Plasmonic antenna enhances spectroscopic studies

Marie Freebody, marie.freebody@photonics.com

The need for technologies that can detect molecules with high sensitivity is important in many fields. For applications ranging from industrial safety to homeland security, single-molecule sensitivity would be a highly valuable tool.

Now an optical antenna developed by a collaborative group at Rice University, the University of Karlsruhe in Germany and the Autonomous University of Madrid in Spain could be the answer to single-molecule spectroscopy, among other applications. The antenna is made from two gold tips separated by a 1-nm gap and relies on an optical effect known as plasmonics to concentrate light down into a tiny space and increase light intensity a thousandfold.

“Plasmonic antenna structures are a way of manipulating light on deep subwavelength scales,” said Douglas Natelson, a condensed matter physicist at Rice University. “This is potentially useful for a variety of photonics applications, including spectroscopies, nonlinear optics, photodetection, photon sources, plasmon-based optical interconnects.”

The plasmonic field between a pair of gold nanotips concentrates light from an incident laser by a factor of 1000 in a newly developed optical antenna. Courtesy of Natelson Lab, Rice University.

The science behind plasmonic enhancement is not yet fully understood. It’s based on a near-field effect in which electrons on the surface of a metal interact with the electromagnetic field of light to create other waves called plasmons.

When two metal surfaces are placed close together, as with the tips in an optical antenna, the combined effect of both plasmon fields greatly enhances the intensity of light in the tiny volume of space between the two tips. The process involves no net energy gain; instead, the plasmons serve simply to redistribute the incident energy into the near field.

“When radio waves hit your car antenna, they make the electrons in the metal wire slosh back and forth, producing a small alternating voltage at the tip of the antenna. Your stereo amplifier magnifies that voltage and for radio demodulates it into sound,” Natelson said. “Incident light excites plasmons in our metal electrodes, producing a little oscillating voltage (in our case, tens of millivolts) between the tips of the incredibly closely spaced electrodes. In this sense, our electrodes act like an antenna, but for light rather than radio waves.”

Because the oscillating voltage can drive a current, the Rice group was able to deduce the optically produced voltage. The details of the system are described in the online edition of the journal Nature Nanotechnology published on Sept. 19, 2010.

A 785-nm continuous-wave diode laser was used to illuminate the gap between the gold tips of the antenna. The incident intensity on the nanogaps is, at most, 22.6 kW/cm². The team found that the intensity in the tiny nanogap region can be more than a million times higher, corresponding to an electric field enhancement of more than 1000.

This colorized scanning electron microscope image depicts the gold tips in the nanoantenna device developed by researchers in the US, Germany and Spain.

Natelson and colleagues have used the plasmonic structure for surface-enhanced spectroscopies and have so far carried out single-molecule Raman spectroscopy. In principle, this could allow detection of chemical analytes with single-molecule sensitivity, although Natelson admits that this remains tricky in practice. Integration with lab-on-a-chip approaches is a natural direction to take, as is using optical antennas for enhanced photodetection.

The group uses similar structures for spectroscopy studies and also is working on some potential sensor applications, a project that is partially supported through LANCER, the Lockheed-Martin Advanced Nanotechnology Center of Excellence at Rice University.