Holograms Gain Efficiency from Metasurface Fabrication Technique

Nanostructured metasurfaces have been incorporated into compact holograms, enabling the production of different images depending on the polarization of incident light. The highly efficient holograms lose very little light in the processing of creating the image, and are expected to improve antifraud holograms as well as those used in displays.

Broadband and chiral binary dielectric metaholograms produce 3D images across different spectrums of light. Courtesy of the Capasso Lab.

"By using incident polarized light, you can see far a crisper image and can store and retrieve more images,” said Harvard University professor Federico Capasso, who coauthored the study. “Polarization adds another dimension to holograms that can be used to protect against counterfeiting and in applications like displays."

Capasso and his team reported broadband operation of dielectric metaholograms from the visible to the near-infrared with efficiency as high as 75 percent in the 1- to 1.4-μm range.

Unlike digital photographs, which capture just the intensity of a field of light around an object and encode it on a chip to produce an image, holograms also capture the phase of light, enabling 3D images. There are several states of polarization. In linearly polarized light, the direction of vibration remains constant while in circularly polarized light it rotates clockwise or counterclockwise, a characteristic known as chirality.

The same hologram illuminated with different polarizations displays two vastly different images. Courtesy of the Capasso Lab.

The Harvard team built silicon nanostructured patterns — nanofins — on a glass substrate, which act as superpixels. Each superpixel responds to a certain polarization state of the incident light, and additional information can be encoded in the hologram by designing and arranging the nanofins to respond differently to the chirality of the polarized incident light.

"Being able to encode chirality can have important applications in information security such as anti-counterfeiting," said researcher Antonio Ambrosio. "For example, chiral holograms can be made to display a sequence of certain images only when illuminated with light of specific polarization not known to the forger."

The researchers said altering the nanofin design could increase image storage and retrieval capacity by employing light with many states of polarization. Additionally, the compact nature of the design means it could be used in portable projectors and wearable optics. The nanostructured metasurface can distinguish between incident polarization using a single layer dielectric surface, the researchers said, meaning it doesn’t require additional optical components such as beam splitters, polarizers and wave plates, as do most polarization imaging systems.

Harvard's Office of Technology Development has filed patents on this and related technologies and is actively pursuing commercial opportunities. The work was supported in part by the Air Force Office of Scientific Research, Google Inc. and Thorlabs Inc., and was published in Science Advances (doi: 10.1126/sciadv.1501258).