Researchers from the National Institute of Standards and Technology (NIST) and the University of Maryland have shown how to make nanoscale measurements of critical properties of plasmonic nanomaterials — the specially engineered nanostructures that modify the interaction of light and matter for applications including sensors, cloaking (invisibility), photovoltaics, and therapeutics.
Plasmonic nanomaterials contain specially engineered conducting nanoscale structures that can enhance the interaction between light and an adjacent material, and the shape and size of such nanostructures can be adjusted to tune these interactions. Theoretical calculations are frequently used to understand and predict the optical properties of plasmonic nanomaterials, but few experimental techniques are available to study them in detail. Researchers need to be able to measure the optical properties of individual structures and how each interacts with surrounding materials directly in a way that doesn't affect how the structure functions.
The research team used photothermal induced resonance (PTIR), an emerging chemically specific materials analysis technique, and showed it can be used to image the response of plasmonic nanomaterials excited by infrared (IR) light with nanometer-scale resolution. The team used PTIR to image the absorbed energy in ring-shaped plasmonic resonators. The nanoscale resonators focus the incoming IR light within the rings' gaps to create "hot spots" where the light absorption is enhanced, which makes for more sensitive chemical identification. For the first time, the researchers precisely quantified the absorption in the hot spots and showed that for the samples under investigation, it is approximately 30 times greater than areas away from the resonators.
Their work further demonstrated the versatility of PTIR as a measurement tool that allows simultaneous measurement of a nanomaterial's shape, size, and chemical composition — the three characteristics that determine a nanomaterial's properties. Unlike many other methods for probing materials at the nanoscale, PTIR doesn't interfere with the material under investigation; it doesn't require the researcher to have prior knowledge about the material's optical properties or geometry.
