Guidestar-free image-guided wavefront shaping

Optical imaging through scattering media is a fundamental challenge in many applications. Recently, breakthroughs such as imaging through biological tissues and looking around corners have been obtained via wavefront-shaping approaches. However, these require an implanted guidestar for determining the wavefront correction, controlled coherent illumination, and most often raster scanning of the shaped focus. Alternative novel computational approaches that exploit speckle correlations avoid guidestars and wavefront control but are limited to small two-dimensional objects contained within the “memory-effect” correlation range. Here, we present a new concept, image-guided wavefront shaping, allowing widefield noninvasive, guidestar-free, incoherent imaging through highly scattering layers, without illumination control. The wavefront correction is found even for objects that are larger than the memory-effect range, by blindly optimizing image quality metrics. We demonstrate imaging of extended objects through highly scattering layers and multicore fibers, paving the way for noninvasive imaging in various applications, from microscopy to endoscopy.

Review of Fresnel Incoherent Correlation Holography with Linear and Non-Linear Correlations

Fresnel incoherent correlation holography (FINCH) is a well-established incoherent imaging technique. In FINCH, three self-interference holograms are recorded with calculated phase differences between the two interfering, differently modulated object waves and projected into a complex hologram. The object is reconstructed without the twin image and bias terms by a numerical Fresnel back propagation of the complex hologram. A modified approach to implement FINCH by a single camera shot by pre-calibrating the system involving recording of the point spread function library and reconstruction by a non-linear cross-correlation has been introduced recently. The expression of the imaging characteristics from the modulation functions in original FINCH and the modified approach by pre-calibration in spatial and polarization multiplexing schemes are reviewed. The study reveals that a reconstructing function completely independent of the function of the phase mask is required for the faithful expression of the characteristics of the modulating function in the image reconstruction. In polarization multiplexing method by cross-correlation, a partial expression was observed, while in spatial multiplexing method by cross-correlation, the imaging characteristics converged towards a uniform behavior.

Visualization of clay minerals at the atomic scale

This review demonstrates that high-resolution transmission electron microscopy (HRTEM) imaging of clay minerals or phyllosilicates with an incident electron beam along the major zone axes parallel to the constituting layers, in which the contrast corresponds to individual cation columns in the images obtained, is indispensable for elucidating the enigmatic structures of these minerals. Several kinds of variables for layer stacking, including polytypes, stacking disorder and the interstratification of various kinds of unit layers or interlayer materials, are common in phyllosilicates. Local and rigorous determination of such variables is possible only with HRTEM, although examination as to whether the results obtained by the HRTEM images from limited areas represent the whole specimen should be made using other techniques, such as X-ray diffraction. Analysis of these stacking features in clay minerals provides valuable insights into their origin and/or formation processes. Recent state-of-the-art techniques in electron microscopy, including incoherent imaging, superior resolutions of ~0.1 nm and low-dose imaging using new recording media, will also contribute significantly to our understanding of the true structures of clay minerals.