(Nanowerk Spotlight) Light passing through most transparent materials, like glass lenses or windows, behaves the same way regardless of which direction it travels. This property, known as Lorentz reciprocity, governs most optical systems except special cases involving magneto-optic effects, nonlinear responses, or temporal modulation.
Scientists can manipulate various properties of light using metasurfaces – engineered surfaces covered with patterns of structures smaller than the wavelength of light. These surfaces can control light’s amplitude, phase (timing of light waves), and polarization (orientation of light’s electric field oscillations). However, creating a single device that processes light differently depending on its direction of travel, while maintaining reciprocity, has remained challenging.
Researchers from KAIST in South Korea have developed an optical device that can produce four distinct light patterns based on illumination direction and polarization state. Their system uses two precisely engineered layers of silicon nanostructures separated by a polymer spacer. The lower silicon layer measures 790 nanometers in height, while the upper layer is 700 nanometers, with a 1,300-nanometer SU-8 polymer spacer between them.
Schematics of a device featuring asymmetric transmission. a) Device operating as a magnifying lens for back-side illumination. b) Device operating as a polarization camera for front-side illumination. The colors represent the polarization states of light, as described in a subsequent section. (Image: Reprinted from DOI:10.1002/adma.202406717, CC BY)
The research team demonstrated their technology by creating surfaces that generate different vectorial holograms – patterns containing both intensity and polarization information. In one configuration, the device produced images of a butterfly and grasshopper when illuminated from the front with specific polarization states, and images of a ladybug and beetle when illuminated from the back with different polarization states.
To overcome fundamental constraints in asymmetric transmission, the researchers partitioned the transmission space into upper and lower half-spaces. They used only the upper half-space as the active target spatial channel, while the lower half-space was blocked or ignored. This approach allowed them to achieve four independent vector functionalities within the maximum target volume.
The experimental devices achieved numerical efficiencies of 31.9%, 52.8%, 44.0%, and 34.4% for the four vectorial holographic images, assuming ideal conditions with unity transmission. These efficiency levels represent the ratio of power in the target spatial channel to input power.
The team enhanced their system’s capabilities by integrating it with computational vector polarizers – arrays of pixelated virtual polarizers. This combination enabled them to encode multiple binary images within single polarization patterns. The system stores information using the way different polarization states interact with three mutually perpendicular polarizer orientations, creating eight distinct possibilities on the Poincaré sphere (a mathematical representation of polarization states).
In their encryption demonstration, the researchers created devices generating 8-by-8 and 10-by-10 pixelated polarization patterns. When measured and decoded using the correct computational polarization filters, these patterns revealed hidden binary images with 85.9% accuracy across 12 decoded images.
The researchers also demonstrated integration with multi-plane holography, generating vectorial holographic images in both the far-field region and at specific distances (Fresnel diffraction regime) from the device. This showed how the technology could create different patterns at various distances while maintaining directional control.
The bi-layer metasurface structure consists of silicon nanoposts with carefully controlled dimensions. The lengths of the major and minor axes of these posts were restricted to between 100 and 350 nanometers for fabrication feasibility. The researchers used materials with specific optical properties: silicon with a complex refractive index of 3.61 + 0.0066i, SiO2 with refractive index 1.45, and SU-8 polymer with refractive index 1.56.
This advance in optical control creates new possibilities for secure communications and data protection. The ability to encode multiple patterns in a single optical element, accessible only under specific illumination conditions, could strengthen security features for authentication applications.
The researchers suggest their technology could be enhanced through integration with active materials, mechanical reconfiguration, or nonreciprocal optical systems. The efficiency could potentially be improved using neural network-based flexible nanostructure design and fabrication-conscious approaches.
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