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SEM images of a "lossless" metamaterial that behaves simultaneously as a metal and a semiconductor. Image credit: Ultrafast and Nanoscale Optics Group at UC San Diego

A newly developed material could help reduce signal losses in photonic devices, while boosting the efficiency of various light-based technologies, including fiber optic communication systems, lasers and photovoltaics.

Engineers at the University of California San Diego have developed this new material that could address the loss of optical signals in devices known as plasmonic metamaterials, one of the biggest challenges in the field of photonics.

Plasmonic metamaterials are engineered at the nanoscale to control light in unusual ways. They can be used to develop exotic devices ranging including quantum computers.

However, an issue with the metamaterials is that they typically contain metals that absorb energy from light and convert it into heat, which wastes a part of the optical signal, lowering the efficiency.

The research was led by electrical engineering professor Shaya Fainman at the UC San Diego Jacobs School of Engineering.

Joseph Smalley, an electrical engineering postdoctoral scholar in Fainman's group and the first author of the study, explained how the team made up for the loss of efficiency.

“We're offsetting the loss introduced by the metal with gain from the semiconductor,” Smalley said in a statement. “This combination theoretically could result in zero net absorption of the signal—a 'lossless' metamaterial.”

The research team shined light from an infrared laser onto the metamaterial and found that depending on which way the light is polarized—which plane or direction all the light waves are set to vibrate—the metamaterial either reflects or emits light.

“This is the first material that behaves simultaneously as a metal and a semiconductor,” Smalley said. “If light is polarized one way, the metamaterial reflects light like a metal, and when light is polarized the other way, the metamaterial absorbs and emits light of a different 'color' like a semiconductor.”

The researchers created the new metamaterial by first growing a crystal of the semiconductor material called indium gallium arsenide phosphide on a substrate. They then used high-energy ions from plasma to etch narrow trenches into the semiconductor, which created 40-nanometer-wide rows of semiconductor spaced 40 nanometers apart.

The last step was to fill the trenches with silver to create a pattern of alternating nano-sized stripes of semiconductor and silver.

According to Smalley, nanostructures with different layers are often made by depositing each layer separately one on top of another. However, the semiconductor material used in the study can’t be grown on top of any substrate or it will have defects.

“Rather than creating a stack of alternating layers, we figured out a way to arrange the materials side by side, like folders in a filing cabinet, keeping the semiconductor material defect-free,” Smalley said.

According to the study, when engineered on scales much smaller than the operating wavelength, metal-semiconductor nanostructures exhibit properties unobtainable in nature.

A uniaxial optical metamaterial described by a hyperbolic dispersion relation can simultaneously behave as a reflective metal and an absorptive or emissive semiconductor for electromagnetic waves with orthogonal linear polarization states, according to the study. 

The team now plans on investigating how much the metamaterial and other versions of it could improve photonic applications that currently suffer from signal loss.

The study was published in Nature Communications.

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