Hard-x-ray lensless imaging of extended objects.

We demonstrate a hard-x-ray microscope that does not use a lens and is not limited to a small field of view or an object of finite size. The method does not suffer any of the physical constraints, convergence problems, or defocus ambiguities that often arise in conventional phase-retrieval diffractive imaging techniques. Calculation times are about a thousand times shorter than in current iterative algorithms. We need no a priori knowledge about the object, which can be a transmission function with both modulus and phase components. The technique has revolutionary implications for x-ray imaging of all classes of specimen.

Transmission microscopy without lenses for objects of unlimited size.

We demonstrate experimentally, for the first time, a new form of lensless microscopy. The image we obtain contains the entire wavefunction emanating from the sample. Large scale, quantitative phase information can be measured, unlike in conventional (Zernike) methods. For light optical experiments, we can dispense with expensive high-quality lenses and the very large working distances available would allow remote monitoring of e.g., environmental cells without compromising resolution. In short wavelength microscopy (X-rays and electrons), where lens components are of very limited numerical aperture, the technique has revolutionary implications: objects of any lateral size or shape can be used and, for transmission electron imaging, resolution down to the scale of the wavelength is likely to be limited only by the presence of atomic vibrations.

Nature of the Stranski-Krastanow transition during epitaxy of InGaAs on GaAs.

We report first quantitative measurements by energy-selected imaging in a transmission electron microscope of In segregation within an uncapped islanded In0.25Ga0.75As layer grown epitaxially on GaAs. This layer has the lowest In concentration at which islanding occurs and, then, only after a flat approximately 3 nm alloy layer has been formed. In buildup by segregation at the surface of this initial flat layer is considered the driving force for islanding and, importantly, the segregation process introduces the characteristic delay seen before the Stranski-Krastanow transition. We observe strong inhomogeneous In enrichment within the islands (up to x(In) approximately 0.6 at the apex) and a simultaneous In depletion in the remaining flat layer.

Inverted electron-hole alignment in InAs-GaAs self-assembled quantum dots.

New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field.

Molecular beam deposition of optical coatings and their characterization.

Significant improvements can be made in the fabrication of optical thin film structures by using molecular beam and ultrahigh vacuum techniques. These lead to the achievement of more stable films and multilayer coatings with improved morphology, density, and resistance to laser-induced damage. The microstructure of the film can be controlled to a high degree by using quasisuperlattice techniques, which also provide a means of refractive index synthesis. This can be applied to simple graded structures or complex periodic gradings as required for Bragglike structures. Etalon filters fabricated using the technique have been used for optical bistability experiments and have exhibited stable operation for periods of many hours under continuous cycling.

Topographical contrast in the transmission electron microscope.

An adaptation of the Foucault method for topographical imaging in the transmission electron microscope is described in detail. The image contrast is produced by selection of electrons which have suffered differential phase retardations in the specimen inner potential. Surface or interface displacements produce bright or dark image contrast, and the ultimate resolution approaches that of the atomic scale. The imaging method is applied in studies of both amorphous and crystalline objects. The possibility of performing quantitative measurements is demonstrated by the estimation of the inner potential of crystalline MgO.