Zero-Refraction Metamaterial Promising for PICs

The first on-chip metamaterial with a refractive index of 0 gives light an infinite phase velocity, which could be advantageous for manipulating light in photonic integrated circuits.

The metamaterial consists of silicon pillar arrays embedded in a polymer matrix and clad in gold film and was developed at Harvard University's John A. Paulson School of Engineering and Applied Sciences. It can couple to silicon waveguides to interface with standard integrated photonic components and chips.

"Integrated photonic circuits are hampered by weak and inefficient optical energy confinement in standard silicon waveguides," said postdoctoral fellow Yang Li. "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 this zero-index material — made of silicon pillar arrays embedded in a polymer matrix and clad in gold film — light experiences no phase advance. Courtesy of Peter Allen/Harvard SEAS.

Light's phase velocity increases or decreases as a function of the refractive index of the material it's moving through. Phase velocity slows down, for example, when a light wave moves from air into water, and speeds up again when it re-enters the air. Air's refractive index is 1, and water's is about 1.3.

In a material with a refractive index of 0, there is no phase advance, meaning light no longer behaves as a moving wave, traveling through space in a series of crests and troughs. Instead it has a constant phase, effectively stretching out into infinitely long wavelengths. The crests and troughs oscillate only as a variable of time, not space. This uniform phase allows the light to be stretched or squished, twisted or turned, without losing energy.

"In quantum optics, the lack of phase advance would allow quantum emitters in a zero-index cavity or waveguide to emit photons which are always in phase with one another," said graduate student Philip Munoz. "It could also improve entanglement between quantum bits, as incoming waves of light are effectively spread out and infinitely long, enabling even distant particles to be entangled."

The research was published in Nature Photonics (doi: 10.1038/nphoton.2015.198).