A University of Pennsylvania professor is exploring an approach to nanotechnology that will allow circuit theory to operate in an entirely new regime--one where "current" is no longer defined as the movement of electrons and holes, but instead as an electromagnetic wave.

If Nader Engheta's theories prove successful in practice--and researchers are already working on experiments to test this--then the work could strike the elusive balance between finding new technologies that can reliably operate at nanometer scales and ensuring that the technologies can bootstrap on decades of knowledge about more-conventional electronics.

For one thing, Engheta said he is interested the possibility of creating switches from metananocircuitry. They could lead to a new kind of optical information processing and, perhaps, a new form of nanoscale computational unit, said Engheta, the H. Nedwill Ramsey Professor of electrical and systems engineering at Penn.

Universiity of Pennsylvania engineering professor Nader Engheta

He is also excited about the idea of "wireless at nanoscales using light." In other words, Engheta said, he'd like to investigate the possibility of optical communication between nanostructures or even cells that could be pressed into service in the same way that RF and microwaves are used at other scales.

George Eleftheriades, professor of electrical and computer engineering and a Canada research chair at the University of Toronto, said Engheta's work provides "a vision, consisting of building blocks, along with instructions on how to arrange them together to enable transplanting well-known passive inductor-capacitor-resistor [LCR] electrical networks to the optical domain. This includes the direct optical realization of filters, antennas, power-distribution networks, microwave transmission-line metamaterials and many more."

The building blocks in Engheta's world are dielectric nanoparticles, Eleftheriades explained. Conventional dielectric nanoparticles--those with positive permittivity--"can realize optical capacitors," he said, whereas negative plasmonic nanoparticles, which have negative permittivity, can realize optical inductors and resistors.

"What makes these different from conventional electronic networks," he said, "is that instead of thinking in terms of a conduction current, one should think in terms of the displacement current, which indeed can 'flow' in free space and in dielectric materials."

New kind of circuit board

Engheta's theory relies on three basic ideas. The first is that nanoparticles of various materials have properties that can be matched to electronic equivalents (such as resistance, inductance and capacitance). Further, the nano- particles can be thought of as "lumped components" that can be connected together into circuits by using additional guiding structures. Finally, the concept of metamaterials--in which composite materials exhibit properties that are dictated by their nanoscale structures rather than their chemistry--is crucial for the design of efficient devices.

To understand how these three ideas work together, it helps first to think about a lone nanoparticle made of some nonmagnetic material, its diameter a small fraction of an optical wavelength. After analyzing this using Maxwell's equations and then equating the electric displacement current density with current, it turns out that if the real part of the material permittivity, Re(e), is greater than zero, then the particle acts as a capacitor for the incoming light. If Re(e) is less than zero, then it acts as an inductor. Finally, if the imaginary part of the permittivity is not equal to zero and so energy is lost (whatever the real part is), then the element can be thought of as having resistance.

Of course, even if the optical and electronic domains can be made equivalent theoretically, the two are very different in practical terms. Electronics do not tend to be leaky; the air or insulator between components prevents current loss. Unfortunately, light cannot be kept from escaping in the same way. To guide the waves, an extra layer of structure is required. Layers of material with a very low permittivity--much smaller than that of a vacuum--can act as terminals, while layers with high permittivity act to prevent propagation. Once these wires and barriers are in place, then networks of devices can be created.