Published:December 22, 2010
r2010 American Chemical Society
dx.doi.org/10.1021/nl102657m|Nano Lett. 2011, 11, 355–360
Extraordinary Optical Transmission Brightens Near-Field Fiber Probe
Lars Neumann,†Yuanjie Pang,§Amel Houyou,†Mathieu L. Juan,†Reuven Gordon,§and
Niek F. van Hulst*,†,‡
†ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain
‡ICREA-Instituci? o Catalana de Recerca i Estudis Avanc -ats, 08015 Barcelona, Spain
§Department of Electrical and Computer Engineering, University of Victoria, Victoria, British Columbia V8W 3P6, Canada
ABSTRACT: Near-field scanning optical microscopy (NSOM)
offers high optical resolution beyond the diffraction limit for
various applications in imaging, sensing, and lithography; how-
ever, for many applications the very low brightness of NSOM
aperture probes is a major constraint. Here, we report a novel
NSOM aperture probe that gives a 100? higher throughput and
40? increased damage threshold than conventional near-field
apertureprobes. Thesebrighter probes facilitatenear-fieldimaging
of single molecules with apertures as small as 45 nm in diameter.
We achieve this improvement by nanostructuring the probe and by employing a novel variant of extraordinary optical transmission,
measured transmission spectra. Due to their significantly increased throughput and damage threshold, these resonant configuration
probes provide an important step forward for near-field applications.
KEYWORDS: Extraordinary optical transmission, near-field scanning optical microscopy, nanophotonics, nanoplasmonics, nanoima-
ging, waveguide resonance, focused ion beam
achieve an optical resolution well beyond the diffraction limit.
imaging,1,2material science,3-6and nanolithography.7-9Tech-
nically, a NSOM probe is usually realized by a single, subwave-
length aperture that is formed by tapering an optical fiber and
Bethe's theory for optical transmission through a subwavelength
the transmission with the aperture diameter, necessitating a
trade-off between resolution and brightness.10Apart from the
transmission through the aperture, the absolute throughput is
limited by the low damage threshold of conventional near-field
probes. Here, the light delivery through the taper to the aperture
depends greatly on the actual taper shape, its length, and the
quality of the metal layer (aluminum) preventing light leakage.
These limitations result in an optical throughput of near-field
probes of typically only 10-5to 10-7.11-13
through subwavelength apertures. The transmission through a
single aperture was enhanced by reshaping the aperture.14-16
Aperture arrays have shown extraordinary optical transmission
(EOT),17and related to EOT is the beaming of light from single
apertures flanked with a periodic structure.18Reshaping and
ear-field scanning optical microscopy (NSOM) combines
optical microscopy with scanning probe microscopy to
films, which are also required for the aperture arrays in standard
EOT. Essential for NSOM, however, is a tiny end face that,
is directly responsible for resolution and sensitivity. Thus, the
aforementioned approaches do not provide a practical solution.
particle to concentrate the electric field and thus provides this tiny
field, modulation schemes are necessary to overcome the strong
background illumination.20Few approaches have been made to
enhance transmission and brightness while maintaining resolution
incorporation of antenna structures with the aperture21,22or a
photonic crystal23and the excitation of plasmons along and inside
In this report we demonstrate a NSOM probe that offers both
aperture and thus allows sufficient brightness to image single
molecules with an aperture of 45 nm diameter. We have imple-
array of apertures or a grating structure;26-28instead, it uses
resonant coupling to the adjacent waveguide. So far, this wave-
guide resonance EOT (WR-EOT) has only been demonstrated
July 29, 2010
October 30, 2010
dx.doi.org/10.1021/nl102657m |Nano Lett. 2011, 11, 355–360
for coupling between two waveguides and the microwave
regime.29Here, we demonstrate a nanoscale version WR-EOT
providing enhanced transmission through a NSOM probe at
visible to near-IR wavelengths. By varying the geometry of the
sible for the WR-EOT. We also show that the absolute through-
put is enhanced by this nanostructuring of the tapered fiber.
While these probes have a slightly wider base than conventional
metal-coated tapered fiber probes, they show 40? improvement
in the optical damage threshold and a 100? enhancement in
Figure 1a shows a schematic and Figure 1b a side view
scanning electron microscopy (SEM) image of the metal-coated
tapered fiber with an aperture in the end face. The taper was
fabricated by heat-pulling an optical fiber.11A 220 nm thick
aluminum coating was deposited around the fiber to prevent light
leakage, which would otherwise add a strong background to the
aperture signal. Aluminum was chosen here as it offers the smallest
optical penetration depth and thus prevents leakage effectively.
milling. In contrast to conventional NSOM fibers, the tapered fiber
was not cut such that it directly forms a subwavelength aperture.
Instead it was cut at a much larger diameter of the taper dt
Panels a-c of Figure 2 show the stages of the fabrication
process.The fiberinFigure 2a was cut at afinal taper diameter dt
as shown in Figure 2b. The choice of gold as the material for the
end face was motivated by its favorable optical properties in the
red and infrared where our optical measurements were carried
out. Inafinalstepasingle subwavelength aperture with diameter
dawas milled into the gold layer, nominally at the center (with a
variance of 30 nm in placement). Figure 2c shows a SEM image
end face. Calibration of the milling process ensured that the
aperture was milled through the gold layer only and not into the
glass fiber core, avoiding spurious effects on the resonance.31
Apertures with diameters as small as 45 nm were fabricated,
which is well within the cutoff regime where Bethe's theory is
typically applied. Several different diameter fiber probes were
from 400 to 730 nm (Figure 2d-f).
The relative transmission of the fabricated enhanced NSOM
probes and, for comparison, also of conventional NSOM probes
put was determined by coupling light with known intensity at a
wavelength of 647 nm into the fiber and recording the trans-
mitted intensity with a large photodetector placed in very close
proximity to the fiber end with the aperture. As variations in the
probes first in conventional NSOM configuration; afterward, we
processed these probes into the enhanced configuration and
recorded the throughput again.
Figure 3 clearly shows the roughly 2 orders of magnitude
improved transmission for enhanced probes compared to con-
ventional fiber aperture probes. For a typical aperture size of
around 100 nm, the throughput in the resonant configuration
was determined to be 10-3, while conventional NSOM probes
showed a throughput of approximately 10-5.11,12The enhanced
configuration allows a reduction in aperture diameter to as little
as 45 nm, while still yielding a throughput of ∼10-5.
To provide a more quantitative comparison with the experi-
mental results, as well as further insight into the underlying
Figure 1. Schematic and SEM image of the EOT near-field fiber probe.
(a) The fiber is tapered and then coated with a 220 nm aluminum layer
to prevent light leakage. We adjust the final taper diameter dt, which
determines the resonant TM mode cutoff-wavelength, by focused ion
gold layer that is evaporated onto the end face. Inset: Conventional
NSOM fiber. (b) The SEM image shows the final configuration with an
aperture of diameter da= 110 nm. The scale bar is 500 nm.
cut by focused ion beam (FIB) milling at the desired taper diameter dt,
here 370 nm, which determines the wavelength of the TM mode cutoff.
we mill the final aperture into the gold layer. The diameter of the
aperture dais held constant at 110 ( 10 nm in all fiber probes. The
the sum of the taper diameter and coating increases while the taper
diameter is varied to achieve different cutoffs of the resonant TM mode.
The presented fiberprobes havetaper diameters of (d) 420 nm, (e) 525
nm, and (f) 724 nm. The scale bar is 500 nm.
dx.doi.org/10.1021/nl102657m |Nano Lett. 2011, 11, 355–360
physics of WR-EOT, comprehensive finite-difference time do-
main (FDTD) simulations were performed. For these simula-
tions, amodesourcewith TE11mode profile was incident on the
aperture from inside the fiber and the transmission through the
gold film and aperture was measured. Perfectly matched layers
were placed far enough from the fiber to ensure no spurious
effects from coupling to evanescent waves, as confirmed by
convergence studies. Convergence studies were also performed
on the mesh size to ensure that the impact of plasmonic effects
associated with penetration into the metals was accurately
captured. We correct for any losses that were not included in
absolute values in the experimental data. As shown in Figure 3,
the simulated throughput values for smaller aperture sizes follow
optical transmission for a 50 nm aperture in our setup is still
significantly larger than for a 100 nm aperture in a conventional
NSOM probe with a 10? taper angle. Therefore, 2 orders of
magnitude enhancement in the coupling throughput of the laser
power or equivalently a reduction in aperture diameter by over a
factor 2 while maintaining the excitation power is achieved with
The brightness of conventional aperture probes is in practice
limited by their damage threshold that restricts the high power
needed to compensate for the low throughput. For our novel
probe design, with a higher throughput, one would expect less
heat to be absorbed by the coating and thus a higher damage
fiber and recorded the transmitted power up to damage simulta-
mode and thus the absolute power coupled into the fiber cannot
be determined with full accuracy. Therefore, once threshold was
reached and the aperture is destroyed, we removed the tapered
recorded intensity inside the fiber and above the taper at which
the aperture is destroyed.13For the fiber in Figure 2e, with an
to be 8 mW inside the fiber, or, assuming a typical coupling
efficiency for NSOM setups of 5%, 160 mW at the launch. For
comparison, the conventional aperture probes have a damage
threshold of typically less than 0.2 mW inside the fiber or,
equivalently, less than 4 mW at the launch. Therefore, the pre-
sentedconfiguration improves thedamagethresholdbyapproxi-
mately 40?. We explain this improved damage threshold by a
efficiency and lower optical intensities due to the broader probe
design and the cutoff of the lowest order mode, effectively
reducing the heat load on the metal coating.
We have measured the performance of the enhanced probe
configurationina home-built NSOM setupon singlefluorescent
molecules. TDI molecules dissolved in toluene/PMMA solution
were spin coated on a glass slide and excited through the near-
was scanned over the sample with nanometric accuracy and the
fluorescence signal of single molecules recorded with an ava-
lanche photodetector. Figure 4 shows fluorescence measure-
ments of single TDI molecules. The fluorescence spot shows a
confinement to 61 ( 3 nm (FWHM), which is close to the
aperture diameter of 45 ( 10 nm, as determined from SEM
images of this specific fiber. The slightly larger size of the
fluorescence spot is attributed to the penetration of the electric
and to the finite aperture-molecule distance.
So far, we have assumed that the observed increase in
of a subwavelength taper in this probe design. However, what is
the contribution of the WR-EOT? In an effort to determine its
effect, we measured the out-coupling transmission and the in-
coupling collection spectrum of the probe with both a super-
continuum and a thermal source. These two different config-
urations yielded nearly identical transmission spectra, when
normalized to the light source and detection efficiency. Due to
the brightness of the supercontinuum source, the results for the
out-coupling transmission spectrum are presented here. For
comparison, the transmission spectra of the fibers before the gold
conventional tapered NSOM aperture probes with the same final
Figure 5a shows typical measured and simulated transmission
spectra of two of the fabricated fiber probes. As the reported
EOT phenomena is associated with mode cutoff in the fiber
waveguide, we expect the longest-wavelength peak to result
from EOT and thus to depend on the final taper diameter.
aperture diameter at a wavelength of 647 nm. The transmission in the
conventional NSOM probes (squares) for comparable aperture sizes
and agrees wellwith ournumerical simulations(crosses, line isguide for
the eye). The simulated throughput is fitted to the experimental data in
order to account for losses not included in the simulations. Some of the
fiber probes were first fabricated as conventional NSOM probes and
then processed into the enhanced configuration; data points belonging
to the same probe are connected by gray lines.
probe with an aperture diameter of 45 nm reveals the improved
resolution. The molecules were embedded in a PPMA matrix and were
excited at a wavelength of 647 nm. The spatial width of the fluorescence
spot is 61 ( 3 and 62 ( 3 nm, respectively. The scale bar is 500 nm.
dx.doi.org/10.1021/nl102657m |Nano Lett. 2011, 11, 355–360
This assumption is confirmed as an almost linear dependence
Figure 5b. Each fiber probe shows a distinct transmission peak
that shifts to longer wavelengths with increasing diameter of the
taper, and the calculations show good quantitative agreement
with the measurements in the location of the resonances
in Figure 5a.
This peak in transmission is the result of WR-EOT. The WR-
EOT occurs for wavelengths above the cutoff of one of the higher-
order TM modes in the waveguide. Approaching the cutoff, the
of the aperture as it is made infinitesimally small. Therefore, the
transmission phenomenon may be considered as a problem of
impedance matching, where the energy builds up resonantly in the
TM mode above its cutoff. This phenomenon is similar to EOT in
aperture arrays, where surface waves store the energy to allow for
enhanced transmission at wavelengths longer than the Wood's
for WR-EOT is that the mode inside the fiber has a divergent
transverse magnetic field at its cutoff wavelength, which enhances
the coupling to the aperture through Bethe's theory.26Various TM
modes may be chosen to satisfy these criteria; however, the TM11
mode was selected in this case because it is the lowest order mode
thatallows for concentric apertures(i.e., it has a nonzerotransverse
magnetic field in the center of the waveguide).27This cutoff mode
tip; therefore, it is equivalent to a surface wave, and its energy is
stored near the surface with the aperture. This is analogous to the
surface waves in EOT of hole arrays, except that here the geometry
of the fiber replaces the periodicity of the array. The mode cutoff,
which is analogous to the Wood's anomaly wavelength in the array
case, is given by32
where nfis the refractive index of the fiber, dtis the final taper
is a perfect electric conductor. Figure 5b shows the location of the
the trend, yet it is clear that the peak resonant transmission occurs
for wavelengths slightly longer than the cutoff of the TM11mode.
As emphasized above, the TM11cutoff mode plays a critical
role inthe WR-EOT as it stores energy at the resonant peak, and
it is crucial to verify that the resonant peak corresponds to a
cutoff mode. Figure 6a shows numerical simulations of the
z-component of the electric field inside the two fiber probes
the fiber, matches that of the TM11. In Figure 6b we have also
monitored the field at two interfaces and at the center of the
aperture. These have very similar field intensity profiles, repre-
sented by the enhanced field confined to the aperture region,
with a dominant resonant background from the TM11mode. To
distinguish our excitation mode from the TM11cutoff mode, we
also monitored the Poynting vector (obtained by the cross
product of the transverse electric and magnetic fields) 150 nm
inside the fiber in Figure 6a. As expected, the Poynting vector
mode by which we excite the fiber. Fabrication margins and a
resulting small offset of the aperture on the end face were
included in the simulations and cause the slight asymmetry of
the field distribution. For wavelengths 100 nm below and above
the resonance wavelength, the resonance is 10? weaker. Experi-
mentally, in Figure 5a we observed a 2- to 3-fold increase in
intensity at the resonance wavelengths compared to the non-
resonant contribution in the spectra.
The 100? total improvement in throughput over a conven-
tional NSOM fiber probe is due to the effect of EOT and an
with both being a consequence of carefully nanostructuring the
probe. The absence of a strongly attenuating subwavelength
taper, present in conventionalNSOM fiber probes, improves the
coupling through the taper region to the aperture, effectively
increasing the magnitude of the electric field at the aperture; the
effect of EOT is to enhance the coupling through the subwave-
length aperture itself. As the shape of the subwavelength taper
and thus the strength of the attenuation depend strongly on the
also improves the fabrication yield as compared to conventional
probes. The challenge of matching the probe dimensions to
a specific wavelength depends merely on the quality of the
In conclusion, we have experimentally demonstrated a novel,
efficient near-field probe that shows an improvement of a
Figure 5. (a) Simulations (top row) and experimentally recorded
transmission spectra (bottom row), are shown exemplary for two fiber
(right column). The dashed lines indicate the transmission peak where
the TM11 mode at cutoff in the tapered fiber waveguide couples
resonantly to the aperture and extraordinary transmission is observed.
(b) The spectral positions of the transmission peaks attributed to mode
cutoff. The numerical simulations (crosses) are in good agreement with
the experimental data (circles) and confirm the existence of the
waveguide resonant extraordinary optical transmission. Indicated by
predicted by eq 1 for a perfect electric conductor.
dx.doi.org/10.1021/nl102657m |Nano Lett. 2011, 11, 355–360
40? enhanced damage threshold and a 100? enhanced through-
put over conventional tapered fiber probes. We have realized
single molecules with this resolution, which confirms the subwave-
length confinement of the transmitted light and the practicality of
to the high probe efficiency is a new type of EOT that relies solely
on the coupling between the fiber waveguide at mode cutoff and
the aperture, as confirmed with comprehensive electromagnetic
simulations. We have shown that the spectral position of the
so that the peak wavelength could be tuned while maintaining the
are envisioned by exploiting the cutoff resonances of lower order
Potential applications that benefit from the high efficiency of
the demonstrated NSOM probe include imaging, sensing (Raman
scattering33), nanolithography, and trapping and manipulation of
nanoscale particles, such as viruses.34EOT on a single aperture has
applications in high sensitivity sensing,35as had been studied
extensively in aperture arrays.36We also believe that this configura-
or two-photon luminescence.38,39
This research was funded by the Spanish Ministry of Science
and Innovation (MICINN) through programs FIS2009-08203
and CSD2007-046 “NanoLight”, Fundaci? o CELLEX Barcelona,
and the European Research Council (NvH, ERC-Advanced
Grant). L.N. was partially funded by the Commission for
Universities and Research of the Department of Innovation,
Universities and Enterprises of the Catalan Government and
the European Social Fund. R.G. and Y.P. acknowledge support
from the Natural Sciences and Engineering Research Council
(NSERC) of Canada Discovery Grant. Y.P. acknowledges sup-
port from the Jarmila Vlasta Von Drak Thouvenelle Graduate
Scholarship. R.G. acknowledges support from a visiting profes-
sorship from the Ag? encia de Gesti? o d'Ajuts Universitaris I de
Recerca (AGAUR), Generalitat de Catalunya.
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