"This on-chip metamaterial opens the door to exploring the physics of zero index and its applications in integrated optics"Eric Mazur

Researchers in the US have designed and fabricated an on-chip metamaterial with a refractive index of zero that allows the phase of light to travel infinitely fast. This new metamaterial could help in the analysis of the physics of zero index and its applications in integrated optics, as well as photonic devices using light to carry large amounts of information quickly that could replace current electronic devices.

For optical connections to be standardized in telecommunications and computing, it is necessary to manipulate light at the nanoscale. Although metamaterials with a refractive index of zero exhibit physical properties such as infinite phase velocity and wavelength, they cannot be implemented on a photonic chip. However, a study by a team from the John A. Paulson School of Engineering and Applied Sciences at Harvard, which was published in Nature Photonics [Li et al. Nature Photon. (2015) DOI: 10.1038/nphoton.2015.198], has explored the fact that the speed of light can also be measured by how fast the crests of a wavelength progress – that is, its phase velocity.

One of the properties of phase velocity is that it can increase or decrease depending on what it is material moving through. This differential in the speed of light wave crests in a material is expressed by the refraction index, so that the higher the index, the more the material is interfering with the propagation of the wave crests of light. If the refraction index is reduced to zero, there is no phase advance and light stops behaving like a wave moving through space in a series of crests and troughs.

Rather, the zero-index material creates a constant phase that consists of either all crests or all troughs, and which elongates into infinitely long wavelengths. In addition, these crests and troughs oscillate only as a variable of time and not space. In this uniform phase, light can then be manipulated from one chip to another, and twisted, turned, stretched or squished all without the loss of energy.

The metamaterial is made up of silicon pillar arrays embedded in a polymer matrix and then covered in gold film, and can combine with silicon waveguides to interface with standard integrated photonic components and chips. As first author Yang Li commented, “This zero-index metamaterial offers a solution for the confinement of electromagnetic energy in different waveguide configurations because its high internal phase velocity produces full transmission, regardless of how the material is configured.”

In quantum optics, such a lack of phase advance also allows quantum emitters in a zero-index cavity or waveguide to emit photons that are always in phase with each other, and improves entanglement between quantum bits, since incoming waves of light are spread out and infinitely long.