I’ve joined a group of folks on Twitter who have vowed to read roughly a paper a day, and will summarize my reading here occasionally.  Part 1 can be read here.  Links are provided for those with university access who are interested in reading more.

1/12: Shadow effects in spiral phase contrast microscopy, A. Jesacher, S. Fürhapter, S. Bernet, and M. Ritsch-Marte (2005).  It turns out that filtering an image through an optical vortex phase mask performs excellent edge enhancement of the image!  In this paper, the authors demonstrate that this enhancement can be tweaked to provide directional “shadow effects,” allowing different edges to be selectively highlighted.

1/13: Radial Hilbert transform with Laguerre-Gaussian spatial filters, C-S. Guo, Y-J. Han, J-B. Xu, and J. Ding (2006). Obviously I’m working on the section of my book discussing using vortex masks for edge enhancement!  In this paper, the authors look at using Laguerre-Gaussian filters, which represent very “pure” vortex states, for this filtering.

1/14: Image processing with the radial Hilbert transform: theory and experiments, J.A. Davis, D.E. McNamara, D.M. Cottrell, and J. Campos (2000).  This was the first paper about using vortex filters for edge enhancement.

1/15: The angular momentum of light inside a dielectric, M. Padgett, S.M. Barnett and R. Loudon (2003).  One of the fascinating debates related to light, still not completely resolved over 100 years later, is the nature of light’s momentum inside a transparent material.  Some say the momentum goes up, while others say it goes down — this is known as the Abraham-Minkowski controversy.  A similar paradox arises for angular momentum, and this paper is an excellent discussion of the issues.

1/16: Optical vortex coronagraph, G. Foo, D.M. Palacios, and G.A. Swartzlander, Jr. (2005).  The hunt for planets around distant stars is extremely difficult because the star is so much brighter than the planet.  Traditionally, this has been solved by physically blocking the central part of the image and the light coming from it, but this is imperfect.  In this paper, it is shown that an appropriately-designed “coronagraph” using a vortex mask can produce an almost perfect nulling of starlight, allowing planets to be readily seen.

1/17: The plasmonic memresistor: a latching optical switch, C. Hoessbacher, Y. Fedoryshyn, A. Emboras, A. Melikyan, M. Kohl, D. Hillerkuss, C. Hafner, and J. Leuthold (2014).  I’ve blogged about surface plasmons before: they are electric charge density waves that can propagate on the surface of certain metals, and possess many useful properties.  In this case, it is shown how they can be applied in making an optical memresistor.  You may have heard of ordinary electrical resistors, that can restrict the flow of current; a memresistor can have its resistance changed by an external control and locked into place, like a “memory.”  An optical memresistor provides memory resistance to light instead of electricity, making it a potentially important tool in making optical computers.

1/18: Intrinsic and extrinsic nature of the orbital angular momentum of a light beam, A.T. O’Neil, I. MacVicar, L. Allen, and M.J. Padgett (2002).  This paper is highly technical! You may be aware that angular momentum comes in two types — orbital, like the Earth orbiting the sun, and spin, like the Earth orbiting around its own axis.  Spin angular momentum is said to be intrinsic, meaning that every observer agrees on its value, regardless of the way they measure it, while orbital angular momentum is extrinsic, and depends on measurement circumstances.  This paper discusses the circumstances under which extrinsic angular momentum acts like intrinsic angular momentum.

1/19: Paraxial beams of spinning light, M.V. Berry (1998).  A classic paper about orbital angular momentum, it is filled with a variety of insights, including the observation that the presence of vortices in optical beams does not perfectly correlate with the orbital angular momentum of the beam.

1/20: Measurement of Pancharatnam’s phase by robust interferometric and polarimetric methods, J. C. Loredo, O. Ortíz, R. Weingärtner, and F. De Zela (2009).  One of these days I’ll blog in detail about Pancharatnam’s phase — once I understand it well enough myself!  It is a strange phenomenon: when a light wave is taken through a cyclical change in its polarization and returned to its starting state, it nevertheless maintains a difference from the original state.  No, I don’t understand it well enough to explain it better right now, other than to note that it is precisely analogous to cat-turning: a cat twists its body around and, although it ends up in the same body position as when it started, it has flipped itself over!

1/21: Poincaré vortices, I. Freund (2001).  This paper discusses an interesting technique to measure and find optical vortices by using the polarization of light.

1/22: On measuring the Pancharatnam phase. I. Interferometry, A.G. Wagh , V.C. Rakhecha (1995).  More reading on the Pancharatnam phase.  It turns out to be a rather tricky phenomenon to measure.

1/23: Geometrodynamics of spinning light, K.Y. Bliokh, A.Niv1, V. Kleiner and E. Hasman (2008).  This is a pretty cool phenomenon!  If one sends a beam of light on a helical path through a clear glass cylinder, where the light spirals around the edge of the glass from one end to another, the left- and right-circularly polarized light ends up becoming separated spatially.  The helical path through the cylinder interacts with the helical polarization, producing a path difference.  This is another one I need to blog at some point.

1/24: Orbital angular momentum and nonparaxial light beams, S.M. Barnett and L. Allen (1994).  An interesting conundrum related to the orbital angular momentum of light: when light is non-paraxial, i.e. not very directional, it is much more difficult to decide what part of the light is “spin” angular momentum and what part is “orbit.”  It turns out that this problem was mostly resolved in a later paper, but it is informative to see what the problem was back then.

1/25: On geometrical scaling of split-ring and double-bar resonators at optical frequencies, S. Tretyakov (2007).  And now for something completely different!  Metamaterials are materials structured on a subwavelength scale to have unique optical properties not found in natural materials, such as negative refraction.  It is quite challenging to determine what sort of subwavelength structures will produce the desired effects, however, and this paper looks at several classes of structures.

1/26: Angular Doppler shift, B.A. Garetz (1981).  You probably have heard of the regular Doppler effect, in which relative motion between a source of waves and a detector results in a shift in frequency.  Much less known is the rotational or angular Doppler effect, in which relative rotation between a source and detector results in a frequency shift!  Only special beam types exhibit this shift — in this paper, “spinning” light with circular polarization is studied.

1/27: Photons that travel in free space slower than the speed of light, Giovannini, Romero, Potocek, Ferenczi, Speirits, Barnett, Faccio, Padgett (2015).  This paper, of course, was the subject of my previous blog post, so I don’t need to elaborate here!

1/28: Observation of superluminal behaviors in wave propagation, D. Mugnai, A. Ranfagni, and R. Ruggeri (2000).  One of the peculiar things about the “slow light” paper I read the day before: the “subluminal” structured light they used was once thought to be “superluminal!” I’ll be blogging about this peculiar conundrum in the future.

1/29: A simple demonstration of the Pancharatnam phase as a geometric phase, P. Hariharan, S. Mujumdara and H. Ramachandrana (1999).  Still trying to sort out exactly how the Pancharatnam phase works!

1/30: Detection of a spinning object using light’s orbital angular momentum,
M.P.J. Lavery, F.C. Speirits, S.M. Barnett and M.J. Padgett (2013).  The angular or rotational Doppler effect I mentioned earlier? One of its possible applications is to detect spinning objects, such as rotating molecules, and even measure their speeds and rotational sense.  This paper is one of the first to try and test this application.

More #365papers in a couple of weeks!