Fancy photochemistry for next-gen displays

To make a thin-film hologram, the substrate needs to be rubbery so monomer molecules can travel through it during laser writing. But for the hologram to keep its ability to diffract in the long term, the pattern needs to stay locked in place, which is why researchers at the University of Colorado Boulder introduced an additional curing step that turns rubbery films into glassy ones.

Video credits:
Written and produced by Anne Hylden
Edited and animated by Janali Thompson
Narrated by Chris McCarthy
Series produced by Vangie Koonce, Anne Hylden, Andrew Sobey, and Jefferson Beck
Executive produced by Matthew Radcliff

Research videos from Alexander Osterbaan, Ph.D., and Andrew Sias
Additional video: Shutterstock, Storyblocks
Sound effects: Soundsnap
Music: “ES” by Grenadine from Epidemic Sound

Source article: 
“Multistage Networks for Glassy Holographic Photopolymers”
ACS Applied Materials & Interfaces
Corresponding author: Christopher N. Bowman, Ph.D.

Transcript

Super-thin holographic films like these have had a big problem for 30 years. Scientists can only make the holograms in soft, rubbery materials. But those floppy films aren't stable enough for useful applications like augmented reality screens. 

So, researchers at the University of Colorado Boulder came up with a way to make holographic films that are thinner than a human hair and rigid enough to stand on their own. They used three custom-designed ingredients, with polyurethane forming a rubbery matrix that houses acrylate and epoxide molecules. 

The scientists shine patterned UV light through the film, which makes the acrylate molecules polymerize where the light is most intense. Free acrylate flows to those areas, creating microscopic lines with alternating high and low refractive indexes. This structure diffracts light, giving the material its rainbow shimmer. Then, they activate a reaction between the epoxide molecules, locking the other polymers into place. 

Since the colors you see depend on the angle of diffracted light, engineers could use the film to make transparent displays where projected images are visible to only one person. 

The team is refining their fabrication process to get the best possible optical performance in a mechanically stable film. They recently published their results in ACS Applied Materials & Interfaces.