Dual Coaxial Longitudinal Polarization Vortex Structures.

Carrying orbital angular momentum per photon, the optical vortex has elicited widespread interest. Here, we demonstrate that dual coaxial longitudinal polarization vortices can appear upon a nonparaxial propagation of a tightly focused Pancharatnam-Berry tailored Laguerre-Gaussian beam. Most importantly, it is capable of accessing arbitrary independent topological charges for both vortices, as well as predesigned tunable spacing distances between them.

Extending the detection range of optical vortices by Dammann vortex gratings.

We report a 2D static binary phase Dammann vortex grating that combines the features of a conventional vortex grating and a Dammann grating. This grating uniformly distributes energies among the diffraction orders, so the low-efficiency problem at higher diffraction orders of conventional vortex gratings is resolved and the detection range of the optical vortices (OVs) is greatly increased. Experimental results of OV detection using a fabricated 5×5 Dammann vortex grating are given, and the topological charge detection range from -12 to +12 is achieved. The potential applications of such gratings include transmitting, receiving, and multiplexing OV beams in optical communication systems.

Achromatic design for the generation of optical vortices based on radial spiral phase plates.

A multi-element design scheme is proposed to produce optical vortices of large spectrum width. The key component within the approach is a radially modulated spiral phase plate. Apart from a conventional spiral phase plate having an azimuthal phase function, the proposed element possesses an additional change of phase in the radial direction.

Analysis of surface plasmon interference pattern formed by optical vortex beams.

Following our recent experimental approach to excitation of surface plasmon polaritons induced by optical vortex beams [5], we report further analysis and verification of the surface plasmon interference pattern formed by locally excited standing surface plasmon polaritons in a metal/dielectric film. Our simulation model can be demonstrated by using angular spectrum representation. The generated standing interference pattern has potential as a resolution enhancement technique for sub-diffraction imaging.

Residue orbital angular momentum in interferenced double vortex beams with unequal topological charges.

When two vortex beams with unequal topological charges superpose coherently, orbital angular momentum (OAM) in the two beams would not be cancelled out completely in the interference. The residual OAMs contained by the superposed beam are located at different concentric rings and may have opposite orientations owing to the difference of the charges. The residual OAM can be confirmed by the rotation of microparticles when difference between the charges of two interfering beams is large.

An X-ray study of the cytoplasmic membranes of two gram-positive bacteria.

X-ray diffraction diagrams of partially disordered one-dimensional lattices of isolated bacterial cytoplasmic membranes are described and they provide a basis for suggesting possible molecular structures of bacterial membranes. Biochemical and electron microscope evidence points towards a lipid bilayer with a high degree of fluidity. The protein molecules are in a disordered configuration in the membrane.

Elemental classification in multi-detector stem images using image analysis clustering techniques.

The multi-detector STEM images form a multi-dimensional measurement space where picture elements of morphologically distinct regions cluster and can be separated for classification using image processing clustering techniques. These images are highly correlated and improve the classification only at the high cost of adding dimensionality. Data reduction techniques which take the class separation into account can be used to compress the useful information carried by these images into a few components to which clustering techniques can be successfully applied. At high magnification, a slight displacement is sometimes observed, which can slightly impair the classification result. The redundancy of the information in the quadrant images suggests their use for phase retrieval - which, added to an energy loss data channel, may greatly improve the classification.

Combined microscope for scanning X-ray transmission and surface topography.

A brief description is given of a scanning transmission X-ray microscope (STXM) for use with synchrotron radiation allowing simultaneous X-ray imaging and imaging of surface topography. Surface topography, appropriate to both conducting and non-conducting samples, is sensed by successive "specimen hopping" i.e. planting of the specimen at and withdrawal from a fixed sharp tip, or stylus, formed by the scanning tip of an atomic force microscope (AFM). First experiments are described with 3 keV photons with zone plate resolution of about 100 nm. Preliminary work is also mentioned to add a collimator to improve spatial resolution (in principle to < 20 nm for 300-500 eV photons), by installing a pierced AFM tip at the zone plate focus. The topographic measurement, currently with specimen thickness resolution of about 10 nm, but potential for approximately 1 nm, and feature-dependent in-specimen plane resolution of about 100 nm as limited by the stylus geometry, is an adjunct to the STXM results, to provide information helpful in the interpretation of the X-ray image. Examples are given for simple specimens of the gains in specimen understanding from simultaneous STXM and hopping contact topographic images. Typical applications areas are radiation damage, and dynamic processes like specimen corrosion.

Optical and digital image processing in high-resolution electron microscopy.

A comparison is made of the deblurring by digital and optical processing of a single conventional bright field electron image of ferritin particles distributed on a carbon supporting film. The deblurring functions include the effects of the partial temporal and partial spatial coherence of the electron beam, the phase contrast transfer function of the objective lens and Weiner signal to noise optimisation. Practical aspects of the similar results achieved by the two methods are discussed. The relative advantages and disadvantages of the batch and interactive computer modes for picture processing are considered in an Appendix.

Selective generation of high-order optical vortices from a single phase wedge.

We demonstrate that it is possible to generate high-order optical vortices from a single phase wedge by applying an incident beam with an annular intensity distribution. Various topological charges of optical vortices are realized by a static phase wedge when the position and radius of the annular illumination are changed accordingly.

Generating radial or azimuthal polarization by axial sampling of circularly polarized vortex beams.

A laser beam with circular polarization can be converted into either radial or azimuthal polarization by a microfabricated spiral phase plate and a radial (or azimuthal)-type linear analyzer. The resulting polarization is axially symmetric and is able to produce tightly focused light fields beyond the diffraction limit. We describe in detail the theory behind the technique and the experimental verification of the polarization both in the far field and at the focus of a high numerical aperture lens. Vector properties of the beam under strong focusing conditions were observed by comparing the fluorescence images corresponding to the focal intensity distribution for both radial and azimuthal polarizations. The technique discussed here may easily be implemented to a wide range of optical instruments and devices that require the use of tightly focused light beams.

Multiplexing free-space optical signals using superimposed collinear orbital angular momentum states.

As a proof of concept, we experimentally demonstrate multiplexing of free-space optical signals in multiple channels labeled with different states of orbital angular momentum. The multiplexing process is carried out by a dynamic liquid-crystal spatial light modulator, while the phase function is calculated by an iterative algorithm. A binary amplitude computer-generated hologram serves as a demultiplexer.

Generalized model for orbital angular momentum states generated by parallel aligned phase wedges.

A generalized model of a double-wedge phase element containing two side-by-side parallel phase ramps with arbitrary slope angles is proposed. We study the diffracted optical field of a double-wedge element in terms of orbital angular momentum states. Analytical calculations reveal how the radius and position of the illuminating laser beam affect the composition of orbital angular momentum generated by the phase wedges.