Tuesday 9th August 2022

Addressable metasurfaces for dynamic holography and optical information encryption

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The ability to endow addressable plasmonic pixels with hierarchical dynamics in a programmable manner allows advanced optical information transmission and encryption. As a proof-of-concept experiment, a set of geometric codes has been designed and illustrated in Fig. 4A. Each Arabic number is represented by a symbol, comprising a unique sequence of dots and dashes, similar to the coding principle of Morse codes. All the numbers from 0 to 9 can be expressed by specific holographic images that are reconstructed from the same metasurface based on the mechanism as shown in Fig. 4B. Each unit cell contains three different pixels: P1 (Mg/Pd), P2 (Au), and P3 (Mg/Pd/Cr). The holographic patterns achieved by P1 and P3 are hollow dashes at two different locations, whereas the holographic pattern generated by P2 comprises one solid dash and three dots (see also fig. S4B). The shape of the dots is complementary to that of the hollow dashes so that they can fit exactly in space to form two solid dashes. It is noteworthy that the intensities of the individual holographic patterns are inversely proportional to their pattern areas. Therefore, P2 is applied twice in each unit cell for achieving a uniform intensity distribution within a merged hologram. The reconstructed holographic image of “1” is presented in Fig. 4B. The holographic images of other numbers can be generated through different routes governed by the helicity of light, the sequences of hydrogenation and dehydrogenation, and the reaction duration (see Fig. 4C). We define a coding rule that each number corresponds to a union set of the two holographic patterns in zones I and II, as shown in Fig. 4C. It is known that the sign of the acquired phase profile is flipped when the helicity of the incident light is changed (12). Therefore, two centrosymmetric holographic patterns can occur in zones I and II, respectively. When P1, P2, and P3 are all at the on state, RCP light illumination uncovers “1” in zone II, whereas LCP light illumination gives rise to its centrosymmetric image in zone I, corresponding to “9” (see Fig. 4C and fig. S5, A and B). The centrosymmetry point is indicated by a white circle in each plot, highlighting the location of the zero-order reflected light. When linearly polarized (LP) light is applied, the two holographic patterns appear simultaneously. On the basis of the coding rule, a union set of the holographic patterns in the two zones gives rise to four solid dashes, corresponding to “0” (see Fig. 4C and fig. S5C). Through sequenced hydrogenation and dehydrogenation, the geometric codes corresponding to the rest of the numbers can be achieved, as shown in Fig. 4C. A video that records the dynamic transformation among different numbers can be found in movie S3 (A and B).
Following the Morse code abbreviations, in which different combinations of numbers represent diverse text phrases for data transmission, we demonstrate a highly secure scheme for optical information encryption and decryption using dynamic metasurfaces. As illustrated in Fig. 5, Alice would like to send different messages to multiple receivers, including Tim, Bob, Ted, and so forth. These messages are all encrypted on one metasurface. Identical samples are sent to the receivers. Upon receipt of his sample, together with the customized keys, Tim reads out the information of 88, which means “love and kisses” in Morse codes following LCP/H2(1). Bob first applies LCP/H2(1)/H2(2)/O2 to obtain “7.” After resetting the sample through sufficient dehydrogenation, he reads out “3” following RCP/H2(1)/H2(2)/O2. The decrypted message is therefore “best regards.” Similarly, other receivers can decode their respective messages using the provided keys. In other words, a single metasurface design can be deciphered into manifold holographic messages, given that the sequences of keys are customized. This elucidates an unprecedented level of data compactness and security for transmitting information, especially to a pool of multiple receivers.

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