Polarization‐Controlled Chromo‐Encryption

Abstract

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 ...

Recent progress on structural coloration

“…Over 1700 years ago, Roman artisans created the famous Lycurgus Cup [84] by mixing nano-sized metal particles into glass. Due to the strong modulation effect of SPR on electromagnetic waves, there have been numerous reports of high spatial resolution color filters based on SPR [49,85,86] . In addition to classical plasmonic metasurfaces, researchers have also designed new nanostructures such as gap plasmons [87] and Fabry-Pérot (FP) cavities [88–90] , and this section will briefly discuss them.…”

“…Typically, utilizing the anisotropy of meta-atoms to achieve different colors for various polarization states is the most common optical encryption method [243–247] . Alternatively, multiple colors can be further encoded using encryption techniques such as binary or Morse code [86,248,249] . However, achieving foolproof advanced optical encryption with these methods can be challenging.…”

Polarization-Dependent Thin Films with Biaxial Anisotropic Absorption Constructed by a Single Coating and Subsequent Topochemical Polymerization of Chromophores

“…Optical polarizers have been widely studied and developed by uniaxial orientation in thin films to achieve anisotropic properties. , Due to the low weight, thin thickness, and high processability of the layer, the coating is the preferred means of achieving uniform molecular alignment on macroscopic length scales. , Lyotropic liquid crystals and host–guest complexes can be good candidates as building blocks for various types of optically anisotropic materials for their unique polarization-dependent optical properties. − This can be used in various fields such as E- and O-type polarizers, smart windows, and optical security devices. − Many efforts have been made to obtain the desired broadband absorption in the visible light spectrum through molecular design. − However, there are challenges in achieving broadband absorption for multiplex applications, including 3D optical displays, chiral optical materials, and polarization-based encryption. These challenges arise due to the linear dichroism of conventional thin film polarizers, which induce differential absorbance by varying the polarization direction. − Stacking two or more layers has been attempted to show various aspects of transmission colors according to the polarization direction. , Despite these achievements, a single-layer multichromic film remains an important goal.…”

Color Generation and Polarization-Sensitive Encryption by Laser Writing on Plasmonic Reflector Arrays

“…Leveraging the fine-tuning capabilities of individual unit cells in structural color metasurfaces, various optical encryption methods have been developed. By arranging anisotropic and isotropic nanoaperture or nanorod arrays based on their polarization-dependent plasmonic resonance characteristics, optical images can be encrypted and decrypted with high precision. − High-resolution grayscale images can also be encrypted using spatially variant polarization states of laser beams, which are revealed through a linear polarizer . Mechanically tunable all-dielectric metasurfaces embedded in stretchable substrates have demonstrated polarization insensitivity and full-spectrum response in the visible range .…”