Topological States in Photonic Crystals

Electron microscopy image of topological photonic crystals in a perforated slab of silicon. (Source: AMOLF)

Researchers of AMOLF and TU Delft have seen light propagate in a special material without it suffering from reflections. The material, a photonic crystal, consists of two parts that each have a slightly different pattern of per­forations. Light can propa­gate along the boundary between these two parts in a special way: it is topo­logically protected and, therefore, does not bounce back at imper­fections. Even when the boundary forms a sharp corner, the light follows it without a problem. “For the first time, we have seen these fascinating light waves move at the techno­logically relevant scale of nano­photonics,” says Ewold Verhagen, group leader at AMOLF.

Verhagen and his colla­borator Kobus Kuipers from TU Delft were inspired by electronic materials, where topo­logical insulators form a new class of materials with remarkable behavior. Where most materials are either conductive for electrons or not, topo­logical insulators exhibit a strange form of conduction. “The inside of a topological insulator does not allow electron propa­gation, but along the edge, electrons can move freely”, says Verhagen. “Impor­tantly, the conduction is ‘topologically protected’; the electrons are not impacted by disorder or imperfections that would typically reflect them. So the conduction is profoundly robust.”

In the past decade, scientists have tried to find this behavior for the conduction of light as well. “We really wanted to accomplish topo­logical protection of light propa­gation at the nanoscale and thus open the door to guiding light on optical chips without it being hindered by scattering at imper­fections and sharp corners”, says Verhagen. For their experiments, the researchers used two-dimensional photonic crystals with two slightly different hole patterns. The edge that enables light conduction is the inter­face between the two hole patterns. “Light conduction at the edge is possible because the mathe­matical description of light in these photonic crystals can be described by specific shapes, or more accurately by topology,” Kuipers says. The topology of the two different hole patterns differs and precisely this property allows light conduction at the boundary, similar to electrons in topo­logical insulators. Because the topology of both hole patterns is locked, light conduction cannot be revoked; it is topo­logically protected.”

The researchers managed to image light propa­gation with a micro­scope and saw that it behaved as predicted. Moreover, they witnessed the topology, or mathematical description, in the observed light. Kuipers: “For these light waves the polarization of light rotates in a certain direction, analogous to the spin of electrons in topo­logical insulators. The spinning direction of light determines the direction in which this light propagates. Because polari­zation cannot easily change, the light wave can even flow around sharp corners without reflecting or getting scattered, as would happen in a regular waveguide.

The researchers are the first to directly observe the propa­gation of topo­logically protected light on the technologically relevant scale of nano­photonic chips. By purposely using silicon chips and light of a similar wavelength as used in tele­communication, Verhagen expects to increase the appli­cation prospects. “We are now going to investigate if there are any practical or fundamental boundaries to topo­logical protection and which func­tionalities on an optical chip we could improve with these principles. The first thing we are thinking of is to make the integrated light sources on a photonic chip more reliable. This is important in view of energy-efficient data processing or green ICT. Also, to efficiently transfer small packages of quantum infor­mation, the topological protection of light can be useful. (Source: AMOLF)

Reference: N. Parappurath et al.: Direct observation of topological edge states in silicon photonic crystals: Spin, dispersion, and chiral routing, Sci. Adv. 6, eaaw4137 (2020); DOI: 10.1126/sciadv.aaw4137

Link: Center for Nanophotonics, AMOLF, Amsterdam, Netherlands

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