New metamaterial enables refractive index of zero

20 Oct 2015

Development by Harvard SEAS offers "new possibilities" for integrated optics, telecoms applications and research.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) say they have made it easier to manipulate light at the nanoscale. They have developed the first on-chip metamaterial with a refractive index of zero, meaning that the phase of light can travel infinitely fast.

The new metamaterial was developed in the lab of Eric Mazur, the Balkanski Professor of Physics and Applied Physics and Area Dean for Applied Physics at SEAS, and is described in the journal Nature Photonics.

The metamaterial consists of low-aspect-ratio silicon pillar arrays embedded in a polymer matrix and clad by gold films. This structure can be fabricated using standard planar processes over a large area in arbitrary shapes and can efficiently couple to photonic integrated circuits and other optical elements.

“Light doesn’t typically like to be squeezed or manipulated but this metamaterial permits us to manipulate light from one chip to another, to squeeze, bend, twist and reduce diameter of a beam from the macroscale to the nanoscale,” said Mazur. “This on-chip metamaterial opens the door to exploring the physics of zero index and its applications in integrated optics. It’s a remarkable new way to manipulate light.”

Although this infinitely high velocity seems to break a fundamental rule of relativity, it does not. Besides the absolute speed of light passing from one point to another, light has another speed, measured by how fast the crests of a wavelength move, known as phase velocity. This speed of light increases or decreases depending on the material it passes through.

When light passes through water, for example, its phase velocity is reduced as its wavelengths are squeezed together. Once it exits the water, its phase velocity increases again as its wavelength elongates. How much the crests of a light wave slow down in a material is expressed by the index of refraction; the higher the index, the more the material interferes with the propagation of the wave crests of light.

RI = zero

The Nature Photonics paper explains what happens when a material’s refractive index is reduced to zero: "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, the zero-index material creates a constant phase — all crests or all troughs — stretching out in 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 squashed, twisted or turned, without losing energy. A zero-index material that fits on a chip could have exciting applications, especially in the world of quantum computing.

Yang Li, a postdoctoral fellow in the Mazur Group and first author on the paper, commented, "Integrated photonic circuits are hampered by weak and inefficient optical energy confinement in standard silicon waveguides. 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, 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 Philip Munoz, a graduate student in the Mazur lab and co-author on the paper. “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.”

About the Author

Matthew Peach is a contributing editor to optics.org.