Zero-Index Metamaterials Open New Possibilities for Optical Chips

Will metamaterials improve optical chips?

3 min read
Zero-Index Metamaterials Open New Possibilities for Optical Chips
Illustration: Peter Allen/Harvard SEAS

One class of metamaterials, with a refraction index of zero [pdf], was first created in 2013 and quickly caught the interest of scientists because of the materials’ unique properties. Such a material, when irradiated with light, behaves in a peculiar way, explains Eric Mazur, who leads a team of researchers investigating metamaterials at  Harvard University in Cambridge, MA.  For example, light directed at a planar slab can only pass when its incident angle is exactly 90 degrees. Says Mazur:

The light sets up a response in the material so that combined with the incident electromagnetic field the field in the material has the same phase throughout the material—not unlike the wheels of a carriage in a movie can appear at rest due to the interplay of the frequency of rotation and the frequency of the movie. This complete coherence in space, results in light with infinite wavelength and infinite phase velocity.

Since the phase of the light is the same in all of the metamaterial, it looks like the sinusoidal field has a wavelength that is streched to infinity and the phase propagates instantaneously. These two properties allow the light to be controlled in an unprecedented way in the very small space available on optical chips; it can travel through exteremely narrow channels or waveguides, and go around sharp corners without losing energy. 

Now Mazur and his team at Harvard, working with researchers at Peking University in Beijing, report in the 19 October online edition of Nature Photonics the creation of an on-chip metamaterial with a refractive index of zero.

Fabricating such a metamaterial on a chip allows integration with other nanofabrication techniques for on-chip light manipulation, says Mazur.  He and his collaborators created a metamaterial layer consisting of silicon pillars embedded in a polymer matrix and covered on both sides with gold film, deposited on a silicon substrate.

Such a zero-index metamaterial layer can fill in many of the potholes engineers might otherwise face on the road to future photonic chips. One of those bumps in the road is is the coupling of light into the small structures—smaller than the diffraction limit of light—on optical chips. The team devised the concept of a "super coupler” [pdf] in which light is transported through zero-index materials that can deal with small sizes and sharp angles. This will reduce the size of optical connections, and eliminate losses in the transmission of light, says Yang Li, a member of Mazur's research team. 

“If we put a zero-index metamaterial into a waveguide made of mirrors, we can achieve a high-efficiency transmission, regardless of the length, squeeze, shape, twisting, or bending of the waveguide. These are phenomena that we are not able to achieve in the microwave and optical regimes by using regular waveguides,” says Li. A demonstation of a super coupler is next on the books, says Li. 

A second application is phase matching in nonlinear optics, which is the study of phenomena in matter caused by light that are not proportional to the intensity of the light. Optical processing will require separate light beams to interact with each other. Two light beams can only interact with each other by nonlinear processes, and only when the momentum of the outgoing photons matches the momentum of incoming photons, says Mazur. 

“Zero-index materials make this particularly easy because the momentum vector of light in a zero-index material is zero. This relaxes some of the constraints on nonlinear optical processes at the nanoscale,” says Mazur.

Applications in optical quantum computers may be promising because all the light emitters in a zero-index material must oscillate in phase. “Take quantum emitters, such as erbium ions, and you can have them entangled over much larger distances that you can have in any other type of environmemt,” says Mazur. 

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