FiO 2008: Day three

Oct 23 2008 Published by under Optics, Science news

All the 'big ticket' events took place on the first and second days, so by day 3 I settled into listening to some shorter talks on various subjects.  Also, day 3 is about the time I start trying to actually get some work done and start drifting away from sitting in sessions all day.  I did manage to sit in on a number of talks on plasmonics and metamaterials, and I briefly summarize some of the substance below.

First, let me give a brief reminder.  In recent years researchers have become fascinated with developing new optical materials which behave in unusual ways not typically seen in nature.  Such materials are broadly known as "metamaterials", though the term is used more or less restrictively by different people.  The metamaterial 'storm' began to build in 1999 with the publication by T. Ebbesen's group of a paper which demonstrated that much, much more light can be passed through an array of small holes embedded in a metal plate than the traditional theory predicted - at least a factor of 10 more.  This result was attributed to the presence of surface plasmons on the metal surface: waves of electron density, with an accompanying electromagnetic field.  The interaction of the surface waves with the hole arrays produces a resonance effect with can increase the throughput through the hole array.  I've talked about extraordinary optical transmission before; you can read some details here.  This discovery energized the study of surface plasmons, and it has grown into its own field of optics known as plasmonics.

Not too long after (in 2001), J.B. Pendry wrote a theoretical paper in which he suggested that a material with negative refractive index could be used as a perfect lens of (theoretically) infinite resolution!  Because a lens can only focus light to a spot of size roughly equal to the wavelength of light, a lens cannot resolve (distinguish) two objects which are separated by distance closer than a wavelength - the two 'image spots' will be blurred together.  Pendry's result suggested that this limit could be beaten, with the right materials.

But the materials are the trick: negative refractive index materials don't exist in nature (and some authors initially argued that they couldn't exist at all!)  To make a negative refractive index, one would need to manipulate the structure of matter on the nanometer (billionth of a meter) scale.  If one can do this, one can make light do lots of 'tricks' which traditionally would have been though impossible.  Such unusual, unnatural materials are referred to as metamaterials, and the study of them has exploded in recent years.

Plasmonics and metamaterials are closely related, as a lot of metamaterial effects arise from manipulating the material structure so that plasmons are produced.  The two fields are therefore closely related and typically have intermingled conference sessions.

The first session I attended Tuesday was the 'Metamaterials I' session.  I didn't catch all of the talks, but I listened to the following:

  • M. Wegener, Toy model for metamaterials incorporating gain.  This theoretical and computational talk attempted to construct a simple model for metamaterials which have gain, i.e. which amplify optical waves which pass through them.  A simple 'toy model' (a model which is oversimplified) was compared to exact numerical solutions.
  • J.M. Lee, Electric and magnetic activities in metal nanocluster metamaterial.  One of the biggest problems in metamaterial construction is making structures of significant size.  These researchers used a technique of 'colloidal self-assembly'.  Basically, their material is made of a 2-D array of clusters of metal nanoparticles.  Basically, laser lithography is used to draw a collection of pits in a surface; then, a solution of metal particles are allowed to dry on this surface, and the particles settle into the holes, forming the array.  The researchers were able to get a magnetic response (see yesterday's summary) in the visible wavelength regime.
  • D. Powell, Nonlinear physics of metamaterials.  When light of high intensity is incident upon a material, one induces a nonlinear response, i.e. the response of the material is not directly proportional to the strength of the field illuminating it.  Nonlinear optics is a huge field of research, which only came about with the introduction of the laser.  This presentation dealt with a collection of nonlinear effects in metamaterials.  The one which caught my eye was hysteresis effects in metamaterials: the ability to 'switch' on and off a negative refractive index.
  • F. Intravaia, Surface plasmons and casimir forces between metamaterials.  The casimir force is a fascinating quantum mechanical effect.  In short, one can demonstrate that a pair of highly conducting surfaces, brought very close together, will experience an attractive force, i.e. they will be attracted to one another.  This phenomenon arises, in a very real sense, from nothing!  According to quantum electrodynamics, virtual photon pairs are being created/destroyed all the time, even in empty space.  The metal plates of the casimir effect, in essence, 'squash' these virtual particles which, although they are never directly seen, can still result in an attractive force between the plates.  But what happens if the metal surfaces can support surface plasmons?  The researchers demonstrated theoretically that, under certain circumstances, the possibility of surface plasmons leads to a repulsive force between the metal plates!  This is a fascinating and unambiguous departure from the traditional casimir theory, if true.
  • A. Huck, Generation of non-classical surface plasmon polaritons.  The first talk in the second session concerned non-classical effects involving surface plasmons.  As I mentioned yesterday, the field of quantum optics concerns the quantum-mechanical behavior of light; as surface plasmons are generated by light, it stands to reason that one could generate non-classical plasmon behavior.  The researchers here did experimental work suggesting the generation of non-classical surface plasmon states.

In the afternoon, I attended a couple of talks in the '3-D metamaterials' session.  As mentioned, the construction of metamaterials requires the manipulation of materials on a billionth of a meter scale.  Most work so far has consisted of constructing thin, effectively two-dimensional materials.  Making a 'bulk' metamaterial, i.e. one with some volume to it, is much more difficult.  The session concerned attempts to make a material with some thickness.

  • S. Zhang, Negative refractive index in bulk metamaterials.  This was a discussion of the 'fishnet' structure which I've talked about in a previous post.  The group managed to layer roughly 21 fishnets on top of each other, and their studies suggest that is approaching the 'bulk' limit.  It is thought that some 25-30 layers of fishnets will result in bulk metamaterial properties.
  • T. Paul, Anomalous diffraction and imaging properties of metamaterials.  This talk was quite interesting!  The 'fishnet' structures just mentioned are, in effect, anisotropic materials: light propagating along the surface of the fishnets behaves very differently from light which travels directly through the fishnets.  The authors of this talk suggest that this anisotropy will wreck lots of possible applications of metamaterials, such as the 'perfect lens' of Pendry mentioned earlier.  The take-away point is that it is much harder to make a metamaterial which will do 'super' things than many people perhaps initially thought.

Still no internet in my hotel room, so posts are still a little spotty.  I'm hoping to do one more post on the meeting substance, though.

4 responses so far

  • SH says:

    As a very new PhD student starting in the field of metamaterials, your coverage has been fun and interesting to follow. Thanks!
    I'm curious, could you give an example of nonclassical behaviour of surface plasmon-polaritons from that talk?

  • SH: Thanks for the comment! The talk was a relatively short one, and my impression is that the work was rather preliminary. What they did, according to my notes, is generated a squeezed state of light, allowed that light to convert to plasmons and back to light, and measured the nature of the output squeezed state. The maintaining of the squeezing suggests that the plasmon state was itself 'squeezed'.

    The only other non-classical plasmon effect I've seen in print was done a few years ago by J.P. Woerdman's group, who demonstrated that entangled states could be modified by plasmonic interaction.

    Good luck with the PhD work!

  • SH says:

    Aha, thanks for the talk description and the good luck!
    Sounds like I can safely go on "ignoring" any nonclassical behaviour of surface plasmons, even though it would be fun and fascinating if there were more subtler aspects to be exploited in fields other than quantum optics.

  • SH: You're welcome! I've done a lot of plasmon simulation work, and that which has been compared to experiment has agreed quite well. My impression is roughly that quantum effects can be ignored if one doesn't look at 4th-order statistics of the light (e.g. photon counting), and if your plasmonic material has a large, effectively infinite source of electrons. If your sample gets so small that the number of free electrons gets small, quantum stuff could come into play.