Multicolor Holography for Compact 3D Displays

Theoretical (left) and experimental (right) holographic image comparison of a multicolor hologram. (Source: OSA / Duke U.)

Researchers have developed a new approach to multi­color holo­graphy that could be used to make 3D color displays for augmented reality glasses, smart­phones or heads-up displays without any bulky optical components. The researchers from Duke Uni­versity encoded a multi­color image onto a 300-by-300 micron hologram in a 2D waveguide structure, a very thin structure that guides light. The computer-gene­rated hologram produces complex multi­color holo­graphic images when the grating coupler is illu­minated by red, green and blue light.

“The hologram could be embossed directly onto the lenses of aug­mented reality glasses to project an image directly into the pupil of the eye without requiring any bulky lenses, beam splitters or prisms,” said Daniel L. Marks, a member of the research team. “It could also be used to project a 3D image from a smart­phone onto a wall or nearby surface.”

The new fabri­cation method encodes holograms in a material that is compatible with inte­grated photonics tech­nology. This means that the holo­graphic devices are easy to mass manu­facture with the same fabri­cation methods used to make computer chips. The hologram producing elements could be incor­porated into tiny chip-based devices that also house the light sources required to create the 3D images.

The new multi­color holo­graphy technique is based on computer-generated holograms. Unlike tradi­tional holo­graphy, which requires a physical object and laser beams to create the inter­ference pattern necessary to form a holo­graphic image, computer-generated holo­graphy generates inter­ference patterns digitally. Computer generated holograms provide high-reso­lution 3D images, but it has proven difficult to create them in more than one color. The Duke team overcame this challenge by fabri­cating a grating and a binary hologram in a waveguide made of a light-sensitive material. They developed a way to integrate the inter­ference patterns for red, green and blue into a single binary hologram pattern.

“One of the difficult parts of making a multi­color display is combining the colors and then precisely sepa­rating them to generate a full color image,” said the principal inves­tigator Zhiqin Huang. “With our approach this is all done all in one step on a single surface without any beam splitters or prisms. This makes it extremely amenable to inte­gration into portable devices.” Another important achieve­ment was creating the holo­graphic device in a wave­guide structure. “Others who have tried to create multi­color computer-generated holo­grams didn’t use a waveguide, which makes it a challenge to integrate the structure into a device,” said David R. Smith, leader of the research team. “Our design offers easier and more flexible inte­gration with a form factor small enough for augmented reality and other displays.”

The researchers used their new holo­graphy method to encode interference patterns for static multi­color holograms of an apple, a flower and a bird. The resulting holo­graphic images all matched well with theo­retical predictions. Although they fabricated very small holograms for the demon­stration, the researchers say that the technique could be easily scaled up to create larger displays. They also believe their approach could be incor­porated with existing technologies – such as those used to make liquid crystal displays – to create dynamic images.

The researchers are now working to optimize the tech­nology by reducing the light lost by the structures that encode the holograms. They also point out that incor­porating the structures into a single inte­grated device with lasers would be neces­sary to make the technique practical. (Source: OSA)

Reference: Z. Huang et al.: Out-of-Plane Computer-Generated Multicolor Waveguide Holography, Optica, 6, 119 (2019); DOI: 10.1364/OPTICA.6.000119

Link: Novel Electromagnetic Media Group (D. R. Smith), Duke University, Durham, USA

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