Andrew Maiden

An improved ptychographical phase retrieval algorithm for diffractive imaging

The ptychographical iterative engine (or PIE) is a recently developed phase retrieval algorithm that employs a series of diffraction patterns recorded as a known illumination function is translated to a set of overlapping positions relative to a target sample. The technique has been demonstrated successfully at optical and X-ray wavelengths and has been shown to be robust to detector noise and to converge considerably faster than support-based phase retrieval methods. In this paper, the PIE is extended so that the requirement for an accurate model of the illumination function is removed.

Ptychographic electron microscopy using high-angle dark-field scattering for sub-nanometre resolution imaging

Diffractive imaging, in which image-forming optics are replaced by an inverse computation using scattered intensity data, could, in principle, realize wavelength-scale resolution in a transmission electron microscope. However, to date all implementations of this approach have suffered from various experimental restrictions. Here we demonstrate a form of diffractive imaging that unshackles the image formation process from the constraints of electron optics, improving resolution over that of the lens used by a factor of five and showing for the first time that it is possible to recover the complex exit wave (in modulus and phase) at atomic resolution, over an unlimited field of view, using low-energy (30 keV) electrons. Our method, called electron ptychography, has no fundamental experimental boundaries: further development of this proof-of-principle could revolutionize sub-atomic scale transmission imaging.

Superresolution imaging via ptychography

Coherent diffractive imaging of objects is made considerably more practicable by using ptychography, where a set of diffraction patterns replaces a single measurement and introduces a high degree of redundancy into the recorded data. Here we demonstrate that this redundancy allows diffraction patterns to be extrapolated beyond the aperture of the recording device, leading to superresolved images, improving the limit on the finest feature separation by more than a factor of 3.

Optical ptychography: a practical implementation with useful resolution

Quantitative phase microscopy offers a range of benefits over conventional phase-contrast techniques. For example, changes in refractive index and specimen thickness can be extrapolated and images can be refocused subsequent to their recording. In this Letter, we detail a lensless, quantitative phase microscope with a wide field of view and a useful resolution. The microscope uses the recently reported coherent diffractive imaging technique of ptychography to generate images from recorded diffraction patterns.

Ptychographic transmission microscopy in three dimensions using a multi-slice approach

Generally, methods of three-dimensional imaging such as confocal microscopy and computed tomography rely on two essentials: multiple measurements (at a range of focus positions or rotations) and a weakly scattering specimen (to avoid distortion of the focal spot in the confocal microscope or to satisfy the projection approximation in tomography). Here we show that an alternative form of multi-measurement imaging, ptychography, can be extended to three dimensions and can successfully recover images in the presence of multiple scattering and when the projection approximation is not applicable. We demonstrate our technique experimentally using visible light, where it has applications in imaging thick samples such as biological tissues; however the results also have important implications for x ray and electron imaging.

Soft X-ray spectromicroscopy using ptychography with randomly phased illumination

Ptychography is a form of scanning diffractive imaging that can successfully retrieve the modulus and phase of both the sample transmission function and the illuminating probe. An experimental difficulty commonly encountered in diffractive imaging is the large dynamic range of the diffraction data. Here we report a novel ptychographic experiment using a randomly phased X-ray probe to considerably reduce the dynamic range of the recorded diffraction patterns. Images can be reconstructed reliably and robustly from this setup, even when scatter from the specimen is weak. A series of ptychographic reconstructions at X-ray energies around the L absorption edge of iron demonstrates the advantages of this method for soft X-ray spectromicroscopy, which can readily provide chemical sensitivity without the need for optical refocusing. In particular, the phase signal is in perfect registration with the modulus signal and provides complementary information that can be more sensitive to changes in the local chemical environment.

Ptychographic microscope for three-dimensional imaging

Ptychography is a coherent imaging technique that enables an image of a specimen to be generated from a set of diffraction patterns. One limitation of the technique is the assumption of a multiplicative interaction between the illuminating coherent beam and the specimen, which restricts ptychography to samples no thicker than a few tens of micrometers in the case of visible-light imaging at micron-scale resolution. By splitting a sample into axial sections, we demonstrated in recent work that this thickness restriction can be relaxed and whats-more, that coarse optical sectioning can be realized using a single ptychographic data set. Here we apply our technique to data collected from a modified optical microscope to realize a reduction in the optical sectioning depth to 2 μm in the axial direction for samples up to 150 μm thick. Furthermore, we increase the number of sections that are imaged from 5 in our previous work to 34 here. Our results compare well with sectioned images collected from a confocal microscope but have the added advantage of strong phase contrast, which removes the need for sample staining.

Extended ptychography in the transmission electron microscope: Possibilities and limitations

The extended-ptychographical iterative engine (e-PIE) is a recently developed powerful phase retrieval algorithm which can be used to measure the phase transfer function of a specimen and overcome conventional lens resolution limits. The major improvement over PIE is the ability to reconstruct simultaneously both the object and illumination functions, robustness to noise and speed of convergence. The technique has proven to be successful at optical and X-ray wavelengths and we describe here experimental results in transmission electron microscopy supported by corresponding simulations. These simulations show the possibilities - even with strong phase objects - and limitations of ptychography; in particular issues arising from poorly-defined probe positions.

Quantitative electron phase imaging with high sensitivity and an unlimited field of view.

As it passes through a sample, an electron beam scatters, producing an exit wavefront rich in information. A range of material properties, from electric and magnetic field strengths to specimen thickness, strain maps and mean inner potentials, can be extrapolated from its phase and mapped at the nanoscale. Unfortunately, the phase signal is not straightforward to obtain. It is most commonly measured using off-axis electron holography, but this is experimentally challenging, places constraints on the sample and has a limited field of view. Here we report an alternative method that avoids these limitations and is easily implemented on an unmodified transmission electron microscope (TEM) operating in the familiar selected area diffraction mode. We use ptychography, an imaging technique popular amongst the X-ray microscopy community; recent advances in reconstruction algorithms now reveal its potential as a tool for highly sensitive, quantitative electron phase imaging.

Nonplanar photolithography with computer-generated holograms.

We outline a method for accomplishing photolithography on grossly nonplanar substrates. First we compute an approximation of the diffraction pattern that will produce the desired light-intensity distribution on the substrate to be patterned. This pattern is then digitized and converted into a format suitable for manufacture by a direct-write method. The resultant computer-generated hologram mask is then used in a custom alignment tool to expose the photoresist-coated substrate. The technique has many potential applications in the packaging of microelectronics and microelectromechanical systems.