Australian scientists have fabricated an ultra-thin semiconductor film which perfectly absorbs light, without reflecting any of it or letting it through.
This breakthrough has implications for devices like photodetectors, optical switches, modulators and transducers, including infrared devices.
It is not unheard of for thin materials to perfectly absorb light, with zero reflection and zero transmission. However, until now, this has required the use of exotic materials, metamaterials, thick metallic gratings, or complex nanostructures.
The breakthrough, spearheaded by researchers at the University of Sydney, found that it is possible to use conventional, weakly-absorbing semiconductors, in conjunction with ultrathin grating structures, to absorb nearly 99% of light. Additionally, the structure of the grating is such that it is much simpler to design and fabricate.
To test their theories, the researchers started on a base of polished silicon wafer. They then used thermal evaporation to place a 130 nm reflective layer of silver, then a 245 nm thick spacer layer of silicon dioxide, followed by a 41 nm thick layer of antimony sulphide.
According to co-author of the paper, Professor Martijn de Sterke from the University of Sydney’s School of Physics, the researchers then etched a pattern of thin grooves, or “gratings” into the antimony sulphide, which then was responsible for much of the subsequent light-absorbing capabilities.
“By etching thin grooves in the film, the light is directed sideways and almost all of it is absorbed, despite the small amount of material – the absorbing layer is less than 1/2000th the thickness of a human hair,” he said.
Without the gratings, the material absorbed just 7.7% of the light. After patterning the semiconductor layer, the researchers found the grating absorbed 98.9% of the light.
This dramatic improvement in efficiency could deliver cost and size savings to things like infrared detectors, which today can cost tens if not hundreds of thousands of dollars.
A key part of these devices, which form the basis of night vision goggles, infrared telescopes, and thermal imaging used by military, medical and industrial applications, is the light absorber, which adds both bulk and cost, and requires constant power to keep the temperature down. An ultrathin absorber could reduce these drawbacks.
Such an improvement could make infrared technology easier to use and cheaper, potentially saving millions of dollars in the defence industry and other areas, and also boost the availability of the technology in a host of new areas, such as agriculture.
Another advantage of this breakthrough is the use of conventional semiconductors in the absorber, which is not only cheaper, but also improves compatibility with optoelectronic applications, allowing the possibility of extracting a photocurrent or measuring the photoresistivity.
The findings are a result of collaborative work between the University of Sydney, UTS, and ANU, including use of the NeCTAR cloud at NCI, Australia’s national supercomputer centre. The work was also funded by the Australian Renewable Energy Agency and the Australian Research Council.