Optical antennas can amplify signals by a million times or more using lasers to induce quantum tunneling between sub-nanometer gaps between metal electrodes, according to researchers at Rice University who say they have accurately characterized optical antennas, which promise to enable single-molecule sensors and other advanced non-linear optical applications.
Condensed matter physicist Doug Natelson and graduate student Dan Ward have found a way to make an optical antenna from two gold tips separated by a nanoscale gap that gathers light from a laser. The tips “grab the light and concentrate it down into a tiny space,” Natelson said, leading to a thousand-fold increase in light intensity in the gap.
Metal nanostructures act as powerful optical antennas because collective modes of the electron fluid in the metal are excited when light strikes the surface of the nanostructure. These excitations, known as plasmons, can have evanescent electromagnetic fields that are orders of magnitude larger than the incident electromagnetic field. The largest field enhancements often occur in nanogaps between plasmonically active nanostructures but it is extremely challenging to measure the fields in such gaps directly. These enhanced fields have applications in surface-enhanced spectroscopies nonlinear optics and nanophotonics. Here we show that nonlinear tunnelling conduction between gold electrodes separated by a subnanometre gap leads to optical rectification, producing a d.c. photocurrent when the gap is illuminated. Comparing this photocurrent with low-frequency conduction measurements, we determine the optical frequency voltage across the tunnelling region of the nanogap, and also the enhancement of the electric field in the tunnelling region, as a function of gap size. The measured field enhancements exceed 1,000, consistent with estimates from surface-enhanced Raman measurements. Our results highlight the need for more realistic theoretical approaches that are able to model the electromagnetic response of metal nanostructures on scales ranging from the free-space wavelength, λ, down to ~λ/1,000, and for experiments with new materials, different wavelengths and different incident polarizations.
“An antenna is a metal structure that interacts with radiation, leading to the production of an oscillating voltage,” said Rice Professor Doug Natelson. “In our situation, the electromagnetic waves are light (specifically at a wavelength of 785 nanometers in the experiment), and those light waves cause the electrons in our little metal electrodes to slosh around, producing a changing voltage across our nanogap. In that sense it really is an antenna, only for light, rather than for radio waves.”
Sensors using the effect could sense even single molecules by harnessing the radiation intensity in the sub-nanometer gap between electrodes, which Rice measured to be “hundreds of thousands or millions of times higher than that from the incident laser,” said Natelson. “For example, closely spaced metal nanoparticles have been used to enhance fields sufficiently in the interparticle gap to allow single-molecule Raman spectroscopy.”
Closely spaced metal electrodes act as optical antennas because their electrons can be excited with a laser, inducing plasmons—collective oscillations of the free electrons—whose evanescent electromagnetic fields are thousands of times stronger than the incident light. Unfortunately, these fields have been very difficult to measure and characterize. Now Natelson and doctoral candidate Dan Ward have found a relatively easy way to measure the fields between sub-nanoscale electrodes on optical antennas.