Gray Level Image Encoding in Plasmonic Metasurfaces
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Gray Level Image Encoding in Plasmonic Metasurfaces Ting Zhang 1,2 & Steve Blair 2 Received: 16 October 2019 / Accepted: 11 March 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Plasmonic metasurfaces have been widely used for image and color representation using nanoscale structures. By designing the size or shape of nanostructures, the phase or amplitude of transmitted or reflected electromagnetic spectrum can be manipulated via control of resonant wavelength, which results in multiple colors and can be used for color and/or image generation. Instead of color printing, we introduce a new approach from which images with multiple gray scales can be generated and hidden under light with specific wavelengths. In this work, plasmonic structures of nanorods with identical geometries are fabricated in arrays on a glass substrate, but the nanorods comprise binary alloys of aluminum and titanium; therefore, grayscale image information is encoded with the material composition. Under white light illumination with polarization along the long axis of a nanorod, the response of the nanorods has essentially the same resonant frequency, but with varying magnitudes of transmission. Based on this, using nanorods with four different compositions, four gray level imaging has been achieved. Since nanorods are sensitive to the polarization and spectral composition of the incident light, under unpolarized white light illumination, the image disappears. Therefore, this technique is promising in image encryption. We also show that the manipulation of light at the resonant wavelength by this method occurs in amplitude, not phase. Keywords Plasmonics . Metasurface . Image-encoding
Plasmonic resonance occurs through the interaction of electromagnetic waves with electron oscillations in metals. Metal nanostructures fabricated with varied shapes, dimensions, orientations, and materials lead to manipulations of the amplitude and phase of electromagnetic waves [1, 2]. Applications for such plasmonic metasurfaces abound in new optical devices, such as flat and ultrathin optical components, waveguides, invisibility cloaks, and holograms [3–12]. In particular, plasmonic metasurfaces have emerged in application to color image generation [13–23]. By tailoring an electromagnetic wave’s spectrum using nanostructures with spatially varying sizes or shapes, color images with high resolution and gamut can be generated [15–18]. Furthermore, encryption of images has been demonstrated using dynamic plasmonic Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11468-020-01151-5) contains supplementary material, which is available to authorized users. * Steve Blair [email protected] 1
Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA
2
Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84112, USA
metasurface devices [19–25]. Image hiding and generation have been achieved by switching the refractive in
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