A red apple with green leaves -- it seems real enough to pick up in one's hand, but there is nothing there to actually touch. This is because the apple is a hologram, a three-dimensional image projected by light. In April 2011, Chief Scientist Satoshi Kawata of RIKEN, the head of the Nanophotonics Laboratory at the Advanced Science Institute in Wako, along with his colleague Miyu Ozaki, a visiting scientist of RIKEN, succeeded in developing a novel holography principle distinct from conventional methods which makes it possible to reconstruct a full-color three-dimensional object. The key to their success resided in superposing a thin silver film on which surface plasmons -- collective oscillations of free electrons within a metal -- were excited. In recent years, research on surface plasmons has rapidly expanded against a background of remarkable advances in nanotechnology, helping establish the subject as a field of engineering. In addition to holograms, Kawata has also undertaken groundbreaking research on surface plasmon applications in metallic nano-lens technology.
Extracting the three primary colors from white light using surface plasmon resonance
"The key to successfully extracting RGB separately from white light resides in superposing a thin silver film onto light-sensitive material. In doing so, surface plasmon polaritons (SPPs) are excited on the thin film," says Ozaki.
A metal contains a great number of free electrons that are oscillating together while simultaneously interacting with each other. The quantum of this collective oscillation of free electrons in a metal is called a plasmon. A plasmon is always accompanied by a photon or electromagnetic field. On metal surfaces, plasmons and photons propagate along the surface, and this combination is called surface plasmon polariton (SPP). When a beam of monochromatic light at a particular wavelength is applied to a thin metal film through a prism, it is totally reflected at angles of incidence between the critical angle (θ c) and 90 degrees. At the same time, dim light exists close to the boundary face. The resulting evanescent waves excite SPPs along the surface of the thin metal film. Usually, an SPP is not excited by the light incident on the metal film, except only at an angle for resonating SPPs. At a resonant angle, the energy of the incidental light transfers to the SPPs rather than to the reflected light. We use the evanescent light field associating with SPPs to illuminate a hologram.
Surface plasmon resonance is seen at given parameters of film thickness, material, incident angle of light, and wavelength of light. In the case of white light, which is a blend of many different wavelengths, the angle of incidence varies as a function of wavelength for RGB. "We make the best use of this mechanism. By changing the angle of incidence to a value that accommodates RGB, we can extract light beams in the three colors R, G and B separately from the same white light," Ozaki says.
Prior to developing the full-color holography technique, in 2005 Kawata created a metallic nano- lens based on the principle of surface plasmons. The device is configured with bundles of nano-sized, thin metal wires and arranged in the form of a pin support. When an object is placed on top of the device, the light reflected by the object collides with the tips of the metal wires, producing surface plasmons. These plasmons carry information about the object and travel through the metal wires resulting in the reconstruction of an image on the opposite side. Due to the wires' thinness, greater detail about the object can be transmitted, which produces a higher resolution of the reconstructed image. Theoretically, images can be transmitted at a resolution of 1 nm. However, the device created by Kawata and colleagues was only able to produce fixed-size images, and the colors of transferable images were limited to discrete wavelengths.
Later in 2008, Kawata published a new idea for a metallic nano-lens capable of providing enlarged images of objects at a resolution of several nanometers in Nature Photonics. The new device is currently under fabrication. "The features of bundling many metal wires and transferring surface plasmons through metal wires are the same as those I announced in 2005," says Kawata. Improvements include the capability of enlarging observed images to macroscopically visible sizes by arranging metal wires into a fan formation, and cutting the metal wires at some points to form nano-sized gaps. The gaps help to broaden the waveband, which can be covered. Information on the object is transferred through the wires as surface plasmons, and as light through the wire-to-wire gaps, and the plasmons are eventually released as light to image the object. In the case of gapless wires, the number of oscillations that permit surface plasmon resonance is subject to limitations, and the surface plasmons gradually lose oscillation energy. By providing gaps, the energy attenuation is prevented, and all wavelengths over the entire band of visible light are covered, so the object can be examined in full color. We have demonstrated this through theoretical calculations and computer simulations."
Because the principle predicts that metallic nano-lenses will also work in water, further advances would enable examination of living cells on the nanoscale, offering expectations of applications in bioengineering and nanolithography in the future. "Plasmonics has a wide range of applicable fields, and some researchers are working on developing medical applications. For example, a cancer therapy may be feasible in which microparticles of silver-coated silica are delivered to the affected part of the body, and a beam of near-infrared rays is applied there to attack the cancer cells with plasmons. Because effective near-infrared rays are available in sunlight, sunbathing could possibly serve as a treatment," says Kawata.
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