Lens-Free, Multicolor Holography Technique Could Enable Compact 3D Displays

A holography technique based on computer-generated holograms (CHGs), developed by a team at Duke University, produces complex, multicolor holographic images without any bulky optical components. The researchers encoded a multicolor image onto a 300- by 300-μm hologram in a 2D waveguide structure and demonstrated a multicolor CGH in an all-dielectric waveguide metasurface system.

“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 professor David R. Smith. “Our design offers easier and more flexible integration with a form factor small enough for augmented reality and other displays.”

To create multicolor holographic images, the researchers fabricated a grating coupler and a binary hologram in a very thin waveguide structure. The resulting structure combines the colors and then precisely separates them to generate a full-color image. Courtesy of Duke University Electrical & Computer Engineering.

For the system structure design, the researchers multiplexed several wavelengths through a single grating into a waveguide and demultiplexed them via a CGH. Light beams from three different color laser sources (red, green, and blue) were coupled into the waveguide via the single grating. The grating was used to combine wavelengths, obviating the need for external beam splitters or prisms. The structures for all three wavelengths were fabricated in a single lithography step, ensuring excellent registration and reproduction of features.

The researchers said that although CGHs provide high-resolution images, they can be challenging to create with more than one color. “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 researcher Zhiqin Huang. “With our approach this is all done in one step on a single surface without any beamsplitters or prisms. This makes it extremely amenable to integration into portable devices.”

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, obtained at working wavelengths in the optical range, matched well with theoretical predictions. Although the researchers fabricated very small holograms for the demonstration, they said that the technique could be easily scaled up to create larger displays. They also believe their approach could be incorporated with existing technologies to create dynamic images.

Researchers developed a new way to create multicolored computer-generated holograms. The experimental images they created matched well with the ones predicted by theory. The waveguide structures created using the new approach could offer easy integration and a form factor small enough for augmented reality and other displays. Courtesy of Duke University Electrical & Computer Engineering.

The materials used in the system are dielectric and thus compatible with current integrated photonics technologies. This means that holographic devices using the system from Duke could be manufactured using the same fabrication methods that are used to make computer chips. The hologram-producing elements could be incorporated into chip-based devices that also house the light sources required to create the 3D images.

The new system could be used to make 3D color displays for augmented reality (AR) glasses, smartphone holographic displays, or heads-up displays (HUDs). “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, beamsplitters, or prisms,” said professor Daniel Marks. “It could also be used to project a 3D image from a smartphone onto a wall or nearby surface.”

The researchers are working to optimize the technology by reducing the light lost by the structures that encode the holograms. To make the technique practical, it will be necessary to incorporate the structures into a single integrated device with lasers, they said.

The research was published in Optica (https://doi.org/10.1364/OPTICA.6.000119).

A Duke University team led by David R. Smith has developed a new approach to multicolor holography that could be used to make 3D color displays for augmented reality glasses, smartphones, and heads-up displays without any bulky optical components. Courtesy of Duke University Electrical & Computer Engineering.