Urbana-Champaign, IL--Resonantly optically excited periodic bow-tie nanoantenna (BNA) arrays can concentrate light a thousandfold more than can individual nanoantenna bow-ties. The increased UV-visible optical response could lead to highly efficient solar cells, as well as higher-resolution optical imaging of nano-sized objects such as proteins and DNA molecules.
A group from from the University of Illinois led by Nicholas Fang and Kimani Toussaint has demonstrated the thousandfold increase in response. Each BNA consists of two triangular pieces of gold with their tips facing each other in a bow-tie shape. They take energy from an illuminating laser beam and compress it into the nanometer gap separating the two triangles. The result is a concentrated spot of light that is many times more intense than the incoming beam.
The group fabricated 50-nm-thick gold BNAs composed of two equilateral triangles with 140 nm sides separated by a 20 nm gap and acquired the emission spectra when illuminated with 780 nm laser light using a camera made by Andor (Belfast, Ireland). When individual antennas were gathered into arrays with 500 nm center-to-center spacing, they found that the large local intensity enhancement of the single BNA was boosted by a factor of 1,000. In addition, the resonantly excited arrays exhibited uniform emission over a spectral region of more than 250 nm.
Studies have suggested that nanoantenna-based solar-energy-collection devices could have a conversion efficiency up to 80%.
Antoine Varagnat, a product specialist at Andor, noted that Andor’s "iDus" CCD platform, used in the research, is well-suited to the study of the key mechanisms at the origin of nanoantennas unique properties, namely nonlinear second-harmonic generation (SHG) and complex photoluminescence. The camera's high UV to near-IR response, low noise, and high dynamic range allow the analysis of a wide range of intensities of these broadband phenomena, providing the accurate information essential to the fine-tuning and optimization of the amplification properties of these nanoantennas. The UV-enhanced back-illuminated CCD was a good fit for the team’s 350 to 660 nm detection requirement, adds Varagnat.
Kaspar D. Ko et al., Nano Letters 11, p. 61 (2011).
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