Science - Japan's RIKEN team have altered the way a hologram is recorded and displayed to ensure that the color of the image remains the same at all angles.
The recently emerging three-dimensional (3D) displays in the electronic shops imitate depth illusion by overlapping two parallax 2D images through either polarized glasses that viewers are required to wear or lenticular lenses fixed directly on the display. Holography, on the other hand, provides real 3D imaging, although usually limiting colors to monochrome. The so-called rainbow holograms—mounted, for example, on credit cards—are also produced from parallax images that change color with viewing angle. We report on a holographic technique based on surface plasmons that can reconstruct true 3D color images, where the colors are reconstructed by satisfying resonance conditions of surface plasmon polaritons for individual wavelengths. Such real 3D color images can be viewed from any angle, just like the original object.
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A conventional hologram is effectively an interference pattern recorded on a photographic plate when laser light bounces off an object of interest. Re-illuminating the plate causes interference that reconstructs wavefronts that appear to have scattered off the object - so it looks 3D. But its colour depends on its viewing angle.
To overcome this, Kawata, Miyu Ozaki and Jun-ichi Kato harnessed a quantum surface effect. Metal films contain free electrons that oscillate on the surface and interact with incoming photons. Called a surface plasmon polariton, this surface wave is confined within a light wavelength of the surface and can be harnessed to cause interference effects. By recording their holograms on 55-nanometre-thick metal films with red, green and blue lasers, they can ensure that the 3D image anybody sees is always the same colour - from any angle.
"Currently 3D TV receivers, 3D games machines and 3D movie theatre screens create an illusion using left and right eye images reconstructed by the brain," says Kawata. "We are creating an optical field in 3D from the actual object in natural colour - there is no illusion."
He hopes their technique will feed into research on new ways to make glasses-free 3D moving picture screens, as well as making holograms look more realistic.
Our results show that plasmon color holography provides a view of an object or a scene seen naturally and vitally with white-light illumination. A typical amplitude modulation in plasmon hologram is ~25 nm, which is much thinner compared with Lippmann-Denisyuk’s color hologram based on Bragg diffraction in volume. The rainbow holograms mounted, for example, on credit cards also reconstruct with white light, where color varies with viewing angle but not with the color distribution in the object. Plasmon holography is advantageous in terms of background-beam–free reconstruction because the illumination light is totally reflected back at the hologram. Plasmon holography does not suffer from the ghost produced by the diffraction of ambient light or higher orders of diffraction, because those components are not coupled with SPPs.
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