t. This paper surveys some of the work our group has done in electrical impedance tomography.

Electrical Impedance Tomography

This Review article provides an overview of the fundamental origins and important applications of the main spin–orbit interaction phenomena in modern optics that play a crucial role at subwavelength scales. Light carries both spin and orbital angular momentum. These dynamical properties are determined by the polarization and spatial degrees of freedom of light. Nano-optics, photonics and plasmonics tend to explore subwavelength scales and additional degrees of freedom of structured — that is, spatially inhomogeneous — optical fields. In such fields, spin and orbital properties become strongly coupled with each other. In this Review we cover the fundamental origins and important applications of the main spin–orbit interaction phenomena in optics. These include: spin-Hall effects in inhomogeneous media and at optical interfaces, spin-dependent effects in nonparaxial (focused or scattered) fields, spin-controlled shaping of light using anisotropic structured interfaces (metasurfaces) and robust spin-directional coupling via evanescent near fields. We show that spin–orbit interactions are inherent in all basic optical processes, and that they play a crucial role in modern optics.

Spin-orbit interactions of light

A self-scanned 1024 element photodiode array and minicomputer are used to measure the phase (wavefront) in the interference pattern of an interferometer to lambda/100. The photodiode array samples intensities over a 32 x 32 matrix in the interference pattern as the length of the reference arm is varied piezoelectrically. Using these data the minicomputer synchronously detects the phase at each of the 1024 points by a Fourier series method and displays the wavefront in contour and perspective plot on a storage oscilloscope in less than 1 min (Bruning et al. Paper WE16, OSA Annual Meeting, Oct. 1972). The array of intensities is sampled and averaged many times in a random fashion so that the effects of air turbulence, vibrations, and thermal drifts are minimized. Very significant is the fact that wavefront errors in the interferometer are easily determined and may be automatically subtracted from current or subsequent wavefrots. Various programs supporting the measurement system include software for determining the aperture boundary, sum and difference of wavefronts, removal or insertion of tilt and focus errors, and routines for spatial manipulation of wavefronts. FFT programs transform wavefront data into point spread function and modulus and phase of the optical transfer function of lenses. Display programs plot these functions in contour and perspective. The system has been designed to optimize the collection of data to give higher than usual accuracy in measuring the individual elements and final performance of assembled diffraction limited optical systems, and furthermore, the short loop time of a few minutes makes the system an attractive alternative to constraints imposed by test glasses in the optical shop.

Digital wavefront measuring interferometer for testing optical surfaces and lenses

High-quality materials are critical for advances in plasmonics, especially as researchers now investigate quantum effects at the limit of single surface plasmons or exploit ultraviolet- or CMOS-compatible metals such as aluminum or copper. Unfortunately, due to inexperience with deposition methods, many plasmonics researchers deposit metals under the wrong conditions, severely limiting performance unnecessarily. This is then compounded as others follow their published procedures. In this perspective, we describe simple rules collected from the surface-science literature that allow high-quality plasmonic films of aluminum, copper, gold, and silver to be easily deposited with commonly available equipment (a thermal evaporator). Recipes are also provided so that films with optimal optical properties can be routinely obtained.

http://pubs.acs.org/doi/pdf/10.1021/ph5004237

Plasmonic Films Can Easily Be Better: Rules and Recipes

The electron is spherical — well, nearly. The standard model of particle physics predicts a slightly aspheric electron, with a distortion characterized by the electric dipole moment (EDM) that is far too small to be detected at current experimental sensitivities. However, some extensions to the standard model predict much larger EDM values that should be detectable. New experiments, using the dipolar ytterbium fluoride rather than spherical thallium, achieve the highest precision measurement of the EDM to date. At this new level of precision the EDM is consistent with zero, and the electron is indeed a sphere. This finding should help to constrain theories of particle physics and cosmology beyond the standard model. The electron is predicted to be slightly aspheric1, with a distortion characterized by the electric dipole moment (EDM), de. No experiment has ever detected this deviation. The standard model of particle physics predicts that de is far too small to detect2, being some eleven orders of magnitude smaller than the current experimental sensitivity. However, many extensions to the standard model naturally predict much larger values of de that should be detectable3. This makes the search for the electron EDM a powerful way to search for new physics and constrain the possible extensions. In particular, the popular idea that new supersymmetric particles may exist at masses of a few hundred GeV/c2 (where c is the speed of light) is difficult to reconcile with the absence of an electron EDM at the present limit of sensitivity2,4. The size of the EDM is also intimately related to the question of why the Universe has so little antimatter. If the reason is that some undiscovered particle interaction5 breaks the symmetry between matter and antimatter, this should result in a measurable EDM in most models of particle physics2. Here we use cold polar molecules to measure the electron EDM at the highest level of precision reported so far, providing a constraint on any possible new interactions. We obtain de = (−2.4 ± 5.7stat ± 1.5syst) × 10−28e cm, where e is the charge on the electron, which sets a new upper limit of |de| < 10.5 × 10−28e cm with 90 per cent confidence. This result, consistent with zero, indicates that the electron is spherical at this improved level of precision. Our measurement of atto-electronvolt energy shifts in a molecule probes new physics at the tera-electronvolt energy scale2.

https://slacportal.slac.stanford.edu/sites/ard_public/people/vibh/Documents/EDM/nature10104-2011-ImperialCollege.pdf

Improved measurement of the shape of the electron

A novel method, utilizing Lissajous figures and ellipse fitting, of calculating the phase difference between interferograms obtained from a phase shifting interferometer is described. Intensity bias and intensity modulation of interferograms are also calculated using this technique. Two new phase extraction algorithms are presented which use intensity and phase step information to calculate a surface from interferograms acquired with uneven phase steps. One algorithm allows surface phase to be calculated from two interferograms. Preliminary results from the Lissajous figure technique and the presented algorithms are discussed.

http://iopscience.iop.org/article/10.1088/0957-0233/3/10/003/pdf

Phase step measurement and variable step algorithms in phase-shifting interferometry

Two novel phase extraction algorithms, in which step sizes need not be known or equal, are described. One algorithm requires a minimum of five interferograms from which intensity offset and intensity modulation are calculated at each pixel and the relative phase is calculated for all pixels with respect to a reference pixel. The other algorithm requires two phase stepping mechanisms and a minimum of ten interferograms. The intensity characteristics and reference phase step are calculated at each pixel and a previously published algorithm is used for the final phase calculation.

https://iopscience.iop.org/article/10.1088/0957-0233/5/6/003/pdf

Phase-step insensitive algorithms for phase-shifting interferometry

Material removal rate prediction for ultrasonic drilling of hard materials using an impact oscillator approach

Conventional and digital holographies are proving to be increasingly important for studies of marine zooplankton and other underwater biological applications. This paper reports on the use of a subsea digital holographic camera (eHoloCam) for the analysis and identification of marine organisms and other subsea particles. Unlike recording on a photographic film, a digital hologram (e-hologram) is recorded on an electronic sensor and reconstructed numerically in a computer by simulating the propagation of the optical field in space. By comparison with other imaging techniques, an e-hologram has several advantages such as three-dimensional spatial reconstruction, non-intrusive and non-destructive interrogation of the recording sampling volume and the ability to record holographic videos. The basis of much work in optics lies in Maxwell's electromagnetic theory and holography is no exception: we report here on two of the numerical reconstruction algorithms we have used to reconstruct holograms obtained using eHoloCam and how their starting point lies in Maxwell's equations. Derivation of the angular spectrum algorithm for plane waves is provided as an exact method for the in-line numerical reconstruction of digital holograms. The Fresnel numerical reconstruction algorithm is derived from the angular spectrum method. In-line holograms are numerically processed before and after reconstruction to remove periodic noise from captured images and to increase image contrast. The ability of the Fresnel integration reconstruction algorithm to extend the reconstructed volume beyond the recording sensor dimensions is also shown with a 50% extension of the reconstruction area. Finally, we present some images obtained from recent deployments of eHoloCam in the North Sea and Faeroes Channel.

https://www.jstor.org/stable/pdfplus/25190786.pdf

Underwater digital holography for studies of marine plankton

Optical holography provides non-intrusive, non-destructive recording of living, motile organisms and particles in their natural environment. High-resolution, three-dimensionality and `optical sectioning' of the image allow identification of species and enumeration of size, orientation and spatial distribution. We have developed a submersible holographic camera to simultaneously expose in-line and, uniquely, off-axis holograms to record organisms from a few micrometres upwards (in-line), in concentrations down to a few tens of particles per cubic metre (off-axis). It can record up to 50 000 cm3 of the water column in a single off-axis hologram and 3500 cm3 in an in-line hologram. We describe the first sea deployment of the camera and the successful recording of holograms of plankton and marine snow to a depth of 100 m water.

https://iopscience.iop.org/article/10.1088/0957-0233/12/8/101/pdf

Michael A. Player

Papers

Phase step measurement and variable step algorithms in phase-shifting interferometry

Phase-step insensitive algorithms for phase-shifting interferometry

Material removal rate prediction for ultrasonic drilling of hard materials using an impact oscillator approach

Underwater digital holography for studies of marine plankton

Simultaneous in-line and off-axis subsea holographic recording of plankton and other marine particles