Light goes 'infinitely fast' with new on-chip material
19 October 2015
Harvard researchers have designed the first on-chip metamaterial with a refractive index of zero, meaning that the phase of light can travel infinitely fast.
Photonic devices, which use light to transport large amounts of information quickly, will enhance or even replace the electronic devices that are ubiquitous in our lives today. But there’s a step needed before optical connections can be integrated into telecommunications systems and computers: researchers need to make it easier to manipulate light at the nanoscale.
Researchers at the Harvard School of Engineering and Applied Sciences (SEAS) claim to have done just that, designing the first on-chip metamaterial with a refractive index of zero, meaning that the phase of light can travel infinitely fast.
This new metamaterial was developed in the lab of Professor Eric Mazur and is described in the journal, Nature Photonics.
“Light doesn’t typically like to be squeezed or manipulated but this metamaterial permits you to manipulate light from one chip to another, to squeeze, bend, twist and reduce diameter of a beam from the macroscale to the nanoscale,” says Mazur. “It’s a remarkable new way to manipulate light.”
Although this infinitely high velocity sounds like it breaks the rule of relativity, it doesn’t. Nothing in the universe travels faster than light carrying information. But 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’s moving through.
Water, for example, has a refraction index of about 1.3, but when the refraction index is reduced to zero, interesting things start to happen. In a zero-index material, there is no phase advance, meaning light no longer behaves as a moving wave. 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 shortened, 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.
“Integrated photonic circuits are hampered by weak and inefficient optical energy confinement in standard silicon waveguides,” says Yang Li, a postdoctoral fellow in the Mazur Group and first author of the Nature Photonics paper. “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.”
The metamaterial consists of silicon pillar arrays embedded in a polymer matrix and clad in gold film. It can couple to silicon waveguides to interface with standard integrated photonic components and chips.
“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,” says Philip Munoz, a graduate student in the Mazur lab and co-author of 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.”
“This on-chip metamaterial opens the door to exploring the physics of zero index and its applications in integrated optics,” Mazur adds.