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Fiber-optic cables are the backbone of the worldwide communications system. By carrying light rather than electrical pulses, they transmit information faster and more efficiently than copper wires. The devices those cables connect still use electrical wires, though, so the optical signals have to be converted at either end of their journey. Replacing device electronics with photonic analogues would both improve information-transfer capabilities and avoid resistive heating.

In a traditional fiber-optic cable, a core, usually a 10–100 µm glass fiber, is surrounded by a cladding material that has a lower refractive index than the core and confines light using total internal reflection. That’s fine for long-distance travel, but it requires material that’s not used for light transmission. If photonic circuits are to replace electronic ones, those cables will have to be shrunk down and packed onto chips to make integrated photonic circuits. The cladding places a fundamental limit on how tightly packed the cores can be: If the light-blocking layer between them is thinner than λ/2, where λ is the wavelength of the light, light will leak into adjacent cores at a considerable rate.

Now Tongtong Song and Hongchen Chu at Nanjing University in China, Jie Luo at Soochow University in China, and their colleagues have used a photonic metamaterial to construct a waveguide array with light-carrying channels that don’t require a separation layer. Experiments and simulations demonstrate that the zero-separation waveguide array (ZSWA), illustrated in the opening figure, confines light to individual channels and efficiently directs it around sharp corners.

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