Light spot
Bow-tie
antenna
(conductor)
Peak feld
concentrated
in feed gap
Surface
plasmons
PHOTONIC FRONTIERS: OPTICAL ANTENNAS
Optical antennas concentrate
light and direct beams
JEFF HECHT, contributing editor
Radio antennas played a crucial role
in electronics, converting electric signals into radio waves at transmitters
and converting the waves back into
electrical signals at receivers. Now
antennas are being shrunk orders of
magnitude to nanoscale for photonic applications.
Optical antennas rely on the same
principles of electromagnetic theory
as radio antennas. Oscillating electric charges collectively radiate electromagnetic waves in a pattern that
depends on the antenna structure and
the oscillation frequency. Receiving
antennas absorb incoming radiation,
converting it into heat and oscillating charges. But although the principles are the same, the frequencies are
hundreds of thousands times higher,
so the workings of optical antennas
differ in important ways from those
of radio antennas.
So far, most optical an-
tenna work has focused on
proof-of-principle demonstra-
tions, says Palash Bharadwaj
of the Swiss Federal Institute
of Technology (Zurich,
Switzerland). The results have
been promising, but techno-
logical applications have been
slowed by the need to
make wavelength-scale
components accurate
to within 10 nm. 1
Optical antenna
fundamentals
Electromagnetic theory
scales simply with wavelength, and
more than half a century ago Richard
Feynman envisioned nanoscale antennas when he told the American
Physical Society, “there’s plenty of
room at the bottom.” He wondered
if arrays of little antennas might stick
up from circuits measuring 100 to
1000 nm. “Is it possible, for example,
to emit light from a whole set of antennas, like we emit radio waves from
an organized set of antennas to beam
radio programs to Europe? The same
thing would be to beam the light out
in a definite direction with a very high
intensity,” he said. 2
However, Feynman recognized that
material response to light fre-
quencies differs greatly from
that to radio waves. Electrical
currents in conductors can eas-
ily vary at the gigahertz rates
needed for radio antennas, but
not at the hundreds of terahertz of
light waves. The oscillating charges
in optical antennas are surface plas-
mons, which move very fast, but con-
ductivity is low at optical frequencies.
Surface plasmons require much higher
electron mobility than radio antennas,
so few conductors can serve as opti-
cal antennas, notably silver and gold.
Importantly, optical excitation of
a subwavelength antenna can produce surface plasmons much smaller than the light wave, as shown in
Fig. 1 for a bow-tie antenna. “It’s
cool because it allows you to make
up for the mismatch between the
size of the photon, a wavelength
of visible light, and true nanoscale
materials,” says James Schuck, director of the Molecular Foundry at
the Lawrence Berkeley Laboratory
(Berkeley, CA). Optical antennas
also are being explored for applications including beam steering and
direction.
Like their radio-frequency counterparts,
optical antennas can collect energy
on subwavelength scales, enhancing
absorption for a range of applications
such as heating, spectroscopic probing,
and light detection.
