Researchers at the Missouri University of Science & Technology have designed a new nanoscale material that can transmit light -- at least its phase front -- faster than the 186,000 miles per second it usually takes to go through air.
That speed through air, or through the vacuum in outer space, is what we usually think of as the speed of light. But light travels at different speeds depending on the material it is passing through. The metric for expressing this difference is the effective permittivity of light: the ratio of light's speed through air to its speed when passing through another material. Its effective permittivity is 1.0 when passing through air. When passing through glass, effective permittivity is 2.25.
What's new about this new material is that it's achieved an effective permittivity of zero. The ratio is called epsilon-near-zero, but it's essentially zero, Xiaodong Yang, assistant professor in the department of mechanical and aerospace engineering and one of the material's inventors, told Design News. That means light is traveling infinitely fast. "This does not mean the energy velocity of light passing through this material goes to infinity, or faster than the speed of light through air," Yang told us. "It's only the phase velocity of light that reaches infinity."
A new nanoscale material can transmit light faster than the 186,000 miles per second it usually takes to travel through air. The "meta-atom" is made of glass layered on gold in a stairstep-like structure that straightens and speeds up light waves, making possible a broadband waveguide with increased bandwidth.
(Source: Missouri University of Science and Technology)
The new material the team designed is an engineered "meta-atom" measuring 100 nm x 25 nm made of silicon oxide, or glass, layered on top of gold in a stairstep-like structure. The meta-atom combines these two materials in an exact mix. "The effective permittivity of glass is positive, at plus 2.25, and for gold it's negative, at around minus 10.0," said Yang. "When you combine them in the right mix, the sum of the two is zero, so you get a permittivity of zero." The researchers have published their work in an article (purchase or subscription only) in the journal Physical Review B.
The researchers created mathematical models of the meta-atoms and in a simulation, stacked 10 of them together. They then passed light through the stack at different frequencies. When light passed through in the range between 540 THz and 590 THz, the shape of the light wave flattened and stretched until it became straight. This is the opposite of what happens to light waves when they pass through other materials, such as a glass or water, which typically cause diffraction by compressing the waves.
Because the light waves stretch when passing through the meta-atoms, they can tunnel through extremely small paths in the material within this 50 THz bandwidth. The stairstep structure has three steps, each with a different height of the gold layer. This creates three different paths or channels, each allowing light to pass through at a different wavelength. "When you combine the three you have a broadband, or wideband, instead of a limited single frequency," said Yang. "You can use any three wavelengths within that 50 THz range."
The team has already built a thin-film wafer of 13 stacked meta-atoms, using an electron-beam deposition process. That material is a uniform layer for a single wavelength. "The stairstep structure is hard to build, because these are very precise, controlled structures," said Yang. "So we are thinking about how to do this. It should be possible soon with state-of-the-art nanofabrication methods. For example, I can now make a uniform gold structure first, then mill it using a focused ion beam to get the stairsteps, then coat the glass on top and make it flat. That's one cycle, which can be repeated in many cycles. That procedure is doable."