S
upporting
Information
Nanoscale Conducting Oxide
PlasMOStor
Ho W. Lee
1,2,
†
, Georgia Papadakis
1
, Stanley P. Burgos
1,
2
, Krishnan Chander
1
,
Arian Kriesch
1,
3
, Ragip Pala
1,
2
, Ulf Peschel
3
and Harry A. Atwater
1,
2,
†
1
T
homas J. Watson Laboratories of
Applied Physics, California Institute of Technology, United
States
2
Kavli Nanoscience Institute,
California Institute of Technology, United
States
3
Inst. of Optics, Information and Photonics & Graduate School in Advanced Optical Technologies,
Friedrich
-
Alexander
-
University
Erlangen
-
Nuremberg, Germany
†
Corresponding
author e
-
mail:
hwlee@caltech.edu
,
haa@caltech.edu
Keywords
Plasmonics
, transparent conducting oxide, modulator, field
-
effect modulation, epsilon
-
near
-
zero material,
nanocircuits, plasmonic slot waveguide
, active plasmonics
1.
Cross
-
polarization far
-
field m
easurement setup
Figure. S
1
:
(a)
Schematic of the
cross
-
polarization far
-
field imaging
system used for measuring the plasmon
mode properties
.
Far
-
field CCD camera image (
b
) with and (
c
) without uncollimated light irradiating onto the
sample. The input beam is carefully aligned to the nanoantenna for effic
ient excitation through the illumination
scheme
.
A
high
-
magnification
cross
-
polariza
tion far
-
field imagi
ng setup is used for
characterizing
the optical
properties of the propagating plasmonic mode
.
1,2
A tunable near infrared laser
is directed through a
polarizer and a 50/50 non
-
polarizing beam splitter. The
collimated
light
is then
focused
using a
100x
0.9
NA
microscope
objective
onto
the nanoantenna to excite the gap
-
plasmon mode
of the waveguide (Fig.
S1a)
. T
he diame
ter of the co
llimated beam was measured
as
FWHM = 1.1 μm
using
an InGaAs
CCD
camera and the effective numeric aperture of the experimental focal spot was dete
rmined for all
subsequent eval
uation steps.
The
output
signal is
imaged
in reflection mode
using the infrared
camera.
Since
the waveguide
is
designed with
a 90
°
bend, the linearly polarized emission from
the
output
of the
waveguide is
orthog
o
nal to
that of
the input
polarization
,
thus
suppressing back
-
reflection
s
coming
from
the incident beam
,
providing a good si
gnal
-
to
-
noise ratio for the
desired
output signal.
A
n
uncollimated
light source together with a
thin glass slide are
used
to irradiate
a
small amount of light onto the sample
for locating the waveguide and antenna positions. As show
n
in Fig. S1
b
,
c
, the in
put beam is
possible
to
be
align
ed
precisely to the nanoantenna using a xyz micro
-
position stage with the illumination scheme.
2
.
Tunable permittivity
of
Indium Tin Oxide (
ITO
)
Fig
ure
. S
2
:
Calculated complex permittivity of ITO for different carrier concentration of the material using
Drude
-
Lorentz model
.
To calculate the tunable permittivity of the
ITO
material
with different carrier concentrations
, we use the
Drude
-
Lorentz model
as descri
bed in
the
following,
where
ε
is the
ITO material
permittivity,
ε
∞
is the high frequency permittivity
(ε
∞
= 3.9 for ITO)
,
1,2
,
ε
o
is
the
free space permittivity,
ω
is the angular frequency
in rad/s
,
ω
p
is the plasma frequency, and
Γ
is the
electron scattering rate
,
m*
is the effective mass (
m
*=0.35
m
o
for ITO, where
m
o
is the rest mass of
electron)
,
3,4,
5
n
is the carrier concentration of the ITO, and
e
is the electron charge.
The calculated complex permittivity of ITO
is
shown in Fig. S2. It is clear from the figure that the
material dispersion shift
s
significantly to shorter wavelength with higher carrier concentration. The
plasma frequencies shift all the way from near
-
IR (
n
~10
19
cm
-
3
) to UV r
ange (
n
~10
21
cm
-
3
) depending on
the carrier concentration of the material.
Considering
the optical wavelength
of
λ
0
=15
50nm,
the real part
of permittivity can tune from positive (dielectric property) to negative (metallic property) with different
carrier concentrations, showing the promising tunablility of TCO for tunable plasmonic applications. Note
that the carrier concentrati
on of
the
ITO film can be tuned by changing the fabrication condition
s
, for
instance the oxygen concentration during sputtering. To show the effect, we fabricated different thin film
s
of ITO (thickness = 300nm) onto a silica glass substrate with different
sputtering condition
s
(corresponding to
different
carrier
concentration
s
of
n
~ 10
19
,
10
20
and 10
21
c
m
-
3
). As seen in the inset of
Fig. S2b, different colors are observed from samples due to the different absorption of the materials,
indicating the strong
tunability
with carrier concentration of material. Instead of changing the fabrication
condition, we tak
e use the tunable optical properties of TCO by applying
a
gate voltage to actively change
the carrier concentration for developing efficient plasmonic modulation as discussed in the main text.
3.
IV measuremen
ts
for
determining
the breakdown field
s
of the
oxide layers
Figure. S
3
:
(a)
Schematic of the IV measurement for determining the breakdown fields of different thicknesses
of
Al
2
O
3
insulation layers
.
(b)
Measured IV curves for
Al
2
O
3
with thickness of 2 nm, 5 nm and 10 nm
.
It is important to determine the breakdown voltages for various thicknesses of the Al
2
O
3
layer until a
current passes between the ITO and gold layers
(Fig. S3a)
. To determine this quantity, a layer of gold
(200 nm) is deposited onto a SiO
2
substrate by Ebe
am deposition, and an atomic layer deposition (ALD)
machine was used to deposit a smooth, thin layer of Al
2
O
3
on a small region of the sample
(area ~
4
mm
2
,
on top of which a layer of ITO (thickness = 220
nm) was placed through sputtering. As
shown in Fig.
S3a,
probes were placed on the gold and ITO layers to produce a voltage difference across the layers and
to
measure the current. The breakdown voltages, as determined from taking IV measurements for
different Al
2
O
3
thicknesses, increase with increasing th
icknesses as expected (
Fig. S3b
). The breakdown
electric fields for the 2, 5, and 10 nm layers are 6.25, 6, and 4 MV/cm respectively, which are in the same
ord
er of magnitude as the value reported elsewhere
.
6
It should be noted that for large area of Al
2
O
3
(e.g.,
>
1
cm
2
), there is
a
possibility of pinholes in the
Al
2
O
3
from the ALD, thus allowing conductivity between
the layers
of gold and ITO
.
With the small waveguide dimension (~ 25 x 25 μm
2
) as discussed in the main
text (inset of Fig. 3), the Al
2
O
3
pr
ovided a good electrical inso
lation, thus field
-
effect dynamics can be
used for efficient modulation.
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