What is Plasmons

The appearance of light through microscopic holes in metal foil made scientists aware of the existence of plasmons. Now, perforated metal foil is being used in a new type of biosensor that can trace a virus in blood and saliva samples.

The perforations are coated with different proteins that bind to antibodies known to be related to particular viruses. If antibodies accumulate on top of a group of holes, the plasmons cannot pull the light through, allowing the virus to be seen.

A whole new light

Plasmons are electron waves that are generated when light is aimed at a metal surface. Because their wavelengths are at least 10 times shorter than the wavelengths of visible light, they can carry both light and information through even the most microscopic passageways and networks.

Thomas Ebbesen held a thin sheet of gold foil up against the light and watched, surprised, as the light filtered through the foil. It must be an error, he thought, because according to everything we know about classical optics, that should not have been possible.

A Norwegian-born physical chemist working for the U.S.-based NEC Research Institute, Ebbesen knew that the 100 million tiny holes in the foil were 200 times smaller than the diameter of a human hair, and thus much smaller than the wavelength of visible light. In other words, the light could not possibly have squeezed through the holes. So then what, he wondered, was going on?

Almost 10 years passed from his discovery in 1989 until he and his colleagues found the answer. But it was worth the wait — the cause of the seemingly magic light is now the hottest phenomenon in nanotechnology, and it just might revolutionize everything from digital cameras to solar energy to cancer therapy.

Many uses for plasmons

The light that Ebbesen aimed at the gold foil stimulated a previously unknown type of electron waves called surface plasmons, whose wavelengths are at least 10 times shorter than those of visible light; this is what allowed the light to pass through the tiny holes in the foil. Since then, studies have shown that, under the right conditions, plasmons can be activated on all metal surfaces.

Once the short waves have been forced to oscillate, they can trap more light and carry it on, which will enable scientists to manipulate light on the nanoscale level.

Plasmon-generating perforated metal foil is now being used for a variety of practical applications. For example, depending on the holes’ size, shape and distance from one another, the plasmons will only focus and transmit light of a particular frequency and thus its color. This can intensify unicolored light from light diodes and lasers and can also help improve the light intake and, consequently, the quality of digital cameras.

Plasmons can also be used to serve more pressing purposes, for the greater benefit of humanity. Scientists at Boston University used perforated metal foil to develop a new biosensor that can detect deadly viruses such as Ebola and its close relative, Marburg, in blood or saliva samples. The device can be used for screening in airports or to track the disease in places where outbreaks typically occur. Plasmons will not only allow scientists to determine almost instantly whether someone is infected, but also how advanced the illness is.

Plasmons light up solar cells

And these are not the only uses for plasmons. The small, energy-dense electron waves can also provide us with cheaper and more efficient solar cells, thanks to several different technologies that have recently been developed.

One of these, based on nanoparticles of metals such as gold or silver, may finally make solar energy cost-effective. The basic principle is that plasmons can function essentially like tiny antennas, trapping and concentrating sunlight in the same way that a radio antenna captures radio waves. The solar cells are designed to send the light rays back and forth several times through the power-generating layers, with the majority of the light remaining inside the cell. The intent is for the cell to use less silicon, which makes up 40 percent of the price of a solar panel today.

Another technology involves using solar cells in which nanosize metallic projections are studded at regular intervals along the solar cell’s bottom layer of metal. When sunlight falls on the projections, plasmons emerge and roll like waves across the bottom layer, from one metal stud to the next. As the incoming light is attached to the plasmons, it is bent 90 degrees, forcing the light beams to move sideways through the solar cell’s thin silicon layer. This allows long wavelengths of the sunlight to be trapped, generating more power.

Solar cells must be two to five times cheaper before solar energy can offer a competitive advantage over fossil fuels such as oil and natural gas. If this cost-effectiveness can be realized, solar panels will be a dim ate-friendly source of power — plasmonic solar cells have the potential to make this happen,

Saving time and lives

In the longer term, plasmons may make electronics faster than ever. Scientists think that these electromagnetic waves can be a highly efficient connection between the global data communication that is transmitted via fiber-optic cables and local data processing in the electronic chips of computers and cell phones.

The shift from light to power is now rather inefficient because of the energy required to convert light signals into the electrical signals that command a computer or cell phone to produce sound, text and images. It would be cheaper and faster to bypass the conversion process and instead send the light signals directly into the electronic calculation units of the device’s computer chips.

Plasmons offer an ideal means for doing this. They are able to maintain the high frequencies of light of several hundred gigahertz, which allows them to carry massive amounts of data. And because their wavelengths are up to 10 times shorter, they can transmit the information they hold directly into even the most microscopic of computer chips.

One area in which plasmons could make a meaningful difference in the very near future is cancer therapy. The chemotherapy now used to treat cancer can cause side effects that make the therapy seem worse than the disease itself And the therapy often doesn’t result in a true cure, but simply keeps the disease at bay. But researchers at Rice University in Houston are using plasmonics and making progress on a treatment that could someday eliminate tumors completely — without the awful side effects.

In a recent experiment, the scientists treated mice with cancerous tumors via a new method in which they used plasmonic nanoparticles of gold to heat the tumors. All signs of the cancer disappeared in 10 to 12 days, and the mice suffered no side effects, as gold nanoparticles are nontoxic and are excreted by the body over time. The technique is currently being tested on human patients suffering from neck and brain cancer; if the results are as successful as they were in the mice, the potential cancer killer could be ready for widespread use in a matter of years — and could even be expanded to treat people with lung, breast, prostate and pancreatic cancer It’s cost-effective too. Near-infrared lasers are standard equipment, and the amount of gold used is modest; a typical wedding ring contains enough gold to treat a dozen cancer patients.

Given the scope, array and potential benefit of new plasmon-based technologies, Thomas Ebbesen’s accidental discovery was no error; it was a gift.

Plasmons used in ancient objects d’art

Throughout antiquity, glassmakers have used plasmonics — albeit unwittingly. The most famous example is the Lycurgus Cup, which was made in the Roman Empire in the 4th century CE.

The glassmakers mixed tiny metal particles into the glass while they were making it. The result is that, when the cup is illuminated with ordinary white light (which contains all the wavelengths of visible light), the plasmons begin to circulate on the surface of the metal particles. If the light source is outside the cup, the long red wavelengths pass through the glass, while the shorter blue and green wavelengths are absorbed, making the cup appear to be green. If the light source is inside the cup, the green and blue colors are reflected toward the inside of the cup, while the red waves pass right through, producing the striking red glow seen from the outside. Read ‘Dark plasmons’ transmit energy