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By Jennifer Ouellette
'Nanorice' shown in the images above, can be used to map the surfaces of biological cells.
A “plasmon microscope” formed with a glycerin drop (left) creates an image of a 30 μm x 30 μm array of “nanoholes” (blue square), in which the triplets of the 100 nm diameter holes can be resolved (right).
When light strikes a metallic surface, it generates electron waves, called plasmons. This remarkable effect was discovered in 1897 by Robert W. Wood, a physics professor at Johns Hopkins University . Wood was also the first person to unwittingly record the energy lost as heat by plasmons skimming along the surface of metals in 1902.
It took 40 years for Italian physicist Ugo Fano to provide an explanation: metals are not perfect conductors. A conducting surface can guide light as a 2D surface wave plasmons are also known as two-dimensional light–and those waves absorb energy. Hence, Wood's anomalous observations of energy loss in the light reflected from metallic surfaces. Their unusual properties make surface plasmons extremely promising for a wide range of applications, including plasmon microscopes and “super-lenses,” plasmon-based nanoparticle biosensors, and electronic circuits capable of operating at optical frequencies.
Cutting-Edge Microscopes . Plasmons can enable scientists to see fine details that were previously undetectable. For instance, a team of scientists at the University of Maryland led by Igor Smolyaninov, along with colleagues at Queen's University in Belfast, Ireland, are developing a two-dimensional plasmon microscope, ideal for imaging living cells, that could operate much like a point-and-shoot camera and reveal much more detail that currently available with existing imaging techniques. They were able to image tiny objects with spatial resolutions of 60 nm, further reduced to 30 nm with a bit of mathematical tweaking. The UMD team believes they can improve the resolution even further, down to around 10 nm.
A sample is placed on a metal-coated glass surface and covered with a drop of glycerin. Laser light shines through the glass and produces surface plasmons in the metal. The glycerin acts like a parabolic dish that can collect plasmons sprayed out from the sample at its focal point. It then forms them into something like a "plasmon beam" that goes back down towards the metal surface. Part of that beam bounces back up and can be seen with a regular light microscope. The performance is close to what an electron microscope might achieve, but involves no vacuum, high voltage or elaborate specimen preparation.
Other planned improvements include replacing the micromanipulators currently used to adjust the glycerin droplet's shape by hand with solid mirrors etched on the glass using lithography — an important step towards building a practical device. This would enable scientists to buy these special glass slides to use with any microscope: any lab could achieve electron microscopy resolution for the cost of a regular microscope. Nor would there be a need for special sample preparation. Movies might even be possible, since the image is taken all at once, rather than one pixel at a time. State-of-the-art lithography cannot yet produce sufficiently smooth mirrors to match that of the glycerin drop's surface, but the UMD researchers are hopeful that the technology will continue to improve rapidly to make it possible in the near future.
Super-Lensing . Other researchers are exploiting plasmons to create “super lenses,” relying on tiny nanoparticles to amplify and focus the light shining on a given sample. Scientists at the University of Texas, for example, have built a "super lens" and used it in a device to take pictures just below the surface of thin material substrates. According to Gennady Shvets, by combining his “super lens” with near-field scanning optical microscopy, he was able to achieve microscope resolutions as good as 1/20th of a wavelength in the mid-infrared range of light.
This in turn enabled him to observe “giant transmission,” in which light falls on a surface covered with holes much smaller than the wavelength of the light. Even though the total area of the holes comprised a mere 6% of the total surface area, 30% of the light nonetheless came through, thanks to the presence of plasmons. It is very difficult to image objects smaller than half the wavelength of the light being used for the imaging, but Shvets and his colleagues were able to achieve much higher resolution because there was less diffraction.
Meanwhile, at Rice University, researchers have created rice-shaped particles of gold and iron oxide, called “nanorice”–so named because when magnified the structures resemble tiny grains of rice that they hope to attach to the probe tips of scanning microscopes to map out the surfaces of biological cells. According to group leader Naomi Halas, nanorice is similar to an earlier structure she invented in 1998 called nanoshells. Both are made of a non-conducting core covered by a metallic shell. Changing the shape of a metal at the nanoscale enables researchers to modify the properties of the plasmon waves produced. Spheres and rods are the most optically useful shapes, and nanorice combines the best properties of both.
Nanoshells and nanorice can also serve as “super-lenses,” amplifying light waves and focusing them to spots far smaller than a wavelength light. In fact, Halas reports that nanoshells are about 10,000 times more effective at surface-enhanced Raman spectroscopy (SERS) than traditional methods. Raman scattering is routinely used by medical researchers, drug designers, and chemists to determine the precise chemical makeup of materials. So single grains of nanorice could provide the needed field intensities to characterize biomolecules like proteins and DNA that adsorb on a particle. They could also be used not just to identify, but eradicate cancer cells in rats.
Integrated “Plasmonics.” To date, it's proven difficult to combine photonic components — such as fiber optic cables — with electronic components like wires and transistors because of their mismatched capabilities and size scales. Photonic components can carry a lot of data — witness the explosion in broadband data transmission rates — but are bulkier than electronic components, which in turn can carry less data. Ideally scientists would like to be able to combine the best features of both onto a single chip in an emerging new discipline known as plasmonics.
Plasmons might be the key to achieving true integration on a single chip, since they operate at optical frequencies — typically 100,000 times greater than the frequency of even the most cutting-edge microprocessors — and the higher the frequency of the wave, the more information you can transport over it. Yet they take up much less space because their wavelengths are much smaller than the light used to create them. In such devices, light would be converted into plasmons, which propagate along a metallic surface with a wavelength smaller than the original light. The plasmons could then be processed with their own 2D optical components– mirrors, waveguides, or lenses–and then later converted back into light, or into electrical signals.
Nader Engheta of the University of Pennsylvania believes that nanoparticles–including those capable of supporting plasmon excitations–could be configured to act as nanometer-scale capacitors, resistors and inductors: the basis elements of an electrical circuit. But in this case, the circuit would operate not at radio or microwave frequencies, but at optical frequencies. This would enable the further miniaturization (down to about 30-50 nm) of optical components and the direct processing of optical signals with nano-antennas, nano-circuit filters, nano-waveguides, and nano-resonators. It could even lead to possible applications in nano-computing, nano-data storage, molecular signaling and molecular-optical interfacing.
Physicists in Denmark and France led by Sergey Bozhevolnyi of the University of Aalborg have developed a waveguide that could allow light at fiber-optic wavelengths to be "squeezed" to below the diffraction limit, allowing it to pass though small regions such as channels on a chip without being significantly lost.
Bozhevolnyi's team used a new class of surface plasmons called channel plasmon-polaritons — electromagnetic waves that originate at the interface of a metal and an insulating dielectric such as air. These plasmons can guide and manipulate light along the bottom of V-shaped grooves in a gold film without significant propagation losses. This is because the surface plasmons remain tightly bound to the interface and thus concentrate the light into a volume that is less than one wavelength across.
Channel plasmon-polaritons can be used to transmit light signals for wavelengths of around 1.5 microns — just right for telecommunications applications. Furthermore, the propagation length of a plasmon at a planar gold-air interface is around 1 mm, which is long enough to optically connect two devices on a chip.
Cloak and Dagger. In 2005, scientists at the University of Pennsylvania announced that they could potentially use plasmon coatings as a cloaking device to render objects invisible by creating a kind of "shell" around the object. Plasmon waves limit light scattering off an object because they resonate at the same frequency as the light striking them, so they cancel each other out. This makes the object in question very difficult to detect.
The roots of their research date back to 1998, when researchers led by Thomas Ebbesen of the Louis Pasteur University in Strasbourg, France shone light on a sheet of gold foil that contained millions of tiny holes. The holes were smaller than the wavelength of the light, and Ebbesen expected no light to get through. Instead, more light came out the other side than what hit the holes. Follow-up research found that plasmons were snagging light and stuffing it through the holes. When the energy and momentum of the photons match the energy and momentum of the plasmons, the photons are absorbed and radiated again on the other side.
Practically speaking, the technology, if developed, might be used in antiglare materials or to improve microscopic imaging. A more futuristic goal is that an entire aircraft might be made transparent to radio waves or some other long-wavelength detector.
But cloaking ability would depend on an object's size, so that only with very small things — items that are already microscopic or nearly so — could the visible light be rendered null. A human could be made impossible to detect in longer-wavelength radiation such as microwaves, but not in visible light.
Anything not perfectly ball-shaped presents additional problems. The researchers' calculations suggest "homogeneous spherical objects" in the nanoscale range could be rendered optically invisible.
More than 100 years after their serendipitous discovery, an increasing number of researchers are catching an electron wave. As a result surface plasmons are emerging as a critical element in many next-generation technological applications because of the remarkable properties. Further research and development is needed before such applications become truly enabled, but the future of plasmons appears as bright as a shiny metallic mirror.
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