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Addressable metasurfaces for dynamic holography and optical information encryption
Jianxiong Li, Simon Kamin, Guoxing Zheng, Frank Neubrech, Shuang Zhang, Na Liu
TL;DR
Holographic images have largely remained static because their phase and amplitude are fixed, limiting dynamic optical information processing. This paper uses chemically active metasurfaces with independently controlled pixels to create dynamic holograms, enabling customized data storage and optical communication functionalities.
Problem
Holographic images have been restricted to static operation because their phase and amplitude are fixed, while imparting complex functionality remains challenging.
Method
The authors employ chemically active metasurfaces whose plasmonic pixels reversibly switch through hydrogenation and oxygen-driven dehydrogenation.
Results
Customized metasurfaces and keys increase conveyed data quantity while enabling compact optical applications and broader metasurface functionality.
Takeaways & Limitations
Independent control of dynamic pixels supports novel data storage and optical communication systems with high spatial resolution and data density.
Abstract
from arXiv · showhide
Metasurfaces enable manipulation of light propagation at an unprecedented level, benefitting from a number of merits unavailable to conventional optical elements, such as ultracompactness, precise phase and polarization control at deep subwavelength scale, and multifunctionalities. Recent progress in this field has witnessed a plethora of functional metasurfaces, ranging from lenses and vortex beam generation to holography. However, research endeavors have been mainly devoted to static devices, exploiting only a glimpse of opportunities that metasurfaces can offer. We demonstrate a dynamic metasurface platform, which allows independent manipulation of addressable subwavelength pixels at visible frequencies through controlled chemical reactions. In particular, we create dynamic metasurface holograms for advanced optical information processing and encryption. Plasmonic nanorods tailored to exhibit hierarchical reaction kinetics upon hydrogenation/dehydrogenation constitute addressable pixels in multiplexed metasurfaces. The helicity of light, hydrogen, oxygen, and reaction duration serve as multiple keys to encrypt the metasurfaces. One single metasurface can be deciphered into manifold messages with customized keys, featuring a compact data storage scheme as well as a high level of information security. Our work suggests a novel route to protect and transmit classified data, where highly restricted access of information is imposed.
MAIN TEXT · Introduction
Metasurface holograms offer precise, ultrathin optical control but are typically static because fabrication fixes their phase and amplitude profiles. This work introduces chemically active magnesium metasurfaces with independently dynamic plasmonic pixels, enabling visible-frequency holograms with multiplexed, sequenced, and encrypted functionality.
- Introduction: The work addresses the central limitation that metasurface holograms are generally static because their phase and amplitude profiles are fixed after fabrication.This limitation motivates the transition toward dynamic metasurface holography.
- Introduction: Dynamic reconfigurability at visible frequencies requires multiplexed addressable subwavelength pixels with independently controllable dynamic functions.These requirements are identified as significant remaining challenges.
- Introduction: The authors overcome these challenges using chemically active magnesium metasurfaces whose plasmonic pixels exploit magnesium hydrogenation/dehydrogenation characteristics.The chemical reactions provide the basis for dynamic pixel operation.
- Introduction: The platform demonstrates a series of dynamic metasurface holograms operating at visible frequencies with novel functionalities.The demonstrated functionalities include static and dynamic patterns, sequenced dynamics, and encrypted holography.
- Introduction: One demonstrated hologram combines static and dynamic patterns within the same metasurface platform.This is presented as one of the platform’s novel holographic functionalities.
- Introduction: Another demonstrated hologram exhibits differently sequenced dynamics, extending the platform beyond a single switching behavior.The introduction identifies sequenced dynamics as a distinct demonstrated functionality.
- Introduction: An encrypted hologram can be deciphered into manifold messages, providing a dynamic metasurface route for optical information security.The encrypted hologram is listed among the demonstrated visible-frequency functionalities.
Results
Addressable Mg-based plasmonic pixels enable reversible, chemically controlled holographic switching and hierarchical dynamics at visible frequencies. These capabilities support multiplexed optical encoding and customized-key encryption, allowing one metasurface to yield multiple messages.
- Phase-transition of Mg nanorod: Mg nanorods reversibly switch their plasmonic response between “on” and “off” through hydrogenation and oxygen-driven dehydrogenation, forming dynamic plasmonic pixels.The metal-to-dielectric transition produces MgH2 and is reversible through dehydrogenation using oxygen.
- Dynamic metasurface holograms: Multiplexed dynamic and static pixels reconstruct separate “harmony” and “peace” holograms, with hydrogenation suppressing “harmony” while oxygen restores it and leaves “peace” unchanged.The static P2 response remains constant throughout hydrogenation and dehydrogenation, while dynamic P1 exhibits slight recovery hysteresis.
- Dynamic metasurface holograms: Different reaction kinetics independently switch P1 and P3 pixels, producing four distinct holographic states that sequence the Marie Curie portrait and Po/Ra patterns.A 1-nm chromium capping layer slows P3 hydrogenation and dehydrogenation, creating delayed switching relative to P1.
- Optical information processing and encryption: Customized helicity, hydrogenation, oxygen, and reaction-duration keys decode manifold holographic messages from one metasurface, enabling compact and secure multi-receiver information transmission.Different receivers recover messages such as “love and kisses” and “best regards” using customized key sequences.
Discussion
Customizing metasurfaces and dynamic pixels could increase conveyed data quantity, enable more dynamic holographic hierarchies, and combine diverse patterns for secure complex information. The addressable-pixel technique may support high-resolution, high-density data storage, optical communication, cryptography, and broader dynamic optical functions.
- Customizing metasurfaces and combining numbers, letters, and pictures could substantially increase conveyed data quantity and encrypt complex information with higher security.Dynamic pixels with different Cr cap thicknesses may introduce more dynamic holographic hierarchies.
- Independent control of addressable dynamic pixels could enable high-spatial-resolution, high-data-density storage and optical communication using ultra-thin devices.The authors identify these capabilities as useful for modern cryptography and security applications.
- The scheme could provide compact optical elements for dynamic beam steering, focusing, shaping, and optical vortex generation.These extensions would broaden the functionality of current metasurface systems.
Materials and Methods
The metasurfaces were fabricated by multilayer electron-beam lithography, metal deposition, and lift-off, then characterized during hydrogenation and dehydrogenation under controlled gas conditions. Holograms were designed as phase-only patterns using the Gerchberg–Saxton algorithm and simulated with Rayleigh–Sommerfeld diffraction.
- Fabrication: Multistep electron-beam lithography defined Au nanorods and alignment markers on SiO2/Si, followed by chromium/Au deposition and lift-off.The Au nanorods measured 200 nm × 80 nm × 50 nm.
- Fabrication: A second aligned layer incorporated 3 nm Ti, 50 nm Mg, 5 nm Ti, and 10 nm Pd deposited by electron-beam evaporation and lift-off.Computer-controlled alignment used Au markers to define the second structural layer.
- Optical characterization: Scattering spectra of individual Mg nanorods before and after hydrogenation were measured by dark-field microspectroscopy with polarization parallel to the rod axis.Spectra were normalized to a bare substrate.
- Design of the metasurface holograms: Phase-only holograms used 600 nm unit cells, 600 μm periodicity, a 17° off-axis angle, and Gerchberg–Saxton design with Rayleigh–Sommerfeld simulation.A 2×2 periodic holographic array was used to avoid laser speckles.
H2: Supplementary Materials
The supplementary materials document the metasurface’s dynamic optical behavior, multiplexing principle, polarization-dependent reconstruction, and hologram quality. Movies further show static-to-dynamic evolution, sequenced dynamics, and transformations among different numbers.
- H2: Supplementary Materials: Supplementary figures show dynamic Mg nanorod scattering during hydrogenation, high-quality holographic patterns, and the multiplexed metasurface principle.
- H2: Supplementary Materials: Reconstructed holographic patterns are presented in different zones under LCP, RCP, and LP illumination.
- H2: Supplementary Materials: Additional movies depict hologram evolution between static and dynamic states and through sequenced dynamics.
- H2: Supplementary Materials: Dynamic transformation among different numbers is demonstrated in the metasurface hologram.
Figures
The figures demonstrate chemically addressable metasurface pixels that dynamically switch holographic patterns, support independently sequenced features, and enable multiplexed optical information encryption. Multiple optical and chemical keys allow customized messages to be decrypted from a single metasurface.
- Dynamic metasurface pixels: Mg nanorods switch from a strong plasmonic resonance to a nearly featureless spectrum after hydrogenation, forming the dynamic pixel mechanism.The hydrogen-responsive rods are capped with Ti/Pd and transition between Mg and MgH2 states.
- Dynamic and static holograms: Hydrogenation switches the ‘harmony’ hologram on/off while the static ‘peace’ hologram remains unchanged.The hybrid metasurface combines dynamic Mg/Pd pixels with static Au pixels.
- Differently sequenced dynamics: Differently sequenced dynamic pixels independently switch the Marie Curie portrait and the Po and Ra chemical-element symbols.The two pixel types use different reaction kinetics, enabling independently controlled transitions.
- Dynamic geometric codes: A single metasurface multiplexed with three pixel sets reconstructs geometric-code holograms for Arabic numbers 0–9.The geometric-code scheme uses P1 Mg/Pd, P2 Au, and P3 Mg/Pd/Cr pixels, with P2 used twice per unit cell for uniform intensity.
- Optical information encryption: Customized helicity, H2, O2, and reaction-duration keys let multiple receivers decrypt different messages from one shared metasurface.The proof-of-concept assigns distinct messages to recipients including Tim, Bob, and Ted.