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OPTICAL ANTENNAS continued
ing. Active beam steering is preferred
for many applications, but the elements
could not be packed closely together using conventional optical couplers and
phase shifters, which are tens to hundreds of wavelengths long.
Michael Watts and colleagues at MIT
overcame those limits by fabricating
light-delivery waveguides and emitting
antennas in silicon on complementary
material. They fabricated a passive 64 ×
64-element array in which a single waveguide running along one side of the array splits input light among secondary
waveguides serving the rows, and each
of those waveguides divides the light
among antennas along one row. Figure
3 shows a part of this array. To make a
steerable array with 8 × 8 elements, they
added silicon electronic connections and
applied heat to the waveguides to shift
the phase actively using the thermo-op-tic effect. 10
Proof-of-principle demonstrations have
yielded exciting results, but actual applications of optical antennas are few
and far between. The ability to focus
hot spots of energy onto 10 nm spots
is attractive for optical imaging and
for spectroscopic probes, says Schuck.
Enhancing optical fields in small volumes is attractive for ultra-sensitive detection, such as spotting toxins before
they reach dangerous concentrations.
One long-term possibility is developing
nanoantennas that could be concentrated into tumor cells, where they could
selectively absorb enough energy to kill
the cells. Both Schuck and Bharadwaj
see heat-assisted magnetic recording for
a new generation of ultra-high-density
hard drives as a very promising application for optical antennas.
Many other applications may be pos-
sible in the long term, such as beam
steering to switch optical signals on
chips. As Feynman said, there is plenty
of room at the bottom. But much work
remains to turn those visions into prac-
1. P. Bharadwaj, B. Deutsch, and L. Novotny,
Adv. Opt. Photon., 1, 438–483 (2009);
doi: 10.1364/aop. 1.000438.
2. R. Feynman, “There’s plenty of room at the
bottom,” Eng. Sci., 23, 5, 22 (May 1960).
3. W. Bao et al., Science, 338, 1317 (2012);
4. S. Kim et al., Nature, 453, 757 (June 5, 2008);
5. Z. Fang et al., Nano Lett., 13, 1736-1742
6. L. Cao et al., Nano Lett., 10, 4, 1229–1233
(2010); doi: 10.1021/nl9037278.
7. M. W. Knight et al., Science, 332, 702 (2011);
8. N. Yu et al., IEEE J. Select. Topics Quantum
Electron., 19, 3, 4700423 (May/June 2013);
9. N. Yu et al., Nat. Mater., doi: 10.1038/
nmat2822 (Aug. 8, 2010).
10. J. Sun et al., Nature, 493, 195–199 (Jan. 10,
2013); doi: 10.1038/nature11727.