Improved holograms for fraud protection, wearable optics

Scientists have developed a new way to create compact holograms which work across the spectrum of light, and may improve protection against fraud as well as lead to better entertainment displays.
Holograms help protect credit cards, driver's licenses etc from fraud - in grocery store scanners and biomedical devices.
Even though holographic technology has been around for decades, researchers still struggle to make compact holograms more efficient, complex and secure.
Researchers at the Harvard University in US have programmed polarisation into compact holograms.
These holograms use nanostructures that are sensitive to polarisation (the direction in which light vibrates) to produce different images depending on the polarisation of incident light.

This advancement, which works across the spectrum of light, improves anti-fraud holograms as well as those 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 Capasso, Professor at Harvard.
"By using incident polarised light, you can see far a crisper image and can store and retrieve more images," said Capasso.
"Polarisation adds another dimension to holograms that can be used to protect against counterfeiting and in applications like displays," he said.
Holograms, like digital photographs, capture a field of light around an object and encode it on a chip. However, photographs only record the intensity of light while holograms also capture the phase of light, which is why holograms appear three-dimensional.

"Our holograms work like any other but the image produced depends on the polarisation 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.
There are several states of polarisation. In linearly polarised light the direction of vibration remains constant while in circularly polarised light it rotates clockwise or counterclockwise.
The team built silicon nanostructured patterns on a glass substrate, which act as superpixels. Each superpixel responds to a certain polarisation 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 direction of rotation of the polarised incident light.

Since this system is compact, it has application in portable projectors, 3D movies and wearable optics, researchers said.
The research appears in the journal Sciences Advances.