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.

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.

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.

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.

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.

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.

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.

Variable-radius focused optical vortex with suppressed sidelobes

We propose a design for a phase mask for generating an optical vortex with suppressed sidelobes in the focal plane where the radius of the intensity ring is variable. A radial modulation added to conventional phase mask exp(iltheta) projects the light diffracted from different annular zones into a single intensity ring in the focal plane.

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.

Collinear superposition of multiple helical beams generated by a single azimuthally modulated phase-only element

We propose a highly efficient approach to generating multihelix beams that contain more than one helical mode, and the power distribution over helical modes is adjustable. A multihelix beam embedded with three collinear helical modes is demonstrated by use of a spatial light modulator.

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.

The effect of soft X-radiation on myofibrils

Myofibrils, the contractile organelles from striated muscles, have been examined in the X-ray microscope to determine the effect of radiation on their function and structure. Using X-rays of energy 350-385 eV in the water window we find that after an exposure to 7.5 x 10(5) photons/micron2 (calculated to give an absorbed dose of 20,000 Gy) the myofibrils will no longer contract. The use of the free radical scavenging agent, DMSO, gives some protection to the fibrils. It has also been found that after this much irradiation the fibrils lose up to 20% of their mass. Further substantial mass loss occurs on subsequent irradiation. After 25 times the loss-of-function exposure only 30% of the mass remains. Analysis of a series of images of the same myofibril covering this range of exposures shows that the mass is preferentially lost in some areas of the structure and consequently significant structural changes occur.

X-Ray Microscopy at Suboptical Resolution: Direct Observation of the 65-nm Periodicity in Collagen Fibrils

Using a Fresnel zone plate we demonstrate for the first time the direct visualization by x-ray microscopy of suboptical regularity in a biological specimen, namely the 65-nm axial periodicity of tendon collagen. This resolution test demonstrates a resolving power of about 20λ; a resolving power of <10λ is in prospect.

Influence of polarization state and absorption gratings on photorefractive two-wave mixing in GaAs

The polarization dependence of two-wave mixing for two common crystal orientations is described and theoretical and experimental results given. The direction of energy exchange can be determined by the polarization of the two beams. Absorption gratings are proposed to explain fully the experimental results.

X-ray microscopy and X-ray imaging

Within a framework of an overview of the current status and potential of X-ray microscopy, a description is given of the development of the King's College scanning instrument which produced its first images in September, 1986. The instrument was mounted on the newly-built undulator beam line at the UK Science and Engineering Research Council's SRS synchrotron. There are consequently three sites worldwide where high-resolution X-ray microscopes with zone-plate optics are in operation. The other sites are BESSY-Berlin and NSLS-Brookhaven.

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.

Multiple signals in STEM

A brief description is given of the eight image signals that can be simultaneously recorded in digital form using an appropriate data recording system and high resolution STEM; this involves a set of detectors covering bright and dark field. Details are provided of the collection and manipulation of image data for STEM, including on-line estimation of off-focus distance, on-line assessment of astigmatism, on-line signal to noise ratio in any or all detectors; examples of all three procedures are given. The use of the data recording system is discussed together with some aspects of image display, including the use of colour. A method is introduced of providing, using the appropriate hardware, a prescribed colour map (image intensity converted to colour) using replaceable PROMS. Two off-line procedures are considered. One is the Karhunen-Loeve transform and its application to bringing together the information contained within the multiple signals, and the other is the discrete cosine transform by which the storage problem for digital image storage may be reduced, according to the image, by between 15 and 30 times.

Phase assignment to diffraction patterns based on the analytic properties of scattered fields applied to the structure of nerve myelin

An analysis of published X-ray diffraction data from nerve myelin is given based on the properties of analytic functions. Functions defined by a finite Fourier transform may be described by their distribution of zeros. This description allows a phase function to be determined from real data, which is unique in principle. A solution to the phase assignment is given and compared with corresponding published solutions derived by other methods. The strong measure of agreement for the phases of the first nine diffraction orders, and the stability of this agreement against the efforts of experimental error, leads to the conclusion that these phases are probably correct.

Phase retrieval using two intensity measurements in the complex plane

A new method of using an exponential filter to generate an intensity distribution in the complex plane is presented. This intensity, with the intensity on the real axis, allows the phase of the scattered field to be determined from its zero locations. It is shown that knowledge of all zero coordinates may not be required.

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.

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.

Tuberculin activity of mycobacterial fractions obtained by chromatography

Commercial purified protein derivatives (PPD), old tuberculin (OT), the bacillary extract, and the culture filtrate of Mycobacterium tuberculosis H37Ra were submitted to Sephadex G-25 and diethylaminoethyl (DEAE)-cellulose chromatography. The ability of the fractions obtained to elicit delayed dermal hypersensitivity in M. tuberculosis H37Ra-infected guinea pigs was studied. Skin tests with Sephadex fractions in M. tuberculosis H37Ra-infected guinea pigs showed that the tuberculin activity was localized in the first fraction. All other Sephadex fractions were nonessential and nonspecifically irritating. Fractions from chromatography of Sephadex G-25 fraction 1 on DEAE-cellulose columns showed that all but the first were able to elicit delayed hypersensitivity reactions. There was a variability in the capacity to elicit the tuberculin reaction according to the fraction injected and the stage of tuberculous infection in guinea pigs. Compared to the others, the seven lots of commercial PPD were variable in composition and content. They contained both essential and nonessential materials for the tuberculin reaction. Sephadex fraction 1 would appear to be a better tuberculin as it excludes nonessential nonspecifically irritating elements and contains the complement able to elicit the tuberculin reaction. Its methodological simplicity would be economically advantageous.

New focusing reflectometer for measuring optical constants of thin films

A focusing reflectometer for measuring the optical constants of thin films by the (Rø) method is described. The focusing reflectometer has a larger effective aperture than typical planar reflectometers, and, unlike planar reflectometers, the illuminated area of the specimen is independent of the angle of incidence. The intensity of the incident light is not measured so the reflectometer is limited to wavelengths where the reflectivity of the specimen varies rapidly with the angle of incidence. Results are given for evaporated gold films in the energy range 6.0 eV to 12.0 eV.