New Hologram Produces 3D Images Across Different Spectrums of Light, Will Improve Fraud Protection

Holograms are those shiny, metallic patterns with ghostly images floating inside them that are used to defeat counterfeiters. Credit cards and driver's licenses have holograms on them too and software packages also frequently have hologrammatic seals to prove their authenticity.

According to Science Daily, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences have programmed polarization into compact holograms. The holograms use nanostructures that are sensitive to polarization, which is the direction in which light will vibrate, to produce a variety of images depending on the polarization of incident light. This advancement works across the spectrum of light, improves anti-fraud holograms as well as those that are used in entertainment displays.

"The novelty in this research is that by using nanotechnology, we've made holograms that are highly efficient, meaning that very little light is lost to create the image," said Federico Capassothe Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering and senior author of the paper. "By using incident polarized light, you can see far a crisper image and can store and retrieve more images," Capasso added.

The Office of Technology Development in Harvard filed patents for this and its related technologies and is now actively pursuing commercial opportunities. India Today has mentioned that holograms, like digital photos capture a field of light around an object and encode it on a chip. But photographs only record the intensity of light, while holograms also capture the phase of light, making it appear three-dimensional.

"Our holograms work like any other but the image produced depends on the polarization state of the illuminating light, providing an extra degree of freedom in design for versatile applications," said Mohammadreza Khorasaninejad, postdoctoral fellow in the Capasso Lab and first author of the paper. Experts have explained that in linearly polarized light, the direction of vibration remains constant, while circularly polarized light rotates clockwise and counterclockwise.

The team also built silicon nanostructured patterns on a glass substrate that act as superpixels. Each superpixel responds to a certain polarization state of the incident light. Even more information can be encoded in the hologram by designing and arranging the nanofins to respond differently to the chirality of the polarized incident light.

"Being able to encode chirality can have important applications in information security such as anti-counterfeiting," said Antonio Ambrosio, a research scientist in the Capasso Lab and co-first author. "For example, chiral holograms can be made to display a sequence of certain images only when illuminated with light of specific polarization not known to the forger." He also said that using different nanofin designs in the future could make an individual will be able to store and retrieve far more images by employing light with many polarization. "Because this system is compact, it has application in portable projectors, 3D movies and wearable optics," Capasso added.

"Modern polarization imaging systems require cascading several optical components such as beam splitters, polarizers and wave plates," said Ambrosio. "Our metasurface can distinguish between incident polarizations using a single layer dielectric surface."