NATURE PHOTONICS | VOL 4 | MAY 2010 | www.nature.com/naturephotonics 267

news & views

Finding convenient ways to control the radiation emitted by tiny light sources such as single molecules and

quantum dots is essential for harnessing electromagnetic energy at the nanoscale. Th is situation is not dissimilar to that of 100 years ago, when an effi cient means of directional transmission and detection of radiowaves was needed. Th e solution came in 1926 from two professors at Tohoku Imperial University in Japan — Hidetsugu Yagi (Fig. 1) and Shintaro Uda — who devised the famous Yagi–Uda antenna1–3. Th is now iconic design is still used in many applications, and is the principal design of television antennas found on rooft ops around the world.

Now, reported in Nature Photonics4, Terukazu Kosako and co-workers from Hiroshima University in Japan revisit the work of Yagi and Uda, but at the nanometre-scale and in the optical domain. Th e result is the fi rst nanoscale Yagi–Uda antenna operating at visible wavelengths. Th e antenna exploits a carefully designed array of gold nanoparticles to perform strong directional control of red light (662 nm) injected at a ‘feed element’. Th e approach is analogous to the way a conventional Yagi–Uda antenna uses electrically conducting rods to achieve directional emission of microwaves and radiowaves. In the work of Kosako et al., the resonances of electrical wires are mimicked by the plasmonic responses of gold nanoparticles. Th e size and location of the nanoparticles, in analogy with the elements in the original Yagi–Uda antenna, are carefully designed to achieve constructive interference in one direction, and the radiofrequency electronic circuit feed is emulated using the polarization dependence of a nanorod.

Radiowave transmission was fi rst explored by Marconi in 18955, who proved it was possible to send information using electromagnetic waves, and received the Nobel Prize for Physics in 1909 for his contribution to radio communications. One of the major problems that Marconi faced in his experiments was the weakness of the signals to be received. For example, to achieve transatlantic radio transmission,

a 154-metre-tall antenna needed to be used as the receptor6. In wireless communications, the weakening of a signal with transmission distance mainly arises from diff ractional divergence, with the amplitude of a signal emitted from a point source falling with the inverse of the propagation distance. One way of overcoming this issue is to use several sources that are tuned in phase, allowing signals to interfere constructively in a given direction and therefore achieve directionality. However, this solution requires the use of many stabilized electrical circuits, making it impractical and costly.

In 1926, Yagi and Uda introduced the concept of ‘parasitic elements’ to solve this issue: just as a mirror refl ects the light coming from a source, a length of electrical conductor placed at some distance from a feed wire functions as a secondary source of radiation due to the currents induced by the incident radiation. Yagi and Uda showed that a single circuit can generate currents in several metallic elements, paving the way for the design of simple and cost-eff ective multi-element antennas for both the

transmission and detection of radiowaves. Th e approach turned out to be so useful for capturing weak signals that the design (Fig. 1) has hardly changed at all since its inception.

Although scientists in the USA and Europe were quick to recognize the benefi ts of the Yagi–Uda design7 — with the British implementing it for use on airborne radars during the Second World War — the invention received little attention in Japan. Somewhat ironically, Yagi was a scientifi c consultant for the Japanese military during this time, and he later expressed frustration that his design had not been adopted earlier by his own country. In fact, the Japanese military realized the full potential of the invention only aft er a captured British radar technician revealed information about the British radar antenna, which was code-named “Yagi”. Th e technician explained that Yagi was the name of the one of the Japanese Professors who developed the antenna design. Today, the Yagi–Uda antenna is still the reference antenna for TV reception and radio transmission, and the inventors have received the recognition they deserve.

Th e directional operation of an antenna, for boosting either the reception or transmission of a signal, results from careful control of the incident radiation’s phase. A metallic wire resonates strongly if its length matches half of the operating wavelength. Shorter wires tend to have oscillations that lead in phase (relative to the driving fi eld), whereas fi elds that build up on longer wires tend to lag because they are inductively detuned. Yagi and Uda combined these principles to create a multi-element antenna design that demonstrates strong coherent interference in a particular direction, thus boosting transmission or detection. Th e design consists of a longer parasitic element — the refl ector — which is parallel and to the left of the feed element, and shorter parasitic elements — the directors — to the right of the feed element. Th e phase relationship between the refl ector, the feed and the directors results in constructive interference between the waves (Fig. 2a). Tuning the distances between all elements provides an emission

NANO-OPTICS

Yagi–Uda antenna shines brightThe demonstration of a Yagi–Uda nano-antenna that operates at visible wavelengths gives hope for a convenient

means of directing radiation patterns from nanoscale light sources such as single molecules and quantum dots.

Geoff roy Lerosey

Figure 1 | Hidetsugu Yagi holding a short-wave

Yagi–Uda antenna that has one refl ector and

two directors.

YA

GI

AN

TE

NN

A

nphoton_.2010.Run_MAY10.indd 267nphoton_.2010.Run_MAY10.indd 267 10.4.16 0:19:48 PM10.4.16 0:19:48 PM

© 20 Macmillan Publishers Limited. All rights reserved10

268 NATURE PHOTONICS | VOL 4 | MAY 2010 | www.nature.com/naturephotonics

news & views

gain in the end-fi re direction (right) that is roughly proportional to the total length of the antenna.

Because of the simplicity and apparent scalability of the concept, Yagi–Uda antennas have been designed for use at many frequencies. Although they have caught the attention of the optics community8,9, severe fabrication issues and experimental limitations associated with scaling down the design have inhibited any proof-of-concept demonstrations at visible wavelengths. However, there are various teams that currently use nanoparticle antennas to confi ne and enhance optical fi elds at the nanoscale. Th ese schemes consist of two coupled resonant nanoparticles separated by a small gap to capture propagating waves and create an intense fi eld localized between them10,11, similar to how a dipole antenna harnesses energy to feed a radio receiver. An important diff erence between a microwave dipole and its optical equivalent arises from the fi nite permittivity of metals in the visible range. Th is gives rise to the ‘localized surface plasmon resonance’ (LSPR) — a resonance that does not depend solely on the length of the rod itself, but rather on the shape and dielectric constant of the nanoparticle. Th e LSPR is also polarization-dependent, provided that the particle is not symmetrical in shape.

Kosako et al. take advantage of the presence of the LSPR at visible wavelengths to realize their nanoscale Yagi–Uda antenna. In contrast with earlier studies10,11, they aim not to concentrate an optical fi eld at the nanoscale, but rather to achieve directional emission using a nano-antenna. Such a device could allow non-directional light sources, such as single molecules, quantum dots or other localized emitters, to become directional emitters. Th e directivity off ered by the Yagi–Uda nano-antenna should increase the amount of light that can be collected for analysis, thereby increasing energy effi ciency. Furthermore, its resonant nature could enhance the spontaneous emission rate of local emitters due to the Purcell eff ect, off ering improved optical detection and characterization of single molecules.

Th e work of Kosako et al. constitutes the fi rst step towards this goal. Th e antenna consists of a linear array of fi ve gold nanorods, which were lithographically fabricated on a glass substrate and embedded in SiOx. Figure 2b shows a scanning electron microscope image of one antenna in the array. Th e refl ector (left ) and the three directors (right) have their long dimension aligned perpendicularly to the antenna axis, and all have the

same polarization dependence. Th e feed particle is tilted and hence has a diff erent polarization dependence. Th e authors also showed that tuning the resonant frequency of a nanorod can be done easily by changing its aspect ratio9. For the sake of fabrication simplicity, they chose to keep a constant cross-section and vary only the length of the nanorods. Th e feed particle’s length was set to 105 nm for a resonance at 655 nm, whereas the refl ector and directors were 125 and 75 nm long, respectively. Th e refl ector element was therefore tuned to a redder resonance of around 770 nm, and the directors tuned to a bluer resonance of around 610 nm. To achieve maximum directivity9, the distance between the refl ector and feed was set to 125 nm, and the remaining spacings were set to 150 nm.

Just as Yagi and Uda designed their antenna to be fed at only one element and hence use a single electrical circuit, the eventual goal of Kosako et al. is to couple a single emitter to the antenna through only the feed element. Because the whole structure is embedded in SiOx and sits on a glass substrate, the authors emulated this behaviour by using external illumination that excites only the feed element.

To achieve this they used a clever trick that involved taking advantage of the polarization dependence of the nanorods. Th e feed was tilted to 45° in the antenna plane, which permitted its independent excitation through an incident electric fi eld

that was polarized perpendicularly to the long dimensions of the other elements. Although the other elements can also be excited resonantly by this polarization along their short axis, this would occur at much shorter wavelengths. Hence, measuring the radiation pattern of the antenna in a cross-polarized scheme ensures that only the feed couples with the incident light.

Polarized light resonantly excites the feed, which, due to its diagonal orientation, couples to the cross-polarized oriented LSPR of the other elements. Owing to the lagging phase response of the refl ector and the leading response of the directors, constructive interference of the light emitted by the whole structure then occurs, predominantly in one direction. Th e team collected the light emitted from the antenna using a polarizer and confi rmed directional emission, with a tenfold ratio between the forwards and backwards directions. Th e work proves that basic radio engineering principles can be transferred to the optical domain, albeit at the price of tedious fabrication procedures, and provided that plasmonic eff ects are carefully accounted for.

Kosako et al. have shown experimentally that the historic multi-element directional Yagi–Uda antenna design can be fabricated for use at visible wavelengths. In doing so, they have also shown that such nanotools are not restricted to the imaginative minds of theoretical physicists or experts of numerical simulations. Given the rich history of its ancestor, it seems likely that nano-Yagi–Uda antennas will fi nd many applications in the optical domain, such as in sensing and detection. Th e next step of directly coupling a single emitter to such an antenna would certainly constitute another major breakthrough. ❐

Geoff roy Lerosey is at the Institut Langevin, CNRS - ESPCI ParisTech, 10 rue Vauquelin, 75005, Paris, France.e-mail: geoff roy.lerosey@espci.fr

References

1. Uda, S. J. IEE 273–282 (1926).2. Uda, S. J. IEE 1209–1219 (1927).3. Yagi, H. Proc. IRE 16, 715–740 (1928).4. Kosako, T., Kadoya, Y. & Hofmann, F. Nature Photon.

4, 312–315 (2010).5. Marconi, G. Phys. Z. 3, 532–534 (1901).6. Marconi, G. Nature 86, 600–605 (1911).7. Balanis, C. A. Antenna Th eory Analysis and Design 3rd edn,

Ch. 10, 577 (Wiley, 2005).8. Li, J., Salandrino, A. & Engheta, N. Phys. Rev. B 76, 245403 (2007).9. Hofmann, H. F., Kosako, T. & Kadoya, Y. New J. Phys.

9, 217–228 (2007).10. Fromm, D. P., Sundaramurthy, A., Schuck, P. J., Kino, G. &

Moerner, W. E. Nano Lett. 4, 957–961 (2004).11. Mühlschlegel, P., Eisler, H.-J., Martin, O. J. F., Hecht, B. &

Pohl, D. W. Science 308, 1607–1609 (2005).

Published online: 14 March 2010

Reflector Feed

Directors

100 nm

b

a

Directors

Feed

Reflector

Figure 2 | The Yagi–Uda antenna. a, Schematic of

the operating principle of the original Yagi–Uda

antenna for radiofrequencies. b, A scanning

electron microscope image of the nanoparticles

used in the nano-Yagi–Uda antenna of

Kosako et al.4, operating at 662 nm.

nphoton_.2010.Run_MAY10.indd 268nphoton_.2010.Run_MAY10.indd 268 10.4.16 0:19:50 PM10.4.16 0:19:50 PM

© 20 Macmillan Publishers Limited. All rights reserved10