Harvard team claims hologram technology advance

Researchers in the US say they have developed a new type of hologram that is more "efficient, complex and secure" than current forms.

The team from the John A Paulson School of Engineering and Applied Sciences at Harvard University say the trick is to use nanostructures called 'meta surfaces' to make the holograms behave differently depending on the polarization of light hitting them.

"By using nanotechnology, we've made holograms that are highly efficient, meaning that very little light is lost to create the image," said Federico Capasso, the senior author of the paper which is published in the journal Science Advances.

"By using incident polarized light, you can see far a crisper image and can store and retrieve more images," he added. "Polarization adds another dimension to holograms that can be used to protect against counterfeiting and in applications like displays."

"The polarization state of the illuminating light provides "an extra degree of freedom in design", according to Mohammadreza Khorasaninejad, co-first author of the paper.

There are several states of polarization. In linearly polarized light the direction of vibration remains constant while in circularly polarized light it rotates clockwise or counter-clockwise. The direction of rotation is known as its chirality.

The team built silicon nanostructured patterns on a glass substrate, which 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 so-called 'nanofins' (pictured below) 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, 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."

Using fabrication methods such as deep ultraviolet lithography and nanoimprinting it should be possible to mass produce the holograms, note the authors.

Harvard's Office of Technology Development has filed patents on this and related technologies and says it is actively pursuing commercial opportunities.

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