Holograms record and display realistic images that appear to float over a surface. One challenge has been how to get those images to move. Now, researchers have created a material that can display different two-dimensional holographic images as the material is stretched (Nano Lett. 2017, DOI: 10.1021/acs.nanolett.7b00807). The technology takes a step toward animated holography, the researchers say.
Typical holograms are made from materials such as photopolymers that form cross-links when exposed to light. Once the cross-links are made in the materials, the resulting hologram image is fixed, says Ritesh Agarwal, a materials scientist at the University of Pennsylvania.
Agarwal and his colleagues created their stretchy hologram out of a so-called metasurface: a thin film that contains nanostructures designed to shape and reflect light. Their metasurface consisted of a square of polydimethylsiloxane (PDMS) embedded with precisely placed gold nanorods.
To make the new holograms, Agarwal and his team first calculated how much an image’s size and distance from the surface would change as the hologram was stretched. Then they used computer programs to generate designs based on those calculations.
The group deposited gold nanorods onto a silicon wafer using those designs, encoding up to three different images in the 100-μm-wide material. They then coated the wafer in PDMS, cured the polymer, and peeled it off with the nanorods embedded inside. The images appear at different distances from the hologram’s surface when it is illuminated with red laser light. As the material is stretched, each image takes turns coming into focus.
Agarwal envisions switchable holograms that can show animations; however, the calculations required to encode tens or hundreds of sequential images would be much more complicated.
Holograms that switch between two or three images might be sufficient for some applications, says Thomas Zentgraf, a physicist at the University of Paderborn. But he believes the more likely applications for metasurface holograms will involve encoding data rather than images.