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Nanometre-scale design squeezes light

02 May 2013

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A research team has designed a way to engineer atoms capable of funneling light through ultra-small channels. According to them, the process they developed is similar to squeezing an elephant through a pinhole.

The study, conducted by Missouri University of Science and Technology (Missouri S&T) scientists, is the latest in a series of recent findings related to how light and matter interact at the atomic scale, and it is the first to demonstrate that the material—a specially designed "meta-atom" of gold and silicon oxide—can transmit light through a wide bandwidth and at a speed approaching infinity. The meta-atoms' broadband capability could lead to advances in optical devices, which currently rely on a single frequency to transmit light, the researchers said.

"These meta-atoms can be integrated as building blocks for unconventional optical components with exotic electromagnetic properties over a wide frequency range," according to Jie Gao and Xiaodong Yang, assistant professors of mechanical engineering at Missouri S&T, and Lei Sun, a visiting scholar at the university. The researchers created mathematical models of the meta-atom, a material 100nm wide and 25nm tall that combined gold and silicon oxide in stair-step fashion. A nanometre is one billionth of a metre and visible only with the aid of a high-power electron microscope.

In their simulations, the researchers stacked 10 of the meta-atoms, then shot light through them at various frequencies. They found that when light encountered the material in a range between 540THz and 590THz, it "stretched" into a nearly straight line and achieved an "effective permittivity" known as epsilon-near-zero.

Effective permittivity refers to the ratio of light's speed through air to its speed as it passes through a material. When light travels through glass, for instance, its effective permittivity is 2.25. Through air or the vacuum of outer space, the ratio is one. That ratio is what is typically referred to as the speed of light.

However, as light passes through the engineered meta-atoms described by Gao and Yang, its effective permittivity reaches a near-zero ratio. In other words, through the medium of these specially designed materials, light actually travels faster than the speed of light. It travels "infinitely fast" through this medium, Yang said.

The meta-atoms also stretch the light. Other materials, such as glass, typically compress optical waves, causing diffraction.

This stretching phenomenon means that "waves of light could tunnel through very small holes," Yang said. "It is like squeezing an elephant through an ultra-small channel."

The wavelength of light encountering a single meta-atom is 500nm from peak to peak, or five times the length of Gao and Yang's specially designed meta-atoms, which are 100nm in length. While the Missouri S&T team has yet to fabricate actual meta-atoms, they say their research shows that the materials could be built and used for optical communications, image processing, energy redirecting and other emerging fields, such as adaptive optics.

Last year, Albert Polman at the FOM Institute for Atomic and Molecular Physics in Amsterdam and Nader Engheta, an electrical engineer at the University of Pennsylvania, developed a tiny waveguide device in which light waves of a single wavelength also achieved epsilon-near-zero. But the Missouri S&T researchers' work is the first to demonstrate epsilon-near-zero in a broadband of 50THz.

"The design is practical and realistic, with the potential to fabricate actual meta-atoms," said Gao. "With this research, we filled the gap from the theoretical to the practical," Yang added.

Through a process known as electron-beam deposition, the researchers have built a thin-film wafer from 13 stacked meta-atoms. But those materials were uniform in composition rather than arranged in the stair-step fashion of their modelled meta-atoms.




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