Nature 496, 196–200 (2013)

Credit: © 2013 NPG

The topological insulator, a new phase of matter whose surface conducts electricity but whose interior is an insulator, has attracted intense interest in recent years. Now, researchers from Technion in Israel and Friedrich-Schiller Universität in Germany have fabricated a photonic analogue that transports visible light along its edges. The optical structure of this device consists of a hexagonal array of helical waveguides, each of which has an elliptical cross-section (major axis of 11 μm, minor axis of 4 μm). The waveguides were written into silica by femtosecond laser writing. When red light with a wavelength of 633 nm was sent into the upper edge of the array, it remained on the outermost edge of the array but propagated clockwise around the edge of the structure. The researchers report that the velocity at which the light travels around the structure's perimeter depends on the radius of the waveguide helix, reaching a maximum at a radius of 10.3 μm.