Phase calibration target for quantitative phase imaging with ptychography.

Quantitative phase imaging (QPI) utilizes refractive index and thickness variations that lead to optical phase shifts. This gives contrast to images of transparent objects. In quantitative biology, phase images are used to accurately segment cells and calculate properties such as dry mass, volume and proliferation rate. The fidelity of the measured phase shifts is of critical importance in this field. However to date, there has been no standardized method for characterizing the performance of phase imaging systems. Consequently, there is an increasing need for protocols to test the performance of phase imaging systems using well-defined phase calibration and resolution targets. In this work, we present a candidate for a standardized phase resolution target, and measurement protocol for the determination of the transfer of spatial frequencies, and sensitivity of a phase imaging system. The target has been carefully designed to contain well-defined depth variations over a broadband range of spatial frequencies. In order to demonstrate the utility of the target, we measure quantitative phase images on a ptychographic microscope, and compare the measured optical phase shifts with Atomic Force Microscopy (AFM) topography maps and surface profile measurements from coherence scanning interferometry. The results show that ptychography has fully quantitative nanometer sensitivity in optical path differences over a broadband range of spatial frequencies for feature sizes ranging from micrometers to hundreds of micrometers.

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.

Coherent optical control of the spin of a single hole in an InAs/GaAs quantum dot.

We demonstrate coherent optical control of a single hole spin confined to an InAs/GaAs quantum dot. A superposition of hole-spin states is created by fast (10-100 ps) dissociation of a spin-polarized electron-hole pair. Full control of the hole spin is achieved by combining coherent rotations about two axes: Larmor precession of the hole spin about an external Voigt geometry magnetic field, and rotation about the optical axis due to the geometric phase shift induced by a picosecond laser pulse resonant with the hole-trion transition.

Phonon-induced Rabi-frequency renormalization of optically driven single InGaAs/GaAs quantum dots.

We study optically driven Rabi rotations of a quantum dot exciton transition between 5 and 50 K, and for pulse areas of up to 14π. In a high driving field regime, the decay of the Rabi rotations is nonmonotonic, and the period decreases with pulse area and increases with temperature. By comparing the experiments to a weak-coupling model of the exciton-phonon interaction, we demonstrate that the observed renormalization of the Rabi frequency is induced by fluctuations in the bath of longitudinal acoustic phonons, an effect that is a phonon analogy of the Lamb shift.