High confinement in plasmonic waveguides usually comes along with high loss. We present experiments on a new approach, which allows to tune adiabatically between high confinement and low loss waveguides, connected to optical Yagi-style antennas

Atwater, Harry A.

Burgos, Stanley

Kriesch, Arian

Peschel, Ulf

Ploss, Daniel

Wen, Jing

Caltech Authors - Main

QM2K.3.pdf CLEO Technical Digest © OSA 2012
Plasmonic nanocircuitry with embedded
subwavelength waveguides and Yagi-style antennas
Arian Kriesch1,2,3, Stanley Burgos3, Daniel Ploss1,2, Jing Wen1,2,
Ulf Peschel1,2, Harry A. Atwater3
1Nonlinear Optics and Nanophotonics, Max Planck Institute for the Science of Light, Guenther-Scharowsky-Str. 1
Bau 24, 91058 Erlangen, Germany.
2Institute of Optics, Information and Photonics, Cluster of Excellence Engineering of Advanced Materials and
Erlangen Graduate School in Advanced Optical Technologies, University of Erlangen-Nuremberg,
Guenther-Scharowsky-Str. 1 Bau 24, 91058 Erlangen, Germany.
3Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, MS 128-95, Pasadena, CA
91125, USA.
arian.kriesch@mpl.mpg.de
Abstract: High confinement in plasmonic waveguides usually comes along with high loss.
We present experiments on a new approach, which allows to tune adiabatically between high
confinement and low loss waveguides, connected to optical Yagi-style antennas.
© 2008 Optical Society of America
OCIS codes: (250.5403) Plasmonics; (250.5300) Photonic integrated circuits; (240.6680) Surface plasmons.
1. Confinement vs. loss in plasmonic waveguides
Plasmonic waveguides allow for guiding light in structures with a confinement of the electromagnetic field, slightly
above [1] to even far below the diffraction limit [2].
In plasmonics, an increase of confinement usually comes along with growing propagation losses due to a higher
field overlap with the lossy metal, thus limiting the applicability for optical nano-circuitry applications.
Plasmonic gap [3, 4] or slot waveguides [5] have proved to achieve comparably maximum field confinement. How-
ever, the minimum loss limit of typical SPP air-gap waveguides, structured into metal layers on a dielectric substrate,
is given by the cutoff of their guided modes due to a drop of their effective refractive index below that of the substrate.
2. Embedded plasmonic gap waveguides with adiabatically tunable loss
re
fle
ct
or
1 µm
(a) (b)
(a‘)
Ag
Ag
SiO2
1300 1400 1500 1600 1700
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
wavelength in nm
reflected
transmitted
into waveguide
no
rm
al
iz
ed
 p
ow
er
 fl
ow
Fig. 1. (a) Optimized optical Yagi-style antenna. Field distribution |Ey| in the substrate plane, ob-
tained from a full 3D FDTD simulation. Simulating the excitation with a Gaussian beam of N.A.
0.9 from air (red circle). (a’) Field distribution in a cross section with 300nm×300nm gap size. (b)
Reflection, transmission and coupling efficiency of an antenna, which is optimized for λ = 1550nm.
Here we present numeric calculations and experimental results on a novel approach to achieve highly subwavelength
(down to around 100nm) integration of guided plasmonic waves, which allow for an adiabatic transition to waveguides
with considerably reduced loss and slightly lower confinement.
These waveguides are fabricated by embedding metallic inlays into a silica substrate, or alternatively into a silicon on
insulator (SOI) template. Filling the gap of a usual plasmonic gap waveguide with a dielectric, increases the effective
QM2K.3.pdf CLEO Technical Digest © OSA 2012
(b)
5 µm
pos. resist
silica substrate
e beam, devel. 
and rie
Ag deposition
liftoff
embedded w.g.
(a)
Fig. 2. (a) Fabrication process for embedded SPP waveguides. (b) SEM of a Ag in quartz sample
with a optical Yagi-style antenna and a short waveguide for testing the spectral properties.
index of the guided modes. Thus, the cutoff gap width of the guided mode is shifted to far higher values, allowing for
SPP waveguides with much lower loss and slightly lower, but still subwavelength, confinement. Propagation lengths of
20µm with confinement of 300nm at a wavelength of 1550nm are observed. These waveguides show low bend losses
(below 15% for R= 2µm) and directional coupling with short coupling length of few microns is feasible. Particularly
interesting is the ability to connect low-loss with high-confinement waveguides to a single directional coupler.
3. Connected efficient Yagi-type optical antennas
Coupling into plasmonic gap waveguides with matched optical antennas has proved to be an efficient way to probe SPP
circuitry components [3, 4]. Here we combined the embedded SPP waveguides with highly efficient optical antennas,
which facilitate the principle of a directional Yagi antenna (fig.1 a) to achieve efficiencies up to 22% (from air) and
27% (from dielectric) when coupling a focused beam of light (N.A.=0.9) into the waveguides (fig.1 b).
To increase the coupling efficiency, the waveguide gap is tapered towards the antenna. A Yagi-style optical reflector
reduces antenna emission into the sample plane and therefore strongly enhances antenna efficiency.
Antenna design and geometry parameters were found and optimized for optimum coupling of a focused Gaussian
beam with NA = 0.9 and λ0 = 1550nm using 3D FDTD calculations with a particle swarm optimization (PSO)
algorithm and frequency domain FEM calculations 1.
4. Fabrication and measurement scheme
We fabricated the embedded SPP waveguides together with the Yagi style antennas in a single combined e-beam, RIE
and liftoff fabrication process (fig. 2 a). The resulting samples (fig. 2 b) offer plasmonic structures, embedded in a
silica matrix. At the same time a plane surface across the whole sample allows to scan the electromagnetic near field
from all SPP circuitry components with a Near Field Scanning Optical Microscope (NSOM) without being obstructed
by topographic features.
Respective measurements are in progress and will be shown at the conference.
References
1. S. I. Bozhevolnyi, E. Devaux, V. S. Volkov, J.-Y. Laluet, and T. W. Ebbesen, “Channel plasmon subwavelength
waveguide components including interferometers and ring resonators.” Nature 440, 508–11 (2006).
2. W. Cai, W. Shin, S. Fan, and M. L. Brongersma, “Elements for Plasmonic Nanocircuits with Three-Dimensional
Slot Waveguides,” Adv. Mater. 22, 5120–5124 (2010).
3. J. Wen, P. Banzer, A. Kriesch, D. Ploss, B. Schmauss, and U. Peschel, “Experimental cross-polarization de-
tection of coupling far-field light to highly confined plasmonic gap modes via nanoantennas,” Applied Physics
Letters 98, 101,109 (2011).
4. J. Wen, S. Romanov, and U. Peschel, “Excitation of plasmonic gap waveguides by nanoantennas,” Optics Ex-
press 17, 5925 (2009).
5. J. A. Dionne, L. A. Sweatlock, H. A. Atwater, and A. Polman, “Plasmon slot waveguides: Towards chip-scale
propagation with subwavelength-scale localization,” Physical Review B 73 (2006).
1Lumerical FDTD solutions and Comsol multiphysics.


Embedded plasmonic waveguides with Yagi-style antennas

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