Coherent Diffractive Imaging Research Papers (original) (raw)

Digital holographic microscopy based on Gabor in-line holography is a well-known method to reconstruct both the amplitude and phase of small objects. To reconstruct the image of an object from its hologram, obtained under illumination by monochromatic scalar waves, numerical calculations of Fresnel integrals are required. To improve spatial resolution in the resulting reconstruction, we resample the holographic data before application of the reconstruction algorithm. This procedure amounts to inverting an interpolated Fresnel diffraction image to recover the object. The advantage of this method is demonstrated on experimental data, for the case of visible-light Gabor holography of a resolution grid and a gnat wing.

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- Digital Holography, Coherent Diffractive Imaging

Lensless imaging is a high potential and currently intensely targeted research goal, in view of those fields of applications for which aberrationfree high-resolution lenses are not available, for example for x-ray imaging. A recently proposed (direct inversion) variant of lensless imaging combines the advantages of two classical routes toward lensless imaging, the high-resolution characteristics of iterative object reconstruction, and the direct and deterministic nature of holographic reconstruction. Here, we use a simple standard optical setup using visible wavelength, a lithographic test object and a phase-shifting reference object to demonstrate the approach. Importantly, we show that a phaseshifting reference object, instead of the absorption mask proposed earlier, is sufficient for object reconstruction. This is relevant in view of the much easier implementation in future x-ray applications.

- by Sergey Podorov
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- Digital Holography, Optical Imaging, Coherent Diffractive Imaging

In-line holography of particle fields suffers from image deterioration caused by intrinsic speckle noise. This is not accounted for by previous theoretical treatments based on the scattering of a single particle, nor has there been any quantitative description of this noise except for an empirical criterion by Royer [Nouv. Rev. Opt. 5, 87 (1974)] based on geometrical obscuration. We develop a theoretical model for in-line holography of multiple particles, using diffraction theory and statistical analysis, and show that the virtual image of the particle ensemble is the dominant source of speckle in reconstruction. We quantify the effect of speckle with a signal-to-noise ratio (SNR). The SNR is found to depend on a speckle parameter (which embodies particle diameter, concentration, and sample depth) and on the film gamma. Experimental results show reasonably good agreement with our model. The SNR equation provides prediction of image quality and thence application limits of in-line holography for particle fields. The fundamental understanding obtained here points not only to constraints but also to possible improvements in experimental procedures.

Ultrafast electron diffractive imaging of nanoscale objects such as biological molecules 1,2 and defects in solid-state devices 3 provides crucial information on structure and dynamic processes: for example, determination of the form and function of membrane proteins, vital for many key goals in modern biological science, including rational drug design 4 . High brightness and high coherence are required to achieve the necessary spatial and temporal resolution, but have been limited by the thermal nature of conventional electron sources and by divergence due to repulsive interactions between the electrons, known as the Coulomb explosion. It has been shown that, if the electrons are shaped into ellipsoidal bunches with uniform density 5 , the Coulomb explosion can be reversed using conventional optics, to deliver the maximum possible brightness at the target 6,7 . Here we demonstrate arbitrary and real-time control of the shape of cold electron bunches extracted from laser-cooled atoms. The ability to dynamically shape the electron source itself and to observe this shape in the propagated electron bunch provides a remarkable experimental demonstration of the intrinsically high spatial coherence of a cold-atom electron source, and the potential for alleviation of electron-source brightness limitations due to Coulomb explosion 6 .