Role of the illumination spatial-frequency spectrum for ptychography

We demonstrate how the spatial-frequency spectrum of the x-ray illumination affects the reconstruction signal-to-noise ratio and resolution of ptychographic imaging. The spatial-frequency spectrum of a focused x-ray probe is enhanced by partially clipping the beam with an aperture near its focus. This approach presents a simple way of enhancing the illumination spectrum without demanding extra efforts in optics fabrication, and we experimentally demonstrate that it provides an improvement in image quality and resolution.

Figure 1

Experimental setup allowing three illuminations: (a) pinhole alone, (b) FZP focused beam, and (c) FZP focus with a pinhole perturbation.

Figure 2

(a) Complex-valued representation of the reconstructed pinhole illumination at the plane of the object where brightness and hue represent amplitude and phase, respectively. (b) Pinhole diffraction pattern without an object on a base-10 logarithmic scale with the exposure time and total photon count indicated in the figure. (c) $3\times 3$ $\mu$m${}^2$ inset of the phase of the reconstruction using the pinhole illumination. Exposure time per point and total photon density for this measurement are indicated on the figure. (d)–(f) Same as above but for the FZP illumination. (g)–(h) Same as above but for the FZP illumination with a pinhole partially clipping the beam. Total exposure time, neglecting overhead, for a $5\times 5$ $\mu$m${}^2$ was (c) 332 seconds, (f) and (i) 8.3 seconds.

Figure 3

Solid curves show the FRC of reconstructions of two independent measurements for (a) pinhole illumination, (b) FZP, and (c) FZP with a pinhole clipping and perturbing the beam. Dashed curves show the 1/2 bit threshold curve (Ref. 21) with the estimated half-period resolution given by the intersection with the FRC curves.