Skulls in the Stars

I’ve joined a group of folks on Twitter who have vowed to read roughly a paper a day for an entire year, and will summarize my reading here occasionally.  Part 1 can be read here, part 2 can be read here, and part 3 can be read here.  Links are provided for those with university access who are interested in reading more.  These posts are a bit more technical than my usual fare, so feel free to ignore if you’re not an optics enthusiast! More fun stuff to come soon.

2/23: Circularly symmetric operation of a concentric‐circle‐grating, surface‐ emitting, AlGaAs/GaAs quantum‐well semiconductor laser, T. Erdogan, O. King, G.W. Wicks, D.G. Hall, E.H. Anderson and M.J. Rooks (1992).  I’ve been reading a lot about “vector beams” these past few weeks, in which the polarization of light either points radially from the center of the beam or circulates azimuthally around it.  This paper, which describes a new laser design with azimuthal polarized light, sparked the modern interest in such beams.

2/24: Focusing of high numerical aperture cylindrical vector beams, K.S. Youngworth and T.G. Brown (2000).  So what good are “vector beams,” that possess unusual polarization? This paper, combining theory and experiment, noted how radial beams produce a strong longitudinal electric field at focus, which can be used to accelerate charged particles.

2/25: The electric and magnetic polarization singularities of paraxial waves, M.V. Berry (2004).  I’ve been extensively studying the “polarization singularities” of light for my chapter on the subject in my singular optics book.  Basically, points in a field where light is circularly polarized may be considered “singular” and distinct from other points.  This paper is one in a long set of articles studying the properties of such singularities.

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I’ve joined a group of folks on Twitter who have vowed to read roughly a paper a day for an entire year, and will summarize my reading here occasionally.  Part 1 can be read here, and part 2 can be read here.  Links are provided for those with university access who are interested in reading more.

One note: I’ve been using twitter, for the most part, to record which papers I’ve read, but I’ve been really bad at it!  In some cases, I’ve ended up “filling in” papers that I read to make up for those I’ve lost track of, and the dates between twitter and here may not always agree.

1/31: Rotational frequency shift, I. Bialynicki-Birula and Z. Bialynicka-Birula (1997).  I’ve mentioned the “angular Doppler effect” before, in which circularly polarized light undergoes a frequency shift when the source or detector is rotated.  It also turns out that vortex beams, with a “twist phase,” experience such a rotational shift as well!  This is something I’m gearing up to blog about in the near future.

2/1: Radiation pressure on a free liquid surface, A. Ashkin and J. M. Dziedzic (1973).  There is a long-running controversy in optics: does the momentum of light increase, or decrease, on entering a transparent medium?  We still don’t have a definite answer, but this paper in the 70s made an ingenious test.  By shining a beam of light onto a liquid from above, they found that the liquid bulged outward slightly, suggesting that the momentum increases.  Others have found other effects…

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Over the years, there has been a lot of hype about the possibility of “superluminal” light: namely, light than can travel faster than the vacuum speed of light meters/second, which is overwhelmingly considered the absolute speed limit of the universe.  I’ve talked about superluminal light before, and the related discoveries always turn out to be somewhat less spectacular than they sound, though they always provide some strange insights into the physics of light.

Recently, though, an even stranger discovery has been making the rounds in the press*: an experiment performed by researchers in Glasgow and Edinburgh** shows that appropriately-prepared light particles (photons) can travel even slower than the vacuum speed of light — in vacuum!

Kind of like its superluminal siblings before it, this discovery has caused a bit of confusion.  In this post I’ll try and shed some light on the phenomenon, showing why it is both less and more than it appears.

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