A Monopole Antenna at Optical Frequencies: Single-Molecule Near

3010 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 11, NOVEMBER 2007

A Monopole Antenna at Optical Frequencies:Single-Molecule Near-Field Measurements

Tim H. Taminiau, Frans B. Segerink, and Niek F. van Hulst

Abstract—We present a monopole antenna for optical frequen-cies ( 600 THz) and discuss near-field measurements with singlefluorescent molecules as a technique to characterize such antennas.The similarities and differences between near-field antenna mea-surements at optical and radio frequencies are discussed in detail.Two typical antenna properties, polarization selectivity and reso-nances, are studied for the optical monopole by direct near-fieldmeasurements and finite integration technique calculations. Theantenna is driven by the local field of a sub-wavelength aperture.This gives rise to a dependence of the antenna response on the ori-entation of the local field vector, in an analogous way to the polar-ization selectivity of linear wire antennas. The antenna resonancesare studied by varying the antenna length. Typical monopole res-onances are demonstrated. The finite conductivity of metals at op-tical frequencies (in combination with the antenna radius) causesthe wavelength of the surface charge density oscillation (surfaceplasmon polariton) along the antenna to be shortened in compar-ison to the free space wavelength. As a result, resonances for theoptical monopole antenna occur at much shorter relative lengthsthan for conventional radio monopole antennas.

Index Terms—Monopole antennas, near fields, optical antennas,optical resonance, plasmons, surface plasmon.

I. INTRODUCTION

THE traditional function of antennas is two-fold: the efficientand selective coupling of far-field radiation in sub-wave-

length waveguides and, vice versa, the efficient and selectivecoupling of energy from waveguides into directed far-field radi-ation. A direct optical analogue is of interest for novel integratedoptical circuits, where optical antennas could be used to couplelight fromandintoplasmonicwaveguides[1].Possiblyevenmoreimportant, the general ability of antennas to efficiently collectenergy in volumes much smaller than allowed by the diffractionlimit enables the application of traditional optical techniques,such as microscopy [2], [3], lithography [4] and spectroscopy [5],on thenanoscale. Ina reciprocalway, theemissioncharacteristicsof sub-diffraction-limit volumes of matter or single quantumemitters can be controlled by optical antennas [6]–[9].

Manuscript received March 12, 2007; revised April 24, 2007. This workwas supported by the Specific Target Research Project (STRP) ASPRINT(NMP-CT-2003-001601) in the 6th Framework Program of the EuropeanCommunity.

T. H. Taminiau is with the Institut de Ciencies Fotoniques (ICFO), Mediter-ranean Technology Park, 08860 Castelldefels (Barcelona), Spain (e-mail: [email protected]).

F. B. Segerink is with the Optical Sciences, MESA+ Institute for NanoTech-nology, University of Twente, 7500AE Enschede, The Netherlands.

N. F. van Hulst is with the Institut de Ciencies Fotoniques (ICFO), Mediter-ranean Technology Park, 08860 Castelldefels (Barcelona), Spain, and also withthe Instituciò Catalana de Recerca i Estudis Avanats (ICREA), 08015 Barcelona,Spain.

Digital Object Identifier 10.1109/TAP.2007.908561

Because of the above prospects, the optical properties ofmetal nanostructures, which could act as antennas, have beenextensively studied. The plasmon resonances (frequency re-sponse), coupling effects and polarization selectivity of suchnanostructures have been characterized by far-field techniquesthat include scattering spectroscopy [10], [11] and multiphotonluminescence [12]–[14]. However, unlike in the case of radio an-tennas, most applications of optical antennas rely on the specificdistribution of the local field at the antenna or on the coupling ofa local emitter to the antenna. Far-field techniques do not yieldany direct information on the actual characteristics of the antennanear field. To gain insight in the local antenna field, true near-fieldmeasurements for optical antennas are needed. Whereas fortraditional radio antennas near-field measurements are often aneasier, better controlled or cheaper way to obtain the far-fieldbehavior of the antenna [15], such measurements are a seriouschallenge at optical frequencies. Techniques that have been suc-cessfully employed to map local fields at nano structures includescanning probe microscopy [16], local surface modification [4]and single molecule fluorescence detection [3], [17].

Recently, we have introduced a probe-based optical antennaand showed that the antenna is equivalent to the standard radiomonopole antenna by directly probing the antenna near-fieldwith single fluorescent molecules [18]. In this paper we focuson the analogies and differences of antennas and near-fieldmeasurements at optical and radio frequencies. First we dis-cuss optical near-field measurements with single fluorescentmolecules. The conceptual and experimental similarities anddifferences between such measurements and near-field mea-surements at radio frequencies are summarized. Second, wefocus on the optical monopole antenna itself. Insight in theantenna characteristics is gained by finite integration technique(FIT) [19] calculations of the local electromagnetic field distri-butions for both the driving field and the antenna response. Weinvestigate the feeding of the antenna through an optical fiberprobe by near-field measurements and calculations. The resultsare related to standard transmission lines and to the polarizationselectivity of linear wire antennas. The resonance behavior ofthe optical monopole is characterized by varying the antennalength and is compared to the typical monopole resonances.It is shown that the relatively low conductivity of aluminumat optical frequencies causes the relative lengths at which theantenna is resonant to be shortened in comparison with thinperfectly conducting monopole antennas.

II. NEAR-FIELD MEASUREMENTS

A typical transmitting antenna system at radio frequencies(roughly MHz to GHz) consists of a generator (feeding source)

0018-926X/$25.00 © 2007 IEEE

TAMINIAU et al.: A MONOPOLE ANTENNA AT OPTICAL FREQUENCIES 3011

Fig. 1. Antennas and near-field measurements at optical frequencies comparedto at the MHz-to-GHz range. Both schemes for near-field measurements consistof a source, a transmission line, an antenna, a near-field probe and a detectionsystem.

that is connected to the antenna by a transmission line. In near-field measurements, the local antenna field is mapped by movinga probe antenna through the field, usually at a distance of sev-eral wavelengths from the antenna (radiating near field), andrecording the collected power (Fig. 1).

In an analogous way the local field at an optical antenna(400–700 THz) can be obtained. In the scheme presented here(Fig. 1), the source of optical power is a laser and the opticalwave is transported to the antenna by a glass fiber. Singlefluorescent molecules are used as small local field probes [3],[17]. Such molecules can be envisioned as tiny dipole antennas[20] with a resonance frequency that depends on the electronicquantum transitions of the molecule. The energy absorbedby the molecule depends on the overlap of its absorptiondipole moment and the local electric field vector ;the molecular orientation determines the field component thatthe molecule is sensitive for. Part of the absorbed energy isre-emitted as radiation with a lower energy (lower frequency).This spontaneous decay process is called fluorescence. Forsufficiently low powers, the emitted fluorescence intensity (I)is proportional to the excitation intensity

(1)

Raster scanning the molecule close to the antenna and detectingthe fluorescence yields a map of the local antenna field. Notethat, in contrast to the radio antenna case, we are particularlyinterested in mapping the field in the very vicinity of the an-tenna, from right at the antenna surface to roughly a fifth of thewavelength (reactive near field). It is possible to map fields atsuch short distances from the antenna because the fraction of theantenna field absorbed by the single molecule is negligible com-pared to the total field; the disturbance of the molecular dipoleon the antenna response can thus be neglected.

To scan single fluorescent molecules close to the opticalantenna we will use a near-field scanning optical microscope(NSOM), a well developed scanning-probe technique thatuses the locally confined field at a probe for high resolutionoptical microscopy [2], [3], [17]. Before discussing the optical

Fig. 2. Overview of a near-field scanning optical microscope (NSOM). Asample containing isolated single fluorescent molecules is scanned underneatha near-field probe. The insets show an example: a standard fiber probe withaperture diameter of 100 nm and a fluorescence image obtained for that probe(image size 2.15 mm � 2.15 mm). The image consists of a collection of mapsof the different field components of the near-field at the aperture of the fiberprobe.

monopole antenna, the principle of NSOM is introduced andillustrated by an example.

A schematic overview of the NSOM set-up used is shownin Fig. 2. The structure in the inset is a conventional near-fieldaperture probe. Note that the naming is unfortunate in the cur-rent context; the aperture probe should not be confused with thesingle molecules that are used to probe the local field at struc-tures. Such an aperture probe is a sharply pointed glass fiberthat is partly metal coated so that the end consists of a smallsub-wavelength aperture. To create such probes, sharp pointedglass tips are created by controllably heating and subsequentpulling of single mode optical glass fibers (633 nm wavelength)with a commercial fiber puller (Sutter P2000). The tips are to-tally coated, first with a few nanometer thick chromium layerand then with nm aluminum. In a final step the coating atthe very end is removed with a focused ion beam (FIB) so thata well-defined aperture in a flat aluminum end face is formed[21]. Typical aperture diameters are 50 to 100 nm.

The 514 nm (583 THz) laser line of an Ar laser is used asan excitation source. The light is coupled into the fiber probeafter being passed through polarization controlling optics.A sample containing single fluorescent molecules (DiI) em-bedded in a nm thick polymer layer (PMMA) is scannedunderneath the fiber probe by a piezo scanning stage with aposition feedback system with nanometer position accuracy.The molecules are distributed and orientated randomly in thesample. The molecular concentration is chosen such that mostof the molecules are spatially isolated. The distance between


the fiber probe and the sample is regulated by a shear forcebased feedback system. The fiber probe is attached to an oscil-lating quartz tuning fork and the change of the phase differencebetween the driving signal and the tuning fork response as thefiber probe approaches the sample is used to keep a constantprobe-sample distance of around 5 nm [22]. The fluorescenceof the molecules is collected with a 1.3 Numerical Aperture oilimmersion objective and, after the laser excitation light is fil-tered out, detected by a photon-counting avalanche photodiode.

As an example, a fluorescence image, obtained for the apertureprobe shown, is presented in Fig. 2. The image consists of mul-tiple fluorescence spots. Each such a spot is the recorded fluores-cence of one single molecule as it is raster scanned underneaththe aperture probe. The total sample area scanned contains sev-eral molecules resulting in a collection of spots. The distributionreflects the (random) placement of the molecules. The variancein shapes of the spots is caused by the dipole orientation of eachmolecule, which determines the field orientation that is probed.Each of the spots in the image is thus a map of a field componentof the local electric field just under the aperture probe. The solidcircles are maps of the field components that are orientated in theplane of the image (and aperture), while the open circles are mapsof the field component perpendicular to the plane. As the fieldis mapped at a very short distance (around 10 nm or ) fromthe aperture probe, the full width at half maximum of the mapsdirectly reflects the lateral confinement of the local field at theaperture probe. For a smaller aperture diameter, the field is moreconfined and the fluorescence maps are narrower. If, like in thiscase, the local field distribution is well known, the system can beused as a microscope. The lateral confinement then determinesthe obtainable resolution, typically 50–100 nm, i.e., better thanthe (250–300 nm) obtainable with lens based far-fieldmicroscopy. This resolution should not be confused with themuch better resolution with which the molecules map the localfield at the aperture probe (or, alternatively antenna).

III. OPTICAL MONOPOLE ANTENNA

The optical monopole antenna is literally based on the stan-dard aperture probe. Instead of creating a flat end face, an elon-gated aluminum antenna is left standing next to the aperture byFIB milling under two different angles. Fig. 3 shows SEM im-ages of such an optical monopole antenna. The typical antennawidth is around 40 nm and the radius of curvature of the apexaround 20 nm. The antenna length is varied controllably from30 to 140 nm, in order to tune the antenna resonance. In thisconfiguration the local field at the aperture drives the antenna.As such the aperture probe replaces the transmission line of theradio wave equivalent. However, there are important differencesbetween the two. First, the small diameter of the aperture andthe part of the fiber just above it do not allow optical modes topropagate. Thus, in contrast to radio transmission lines, the op-tical waveguide is operated below cutoff and the throughput isvery low. Second, it is expected that not all the components ofthe local aperture field couple efficiently to the antenna. Elec-tric field components that are not directed along the antenna axisor that are not in the immediate vicinity of the antenna remainvirtually unmodified and will result in a background field addi-tional to the antenna response.

Fig. 3. An optical monopole antenna based on the standard aperture fiberprobe: SEM images from (a) side-view and (b) under 52 angle. The aluminumantenna shown here has a length of 70� 6 nm and a diameter of 45� 6 nm.

IV. ANTENNA RESONANCES

Insight into the antenna properties can be obtained bystudying the field distributions in the vicinity of the antenna.Three-dimensional electric and magnetic field distributions areobtained from FIT calculations with commercial software (CSTMicrowave Studio) [23]. For simplicity we model the antennaas an aluminum cylinder with a spherical apex (radius 20 nm)placed next to an aperture (diameter 100 nm) in a 10 nm thickinfinite perfectly electrical conducting (PEC) screen (crosssection: inset Fig. 4). The plate is illuminated from the top bya plane wave ( nm or THz) of variable linearpolarization. It has been shown that such a model, without anantenna, gives accurate results for the local field of an apertureprobe [17]. At 514 nm the dielectric constant of aluminum is

, as determined by fitting a Drude model toexperimental values [24]–[26].

First the antenna resonances are characterized. In the caseof radio antennas it is common to characterize the antenna re-sponse in terms of the operating frequency. It is also commonto take the magnitude of the induced current at the feed pointas a measure of antenna response. In the case of the optical


Fig. 4. The electric field intensity at the antenna apex and the squared magneticfield at the base of the antenna (far side from the aperture) against the antennalength. Both field intensities show the same distinct resonant behavior. The insetshows a cross section of the structure and the locations where the field is probed(both 5 nm from the edges). Values are normalized on the respective maxima.

monopole antenna the current can not easily be measured. In-stead, as explained above, the near field at the antenna apex isprobed. Moreover, the antenna response for varying frequency(or wavelength) is complicated because of several effects. First,varying the frequency changes the aperture diameter to wave-length ratio, which drastically changes the throughput. Further-more, for the actual tapered optical fiber probe the frequencydependence of the throughput is more complicated and hard todetermine. Second, the antenna sharpness relative to the wave-length varies with frequency. Both these relative changes havea drastic effect on the field strength at the antenna apex and thusadd a frequency response of their own to the system. This re-sults in a frequency resonance curve whose maximum does ingeneral not coincide with the resonance of the actual antennaelement. More insight is gained by varying the antenna lengthfor a constant frequency. In that case the two effects mentionedabove are not important as the dimensions relative to the wave-length are constant and thus information on the resonance of theactual antenna element is obtained.

Fig. 4 shows both the electric field intensity at the antennaapex and the square of the magnetic field at the antenna base (onthe side furthest from the aperture) for variable antenna length,at a wavelength of 514 nm (583 THz). The electric field inten-sity near the apex is the quantity probed in the near-field experi-ments. The magnetic field is a measure for the total current, con-duction and displacement. Both curves show distinct resonancesat similar lengths; equivalent information on the resonances isobtained by measuring the current at the base and the field inten-sity at the apex. The first resonance occurs at a length of 70 nm,the second at around 260 nm. Given the operating wavelength of514 nm, these lengths are significantly shorter than the lengths

found for a thin radio frequency monopole antenna, for whichresonances occur close to uneven multiples of (even mul-tiples are anti-resonances, modes without net dipole moment[27]). To explain this, two aspects of the optical antenna have tobe taken into account. First, the length-to-radius ratio for the op-tical monopole antenna is much lower (3.5 at the first resonance)than usual for MHz monopole antennas, where the ratio typi-cally is at least larger than 100. For good conducting antennas,the relatively large radius decreases the resonant length [28].Second, the finite conductivity of aluminum (finite ) at opticalfrequencies reduces the resonant length. For a frequency of 583THz and an antenna radius of 20 nm, the surface charge densitywave along the antenna, the quanta of which are called surfaceplasmon polaritons (SPP), has a wavelength that is significantlyshorter than the free space wavelength of 514 nm. The antennalength for which the SPP mode is at resonance is reduced by thesame factor (see for example dispersion curves for aluminumcylinders [29] and noble metal rods [30]). Thus, resonant op-tical monopole antennas are significantly shorter (relative to thefree-space wavelength) than otherwise equivalent PEC radio an-tennas. Indeed, it can be shown that for a PEC antenna, with ahigh length-to-radius ratio in the same configuration, character-istic monopole antenna resonances are recovered [18].

V. ELECTROMAGNETIC FIELD DISTRIBUTIONS

More insight in the induced antenna response and the drivingfield at the aperture can be gained by examining the field distri-butions for resonant antenna lengths. Since, at resonance, boththe antenna response and the driving field form standing waves(constant phase) and since the response of the antenna lags be-hind the driving field with roughly 90 , it is possible to “sep-arate” the two contributions near the antenna by looking at theinstantaneous fields. Fig. 5 shows the instantaneous electric fieldmagnitude for resonant antennas (lengths of 70 and 260 nm) fortwo different phases, 90 apart. In Fig. 5(a) and (c) the drivingfield, the standing wave formed by the almost total reflectionof the incident plane wave at the aperture, is at its maximum.The field that, as it were, penetrates the aperture, causes a localand radiating background to the antenna response. The actualantenna response, the standing wave along/in the antenna, isshown in Fig. 5(b) and 5(d) and lags roughly 90 behind thedriving field. Close to the antenna, the response is nearly sym-metric around the antenna axis, indicating no significant influ-ence from the aperture.

Further away from the antenna, this no longer holds and, forexample, the far-field radiation pattern is distorted by radiationfrom the aperture (not shown). The electric field outside theantenna is minimum at the antenna base and maximum at theapex. The magnetic field distributions reveal similar informa-tion (Fig. 6): the antenna response is close to symmetric and90 out of phase with the magnetic field from aperture back-ground field. The magnetic field corresponds to the total currentin the antenna. The response currents in the antenna are in turn90 out of phase with the electric field. Spatially, the current ismaximum at the antenna base, the typical feed point, and min-imum at the apex.

The finite conductivity of the optical antenna results in alarge penetration of the field into the antenna; aluminum at


Fig. 5. The instantaneous electric field magnitude near resonant monopole an-tennas (cross section). A plane wave (electric field amplitude 1 V/m) is inci-dent from the top with a polarization in the plane of cross section. The antennalength is equal to the first resonance, 70 nm, in (a) and (b), equal to the secondresonance, 260 nm, in (c) and (d). The driving field is visualized in (a) and (c),� = 0 . The antenna response in (b) and (d), � = 90 .

Fig. 6. The instantaneous magnetic field magnitude near resonant monopoleantennas for the same parameters as Fig. 5. The fields associated with antennaresponse are shown in (a) and (c), those with the driving field in (b) and (d). Themagnetic field in (a) and (c) curls around the antenna and is a measure of thetotal current in the antenna.

583 THz is clearly not a PEC and for these dimensions sur-face impedance boundary conditions are generally not a validapproximation [31]. The wavelength of the SPP modealong the antenna can easily be recognized from the standingwaves in both the current and field distributions. Indeed, asexpected for a monopole antenna, the first resonance occurs ata length of and the second at , where issignificantly shorter than the free space wavelength, resultingin shorter resonance lengths (see also Fig. 4).

In Figs. 5(b) and (d) a strong locally enhanced field is ob-served at the antenna apex. This field originates from the reso-

Fig. 7. Response for an incident wave polarized perpendicular to the planeof the cross section. The instantaneous local electric (a) and (b) and magnetic(c) and (d) field magnitudes for a resonant monopole antenna (first resonance,length 70 nm). The distributions associated with the driving field are shown in(a) and (d), those with the antenna response in (b) and (c). For this polarizationthe antenna is not driven and both the electric and magnetic field magnitude nearthe antenna show no response (b and c).

nant response of the antenna to the driving field. For linear wireantennas, like the monopole, a field component directed alongthe antenna axis is necessary to induce an antenna response. Theoptical monopole is driven by the near field of the aperture andsimilar polarization selectivity is expected. For the local fieldof an aperture a field component along the antenna axis, in thiscase the direction perpendicular to the plane of the aperture, ispresent at the edges of the aperture (see for example the openring patterns in Fig. 2). Moreover it can be shown that this com-ponent is only present on the sides of the aperture that are in thedirection of the polarization of the incident light (in the examplein Fig. 2 circularly polarized light, which contains all in-planepolarization directions, is used, resulting in full circles) [17]. InFig. 5 the polarization is in the plane of the cross section, the an-tenna is driven and a locally enhanced field forms at the apex. Incontrast, in Fig. 7 the polarization is perpendicular to the planeof the cross section; no field component along the antenna ispresent, the antenna is not driven and no field enhancement oc-curs at the antenna apex. The ratio of the field enhancement atthe antenna apex for the two polarization directions of Figs. 5and 7 is larger than 20. The antenna exhibits the expected se-lectivity for the direction of the field vector of the local drivingfield, in analogy to the polarization selectivity for collection offar-field radiation in the case of linear wire antennas.

VI. NEAR-FIELD MEASUREMENT RESULTS

Experimental measurements of the local field at the antennaapex are obtained as described in the discussion of Fig. 2.A sample containing single fluorescent molecules is scanned


Fig. 8. Near-field measurements of the local antenna field by scanning a sampleof single fluorescent molecules underneath the antenna. The polarization of thedriving light is in the direction of the antenna in (a) and perpendicular to that in(b) (see insets, small circle: antenna, large circle: aperture, arrow: polarization).Only for the polarization of (a) the antenna is driven and narrow spots originatingfrom the local antenna field are present (examples are encircled). The scan sizeof the images is 3� 3 �m. The insets (top right) are enlarged images of themarked spot and a cross section over a length of 300 nm is shown. The typicalfull width at half maximum of the intensity of the spots is around 25 nm.

underneath the monopole antenna. Thus, the molecules arescanned through a plane just under the antenna apex andparallel to the plane of the aperture. To investigate the feedingof the antenna, near-field measurements are made for two dif-ferent polarization directions of the driving field. Fig. 8 showsresulting fluorescence images for the antenna of Fig. 3. For thepolarization in the direction of the antenna [Fig. 8(a)] a largeamount of narrow fluorescence spots are present. The locationsof several examples are encircled in the image. Cross sectionsof these spots reveal a typical full width at half maximum(FWHM) of around 25 nm. Such narrow spots can not bethe result of the much larger aperture (100 nm in radius) andthus each narrow spot is a map of the locally enhanced fieldat the antenna apex (radius of curvature 20 nm) [Fig. 5(b)].Like in the discussion of Fig. 2, each map is the result ofscanning one single molecule underneath the antenna apex,the distribution of the maps simply reflects the location ofthe molecules. Together with the narrow maps of the antennafield, an inhomogeneous background signal is present. Thisbackground is caused by the aperture field components thatdo not couple to the antenna [Fig. 5(a) and Fig. 7(a)]. For theperpendicular incident polarization, the antenna is not expected

Fig. 9. Antenna resonance: the antenna response for antennas of differentlengths (data points). The solid line is obtained from the theoretical resonancecurve of Fig. 4. For the open circles the antenna field maps are not distinguish-able, instead a maximum intensity has been estimated from the noise level.

to be driven [Fig. 7(b)] and indeed no narrow spots are found[Fig. 8(b)]. Only the inhomogeneous background is present. Aspredicted by the calculations, a clear dependence of the antennaresponse on the polarization of the driving field is found. Thisconfirms that just like standard linear wire antennas the opticalmonopole is selectively driven by field orientations along theantenna axis. The difference is that here the local field vector atan aperture is used to drive the antenna instead of a plane wavewith polarization along the antenna axis.

The antenna resonances are studied by near-field measure-ments on antennas of different lengths. The acquisition of anintensity measurement of both the antenna field (narrow fluo-rescence maps, FWHM around 25 nm) and the aperture back-ground field (isolated large fluorescence spots, FWHM >80 nm)allows us to characterize the antenna response independent ofthe efficiency (throughput) of the aperture probe. The antennaresponse is defined as the average intensity of the narrow an-tenna maps divided by the average intensity of the accompa-nying large aperture spots. In Fig. 9 the antenna response againstthe antenna length is plotted. The solid line is the theoreticalfield intensity at the apex (Fig. 4) corrected for the field inten-sity of a simple aperture at the corresponding distance. The goodagreement between the calculated curve and the data confirmsthat the resonance found around 75 nm is indeed a typicalmonopole antenna resonance at 583 THz and thus that the rel-ative resonant length for the optical monopole antenna is short-ened in comparison to a thin radio monopole antenna.

VII. CONCLUSION

We have discussed the details of near-field measurementswith single fluorescent molecules as a method to directly mapthe local field of optical antennas in an analogous way to stan-dard near-field antenna measurements. The resonances and po-larization selectivity of optical monopoles have been character-ized by directly probing the local antenna field and by full 3-D


calculations of the magnetic and electric field distributions atthe antenna. Comparing the results to standard monopole an-tennas it is found that characteristic monopole antenna reso-nances occur, however the finite conductivity of the optical an-tenna results in shorter values for the resonance length. A cleardependence of the antenna response on the polarization of thedriving field is found. This is explained by the selective drivingof the antenna by the local field vectors at the aperture, muchlike the polarization selectivity of linear wire antennas.

The results presented here show that concepts of antennatheory can be transferred to the optical domain, when takinginto account differences such as the finite conductivity of metalsat optical frequencies. Although the optical monopole antennadiscussed here is a rather basic example, more complicatedoptical antennas tailored for specific purposes, such as a highdirectivity or certain frequency selectivity, are an interestingprospect. For this, there is a wealth of radio frequency antennaconfigurations available to take inspiration from. Possibly,applications for optical antennas might not be found so muchin communication, where antennas are traditionally used, butinstead in the study of light matter interaction at the nano scale,as for example in molecular biology and chemistry. Note thatthe results presented in Fig. 8(a) are a prime example of thepower of such antennas as they demonstrate optical microscopywith single molecule sensitivity at a resolution of .

ACKNOWLEDGMENT

The authors thank G. W. Hanson for inviting them for thisspecial issue. They also thank L. Kuipers and R. J. Moerlandfor many fruitful discussions. Finally they thank the members ofComputer Simulation Technology (CST), Darmstadt, Germany,for constructive feedback on the use of Microwave Studio.

REFERENCES

[1] S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, and T. W. Ebbesen,“Channel plasmon-polariton guiding by subwavelength metalgrooves,” Phys. Rev. Lett., vol. 95, p. 046802, 2005.

[2] D. W. Pohl, W. Denk, and M. Lanz, “Optical stethoscopy: Imagerecording with resolution �=20,” Appl. Phys. Lett., vol. 44, p. 651,1984.

[3] E. Betzig and T. J. Chichester, “Single molecules observed by near-field scanning optical microscopy,” Science, vol. 262, p. 1422, 1993.

[4] A. Sundaramurthy, P. J. Schuck, N. R. Conley, D. P. Fromm, G.S. Kino, and W. E. Moerner, “Toward nanometer-scale optical pho-tolithography: Utilizing the near-field of bowtie optical nanoantennas,”Nano Lett., vol. 6, p. 355, 2006.

[5] R. M. Stöckle, Y. D. Suh, V. Deckert, and R. Zenobi, “Nanoscale chem-ical analysis by tip-enhanced Raman spectroscopy,” Chem. Phys. Lett.,vol. 318, p. 131, 2000.

[6] J.-J. Greffet, “Nanoantennas for light emission,” Science, vol. 308, p.1561, 2005.

[7] J. N. Farahani, D. W. Pohl, H.-J. Eisler, and B. Hecht, “Single quantumdot coupled to a scanning optical antenna: A tunable super emitter,”Phys. Rev. Lett., vol. 95, p. 017402, 2005.

[8] S. Kühn, U. Håkanson, L. Rogobete, and V. Sandoghdar, “Enhance-ment of single-molecule fluorescence using a gold nanoparticle as anoptical nanoantenna,” Phys. Rev. Lett., vol. 97, p. 017402, 2006.

[9] P. Anger, P. Bharadwaj, and L. Novotny, “Enhancement and quenchingof single-molecule fluorescence,” Phys. Rev. Lett., vol. 96, p. 113002,2006.

[10] D. P. Fromm, A. Sundaramurthy, P. J. Schuck, G. Kino, and W. E.Moerner, “Gap-dependent optical coupling of single “bowtie” nanoan-tennas resonant in the visible,” Nano Lett., vol. 4, p. 957, 2004.

[11] W. Rechberger, A. Hohenau, A. Leitner, J. R. Krenn, B. Lamprecht, andF. R. Aussenegg, “Optical properties of two interacting gold nanopar-ticles,” Opt. Commun, vol. 220, p. 137, 2003.

[12] P. Mühlschlegel, H.-J. Eisler, O. J. F. Martin, B. Hecht, and D. W. Pohl,“Resonant optical antennas,” Science, vol. 308, p. 1607, 2005.

[13] A. Bouhelier, R. Bachelot, G. Lerondel, S. Kostcheev, P. Royer, andG. P. Wiederrecht, “Surface plasmon characteristics of tunable pho-toluminescence in single gold nanorods,” Phys. Rev. Lett., vol. 95, p.267405, 2005.

[14] P. J. Schuck, D. P. Fromm, A. Sundaramurthy, G. S. Kino, and W.E. Moerner, “Improving the mismatch between light and nanoscaleobjects with gold bowtie nanoantennas,” Phys. Rev. Lett., vol. 94, p.017402, 2005.

[15] A. D. Yaghjian, “An overview of near-field antenna measurements,”IEEE Trans. Antennas Propag., vol. AP-34, p. 30, 1986.

[16] A. Bouhelier, M. R. Beversluis, and L. Novotny, “Characterizationof nanoplasmonic structures by locally excited photoluminescence,”Appl. Phys. Lett., vol. 83, p. 5041, 2003.

[17] J. A. Veerman, M. F. Garcia-Parajo, L. Kuipers, and N. F. van Hulst,“Single molecule mapping of the optical field distribution of probes fornear-field microscopy,” J. Microsci., vol. 194, p. 477, 1999.

[18] T. H. Taminiau, R. J. Moerland, F. B. Segerink, L. Kuipers, and N. F.van Hulst, “�=4 Resonance of an optical monopole antenna probed bysingle molecule fluorescence,” Nano Lett., vol. 7, p. 28, 2007.

[19] T. Weiland, “A discretization method for the solution of Maxwell’sequations for six component fields,” Electron. Commun. AEU, vol. 31,p. 116, 1977.

[20] D. W. Pohl, “Near field optics seen as an antenna problem,” presentedat the Near-Field Optics: Principles and Applications, 2nd Asia-PacificWorkshop on Near-Field Optics, Beijing, China, Oct. 20–23, 1999.

[21] J. A. Veerman, A. M. Otter, L. Kuipers, and N. F. van Hulst, “Highdefinition aperture probes for near-field optical microscopy fabricatedby focused ion beam milling,” Appl. Phys. Lett., vol. 72, p. 3115, 1998.

[22] A. G. T. Ruiter, J. A. Veerman, K. O. van der Werf, and N. F. van Hulst,“Dynamic behavior of tuning fork shear-force feedback,” Appl. Phys.Lett., vol. 71, p. 28, 1997.

[23] [Online]. Available: www.cst.com CST Microwave Studio[24] G. Hass and J. E. Waylonis, “Optical constants of evaporated aluminum

in the visible and ultraviolet,” J. Opt. Soc. Am., vol. 50, p. 1133, 1960.[25] L. G. Schultz and F. R. Tangherlini, “The optical constants of silver,

gold, copper, and aluminum .2. The index of refraction-n,” J. Opt. Soc.Am., vol. 44, p. 362, 1954.

[26] L. G. Schultz, “The optical constants of silver, gold, copper, and alu-minum .1. The absorption coefficient-k,” J. Opt. Soc. Am, vol. 44, p.357, 1954.

[27] J. Aizpurua, G. W. Bryant, L. J. Richter, F. J. García de Abajo, B.K. Kelley, and T. Mallouk, “Optical properties of coupled metallicnanorods for field-enhanced spectroscopy,” Phys. Rev. B, vol. 71, p.235420, 2005.

[28] C. A. Balanis, Antenna Theory: Analyses and Design, 3 ed. NewYork: Wiley, 2005.

[29] L. Novotny and C. Hafner, “Light propagation in a cylindrical wave-guide with a complex, metallic, dielectric function,” Phys. Rev. E, vol.50, p. 4094, 1994.

[30] G. Schider, J. R. Krenn, A. Hohenau, H. Ditlbacher, A. Leitner, F. R.Aussenegg, W. L. Schaich, I. Puscasu, B. Monacelli, and G. Boreman,“Plasmon dispersion relation of Au and Ag nanowires,” Phys. Rev. B,vol. 68, p. 155427, 2003.

[31] G. W. Hanson, “On the applicability of the surface impedance integralequation for optical and near infrared copper dipole antennas,” IEEETrans. Antennas Propag., vol. 54, pp. 3677–3685, 2006.

Tim H. Taminiau was born in Eindhoven, TheNetherlands, in 1981. He received the M.Sc. degreein applied physics from the University of Twente,Enschede, The Netherlands, in 2005. Currently, heis working toward the Ph.D. degree at the Institut deCiències Fotòniques (ICFO), Barcelona, Spain.

His research interests include understanding andcontrolling the interaction between light and matter atthe nanometer scale, local fields at nanometer-sizedmetallic structures, and coupling of emitters to op-tical antennas.


Frans B. Segerink was born in Oldenzaal, TheNetherlands, on August 2, 1958. He received theB.Sc. degree in electrotechnical engineering fromHogere Technische School, Enschede, The Nether-lands, in 1981.

Until 1987, he was working in the Optics andGlassfibers Group at the Dr. Neher Laboratory,Leidschendam, The Netherlands, where his main oc-cupations were fiber characterization and automationof measurement setup’s. Since 1987, he has been anemployee of the Applied Optics Group, Department

of Science and Technology, University of Twente, The Netherlands. His currentresearch projects include focused ion beam and e-beam operations, (opto-)electronics and data-acquisition software development, and characterization oflab-conditions, e.g., vibration levels.

Niek F. van Hulst received the Ph.D. degree inmolecular and laser-physics from the University ofNijmegen, The Netherlands, in 1986. His dissertationwas on a crossed molecular beam study of collisioninduced inelastic state-to-state rotational transitionsin molecules of interstellar relevance (NH3, H2CO).

After research in nonlinear optics, organic mate-rials, integrated optics and waveguided frequencydoubling of laser diodes, he became an AssistantProfessor at the University of Twente, The Nether-lands, in 1990, and started research in near-field

optical and atomic force microscopy, nonlinear optics and hyper Raleighscattering. In 1997, he obtained full Professor Chair in Applied Optics, at theScience and Technology Department, MESA+ Institute for NanoTechnology,University of Twente, with focus on single molecule detection, nanophotonics,photonic structures, scanning probe technology and applications in molecularbiology and chemistry. In September 2005, he left Twente to take up an ICREAResearch Professor position and to join the Institute of Photonic Sciences(ICFO) in Barcelona, Spain, as a Senior Group Leader in NanoPhotonics.The ICFO group of Niek van Hulst is active in the fields of nano-scale opticsand single molecule detection. Engineering of the nanoscale optical fieldis pursued both by top-down nanofabrication and by bottom-up molecularengineering. Single molecules are used in a double role as exquisite sensors ofthe local field and building blocks for molecular photonic structures. Researchareas include single molecule detection, near field optics, local field effects,photonic structures, fs-spectroscopy, field shaping, molecular switching andmolecular motors and photophysics/chemistry of macro- and bio-molecules.His specific current projects are: single molecule ultrafast photonics, probinglight propagation at nanostructures, field enhancement and mode density atnanostructures, and single molecular photonic wires, nano-antennas.

Dr. van Hulst received the Shell Stimulation Award in 1997 and the EuropeanScience Award of the Körber Foundation (Hamburg) in 2003 for his work onsingle molecule detection.

Documents

A Monopole Antenna at Optical Frequencies: Single-Molecule Near