Triangular metal wedges for subwavelength
plasmon-polariton guiding at telecom
wavelengths
Alexandra Boltasseva1,*, Valentyn S. Volkov2,5, Rasmus B. Nielsen1, Esteban Moreno3,
Sergio G. Rodrigo4, Sergey I. Bozhevolnyi2,5
1
DTU Fotonik, Department of Photonics Engineering, Nano•DTU, Technical University of Denmark, Building 343,
DK-2800 Kongens Lyngby, Denmark; [email protected]
2
Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, DK-9220 Aalborg Øst, Denmark
3
Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid,
E-28049 Madrid, Spain
4
Departamento de Física de la Materia Condensada, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
5
Institute of Sensors, Signals and Electrotechnics (SENSE), University of Southern Denmark, Niels Bohrs Allé 1, DK5230 Odense M, Denmark
*
Corresponding author: [email protected]
Abstract: We report on subwavelength plasmon-polariton guiding by
triangular metal wedges at telecom wavelengths. A high-quality fabrication
procedure for making gold wedge waveguides, which is also massproduction compatible offering large-scale parallel fabrication of plasmonic
components, is developed. Using scanning near-field optical imaging at the
wavelengths in the range of 1.43 – 1.52 µm, we demonstrate low-loss
(propagation length ~ 120 μm) and well-confined (mode width ≅ 1.3 µm)
wedge plasmon-polariton guiding along triangular 6-µm-high and 70.5°angle gold wedges. Experimental observations are consistent with numerical
simulations performed with the multiple multipole and finite difference time
domain methods.
©2008 Optical Society of America
OCIS codes: (240.6680) Surface plasmons; (250.5403) Plasmonics.
References and links
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1. Introduction
Miniaturization of optical circuits and dense integration of optical devices remain major tasks
in micro and nanotechnology. One of the promising approaches for downscaling optical
components is to use surface plasmon-polaritons (SPPs) [1,2]. These are electromagnetic
excitations supported by metallic structures that offer subwavelength light confinement in the
direction perpendicular to the metallic-dielectric interface [1,2]. Various geometries have been
proposed and experimentally studied to demonstrate subwavelength confinement and efficient
guiding of the plasmon-polariton modes including metal strips with nm-scale thickness [3-5],
subwavelength nanowires [6,7] and metal gap waveguides [8,9]. Among the studied
geometries, a V-shaped metal groove supporting a strongly localized plasmon-polariton mode
called channel plasmon-polariton (CPP) is of particular interest due to high degree of the CPP
mode confinement, relatively low propagation and bending losses, and single mode operation
achievable for the CPP [10-14]. The inverse geometry, namely, a triangular metal wedge
supports a plasmon-polariton mode (wedge plasmon-polariton, WPP) that is, in a way,
complementary to the CPP mode. Edge modes were first studied theoretically for metal
triangular and parabolic edges [15,16], and later, triangular metal wedges were shown (both
theoretically [17,18] and experimentally [17]) to support strongly localized plasmons. It was
recently shown that WPPs, while showing significantly smaller modal size than CPPs, exhibit
similar propagation length as CPPs [18]. At telecom wavelengths, WPP guiding properties
were found to be superior to the ones offered by CPPs, where low losses for CPPs are
achievable for relatively large mode sizes [19]. Moreover, the possibility of light focusing via
the geometry-driven conversion of a standard SPP into a tightly confined WPP was recently
proposed [18].
To our knowledge, only one experiment was performed to investigate the metal wedge
configuration [17]. In the work by Pile and co-workers [17], highly localized WPP modes
supported by a triangular silver wedge were studied in the visible regime, where the mode
propagation length is quite short. Moreover, the structures were fabricated using the focusedion beam (FIB) technique. In the perspective of future applications, the FIB-based approach
has certain limitations due to high cost, complexity and low throughput. In addition, FIB#92255 - $15.00 USD
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fabricated components are hard to interface with optical fibers to achieve efficient end-fire
excitation. Thus, experimental realization of WPP components that are robust, simple to
fabricate and easy to interface with outside world is of great interest for future investigations
of subwavelength plasmon guiding as well as for development of functional plasmonic
devices.
In this work, subwavelength WPP guiding along straight gold wedges is realized and the
WPP propagation is studied both theoretically and experimentally. For fabricating triangular
metal wedge waveguides a wafer-scale, parallel method based on standard UV lithography is
developed. We focus on at telecom wavelengths where WPP losses are much lower[18] than
in case of visible light[17]. The developed fabrication procedure provides waveguides that are
compatible with fiber optics giving the possibility of easy in- and out coupling.
2. Fabrication
Large-scale fabrication of profiled metal surfaces remains a challenging task. While Vgrooves and sharp wedges can easily be made in silicon using standard lithographic and
etching techniques, profiling metal surfaces often requires complex fabrication techniques like
FIB [13,14,17]. In this work, in order to fabricate triangular metal structures, we developed a
flip-technique that allows fabrication of sharp metal wedges. Such geometry can not be
achieved by standard deposition techniques since depositing a metal layer on a sharp tip (for
example, in silicon) would result in smoothening of the edge. A standard fabrication sequence
of lithographic and etching steps to achieve a wedge in a substrate combined with standard
metal deposition can therefore only be used for making rounded-top or trapezium-shaped
wedges [20].
The fabrication steps of the developed procedure are shown schematically in Fig. 1. To
fabricate metal wedges, a silicon dioxide layer (~ 400 nm) is first grown on a silicon substrate
by thermal oxidation. Prior to photoresist (AZ 5214E from MicroChemicals GmbH) spin, a
layer of an adhesion promoter (hexamethyldisilazane, HMDS) is applied in a vapor priming
oven. After hotplate baking, 1.5-µm-thick resist is patterned by standard UV lithography.
Straight-waveguides pattern is then transferred into a SiO2 layer via wet etch in buffered
hydrofluoric acid (BHF) using developed and hard-baked resist as a mask. After silicon
dioxide etch, the resist is stripped from the wafer, leaving SiO2 mask ready for the next
etching step. V-grooves in silicon are afterwards etched via standard KOH-etch process
followed by oxide layer removal in BHF. After wafer cleaning, 500 nm of gold is e-beam
evaporated on the silicon wafer with etched V-grooves. Using electroplating deposition, 53µm-thick layer of nickel is then deposited on top of the gold-covered wafer. After
electroplating step, the wafer is cut into pieces of different lengths containing straight
embedded V-grooves. Removal of silicon substrate (KOH solution) from separate samples
completes the fabrication sequence yielding straight V-wedges of gold (Fig. 2). The
developed procedure provides straight wedges with the fixed apex angle of 70.5◦ (due to wet
etch of silicon). The apex angle of the V-groove made in a substrate can be changed via
oxidation process [21].
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Fig. 1. Schematic of the fabrication steps: (1) a silicon wafer is covered with a layer of silicon
oxide and photoresist, (2) resist is exposed and developed, and the pattern is transferred into the
oxide, (3) V-grooves are etches in silicon, (4) gold is deposited after oxide removal, (5) nickel
is deposited, (6) silicon substrate is dissolved leaving gold 70.5°-wedges.
(a)
(b)
(c)
Fig. 2. Scanning electron microscope image of (a) 8.5-µ m-wide, 6-µ m-high gold wedge
waveguides together with (b,c) close-ups of the fabricated wedges (b – wavy edge at the lower
end is due to charging effects). Marks and facet defects are due to rough sawing of the metal.
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3. Experiment
The experimental setup employed for the sample characterization is essentially the same as in
our previous experiments with CPPs and photonic crystal waveguides [13,22]. It consists of a
collection scanning near-field optical microscope (SNOM) with an uncoated fiber tip used as
a probe and an arrangement for illuminating metal wedge waveguides with tunable (1425–
1500 nm) TM/TE-polarized (the electric field is perpendicular/parallel to the sample surface
plane) radiation by positioning a tapered-lensed polarization maintaining single-mode fiber
[13]. The adjustment of the in-coupling fiber (with respect to the Λ-wedge position) was
accomplished when monitoring the light propagation along the surface with the help of a farfield microscopic arrangement. The idea was to judge upon the WPP excitation and
propagation by observing its scattering out of the surface plane by wedge imperfections. We
found that for both polarizations of incident light the track of radiation propagating along a
wedge waveguide was clearly distinguishable for distances of up to 300 µm from the incoupling wedge facet in the whole range of laser tunability. Far-field experiments included
also adjusting the in-coupling fiber position to maximize the coupling efficiency. After these
adjustments the whole fiber-sample arrangement was moved under the SNOM head. The
intensity distribution near the surface of the wedge was mapped with a sharp SNOM tip that
was scanned along the sample at a constant distance of a few nanometers. The distance was
maintained by shear-force feedback, and the radiation collected by the fiber was detected with
a femtowatt InGaAs photo receiver.
Typical SNOM images recorded at different wavelengths for TM polarization of the
incident light are shown in Figs. 3(b), 3(c) along with the corresponding topographical image
(Fig. 3 (a)). The near-field optical images (obtained at λ ≅ 1440 and 1500 nm) demonstrate
efficient and well-confined WPP guiding. Pronounced intensity variations observed along the
propagation direction (Figs. 3(b), 3(c)) can be most probably attributed to the interference
between the WPP modes and scattered field components (including stray light), similar to that
observed previously in our experiments with CPPs [13]. One can easily recognize the Λwedge waveguide on the topographical image (Fig. 3(a)). It is also seen that the wedge image
is quite irregular showing a zigzag pattern with sudden lateral displacements, which were
caused by shear-force scanning instability due to a rather large wedge height (~ 5 µm). The
near-field optical images obtained simultaneously with the topographical ones display similar
zigzag patterns replicating lateral displacements of the fiber tip (Figs. 3(b), 3(c)). Despite the
aforementioned signal variations and perturbations in topographical and near-field optical
images, one could determine the average full-width-at-half-maximum (FWHM) of the WPP
mode and its propagation length from 32-μm-long SNOM images (Fig. 3(d)). In the
wavelength range 1425–1500 nm the FWHM was practically constant (1.32 ± 0.1 μm)
whereas the WPP propagation length was found to vary between ~ 100 and 120 μm,
depending on both the coupling arrangement and the wavelength used. We attribute these
variations to poor reproducibility of the fiber coupling where the adjustment is done using farfield observations. In this case the detected signal is strongly influenced by the scattered (by
the wedge imperfections) radiation. Note that the wedge tip seen from the topographical
image (Figs. 3(a), 3(d)) appears rather blunt, most probably due to the convolution between
the fiber probe shape and the wedge tip profile. The sharpness of the latter can more reliably
be evaluated from SEM images (Fig. 2) as been on the sub-100-nm scale.
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Fig. 3. Pseudo-color (a) topographical and (b, c) near-field optical images taken with the Λwedge illuminated with TM-polarized light at λ ≅ (b) 1440, and (c) 1500 nm (the WPP
propagates rightwards). The image size: 32 x 10 μm2. (d) Cross sections of the topographical
(stars) and near-field optical (filled and open circles) images of Fig. 3(a) and 3(c) averaged
along 10 lines along the propagation direction or perpendicular to it. The exponential
dependence fitted by the least-square method to the signal dependence along the propagation
direction is also shown.
The experiment was repeated for the TE-polarized incident light by rotating the polarization
maintaining in-coupling fiber by 90º. For this polarization and the in-coupling fiber positioned
symmetrically against the wedge, the WPP excitation is forbidden by symmetry. The typical
topographical and corresponding near-field optical images recorded in this configuration at
1500 nm are shown in Fig. 4. In this case the intensity distribution is very broad and exhibits
indeed a local minimum at the wedge top, showing instead two maxima on both sides of the
wedge. These maxima are related to SPP modes that propagate along two broad (~ 6 µm)
wedge sides and, being polarized perpendicular to the wedge sides, can be efficiently excited
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by TE-polarized light. These modes are somewhat similar to the SPP modes of metal stripes
whose lateral confinement is rather poor [23].
Fig. 4. Pseudo-color (a) topographical and (b) near-field optical images taken with the Λ-wedge
illuminated with TE-polarized light at λ ≅ (b) 1500 nm (the SPP propagates upwards). The
image size: 18 x 32 μm2. (c) Cross sections of the topographical (stars) and near-field optical
(filled circles) images of Fig. 4(a) and 4(b) averaged along 10 lines along the propagation
direction or perpendicular to it.
4. Theory
The corresponding structure used for our modeling is a gold 70°-wedge surrounded by
vacuum that is infinitely long in the propagation direction and rounded with the radius of
curvature r. The gold dielectric constant is represented by a Drude-Lorentz type dielectric
function. The size of the considered structures is sufficiently large so as to use bulk dielectric
functions and neglect additional damping due to electron scattering at the metal surface. The
simulations were carried out using a rigorous electrodynamics computational framework, viz.
the multiple multipole and finite difference time domain methods [18].We calculated the
complex WPP propagation constant and field distribution as a function of the curvature radius
of the wedge tip for the telecom wavelength of 1500 nm, allowing us to determine the mode
size and propagation length (Fig. 5). Here the mode size is defined as the transverse separation
between the locations where the time-averaged electric field of the WPP mode has fallen to
one tenth of its maximum value. The factor 1/10 in this definition is somehow arbitrary but it
is sufficient for our mode characterization purposes and consistent with the previous
calculations [18]. As expected, the mode size and propagation length are increasing with the
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increase of the curvature radius, so that in order to achieve deep subwavelength WPP field
confinement one should realize 10-nm-sharp wedges or even sharper ones (Fig. 5).
Fig. 5. Transverse electric field of the WPP mode at λ = 1.5 µ m for the curvature radius r = (a)
10 and (b) 100 nm. (c) Mode size (circles) and propagation length (triangles) of WPP mode as
a function of the radius curvature (solid lines represent spline-interpolation).
The obtained results indicate the possibility of subwavelength WPP guiding over ~ 100 µm
for reasonable values (10 – 20 nm) of curvature radius (Fig. 5) in agreement with our
experimental observations. Conversely, comparing the experimental results with the
simulations (cf. Figs. 3(c) and 5) indicates that the curvature radius in the fabricated structures
was close to ~25 nm, a value that is consistent with the SEM images of our structures (Fig. 2).
Note that while accurate determination of the curvature radius from SEM images is in
principle possible, it is rather difficult to actually make high-quality cross cuts of the
fabricated structures required for this procedure. Overall, our experimental results are found in
good agreement with the simulations results.
5. Conclusions
We realized subwavelength plasmon-polariton guiding along straight gold wedges that were
fabricated via large-scale parallel process based on standard UV lithography. The developed
fabrication procedure provides waveguides that are compatible with fiber optics. The WPP
propagation was studied both theoretically and experimentally, and the experimental
observations are consistent with numerical simulations performed with the multiple multipole
#92255 - $15.00 USD
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Received 29 Jan 2008; revised 12 Mar 2008; accepted 13 Mar 2008; published 1 Apr 2008
14 April 2008 / Vol. 16, No. 8 / OPTICS EXPRESS 5259
and finite difference time domain methods. Using SNOM characterization, in the wavelength
range 1.43 – 1.52 µm the FWHM and the propagation loss of the WPP mode propagating
along triangular 6-µm-high and 70.5°-angle gold wedges were found to be around 1.3 μm and
120 µm respectively. The results of this work open up the possibility of developing WPPbased waveguide components for different applications ranging from integrated optics to
biosensors.
Acknowledgments
The support from the European Network of Excellence, PLASMO-NANO-DEVICES (FP62002-IST-1-507879), the Danish Research Agency through the NABIIT project (Contract No.
2106-05-033) and the FTP project (Contract No. 274-07-0258) is greatly acknowledged; and
Torben Tang’s assistance and help with electroplating deposition process is highly appreciated
(Department of Manufacturing Engineering and Management, Technical University of
Denmark).
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