High-dynamic-range coherent diffractive imaging: ptychography using the mixed-mode pixel array detector

Abstract

Coherent (X-ray) diffractive imaging (CDI) is an increasingly popular form of X-ray microscopy, mainly due to its potential to produce high-resolution images and the lack of an objective lens between the sample and its corresponding imaging detector. One challenge, however, is that very high dynamic range diffraction data must be collected to produce both quantitative and high-resolution images. In this work, hard X-ray ptychographic coherent diffractive imaging has been performed at the P10 beamline of the PETRA III synchrotron to demonstrate the potential of a very wide dynamic range imaging X-ray detector (the Mixed-Mode Pixel Array Detector, or MM-PAD). The detector is capable of single photon detection, detecting fluxes exceeding 1 × 10(8) 8-keV photons pixel(-1) s(-1), and framing at 1 kHz. A ptychographic reconstruction was performed using a peak focal intensity on the order of 1 × 10(10) photons µm(-2) s(-1) within an area of approximately 325 nm × 603 nm. This was done without need of a beam stop and with a very modest attenuation, while `still' images of the empty beam far-field intensity were recorded without any attenuation. The treatment of the detector frames and CDI methodology for reconstruction of non-sensitive detector regions, partially also extending the active detector area, are described.

Keywords: coherent X-ray diffractive imaging; pixel array detectors; ptychography.

Figure 1

Central region of the KB far-field measured without any attenuators for 0.1 s illumination time. The maximum flux per pixel is 2.0 × 10⁸ photons s⁻¹.

Figure 2

(a) Section from a histogram of analog-to-digital units resulting from an empty beam exposure, with many detector pixels illuminated by single photons only. The vertical red line in the histogram plot marks the offset of 4 ADUs that was used to subtract the zero-photon noise. Its peak can be clearly separated from the single-photon peak at 10.8 ADUs. (b) Scattering pattern of the empty beam that was used for plotting the histogram on the left. In this image, all values below the threshold of 4 ADUs (red line in subfigure on the left) have been set to 0, corresponding to the black pixels containing no ADUs.

Figure 3

(a) Two-dimensional intensity distribution, averaged over all scan points, rescaled into photons per pixel, as measured by the MM-PAD during the ptychographic scan. The black areas correspond to non-sensitive regions, i.e. gaps between active sensor areas, invalid pixels, as well as a larger area on the left which is not measured, but which is required to be considered for the high-resolution reconstruction (see main text). (b) Two-dimensional intensity distribution, averaged over all scan points, as obtained from the numerically reconstructed exit wavefields.

Figure 4

(a) Phase reconstruction of the object function (sample) using a detector area of 300 × 300 pixels. Of this area, the leftmost 85 columns have not been measured (see main text and Fig. 3 ▶). For resolution determination, two line scans in the horizontal (x) and vertical (y) direction are indicated by white lines and plotted as phase versus position in (b). Here, the red lines indicate the fitted error functions, with corresponding FWHM values of 22 nm in the horizontal direction and 27 nm in the vertical direction. (c) Phase reconstruction of the object function using the central 120 × 120 pixels exhibiting the same features as (a), but with a resolution limited by the portion of diffraction data used.

Figure 5

Complex wavefield reconstructed from the high-resolution dataset, numerically propagated into the focal plane. Phase is decoded in color, amplitude in brightness, as per the colorwheel on the lower left.

Figure 6

Reconstructed intensity distribution near the focus (a) in the sagittal (xz) and (c) in the meridional (yz) plane, where the optical axis is oriented in the z-direction. (b) and (d) show corresponding line cuts through the intensity in the focal plane.