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Article
Electro-Optically Tunable Multifunctional Metasurfaces
Ghazaleh Kafaie Shirmanesh, Ruzan Sokhoyan, Pin Chieh Wu, and Harry A Atwater
ACS Nano
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1
Electro-Optically Tunable Multifunctional Metasurfaces
Ghazaleh Kafaie Shirmanesh,
†
Ruzan Sokhoyan,
†
Pin Chieh Wu,
†,‡
and Harry A. Atwater
†,₴,*
†
Thomas J. Watson Laboratory of Applied Physics and
₴
Kavli Nanoscience Institute, California Institute
of Technology, Pasadena, California 91125, USA
‡
Department of Photonics, National Cheng Kung University, Tainan 70101, Taiwan
*
E-mail:
haa@caltech.edu
Abstract:
Shaping the flow of light at the nanoscale has been a grand challenge for nanophotonics over
decades. It is now widely recognized that metasurfaces represent a chip-scale nanophotonics array
technology capable of comprehensively controlling the wavefront of light
via
appropriately configuring
subwavelength antenna elements. Here, we demonstrate a reconfigurable metasurface that is
multifunctional,
i.e.
, notionally capable of providing diverse optical functions in the telecommunication
wavelength regime, using a single compact, lightweight, electronically-controlled array with no moving
parts. By electro-optical control of the phase of the scattered light from each identical individual
metasurface element in an array, we demonstrate a single prototype multifunctional programmable
metasurface that is capable of both dynamic beam steering and reconfigurable light focusing.
Reconfigurable multifunctional metasurfaces with arrays of tunable optical antennas thus can perform
arbitrary optical functions by programmable array-level control of scattered light phase, amplitude, and
polarization, similar to dynamic and programmable memories in electronics.
Keywords:
active metasurface, multifunctional, indium tin oxide, wavefront engineering, beam steering,
focusing meta-mirror
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Rapid advances in control of the phase and amplitude of the light scattered from planar arrays of
nanophotonic elements has stimulated the development of metasurfaces that utilize amplitude/phase-
sensitive scattering to enable wavefront engineering.
1, 2
Metasurfaces are also now demonstrating some of
their potential applications in compact, high-performance, and low-cost optical devices and components,
creating burgeoning interest in photonic integration. To date, metasurfaces have mostly been designed in
an application-specific manner and the design process resulted 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, optical pumping, heating, 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 proven to be a robust, energy-efficient and reversible scheme for tuning active metasurfaces.
4-16
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.
17
However, in spite of advances in active metasurfaces to date, multifunctional
reprogrammable metasurface components have not yet been demonstrated. Realization of a single hardware
device that can provide multiple and --indeed general-- functions would further accelerate the impact of
metasurfaces and their applications. Such multifunctionality can be found in electronics technology that has
benefitted from development of programmable and reprogrammable circuits composed of identical circuit
elements, such as dynamic
18
and static
19
random access memories and field-programmable gate arrays.
20
In
this paper, we demonstrate a state-of-the-art prototypical multifunctional metasurface which can be
electronically programmed to 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 (LiDAR),
21
optical interconnects,
22
and optical communications.
23
Conventional beam steering
devices such as Risley prisms,
24
galvanometer-scanning mirrors,
25
and decentered lenses
26
employ
mechanically moving 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.
27
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 diverse functions unachievable
via
mechanical motion.
Reconfigurable metasurfaces have recently been employed to provide dynamic beam steering in
the microwave and NIR regimes by exploiting microfluidic flows,
28, 29
incorporation of phase-change
materials,
30
and reorientation of liquid crystals.
31
However, the performance of these devices is limited due
to their failure to provide an exquisite control over the phase of the scattered light and accurately generate
a desired phase profile, leaving them unable to demonstrate arbitrary functions. Alternatively, electro-optic
modulation in multiple-quantum-well resonant metasurfaces,
32, 33
an intrinsically ultrafast process, has been
shown to provide high-speed modulation and dynamic beam steering, but to date, a limited phase
modulation range has constrained the achievable beam directivity and steering angle range.
Electro-optically controllable beam-switching has also been demonstrated
via
incorporation of
transparent conducting oxides as active material into metasurfaces.
4, 34, 35
However, individual control over
each metasurface element, which is required for more complex phase distribution patterns, has not been
reported. Other researchers have demonstrated beam steering using waveguide-based thermo-optical phase
shifters coupled to antennas
36-40
or by employing frequency-gradient metasurfaces.
41
These chip-based
antenna arrays can enable beam steering at visible or infrared frequencies, but are application-specific, and
hence, has been unable to achieve more general array functionalities.
Light focusing is another paramount optical function that plays a fundamental role in almost every
optical system such as imaging, microscopy, optical data storage, and optical encryption.
42
Metasurfaces
have given rise to versatile metalenses that can replace bulky conventional lenses by engineering the spatial
variation of field amplitude or phase distribution over arrays of individual metasurface elements at
approximately wavelength-scale or smaller spacing.
43-47
Metalenses have demonstrated the capability to
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perform high-resolution imaging, wavefront shaping for aberration correction, and polarization
conversion.
1, 2, 48
Reconfigurable metasurfaces have been utilized to realize dynamic focusing by variation of the
overall lens optical thickness or curvature,
via
liquid crystal reorientation,
49
microfluidic flow,
50, 51
or elastic
deformation.
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However, these modes of dynamic focusing do not permit precise tailoring of the lens focal
properties by arbitrary phase control of the lens phase elements.
Here, we design and realize a multifunctional electro-optically tunable metasurface that can exhibit
multiple functions in the NIR wavelength regime using a single device,
via
precise tailoring of the phase
profile of an optical aperture. Figure 1a schematically illustrates this metasurface, whose independently
addressable elements enable dynamic control of the wavefront
via
a pixel-by-pixel reconfiguration. Using
this scheme, we demonstrate a reprogrammable metasurface whose function can be reconfigured between
dynamic beam steering (Figure 1b) and dynamic focusing meta-mirrors, achieving a reconfigurable focal
length and numerical aperture (Figure 1c) by tuning of the gate voltages applied to individual metasurface
elements.
Results and Discussions
Design of Electro-Optically Tunable Metasurface Element
Figure 2a,b schematically illustrates the building blocks of our tunable gated field-effect
metasurface, consisting of an Au back-reflector, on top of which 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 ‘fishbone’
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 material that simultaneously exhibits high breakdown field and high
DC permittivity
6
(see Supporting Information, Part 1 for a comparison between the proposed metasurface
design and the dual-gated metasurface design
6
). We apply a DC electric bias between the ITO layer and the
nanoantennas. This causes the ITO layer to undergo a reproducible field-effect-induced index change. By
altering the applied electric field, one can modulate the ITO charge carrier density close to the interface of
the ITO and the gate dielectric. By further increasing the applied 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 (see Supporting Information, Part 2).
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, the metasurface is expected to exhibit large phase
modulation.
Optical Modulation in Electro-Optically Tunable Metasurface Element
Figure 2c shows the reflectance spectrum of the metasurface for different applied biases. As seen
in Figure 2c, at all applied biases, resonant dips are clearly observed at wavelengths close to
=
1500 nm,
our wavelength of interest. Figure 2d,e illustrates the simulated reflectance and phase shift as a function of
applied bias at different wavelengths. Here, phase shift 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 modulation. This demonstrates that both the real and imaginary parts 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 applied bias, we can then pick the
operation wavelength of the beam steering and focusing devices. To accomplish this, we utilize the
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metasurface as a phase modulator, for which the reflectance should ideally remain constant upon change in
the applied bias. Here the operation wavelength of
= 1510 nm is chosen so that we obtain a phase shift of
higher than 270
while the maximum reflectance modulation remains as small as possible (see Methods and
Supporting Information, Part 3). After confirming this tunable response, we experimentally obtained the
reflectance and phase shift of the fabricated metasurface under applied bias. Figure 2f 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 the incident laser beam on the edge of the metasurface nano-antenna array, the
scattered beam is reflected partly from the metasurface and partly from the gold back-plane, resulting in a
lateral shift in the interference fringe patterns of the metasurface and the back-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
acquired due to the applied bias. As seen in Figure 2f, at an operation wavelength of
λ=1522
nm, an actively
tunable continuous phase shift of 0
o
to 274
is accompanied by a non-negligible reflectance modulation.
When analyzing beam steering performance of our multifunctional metasurface, we observe that the
mentioned reflectance modulation results in the increased intensity of the undesired side-lobes in the far-
field radiation pattern (see Supporting Information, Part 12 for the effect of the reflectance modulation on
the beam steering performance of our multifunctional metasurface). Moreover, since the complex dielectric
permittivity of ITO is significantly modulated only in a sub-nm-thick layer, a large tunable phase shift is
observed only when the optical field is tightly confined in this sub-nm-thick ITO active layer. This tight
field confinement results in enhanced absorbance and, hence, reduced reflectance of our active metasurface.
Changing the thickness of the dielectric Al
2
O
3
and HAOL layers, or using transparent conducting oxides
with higher electron mobilities, such as cadmium oxide (CdO),
53
one can increase the reflectance of the
metasurface (see Supporting Information, Part 4).
Once we validated the modulation 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 Figure 3a. In our metasurface
device, nanoantennas 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. Figure 3b is a photomicrograph of the fabricated array, consisting of 96
individually-controllable and identical metasurface pixels (see Methods and Supporting Information, Part
5 for fabrication steps). In order to individually bias each of these metasurface pixels, we designed two
printed circuit boards (PCBs). Figure 3c shows the first PCB with the multifunctional metasurface mounted
on and wire-bonded to it. Each conducting pad on the first PCB is then connected to an external pin on the
second PCB that is shown in Figure 3d. This voltage deriving PCB is capable of providing 100 independent
voltages that can be individually controlled through programming a number of micro-controllers by a
computer (see Methods).
In order to characterize our multifunctional metasurface, we used a custom-built optical setup to
measure reflectance spectrum, phase shift, beam steering profile, and focused beam profile (see Supporting
Information, Part 6-10 for a detailed description of the measurements).
Demonstration of Beam Steering
After validating the wide phase tunability of our metasurface, we designed and demonstrated a
dynamic beam steering device. In order to implement beam steering, we designed the spatial phase profile
of the light reflected from the metasurface by engineering the spatial distribution of the DC bias voltages
applied to the 96 metasurface pixels (see Supporting Information, Part 11).
In order to design the spatial phase profile of the metasurface, we employed a multilevel approximation of
blazed grating approach
54-56
that is widely used for demonstration of beam steering metasurfaces.
31, 32, 34
Here, we discretized the phase shift acquired by the metasurface pixels into four levels 0
, 90
, 180
, and
270° (see Supporting Information, Part 12 for a discussion on the choice of phase distribution). In this
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configuration, the metasurface acts as a diffraction grating with reconfigurable periodicity. Each effective
period, hereafter termed a supercell, consists of the metasurface pixels exhibiting the discretized 4-level
phase shift values. When no bias is applied, we observed 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. By changing the pixel repetition number (
RN
) for each phase shift value
within one supercell, we electrically modulated the effective periodicity of the metasurface array. This
resulted in a shift of the spatial position of the first diffracted order, enabling manipulation of the far-field
radiation.
Figure 4a shows the metasurface spatial phase profiles, for the four-level phase shift with different
RN
values. In Figure 4a, each gray-shaded region determines one supercell in each case. The simulated far-
field pattern of the beam steering device is presented in Figure 4b (see Supporting Information, Part 12 for
simulation methods). It should be noted that the simulations correspond to the dimensions of our fabricated
metasurface that showed an average pitch size of 504 nm. As can be seen, by changing the
RN
value, the
size of the metasurface supercell is electrically modulated, resulting in reconfigurable beam steering with
quasi-continuous steering angles that can be as large as ~70.5° for our metasurface design with a pitch size
of 400 nm (see Supporting Information, Part 12). Figure 4c shows the measured far-field pattern for our
beam steering device. Due to limitations of our measurement setup, steering angles of higher than 23.5°
could not be captured by the imaging system. As a result, the maximum measured steering angle was ~22°,
which corresponds to a repeat number of 2. As 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
with an intensity equal to that of the desired steering angle is observed at negative angles, indicating true
phase gradient beam steering rather than switchable diffraction. This confirms that the beam steering is
obtained as a result of the asymmetric phase gradient introduced by the subwavelength metasurface phase
elements.
Demonstration of Dynamic Focusing Meta-Mirror
Using the same concept of controlling the phase imposed by each individual metasurface pixel, we
were able to demonstrate use of our multifunctional metasurface as a reconfigurable lens by developing
phase profiles for lenses with different focal lengths. Figure 5a-c shows the spatial distribution of the phase
shift (diamond) and the corresponding applied bias voltage (square) required to focus the reflected beam at
focal lengths of 1.5
m, 2
m and 3
m. These values were extracted from the simulated phase shift as a
function of applied bias (see Supporting Information, Part 13). In order to investigate the focusing
performance, we simulated the multifunctional metasurface under the applied bias distributions illustrated
in Figure 5a-c. In our full-wave electromagnetic simulations, we modeled a miniaturized lens with a 20
m
aperture size since simulating the full metasurface at the small mesh sizes required for the ITO layer active
region is beyond our present numerical simulation capability. Figure 5d-f illustrate the far-field pattern of
the beam reflected from our tunable metasurface in the x-z plane. As seen in Figure 5d-f, the metasurface
can clearly focus the reflected light at the focal lengths of 1.5
m, 2
m and 3
m, when appropriate bias
voltages are applied to the individual metasurface pixels.
We then experimentally characterized the dynamic focusing meta-mirror once the focusing
performance of our multifunctional metasurface was confirmed by calculations. We programmed the
voltages applied to each metasurface pixel in order to experimentally achieve the desired phase shift values
(Figure 2f) (see Supporting Information, Part 13). Then the fabricated focusing meta-mirror was
characterized utilizing our multifunctional setup (see Supporting Information, Part 10). Using this setup,
the intensity profile of the reflected beam in the
xy
-plane was recorded. By extracting the cross sections of
the captured intensity profiles at fixed
y
values, we reconstructed the intensity profile of the reflected beam
in the
xz
-plane. Figure 5g-i illustrate the metasurface reflected beam intensity profiles in the
xz
-plane for
the applied bias distributions shown in Figure 5a-c. As can be seen, the fabricated metasurface focuses the
reflected beam at the desired depths. The scale bars in Figure 5g-i were obtained by imaging an object of
known size. When the incident light was polarized perpendicular to the antennas, no focusing was observed
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since no phase modulation could be achieved in that polarization. This observation confirmed that the
captured focusing originated from the metasurface.
Using the same concept of individually-controlled metasurface pixels, we reprogrammed the
applied bias voltages to the metasurface in order to experimentally demonstrate a tunable focusing meta-
mirrors with focal length varying from 15
m to 25
m (see Supporting Information, Part 14).
Conclusions
We have designed and experimentally demonstrated an electrically tunable multifunctional metasurface in
the NIR wavelength range. The multifunctional metasurface is realized
via
field-effect-induced modulation
of transparent conducting oxide active regions incorporated into the metasurface and is capable of
spatiotemporal modulation of the fundamental attributes of light. As a proof of concept, we designed phase
profiles for our multifunctional metasurface to demonstrate beam steering and dynamic focusing using the
same device
via
individually controlling each metasurface pixel. Such a multifunctional metasurface can
initiate integrated on-chip electro-optical devices such as light detection and ranging (LiDAR) systems.
Prior research has shown that the reflectance of the ITO-based active metasurfaces can be considerably
enhanced by utilizing ITO-integrated all-dielectric guided-mode resonance mirror designs.
57
The efficiency
of the multifunctional metasurface can possibly be further improved
via
optimization algorithms.
58-61
It has
been previously shown that optimization algorithms may yield non-trivial structural shapes and metasurface
antenna distributions that yield significantly improved optical performance. In particular, optimization
algorithms may significantly boost the performance of active beam steering metasurfaces.
62
A worthy
direction for future research is to extend the multifunctional metasurface concept demonstrated here to a
two-dimensional phased array architecture. In addition to enabling beam steering and focusing in two
dimensions, such a two-dimensional array could enable fast and energy-efficient programmable devices
such as dynamic holograms, off-axis lenses, axicons, vortex plates, and polarimeters.
Methods
Full-wave simulation of reconfigurable metasurface
Full full-wave electromagnetic calculations for our tunable metasurface were performed using finite
difference time domain optical simulations (FDTD Lumerical). Figure S3a shows the calculated phase shift
spectrum at different applied biases.
After we confirmed that our designed metasurface can provide both reflectance (Figure 3a-c of
main manuscript) and phase (Figure S3a) modulation, we chose the operating wavelength of the device
such that the metasurface could provide a large phase modulation and as modest reflectance modulation as
possible. Figure S3b shows the maximum reflectance modulation and the maximum achievable phase shift
at different wavelengths. As can be seen, at
= 1510 nm, a phase shift larger than 270° is achievable while
the reflectance change is kept to be as modest as possible.
Multifunctional metasurface fabrication
The metasurface fabrication steps are illustrated in Figure S8. In order to fabricate the multifunctional
metasurface device, we first did a standard cleaning process on Si wafers. Then we patterned the outmost
part of the connecting pads as well as some alignment markers using photolithography. After developing
the photoresist, a 10 nm-thick Ti layer followed by 200 nm-thick Au layer was deposited on the samples
using electron beam evaporator. After lifting-off the excess Ti-Au parts, we patterned the back reflector by
electron beam lithography [VISTEC electron beam pattern generator (EBPG) 5000+] at an acceleration
voltage of 100 keV. After developing the electron beam resist, we deposited a 3 nm-thick Cr layer followed
by 80 nm-thick Au layer using electron beam evaporator. After the lift-off process, a 9.5 nm-thick Al
2
O
3
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layer was deposited on the samples using atomic layer deposition (ALD) through shadow masks. After
developing the electron beam resist, the ITO layer was patterned by electron beam lithography, and a 5 nm-
thick ITO layer was deposited on the sample using room-temperature RF magnetron sputtering in Ar/O
2
plasma environment. The deposition pressure is 3 mTorr while the applied RF power is 48W. Once the
excess ITO regions were lifted off, we patterned the contact pads of the ITO layer by electron beam
lithography. A 10 nm-thick Ti layer followed by 200 nm-thick Au layer was then deposited on the samples
using electron beam evaporator after developing the electron beam resist. Afterwards, a 9.5 nm-thick HAOL
layer was deposited on the samples using ALD. The size of the HAOL film was controlled by using shadow
masks during the atomic layer deposition. Then we patterned the antennas by electron beam lithography
and made the inner contact pad connections. Once the electron beam resist was developed, we deposited a
1.5 nm-thick Ge layer followed by a 40 nm-thick Au layer.
Electrical connections to individually control metasurface pixels
In order to individually bias each of 96 different metasurface elements, 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 3c of main manuscript). Each conducting pad on P
1
is then connected to an external pin on the
second PCB (P
2
). This voltage deriving PCB is capable of providing 100 independent voltages that can be
individually controlled (Figure 3d of main manuscript). These independent bias voltages are produced by
programming 12 digital to analog converters (DACs). Every set of three DACs is programmed by an
Arduino Nano micro-controller board based on the ATmega328P (Arduino Nano 3.x). In order to provide
the desired voltages at the output ports of the DACs, the input ports of the DACs are connected to the digital
outputs of the Arduino microcontrollers and are then programmed
via
computer by using the Arduino
Software (IDE).
ASSOCIATED CONTENT
Supporting
Information
The Supporting Information is available free of charge at:
Comparison to the previously proposed design; Calculating electrostatic properties of ITO; Full-wave
simulation of reconfigurable metasurface; Increasing metasurface reflectance level; Multifunctional
metasurface fabrication; Universal measurements setup; Reflectance measurements; Phase shift
measurements; Beam steering measurements; Reconfigurable focusing measurements; Choosing the
number of metasurface pixels; Beam steering metasurface simulations; Spatial phase and voltage
profiles for reconfigurable focusing meta-mirror; Reconfigurable focusing meta-mirror: accessing
longer focal lengths.
Pre-print version is available at: Kafaie Shirmanesh, G.; Sokhoyan, R.; Wu, P. C.; Atwater, H. A.,
Electro-Optically Tunable Universal Metasurfaces. 2019, 1910.02069. arXiv: physics.optics.
http://arxiv.org/abs/1910.02069 (Accessed Oct 4, 2019)
AUTHOR INFORMATION
ORCID
Ghazaleh Kafaie Shirmanesh:
0000-0003-1666-3215.
Ruzan Sokhoyan:
0000-0003-4599-6350.
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Pin Chieh Wu:
0000-0002-5781-9696.
Harry A. Atwater:
0000-0001-9435-0201.
Author Contributions
G.K.S, R.S., and H.A.A conceived the original idea. G.K.S performed the numerical design, device
fabrication, performed the optical measurements, analyzed numerical and experimental data, designed and
build up the PCB for individually electrical control of metasurface elements, helped with the build-up of
optical setup for measurement, and wrote the manuscript. P.C.W built up the optical setup, performed the
numerical simulations for beam steering and focusing. R.S. performed the device physics numerical
calculations, helped with data analysis, and wrote the manuscript. H.A.A. organized the project, designed
experiments, analyzed the results, and prepare the manuscripts. All authors discussed the results and
commented on the manuscript.
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
The authors acknowledge metasurface device fabrication support provided by the Kavli Nanoscience
Institute (KNI).
Funding:
This work was supported by Samsung Electronics and the National Aeronautics and Space
Administration. P.C.W. acknowledges the support from Ministry of Science and Technology, Taiwan
(Grant number: 107-2923-M-006-004-MY3; 108-2112-M-006-021-MY3). P.C.W. also acknowledges the
support in part by Higher Education Sprout Project, Ministry of Education to the Headquarters of University
Advancement at National Cheng Kung University (NCKU).
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FIGURES
Figure 1. Multifunctional metasurface with 96 independently addressable metasurface elements.
Schematic of (a) the multifunctional metasurface whose functionality can be switched between (b)
dynamic beam steering and (c) cylindrical metalens with reconfigurable focal length.
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Figure 2. Unit cell design of the multifunctional metasurface. Schematic of (a) periodic array and (b)
unit cell of the antenna elements. The metasurface is composed of an Au back-reflector, an Al
2
O
3
dielectric layer, an ITO layer, and a hafnium oxide/aluminum oxide laminated (HAOL) gate
dielectric followed by an Au fishbone antenna. The periodicity of the metasurface is
p
= 400 nm, and
the thickness of the back-reflector, Al
2
O
3
, ITO, and HAOL layers are
t
b
= 80 nm,
t
Al
= 9.5 nm,
t
i
= 5
nm, and
t
h
= 9.5 nm, respectively. The width, length, and the thickness of the antenna are
w
a
=130 nm,
l
a
= 230 nm, and
t
a
= 40 nm, respectively and the width of the electrode is
w
e
= 150 nm. (c) Simulated
reflectance spectrum at different bias voltages. Simulated (d) reflectance and (e) phase of the
reflection from the metasurface 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
= 1522 nm such that a phase
shift greater than 270
accompanied by moderate amplitude variation could be obtained.
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