of 13
1
G
ate
-
tunable
conducting oxide
metasurfaces
Yao
-
Wei Huang
1
,
,
Ho Wai Howard Lee
1
,2
,
,+
,
Ruzan Sokhoyan
1
,
Ragip Pala
1
,2
,
Krishnan Thyagarajan
1
,2
, Seunghoon Han
1
,3
,
Din Ping Tsai
4
,
and
Harry A. Atwater
1,
2
,*
1
Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena,
California 91125, United States
2
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United
States
3
Samsung Advanced Institute of Technology
,
Samsung Electronics
,
Suwon
-
si, Gyeonggi
-
do 443
-
803
,
Republic of Korea
4
Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
Abstract
Metasurfaces composed of
planar arrays of
sub
-
wavelength artificial structures show promise for
extraordinary
light
manipulation
; they
have yielded
novel
ultrathin optical components such as
flat
lenses, wave plates,
holographic surfaces and
orbital angular
momentum manipulation and
detection
over a broad range of el
ectromagnetic spectrum
. However the
optical properties of
metasurfaces
developed to date
do not
allow
for
versatile
tun
ability
of reflected or transmitted
wave amplitude and phase after fabrication,
th
us
limiting
their
use
in a wide range of
applications
.
Here, we
experimentally
demonstrate a
gate
-
tunable metasurface that enables
dynamic
electrical
control of the
phase
and
amplitude
of the plane wave reflected from the
metasurface.
Tunability
arises from
field
-
effect
modulation
of the complex
refractive index of
conducting oxide
layers
incorporated into metasurface
antenna elements which are configured
in
a
reflectarray
geometry
.
We measure a
phase shift
of
π
and
~ 3
0% change in the
reflectance by
applying 2.5 V gate bias.
Additionally,
we dem
onstrate
modulation at frequencies exceeding
10
MHz
, and electrical switching of +/
-
1 order diffracted
beam
s
by
electrical control over subgroups
of metasurface elements
, a bas
ic requirement for electrically
tunable beam
-
steering
phased array
metasurface
s
.
The
proposed
tunable metasurface
design with
high optical quality and
high speed
dynamic phase modulation suggest
s
applications in next generation
ultrathin
optical components
for imaging and sensing technologi
e
s
, such as
reconfigurable beam steering devices,
dynamic
holograms, tunable ultrathin lens,
nano
-
projectors, and nanoscale spatial light modulators.
Importantly, our design allows complete integration
with electronics and hence electrical
addressability of individual metasurface elements.
Introduction
Metasurfaces are a
rrays of subwavelength
elements
in which
each element is configured to
control
the
phase
and amplitude
of the
transmitted, reflected, an
d
scattered light
1
-
3
.
By
controlling the phase
shift
and amplitude
change
imposed by
each
metasurface
element, phased
arrays are in principle
achievable
that would enable complex wavefront engineering
.
In the
last
four
years
,
sign
ificant
advances have occurred
,
resulting in
a generalized form of Snell’s law
with ordinary and anomalous refrac
tion, metasurfaces that refract and focus light,
enabling
applications such as
holograms
4
-
7
, o
ptical vortex generation/detection
, u
ltrathin focusing lens
,
p
hotonic
spin Hall effect
8
-
11
, among others
.
Metasurfaces can also
control beam polarization,
yi
elding linear
ly
, elliptical
ly
, circular
ly
or radial
ly
polariz
ed
light
4
,
12
.
Due to the flat nature of
2
metasurfa
ces (typical thickness < 100nm), conventional three
-
dimensional optical elements such
as prisms or lenses can be replaced by their flat, low
-
profile
analogs
.
A
n
important practical
consequence of the fl
at geometry is that
metasurfaces
are straightforwardly fabricated
via
planar
lithographic processing.
Passive
metasurface
s
have been demonstrated
in the
spectral
region
rang
ing
from visible to
microwave
frequencie
s
.
An as
-
yet unrealized milestone in
the
field
is
to achieve an
actively
tunable metasurface
with arbitrary control of phase and amplitude of individual antenna elements,
by
post
-
fabrication electrical modulation which would enable dynamical wavefront control in
thin
flat
optical devices
,
such as
dynamical beam steering, reconfigurable imaging,
tunable
ultrathin lens,
and high capacity data storage.
There have been number of attempts to actively
control the overall response of the metasurface by using various physical mechanisms
13
-
22
.
It has
been shown that one
can actively control transmittance or reflectance of the impinging light.
However
to date there has been no
comprehensive
experimental validation of
the promise for
dynamical
control of
both the
wave
phase
and
the
amplitude
either in reflection
or
transmis
sion
from
metasurfaces operating in the visible or near
-
IR
.
Moreover,
i
t is worth mentioning that
previous works on tunable metasurfaces
show active control
of amplitude
over overall
response
of
the metasurface
without
addressing the issue of individual control of metasurface elements.
Here, we experimentally
demonstrate independent electrical addressability to subgroups of
three
metasurface elements
. This results in electrical switching of +/
-
1 order diffracted beams. Our
design, in principle, allows
addressing individual metasurface elements
and complete integration
with electronics.
Among various
physical
mechanisms
for modulation of complex refra
ctive index
, f
ield
-
effect
modulation
is a distinctly attractive approach because of
the
combined advantages of
high speed
modulation of large numbers of individual me
t
a
surface
elements
and
extremely low power
dissipation.
Field
-
effect modulation is ubiqui
tous in semiconductor electronics, and is the
principle underlying contemporary low power integrated circuit performance. B
ased on
formation of charge depletion or accumulation regions
in
doped
semiconductors
(
e.g., metal
-
oxide
-
semiconductor field
-
effect
transistor
s
and thin
-
film transistor
s
)
field effect modulation can
provide a
sufficiently large
carrier density
change in
a
heavily
-
doped
semiconductor
or
conducting oxide,
which
resul
ts
in
a large
variation of
its
complex
refractive index in the charge
ac
cumulation or depletion regions
23
,
24
.
This phenomenon has been used to demonstrate
electrically controlled
plasmonic
amplitude
modulator
s
in a
metal
-
oxide
-
semiconductor (
MOS
)
configuration,
where coupling of modulator
output power
into a
waveguide
is controlled by an
electrical bias
across the
metal
-
semiconductor
field effect channel
23
-
25
.
T
ransparent conducting
oxide (TCO)
materials
26
,
27
,
such as
indium tin oxide (ITO)
,
have also
been used
as
the active
semiconductor layer
23
,
25
,
28
-
31
. The
results
reported by the
se authors
suggest
that
applying
an
electrical bias between metal and ITO
changes
the sign of
ε
r
,
the
real part of the dielectric
permittivity
of
ITO in the accumulation layer
,
from positive to negative. When
|
ε
r
|
is in
the
epsilon
-
near
-
zero (ENZ)
region,
that is
1<
ε
r
<1
, a
large electric field enhancement
occurs
in the
accumulation layer for
near
-
infrared
wavelength
s
21
,
25
,
32
,
33
, providing an efficient way to
electrically modulate
the
optical properties of nanophotonic devices
with high modulation speed
and low power consumption
.
Here we integrate field
-
effect tunable materials with metasurfaces to demonstrate a
dynamically
tunable
metasurface that allows
for the first time
active control of
both
reflected light
phase
and
ampli
tude
at
near
infrared wavelengths
for applications such as dynamic beam steering.
The
metasurface we study
consists of a
gold back
plane
, an ITO layer followed by an
aluminum
3
oxide
layer on which we pattern
a
gold nanoantenna
array
(Fig. 1
a
).
The identical antennas are
connected either to right or left external gold
electrodes
to
create
electrical
gates
.
Unlike
previous optical frequency
metasurface
s
which
utilized variations
in
the
antenna geometry or
orientation to
impose different phase shi
f
ts for each antenna element
, the metasurface we study
here is periodic
, and
a dynamic control over the
phase shift by
each
metasurface element
is
achieved by appl
ying
bias voltages to
combinations of
adjacent antenna
electrodes
.
Each
metasurface
antenna
e
lement is effectively
an MOS capacitor
with t
he
Au
antenna
serving as a
gate
and ITO
functioning as a field effect channel
(
Inset of
Fig. 1
a
).
When applying
an
electrical
bias between
the
antenna
gate
and
the underlying groun
d
plane,
the carrier
concentration at the
Al
2
O
3
/
ITO interface
increases
or decreases
by forming
a
charge
accumulation
or depletion layer
.
This results in modulation of the complex permittivity of ITO, which alters the interaction of the
incident light with the antenna and modu
lates the reflection from the surface. The variation of the
reflected phase and amplitude at each antenna element is amplified when the real part of the
dielectric permittivity in the ITO accumulation layer changes its sign from positive to negative
(Fig 1
B). Our field effect electrostatics calculations indicate that this condition is satisfied when
the background carrier concentration in ITO is
N
0
= 3
×
10
20
cm
-
3
, with a 5
-
fold increase in the
carrier concentration at the Al2O3/ITO interface
under an
applied bias increase from 0 to 6 V.
Figure
1
b
(bottom)
shows the calculated
ε
r
for different applied biases as a function of distance
from
the
Al
2
O
3
/ITO interface at
a
n operation
wavelength of 1500 nm. The dielectric permittivity
of ITO substantially cha
nges over
the region within
1.5
nm
of the Al
2
O
3
/ITO interface
due to the
formation of accumulation layer
. The gray area highlights the ENZ region of ITO
where
ε
r
acquires values between 1 and
-
1
.
Under
a
positive
bias, the value of
ε
r
at
the
Al
2
O
3
/ITO
interface decreases
,
reaching
the
ENZ
condition
at
an
applied bias of 2.5
V. The thickness of
the
ENZ region
can be estimated as
0.7 nm at applied bias of 6
V. Importantly, when the ENZ
condition holds,
a
large electric field enhancement
is generated
in t
he accumulation layer of ITO.
This can be readily understood from the boundary condition imposing continuity of the normal
component of electric displacement at the
field effect dielectric/channel
interface
25
,
32
,
33
.
A photographic image of the final device
,
fabricated by multilayer deposition and e
-
b
eam
lithography
is shown
in
Figure
1
c
.
One can visually
distinguish the
Au back
plane
, ITO,
electrical connections, and
pads
(
for further fabrication details
,
see
S
upplementary
section S1
)
.
Scanning electron microscope
images of
a
stripe antenna structure are
depicted
in Fig
s
. 1D and
E
.
Adjacent stripe
a
ntennas are
connected electrically
in groups of
t
hree
so that each group is
connected to a different external
gold
pad
.
Th
e
electrical pads are
wire
connected to a compact
chip
carrier
and circuit board
for
electrical
gating
.
C
oupling of ENZ resonance via conducting oxide field
-­‐
effect
.
Using finite element
electromagnetic simulation
method
s
, we simulated reflect
ance
and phase
modulation of the periodically patterned antenna structure under normal
incidence
illumination
with
a transverse magnetic (
TM
)
polarization (H
-
field along the stripes)
.
In our simulations we
considered an array Au stripes of width,
w
= 230 nm, t
hickness,
t
= 50 nm, and periodicity of
p
=
400 nm. The stripe array is on a 5 nm Al
2
O
3
layer placed above a 17 nm ITO layer and a 100 nm
Au back plane
(Fig. 1a
)
.
Figure
s
2
a
and
2
b
show the reflection and phase shift spectra as a
f
unction of applied voltage.
The
black
dashed lines indicate the ENZ region
in the accumulation
layer of ITO
at the Al
2
O
3
/
ITO
interface
,
and
the green line
marks
the
position of
a reflectance
dip
corresponding to the
magnetic
dipole
plasmon
resonance
. With
increasing gate
bias
,
the
magnetic dipole plasmon resonance
couples to the ENZ
region
in the
ITO
accumulation layer
which shifts the
resonance
and
induces
a
significant phase
shift
in reflection
.
4
T
he
calculated
phase
as a function of wavelength
under
an
applied bias from 0 V to 5.5 V is
depicted
in Fig.
2
c
.
As one can see,
t
here are
two
different regimes describi
n
g
coupling
of the
plasmonic resonance with
the ENZ
region
in
ITO.
For an increasing bias from 0 to 2.5V, the
plasmonic resonance shifts to shorter wavelengths due to an increase in the carrier concentration
and reduction of
ε
r
. As the resonance blue shifts, the phase shift at 1500 nm increases. In the case
of applied bia
s of 3.5 V,
ε
r
becomes zero, and applying a higher voltage, the plasmonic
resonance shifts
to longer wavelengths
. This change in the sign of the resonant wavelength shift
can be intuitively understood by looking at the behavior of the optical properties of
the ITO
layer. With increased bias, the carrier concentration in the ITO layer increases and starts
functioning optically as a metal, thus shrinking the thickness of dielectric spacer. Interestingly,
for applied biases exceeding 3.5 V, phase of the reflec
ted beam decrease with increasing
wavelength as opposed to the case of smaller applied bias (0
2.5 V), which shows an increase
in the reflection phase with increasing with wavelength. Therefore there is a resulting phase shift
larger than 180 degrees at
an excitation wavelength of 1500 nm.
To
gain further insight
into
the coupling of
the magnetic dipole plasmon resonance
with
the
ENZ
region
of
the
ITO, the distribution of electromagnetic field at different applied voltages
is
simulated
(
Fig.
2
d, e
)
. At 0
V, ITO
optically
behave
s
as
a
dielectric
, thus
the resonance shows a
distribution of anti
-
parallel electric field and forms magnetic dipole between Au antenna and Au
back plane
.
For
the applied
bias
larger than 2.5
V
,
when
the ENZ condition holds in the
ac
cumulation layer of the
ITO, the enhancement of the
z
-
component of the electric field (
E
z
) in
the accumulation
layer
is
observed (see
second
image
in Fig. 2
e
)
due to
continuity of the
normal
component of electric displacement (
ε
E
) at the interface of
the
two media
. In case of
applied
voltage
of
3.5
V where 1 >
ε
r
> 0 in the accumulation
layer
,
E
z
is enhanced
in a direction
parallel
to
the field
in the Al
2
O
3
and
bulk
ITO (
second
image in Fig. 2
e
). In contrast,
for
applied voltage
of 5.5
V (
third
image
in Fig. 2
e
), since the permittivity of ITO in
some region of
the
accumulation layer
is
negative
(metal
-
like), the
E
z
component
is
antiparallel to
the field
in the
Al
2
O
3
and
bulk
ITO
layers
.
The
enhanced parallel and antiparallel
E
z
due to coupling
to
ENZ
region
further
modifies
the strength of magnetic dipole
and
phase shift
profile
as
can be
seen in
Fig.
2
e
, resulting in the large phase modulation via electrical gating
.
Results of tunable conducting oxide metasurfaces
The
reflectance spectra
and
the
metasurface
phase
shift
are measured
for
different applied biases
as
shown in
Fig.
3
(
c.f.,
S
upplementary
section S
2 for measurement setup
)
.
In
Fig.
3
a
, w
e
observe
that the
resonant reflectance minimum
shift
s
to shorter wavelength
s
when increasing
voltage
as expected from the simulation result
s
(
Fig. 2
a
)
.
The
reflectance
change (normalized
to
the
reflectance
without applied voltage
) is
as high as
28.9
%
for operating wavelengths
near the
resonan
t reflectance minimum
under an applied bias of 2.5 V
(
Fig.
3
b
)
,
indicati
ng
gate
-
actuated
metasurface
reflect
ance
amplitude
modulation.
To measure the phase
shift
,
a
Michelson
interferomet
er
wa
s used
to observe
interferomet
ric
fringes
where the incident beam is
positioned
at the edge of the metasurface so that half of the beam is
reflected
from the metasurface and the
other half is reflected from the bottom gold
back plane
, which acts as a built
-
in phase reference
(
S
upplementary
section S3
-
4
)
.
The
phase shift measurement
la
ser wavelength
w
as
λ
=
157
3
nm.
Phase shifts
were retrieved
at
different
applied
voltages
by fitting and analyzing the interference
fringes (
S
upplementary
)
.
T
he results are shown in
Fig.
3
c
.
It is clear that t
he phase shift increases
with
applied
bias
.
A phase
change
of ~184
degrees is observed
at applied bias of
2.5
V
, which is
in a
good
agreement with simulat
ion
result
s
(
i
nset of
Fig.
3c
)
.
The slight discrepancy may be
attributed to the small geometry
,
dielectric constant
, and the intrinsic material properties (e.g.,
5
work function and effective mass of materials)
difference between the simulated and
experimental structures.
We also
applied
n
egative bias
voltages
which,
in contrast to the positive
bias
voltages
, further dep
lete
the
ITO at the ITO/
Al
2
O
3
interface. In case of
a
negative
applied
bias,
a
red
-
shift
of the
resonan
t reflectan
ce
minimum
is observed, indicating that we can both
increase and decrease
ε
r
near
the
ITO/
Al
2
O
3
interface by applying positive or negative bias
es
,
respectively.
This excludes alternative interpretations of the observed resonance shift
mechanisms
including but not limited to
thermal
(
Joule
)
heating
(
S
upplementary
section S
8
)
.
We
note that the 5 nm Al2O3 layer exhibits electrical breakdown at
2.5
3 V.
To characterize t
he response
frequency
of
our
tunable metasurfaces
,
a
2
V
bias
with
0.5
-
10
MHz
frequency
was applied to
the
sample and
a high speed
InGaAs
detector
was used to detect
the
high
frequency metasurface
reflectance
.
To
ensure high enough reflected light intensity, the
width of the
antenna
wa
s carefully designed such
that the resonan
t reflectan
ce
minimum
is
located at ~ 1650 nm
so
we
can
record
a
sufficien
t
ly high
light intensity at the
telecommunication wavelength
s
(
S
upplementary
section S
9
)
.
As shown in the inset of
Fig.
3d
,
high speed
reflectance
modulation is observed by applying 2
V of 500
kHz AC signal at the
wavelength of 1515 nm
(blue curve)
.
M
odulation speed
s
as high as
10
MHz
were
demonstrated
(see
Fig.
3d
)
. Note that the waveform is
distort
ed
at the highest frequencies
due to the detector
bandwidth
.
T
he
metasurface
at
2
MHz
frequency for
different
applied
biases
was
also
investigated and
a
~ 15
% of
reflectance
amplitude modulation is obtained with
a
2
V applied
voltage
(
S
upplementary
section S
9
)
.
The
maximum
speed of the modulation can be estimated using a simple device physics
calculation, which gives a capacitance value of 14 fF/
μ
m
2
per u
nit area
(see
S
upplementary
section S5)
. For
our
antenna array
, this would yield capacitance and resistance values of 140 fF
and 100
Ω
per wire
(area = 50
μ
m x 0.2
μ
m)
respectively. Such a small capacitance e
nables
modulation speeds up to 11
GHz and switching energies as low as 0.7 pJ/bit
(this estimate does
not include other sources of signal delay resulting from the wiring and RF probe connections)
.
Note that the
individual antenna
can be implemented in a 2
-
dimensional antenna array with a
smaller footprint (0.2
μ
m x 0.2
μ
m) and capacitance (0.5 fF).
With
10
0
Ω
resistance
per pix
el
would
enable
2.5 fJ
switching energies
per bit
,
and up to 3 THz modulation speeds
(although the
speed
could be
limited
by
interconnect with current speed up to ~ 100 GHz)
34
.
We
also
anticipate
that the
modulation characteristics of the metasurfaces
could be
further
improved by using
alte
rnative high
-
k gate dielectrics
(S
upplementary section S13).
As demonstrated in both experimental and simulated re
sults, a large phase change
(
>
180
degrees
)
can be achieved. We further employ the tunable phase shift to
develop a
n
electrically
driven
dynamic phase grating.
The
voltage
-
dependent
far
-
field intensity profiles of a metasurface
phase grating with 64 unit cells consisting of identical patch antennas on
Al
2
O
3
/ITO/Au
planar
layers
were simulated
.
Fig.
4
a
shows the far
field diffracted beam profile of
a
2
-
level phase
grating with a period
icity of
Λ
=
2.4
μm.
At applied bias
of
0
V, the diffracted beam shows
a
direct
ional
reflectance
from
the sample.
While at
the applied bias of
3.5 V,
t
he 2
-
level phase
grating creates two symmetric first
-
order d
iffracted beams
with maximum intensity
at
angles of
-
39 and 39
degrees due to
the
spatial symmetry
of the structure
.
The experimental
ly
measured far
-
field diffracted
beam
intensity is depicted in
Fig. 4b
. It can be seen that the ± first
-
order
diffracted beam
s
appear with applied voltage ~> 1.5
V w
hile the zero
-
order diffracted beam
intensity
reduc
es
with
increasing
voltage
that
agree
s
with the simulation
results
(Fig. 4a
). These
results
manifest electrical tunability of reflection phase at a patch antenna level, which forms a
6
fundamental basis for
tunable optical
phased array metasurfaces.
Note that the slight discrepancy
on the diffracted angles and beam widths between the simulated and measured results can be
attributed to the non
-
parallel
incident light excitation from the high NA objective and t
he slight
non
-
uniformity of the sample.
Importantly, t
he diffracted beam
angle
can
also
be varied by
gating multiple antenna
with
different periodicities
Λ
for steering diffracted beam angles
(Fig. 4
c
-
e
)
. As shown
in Fig. 4c
,
gating of 4, 3, 2
antennas periodically will lead to the grating
periodicity of 3.2, 2.4, 1.6 μm, respectively (separation between each antenna is 400 nm), leading
to the ability of steering diffracted beam angles
(see schematics in Fig. 4
c and d
)
. To show the
tunability
of the diffracted angle for the first order diffraction, the far
-
field intensity of the first
order reflected beam from the metasurface as a function of the incidence diffraction angles are
simulated by gating the metasurface with 3.5 V (Fig.
4e
). It is cl
ear from the figure that the
reflected beams are effectively steering to
wide range of
angles
(> 40
o
) by individually gating of
4, 3, 2
antennas
to
demonstrate
the efficient nanoscale beam steering device.
T
h
ese
efficient
wide
-
angle electronically tunable
beam steering
components
are
necessary
for the development
of next generation ultrathin on
-
chip imaging and sensing
devices
, such as high resolution LIDAR
devices and nanoscale spatial light modulator
s
.
Conclusions
In conclusion,
we present
a comprehensive
experimental demonstration of
a
tunable metasurface
in
near
infrared
wavelength region
. We control the phase and amplitude by gate
-
tunable
conducting oxide field effect dynamic
al permittivity
modulation
.
A phase shift of
184
degrees
and reflectance change of ~
3
0% were measured by applying 2.5 V gate bias
.
A modulation
speed
of up to 10 MHz
(with potential modulation speed up to 11 GHz)
and electrical beam
-
steering of +/
-
1 order diffracted
beam
s were
also demonstrated.
In addition to the fundamental
interest of tunable metasurfaces, these structures have many
potential
applications
for future
ultrathin optical components
, such as dynamic holograms, tunable ultrathin lens, reconfigurable
beam ste
ering devices, nano
-
projectors,
and
nanoscale spatial light modulators
.
Acknowledgments
This work was supported by Samsung
Electronics
. The conducting oxide material synthesis
design and characterization was supported by the U.S. Department of Energy (DOE) Office of
Science grant DE
-
FG02
-
07ER46405 (K.T. and H.A.A.), and
used facilities supported by the
Kavli Nanoscience Institute (KNI) a
t Caltech. Y.W.H. and D.P.T. acknowledge the support from
Ministry of Science and Technology, Taiwan (Grant numbers: 103
-
2911
-
I
-
002
-
594 and 104
-
2745
-
M
-
002
-
003
-
ASP
).
K. T. acknowledges funding from the Swiss National Science
Foundation (Grant number: 151853
).
Author contributions
Y.H.,
H.
W.L., R.S
., and H.A.A.
designed
and conceived
the experiments.
Y.H.
and
H.W.L
.
fabricated the samples.
Y.
H. and
H.W.L. develop
ed
the measurement setup.
Y.H., H.W.L.,
and
R.P.
performed the experiments.
Y.H., H.W.L.,
and
K.T. performed
materials characterizations
.
Y.H.
and
R.S. performed
numerical
simulations.
Y.H., H.W.L., R.S.,
R.P., K.T., S.H.,
and
H.A.A.
wrote the paper. All authors discussed the results and commented on the manuscript.
These authors contributed equally to this work.
* Corresponding author.
E
-
mail:
haa@caltech.edu
+ Current address
: Department of Physics, Baylor University, Waco, Texas 76798, United States
;
Institute of Quantum Sci
ence and Engineering
,
Texas
A&M,
College Station, TX 77843
-
4242,
United States
7
Competing financial interests
The authors declare no competing financial interests.