Encoded multiple holographic images in a metasurface that can be unlocked separately with different polarized light could improve holograms for anti-fraud protection and entertainment. Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) say their novel technique could also offer more control over the manipulation and measurement of polarization. A single metasurface encodes two separate holograms. When illuminated with one direction of polarized light, the metasurface projects an image of a cartoon dog. When illuminated with the perpendicular direction of light, the metasurface projects an image of a cartoon cat. Courtesy of The Capasso Lab/Harvard SEAS. "The novelty of this type of metasurface is that for the first time we have been able to embed vastly different images that don't look at all like each other — like a cat and a dog — and access and project them independently using arbitrary states of polarization," said lead researcher Federico Capasso. Polarization is the path along which light vibrates. Previous research from Capasso and his team used nanostructures sensitive to polarization to produce two different images encoded in the metasurface. However, those images were dependent on one another; two images were created but only one appeared in the field of vision. In their current research, the metasurface is made of titanium dioxide and consists of an array of polarization-sensitive pillars (nanofins) that redirect the incident light. Unlike previous arrays, which were uniform in size, these nanofins vary in orientation, height and width, depending on the encoded images. This hologram is one of two different holographic images encoded in a metasurface that can be unlocked separately with differently polarized light. Courtesy of The Capasso Lab/Harvard SEAS. "Each nanofin has different, precisely controllable polarization properties," said researcher Noah Rubin. "You use this library of elements to design the encoded image." Different polarizations read different elements. The novel metasurface can be encoded with any two images and also unlocked by any two perpendicular polarizations. "You can also embed different functionalities. It can be a lens for one polarization and if you go to a different polarization, it can be a hologram,” said Rubin. “So, this work is a general statement about what can be done with metasurfaces and enables new optics for polarization." The research has been published in the journal Physical Review Letters (doi.org/10.1103/PhysRevLett.118.113901).