Unlocking chirality: New Metasurfaces for Advanced Light Control

EPFL scientists have developed a revolutionary “chiral design toolkit” using artificial optical structures called metasurfaces. This breakthrough allows for precise control over light’s “handedness,” opening doors to advanced applications in data encryption, biosensing, and quantum technologies.

Imagine trying to wear a left-handed glove on your right hand.It simply doesn’t fit. This incompatibility, due to their mirror-image nature, is a prime example of chirality – a basic property that dictates how molecules interact with the world around them.This “handedness” is crucial in biology, with most DNA and sugars being right-handed, while amino acids predominantly exhibit left-handedness.Even slight alterations in a molecule’s chirality can render vital nutrients useless or drugs harmful.

Light itself can possess this property of handedness. When light is circularly polarized, its electric field twists through space in a spiral, either left-handed or right-handed. Because chiral structures interact differently with these two types of light, scientists can analyze how a sample absorbs, reflects, or delays these twisted light beams to determine its own inherent handedness. Though, the effect is incredibly subtle, making precise control of chirality a significant challenge.

Now, a team from the Bionanophotonic Systems Laboratory at EPFL’s School of Engineering, in collaboration with Australian researchers, has engineered a groundbreaking solution: metasurfaces. these are artificial optical structures, essentially 2D lattices composed of meticulously arranged tiny elements called meta-atoms. The key innovation lies in their ability to easily tune their chiral properties. By subtly altering the orientation of these meta-atoms within the lattice, scientists can precisely control how the resulting metasurface interacts with polarized light.

“Our ‘chiral design toolkit’ is elegantly simple, and yet more powerful than previous approaches, which tried to control light through very complex meta-atom geometries,” explains Hatice Altug, head of the Bionanophotonics Lab. “Instead, we leverage the interplay between the shape of the meta-atom and the symmetry of the metasurface lattice.” This innovative approach, published in the prestigious journal Nature Communications, promises to revolutionize various fields.

An Invisible, Dual-Layer Watermark for Enhanced Security

The team’s remarkable metasurface, constructed from germanium and calcium difluoride, features a gradient of meta-atoms with continuously varying orientations across its surface. The intricate interplay between the shape and angles of these meta-atoms, combined with the symmetry of the lattice, collectively fine-tunes the metasurface’s response to polarized light.In a compelling presentation, the scientists successfully encoded two distinct images onto a metasurface optimized for the invisible mid-infrared spectrum. The first image, a vibrant Australian cockatoo, was embedded in the size of the meta-atoms, acting as pixels, and decoded using unpolarized light. The second image, the iconic Swiss Matterhorn, was cleverly encoded in the orientation of the meta-atoms. When exposed to circularly polarized light,the metasurface revealed this second hidden picture.

“This experiment showcased our technique’s ability to produce a dual layer ‘watermark’ invisible to the human eye, paving the way for advanced anticounterfeiting, camouflage, and security applications,” says Ivan Sinev, a researcher at the Bionanophotonic Systems Lab. This invisible, layered security feature could significantly enhance product authentication and data protection.

Beyond encryption, the potential applications of this chiral control extend to the cutting edge of quantum technologies, many of which rely on the precise manipulation of polarized light for computations. Furthermore,the ability to map chiral responses across large surfaces could dramatically streamline biosensing.”We can use chiral metastructures like ours to sense, such as, drug composition or purity from small-volume samples,” Altug adds. “Nature is chiral, and the ability to distinguish between left- and right-handed molecules with such precision opens up exciting new avenues for scientific discovery and technological advancement.”

This breakthrough in controlling chirality through metasurfaces marks a significant leap forward, offering a powerful and versatile tool for innovation across a spectrum of scientific and technological domains.

What are the limitations regarding the viewing angle when attempting to decode the hidden image using polarized light?

Twisted Light Reveals Hidden Images on Flat Chip

Unveiling the Technology: How Twisted Nematic Displays Work

The ability to embed hidden images within seemingly uniform surfaces using light manipulation is a engaging area of optical technology. At the heart of this lies the principle of twisted light, specifically leveraging the properties of twisted nematic (TN) displays. As detailed in research [1],TN mode utilizes the twisting of liquid crystals to alter the polarization of light. This isn’t about creating holograms in the traditional sense; it’s about controlling light transmission based on the alignment of these liquid crystals.

Here’s a breakdown of the core concept:

Liquid Crystal Alignment: Liquid crystals aren’t liquid or solid; they exist in a state between, allowing their molecules to be aligned. In a TN display,these molecules are twisted in a helical structure.

Polarized Light Interaction: When polarized light enters the twisted liquid crystal structure, its polarization is rotated with the twist.

Controlling Light Transmission: By applying an electric field, the twisting can be altered, changing the polarization rotation and, consequently, the amount of light that passes through. This is the basic mechanism for displaying images.

Beyond Visibility: The key to “hidden images” isn’t necessarily creating an image, but controlling the light in a way that reveals a pre-existing pattern or information encoded within the chip’s structure.

The “Flat Chip” Implementation: Encoding and Decoding

The “flat chip” refers to a substrate – frequently enough glass or plastic – containing a layer of liquid crystals. The hidden images aren’t generated by the chip itself, but are revealed through it when illuminated with appropriately polarized light.

Here’s how the process typically works:

1. Microstructure Creation: A microscopic pattern is etched or deposited onto the chip before the liquid crystal layer is applied. This pattern is the “hidden image.” It could be a security feature, a watermark, or even detailed artwork.
2. Liquid Crystal Application: The twisted nematic liquid crystal material is applied over this microstructure.
3. Polarized Light Illumination: When the chip is illuminated with polarized light, the microstructure interacts with the twisted light, causing variations in light transmission.
4. Image Revelation: These variations in light transmission reconstruct the hidden image, making it visible. The angle of the polarized light and the observer’s viewing angle are crucial for optimal visibility.

Applications of Hidden Image Technology

This technology isn’t just a novelty; it has several practical applications:

Security Features: Anti-counterfeiting measures are a major driver. Hidden images can be incorporated into banknotes, ID cards, and product packaging to deter forgery. These images are tough to replicate without knowing the exact microstructure and polarization requirements.

Brand Protection: Brand authentication benefits from this technology. Unique hidden logos or patterns can verify the authenticity of products.

Data Storage: While not mainstream, the principle could be extended to optical data storage, encoding information within the microstructure and retrieving it with polarized light.

Advanced Displays: Research is ongoing into using similar principles for novel display technologies, creating images that are viewable only under specific conditions.

Microscopy & Imaging: The manipulation of polarized light can be used in advanced microscopic techniques to reveal details not visible with standard illumination.

Cost-Effective: The manufacturing process can be relatively inexpensive, especially for large-scale production.

Difficult to Counterfeit: Replicating the precise microstructure requires specialized equipment and expertise.

Subtle Integration: The hidden image is invisible under normal lighting conditions, making it a discreet security feature.

Versatility: A wide range of patterns and images can be encoded.

Compact Size: The technology can be integrated into small form factors,ideal for applications like microchips and security tags.

Practical Considerations & Future Trends

While promising, there are challenges to overcome:

*Viewing Angle sensitivity