Seize the Light: Learning to control photons with metal nanoparticles

To most people, controlling light is as simple as flipping a switch. However, maintaining this control becomes much more complicated when steering light waves through optical circuits 1,000 times smaller than the thickness of a human hair.
Scientists have struggled for decades to find ways of manipulating light at miniscule dimensions. Recent efforts, which have focused on the scale of the nanometer (a nanometer is a billionth of a meter), have been hampered by the fact that a nanometer is only one-thousandth the wavelength of visible light. These dimensions make it virtually impossible for light waves to travel without being scattered in different directions — an effect that has been a major source of frustration for those seeking to advance light-based technologies such as fiber optics and optical data storage.

Now, Argonne scientists are making strides toward understanding and manipulating light at the nanoscale by using the unusual optical properties of metal nanoparticles. A joint experimental and theoretical team has used powerful high-resolution imaging and modeling techniques to characterize these properties at a level of specificity never before attained. The experimental effort is led by Gary Wiederrecht and the theoretical effort by Stephen Gray.
"Using experimental and theoretical approaches, we were able to observe the interaction of light with the surfaces of the metal nanoparticles, explained Gray. "We hope that these studies will lead to the creation of optical technologies that can manipulate light with precision at nanoscale dimensions.

Experiments have shown that these metal nanoparticles, such as extremely tiny spheres of silver or gold, can concentrate large amounts of light energy at their surfaces. This effect is due to surface plasmons — special collections of excited free electrons that exist near the surface of the metal nanoparticles. As a result, scientists believe that this field of energy, called "near-field light, will enable devices to steer light along optical circuits far smaller than is currently possible. With continued research, this technology has the potential to revolutionize everything from telecommunications to high-speed computing, as well as nanoscale chemical and biological sensors.

Photons vs. Electrons

At the source of what we know of as visible light are zillions of photons — packets of electromagnetic waves that travel through space and are absorbed by our eyes. These photons, in addition to allowing us to see on a day-to-day basis, have provided scientists and engineers with a more efficient, faster alternative to current electronic systems.

"In a nutshell, photons move faster than electrons. They are a highly efficient power source just waiting to be harnessed, said Wiederrecht.

Current technologies, such as high-speed computers and Internet routers, rely heavily on electrons — negatively charged particles whose movement generates electrical currents. This negative charge gives scientists an easy way to control the direction of electron flow. However, due to their mass, the particles can create a lot of friction as they travel through wires, causing a buildup of heat that can easily overload an electric circuit.

Photons, by contrast, follow a different set of physical rules. They are massless particles that have no charge — characteristics that make them as slippery as eels when compared to the easily controlled electron.

According to Wiederrecht, replacing electrons with photons would provide a way to overcome mechanical challenges such as friction and heat.

"With the need for high data transmission and small circuit sizes, electric circuits simply generate too much heat and friction to be practical. If we can figure out how photons will behave when they encounter certain materials, we'll be closer to being able to control their movement for use in technological devices, he explained.

Shedding light on light

It was in hopes of tracking down the movements of the slippery photon that Gray and Wiederrecht began their collaborative study in 2003. "We decided to focus on gold and silver particles because their surface plasmons can be excited with visible and near-infrared light. Light with such wavelengths is most compatible with current optical technologies.

Gray and Wiederrecht chose to study these metal particles in isolation and in patterned arrays. The particles, with diameters as small as 25 nanometers, were placed on a glass surface at uniform distance from one another with electron beam lithography — a tool used for etching patterns at the nanoscale. The glass surface was then illuminated with laser light, forming miniscule fields of light energy detectable with an advanced imaging technique called near-field scanning optical microscopy (NSOM).

Unlike conventional imaging techniques, which cannot focus light in an area smaller than its wavelength, near-field scanning optical microscopy employs a nanoscale probe positioned close to the sample surface. One form of this probe, which is used by most scientists, employs a 100-nanometer-sized aperture through which incredibly small packets of light can be "squeezed to illuminate or characterize the optical near-field of a sample.

The Argonne scientists, however, decided to use a second, less common form of near-field microscopy when constructing their experimental design. This form, called "apertureless near-field scanning microscopy, uses a metal or silicon probe positioned a few nanometers from the sample surface. When the sample is illuminated with laser light at a particular angle, near-field light is generated on the glass surface, thus illuminating the metal nanoparticles and exciting the surface plasmons. The nearby probe can further concentrate and scatter light at its tip, becoming much like a miniscule lantern capable of resolving nanoscale dimensions.

Key to the advantages of using apertureless near-field microscopy is its advanced characterization ability. As the probe concentrates near-field light at its tip, it can also scatter photons at a certain angle depending on the nature of the sample being used. This scattering angle can provide scientists with important clues regarding how to control photon flow at such miniscule levels — a capability that has been confirmed by detailed theoretical modeling.

The future of light

Gray and Wiederrecht's study was the first to determine the scattering angle of light when it interacts with gold and silver nanoparticles on glass. Using their unique experimental design, they were able to determine that the metal nanoparticles scattered light at a 20-degree angle from the glass surface. Futhermore, they discovered that more control could be attained over the direction of light travel simply by arranging the nanoparticles in arrays, an encouraging result for using near-field photons in two-dimensional devices such as optical chips. The studies were also the first to use a powerful imaging technique known as atomic force microscopy to obtain simultaneous correlation of the NSOM optical images with nanoparticle array topography. All findings were validated using computational and theoretical methods, and together, they provide specific information as to how near-fields can be used to guide light.

"Gray and Wiederrecht's study is certainly an important one, affirmed Renaud Bachelot, a renowned expert in near-field optics from the University of Troyes, France. "They made a wise choice in using apertureless near-field optical microscopy instead of the method used by most near-field scientists. Their elegant experimental design helped them to obtain data that I have never seen before.

The scientists' work is part of a large-scale nanophotonics program in Argonne's Chemistry Division and Center for Nanoscale Materials. Combining the talents of chemists, physicists, experimentalists and theorists, the program focuses on investigating near-field light phenomena — a new branch of optical science that has only been around for about 15 years.

"Our findings are part of the first step toward creating optical systems at the nanoscale, said Gray. "With more work, technologies such as optical computing and nanolasers may soon be within reach.

The research work by scientists in Argonne's Chemistry Division and Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Basic Energy Sciences.