In the University of Pennsylvania demonstration of a stretchable metasurface hologram, the holographic image moves away from the device, and is enlarged, as the substrate is isotropically stretched. [Image: American Chemical Society]
Scientists at the University of Pennsylvania, USA, have modeled, designed and demonstrated metasurfaces that can display simple holographic images—and can swap in a new image when the surface is isotropically stretched (Nano Lett., doi: 10.1021/acs.nanolett.7b00807). While the technique currently involves only a small number of separate monochrome images on a given surface, the researchers view the work as a confirmation that “metasurfaces on stretchable substrates can serve as a platform for a variety of reconfigurable optical devices.”
Engineered surfaces
Metasurface holograms arise from careful engineering of nanostructural antennas on a surface, in a shape and configuration that records, on subwavelength scales, the phase and amplitude of scattered light field from a given object. As with a conventional hologram, the nanoscale details on the metasurface hologram then reconstruct the original scattered light field when another light beam is shone on the metasurface. The result? A ghostly image that hovers in an “image plane” some distance in front of the surface.
Because of their potentially highly compact, planar designs, metasurface holograms have attracted some attention as a potential platform for information storage and optical-communication applications. There have even been schemes to create metasurface holograms that can change when the polarization of the incident light is switched—but those schemes have been limited to toggling between only two holographic images, and depend on changes in the incident light, a potentially awkward requirement in some applications.
Stretching and the image-plane distance
The Pennsylvania team wanted to see if it could create metasurface holograms that could switch between more than two images, with the switching controlled by a property of the surface rather than the incident light. The researchers began by digging into the mathematics of the phase discontinuities on a given metasurface, and how those discontinuities change when the material is stretched.
Through that work, they were able to determine that if a holographic metasurface is stretched by a factor s, the image plane of the encoded holographic image will move away from the unstretched image-plane position as a function of s2 (see diagram above). That raised the possibility of creating a stretchable metasurface that encoded multiple holographic images—which, observed at a specific distance from the surface, would become visible in succession as the material was increasingly stretched.
Gold nanorods
To test out the concept, the Pennsylvania team computer-designed the phase distribution on a metasurface—consisting of an array of anisotropic gold nanorods—to encode distinct monochrome holograms of three simple shapes, each initially visible at a different image-plane distance from the metasurface itself. They then fabricated the designed metasurface on a silicon handler wafer, and transferred the fabricated surface to a stretchable polydimethylsiloxane (PDMS) substrate.
As the substrate was stretched, each projected holographic image expanded in size and moved away from its original image plane position by a function of the square of the strain—as the team had mathematically predicted. And, also as expected, if the hologram were observed at a fixed distance from the surface, the projected image at that distance would change from one image to another as the substrate was stretched.
The researchers acknowledge that this image multiplexing is currently at a very early stage, and that the proof-of-concept device suffers from noise and is limited to monochrome display. They believe, however, that improvements in fabrication processes, the use of dielectric rather than plasmonic (metal) resonantors, and extension of the technique to multicolor stretchable holograms “may enable practical applications of this device.” Such applications, the team says, could lie in areas such as “virtual reality, flat displays, and optical communication.”