image: Meta-hologram optical device that operated in forward and backward directions.
Hologram techniques are already used in our everyday life. A hologram sticker to prevent from counterfeiting money, Augmented Reality navigation projected in front mirror of a car to guide directions, and Virtual Reality game that allows a user to play in a virtual world with a feeling of live are just a few examples to mention. Recently, thinner and lighter meta-hologram operating in forward and backward directions has been developed. As seen in the movie, Black Panther, people from Wakanda Kingdom communicate to each other through the hologram and, this specific movie scene seems to become reality soon that we can exchange different information with people from different locations.
Junsuk Rho, professor of POSTECH Mechanical Engineering and Chemical Engineering Department with his student, Inki Kim developed a multifunctional meta-hologram from a monolayer meta-holographic optical device that can create different hologram images depending on a direction of light incident on the device. Their research accomplishment has been introduced as a cover story in the January 2020 issue of Nanoscale Horizons.
Televisions and beam projectors can only transmit intensity of lights but holographic techniques can save light intensity and its phase information to play movies in three-dimensional spaces. At this time, if metamaterials are used, a user can change nano structures, size, and shapes as desired and can control light intensity and phase at the same time. Meta-hologram has pixel sizes as small as 300 to 400 nanometers but can display very high resolution of holographic images with larger field of view compared to existing hologram projector such as spatial light modulator.
However, the conventional meta-holograms can display images when incident light is in one direction and cannot when light is in the other direction.
To solve such a problem, the research team used two different types of metasurfaces.1) One metasurface was designed to have phase information when incident light was in the forward direction and the other one to operate when light was in backward direction. As a result, they confirmed that these could display different images in real-time depending on the directions of light.
In addition, the team applied dual magnetic resonances and antiferromagnetic resonances, which are phenomena occurring in silicon nanopillars, to nanostructure design to overcome low efficiency of the conventional meta-hologram. This newly made meta-hologram demonstrated diffraction efficiency higher than 60% (over 70% in simulation) and high-quality and clear images were observed.
Furthermore, the new meta-hologram uses silicon and it can be easily produced by following through the conventional semiconductor manufacturing process.
The meta-hologram operating in both directions, forward and backward, is expected to set a new hologram platform that can transmit various information to multiple users from different locations, overcoming the limits of the conventional ones which could only transmit one image to a limited location.
Junsuk Rho who is leading research on metamaterials said, 'Microscopic, ultrathin, ultralightweight flat optical devices based on a metasurface is an impressive technique with great potentials as it can not only perform the functions of the conventional optical devices but also demonstrate multiple functions depending on how its metasurface is designed. Especially, we developed a meta-hologram optical device that operated in forward and backward directions and it could transmit various visual information to multiple users from different locations simultaneously. We anticipate that this new development can be employed in multiple applications such as holograms for performances, entertainment, exhibitions, automobiles and more."