Optical engineering has had a tremendous impact on our everyday lives, providing us with fiber optic communications and optical data storage. However, manipulating light on the nanoscale level can be a Herculean task, since the nanoscale level is so incredibly tiny – less than one tenth the wavelength of light.
Researchers at Argonne National Laboratory are making strides towards understanding and manipulating light at the nanoscale by using the unusual optical properties of metal nanoparticles, opening the door to microscopic-sized devices such as optical circuits and switches.
Metal nanoparticles, such as extremely tiny spheres of silver or gold, can concentrate large amounts light energy at their surfaces. The light energy confined near the surface is known as the near-field, whereas ordinary light is known as far-field. Many scientists believe that by understanding how to manipulate near-field light, new optical devices could be built at dimensions far smaller than is currently possible. In an effort to characterize near-field behavior, a joint experimental and theoretical study published in the Dec. 25 edition of the Journal of Physical Chemistry B, used powerful high-resolution imaging and modeling techniques to detail how light is localized and scattered by metal nanoparticles.
Current technologies, such as high speed computers and internet routers, rely heavily on electrons flowing through wires in order to function. However, with the ever increasing demand for higher data rates and smaller sizes, the complexity of electrical circuits becomes untenable. According to experimental team leader Gary Wiederrecht, this challenge can be overcome by replacing electrons with photons (units of light), since the wave-like character of photons would reduce obstacles such as heat and friction within a given system. "In a nutshell, photons move faster than electrons," said Wiederrecht. "They are a highly efficient power source just waiting to be harnessed."
"Using experimental and theoretical approaches, we were able to observe the interaction of light with the surfaces of the metal nanoparticles. We hope that this study will lead to the creation of optical technologies that can manipulate light with precision at nanoscale dimensions," explained lead theoretician Stephen Gray.
To obtain a more comprehensive understanding of the near-field, the Argonne researchers used an advanced technique imaging technique known as near-field scanning optical microscopy. The nanoparticles, with diameters as small as 25 nanometers, were placed on a prism and illuminated with laser light, forming a near-field that was detectable with near-field scanning optical microscopy by a nanoscale probe positioned close to the sample's surface. Optical scattering experiments were performed on isolated metal nanoparticles and arrays of metal nanoparticles. Electron beam lithography was used to uniformly place nanoparticles within 100 nanometers from one another. Using a special experimental setup, the team was able to explicitly map the near-field light intensity onto the three-dimensional topography of the metal nanoparticle arrays.
Experimental results yielded a number of valuable findings regarding the character of the near-field. The researchers found that an isolated nanoparticle would scatter light at a 20-degree angle from the prism surface. Furthermore, the researchers found that arrays of nanoparticles scatter light at much smaller angles, an encouraging result for using near-field photons in two dimensional devices such as optical chips. 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.