illuminating a metasurface (encrypted information) with a modulated incident light (key). [15] Another group has patterned the encrypted information by encoding the orbital angular momentum helical phase and Fourier transform lens, such that when decrypting with correct keys (timedependent incident beams) the encrypted information becomes comprehensive. [16] In addition to reconstructing the information in the far field like holograms, encrypted metasurfaces composed of complex-amplitude units can be arranged into gray scale or color images. [17] Combining the channels in the far field and near field makes optical encryption more versatile, like in binary or color QR codes. [18] Since the spectral response of nanostructures is generally sensitive to the polarization orientation it can serve as key to decrypt information. [19] However, these encryption concepts are not yet utilized to their full potential. In this work, we use a simple geometry to demonstrate a plethora of spectral responses, which are then employed for encryption using a quaternary system.Here, we resort to silver (Ag) for its low losses in the visible. [20] Each color pixel (unit cell) contains a rectangular Ag nanorod on a dielectric pedestal that forms an aperture of similar geometry within an Ag mirror underneath (see Experimental Section). The unit cells are placed on a square array with period 300 nm, which prevents grating diffraction orders and Wood-Rayleigh anomalies at the visible wavelengths of interest. [21] The coupling between the nanostructures and the mirror plane is negligible since the pedestal height is larger than 80 nm. [22] The readout scheme is illustrated in Figure 1a, incident light (in black), passing through a linear polarizer and a polarization insensitive beam splitter, impinges on the sample. The reflected light (in red) is then analyzed with another polarizer (analyzer). When both polarizers are parallel with the nanostructures (horizontal, H-direction, Figure 1b), the observed colors are muted, with a low chroma caused by the low Ag absorption. When rotating both polarizers perpendicular to the nanostructures (vertical, V-direction, Figure 1c), all the colors appear grey due to the lack of dimensional variations in this direction. However, the colors become vivid and saturated when both polarizers are in the diagonal direction (+45°, D-direction), Figure 1d. This difference of color contrast controlled by the mere orientation of the polarization is quite baffling, since it contradicts our intuition that the response for diagonal polarization could be inferred from the superposition The response of simple plasmonic nanorods to polarized illumination is studied in detail. Depending on the orientation of that polarization with respect to the symmetry axes of the nanostructure, a chiral response can occur, which can be analyzed through a second polarizer, in order to control the spectral response of the system. Specifically, for the Ag nanorods fabricated here, a broad variety of colors can be produced that cover half ...
