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Holograms can produce strikingly realistic three-dimensional images from light hitting a two-dimensional surface. Aside from some applications as counterfeit-resistant security images used on currency, holograms have mostly remained a novelty. But now researchers are hoping to move holograms out of the realm of museum exhibits and take advantage of their ability to store large amounts of data. A new study demonstrates a gold nanorod material that serves as a versatile, high-density storage medium for holograms and works with a broad spectrum of wavelengths, including infrared light (Nano Lett. 2013, DOI: 10.1021/nl403811d).
The process of storing an image of an object in a hologram is analogous to how photographic film works. Light reflecting off the object is combined with light from a reference laser beam to create an interference pattern, which is recorded in a light-sensitive medium, such as a photopolymer. The light causes microscopic changes in the material to encode the pattern, such that when the finished hologram is read by another laser, an image pops up that can be viewed from different angles.
Besides recording images, these interference patterns also can store a large volume of data that corresponds to layers of 1s and 0s. While the pits on the surface of a DVD store one bit of data, a hologram can store layers and layers of bits in the same area, leading to possibly super-data-rich storage materials.
To realize this high-density digital data storage, researchers need more versatile holographic materials. The optical polymers currently used to store holograms are bulky, says Kuang-Yu Yang of Academia Sinica, in Taiwan, and can only record images made with visible light.
That’s why researchers are turning to a new branch of materials science that’s making a big impact on optics: metamaterials. These materials are metals or semiconductors patterned with features smaller than the wavelength of light. The nanoscale patterns enable the metamaterials to interact with light in exotic ways that natural materials can’t match—for example, as in invisibility cloaks that work by refracting or bending light backwards (Science 2006, DOI: 10.1126/science.1133628). In 2012, physicist David R. Smith of Duke University made the first holograms based on metamaterial designs (Nat. Mater. 2012, DOI: 10.1038/nmat3278). His holograms produced images when light was transmitted through the metamaterial, which was made of layers of metal. The layered material absorbed most of the light and lost it to heat, leading to a very faint hologram.
So Yang, Din Ping Tsai of National Taiwan University, and their colleagues decided to make a reflective holographic surface instead of a hologram that absorbs and then transmits the light. Their hologram is made of gold nanorods of varying lengths patterned on a reflective gold mirror. The rods and the mirror are separated by an optical spacer made of magnesium fluoride.
Instead of recording images of real objects, the team created patterns of the nanorods so that when light hit the materials’ surfaces it created simple patterns, such as words. These reflective holograms are more efficient than the previous meta-holograms, reflecting about 18% of the light. The Duke metamaterial hologram transmitted 6%.
The pixels in the new meta-hologram are about 1.5 μm across, compared to 10 μm in a typical polymer hologram—a size difference that could translate into greater storage capacity. And, unlike conventional holograms, the meta-hologram works in the infrared region, which could be useful for biological imaging applications. By changing the size and shape of the gold patterns, it should be possible to make the technology work with wavelengths from microwave light through the whole visible light spectrum, Yang says.
Smith says the NTU researchers’ metamaterial holograms reflect more light, showing the promise of this kind of design. These metamaterial holograms are some of the first of their kind and don’t yet have any particular advantages over conventional ones, Smith says. Metamaterials like the ones made at NTU will lead to new types of holographic optics that perform better and do things existing technologies cannot, he says.
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