Plasmonic absorbers are gaining significant attention for applications such as photo/thermal detectors, solar energy conversion, and infrared imaging because of their exceptional ability to concentrate electromagnetic energy and trap it into thin layers to generate hot electrons. These absorbers are a determining factor in the performance of the whole system, making their efficiency and bandwidth of absorption crucial. A research team from Nanjing University, China, has now fabricated a broadband plasmonic absorber with average measured absorbance of 99% across wavelengths ranging from 400 nm to 10 µm.

As reported in a recent issue of Science Advances (doi:10.1126/sciadv.1501227), Lin Zhou, Yingling Tang, and Jia Zhu from the National Laboratory of Solid State Microstructures, Nanjing University, and collaborators from the University at Buffalo, The State University of New York, as well as the University of Wisconsin–Madison, created their plasmonic absorbers by self-assembling gold nanoparticles on a nanoporous template using a physical vapor deposition (PVD) process. A nanoporous alumina template with pore sizes of 30–400 nm provides a percolated scaffold and is used to control the deposition of the gold nanoparticles during PVD. The size of the gold nanoparticles could be changed by varying the pore size of the nanoporous template and the gas pressure of the PVD system. It was found that the gold nanoparticles were deposited on the surface of the template forming a metallic film, and also on the side walls of the pores as randomly distributed aggregates. The latter creates a gradual size distribution along the deposition path. This random size distribution, as shown in the figure, is critical for efficient and broadband absorption because it generates multiple overlapping plasmonic modes.

The absorption spectra of a bare nanoporous template and two gold sputtered templates with different pore diameters were measured over the wavelength range of 400 nm to 10 µm, and compared to simulated results to investigate the possible mechanism of absorption. The sample with the larger pore diameter of D = 365 nm showed an average measured absorbance of about 99% in the visible-near-infrared regime (400 nm to 2.5 µm) and greater than 99% in the mid-infrared regime (2.5–10 µm). “We are very excited about this work, particularly because this is the darkest metal reported so far,” says Zhu, who is the chief investigator. “A combination of extraordinary absorption with other properties of metals can open up tremendous opportunities, such as photocatalysis, sensing, and desalination.”

These plasmonic absorbers were tested for use in solar steam generation and demonstrated over 90% conversion efficiency at a solar irradiation of 4kW/m². Shanhui Fan from Stanford University says, “This is innovative work demonstrating an important application in energy technology of nanophotonic concepts. I look forward to seeing this scaled up into a practical system.”

The researchers believe that with more advancements in design and fabrication of different templates along with low-cost plasmonic materials like aluminum, large-scale manufacturing of complex nanoscale architectures will be possible for a diverse set of potential application fields.