HOUSTON – 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
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
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
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.