## #365papers, part 3!

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…

Posted in Optics | 1 Comment

## Dr. SkySkull and the mystery of the subluminal superluminal light!

References in a scientific paper are supposed to answer questions, not raise them, but sometimes they inadvertently create a minor mystery for the reader.  A few weeks back, I blogged about the curious phenomenon of subluminal vacuum beams of light, i.e. pulsed beams of light that travel slower than the vacuum speed of light c = 3 × 108 meters/second even in vacuum.  One of the pulses beams tested experimentally, a so-called Bessel beam, has had its speed measured extensively in the past — however, the original paper* on the speed of a Bessel beam, published in 2000, refers to it as a superluminal beam of light!  This paper contains both theoretical and experimental work verifying their result, which I should say at the get-go is all correctly done.

There was no explanation in the subluminal paper for this discrepancy — how can a pulse of light moving slower than the vacuum speed c also be considered as moving faster than the vacuum speed c?  The answer leads us to some interesting aspects of Einstein’s special relativity as well as optics — Dr. SkySkull is on the case!

Posted in ... the Hell?, Optics | 4 Comments

## Robert Aickman’s Dark Entries

It took me far longer than it should have, but I have finally read a collection of short stories by Robert Aickman (1914-1981).  Though the 1960s and 1970s, he published 48 supernatural tales, some of which are acknowledged as classics, that were collected in 8 volumes.  These collections were hard to find until last year, when new editions finally came out.  I picked up the first of these, Dark Entries, and devoured it last week.

I was doubly intrigued to read Aickman’s work.  Not only is he highly regarded by some of the greatest weird fiction writers of our time such as Neil Gaiman, Ramsey Campbell and Peter Straub, he comes from an impressive lineage: his grandfather is none other than Richard Marsh, the master of the macabre that I’ve obsessively written about many times on this blog.

On this first outing, though, I have to admit that I was a little underwhelmed.  There are stories of undeniable brilliance, but an equal number of stories that I found primarily baffling.  Let’s summarize each of them…

## #365 papers, part 2!

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.

## So what’s up with that “slower than light” light?

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 $c = 3\times 10^8$ 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.

Posted in Optics | 2 Comments

## Null-field radiationless sources: even more invisible than invisible?

I spend a lot of time talking about invisibility on this blog, as it is a subject near and dear to me: I did my PhD work, completed in 2001, on early historical forms of invisibility.  I like to tell people that I’m an invisibility hipster, and that I worked on invisibility “before it was cool.”

The field has progressed dramatically since I first did my work, and in recent years I’ve been playing a bit of “catch up” with my own invisibility work, trying both to build on the newest insights as well as apply knowledge from the old theories.

This past fall, my student Elisa and I used this approach to ask and answer a curious question: can something be more invisible than invisible?

Our answer was published in a paper titled “Null-field radiationless sources,” that appeared in November in Optics Letters.*  Ordinary invisible objects, including cloaking devices, are objects that don’t scatter any light outside the domain of the object; a potential null-field object will potentially not scatter light inside, as well!