Using experiments designed by Duke engineers, a team of scientists from University College London and Imperial College London has explored the limits of transforming light waves into smaller electrical ones.
Light can carry more information than electricity, but is impractical to use on a small scale. Because fiber optic cables are expensive and optical sensors are bulky, electrical signals are often preferable. A field called plasmonics allows light to be transmitted in the same way electrical signals are sent—for example, through a thin wire.
“Photons are about 1,000 times the size of atoms – getting them to interact with atoms on a one-to-one basis is very impressive,” said Sir John Pendry, chair in theoretical solid state physics at Imperial College. “If you want to compress something that much, the engineering has to be pretty impressive.”
Plasmonic systems are employed in new forms of small scale optical communication—such as lasers—and may have applications in some medical technologies, such as bioassays, which are experiments to determine the effect of particular substances on living organisms.
In plasmonic systems, the clouds of electrons surrounding metal atoms behave like very small pockets of gas or plasma. The surfaces of the clouds are capable of carrying waves of energy like ripples across the surface of a pond.
“When you hit an electron cloud with a photon, you create a surface wave called a plasmon, that you can focus with the right technology,” Pendry explained. “A plasmon is a sort of wobbling at the surface of an atom.”
Similarly, it is far easier to transmit sound energy to the human brain by striking a drum to create pressure waves in the air, than to smash a drumstick directly into someone’s eardrum.
Because the light energy is merely transferred into a sort of plasma wave, the electron clouds vibrate in the visible spectrum and carry the same information that photons would have. Until now, however, it was not known how far such signals could be compressed.
Although Pendry’s team was responsible for much of the theoretical work behind the project, Cristian Ciraci, a postdoctoral fellow at Duke’s Pratt School of Engineering, led a team of designers to create an especially accurate testbed for plasmonic systems that could be used to test the limits of photon-electron interactions.
“When you work with systems on the scale of a few fractions of a nanometer, it’s very hard to get precise control,” he said.
His team designed a system that uses various lengths of carbon chains to suspend single gold atoms a certain distance above extremely thin gold foil. Compounds of differing length were reacted with the gold to control the spacing between the metal atoms. Photons could then be directed between these two gold surfaces to create a plasmonic wave. Their results were revealing, Ciraci said.
“The response of your material is basically proportional to the strength of its electric field,” he said. “When you use a plasmonic system to squeeze the light into this tiny area, you have to keep the electrons from repelling each other.”
If the two metallic atoms are too close together, their electron clouds push with such force on one another that they overlap and block the wave.
“We now have a means of assessing the limitations of plasmonics, since the more advanced response of the electrons in the metal is ultimately what will define the field enhancement obtainable from a nanoparticle system,” David R. Smith, director of the lab in which Ciraci conducted his research, wrote in an email Thursday.
But despite this discovery of a limit on plasmonic systems, Smith remains optimistic.
“There may be much more in these systems to exploit. We are looking forward to seeing if we can unlock more phenomena from this new understanding,” Smith, William Bevan professor of electical and computer engingeering, said.
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