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Trapping rainbows: ultrathin films that absorb light

18 February 2013

Engineers have achieved an advance in photonics that could lead to technological breakthroughs in solar energy and stealth technology.

An view of the hyperbolic metamaterial waveguide, which catches and ultimately absorbs wavelengths (or colour) in a vertical direction (illustration courtesy of the University at Buffalo)

Qiaoqiang Gan, an assistant professor of electrical engineering at the University at Buffalo (UB), and a team of graduate students have developed a 'hyperbolic metamaterial waveguide' which is essentially an advanced microchip made of alternate ultra-thin films of metal and semiconductors and/or insulators. The waveguide halts and ultimately absorbs each frequency of light, at slightly different places in a vertical direction (see illustration), to catch a 'rainbow' of wavelengths.

“Electromagnetic absorbers have been studied for many years, especially for military radar systems,” Dr Gan said. “Right now, researchers are developing compact light absorbers based on optically thick semiconductors or carbon nanotubes. However, it is still challenging to realise the perfect absorber in ultra-thin films with tunable absorption band.

“We are developing ultra-thin films that will slow the light and therefore allow much more efficient absorption, which will address the long existing challenge.”

In their initial attempts to slow light, researchers relied upon cryogenic gases. But these are difficult to work with outside a laboratory.

Before joining UB, Dr Gan helped pioneer a way to slow light without cryogenic gases. He and other researchers at Lehigh University made nano-scale-sized grooves in metallic surfaces at different depths, a process that altered the optical properties of the metal. While the grooves worked, they had limitations. For example, the energy of the incident light cannot be transferred onto the metal surface efficiently, which hampered its use for practical applications.

The hyperbolic metamaterial waveguide solves that problem because it is a large area of patterned film that can collect the incident light efficiently. It is referred to as: an artificial medium with subwavelength features whose frequency surface is hyperboloid. This allows it to capture a wide range of wavelengths in different frequencies, including visible, near-infrared, mid-infrared, terahertz and microwaves.

It could lead to advancements in an variety of fields. For example, the elimination of crosstalk in electronic circuits. The on-chip absorber may also be applied to solar panels and other energy-harvesting devices. It could be especially useful in mid-infrared spectral regions as a thermal absorber for devices that recycle heat after sundown.

And because the on-chip absorber has the potential to absorb different wavelengths at a multitude of frequencies, it could be useful as a stealth coating material.

The authors describe their work in a paper - Rainbow Trapping in Hyperbolic Metamaterial Waveguide - published in the online journal, Scientific Reports.

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