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Material Gives Visible Light an Infinite Wavelength

AMSTERDAM & PHILADELPHIA, Oct. 16, 2013 — A new metamaterial made from stacks of silver and silicon nitride nanolayers gives visible light a nearly infinite wavelength, an international team reports. The material could be used to make novel optical components or more efficient LEDs.

Light’s phase velocity (how the peaks and valleys of the wave move) and group velocity (the transport of energy) dictate how it propagates in a material. Energy transport is limited to the speed of light, according to Einstein’s laws, but there are no physical limitations on the phase velocity. When it reaches zero, meaning there is no movement of the peaks and valleys of the wave, the phase velocity becomes infinite, making the wavelength of light nearly infinite.

Effective permittivity of a layered Ag/SiN-metamaterial for a variety of layer thicknesses, plotted against wavelength. Images courtesy of FOM Institute AMOLF.

While optical metamaterials exist that operate in the microwave and far-IR ranges of the spectrum, such engineered materials with a near-zero permittivity (epsilon near zero, or ENZ) in the visible spectrum have been elusive. But advanced ion- and electron-beam nanolithography methods have now brought them within reach.

“If three-dimensional ENZ metamaterials could be made in the visible, entirely new forms of waveform shaping, light tunneling and spontaneous emission enhancement would become possible over this technologically important spectral range,” the team from the Dutch research institute FOM Institute AMOLF and the University of Pennsylvania said in their paper, which appears in Nature Photonics

An electron microscope image of the top side of the fabricated metamaterial. The silver and silicon nitride layers are clearly visible as bright and dark bands respectively. The top surface of the metamaterial is polished using an ion beam to remove excess silver.

They fabricated their visible-wavelength ENZ metamaterial using focused ion beam milling, a technique that allows control over the structure of a material at the nanoscale. Their metamaterial has a unit cell structure much smaller than the wavelength of light. Because the permittivity (resistance) of silver is negative and silicon nitride’s is positive, they effectively cancel each other out to give the combined material a permittivity equal to zero. With zero resistance, light propagates with an infinite phase velocity.

Using a specially built interferometer, they observed that light propagated through the metamaterial with no significant change of phase at wavelengths from 351 to 633 nm, corresponding to an almost infinite wavelength.

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