High-resolution 3D phase imaging using a partitioned detection aperture: a wave-optic analysis.

Quantitative phase imaging has become a topic of considerable interest in the microscopy community. We have recently described one such technique based on the use of a partitioned detection aperture, which can be operated in a single shot with an extended source [Opt. Lett.37, 4062 (2012)OPLEDP0146-959210.1364/OL.37.004062]. We follow up on this work by providing a rigorous theory of our technique using paraxial wave optics, where we derive fully 3D spread functions for both phase and intensity. Using these functions, we discuss methods of phase reconstruction for in- and out-of-focus samples, insensitive to weak attenuations of light. Our approach provides a strategy for detection-limited lateral resolution with an extended depth of field and is applicable to imaging smooth and rough samples.

Controlling neutron orbital angular momentum.

The quantized orbital angular momentum (OAM) of photons offers an additional degree of freedom and topological protection from noise. Photonic OAM states have therefore been exploited in various applications ranging from studies of quantum entanglement and quantum information science to imaging. The OAM states of electron beams have been shown to be similarly useful, for example in rotating nanoparticles and determining the chirality of crystals. However, although neutrons--as massive, penetrating and neutral particles--are important in materials characterization, quantum information and studies of the foundations of quantum mechanics, OAM control of neutrons has yet to be achieved. Here, we demonstrate OAM control of neutrons using macroscopic spiral phase plates that apply a 'twist' to an input neutron beam. The twisted neutron beams are analysed with neutron interferometry. Our techniques, applied to spatially incoherent beams, demonstrate both the addition of quantum angular momenta along the direction of propagation, effected by multiple spiral phase plates, and the conservation of topological charge with respect to uniform phase fluctuations. Neutron-based studies of quantum information science, the foundations of quantum mechanics, and scattering and imaging of magnetic, superconducting and chiral materials have until now been limited to three degrees of freedom: spin, path and energy. The optimization of OAM control, leading to well defined values of OAM, would provide an additional quantized degree of freedom for such studies.

High-throughput imaging of self-luminous objects through a single optical fibre.

Imaging through a single optical fibre offers attractive possibilities in many applications such as micro-endoscopy or remote sensing. However, the direct transmission of an image through an optical fibre is difficult because spatial information is scrambled upon propagation. We demonstrate an image transmission strategy where spatial information is first converted to spectral information. Our strategy is based on a principle of spread-spectrum encoding, borrowed from wireless communications, wherein object pixels are converted into distinct spectral codes that span the full bandwidth of the object spectrum. Image recovery is performed by numerical inversion of the detected spectrum at the fibre output. We provide a simple demonstration of spread-spectrum encoding using Fabry-Perot etalons. Our technique enables the two-dimensional imaging of self-luminous (that is, incoherent) objects with high throughput in principle independent of pixel number. Moreover, it is insensitive to fibre bending, contains no moving parts and opens the possibility of extreme miniaturization.

Single-exposure surface profilometry using partitioned aperture wavefront imaging.

We demonstrate a technique for instantaneous measurements of surface topography based on the combination of a partitioned aperture wavefront imager with a lamp-based reflection microscope using standard objectives. The technique can operate at video rate over large fields of view, and provides nanometer axial resolution and submicrometer lateral resolution. We discuss performance characteristics of this technique, which we experimentally compare with scanning white light interferometry.

Cross-correlated imaging of single-mode photonic crystal rod fiber with distributed mode filtering.

Photonic crystal bandgap fibers employing distributed mode filtering design provide near diffraction-limited light outputs, a critical property of fiber-based high-power lasers. Microstructure of the fibers is tailored to achieve single-mode operation at specific wavelength by resonant mode coupling of higher-order modes. We analyze the modal regimes of the fibers having a mode field diameter of 60 µm by the cross-correlated (C(2)) imaging method in different wavelength ranges and evaluate the sensitivity of the modal content to various input-coupling conditions. As a result, we experimentally identify regimes of resonant coupling between higher-order core modes and cladding band. We demonstrate a passive fiber design in which the higher-order modal content inside the single-mode guiding regime is suppressed by at least 20 dB even for significantly misaligned input-coupling configurations.

Optimal nonlinear passage through a quantum critical point.

We analyze the problem of optimal adiabatic passage through a quantum critical point. We show that to minimize the number of defects the tuning parameter should be changed as a power law in time. The optimal power is proportional to the logarithm of the total passage time multiplied by universal critical exponents characterizing the phase transition. We support our results by the general scaling analysis and by explicit calculations for the transverse-field Ising model.