In the May 1st issue of Optics Letters, a Korean research group has demonstrated another interesting application of surface plasmon resonances: the optical measurement of neural activity. Though I’m not sure how useful this technique will be in the long run, it shows that surface excitations can be used in sensors in many situations when an ordinary optical wave is not sensitive enough. A description after the fold…
Update: In my haste to finish this “monster” post, I neglected to include an introduction to standing waves, an explanation which is crucial to understanding the experiment. That oversight has been corrected.
A couple of weeks ago I issued a “challenge” to my fellow science bloggers: find, read, and blog about a classic, (preferably pre-WWII) scientific paper. There’s so much interesting historical context and methodological information hidden away that’s worth a second look.
For my part in the challenge, I chose an 1890 paper by Otto Wiener, “Stehende Lichtwellen und die Schwingungsrichtung polarisirten Lichtes,” Ann. Phys. Chem. 38 (1890), 203-243. Loosely translated, the title is, “Standing light waves and the oscillation direction of the polarization of light.”
The experiment that Wiener performed, as we will see, is conceptually simple and elegant. I foolishly thought that this would “translate” into a short, easy to cope with paper. As one can see from the citation above, no such luck: the paper is 40 pages of somewhat antiquated German! I accepted my fate, though, and soldiered on. A description begins below the fold…
Right after “challenging” my fellow science bloggers to find and write about an old scientific paper, I take a hypocritical turn and write about some recent results in the theory of extraordinary optical transmission!
In a paper that came out recently in Nature*, authors Haitao Liu and Philippe Lalanne present a new model for the phenomenon now known as “extraordinary optical transmission”. The relatively simple pen-and-paper model they’ve developed provides results which are quantitatively in agreement with exact numerical simulations, and promises to be a powerful tool in the study of plasmonic nano-optical systems.
But what is extraordinary optical transmission, what are plasmons, and what is the relevance of both to nano-optics? Before I describe the results of the recent publication, I give a background on these questions, and others related to nano-optics.
A couple of years ago, a number of physicists made international news (some descriptions here and here) by proposing that “cloaking devices” were theoretically possible to construct. Two papers appeared consecutively in Science Magazine in May 2006, one by U. Leonhardt of the University of St Andrews, Scotland (Science 23 June 2006: Vol. 312. no. 5781, pp. 1777 - 1780), and the other by J.B. Pendry of Imperial College, London and D. Schurig and D.R. Smith of Duke University (Science 23 June 2006: Vol. 312. no. 5781, pp. 1780 - 1782). Both papers describe how, with the proper materials, one could create devices which ‘guide’ light around a central core region without distortion, effectively making the cloak, and whatever sits in the core, invisible. This idea is illustrated by the figure below, from the Pendry paper, which shows how light rays could be guided around the core:
These papers have generated so much interest that it is fair to say that they have created their own subfield of optical science, what one might call ‘invisibility physics’, and numerous research groups are busy concocting their own invisibility schemes or attempting to construct a Leonhardt/Pendry-style device.
It is interesting to note, however, that the study of objects which are in some sense ‘invisible’ is not really new, and in fact there is a century-long history of scientists studying objects which may be considered, one way or another, undetectable.
I happen to know a lot about the history of such objects, so I thought I’d start yet another long-running series of posts, this one on invisibility physics. We start today with a discussion of what may be the first paper of this type, written by none other than the remarkable physicist Paul Ehrenfest.
In my recent post on the camera obscura, I discussed the optical illusion produced by so-called anamorphic images, i.e. images which only appear normal from a particular point of view. One can readily understand such images from the point of view of geometrical optics, but I thought I’d go a step further and show how a little geometry can be used to construct your own simple anamorphs. In this post we discuss the simplest form of anamorphic image — one constructed from piecewise planar images — and when my sanity returns I’ll contemplate doing posts on other, more complicated distortions.
When I was an undergraduate, one of my professors told the following funny (and probably apocryphal) anecdote (recalled from memory):
A court case was being tried in New Mexico. A group of pornographers were charged with smuggling pornography from Mexico by projecting it across the border to a camera. The defense argued that nothing physical was transported, and in the end the argument boiled down to this: if light moved at a finite speed, the films were being transported; if it moved at infinite speed, the defense was correct. A physicist was brought in to discuss the speed of light but, after a number of figures were presented, the judge interrupted. “When I put my hands over my eyes, the light stops coming immediately, and when I move my hands, it reappears instantly. The speed of light is infinite - the defendants are not guilty!”
The reason I suspect this story is apocryphal is that science has accepted that the speed of light is finite - albeit very large - for centuries. The value, usually denoted c, is approximately 
My attention was recently drawn to this article (h/t Personal Demon and StumbleUpon) in Scientific American: a group of researchers have concocted a relatively simple way to generate an ‘event horizon’ in an optical fiber, analogous to those found in black holes. This technique may make it possible to study, on a tabletop, some of the more intriguing theoretical predictions about black holes. I give a brief description of the theory and experiment below the fold, plus a few observations…
I thought I’d muscle in on Swans on Tea’s turf for a post and discuss an interesting optical illusion that is based just as much on optics as on the idiosyncrasies of the eye itself. While stumbling through StumbleUpon.com, I found an interesting collection of images at 2Loop.com showing ‘3D Painted Rooms’. An example of this is shown below the fold, from 2Loop…
In a previous optics basics post, we discussed challenges associated with trying to define the velocity of a localized wave or ‘pulse’ of light. Traditional measurements of the velocity of an object involve measuring how far Δd an object travels in a certain amount of time Δt; then the velocity is simply
velocity = distance/time = Δd/Δt.
But a wave is an extended disturbance, not definitely associated with any particular point in space, and so measuring Δd becomes tricky. If there is a definite feature of the wave (such as a peak), we can define the velocity by measuring how fast the peak moves. If the wave changes shape (i.e. the peak disappears), as happens when waves propagate in matter, it is not immediately clear how one defines wave velocity.
The answer, as discussed previously, seems to be to define a ‘group’ velocity: we can mathematically characterize the velocity of the overall wave signal by
where Δω is the range of temporal frequencies in the wave pulse and Δk is the range of spatial wavenumbers in the pulse. This measure seemed quite good: under most circumstances, the quantity was less than the vacuum speed of light c, and therefore didn’t violate Einstein’s relativity, and those cases where the group velocity was greater than c seemed to always involve a significant attenuation or distortion of the wave.
However, in 2000 researchers Wang, Kuzmich and Dogariu from the NEC Research Institute shocked the physics and optics community by demonstrating* that materials exist for which the group velocity is greater than c, sometimes much greater than c, and the pulse travels at this higher speed without any obvious distortion or attenuation. What was going on?
This was an interesting bit of science news from last week: according to an article on optics.org (free registration required), a research team from Rensselaer Polytechnic Institute and Rice University has fabricated the ‘blackest’ material ever known, which reflects just 0.045% of the light incident upon it. This beats the previous record of 0.16% that was set by a nickel and phosphorous alloy.
