HANCOCK, N.H. " Researchers at Argonne National Laboratory's Center for Nanoscale Materials have reached new insights in the "near field" that promise to develop a new generation of subwavelength optical components.
Work in the near field, where wavelength does not figure in the equations governing light propagation, may help optical researchers who hope to integrate optical devices into electronic circuits break through the subwavelength barrier.
In the realm where VLSI technology moves to feature sizes smaller than the wavelength of photons, light simply cannot propagate through structures that small. Electrons, however, have no trouble navigating through sub-0.1-micron features due to their much shorter wavelengths.
Researchers at the Argonne Center for Nanoscale Materials (Argonne, Ill.) were using a near-field scanning optical microscope (NSOM) to study individual metallic nanoparticles. The object of the work was to better understand how light can be concentrated into the near field and guided by arrays of nanoparticles. In doing so, they reached a new understanding of how to couple light into an array of nanodots.
While the optical properties of those arrays have been discovered through experiments, the actual interactions taking place at individual particle sites is an unknown. Hoping to clarify that missing component of the theory, the Argonne researchers have been observing the near field around single gold and silver particles between 20 and 40 nanometers in diameter.
"Using experimental and theoretical approaches, we were able to observe the interaction of light with the surfaces of the metal nanoparticles," explained Stephen Gray, one of the Argonne researchers who is building theoretical models of the system. Gray believes the new results will lead to methods for guiding light fields through nanometer-dimensioned regions defined by metal particle arrays.
Individual particles a few nanometers from the tip of an NSOM were illuminated with laser light, which creates a high-intensity near field close to the surface of the particle. The light is concentrated near the surface of the particle through a mechanism called surface plasmon resonance. Discovered only recently, the effect explains why metal particles added to glass will generate intense colors, a technique that has been used since the Middle Ages to build stained-glass windows. The effect is only pronounced in gold, silver and copper nanoparticles.
Plasmons are electron charge density waves that travel along the surface of a metal. While their wavelength and momentum are different from that of light, at certain precise wavelengths of the incident light a resonance between the photons and plasmons occurs that binds the photons to the surface, creating an optical near field. By using the NSOM to make a detailed map of the optical field near the surface of the particle, the researchers discovered that polarization along with wavelength is a big factor in the photon-binding process. Only photons polarized at a perpendicular angle to the substrate showed strong confinement.
A second unexpected discovery was that when photons scatter into the far field, they always travel at an angle of less than 20 degrees from the substrate surface. That effect works in reverse as well, so that incident light at small angles to the substrate will efficiently couple into the near field.
The experiment was repeated with several nanoparticles spaced at 100-nm intervals and the researchers found that the new effects were enhanced by the presence of the other particles.
The surface-plasmon effect was first observed in gold films at NEC Research Institute (Princeton, N.J.) in 1989. The film had been drilled with an array of 300-nm holes, which should have been too small to allow visible light to pass through. Strangely, light seemed to pass through the holes without any attenuation.
The effect remained a mystery until Peter Wolff, an NEC theoretical physicist, developed a mathematical model for surface-plasmon waves in metals. With the model, Wolff was able to show that plasmons were responsible for concentrating the photons into an optical near field in the holes. It is that effect that optical researchers are hoping to harness in the bid to build optical components that operate below the wavelength of light.