In a recent Physics Today article, Mohammad Hafezi and Jacob Taylor reviewed their recent work on creating topological insulators for light (Physics Today, May 2014, p. 68, http://dx.doi.org/10.1063/PT.3.2394). One of the great things about CLEO is that this sort of cutting-edge research is commonly part of the program. And never failing to please, this year’s conference featured a couple of talks on this novel topic in photonic science.
First, let me review just a little bit about topological insulators as I understand them using (as Hafezi and Taylor did) the canonical example of the quantum Hall effect. The system under study is a two-dimensional material containing charged particles. If one applies a magnetic field perpendicular to the sheet, this will cause the particles to undergo circular orbits in the plane. In the center of the material, particles complete their orbits and globally remain stationary; the result is that there is no net transfer of charge and the material is insulating. However, something remarkable happens at the edges: the particles are unable to undergo a full rotation before ramming into a material wall. Then, instead of orbiting in place, the particle bounces off the wall and begins another partial rotation in the same direction. As a result, the particle hops along the edge of the material, and the effect is that there is charge transport and, therefore, a current. What you’re left with is a material that is insulating on the interior and conducting along the edges. The remarkable thing is that now researchers have observed this effect using photonics, and in more way that one!
In one realization presented at the conference, researchers from Technion Israel Institute of Technology and Friedrich-Schiller-Universitat Jena created a periodic array of helically shaped waveguides. The analog of photons propagating down the array of helical waveguides is electrons evolving in time in a lattice of circulating atoms. In this rotating frame, the result is similar to that described above: propagation in the bulk of the material is prohibited, whereas there are propagating states allowed at the edges. The talk gave many examples of how this worked and what could be done with it. For example, in a honeycomb lattice, there are types of edges that allow edges states and some that don’t. However, when the helical waveguides are used, the usually “non-conducting” edges begin to allow the propagation of light. What’s even more remarkable is that light that transits from one edge type to another at a corner does so without scattering; instead it just makes the 90-degree turn an continues its propagation. This is just a taste of some of the remarkable results discussed in this talk, and more can be found in the CLEO abstract and their recent publication (Rechtsman et al., Nature, vol. 496, p. 196, 2013).
The other topological insulator talk I had the privilege of seeing was given by a member of the group that published the Physics Today article. In their work, they realize a topological insulator by utilizing a 2D array of coupled ring resonators. As they note in their article, the important property that they honed in on is that the path length of light traveling in a clockwise direction should be different from that traveling counterclockwise. To achieve this, they made oblong ring resonators, rotated neighboring rings 90 degrees with respect to one another, and offset them from center. The result is that light traveling in one direction take a long-arm path, whereas the short path is taken in the other direction. By using such an arrangement, they showed that edge states can be excited by operating at the correct frequency where bulk propagation is disallowed. In addition, due to the path length asymmetry, propagation in different directions is excited at different frequencies. A very exciting result to be sure. Again, I’m certain I can’t do justice to all the remarkable results, but one can see their CLEO abstract or their recent publication (Hafezi et al., Nature Photonics, vol. 7, p. 1001, 2013).
To my knowledge, these are the only two demonstrations of photonic topological insulators to date. However, this is a very new and exciting field; the theory for photonic topological insulators is less than a decade old. It’s remarkable how quickly these experimental results have been realized. I’m sure we can look forward to even more exciting results in the near future, and I look forward to it.
Disclaimer: Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government and MIT Lincoln Laboratory.