28 research outputs found
Efficient and flexible approach to ptychography using an optimization framework based on automatic differentiation
Ptychography is a lensless imaging method that allows for wavefront sensing
and phase-sensitive microscopy from a set of diffraction patterns. Recently, it
has been shown that the optimization task in ptychography can be achieved via
automatic differentiation (AD). Here, we propose an open-access AD-based
framework implemented with TensorFlow, a popular machine learning library.
Using simulations, we show that our AD-based framework performs comparably to a
state-of-the-art implementation of the momentum-accelerated ptychographic
iterative engine (mPIE) in terms of reconstruction speed and quality. AD-based
approaches provide great flexibility, as we demonstrate by setting the
reconstruction distance as a trainable parameter. Lastly, we experimentally
demonstrate that our framework faithfully reconstructs a biological specimen
Spatial coherence control and analysis via micromirror-based mixed-state ptychography
Flexible and fast control of the phase and amplitude of coherent light,
enabled by digital micromirror devices (DMDs) and spatial light modulators
(SLMs), has been a driving force for recent advances in optical tweezers,
nonlinear microscopy, and wavefront shaping. In contrast, engineering spatially
partially coherent light remains widely elusive due to the lack of tools
enabling a joint analysis and control sequence. Here, we report an approach to
coherence engineering that combines a quasi-monochromatic, thermal source and a
DMD together with a ptychographic scanning microscope. The reported method
opens up new routes to low-cost coherence control, with applications in
micromanipulation, nanophotonics, and quantitative phase contrast imaging
Addressing phase-curvature in Fourier ptychography
In Fourier ptychography, multiple low resolution images are captured and subsequently combined computationally into a high-resolution, large-field of view micrograph. A theoretical image-formation model based on the assumption of plane-wave illumination from various directions is commonly used, to stitch together the captured information into a high synthetic aperture. The underlying far-field (Fraunhofer) diffraction assumption connects the source, sample, and pupil planes by Fourier transforms. While computationally simple, this assumption neglects phase-curvature due to non-planar illumination from point sources as well as phase-curvature from finite-conjugate microscopes (e.g., using a single-lens for image-formation). We describe a simple, efficient, and accurate extension of Fourier ptychography by embedding the effect of phase-curvature into the underlying forward model. With the improved forward model proposed here, quantitative phase reconstruction is possible even for wide fields-of-views and without the need of image segmentation. Lastly, the proposed method is computationally efficient, requiring only two multiplications: prior and following the reconstruction
Broadband ptychography using curved wavefront illumination
We examine the interplay between spectral bandwidth and illumination
curvature in ptychography. By tailoring the divergence of the illumination,
broader spectral bandwidths can be tolerated without requiring algorithmic
modifications to the forward model. In particular, a strong wavefront curvature
transitions a far-field diffreaction geometry to an effectively near-field one,
which is lees affected by temporal coherence effects. The relaxed temporal
coherence requirements allow for leveraging wider spectral bandwidths and
larger illumination spots. Our findings open up new avenues towards utilizing
pink and broadband beams for increased flux and throughput at both synchrotron
facilities and lab-scale beamlines
Fourier ptychography: current applications and future promises
Traditional imaging systems exhibit a well-known trade-off between the resolution and the field of view of their captured images. Typical cameras and microscopes can either “zoom in” and image at high-resolution, or they can “zoom out” to see a larger area at lower resolution, but can rarely achieve both effects simultaneously. In this review, we present details about a relatively new procedure termed Fourier ptychography (FP), which addresses the above trade-off to produce gigapixel-scale images without requiring any moving parts. To accomplish this, FP captures multiple low-resolution, large field-of-view images and computationally combines them in the Fourier domain into a high-resolution, large field-of-view result. Here, we present details about the various implementations of FP and highlight its demonstrated advantages to date, such as aberration recovery, phase imaging, and 3D tomographic reconstruction, to name a few. After providing some basics about FP, we list important details for successful experimental implementation, discuss its relationship with other computational imaging techniques, and point to the latest advances in the field while highlighting persisting challenges
Visualizing the ultra-structure of microorganisms using table-top extreme ultraviolet imaging
Table-top extreme ultraviolet (EUV) microscopy offers unique opportunities
for label-free investigation of biological samples. Here, we demonstrate
ptychographic EUV imaging of two dried, unstained model specimens: germlings of
a fungus (Aspergillus nidulans), and bacteria (Escherichia coli) cells at 13.5
nm wavelength. We find that the EUV spectral region, which to date has not
received much attention for biological imaging, offers sufficient penetration
depths for the identification of intracellular features. By implementing a
position-correlated ptychography approach, we demonstrate a millimeter-squared
field of view enabled by infrared illumination combined with sub-60 nm spatial
resolution achieved with EUV illumination on selected regions of interest. The
strong element contrast at 13.5 nm wavelength enables the identification of the
nanoscale material composition inside the specimens. Our work will advance and
facilitate EUV imaging applications and enable further possibilities in life
science
Advances in laboratory-scale ptychography using high harmonic sources [Invited]
Extreme ultraviolet microscopy and wavefront sensing are key elements for nextgeneration ultrafast applications, such as chemically-resolved imaging, focal spot diagnostics in pump-and-probe experiments, and actinic metrology for the state-of-the-art lithography node at 13.5 nm wavelength. Ptychography offers a robust solution to the aforementioned challenges. Originally adapted by the electron and synchrotron communities, advances in the stability and brightness of high-harmonic tabletop sources have enabled the transfer of ptychography to the laboratory. This review covers the state of the art in tabletop ptychography with high harmonic generation sources. We consider hardware options such as illumination optics and detector concepts as well as algorithmic aspects in the analysis of multispectral ptychography data. Finally, we review technological application cases such as multispectral wavefront sensing, attosecond pulse characterization, and depth-resolved imaging
Ptychography-based characterization of wavelength-tunable vortex beams
We demonstrate monochromatic ptychographic reconstructions of vortex beams within a range of 0.2λ0. Modal decomposition of the vortices reveal that the pu-rity of the dominant LG01 mode exceeds 85% for the full investigated bandwidth
Addressing phase-curvature in Fourier ptychography
In Fourier ptychography, multiple low resolution images are captured and subsequently combined computationally into a high-resolution, large-field of view micrograph. A theoretical image-formation model based on the assumption of plane-wave illumination from various directions is commonly used, to stitch together the captured information into a high synthetic aperture. The underlying far-field (Fraunhofer) diffraction assumption connects the source, sample, and pupil planes by Fourier transforms. While computationally simple, this assumption neglects phase-curvature due to non-planar illumination from point sources as well as phase-curvature from finite-conjugate microscopes (e.g., using a single-lens for image-formation). We describe a simple, efficient, and accurate extension of Fourier ptychography by embedding the effect of phase-curvature into the underlying forward model. With the improved forward model proposed here, quantitative phase reconstruction is possible even for wide fields-of-views and without the need of image segmentation. Lastly, the proposed method is computationally efficient, requiring only two multiplications: prior and following the reconstruction