The Color of Money
Michael Farady knew that the optical properties of bulk metals are very different from the properties of metallic nanoparticles. In his 1857 Bakerian lecture for the Royal Society, London, he further stated that “a mere variation in the size of its [gold] particles [gives] rise to a variety of resultant colours.”
Indeed, at the nanoscale, the optical (and many other) responses of metals depend crucially on size and shape, engendering the publication of a multitude of modern research papers.
According to SPIE Fellow Akhlesh Lakhtakia, editor-in-chief of the Journal of Nanophotonics, a recent paper in the journal, “Nanocrystalline cellulose for covert optical encryption,” has gone further. SPIE members Yu Ping Zhang and Vamsy P. Chodavarapu, along with co-authors Andrew G. Kirk, and Mark P. Andrews, show that the commonplace cellulose that has clothed us for millenniums and on which we have written for centuries, is fluorescent at the nanoscale.
Films of nanocrystalline cellulose (NCC) made by the authors at McGill University (Canada) are brightly iridescent, due to their structural chirality, very much like chiral sculptured thin films and cholesteric liquid crystals. Change the pitch and/or the viewing angle, and the color changes.
At bottom, excited with UV light.
Courtesy of Yu Ping Zhang
Zhang et al. attached an optical brightening agent to the NCC films for intense ultraviolet fluorescence that does not affect the visible iridescence, thereby fabricating a material that can be used to implement security features in banknotes and other negotiable paper. Cellulose comes from plants, is a renewable resource, and is naturally recyclable, all adding to the desirability of NCC.
NCC’s promise as an anti-counterfeiting measure lies in the level of overt encryption this renewable resource can provide because of its iridescent properties.
“Such films offer an additional level of covert encryption because they reflect left-circularly polarized light,” the researchers report. “A third level of (covert) encryption can be included by binding dye molecules that fluoresce when excited by UV light.
“The result is one system that supports multiple potential security features: overt security (iridescence) and covert security (selective circular reflection and fluorescence).”
Says Lakhtakia: “Look out for iridescent banknotes and credit cards in the near future.”
Source: Journal of Nanophotonics 6, 063516 (2012); doi: 10.1117/1.JNP.6.063516
Optical Engineering, the flagship journal of SPIE, will have two special sections on astronomy topics in August 2013.
Manuscripts are due 1 October for a special section on ground-based and airborne telescopes and instrumentation. Guest editor is Helen Hall of NASA Ames Research Center.
Manuscripts are due 1 November for the special section on adaptive optics systems, guest edited by SPIE member Brent Ellerbroek of the Thirty Meter Telescope Observatory.
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