Efficient coupling and field enhancement for the nano-scale: plasmonic needle.

Abstract

Theoretical demonstration of efficient coupling and power concentration of radially-polarized light on a conical tip of plasmonic needle is presented. The metallic needle is grown at the center of radial plasmonic grating, engraved in a metal surface. The electromagnetic field distribution was evaluated by Finite Elements and Finite-Difference-Time-Domain methods. The results show that the field on the tip of the needle is significantly enhanced compared to the field impinging on the grating. The power enhancement exhibited a resonant behavior as a function of needle length and reached values of approximately 10(4). Test samples for few types of characterization schemes were fabricated.

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Efficient coupling and field enhancement for the
nano-scale: plasmonic needle
Alexander Normatov1, Pavel Ginzburg1,*, Nikolai Berkovitch1, Gilad M. Lerman2,
Avner Yanai2, Uriel Levy2 and Meir Orenstein1
1EE department, Technion – Israel Institute of Technology, Technion City, Haifa 32000 Israel
2Department of Applied Physics, The Benin School of Engineering and Computer Science, The Center for
Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel
*gpasha@tx.technion.ac.il
Abstract: Theoretical demonstration of efficient coupling and power
concentration of radially-polarized light on a conical tip of plasmonic needle
is presented. The metallic needle is grown at the center of radial plasmonic
grating, engraved in a metal surface. The electromagnetic field distribution
was evaluated by Finite Elements and Finite-Difference-Time-Domain
methods. The results show that the field on the tip of the needle is
significantly enhanced compared to the field impinging on the grating. The
power enhancement exhibited a resonant behavior as a function of needle
length and reached values of ~104. Test samples for few types of
characterization schemes were fabricated.
©2010 Optical Society of America
OCIS codes: (240.6680) Optics at surfaces: Surface plasmons; (310.6628) Subwavelength
structures, nanostructures
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1. Introduction
Fast development of nano-scale technology enables the examination of basic phenomena in
physics, biology and chemistry, and allows a variety of implementations and applications.
Exploiting e-beam and ion-beam nano-writing techniques, it is possible to fabricate high
quality noble metal nano-structures [1] and study their interaction with light. Metals, having
negative electrical dielectric constant at visible and infra-red domains, support Surface
Plasmons Polaritons (SPPs) [2,3] which may be guided with sub-wavelength confinement and
excite modes of nanometric particles [4]. Nano-focusing of SPPs was proposed and
demonstrated in a variety of configurations: adiabatic conical metal rod [5], chain of metal
spheres with variable radii, tapered [6] or abrupt impedance matched metal/insulator/metal
(MIM) plasmonic waveguides [7]. Guided and localized plasmons may serve to enhance
nonlinearities [8] and radiation efficiency of quantum emitters [9,10]. Arrays of plasmonic
nanowires may also serve as building blocks for metamaterials [11,12]. Recent experiments
exhibited some merits of these focusing for 3D [13] and, mainly, on planar focusing tapers
[14]. A different focusing configuration is the planar plasmonic lens, generating circular
converging SPP waves from a circular aperture [15]. It was shown recently that plasmonic
assisted focusing is much more efficient if the impinging light is radially polarized - better
matching both the circular symmetry of the structure and the plasmon polarization. In addition
it allows for constructive interference of the dominant out-of-plane electric field component at
the center of the structure [16,17, and 18,].
Exploiting these ideas, two groups recently proposed independently focusing structures
with some similarity [19,20]. Here we discuss in detail the configuration of Ref. [20], evaluate
theoretically, design and fabricate a plasmonic structure aimed at efficient launching of
radially polarized incident illumination onto a plasmonic nanowire waveguide (needle) and
subsequently to the needle tip. The proposed plasmonic structure comprises radial gratings,
coupling the incident illumination to SPPs and a plasmonic nanowire waveguide of finite
length, terminated by a short conical tipped segment (placed in the center of the grating). Part
of the excited SPP is focused towards the center of symmetry due to the radial nature of the
structure. These focused SPPs are efficiently matched to excite the highly confined mode of
the plasmonic needle (the anti-symmetrical mode), and further substantial intensity
enhancement occurs as the light is propagating to the apex of the short cone, as predicted in
[5]. This focusing structure model is illustrated schematically in Fig. 1(a). Actual devices,
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shown in Fig. 1(b), were fabricated by a two step process at the same chamber. First, the
radial gratings were generated using focused ion beam (FIB) with a precise control over the
grating height and period. Subsequently a gold needle with a conical tip was grown on the
center by means of low current electron beam assisted local deposition of Au from gas phase
precursor. A variety of device parameters were shown to be controlled: needle height and
diameter (Fig. 1(b)), gratings period and modulation depth which are promising results for on-
going experiments.

Fig. 1. (a) The schematics of the structure proposed for efficient coupling of radially-polarized
light to plasmonic needle apex (b) SEM image of a fabricated device (grooves were etched here
down to the substrate for visual clarity, while in the actual device they were only slightly
etched); upper inset: zoom on the needle; bottom inset: variety of grown needles
The excitation of such high field intensities (hot spots) is important for validating basic
limiting factors of plasmonic power concentration (e.g. nonlocal effects [21]), efficient near
field inspection and writing (nanolithography, memories), and efficient coupling of nano
emitters/absorbers to the far field.
Our paper dwells primarily with the coupling efficiency of a converging circular
plasmonic wave into a vertical finite length nanoplasmonic wire with emphasis on this 2D-1D
conversion as well as on the resonance effects due to the finite length of the wire;
subsequently we deal with further field concentration into the conical tip of the wire and with
issues related to fabrication and measurement strategies. A paper by G. Rui, et. al. [19] deals
primarily with a transmission configuration of a Bragg planar plasmonic structure coupled to a
micrometer sized cone exploring mainly the overall field concentration at the tip of the whole
structure.
The analysis of the structure is done in four steps to distinguish between different
enhancement mechanisms. The first is aimed at evaluation of the field enhancement of a
converging circular plasmonic wave and its coupling efficiency to the needle. The second step
examines the design of circular gratings aimed at efficient coupling of the incident radial
illumination to SPPs. The third explores the influence of the needle height. Finally the
dependence of the plasmonic modes on the excitation polarization was investigated.
2. Coupling of converging circular plasmonic wave to a nano needle
SPP waves were shown to be relatively robust against sharp bending scenarios even for π/2
bends [22,23]. While previous investigations dealt mainly with two-dimensional plasmonic
waveguides, three-dimensional case was not fully addressed. Here we investigate the coupling
efficiency of a converging, radially symmetric, circular SPP wave to nanometric diameter
plasmonic needle. The needle resides in the center of symmetry vertically attached to a thick
gold film, much thicker (1µm) than the penetration depth. The converging plasmonic wave is
launched by applying a circular symmetric plasmonic wave source at the metal plane with
transverse polarization, at the periphery of our calculation domain. At the center the in-plane
plasmonic field exhibits a singularity, and the only non-vanishing electric field component is
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the one perpendicular to the surface. This latter field component – axial for the needle, leads
to efficient excitation of its highly confined anti-symmetrical mode.
Calculations were performed by the Finite Element Method (FEM). The model based on
circular symmetric geometry, benefits from reduction of the computation complexity and thus
allows denser grid and enhanced accuracy. The device parameters were chosen in a range that
matches our actual fabrication and experimental parameters. The needle height was optimized
for highest local electric field enhancement at the tip. The total electric field amplitude is
22
totrz
EEE
=+
with vanishing azimuthal component due to rotational symmetry. The
resulting normalized field amplitude (in logarithmic scale) for a 965nm high gold needle at a
wavelength of 1.55µm is presented in Fig. 2, showing power enhancement of about 103. The
comparison is made for the case with no needle, where the maximum z-directed, field at the
center is Eref, measured just above the substrate.

Fig. 2. Total field amplitude of a converging circular plasmonic wave on a flat gold substrate
coupled to a needle. Red arrows indicate the direction of the Poynting vector.
Evaluation of SPP coupling efficiency was based on comparing the total power (P) flow
along the substrate with the flow along the needle. The value of Pr was integrated along the
line r = 2µm in the air, at the relevant heights (0nm < z < 1µm, shown as a vertical green line),
yielding the substrate SPP power. The value of Pz was integrated along the line z = 350nm in
the air, at the relevant radial distances (55nm < r < 1.2µm, shown as a horizontal pink line).
The integrated power flow includes that of the needle SPP (directed downward) and the
corresponding upward part of the vortex flow, centered at the position approximately
indicated by the white arrows. The sum yields the non-localized portion of needle SPP power
that should be compared with that of the incident substrate SPP. The resulting power coupling
efficiency between the substrate surface (2D) and the needle (1D) is estimated to be >40%.
The power density for the planar SPP is much smaller compared to that of the needle.
The complexity of the coupling mechanism, as it is evident from Fig. 2 implies that the
coupling value can be sensitive to various surface defects within a radial range of at least one
free space wavelength. These defects alter fields associated with the substrate surface SPP
mode and thus may enhance or inhibit their overlap with fields associated with the needle SPP
mode. The needle mode fields are defined by the needle shape which makes the shape a key
parameter for the coupling. Additional numerical experiments exhibited sensitivity of field
enhancement to the needle height and will be discussed below. The shape of the needle tip
was defined according to that of the fabricated experimental samples. The present work does
not include an investigation of the influence of the shape of the needle tip.
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3. Structure design for normal incidence
The parameters of the circular gratings were chosen according to a momentum matching
formula for the second order resonance [16]. The design of the whole structure may be
optimized for front illumination (the side with the needle), as well as for back side
illumination. The first configuration is aimed for experiments where the measurements of the
field concentration is indirect – e.g. by far field monitoring of nonlinear higher harmonic
generation, while the second configuration is designed for direct measurement by near field
scan. Here the first configuration is analyzed, while both versions were fabricated.
A circular gratings structure, similar to the structure used to harvest linearly polarized light
onto a hole for enhanced transmission [24], couples the radially polarized light to converging
circular plasmonic waves. The structure design that incorporates a height-optimized needle
was performed for free space wavelength of 1.55µm and is shown in Fig. 3:

Fig. 3. The height-optimized structure design, implemented for free space wavelength of
1.55µm
The resulting normalized distribution of the real part value of the field components is
depicted in Fig. 4: The Er field distribution in Fig. 4(a) shows both the standing wave pattern
of the radial illumination and, in correspondence with Fig. 4(b), the vortex field distribution as
was indicated in Fig. 2. It must be noted that the color-scales of Fig. 4 are fitted to
accommodate representation of weaker fields remote from the needle and thus at the needle
tip the field values are saturated. The Ez values at the needle tip correspond to power
enhancement of the order of 104, as will be demonstrated in the next section. Here Eref is the z
directed field in the center of the structure when the needle is not present.

Fig. 4. Normalized value of the electric field components is shown for the height-optimized
needle. (a) The Er field; (b) The Ez field.
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4. Resonant behavior of field enhancement
The basic phenomenon, leading to the field enhancement at the tip, is the narrowing of the
SPP mode when propagating to the needle tip. In contrary to the theoretical ideal adiabatic
infinitely tipped cone of Ref [5], where no back reflection from the tip is expected due to SPP
adiabatic stopping, the conical part of our needle is finite. Thus reflection from the apex is
exhibited, depending on the specific tip shape and wavelength. In addition, the needle base is
not completely impedance matched to the planar SPP structure and consequently partial
refection occurs there as well. This turns the needle into a Fabry-Perot (FP) like resonator
where the reflection phases are of great importance and the longitudinal modes that are
supported by the FP resonators obey the following equation [25]:

12
42
avg
hN
πλθθπ
++=
(1)
where h is the needle height, λavg the averaged SPP wavelength (λ varies along the needle
according to the local diameter), and θ1, 2 are reflection phases from the top and the bottom of
the needle. This resonance can be illustrated by varying the plasmonic needle height and
measuring Ez field enhancement at the conical tip (Er = 0 there), relative to the field in the
center of the gratings structure with no needle. The results showing the ratio of field
enhancement are shown in Fig. 5. The difference in the plasmonic needle height between
consecutive peaks (resonances), is about 700nm. This means that λavg, which ideally should be
equal to a half of the free space wavelength, corresponds to averaged SPP effective index of
~1.1 (when the refection phases are assumed to be non-dispersive). Largest field enhancement
corresponding to the investigated positive defect was about 150. Figure 6 shows the field
distributions, corresponding to the peaks, represented in Fig. 5.

Fig. 5. Ez field enhancement at the needle tip as a function of needle height for λ0 = 1550nm.
The absolute value of Etot is
22
totrz
EEE
=+
, as the incident illumination is radially
polarized and the structure has cylindrical symmetry.
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Fig. 6. Field distribution cross section corresponding to the local maxima of Fig. 5. For
visualization, the relative field values are shown as log(|Etot|/|Eref| + 1) (a) at 300nm (b) at
950nm (c) at 1670nm.
Figure 6(a) confirms that at the first (electrostatic) resonance of the structure the needle
height is small compared to the excitation wavelength. Figures 6(b), (c) show the second and
third structure resonances or first and second cavity (retarded) resonances. Additional
optimizations may further enhance the field.
5. Needle modes and their dependence on impinging polarization
The nano needle supports two bound modes with symmetric and antisymmetric profiles [26].
The mode with antisymmetric magnetic field profile is the mode of interest here since it has
the highly confined field. Radial polarization illumination exhibits an antisymmetric field
component (Hφ) around the center, matching the required needle excitation. On the other
hand, linear (circular) polarization excites the symmetric and much less confined mode.
Differences in confinement between these two excitation polarizations are calculated by
Finite Difference Time Domain method [27] and shown in Fig. 7. Comparing Fig. 7(a) - two
Fabry Perot lobes, and 7(c) - three Fabry Perot lobes, reveals that the effective wavelength at
the tip is much smaller under radial polarization illumination – as expected by the confined
mode dispersion. Also, the much higher confinement is evident. Comparison between Fig. 7
(b) and (c) shows that only for radial polarization the strong EZ component (along the tip)
resides at the tip.

Fig. 7. Field distribution for 950nm high tip. (a) transverse field excited by linear polarization
illumination (b) longitudinal field obtained by linear polarization illumination (c) transverse
field by radial polarization illumination (d) longitudinal field by radial polarization
illumination. The color scaling of (b) and (d) is normalized and for (a) and (c) is proportional to
(b) and (d) respectively.
6. Outlook and conclusions
On-going measurements are aimed at the evaluation of the field enhancement as a function of
excitation wavelength, polarization and design parameters. The resonant sensitivity of the
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structure, as discussed above, (significant enhancement for restricted wavelength / needle
length combinations) will be employed to eliminate the background of the non enhanced
fields. The enhanced Au based second harmonic generation by a pulsed excitation at 1.5µm
and 0.85µm regimes and the actual field distributions will be measured by NSOM.
In conclusion, we propose a configuration of a nanometric plasmonic needle, surrounded
by a radial gratings, yielding power enhancement factor of the order of 104. The fabricated
samples are currently under measurements. High coupling efficiency of this structure to single
quantum emitters may open possibilities to realize quantum memories and quantum gates in
the near future. The high field concentration at the nano-scale opens possibilities to test basic
nonlinear properties of isolated nano-structures.
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