Photonic devices could see a reduction in optical signal loss as a result of the discovery of a plasmonic metamaterial that compensates for loss of light energy by using a semiconductor to act as a light emitter. Plasmonic metamaterials typically contain metals that absorb energy from light and convert it into heat, wasting a portion of the optical signal and lowering its efficiency. “ SEM images of a “lossless” metamaterial that behaves simultaneously as a metal and a semiconductor. Courtesy of Ultrafast and Nanoscale Optics Group at UC San Diego. Using a multilayer architecture, researchers at the University of California, San Diego grew a crystal of the semiconductor material (InGaAsP) on a substrate. They then used high-energy ions from plasma to etch narrow trenches into the semiconductor, creating 40-nm-wide rows of semiconductor, spaced 40 nm apart. 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,” said researcher Joseph Smalley. “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.” When light from an IR laser was shined onto the metamaterial, the researchers found that, depending on which direction the light waves were polarized, the metamaterial either reflected or emitted light. “This is the first material that behaves simultaneously as a metal and a semiconductor. 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,” Smalley said. Through micro-photoluminescence measurements, absorption anisotropies greater than a factor of 10 and degree-of-linear polarization of emission greater than 0.9 were observed. Hyperbolic dispersion was verified with numerical simulations that modeled the metasurface as a composite nanoscale structure, and according to the effective medium approximation. Results of experiments showed a greater than 350 percent emission intensity enhancement relative to the bare semiconducting quantum wells. 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. The discovery has the potential to improve the efficiency of light-based technologies including fiber optic communication systems, lasers and photovoltaics. “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 Smalley. The research was published in Nature Communications (doi:10.1038/ncomms13793).