Abstract-Bistable Physical Geometries for Terahertz Plasmonic Structures Using Shape Memory Alloys

http://onlinelibrary.wiley.com/doi/10.1002/adom.201601008/full

Shape memory alloy foils that are appropriately patterned are cycled between two different metal foil geometries resulting in two different terahertz (THz) plasmonic responses. This is accomplished by using patterned foils of a nickel–titanium alloy (Nitinol) that switches between the martensite phase below 31 °C, yielding one physical geometry, and the austenite phase, when the foil is heated above 51 °C, yielding a second physical geometry. In order to enable this reproducible switching, the sample is initially put through a two-way training procedure, through which the two different desired physical geometries are imprinted. Specifically, the metal foils are trained to switch between a sinusoidal corrugation, either 1D or 2D, at close to room temperature and a flat metal sheet above the austenite phase transition temperature. The foils are found to switch reproducibly between geometries over at least 100 thermal cycles. Using THz time-domain spectroscopy, the transmission properties of the foils are measured as a function of incident polarization and foil geometry. The changes in spectrum are explained qualitatively and through numerical simulation.

Abstract-Terahertz Plasmonic Structures Based on Spatially Varying Conductivities

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1. Barun Gupta¹, 
2. Shashank Pandey¹,
3. Sivaraman Guruswamy², 
4. Ajay Nahata^1,*

Article first published online: 7 MAR 2014

DOI: 10.1002/adom.201400018

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

http://onlinelibrary.wiley.com/doi/10.1002/adom.201400018/abstract

Terahertz plasmonic structures are demonstrated in which the conductivity of the metallic film is varied spatially in order to further enhance the response. Using a commercially available inkjet printer, in which one cartridge is filled with conductive silver ink and a second cartridge is filled with resistive carbon ink, computer generated drawings of plasmonic structures are printed in which the individual printed dots can have differing amounts of the two inks, thereby creating a spatial variation in the conductivity. The silver ink has a DC conductivity that is only a factor of six lower than bulk silver, while the carbon ink acts as a lossy dielectric at THz frequencies. Both inks sinter at room temperature immediately after contact with the plastic film. Using a periodic array of subwavelength apertures as a test structure, patterns printed with different fractional amounts of the two inks show dramatically different enhanced optical transmission properties. These differences arise from changes in the propagation loss properties as a function of conductivity. This data is used to design and fabricate aperture arrays in which the conductivity varies spatially. The resulting plasmonic effect is found to dramatically alter the spatial beam profile of the transmitted THz radiation, as measured by THz imaging.