1
Electro
-
Optically Tunable Universal Metasurfaces
Ghazaleh Kafaie Shirmanesh
1
,
Ruzan Sokhoyan
1
,
Pin Chieh Wu
1,2
,
and Harry A. Atwater
1,3
*
1
Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125,
USA
2
Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan
3
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, USA
*
Corresponding author:
haa@caltech.edu
Molding the flow of light at
the
nanoscale has been a grand challenge of nanophotonics for decades.
It is now widely recognized that metasurfaces represent a chip
-
scale nanophotonics array technology
capable of comprehensively controlling the wavefr
ont of light via appropriately configuring
subwavelength antenna elements.
Here, we demonstrate a reconfigurable metasurface that is
universal, i.e., notionally capable of providing diverse optical functions in the telecommunication
wavelength regime, usin
g a compact, lightweight, electronically
-
controlled array with no moving
parts. By electro
-
optical control of the phase of the scattered light from identical individual
metasurface elements, we demonstrate a single prototype universal programmable metasurf
ace that
is capable of both dynamic beam steering and reconfigurable light focusing using one single device.
Reconfigurable universal metasurfaces with arrays of tunable optical antennas thus can perform
arbitrary optical functions by programmable array
-
le
vel control of scattered light phase, amplitude,
and polarization, similar to dynamic and programmable memories in electronics.
R
apid
advances in
control of
the
phase and amplitude of
the
light scattered from planar
arrays of nanophotonic elements
has sti
mulated
the development of metasurfaces that utilize
amplitude/
phase
-
sensitive scattering to enable wavefront engineering
1, 2
.
Metasurfaces are also
now de
monstrating some of their potential applications in
compact, high
-
performance, and low
-
cost optical devices
and
components
,
creat
ing
burgeoning interest in photonic integration.
To date
,
metasurfaces have
mostly
been designed in an application
-
specific man
ner and the design process
result
ed
in bespoke architectures tailored to particular applications
. Dynamical control of the
properties of the scattered light
is
possible by using tunable metasurfaces, for which external
stimuli such as
electrical biasing
, o
ptical pumping,
heat
ing
, or elastic strain can give rise to changes
in the dielectric function or physical dimensions of the metasurface elements
3
, thereby modulating
the antenna phase and amplitude response. Among these mechanisms, electrical tuning has been
pro
ven to be a robust,
high speed
, energy
-
efficient and reversible scheme for tuning active
metasurfaces
4
-
11
.
The ability of metasurfaces to spectrally, temporally, or spatially manipulate the wavefront
of light
with
very high spatial resolution, is expected to accelerate miniaturization of optical
devices and integration of optical systems
12
.
However, i
n spite of advances in active metasurfaces
to date, universally reprogrammable metasurface components have not yet been demonstrated.
Realization of a single hardware device that can provide multiple and
--
ind
eed general
--
functions
would further accelerate the impact of metasurfaces and their applications. Such universality can
be found in
electronics technology
that
has benefitted from development of programmable and
reprogrammable circuits
composed of identi
cal circuit elements,
such as
dynamic
13
and
static
14
random access memories and field
-
programmable gate arrays
15
.
In
this
paper, we demonstrate a
state
-
of
-
the
-
art
prototypical ‘universal’ metasurface which can be electronically progr
ammed to
2
achieve
two of the most
essential
functions identified to date for metasurfaces, namely, beam
steering and focusing of light
.
Optical beam steering is the key element of a broad range of optical systems such as
light
detection and ranging (
L
i
DAR
)
16
, optical
interconnects
17
, and optical
communications
18
.
Conventional beam steering devices such as
Risley
prisms
19
, galvanometer
-
scanning mirrors
20
,
and decentered lenses
21
employ
mechanically mov
ing
optical
components
to steer the incident
light. Although mechanical beam steering systems provide wide steering angular range and large
number of resolvable beam directions, they suffer from low
steering speed due to the
inertia
of
their moving parts
and
the weight of their
mechanical
components
22
.
The availability of electronic
beam steering arrays at
near infrared
(
NIR
)
wavelengths with scanning frequencies
above
the MHz
range
could replace mechanical components with compact
and
lightweight optoelectronic
alternatives and enable new functions unachievable via mechanical motion.
Reconfigurable metasurfaces have recently been employed to provide dynamic beam
steering in the mic
rowave and NIR regimes by
exploiting
microfluid
ic flow
23, 24
,
incorporation of
phase
-
change materials
25
,
and
reorientation of
liquid crystals
26
.
However, the performance for
these
devices
is limited by low (~kHz) switching speeds
. Alternatively, electro
-
optic modulation
in multiple
-
quantum
-
well
resonant
metasurfaces
27
, an intrinsically ultrafast process, has been
shown to provide high
-
speed dynamic beam steering, but to date
,
a limited phase modulati
on range
has constrained the achievable beam directivity and steering angle range.
Electro
-
optically controllable
beam switching
ha
s
also been demonstrated via
incorporation of transparent conducting oxides as active media into
the
metasurfaces
4, 28, 29
.
However, individual control over each metasurface element, which is required for more complex
phase distribution patterns, ha
s
not been reported.
Other researchers have demonstrated beam
steering using waveguide
-
bas
ed thermo
-
optical phase shifters
coupled to antennas
30
-
34
, or by
employing
f
requency
-
gradient
metasurfaces
35
.
These chip
-
based antenna arrays can enable beam
steering at
visible or
infrared
frequen
cies, but are application
-
specific, and
hence,
has been
unable
to achieve more general array functionalit
ies
.
L
ight focusing
is a
nother
paramount
optical
function
that
plays a
fundamental
role
in
almost every optical
system
such as
imaging, microscopy, op
tical data storage
,
and
optical
encryption
36
.
Metasurfaces have given rise to versatile
meta
lenses that can replace bulky
conventional lenses by engineering abrupt phase delays
i
ntroduced by
individual metasurface
elements and
phase gradients across
antenna
array
s
37
-
39
.
I
n order to focus light
using metasurfaces
,
the spatial variation of
field
amplitude
or
phase distribution has to be
care
fully controlled
over
arrays of
resonant
elements at approximately wavelength
-
scale or smaller spacing
.
Metalenses
have demonstrated the
capability
to perform high
-
resolution imaging, wavefront shaping for
aberration correction, and polarization conversion
1, 2, 40
.
Reconfigurable
metasurfaces have been utilized to realize dy
namic focusing
by variation
of the overall lens optical thickness or curvature, via
liquid
crystal
reorientation
41
, microfluidic
flow
42, 43
, or
elastic deformation
44
.
We note these modes of dynamic focusing do not permit precise
tailoring of the lens focal properties by arbitrary phase control of the lens elements.
Alternatively,
electro
-
thermo
-
op
tically controllable lenses have been proposed to precisely engineer the optical
wavefronts at the microscale
45
. However, such a
d
ynamic wavefront control
at the subwavelength
scale, an essential requirement to achieve high resolution imaging, has not yet been
reported
.
Here, we design and
demonstrate
a
universal
electro
-
optically tunable metasurface
that can
exhibit
multiple functions
i
n the
NIR
wavelength
regime
using
a single
device
, via precise tailoring
of the phase profile of an optical aperture
.
Fig
ure
1
a
schematic
ally
illustrate
s
this
metasurface
,
3
whose
independently
addressable elements
enable
dynamic
control
of
t
he wavefront
via
a pixel
-
by
-
pixel reconfiguration
. Using this scheme
,
we demonstrate a reprogrammable metasurface
whose function can be reconfigured
between dynamic beam steering and
dynamic
cylindrical
metalens
,
achieving
a reconfigurable
focal length and
numerical aperture
by tuning
the
gate
voltages applied to individual metasurface elements.
Figures
1
,
b
and
c
schematically illustrate the building blocks of our tunable gated field
-
effect metasurface, consisting of an Au back
-
reflector, on top of whic
h an Al
2
O
3
layer is deposited.
The Al
2
O
3
layer acts as a dielectric spacer, adding a degree of freedom for the metasurface optical
mode profile design. This layer is followed by deposition of an
indium
-
tin
-
oxide (ITO)
layer, a
gate dielectric
,
and Au ‘fish
bone’ nanoantennas. The fishbone nanoantennas are comprised of
patch antennas that are connected together by Au stripes, which also serve as gate voltage control
electrodes.
The gate dielectric is a hafnium/aluminum oxide nanolaminate (HAOL), a hybrid
mate
rial that simultaneously exhibits high breakdown field and high DC permittivity
6
. We apply
a DC electric bias between the ITO layer and the nanoantennas.
This cau
ses the ITO layer to
undergo a reproducible
field
-
effect
-
induced
index change.
By altering the applied electric field,
one can modulate the
ITO
charge carrie
r density close to the interface of the ITO and the gate
dielectric. By further increasing the appl
ied bias, the real part of the dielectric permittivity in an
accumulation layer located within ITO takes values between
-
1 and +1, yielding an epsilon
-
near
-
zero (ENZ) condition. In the ENZ regime, the ITO layer permittivity is varied at NIR wavelengths
by
changing the applied DC bias
.
The width and length of the antenna, and the width of the
electrode are designed so that
a magnetic dipole plasmon resonance occurs at
the
wavelengths
coinciding with the
ENZ regime for ITO,
operating in the telecommunication
wavelength regime.
As a result
of the
spectral overlap of the ENZ regime of
ITO and
the geometrical resonance
of the
metasurface
,
t
he metasurface
is expected to
exhibit large phase modulation
.
Figure 1
.
a,
Schematic of the universal metasurface whose fun
ctionality can be switched between dynamic
beam steering and cylindrical metalens with reconfigurable focal length.
Schematic of (
b
) periodic array
and (
c
) unit cell of the antenna elements. The metasurface is composed of an Au back
-
reflector, an Al
2
O
3
die
lectric layer, an ITO layer, and a hafnium oxide/aluminum oxide laminated
(HAOL)
gate dielectric
V
1
V
2
V
25
V
96
θ
1
θ
2
θ
3
f
1
f
2
f
3
Back reflector
Antenna
Al
2
O
3
ITO
HAOL
Electrode
V
x
y
z
b
c
d
f
a
c
f
e
Voltage (V)
-
6
-
4
-
2
0
2
4
6
Voltage (V)
-
6
-
4
-
2
0
2
4
6
Voltage (V)
-
6
-
4
-
2
0
2
4
6
4
followed by an Au fishbone antenna. The periodicity of the metasurface is 400 nm, and the thickness of the
back
-
reflector, Al
2
O
3
, ITO, and
HAOL
layer
s
are 80 n
m,
9.5 nm, 5 nm, and 9.5 nm, respectively. The width,
length, and the thickness of the antenna are 130 nm, 230 nm, and 40 nm, respectively and the width of the
electrode is 150 nm. Simulated (
d
) reflectance and (
e
) phase of the reflection from the metasurf
ace as a
function of applied voltage for different wavelengths.
f,
Measured reflectance (blue curve) and phase shift
(red curve) as a function of applied bias voltage
.
The operation wavelength of the fabricated device was
chosen to be
l
=1522 nm
such that a
phase shift of greater than 270
°
,
accompanied by a modest reflectance
change could be obtained.
Fig
ures
1
,
d
and
e
, respectively,
illustrate the simulated reflectance
and phase shift
as a
function of applied bias
at different wavelengths.
Here, phase shif
t is defined as a difference
between the phases of
the
reflected and incident plane waves calculated at the same spatial point.
As can be seen, when the external bias is changed, we observe a reflectance change that is
accompanied
by
significant phase mod
ulation. This demonstrates that both the real and imaginary
part
s
of the refractive index of the active region in the ITO layer are modulated by
the
applied bias.
Once we obtained the reflectance and phase shift spectrum of the designed metasurface under
a
pplied bias, we
can
then
pick the operation wavelength of the beam steering and focusing devices.
To accomplish
this, we
utilize the metasurface as a phase modulator
where
the reflectance
should
ideally
remain
constant
upon
chang
e in
the applied bias. Here
the operation wavelength
of
l
=
1510
nm
is chosen
so that we obtain a phase shift of higher than 270
°
while the maximum reflectance
modulation remains
as
modest
as possible
. After confirming th
is
tunable
response
,
we
experimentally obtained
the reflectance
and phase shift of
the fabricated
metasurface under applied
bias
. Fig
ure
1
f
illustrates the measured reflectance (
blue curve
) and phase shift (
red curve
) as a
function of applied bias
. In order to experimentally evaluate the
reflection
phase shift from the
metasurface, we used a Michelson interferometer system
6, 8
.
By focusing t
h
e incident laser beam
on the edge of the metasurface nanoantenna array
,
the scattered beam
is
reflected partly from the
metasurface and partly from the gold back
-
plane
, resulting in
a lateral shift in the in
terference
fringe patterns of the metasurface and
the bac
k
-
reflector when changing the applied bias
. By fitting
these two cross sections to sinusoidal functions, and obtaining the relative delay between the fitted
sinusoidal curves when changing the applied voltage, we could retrieve the phase shift acqu
ired
due to the applied bias.
As can be seen in Fig. 1f, a significant phase shift of 273.81
°
accompanied
by a modest reflectance modulation is attainable at the device operation wavelength by electrically
biasing the metasurface.
Once we validated the mod
ulation performance of the individual metasurface elements, we
investigated the metasurface array beam steering and focusing performance.
Scanning electron
microscopy (SEM) images of the fabricated metasurface nanoantennas are shown in Fig.
2a
.
In
our meta
surface device, n
anoantennas are electrically bus
-
connected together in one direction,
forming equipotential antenna rows, referred to here as a metasurface pixel. Then each pixel is
individually controlled by a separate applied gate voltage.
Fig
ure
2b
is
a photo
micrograph
of the
fabricated
array, consisting of
96 individually
-
controllable
and identical
metasurface
pixels
.
In
order to individually bias each of
these
metasurface
pixels
, we designed two printed circuit boards
(PCBs). The sample is mounted on
the first PCB (P
1
), and 96 individual metasurface elements as
well as 4 ITO connecting pads (to be used as the ground) are wire
-
bonded to 100 conducting pads
on the PCB. Figure 2c shows the first PCB with the multifunctional metasurface mounted on and
wire
-
bonded to it. Each conducting pad on P
1
is then connected to an external pin on the second
PCB (P
2
) that is shown in Fig. 2d. This voltage deriving PCB is capable of providing 100
5
independent voltages that can be individually controlled
through programming
a number of
micro
-
controller
s
by a
computer
.
Then
,
i
n order to characterize our universal metasurface, we used a custom
-
built optical setup
that
could
measure reflectance spectrum, phase shift, beam steering
profile
, and
focused beam profile
.
Figure 2.
a
, SEM image of the nanoantennas of the fabricate
d gate
-
tunable metasurface for the
demonstration of dynamic beam steering and a reconfigurable metalens. The scale bars from left to right
are 200
μ
m, 50
μ
m, and 500 nm respectively.
b
, Photographic image of the universal metasurface with 96
independently
addressable elements. Scale bar is
5
m
m.
c
, Sample mounting PCB to which we wire
-
bond
the universal metasurface pads (P
1
). 96 metasurface elements’ pads and 4 ITO pads are wire
-
bonded from
the sample to 100 conducting pads on P
1
. Scale bar is
10
mm
.
d
, Vol
tage deriving PCB
(
P
2
)
that provides
100 voltages controlled by programming
micro
-
controllers
. Scale bar is
20
mm
.
Demonstration of Beam Steering
After validating
the
wide phase tunability
of
our
metasurfac
e,
we designed and
demonstrated a dynamic beam s
teering device. In order to implement beam steering
,
we used a
blazed grating approach to
design
the spatial profile of the
phase of the
light reflected from the
metasurface
by engineering
the spatial distribution of the DC bias voltages applied to the 96
metasurface pixels
.
When no bias
wa
s
applied,
we
observe
d
only the zeroth order diffracted beam in the Fourier
plane. In other words, the subwavelength period of the metasurface results in
an
absence of higher
-
order diffracted beams at zero bias. Then we
designed the spatial phase profile of the beam steering
metasurface by discretizing the phase shift acquired by the metasurface pixels into four levels 0
°
,
90
°
, 180
°
, and 270°.
Each effective period, hereafter termed a supercell, consists of the
metasurfac
e pixels exhibiting the discretized 4
-
level phase shift values
. By changing the
pixel
repetition number
(RN)
for
each phase
shift
value
within one supercell, we electrically
modulated
6
the effective periodicity of the metasurface array. This resulted in
a
s
hift
of
the spatial position of
the first diffracted order, enabling manipulation of
the
far
-
field radiation.
Figure
3
a
shows
the
metasurface
spatial
phase
profile
s, for
the four
-
level phase shift with
different
RN
values. In Fig.
3
a
, each gray
-
shaded reg
ion determines one supercell in each case.
The
simulat
ed
far
-
field pattern of the beam steering device is presented in Fig.
3
b
.
As can be seen,
by changing the
RN value
, the
size of the metasurface supercell
is
electrically
modulated
,
resulting
in
reconfig
urable beam steering with quasi
-
continuous steering angles
that can be
as
large as
~
70.5
°
. Fig
ure
3
c
show
s
the measured far
-
field pattern
for
our
fabricated
beam steering device
whose SEM images showed
an
average pitch size of 504 nm
.
D
ue to limitation
s
of
our measurement setup, steering angles of higher than 23.5° could
not be capture
d
by the imaging system. As a result, the maximum measured steering angle was
~22°
,
which corresponds to
a repeat number of
2.
A
s expected, by increasing the effective period
of the metasurface, the
beam
angle becomes smaller.
We also note
that for each
RN
value,
no
diffracted order
is observed
at
negative angles
, indicating true phase gradient beam steering rather
than switchable diffraction
. This confirms that the beam steeri
ng is obtained as a result of the
a
symmetric phase gradient i
n
troduced by the subwavelength metasurface
phase
elements.
Figure 3.
a
, The spatial phase distribution
s
of the metasurface elements with different
RN values
that
are
used to create phase gradie
nts resulting in beam steering.
b
, Simulation results of the beam steering
metasurface obtained through analytical calculations. Changing the
RN value
, the steering angles of 70.68
°
(RN=1),
28.14
° (RN=2),
13.62
° (RN=4), and
9.02
° (RN=6) were obtained
throu
gh
calculations
.
c
,
Experimental results of the beam steering metasurface. Changing the
RN value
, we could obtain the steering
angles of 22.19
° (RN=2),
10.91
° (RN=4),
and
7.40
° (RN=6)
. Each steering angle corresponds to the spatial
phase distribution of th
e same color presented in
a
.
Demonstration of Dynamic Metalens
Using
the same concept of
controlling the
phase
imposed by
each individual
metasurface
pixel
,
we were able to
demonstrate
the
use of our universal metasurface as a reconfigurable lens
a
c
b
0
1
2
3
4
5
Efficiency