Coherent-Pulse 2D Crystallography Using a Free-Electron Laser X-Ray Source

Coherent diffractive imaging for the reconstruction of a two-dimensional (2D) finite crystal structure with a single pulse train of free-electron laser radiation at 7.97 nm wavelength is demonstrated. This measurement shows an advance on traditional coherent imaging techniques by applying it to a periodic structure. It is also significant that this approach paves the way for the imaging of the class of specimens which readily form 2D, but not three-dimensional crystals. We show that the structure is reconstructed to the detected resolution, given an adequate signal-to-noise ratio.

Figure 1

A sketch of the experiment. The beam from the beam line first interacts with the sample, and then the diffracted radiation propagates to a CCD detector. To enhance the measured resolution of the data the direct beam was incident near the corner of the detector.

Figure 2

(a) Far-field diffraction data measured from a single train of 21 femtosecond pulses from the FEL. (Inset) Enlarged region of diffraction pattern. (b) A symmetrized version of (a) used for reconstruction. The region in black corresponds to missing data covered by the beamstop. The white dot (in both panels) corresponds to the center of the incoming beam.

Figure 3

(a) SIM image of the finite, periodic structure. (b) Support region used for the reconstruction of the diffraction pattern. The large rectangular support (dashed line) was used in the reconstruction shown in (c). The support in the form of 25 rectangular boxes was used in the reconstruction shown in (d). (c) The initial reconstructed image with the original data binned $3\times 3$. (d) The final reconstructed image using the original data binned $5\times 5$. The scale bar is $2\mu \mathrm{m}$ in length.