Ultrafast Coherent Diffractive Imaging at FLASH

Abstract

Using the FLASH facility we have demonstrated high-resolution coherent diffractive imaging with single soft-X-ray free-electron laser pulses [1]. The intense focused FEL pulse gives a high resolution low-noise coherent diffraction pattern of an object before that object turns into a plasma and explodes. Our experiments are an important milestone in the development of single-particle diffractive imaging with future X-ray free-electron lasers [2, 3]. Our apparatus provides a new and unique tool at FLASH to perform imaging of biological specimens beyond conventional radiation damage resolution limits [2, 4] and to acquire images of ultrafast processes initiated by an FEL pulse or other laser pulse. Coherent diffractive imaging is an ideal method for high-resolution ultrafast imaging with an FEL. Since no optical element is required, the method can in principle be scaled to atomic resolution with short enough wavelength. Spatial and temporal coherence are necessary to ensure that the scattered light waves from all positions across the sample are correlated when they interfere at the detector, giving rise to a coherent diffraction pattern that can be phased and inverted to give a high-resolution image of the sample. In contrast to crystals, where scattering from the many unit cells constructively interfere to givemore » Bragg spots, the coherent diffraction pattern of a non-periodic object is continuous. Such a coherent diffraction pattern contains as much as twice the information content of the pattern of its crystallized periodic counterpart--exactly the amount of information needed to solve the phase problem and deterministically invert the pattern to yield an image of the object [5, 6]. The computer algorithm that performs this function replaces the analogue computations of a lens: summing the complex-valued amplitudes of scattered waves to form an image at a particular plane. Our experimental geometry is shown in Fig. 1. We focus a coherent X-ray pulse from the FLASH source onto the sample and record the far-field diffraction pattern of the object on an area detector (a direct-detection CCD chip) centered on the forward direction. The CCD is protected from destruction by the intense forward scattered beam by a mirror that reflects only the diffracted light onto the detector; the direct beam harmlessly passes through a hole in the mirror. The mirror is coated with a resonant X-ray multilayer coating. We fabricated the coating so that the layer period varies across the mirror in such a way that only in-band X-rays propagating from near the sample interaction point are efficiently reflected. In this way the mirror is a very effective filter that rejects noise such as broadband emission from the sample (e.g. when it turns into a plasma and explodes) and off-axis stray light from scattering and emission of beam line components. This arrangement was crucial here to record clean single-pulse diffraction patterns that could be phased and inverted.« less