Holography is one of the more dramatic forms of photography, in which a three-dimensional image is stored on a photographic plate in the form of interference fringes. Researchers at Purdue University in Indiana have developed a different approach, in which a 3D image is stored in a structure of thousands of V-shaped nanoantennas etched into an ultrathin gold foil. The new approach dramatically shrinks the size of a hologram, potentially enabling photonic and plasmonic devices and optical switches small enough to be integrated into computer chips.
A transmission hologram is viewed by passing light through the pattern of amplitude and phase changes recorded within a photographic emulsion. The transmitted light, in passing through the emulsion, is converted into an accurate replica of the light field corresponding to the light scattered from the original object. Alexander Kildishev, an Associate Professor of Electrical and Computer Engineering at Purdue University, recognized that this same feat can be accomplished by using a planar array of antennas to substitute for the photographic emulsion.
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An antenna absorbs passing electromagnetic waves by converting them into electrical currents in its structure. These electrical currents then cause the antenna to reemit the original signal. However, this process does not occur with 100 percent efficiency, so the amplitude of the wave on the far side of an antenna has changed. The re-emission also does not take place instantly; a time delay is involved which changes the phase of the original wave. If you have an array of antennas, each of which produces carefully adjusted amplitude and phase variations, they can collectively supply the information required to reconstruct a holographic 3D image.
The picture above shows the workings of a holographic prism using the new Purdue design. Light is moving upward in the figure, with the colors indicating the local phase of the light. Initially, the light is moving through the lower glass layer, with the wavefront oriented along the dashed white line, so the motion of the wave is slightly to the right of vertical. When the light hits the nanoantenna layer, the various shapes of nanoantennas shown at the bottom provide different degrees of phase shift. The result in this simple case is that the light is refracted so that it moves about 30 degrees left of a vertical path. Quite profound modifications of the incident light can be accomplished using more complex patterns of nanoantennas.
Nanoantenna holograms not only provide a way to store and display 3D images, but also represent one of the most flexible resources for directing light within optical devices and systems. However, for many such applications conventional holograms are too bulky, and create their images at too great a distance for easy integration into, for example, micro-scale optical computers.
Plasmonic metamaterials offer great potential for such applications. A plasmon (more properly a surface plasmon polariton) is a hybrid excitation between light and surface electrons at a metal surface. As the electrons move slower than light, a plasmon has a smaller wavelength than did the light whose information it carries. Such structures enable subwavelength optical devices to be used in a host of applications.
Holograms can be made using plasmonic metamaterials by fabricating an array of nanoantennas in a thin (30 nm) gold film on a glass surface. For imaging in red light, each antenna is about 100 nm in size, and has a shape designed to provide a desired amplitude and phase shift at its location. As the red light used in the Purdue experiments has a wavelength of 676 nm (the source is a krypton-argon gas laser), the nanoantennas are far smaller than the light photons. However, they are just the right size to phase shift red surface plasmons.
Once the nanoantenna array is formed using ion beam milling, laser light is transmitted through the sheet of nanoantennas, to create a hologram which is 10 microns above the metasurface. As a demonstration, the scientists created a hologram of the word PURDUE, which was less than 100 microns wide, or about the width of a human hair. The width of the optical lines making up the letters is about a micron in size, a remarkable resolution for a red light hologram whose nanoantenna structure measures only about 20 wavelengths across the entire letter.
The Purdue research team has demonstrated a remarkable new method for controlling light using plasmonic metasurfaces. Nanophotonic devices fabricated using such structures could provide a class of devices capable of using single photons in switching or routing for advanced optical computers, combining the enormous bandwidth of optical signals with the compressed wavelengths of surface plasmons. The result could be a generation of substantially faster computers and telecommunication photonics, as well as new sensors and high-resolution displays.
The team's research is published in the journal Nature Communications.
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