Authors: Mikael C. Rechtsman1, Julia M. Zeuner2, Yonatan Plotnik1, Yaakov Lumer1, Mordechai Segev1 and Alexander Szameit2
1Department of Physics and the Solid State Institute, Technion – Israel Institute of Technology, Haifa, Israel
2Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Germany
There is a topological revolution going on in solid-state physics. At its heart are ‘topological insulators’ (TIs), a class of materials that have electrical properties unlike anything else that has been seen before [1–4]. In TIs, electrical current flows only on the edges or surfaces and not through the bulk of the material: this is why the titles of many popular science articles on the topic are variants of “Physics on the edge.” The atoms making up these materials are, for example, Bismuth Selenide (for the three-dimensional case), and Mercury Telluride (for the two-dimensional case).
Alexander Szameit of Friedrich-Schiller-Universität Jena
The two-dimensional TI has perhaps the most bizarre behavior: electrical current flowing on its edge does not get obstructed by imperfections of the lattice. Any kind of defect or disorder cannot scatter the electrons – allowing them to flow in a smooth and seamless way. Essentially because of this robustness, the condensed matter and solid-state physics community has been convinced that these are going to be the materials at the core of futuristic technologies like robust quantum computation and spintronic devices.
Just as electronics uses the flow of electrons to manipulate information, the field of photonics explores the use of light to do the same thing. Examples of photonic technologies are optical fibers (which transmit internet signals across oceans) and DVD players (think “optical drives”), among many, many others. Physicists in the 1980s realized that light could be manipulated with incredible control in “photonic crystals" [5,6], which are lattices made out of some transparent material (for example, glass, polystyrene, or even silicon). Photonic crystals are central in many emerging photonic technologies, and have the ability to manipulate light in much the same way that semiconductors can control and manipulate electrons in computer chips.
This begs the question: can photons be topologically protected from disorder the same way that electrons can in TIs? Can topological robustness be realized photonic crystals just as it was in solids? As Raghu and Haldane showed in a theoretical paper in 2008 , the answer is yes – all you need to do is make a photonic crystal out of a gyromagnetic material (like, for example, Yttrium Iron Garnet - YIG), and apply an external constant magnetic field. The external field acts to break “time-reversal symmetry,” meaning that if light is flowing forward, it could be prevented from being scattered (due to the unavailability of a backwards-propagating wave).
Raghu and Haldane’s theory was proven correct by an experiment carried out in Marin Soljacic’s group in 2009 [8,9]. But there was a catch: because of the weakness of the gyromagnetic effect at optical wavelengths, their experiment could only be done at microwave frequencies. That meant that the photonic crystals involved could be (roughly) no smaller than the metallic grating you can find on the front cover of your microwave oven. Another scheme – in other words, different physics – would have to be found in order to scale down to micron scales and beyond.
There were quite a number of ideas put forward on how to exactly to scale down in order to make photonic topological insulators with light [10–12]. Our team of researchers (a collaboration between the Segev group at the Technion – Israel Institute of Technology in Haifa and the Szameit group at Friedrich-Schiller University in Jena, Germany) conceived of a new way of doing exactly this, and then implemented it in the lab . The group used an array of waveguides made of fused silica – i.e., glass – to create a honeycomb lattice. The waveguides – which are essentially wires for light – were made to propagate in a helical fashion, rather than straight (see Fig. 1). The helicity is the key ingredient – it allows the light to differentiate between diffracting off to the left or to the right, and only thus can topological protection be achieved. An animation of a simulation in which light misses a defect and then goes around a corner is included below. Full experimental results are available in a recent publication in the journal Nature .
Figure 1: This is a schematic diagram of a photonic topological insulator realized in Ref. 13. Each helix represents a waveguide that guides light just as a wire guides electrical current. Each waveguide is about 6μm in diameter and the spacing between waveguides is 15μm.
Caption for Animation: This animation is a simulation of how light propagates through a photonic topological insulator. As light travels down the waveguides, it encounters a defect in the form of a missing waveguide and simply bypasses it without getting scattered. Then, light encounters the corner of the array and simply goes up along the edge because it cannot go back. The reason that the whole array is spinning in a circular way is due to the helicity of the waveguides.
The realization of topological protection of light will impact photonic crystal science and beyond – especially by making photonic devices more robust to disorder, and hence – hopefully – cheaper to manufacture. The optical setting will also enable probing new topological physics that may never have been accessed otherwise. Perhaps the most beautiful aspect of photonic TIs is that they are another example of “emergent physics”  – the idea that in complex systems, physics can be much more than the sum of its parts.
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