Shine a light: material behaves simultaneously as metal and semiconductor

The discovery by engineers at the University of California San Diego is claimed to address one of the biggest challenges in the field of photonics, namely minimising loss of optical signals in plasmonic metamaterials.

Plasmonic metamaterials are materials engineered at the nanoscale to control light and can be used to develop devices ranging from invisibility cloaks to quantum computers. Metamaterials typically contain metals that absorb energy from light and convert it into heat. Consequently, part of the optical signal gets wasted, lowering the efficiency.

In a recent study published in Nature Communications, a team of photonics researchers led by electrical engineering professor Shaya Fainman at the UC San Diego Jacobs School of Engineering demonstrated a way to make up for these losses by incorporating a light emitting semiconductor material into the metamaterial.

"We're offsetting the loss introduced by the metal with gain from the semiconductor. This combination theoretically could result in zero net absorption of the signal - a 'lossless' metamaterial," said Joseph Smalley, an electrical engineering postdoctoral scholar in Fainman's group and the first author of the study.

In their experiments, the researchers are said to have shined light from an infrared laser onto the metamaterial. They found that depending on which way the light is polarised the metamaterial either reflects or emits light.

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

Researchers created the new metamaterial by first growing a crystal of the semiconductor material indium gallium arsenide phosphide on a substrate. They then used high-energy ions from plasma to etch narrow trenches into the semiconductor, creating 40nm rows of semiconductor spaced 40nm apart. Finally, they filled the trenches with silver to create a pattern of alternating nano-sized stripes of semiconductor and silver.

"This is a unique way to fabricate this kind of metamaterial," Smalley said.

Nanostructures with different layers are often made by depositing each layer separately one on top of another, Smalley explained, but the semiconductor material used in this study can't be grown on top of any substrate - like silver - otherwise 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."

As a next step, the team plans to investigate how much this metamaterial and other versions of it could improve photonic applications that currently suffer from signal losses.