Quantitative phase microscopy of thick objects

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Microscopes have always had trouble imaging thick specimens. The problem is that when the microscopist tries to focus on a plane deep within such objects, the image is swamped by out-of-focus background generated by the rest of the sample - and the higher the resolution required the worse this problem gets. The simplest solution is to physically slice the specimen very thinly, but this is time consuming and disruptive, destroying the sample's native state. The aim of this research is to investigate computational methods for achieving the same slicing effect, without the need to physically alter the specimen. We hypothesise that this can be achieved using a relatively new technique called ptychography, in combination with the sample rotation idea and algorithms from tomography. By unravelling the complex, multiple-scattering interaction of an illumination source with a thick specimen, we hope to stretch the 3D imaging capabilities of this hybrid approach beyond the limits imposed upon the conventional forms of these two techniques. Potential applications for the research stretch across the spectrum of microscope modalities. At infrared wavelengths detecting small defects in semiconductor wafers is an exciting application, currently beyond the reach of existing light-based inspection devices - the end result would be improved understanding of the lithographic process and better yields. For visible light, a primary example is the imaging of thick tissue specimens, a long-standing bugbear for biologists, who would benefit greatly from the ability to perform stain-free in vivo imaging at micron-scale resolutions. At the X-ray end of the spectrum, applications include the imaging of the lacuna-canalicular network of bone fragments and soft X-ray whole cell imaging, where both accuracy and achievable resolution will be greatly improved if the thickness limit inherent to the current imaging methods can be surpassed. For electrons, our research could improve the imaging of key material properties of semiconductors, such as doping concentrations and magnetic fields, measurement of which is vital to the development of future electronic devices.