam focused using Kirkpatrik-Baez mirrors. A silver nanocube with an edge length of $\sim 100\text{nm}$ is illuminated with the x-ray beam focused to a $\sim 1\mu \text{m}$ spot at 12 keV. A high-contrast symmetric diffraction pattern of the nanocube is observed in the forward far field. An image of the nanocube is successfully reconstructed by an iterative phasing method and its half-period resolution is 3.0 nm. This method does not only dramatically improve the spatial resolution of x-ray microscopy but also is a key technology for realizing single-pulse diffractive imaging using x-ray free-electron lasers.

Figure 1

(Color) (a) Experimental setup for coherent x-ray diffraction pattern measurements of silver nanocube using x rays focused by KB mirrors. The nanocube was placed at the center of the beam waist. (b) Intensity distribution profile along vertical and horizontal directions of the focused beam. The profile was derived by differentiating the x-ray absorption distribution of the gold wires at 250 nm intervals. (c) SEM image of silver nanocubes.

Figure 2

(Color) (a) Coherent diffraction pattern of silver nanocube in $1251\times 1251\text{pixels}$. $\mathit{q}$ is defined as $\left|\mathit{q}\right|=2\text{sin}\left(\Theta /2\right)/\lambda$, where $\Theta$ is the scattering angle and $\lambda$ is the x-ray wavelength. (b) Expanded image of the area inside the square in (a). (c) $q$ dependence, along the white line indicated in (a), of photon numbers detected at one pixel of the CCD detector for the high-$q$ diffraction measurement.

Figure 3

(a) Five projection images of the silver nanocube reconstructed from the diffraction pattern in Fig. 2a, which were started from different electron-density maps. (b) Final image derived by averaging the five images of (a) in which a line scan through an edge shown in the inset demonstrates the resolution of $\sim 3\text{nm}$. The reconstructed images are normalized by the maximum value of image intensities and are displayed in gray scale. (c) PRTF for the reconstructed image of (b). The half-period resolution of the image of (b) is 3.0 nm, where the PRTF drops to a value of $1/e$. (d) SEM image of the silver nanocube.