Squeezing light into narrow spaces

The spectrum of visible light restricts what we are able to see to those frequencies that fall in the narrow range between ultra-violet and the infrared with a small overlap into the two extremes. Objects that are smaller than the smallest wavelength of visible light can not be seen by human eyes, nor can they be resolved by conventional microscopes that focus light waves.

Similarly, the diameter of an optical fiber cable needs to be at least one-half of the wavelength of the light wave which is transmitted through it. This places a constraint on their application in micro-circuits, since that diameter is much larger than the dimensions of the conducting strips in the smallest electronic devices.

Research in the 1980's uncovered an interesting phenomenon. It was found that when light was directed at the space between a conducting material such as a metal and a non-conducting material or dielectric such as glass a resonant oscillating wave was generated by the electrons on the metal's surface, much like the ripples on the surface of a pond when a pebble is dropped into it. The frequency of the oscillations matched the frequency of the electromagnetic field outside the metal. The metal conductor's surface oscillations have been given the term surface plasmons, and the field of study is called plasmonics.

Using empirical data, researchers have now engineered a metal-dielectric interface which generates surface oscillations at the same frequency as the outside electromagnetic field but with a much shorter wavelength. In fact, Hideki Miyazaki of the National Institute for Materials Science in Japan has squeezed red light with a normal wavelength of 651 nanometers into a plasmon slot waveguide that is only three nanometers thick and 55 nanometers wide. The effective wavelength of the surface plasmon travelling through the gap was 51 nanometers.

Using fabrication techniques similar to those used to manufacture integrated circuits it is possible to mass-produce nano-scale plasmonic devices. There is a tremendous advantage to incorporating plasmonics into micro-electronic circuitry. The frequency of an optical signal is more than 400,000 Gigahertz, and such a circuit could carry huge amounts of data for a given instant. Furthermore, such a circuit might be even simpler overall, since there would be little to none of the stray capacitance or inductive effects which must be compensated for in conventional micro-circuitry. Harry A. Atwater and his group at the California Institute of Technology and others have laid the groundwork for and have developed primitive versions of the plasmonic transistor. More research in this area is needed to improve their performance but the future of micro-photoelectronics looks promising.