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.
Though all of this is theoretically sound, there is a problem: At optical wavelengths, the ideal materials to implement such circuits don't really exist in nature. The advent of metamaterials, however, may banish that concern. Scientists have already shown that by embedding nanoscale structures of one material inside another, resonances and other interactions can change the bulk properties of the material as a whole. Most famously, this has been demonstrated for negative-refractive-index materials–those that bend light in the opposite direction to conventional, optically dense materials.
Making it real
Engheta and his team have done simulations of various circuits, including an optical version of a Yagi-Uda antenna structure. It's still unclear, however, whether his ideas can be implemented in practice. Some of the metamaterials that will be required to make them work well have not been invented yet, never mind fabricated. On the other hand, negative-refractive-index materials were demonstrated less than a decade after John Pendry, a professor of theoretical physics at Imperial College London, proposed (hugely controversially) that they were possible. Such precedent may bode well for Engheta's vision, some believe.
Two teams are already at work trying to demonstrate the basic nanocircuit principles. With his colleagues, Rohit Prasankumar of the Center for Integrated Nanotechnologies at Los Alamos National Laboratory is working on optical nanoantennas that he says should operate as lumped nanocircuit elements at visible wavelengths. “We are currently fabricating these nanoantennas and hope to test their operation as nanocircuits using optical scattering experiments shortly afterward,” Prasankumar said. “Sub- sequent experiments will include design, fabrication and testing of more-complex nanocircuits to achieve a desired functionality”–for example, nanotransmission lines.
Prasankumar sees the endeavor as “one of the most exciting developments to emerge from research into metamaterials and their applications in the last few years, particularly if we are successful in making Prof. Engheta's theoretical ideas a reality. I am excited to be working on this project, and hope to have a working optical nanocircuit in the near future.”
Penn's physics department is also working on the problem. “We plan to construct specially designed grating structures with periods much less than the operating wavelengths, and then experimentally verify the performance of such nanostructures in terms of optical reflection and transmission,” said Penn physicist Marija Drndic.
According to Engheta's predictions, such nanostructures should act as optical filters at nanoscales, she said–for example, bandpass or bandstop filters depending on incident polarization. If successful, Drndic said, the experiment will show “that his concept of lumped circuit elements at optical frequencies will indeed provide useful recipes for design of optical nanocircuits with various functionalities.”
Though generally effusive about the work, Eleftheriades at the University of Toronto sees some challenges ahead for researchers. Specifically, “plasmonic materials [such as silver and gold] can be lossy when used as interconnects,” he said, “and the integration of these optical LCR nanocircuits with active devices such as lasers can be challenging.”
Engheta agreed with that analysis, particularly the problem of material loss, but also said he sees huge potential for metananocircuits in the future. n
Sunny Bains is a scientist and technology journalist based in London.