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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 frequencies ( 600 THz) and discuss near-field measurements with single
fluorescent molecules as a technique to characterize such antennas.
The similarities and differences between near-field antenna measurements at optical and radio frequencies are discussed in detail.
Two typical antenna properties, polarization selectivity and resonances, are studied for the optical monopole by direct near-field
measurements and finite integration technique calculations. The
antenna is driven by the local field of a sub-wavelength aperture.
This gives rise to a dependence of the antenna response on the orientation of the local field vector, in an analogous way to the polarization selectivity of linear wire antennas. The antenna resonances
are studied by varying the antenna length. Typical monopole resonances are demonstrated. The finite conductivity of metals at optical frequencies (in combination with the antenna radius) causes
the wavelength of the surface charge density oscillation (surface
plasmon polariton) along the antenna to be shortened in comparison to the free space wavelength. As a result, resonances for the
optical monopole antenna occur at much shorter relative lengths
than for conventional radio monopole antennas.
Index Terms—Monopole antennas, near fields, optical antennas,
optical resonance, plasmons, surface plasmon.
I. INTRODUCTION
T
HE traditional function of antennas is two-fold: the efficient
and selective coupling of far-field radiation in sub-wavelength waveguides and, vice versa, the efficient and selective
coupling of energy from waveguides into directed far-field radiation. A direct optical analogue is of interest for novel integrated
optical circuits, where optical antennas could be used to couple
light from and into plasmonic waveguides [1]. Possibly even more
important, the general ability of antennas to efficiently collect
energy in volumes much smaller than allowed by the diffraction
limit enables the application of traditional optical techniques,
such as microscopy [2], [3], lithography [4] and spectroscopy [5],
on the nano scale. In a reciprocal way, the emission characteristics
of sub-diffraction-limit volumes of matter or single quantum
emitters can be controlled by optical antennas [6]–[9].
Manuscript received March 12, 2007; revised April 24, 2007. This work
was supported by the Specific Target Research Project (STRP) ASPRINT
(NMP-CT-2003-001601) in the 6th Framework Program of the European
Community.
T. H. Taminiau is with the Institut de Ciencies Fotoniques (ICFO), Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain (e-mail: tim.
taminiau@icfo.es).
F. B. Segerink is with the Optical Sciences, MESA+ Institute for NanoTechnology, University of Twente, 7500AE Enschede, The Netherlands.
N. F. van Hulst is with the Institut de Ciencies Fotoniques (ICFO), Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain, and also with
the Instituciò Catalana de Recerca i Estudis Avaņats (ICREA), 08015 Barcelona,
Spain.
Digital Object Identifier 10.1109/TAP.2007.908561
Because of the above prospects, the optical properties of
metal nanostructures, which could act as antennas, have been
extensively studied. The plasmon resonances (frequency response), coupling effects and polarization selectivity of such
nanostructures have been characterized by far-field techniques
that include scattering spectroscopy [10], [11] and multiphoton
luminescence [12]–[14]. However, unlike in the case of radio antennas, most applications of optical antennas rely on the specific
distribution of the local field at the antenna or on the coupling of
a local emitter to the antenna. Far-field techniques do not yield
any direct information on the actual characteristics of the antenna
near field. To gain insight in the local antenna field, true near-field
measurements for optical antennas are needed. Whereas for
traditional radio antennas near-field measurements are often an
easier, better controlled or cheaper way to obtain the far-field
behavior of the antenna [15], such measurements are a serious
challenge at optical frequencies. Techniques that have been successfully employed to map local fields at nano structures include
scanning probe microscopy [16], local surface modification [4]
and single molecule fluorescence detection [3], [17].
Recently, we have introduced a probe-based optical antenna
and showed that the antenna is equivalent to the standard radio
monopole antenna by directly probing the antenna near-field
with single fluorescent molecules [18]. In this paper we focus
on the analogies and differences of antennas and near-field
measurements at optical and radio frequencies. First we discuss optical near-field measurements with single fluorescent
molecules. The conceptual and experimental similarities and
differences between such measurements and near-field measurements at radio frequencies are summarized. Second, we
focus on the optical monopole antenna itself. Insight in the
antenna characteristics is gained by finite integration technique
(FIT) [19] calculations of the local electromagnetic field distributions for both the driving field and the antenna response. We
investigate the feeding of the antenna through an optical fiber
probe by near-field measurements and calculations. The results
are related to standard transmission lines and to the polarization
selectivity of linear wire antennas. The resonance behavior of
the optical monopole is characterized by varying the antenna
length and is compared to the typical monopole resonances.
It is shown that the relatively low conductivity of aluminum
at optical frequencies causes the relative lengths at which the
antenna is resonant to be shortened in comparison with thin
perfectly 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
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Fig. 1. Antennas and near-field measurements at optical frequencies compared
to at the MHz-to-GHz range. Both schemes for near-field measurements consist
of a source, a transmission line, an antenna, a near-field probe and a detection
system.
that is connected to the antenna by a transmission line. In nearfield measurements, the local antenna field is mapped by moving
a probe antenna through the field, usually at a distance of several wavelengths from the antenna (radiating near field), and
recording 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 optical
wave is transported to the antenna by a glass fiber. Single
fluorescent 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 electronic
quantum transitions of the molecule. The energy absorbed
by the molecule depends on the overlap of its absorption
and the local electric field vector
;
dipole moment
the molecular orientation determines the field component that
the molecule is sensitive for. Part of the absorbed energy is
re-emitted as radiation with a lower energy (lower frequency).
This spontaneous decay process is called fluorescence. For
sufficiently low powers, the emitted fluorescence intensity (I)
is proportional to the excitation intensity
(1)
Raster scanning the molecule close to the antenna and detecting
the fluorescence yields a map of the local antenna field. Note
that, in contrast to the radio antenna case, we are particularly
interested in mapping the field in the very vicinity of the antenna, from right at the antenna surface to roughly a fifth of the
wavelength (reactive near field). It is possible to map fields at
such short distances from the antenna because the fraction of the
antenna field absorbed by the single molecule is negligible compared to the total field; the disturbance of the molecular dipole
on the antenna response can thus be neglected.
To scan single fluorescent molecules close to the optical
antenna we will use a near-field scanning optical microscope
(NSOM), a well developed scanning-probe technique that
uses the locally confined field at a probe for high resolution
optical microscopy [2], [3], [17]. Before discussing the optical
Fig. 2. Overview of a near-field scanning optical microscope (NSOM). A
sample containing isolated single fluorescent molecules is scanned underneath
a near-field probe. The insets show an example: a standard fiber probe with
aperture 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 maps
of the different field components of the near-field at the aperture of the fiber
probe.
2
monopole antenna, the principle of NSOM is introduced and
illustrated by an example.
A schematic overview of the NSOM set-up used is shown
in Fig. 2. The structure in the inset is a conventional near-field
aperture probe. Note that the naming is unfortunate in the current context; the aperture probe should not be confused with the
single molecules that are used to probe the local field at structures. Such an aperture probe is a sharply pointed glass fiber
that is partly metal coated so that the end consists of a small
sub-wavelength aperture. To create such probes, sharp pointed
glass tips are created by controllably heating and subsequent
pulling of single mode optical glass fibers (633 nm wavelength)
with a commercial fiber puller (Sutter P2000). The tips are totally coated, first with a few nanometer thick chromium layer
nm aluminum. In a final step the coating at
and then with
the very end is removed with a focused ion beam (FIB) so that
a 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 as
an excitation source. The light is coupled into the fiber probe
after being passed through polarization controlling optics.
A sample containing single fluorescent molecules (DiI) emnm thick polymer layer (PMMA) is scanned
bedded in a
underneath the fiber probe by a piezo scanning stage with a
position feedback system with nanometer position accuracy.
The molecules are distributed and orientated randomly in the
sample. The molecular concentration is chosen such that most
of the molecules are spatially isolated. The distance between
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 11, NOVEMBER 2007
the fiber probe and the sample is regulated by a shear force
based feedback system. The fiber probe is attached to an oscillating quartz tuning fork and the change of the phase difference
between the driving signal and the tuning fork response as the
fiber probe approaches the sample is used to keep a constant
probe-sample distance of around 5 nm [22]. The fluorescence
of the molecules is collected with a 1.3 Numerical Aperture oil
immersion objective and, after the laser excitation light is filtered out, detected by a photon-counting avalanche photodiode.
As an example, a fluorescence image, obtained for the aperture
probe shown, is presented in Fig. 2. The image consists of multiple fluorescence spots. Each such a spot is the recorded fluorescence of one single molecule as it is raster scanned underneath
the aperture probe. The total sample area scanned contains several molecules resulting in a collection of spots. The distribution
reflects the (random) placement of the molecules. The variance
in shapes of the spots is caused by the dipole orientation of each
molecule, which determines the field orientation that is probed.
Each of the spots in the image is thus a map of a field component
of the local electric field just under the aperture probe. The solid
circles are maps of the field components that are orientated in the
plane of the image (and aperture), while the open circles are maps
of the field component perpendicular to the plane. As the field
) from
is mapped at a very short distance (around 10 nm or
the aperture probe, the full width at half maximum of the maps
directly reflects the lateral confinement of the local field at the
aperture probe. For a smaller aperture diameter, the field is more
confined and the fluorescence maps are narrower. If, like in this
case, the local field distribution is well known, the system can be
used as a microscope. The lateral confinement then determines
the obtainable resolution, typically 50–100 nm, i.e., better than
(250–300 nm) obtainable with lens based far-field
the
microscopy. This resolution should not be confused with the
much better resolution with which the molecules map the local
field at the aperture probe (or, alternatively antenna).
Fig. 3. An optical monopole antenna based on the standard aperture fiber
probe: SEM images from (a) side-view and (b) under 52 angle. The aluminum
antenna shown here has a length of 70 6 nm and a diameter of 45 6 nm.
6
6
III. OPTICAL MONOPOLE ANTENNA
The optical monopole antenna is literally based on the standard aperture probe. Instead of creating a flat end face, an elongated aluminum antenna is left standing next to the aperture by
FIB milling under two different angles. Fig. 3 shows SEM images of such an optical monopole antenna. The typical antenna
width is around 40 nm and the radius of curvature of the apex
around 20 nm. The antenna length is varied controllably from
30 to 140 nm, in order to tune the antenna resonance. In this
configuration the local field at the aperture drives the antenna.
As such the aperture probe replaces the transmission line of the
radio wave equivalent. However, there are important differences
between the two. First, the small diameter of the aperture and
the part of the fiber just above it do not allow optical modes to
propagate. Thus, in contrast to radio transmission lines, the optical waveguide is operated below cutoff and the throughput is
very low. Second, it is expected that not all the components of
the local aperture field couple efficiently to the antenna. Electric field components that are not directed along the antenna axis
or that are not in the immediate vicinity of the antenna remain
virtually unmodified and will result in a background field additional to the antenna response.
IV. ANTENNA RESONANCES
Insight into the antenna properties can be obtained by
studying the field distributions in the vicinity of the antenna.
Three-dimensional electric and magnetic field distributions are
obtained from FIT calculations with commercial software (CST
Microwave Studio) [23]. For simplicity we model the antenna
as an aluminum cylinder with a spherical apex (radius 20 nm)
placed next to an aperture (diameter 100 nm) in a 10 nm thick
infinite perfectly electrical conducting (PEC) screen (cross
section: inset Fig. 4). The plate is illuminated from the top by
nm or
THz) of variable linear
a plane wave (
polarization. It has been shown that such a model, without an
antenna, gives accurate results for the local field of an aperture
probe [17]. At 514 nm the dielectric constant of aluminum is
, as determined by fitting a Drude model to
experimental values [24]–[26].
First the antenna resonances are characterized. In the case
of radio antennas it is common to characterize the antenna response in terms of the operating frequency. It is also common
to take the magnitude of the induced current at the feed point
as a measure of antenna response. In the case of the optical
TAMINIAU et al.: A MONOPOLE ANTENNA AT OPTICAL FREQUENCIES
Fig. 4. The electric field intensity at the antenna apex and the squared magnetic
field at the base of the antenna (far side from the aperture) against the antenna
length. Both field intensities show the same distinct resonant behavior. The inset
shows 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. Instead, as explained above, the near field at the antenna apex is
probed. Moreover, the antenna response for varying frequency
(or wavelength) is complicated because of several effects. First,
varying the frequency changes the aperture diameter to wavelength ratio, which drastically changes the throughput. Furthermore, for the actual tapered optical fiber probe the frequency
dependence of the throughput is more complicated and hard to
determine. Second, the antenna sharpness relative to the wavelength varies with frequency. Both these relative changes have
a drastic effect on the field strength at the antenna apex and thus
add a frequency response of their own to the system. This results in a frequency resonance curve whose maximum does in
general not coincide with the resonance of the actual antenna
element. More insight is gained by varying the antenna length
for a constant frequency. In that case the two effects mentioned
above are not important as the dimensions relative to the wavelength are constant and thus information on the resonance of the
actual antenna element is obtained.
Fig. 4 shows both the electric field intensity at the antenna
apex and the square of the magnetic field at the antenna base (on
the side furthest from the aperture) for variable antenna length,
at a wavelength of 514 nm (583 THz). The electric field intensity near the apex is the quantity probed in the near-field experiments. The magnetic field is a measure for the total current, conduction and displacement. Both curves show distinct resonances
at similar lengths; equivalent information on the resonances is
obtained by measuring the current at the base and the field intensity at the apex. The first resonance occurs at a length of 70 nm,
the second at around 260 nm. Given the operating wavelength of
514 nm, these lengths are significantly shorter than the lengths
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found for a thin radio frequency monopole antenna, for which
(even mulresonances occur close to uneven multiples of
tiples are anti-resonances, modes without net dipole moment
[27]). To explain this, two aspects of the optical antenna have to
be taken into account. First, the length-to-radius ratio for the optical monopole antenna is much lower (3.5 at the first resonance)
than usual for MHz monopole antennas, where the ratio typically 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 optical
frequencies reduces the resonant length. For a frequency of 583
THz and an antenna radius of 20 nm, the surface charge density
wave along the antenna, the quanta of which are called surface
plasmon polaritons (SPP), has a wavelength that is significantly
shorter than the free space wavelength of 514 nm. The antenna
length for which the SPP mode is at resonance is reduced by the
same factor (see for example dispersion curves for aluminum
cylinders [29] and noble metal rods [30]). Thus, resonant optical monopole antennas are significantly shorter (relative to the
free-space wavelength) than otherwise equivalent PEC radio antennas. Indeed, it can be shown that for a PEC antenna, with a
high length-to-radius ratio in the same configuration, characteristic monopole antenna resonances are recovered [18].
V. ELECTROMAGNETIC FIELD DISTRIBUTIONS
More insight in the induced antenna response and the driving
field at the aperture can be gained by examining the field distributions for resonant antenna lengths. Since, at resonance, both
the antenna response and the driving field form standing waves
(constant phase) and since the response of the antenna lags behind the driving field with roughly 90 , it is possible to “separate” the two contributions near the antenna by looking at the
instantaneous fields. Fig. 5 shows the instantaneous electric field
magnitude for resonant antennas (lengths of 70 and 260 nm) for
two different phases, 90 apart. In Fig. 5(a) and (c) the driving
field, the standing wave formed by the almost total reflection
of the incident plane wave at the aperture, is at its maximum.
The field that, as it were, penetrates the aperture, causes a local
and radiating background to the antenna response. The actual
antenna response, the standing wave along/in the antenna, is
shown in Fig. 5(b) and 5(d) and lags roughly 90 behind the
driving field. Close to the antenna, the response is nearly symmetric around the antenna axis, indicating no significant influence from the aperture.
Further away from the antenna, this no longer holds and, for
example, the far-field radiation pattern is distorted by radiation
from the aperture (not shown). The electric field outside the
antenna is minimum at the antenna base and maximum at the
apex. The magnetic field distributions reveal similar information (Fig. 6): the antenna response is close to symmetric and
90 out of phase with the magnetic field from aperture background field. The magnetic field corresponds to the total current
in the antenna. The response currents in the antenna are in turn
90 out of phase with the electric field. Spatially, the current is
maximum at the antenna base, the typical feed point, and minimum at the apex.
The finite conductivity of the optical antenna results in a
large penetration of the field into the antenna; aluminum at
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 11, NOVEMBER 2007
Fig. 5. The instantaneous electric field magnitude near resonant monopole antennas (cross section). A plane wave (electric field amplitude 1 V/m) is incident from the top with a polarization in the plane of cross section. The antenna
length is equal to the first resonance, 70 nm, in (a) and (b), equal to the second
resonance, 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 monopole
antennas for the same parameters as Fig. 5. The fields associated with antenna
response are shown in (a) and (c), those with the driving field in (b) and (d). The
magnetic field in (a) and (c) curls around the antenna and is a measure of the
total current in the antenna.
583 THz is clearly not a PEC and for these dimensions surface impedance boundary conditions are generally not a valid
approximation [31]. The wavelength of the SPP mode
along the antenna can easily be recognized from the standing
waves in both the current and field distributions. Indeed, as
expected for a monopole antenna, the first resonance occurs at
and the second at
, where
is
a length of
significantly shorter than the free space wavelength, resulting
in shorter resonance lengths (see also Fig. 4).
In Figs. 5(b) and (d) a strong locally enhanced field is observed at the antenna apex. This field originates from the reso-
Fig. 7. Response for an incident wave polarized perpendicular to the plane
of 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 polarization
the antenna is not driven and both the electric and magnetic field magnitude near
the antenna show no response (b and c).
nant response of the antenna to the driving field. For linear wire
antennas, like the monopole, a field component directed along
the antenna axis is necessary to induce an antenna response. The
optical monopole is driven by the near field of the aperture and
similar polarization selectivity is expected. For the local field
of an aperture a field component along the antenna axis, in this
case the direction perpendicular to the plane of the aperture, is
present at the edges of the aperture (see for example the open
ring patterns in Fig. 2). Moreover it can be shown that this component is only present on the sides of the aperture that are in the
direction of the polarization of the incident light (in the example
in Fig. 2 circularly polarized light, which contains all in-plane
polarization directions, is used, resulting in full circles) [17]. In
Fig. 5 the polarization is in the plane of the cross section, the antenna is driven and a locally enhanced field forms at the apex. In
contrast, in Fig. 7 the polarization is perpendicular to the plane
of the cross section; no field component along the antenna is
present, the antenna is not driven and no field enhancement occurs at the antenna apex. The ratio of the field enhancement at
the antenna apex for the two polarization directions of Figs. 5
and 7 is larger than 20. The antenna exhibits the expected selectivity for the direction of the field vector of the local driving
field, in analogy to the polarization selectivity for collection of
far-field radiation in the case of linear wire antennas.
VI. NEAR-FIELD MEASUREMENT RESULTS
Experimental measurements of the local field at the antenna
apex are obtained as described in the discussion of Fig. 2.
A sample containing single fluorescent molecules is scanned
TAMINIAU et al.: A MONOPOLE ANTENNA AT OPTICAL FREQUENCIES
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Fig. 9. Antenna resonance: the antenna response for antennas of different
lengths (data points). The solid line is obtained from the theoretical resonance
curve of Fig. 4. For the open circles the antenna field maps are not distinguishable, instead a maximum intensity has been estimated from the noise level.
Fig. 8. Near-field measurements of the local antenna field by scanning a sample
of single fluorescent molecules underneath the antenna. The polarization of the
driving 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 originating
from the local antenna field are present (examples are encircled). The scan size
of the images is 3 3 m. The insets (top right) are enlarged images of the
marked spot and a cross section over a length of 300 nm is shown. The typical
full width at half maximum of the intensity of the spots is around 25 nm.
2
underneath the monopole antenna. Thus, the molecules are
scanned through a plane just under the antenna apex and
parallel to the plane of the aperture. To investigate the feeding
of the antenna, near-field measurements are made for two different polarization directions of the driving field. Fig. 8 shows
resulting fluorescence images for the antenna of Fig. 3. For the
polarization in the direction of the antenna [Fig. 8(a)] a large
amount of narrow fluorescence spots are present. The locations
of several examples are encircled in the image. Cross sections
of these spots reveal a typical full width at half maximum
(FWHM) of around 25 nm. Such narrow spots can not be
the result of the much larger aperture (100 nm in radius) and
thus each narrow spot is a map of the locally enhanced field
at the antenna apex (radius of curvature 20 nm) [Fig. 5(b)].
Like in the discussion of Fig. 2, each map is the result of
scanning one single molecule underneath the antenna apex,
the distribution of the maps simply reflects the location of
the molecules. Together with the narrow maps of the antenna
field, an inhomogeneous background signal is present. This
background is caused by the aperture field components that
do not couple to the antenna [Fig. 5(a) and Fig. 7(a)]. For the
perpendicular incident polarization, the antenna is not expected
to be driven [Fig. 7(b)] and indeed no narrow spots are found
[Fig. 8(b)]. Only the inhomogeneous background is present. As
predicted by the calculations, a clear dependence of the antenna
response on the polarization of the driving field is found. This
confirms that just like standard linear wire antennas the optical
monopole is selectively driven by field orientations along the
antenna axis. The difference is that here the local field vector at
an aperture is used to drive the antenna instead of a plane wave
with polarization along the antenna axis.
The antenna resonances are studied by near-field measurements on antennas of different lengths. The acquisition of an
intensity measurement of both the antenna field (narrow fluorescence maps, FWHM around 25 nm) and the aperture background field (isolated large fluorescence spots, FWHM >80 nm)
allows us to characterize the antenna response independent of
the efficiency (throughput) of the aperture probe. The antenna
response is defined as the average intensity of the narrow antenna maps divided by the average intensity of the accompanying large aperture spots. In Fig. 9 the antenna response against
the antenna length is plotted. The solid line is the theoretical
field intensity at the apex (Fig. 4) corrected for the field intensity of a simple aperture at the corresponding distance. The good
agreement between the calculated curve and the data confirms
that the resonance found around 75 nm is indeed a typical
monopole antenna resonance at 583 THz and thus that the relative resonant length for the optical monopole antenna is shortened in comparison to a thin radio monopole antenna.
VII. CONCLUSION
We have discussed the details of near-field measurements
with single fluorescent molecules as a method to directly map
the local field of optical antennas in an analogous way to standard near-field antenna measurements. The resonances and polarization selectivity of optical monopoles have been characterized by directly probing the local antenna field and by full 3-D
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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 55, NO. 11, NOVEMBER 2007
calculations of the magnetic and electric field distributions at
the antenna. Comparing the results to standard monopole antennas it is found that characteristic monopole antenna resonances occur, however the finite conductivity of the optical antenna results in shorter values for the resonance length. A clear
dependence of the antenna response on the polarization of the
driving field is found. This is explained by the selective driving
of the antenna by the local field vectors at the aperture, much
like the polarization selectivity of linear wire antennas.
The results presented here show that concepts of antenna
theory can be transferred to the optical domain, when taking
into account differences such as the finite conductivity of metals
at optical frequencies. Although the optical monopole antenna
discussed here is a rather basic example, more complicated
optical antennas tailored for specific purposes, such as a high
directivity or certain frequency selectivity, are an interesting
prospect. For this, there is a wealth of radio frequency antenna
configurations available to take inspiration from. Possibly,
applications for optical antennas might not be found so much
in communication, where antennas are traditionally used, but
instead in the study of light matter interaction at the nano scale,
as for example in molecular biology and chemistry. Note that
the results presented in Fig. 8(a) are a prime example of the
power of such antennas as they demonstrate optical microscopy
.
with single molecule sensitivity at a resolution of
ACKNOWLEDGMENT
The authors thank G. W. Hanson for inviting them for this
special issue. They also thank L. Kuipers and R. J. Moerland
for many fruitful discussions. Finally they thank the members of
Computer Simulation Technology (CST), Darmstadt, Germany,
for constructive feedback on the use of Microwave Studio.
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Tim H. Taminiau was born in Eindhoven, The
Netherlands, in 1981. He received the M.Sc. degree
in applied physics from the University of Twente,
Enschede, The Netherlands, in 2005. Currently, he
is working toward the Ph.D. degree at the Institut de
Ciències Fotòniques (ICFO), Barcelona, Spain.
His research interests include understanding and
controlling the interaction between light and matter at
the nanometer scale, local fields at nanometer-sized
metallic structures, and coupling of emitters to optical antennas.
TAMINIAU et al.: A MONOPOLE ANTENNA AT OPTICAL FREQUENCIES
Frans B. Segerink was born in Oldenzaal, The
Netherlands, on August 2, 1958. He received the
B.Sc. degree in electrotechnical engineering from
Hogere Technische School, Enschede, The Netherlands, in 1981.
Until 1987, he was working in the Optics and
Glassfibers Group at the Dr. Neher Laboratory,
Leidschendam, The Netherlands, where his main occupations were fiber characterization and automation
of measurement setup’s. Since 1987, he has been an
employee of the Applied Optics Group, Department
of Science and Technology, University of Twente, The Netherlands. His current
research projects include focused ion beam and e-beam operations, (opto-)
electronics and data-acquisition software development, and characterization of
lab-conditions, e.g., vibration levels.
Niek F. van Hulst received the Ph.D. degree in
molecular and laser-physics from the University of
Nijmegen, The Netherlands, in 1986. His dissertation
was on a crossed molecular beam study of collision
induced inelastic state-to-state rotational transitions
in molecules of interstellar relevance (NH3, H2CO).
After research in nonlinear optics, organic materials, integrated optics and waveguided frequency
doubling of laser diodes, he became an Assistant
Professor at the University of Twente, The Netherlands, in 1990, and started research in near-field
3017
optical and atomic force microscopy, nonlinear optics and hyper Raleigh
scattering. In 1997, he obtained full Professor Chair in Applied Optics, at the
Science 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 molecular
biology and chemistry. In September 2005, he left Twente to take up an ICREA
Research 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 optics
and single molecule detection. Engineering of the nanoscale optical field
is pursued both by top-down nanofabrication and by bottom-up molecular
engineering. Single molecules are used in a double role as exquisite sensors of
the local field and building blocks for molecular photonic structures. Research
areas include single molecule detection, near field optics, local field effects,
photonic structures, fs-spectroscopy, field shaping, molecular switching and
molecular motors and photophysics/chemistry of macro- and bio-molecules.
His specific current projects are: single molecule ultrafast photonics, probing
light propagation at nanostructures, field enhancement and mode density at
nanostructures, and single molecular photonic wires, nano-antennas.
Dr. van Hulst received the Shell Stimulation Award in 1997 and the European
Science Award of the Körber Foundation (Hamburg) in 2003 for his work on
single molecule detection.