ARTICLE
Dynamic beam steering with all-dielectric electro-
optic III
–
V multiple-quantum-well metasurfaces
Pin Chieh Wu
1,2
, Ragip A. Pala
1
, Ghazaleh Kafaie Shirmanesh
1
, Wen-Hui Cheng
1
, Ruzan Sokhoyan
1
,
Meir Grajower
1
, Muhammad Z. Alam
1
, Duhyun Lee
1,3
& Harry A. Atwater
1,4
Tunable metasurfaces enable dynamical control of the key constitutive properties of light at a
subwavelength scale. To date, electrically tunable metasurfaces at near-infrared wavelengths
have been realized using free carrier modulation, and switching of thermo-optical, liquid
crystal and phase change media. However, the highest performance and lowest loss discrete
optoelectronic modulators exploit the electro-optic effect in multiple-quantum-well hetero-
structures. Here, we report an all-dielectric active metasurface based on electro-optically
tunable III
–
V multiple-quantum-wells patterned into subwavelength elements that each
supports a hybrid Mie-guided mode resonance. The quantum-con
fi
ned Stark effect actively
modulates this volumetric hybrid resonance, and we observe a relative re
fl
ectance mod-
ulation of 270% and a phase shift from 0° to ~70°. Additionally, we demonstrate beam
steering by applying an electrical bias to each element to actively change the metasurface
period, an approach that can also realize tunable metalenses, active polarizers, and
fl
at spatial
light modulators.
https://doi.org/10.1038/s41467-019-11598-8
OPEN
1
Thomas J. Watson Laboratory of Applied Physics, Calif
ornia Institute of Technology, Pasadena, CA 91125, USA.
2
Department of Phot
onics, National
Cheng Kung University, Tainan 70101, Taiwan.
3
Samsung Advanced Institute o
f Technology, Suwon, Gyeonggi-do 443-803, South Korea.
4
Kavli
Nanoscience Institute, California Institute o
f Technology, Pasadena, CA 91125, USA. Correspond
ence and requests for materials should be addresse
d
to P.C.W. (email:
pcwu@gs.ncku.edu.tw
) or to H.A.A. (email:
haa@caltech.edu
)
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
1
1234567890():,;
A
chieving versatile dynamical control of the key con-
stitutive properties of light at the nanoscale is a grand
challenge for nanophotonics. In the last several years,
metasurfaces have shown extraordinary promise to achieve such
comprehensive control over the characteristics of scattered light.
Metasurfaces can be viewed as arti
fi
cially designed arrays of
subwavelength optical scatterers, where each scatterer introduces
abrupt changes to the phase, amplitude, or polarization of the
re
fl
ected or transmitted electromagnetic waves
1
–
3
. Thus, meta-
surfaces offer the ability to control the wavefront of the scattered
light, thereby creating new
fl
at optics and ultrathin optoelectronic
components
4
,
5
. To date, metasurfaces have been used to
demonstrate a number of low-pro
fi
le optical components with
important capabilities, including focusing
6
–
9
, polarization control
and detection
10
–
12
, holograms
13
–
15
, and quantum light
control
16
,
17
.
Among the large volume of experimental reports about meta-
surfaces, most demonstrated so far are passive. For passive
metasurfaces, the light-scattering characteristics are de
fi
ned by
the geometry and arrangement of subwavelength scatterers,
fi
xed
at the time of fabrication. In contrast to passive metasurfaces,
actively tunable metasurfaces can realize multiple functions
18
–
20
,
serving as low-pro
fi
le nanophotonic devices capable of beam
steering, active polarization switching, and formation of recon
fi
-
gurable metalenses.
So far, a number of different methods have been used to realize
tunable metasurfaces, commonly by incorporating an active
material into the metasurface structure. The dielectric permit-
tivity of the active material is then dynamically controlled via
application of an external stimulus, such as an electrical bias
21
,
22
,
laser pulse
23
, or heat input
24
. Recon
fi
gurable metasurfaces, which
are based on incorporating active materials into otherwise passive
antenna arrays, are hereafter referred to as hybrid metasurfaces.
For example, incorporation of monolayer graphene into a plas-
monic metasurface can enable active tuning of the spectral
response by electrically tuning the Fermi level of the graphene
sheet
25
–
27
. Electrical tuning of the coupling between metasurface
resonances and intersubband transitions in multiple-quantum-
wells (MQWs) has also been explored for applications, such as
tunable
fi
lters
28
and optical modulators at mid-infrared
wavelengths
29
,
30
. To achieve active metasurface performance at
visible and near-infrared (NIR) wavelengths, the integration of
metasurfaces with phase-change materials or liquid crystals has
enabled the demonstration of phase modulation
31
and active
beam switching
32
,
33
. Modulation of the dielectric permittivity
near the epsilon-near-zero (ENZ) transition in doped transparent
conducting materials can yield large optical modulation of the
scattered light
34
, and to date the ENZ transition in indium tin
oxide
35
–
37
and titanium nitride
38
has been exploited to elec-
trically tune the properties of scattered/emitted light. These
hybrid metasurfaces
34
–
37
operate by spectrally overlapping the
geometrical antenna resonance and the ENZ permittivity regime,
and also spatially overlapping the metasurface element mode
pro
fi
le with the tunable permittivity transparent conducting
material. To enable a widely tunable optical response, strong local
fi
eld con
fi
nement and enhancement in the active material is
required. Prior research has also combined tunable metasurface
optics with microelectromechanical systems (MEMS) technology
to demonstrate varifocal lenses
39
. Moreover, previous work has
shown that fabricating metasurfaces on elastomeric substrates
may yield adaptive metalenses
40
, strain-multiplexed meta-holo-
grams
41
, and an active control of the structural color
42
. However,
in MEMS-based and mechanically stretchable substrate modula-
tion approaches, control of the optical response is achieved by
changing the distance between either adjacent metasurface
elements
43
,
44
or entire element arrays
39
, and requires a
mechanical transducer, which limits the frequency bandwidth.
While interesting, these approaches are not able to yield versatile
active control over the scattered light wavefront over a wide
frequency range. This condition can only be realized by electronic
tuning the optical response of each metasurface element.
Fabricating metasurface elements directly in an active material
could substantially simplify the metasurface design and facilitate
the fabrication process. For example, prior research has used
phase-change materials as metasurface building blocks to achieve
actively tunable optical responses
19
,
45
,
46
. The ability to rewrite
metasurface patterns incorporating phase-change materials with a
pump laser has enabled the demonstration of multiple functions
when using a single sheet of either GST or VO
2
19
,
46
. However, the
tuning speed of the phase-change-material-based tunable meta-
surfaces is usually slow, because the phase transition speed is
typically limited by the thermal response time in material heating.
Previously, a GaAs all-dielectric tunable metasurface
23
has
been realized by actively refractive index tuning resulting from
free carrier generation via an optical pump
23
. This approach
enables a picosecond response time, but the requirement of an
ultrafast pump laser beam is not desirable for many low-power
compact nanophotonic applications. Under optical pumping, the
area for refractive-index modulation is determined by the size of
focused laser spot, and is relatively large, limiting the possibility
to achieve control of individual metasurface elements. To achieve
an independent control of each metasurface element, it is pre-
ferable to modulate the optical response of the metasurface
electrically rather than optically. Prior research has shown that a
patterned graphene layer under applied bias voltage can be used
to actively modulate the properties of the scattered light
47
,
48
.So
far, the actively tunable optical response of the patterned gra-
phene layer has only been demonstrated in the mid-infrared
wavelength regime because of the achievable carrier densities in
doped graphene. Thus it remains an outstanding research chal-
lenge to develop an active metasurface platform operating in the
visible or NIR wavelength range that would dynamically tailor the
wavefront of scattered light by modulation of individual antenna
elements.
Here, we describe an electrically tunable metasurface, which
utilizes III
–
V compound semiconducting MQW structures as
resonant elements. The amplitude and phase of the light re
fl
ected
from the metasurface can be continuously tuned by applying a
DC electric
fi
eld across the MQW metasurface elements, with a
tunable optical response from the quantum-con
fi
ned Stark effect
(QCSE)
30
,
49
. The QCSE enables electro-optic modulation of the
MQW complex refractive index, most strongly at wavelengths
near the MQW bandgap. In our metasurface design, each MQW
resonator supports a hybrid-resonant mode with a relatively high-
quality factor, enabling optical modulation under applied bias.
Using this active device concept, we experimentally demonstrate
beam steering by electrically controlling the optical response of
individual metasurface elements. The QCSE is widely used in
high-performance electro-optical components such as high-speed
modulators
50
. Thus, our approach combines the well-established
MQW technology with subwavelength antennas to creating an
active metasurface platform for diverse nanophotonics
applications.
Results
Characterization of MQW
. We utilize an epitaxial III
–
V het-
erostructure consisting of a GaAs substrate, distributed Bragg
re
fl
ector (DBR) and a 1.23 -
μ
m-thick undoped MQW layer (see
inset of Fig.
1
a). The DBR is comprised 20 pairs of alternating
layers of n-doped Al
0.9
Ga
0.1
As (76.5 nm) and n-doped GaAs
(65 nm) with the n-doped Al
0.9
Ga
0.1
As as the topmost layer. A
ARTICLE
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11598-8
2
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
50- nm-thick p-doped GaAs contact layer with a carrier density of
10
19
cm
−
3
is grown on top of the MQWs (not shown in the inset
of Fig.
1
a). Figure
1
a illustrates the measured re
fl
ectance spectrum
of the planar MQW/DBR/GaAs structure. As seen in Fig.
1
a, for
wavelengths ranging from 915 nm to 990 nm, the re
fl
ectance is
close to 100%, indicating that the DBR acts as a high-quality
mirror in this wavelength range. We also observe a sharp
re
fl
ectance dip at a wavelength of ~915 nm. This re
fl
ectance dip
originates from near-bandgap absorption in the MQW layer. As a
next step, we investigate the tunable optical response of the
MQW layer. Due to the QCSE, the interband transition energy is
shifted by applying a DC electric
fi
eld across the quantum wells
resulting in bias-induced MQW complex refractive index mod-
ulation
49
. For our quantum well heterostructures (Supplementary
Fig. 1a), the expected modulation of the real part of the refractive
index is on the order of
Δ
n
=
0.01
51
. To be able to experimentally
observe this small variation of the real part of the refractive index,
we integrated a Fabry
–
Pérot resonant cavity around the MQWs
whose top mirror was formed by depositing a 35-nm-thick
semitransparent Au
fi
lm on top of the MQWs. To improve the
adhesion of Au to the top p-doped GaAs layer, we
fi
rst deposited
a 2-nm-thick Ti
fi
lm before depositing the Au
fi
lm (see the inset
of Fig.
1
b). Figure
1
b plots the measured re
fl
ectance spectrum of
the fabricated DBR/MQW/Au Fabry
–
Pérot cavity. As seen in
Fig.
1
b, the Fabry
–
Pérot cavity exhibits a narrow resonance at a
wavelength of 932.7 nm. This narrow resonance enhances the
optical modulation caused by the variation of the complex
refractive index of the MQWs under applied bias.
Once we measured the re
fl
ectance spectrum of the DBR/MQW/
Au planar heterostructure, we then measured its re
fl
ectance
modulation under applied bias. To facilitate bias application, we
deposited ohmic contacts on the topmost p-doped GaAs layer
[Ti (20 nm)/Pt (30 nm)/Au (300 nm)] and at the bottom of the
n-doped GaAs substrate [Ge (43 nm)/Ni (30 nm)/Au (87 nm)].
We then measured the re
fl
ectance from our Fabry
–
Pérot resonant
MQW sample, when the GaAs substrate (low potential) and the
top ohmic contact (high potential) are biased with respect to each
other (see the inset of Fig.
1
b). Figure
1
c shows the map of the
measured re
fl
ectance as a function of wavelength and applied
bias. When the external bias is applied, we observe a shift of the
resonant wavelength that is accompanied with a signi
fi
cant
re
fl
ectance modulation. This demonstrates that both the real and
imaginary part of the refractive index of the MQWs are
modulated by applied bias. To study the tunable optical response
of the MQWs at different wavelengths, the position of the
Fabry
–
Pérot resonance has to be shifted to the desired spectral
position. We achieved this by spin coating a thin PMMA layer
with a pre-de
fi
ned thickness between the Au
fi
lm and the MQW
layer so as vary the cavity length. The spectral position of the
Fabry
–
Pérot resonance thus varies with the thickness of the
PMMA spacer layer in the cavity (Supplementary Fig. 1b). Our
analysis shows that, as expected, larger optical modulation is
observed at shorter operation wavelengths near the semiconduc-
tor band edge (Supplementary Note 1). From these measure-
ments, we concluded that the optimal wavelength to observe a
large re
fl
ectance modulation was between 915 nm and 920 nm.
Design and characterization of all-dielectric tunable metasur-
face
. Once we had identi
fi
ed the optimal operation wavelength
for observing the tunable optical response of MQWs, we designed
and fabricated our tunable metasurface. Since our MQWs exhibit
relatively modest refractive index change under applied bias, the
designed metasurface element has to support high-quality reso-
nant mode near the semiconductor band edge to exhibit sig-
ni
fi
cant optical modulation under applied bias. The fundamental
electric or magnetic dipole modes of typically utilized dielectric
resonators do not possess suf
fi
ciently high-quality factors. Fig-
ure
2
a shows the schematic of our electrically tunable all-
dielectric III
–
V MQW resonator-based metasurface. The reso-
nator design has a double-slit structure, where the double slits
have been partially etched into the MQW layer (inset of Fig.
2
b).
We choose the structural parameters of our resonators such that
these slits support a guided mode (GM) resonance that hybridizes
with a higher-order Mie resonance, at a wavelength slightly
beyond the MQW band edge absorption wavelength. The geo-
metrical parameters of the metasurface elements are summarized
in the caption of Fig.
2
. The inherently large real part of the
refractive index (
n
≈
3.62) of our MQWs enables us to design
subwavelength resonators (metasurface elements), which are only
700 -nm wide. The simulated re
fl
ectance spectrum of our meta-
surface is shown in Fig.
2
b. As seen in Fig.
2
b, the metasurface
supports two distinct resonant modes at wavelengths of 915.9 nm
and 936.3 nm. Figure
2
c and d shows the
z
-component of electric-
and magnetic
fi
eld intensities in our metasurface element at both
resonant wavelengths. The calculated
fi
eld pro
fi
les show that at a
wavelength of 915.9 nm, the metasurface element supports a
high-order Mie resonance (left images in Fig.
2
c, d). The multi-
pole decomposition analysis
52
–
54
shows that the supported high-
order Mie resonant mode is dominated by the magnetic octupolar
mode (Supplementary Note 2). In addition, at the same wave-
length, our metasurface element supports a GM resonance pro-
pagating along
x
direction, resulting in an electric
fi
eld that leaks
into the air gaps separating the metasurface elements (Supple-
mentary Note 3). Hence, the resonant mode supported by the
metasurface element at a wavelength of 915.9 nm can be inter-
preted as a coupling of a Mie resonance and a GM resonance,
which is referred to here as a hybrid Mie-GM resonance. Note
that the coupling of two resonant modes normally results in
mode splitting. In our case, the mode splitting can be seen when
0
–10
932.3
932.7
933.1
933.5
933.9
0
10
20
30
40
50
60
70
–8
–6
–4
–2
0
20
40
60
80
100
Reflectance (%)
Wavelength (nm)
915
925
935
945
955
965
975
985
860
890
920
950
980
1010 1040
0
20
40
60
80
100
Reflectance (%)
Wavelength (nm)
GaAs
DBR
QW
GaAs
DBR
QW
c
b
a
Wavelength (nm)
Voltage (V)
Reflectance (%)
Fig. 1
Optical performance of MQWs. Measured re
fl
ectance spectra of (
a
) a bare DBR/MQW and (
b
) a DBR/MQW/Ti/Au Fabry
–
Pérot cavity under no
applied bias. The insets show the schematics of corresponding structures. The light orange area in (
a
) indicates the wavelength range shown in (
b
).
c
Measured re
fl
ectance of the DBR/MQW/Ti/Au Fabry
–
Pérot cavity as a function of wavelength and applied voltage
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11598-8
ARTICLE
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
3
extending the simulation range to the shorter wavelengths
(Supplementary Note 4). We observe another resonance at a
wavelength of 936.3 nm, which can be interpreted as a
Fabry
–
Pérot resonance coupled to a GM resonance (Fig.
2
c;
Supplementary Fig. 7b). The Fabry
–
Pérot resonance-like mode
propagates along
z
direction of our 1.23-
μ
m tall resonators and
couples with a GM resonance propagating along the
x
direction of
our 700-nm-wide resonators. Full-wave simulations suggest that
the GM resonance at the wavelength of 915.9 nm arises mainly
from the partially etched double-slit structures, while the one at
936.3 nm is mostly attributable to the MQW slab (Supplementary
Note 3).
Next, we fabricated a tunable metasurface and experimentally
investigated its tunable optical response. We fabricated our
metasurface by electron-beam lithography and inductively
coupled plasma-reactive ion etching (ICP-RIE) etching (see the
Methods section). Figure
3
a shows a scanning electron micro-
scopy (SEM) image, in which double slits are observed on the top
of a MQW slab. Figure
3
b shows the measured re
fl
ectance spectra
of the fabricated metasurface under different applied biases (see
Supplementary Fig. 9 for details of optical setup). At zero bias,
two resonant dips are clearly observed, which is consistent with
our simulation results shown in Fig.
2
b. When we decrease the
applied bias from 0 V to
̶
10 V, we observe a signi
fi
cant red shift
of the shorter-wavelength resonance, which corresponds to the
hybrid Mie-GM resonance. Moreover, we observe a simultaneous
increase of re
fl
ectance intensity with decreased bias. Under an
applied bias, both the real and imaginary parts of the complex
refractive index of the quantum well are modulated. The observed
red shift of the resonance indicates an increase of the real part of
the refractive index. The modulation of the real part of the
refractive index is responsible for the bias-induced phase shift of
the re
fl
ected light. On the other hand, the change of the
re
fl
ectance at the resonance dip is caused by the modulation of
the imaginary part of the refractive index. Therefore, our hybrid
Mie-GM resonance tunable metasurface can be used as an
ef
fi
cient amplitude modulator. When analyzing the behavior of
the re
fl
ectance at a wavelength of 938 nm, we observe that the
shift of the resonance dip is negligible, while the re
fl
ectance at this
wavelength is slightly decreased. At wavelengths longer than
940 nm, high-quality resonances are absent and the index change
is smaller, so we observe no signi
fi
cant re
fl
ectance modulation at
these wavelengths. Since our metasurface exhibits much stronger
optical modulation at wavelengths corresponding to the hybrid
Mie-GM resonance, we focused our characterization on the
tunable resonance at shorter wavelengths. To gain further insight,
we plot the relative re
fl
ectance as a function of wavelength and
applied bias (Fig.
3
c). The relative re
fl
ectance is de
fi
ned as [
R
(
V
a
≠
0V)
–
R
(
V
a
=
0 V)]/
R
(
V
a
=
0V)
=
Δ
R
/
R
0
, where
V
a
is the
applied voltage. As mentioned above, in Fig.
3
c, we limit the
wavelength range between 915 nm and 925 nm. When
V
a
decreases from 0 V to
−
10 V, we observe a strong relative
re
fl
ectance modulation. In particular, at a wavelength of 917 nm,
we observe a relative re
fl
ectance modulation as high as 270%. The
relative re
fl
ectance modulation decreases to about
−
45% at a
wavelength of 925 nm. Thus, the proposed III
–
V MQW
resonator-based metasurface is a promising candidate for tunable
amplitude modulation. It is worth mentioning that we observe
about
+
20% and
−
30% absolute re
fl
ectance modulation [de
fi
ned
as
R
(
V
a
≠
0V)
–
R
(
V
a
=
0 V)] at wavelengths of ~917 nm and
~924 nm, respectively (Supplementary Fig. 10). Although they are
quantitatively comparable, the phase modulation and desired
optical functionality can only be observed when a high-quality
factor resonance is present (which shows higher relative
re
fl
ectance modulation), as can be seen in the following sections.
In addition, since the amplitude modulation is achieved via the
electro-optic effect rather than charge-carrier injection (due to the
low leakage current density in our samples, see Supplementary
Note 11), the intrinsic modulation frequency of our device can be
MHz (Supplementary Fig. 12) or substantially higher
30
,
49
,
50
.
We also experimentally evaluated the phase shift of the
re
fl
ected beam under applied bias at wavelengths of 917 nm and
924 nm using a Michelson interferometer system
31
,
35
,
37
. The
incident laser spot was positioned to illuminate the edge of the
resonator-based metasurface. As a result, part of the incident
beam is re
fl
ected from the metasurface, while the other part is
b
a
c
Max.
Min.
Max.
Min.
E
z
field intensity
Magnetic field intensity
d
GaAs
DBR
I
0
e
–
0
I
r
e
–
r
MQW resonators
x
y
915
920
925
930
935
940
945
950
0
10
20
30
40
50
60
70
80
Reflectance (%)
Wavelength (nm)
w
1
g
t
w
2
w
h
p
g
Fig. 2
Resonant modes in MQW-based metasurface.
a
Schematic for all-dielectric MQW metasurface. The metasurface consists of a n-doped
GaAs substrate, a distributed Bragg re
fl
ector (DBR), and a 1230-nm-thick MQW layer. There is a 50-nm-thick p-doped GaAs layer with doping level of
10
19
cm
−
3
that is grown on top of the MQWs as a top contact.
b
Simulated re
fl
ectance spectrum of a Mie-GM resonant metasurface under an
x
-polarized normal illumination. The inset shows the schematic of the metasurface unit element. The unit element dimensions are de
fi
ned as follows:
w
1
=
110 nm,
w
2
=
210 nm,
w
=
180 nm,
g
=
100 nm,
t
=
1230 nm, and
h
=
40 nm. The periodicity
p
is 910 nm.
c
,
d
show the spatial distributions of the
E
z
and magnetic
fi
eld intensities at the wavelengths corresponding to the resonant dips
ARTICLE
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11598-8
4
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
re
fl
ected from the unpatterned MQW heterostructure, and the
phase shift was determined using the unpatterned MQW
heterostructure as a built-in phase self-reference. By processing
and
fi
tting the interference fringes captured by a camera, we are
able to calculate the relative displacement of interference fringes
between the hybrid Mie-GM resonator region and the unpat-
terned region. From this, we retrieved the phase shift acquired
due to the applied bias. Figure
3
d shows the measured phase shift
as a function of applied bias at wavelengths of 917 nm (red dots)
and 924 nm (blue dots). When the applied bias is decreased from
0Vto
−
7 V, we observe a continuous increase in phase shift by
about 70° at a wavelength of 917 nm. The phase shift decreases to
about 50° when the applied bias is further decreased to
−
10 V.
The phase modulation becomes weaker for wavelengths away
from the hybrid Mie-GM resonance. For example, at a
wavelength of 924 nm, the largest phase shift reduces to 12°,
which we observed at an applied bias of
−
10 V. This modest
phase shift is also accompanied by a weaker relative re
fl
ectance
modulation of
−
45% (Fig.
3
c). These results are consistent with
the Kramers
–
Kronig relation: a large change of the real part of the
refractive index (phase shift) is accompanied with a signi
fi
cant
modulation of the imaginary part of the refractive index
(re
fl
ectance modulation).
Electrical switching of beam diffraction
. As experimentally
demonstrated, an extremely strong relative re
fl
ectance modula-
tion (~270%) with a phase shift of ~70° can be achieved by
electrically biasing the metasurface. As a next step, we patterned
the edges of the metasurface to selectively apply a bias to inde-
pendent groups of metasurface elements, enabling active control
of re
fl
ectance of the independent element groups. This enables us
to demonstrate an electrically switchable grating resulting in
dynamic beam diffraction, which was detected as a far-
fi
eld
radiation pattern. To create the switchable diffraction grating, we
fabricated a metasurface with similar structural dimensions as the
one described in the inset of Fig.
2
b, but we electrically connected
in parallel the resonant stripes in groups of three, and leave the
adjoining group of three resonant stripes isolated, as shown in
Fig.
4
a. Under zero applied bias, we observe a single output beam
in the Fourier plane that is re
fl
ected normally corresponding to
the zeroth-order diffracted beam. Higher-order diffracted beams
are absent since the period of our metasurface,
p
=
Λ
=
910 nm, is
subwavelength at 0 V bias (see details in Supplementary Note 6).
When we apply a negative bias voltage, the re
fl
ectance of the
electrically connected MQW resonators increases, causing an
effective increase in the period of the metasurface array (6 ×
p
=
Λ
'
=
5460 nm). This increased period creates
fi
rst-order diffracted
beams which appear at an angle de
fi
ned by the grating equa-
tion:
θ
¼
sin
1
m
λ
p
g
, where
p
g
is the period of the grating and
m
is
the diffraction order. Figure
4
b shows the SEM image of the
fabricated device. Figure
4
c shows the schematic optical setup
used for measurement of the far-
fi
eld radiation pattern in the
Fourier space. We utilized an uncollimated white light source
from a halogen lamp to visualize the sample surface. When
measuring the far-
fi
eld radiation pattern, we use a coherent NIR
laser beam (Toptica Photonics CTL 950) as a light source. The
laser beam was focused using a long working distance objective
with ×10 magni
fi
cation and 0.28 numerical aperture. The radia-
tion pattern is captured directly by the CCD camera, which is
positioned in the Fourier plane. Figure
4
d and e shows experi-
mentally measured diffraction patterns for different applied bias
voltages. The dynamic diffraction pattern measurements have
b
a
d
c
–10
–8
–6
–4
–2
0
0
20
40
60
Phase shift (
°
)
Voltage (V)
Voltage (V)
915
920
925
930
935
940
945
950
0
10
20
30
40
50
60
–6 V
–7 V
–8 V
–9 V
–10 V
Reflectance (%)
Wavelength (nm)
0 V
–1 V
–2 V
–3 V
–4 V
–5 V
2
μ
m
1.29
μ
m
917 nm
924 nm
Δ
R
/
R
(%)
915
917
919
921
923
925
–50
0
50
100
150
200
250
0
–2
–4
–6
–8
–10
Wavelength (nm)
Fig. 3
Experimental veri
fi
cation of optical modulation in MQW resonators.
a
SEM image of MQW-based hybrid Mie-GM resonators with double slits.
b
Measured re
fl
ectance spectra of the resonator array for different applied bias voltages. The incoming light is polarized perpendicularly to the MQW
stripes.
c
Measured relative re
fl
ectance of the hybrid Mie-GM resonant metasurface as a function of wavelength and applied voltage. We consider the
wavelength range from 915 nm to 925 nm with a step of 1 nm.
d
Measured phase modulation at two different wavelengths. Red: 917 nm, blue: 924 nm. Each
data point corresponds to an average phase shift measured at four different positions on the sample, while each error bar indicates the standard devia
tion
of the obtained four data points
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11598-8
ARTICLE
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
5
been performed at a wavelength of 917 nm, which corresponds to
the hybrid Mie-GM resonant mode supported by the unit ele-
ments of the metasurface. As seen in Fig.
4
d, we observe the
fi
rst-
order diffracted beam only in the cases when the contrast of the
re
fl
ectance between the resonator groups (i.e., the difference in
their refractive indices) becomes signi
fi
cantly large. For applied
bias voltages between 0 V and
−
3 V (0 V
≥
V
a
≥
̶
3 V) we observe
specular re
fl
ection from the metasurface. For applied bias voltages
below
−
3V(
V
a
≤
̶
3 V), the
fi
rst-order diffracted beam appears
at an angle of about 9.66° in the Fourier plane. Interestingly, the
intensity of the
fi
rst-order diffracted beam saturates when the
absolute value of the applied bias is lower than
−
6.5 V (Fig.
4
d).
We do not observe the
fi
rst-order diffracted beam when the
incident wavelength is switched to 924 nm (Supplementary
Note 8). This is expected because there is no signi
fi
cant re
fl
ec-
tance and phase modulations at this wavelength.
Dynamic beam steering with all-dielectric electro-optic MQW
metasurface
. Apart from switchable beam diffraction, we
experimentally demonstrated beam steering, which requires
control over individual metasurface elements. The spatial position
of the
fi
rst diffraction order can be effectively shifted when the
periodicity of metasurface is changed, enabling manipulation of
far-
fi
eld radiation. To realize dynamic beam steering, we designed
and fabricated another metasurface in which each unit element is
electrically isolated by fully etching the air gap between the
quantum well slabs. Due to the high refractive index and large
thickness of the MQW slabs, the resonant mode is sensitive to
minor variations of metasurface structural parameters. This
results in different electromagnetic
fi
eld pro
fi
les between the
fi
rst
and the second hybrid Mie-GM metasurfaces (Fig.
2
c,
d;
Sup-
plementary Fig. 14). Our beam steering metasurface also supports
a hybrid Mie-GM resonant mode at a wavelength near the band
edge absorption of the MQWs, yielding re
fl
ectance and phase
modulation (Supplementary Note 9). Figure
5
a
–
d shows the
images of the fabricated sample where 64 unit MQW resonator
elements are electrically connected to a printed circuit board
(PCB) via wire bonding, and each element is independently
controlled (see the Methods section). We
fi
rst examined the
spectral response as well as the active optical modulation of this
metasurface by applying an identical electrical bias to all the array
elements. This beam steering metasurface sample yielded about
80% relative re
fl
ectance modulation with a phase shift of ~42°
(Supplementary Fig. 15). Next, by individually addressing each
metasurface element, we steered the re
fl
ected beam, as seen in
Fig.
5
e, where the
fi
rst-order diffraction angle becomes smaller
as the metasurface periodicity is increased via electrical control.
Our numerical simulations show similar far-
fi
eld radiation pat-
terns (Supplementary Fig. 16a). Note that the sidelobes appear
around the zeroth-order diffraction beam are from the
fi
nite
aperture effect, which can be seen in both measured and simu-
lated results. By characterizing the measured and simulated far-
fi
eld radiation patterns with a larger total number of unit
elements (Fig.
5
e; Supplementary Fig. 16), the
fi
rst-order dif-
fraction peaks can be picked out, as indicated by black arrows in
Fig.
5
e. In addition, as seen from our simulations, the width of
both zero- and
fi
rst-order diffracted beams is narrower when the
total number of unit elements is larger (Supplementary Fig. 16b).
Discussion
In summary, we have demonstrated an all-dielectric active
metasurface platform based on an electro-optic effect in III
–
V
compound semiconducting MQWs. Our metasurface consists of
an array of two-dimensional hybrid Mie-GM resonators which
exhibit an actively tunable optical response under applied bias in
the NIR wavelength range. By applying a DC electric
fi
eld across
the hybrid Mie-GM resonators, we have experimentally observed
a relative re
fl
ectance modulation of ~270% at a hybrid Mie-GM-
–15
–10
–5
0
5
10
15
Normalized intensity
Angle (
°
)
0.0
0.5
1.0
0.5
1.0
λ
= 917 nm
GaAs
DBR
2
μ
m
V
= 0
V
≠
0
Λ′
a
e
b
GaAs
DBR
Λ
Angle (°)
NIR tunable
laser
Metasurface
White light
source
Beam expander
M
M
ND
I
L
BS
P
BS
O
M
CCD camera
c
V
a
= 0 V
V
a
= –10 V
+1
–1
Normalized intensity
Voltage (V)
d
0
–2
–4
–6
–8
–10
–16
–12 –8 –4
0
4
8
12 16
0.5
0.4
0.3
0.2
0.1
0
Fig. 4
Demonstration of switchable diffraction grating.
a
Schematic of the dynamic diffraction grating, which can be realized by changing the grating
periodicity via an appropriate bias application.
b
SEM image of the fabricated sample, which we used for the demonstration of the dynamic beam switching.
c
Optical setup for far-
fi
eld radiation pattern measurements. M: mirror, ND: neutral density
fi
lter, I: iris, BS: beam splitter, O: objective with 0.28 numerical
aperture, L: lens.
d
Experimentally measured intensity of the scattered light in the far-
fi
eld as a function of diffraction angle and applied voltage. The
fi
rst-
order diffracted beam appears when the applied voltage is lower than
−
3 V. The plotted diffracted light intensity is normalized to the light intensity at 0°.
The light intensity is plotted for a wavelength of 917 nm.
e
Intensity of the scattered light as a function of the diffraction angle at an applied bias of 0 V (top
panel) and at an applied bias of
−
10 V (bottom panel). The ±1 diffraction order clearly appears when the applied voltage is decreased to
−
10 V
ARTICLE
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11598-8
6
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
resonant wavelength of 917 nm. We have also measured a con-
tinuous phase shift from 0° to 70° at a wavelength of 917 nm. To
demonstrate a dynamic diffraction grating utilizing this large
re
fl
ectance modulation with modest phase shift, we have elec-
trically connected metasurface elements in groups of three and
actively changed the metasurface period by applying DC electric
fi
eld across the hybrid Mie-GM resonators. As a result, we
have been able to electrically switch on and off the
fi
rst-order
diffracted beam. Finally, as a proof-of-concept, we further
experimentally demonstrate a dynamic beam steering with elec-
trical and individual control of each unit element over the MQW
metasurface array.
In our work, our starting point is a monolithically grown
III
–
V compound semiconducting wafer, which we pattern by
using electron-beam lithography and dry etching. This is quite
different from the case of hybrid active metasurfaces, where the
metasurface cannot be grown monolithically. Monolithically
grown MQW active metasurfaces can potentially be integrated
with existing light emitting devices, such as vertical-cavity
surface-emitting lasers (VCSELs). Such an integrated device
could serve as a base for future on-chip light detection and
ranging systems. Since the tunable optical response of MQWs is
based on an electro-optic effect, the proposed metasurface
platform also offers the bene
fi
t of high modulation speed. The
presented active metasurface platform may be useful for the
realization of dynamically tunable ultrathin optical compo-
nents, such as tunable metalenses with recon
fi
gurable focal
lengths and numerical apertures, on-chip beam steering devi-
ces, active polarizers, and
fl
at spatial light modulators. The
performance of the proposed all-dielectric metasurface with
hybrid Mie-GM resonance can be further improved by utilizing
alternative quantum well systems which exhibit larger mod-
ulation of the real part of the refractive index and lower optical
loss as compared to the quantum well used in the present
work
55
,
56
(Supplementary Note 13).
Methods
Numerical simulation
. All numerical simulations were carried out using
fi
nite
difference time domain (FDTD) method (Lumerical). When designing our MQW
resonators, we used the perfectly matched layer (PML) boundary condition in
z
direction and the periodic boundary conditions in
x
direction. Hence, the calcu-
lations of the re
fl
ectance spectrum of the MQW resonators were performed in the
array con
fi
guration. The MQW resonators were assumed in
fi
nite in
y
direction. In
our electromagnetic calculations, we assumed that the incoming light impinged
normally on the metasurface. That is, the incoming electromagnetic wave propa-
gated along the
z
direction. For simplicity, the refractive indices of the n-
Al
0.9
Ga
0.1
As, n-Al
0.31
Ga
0.69
As, GaAs
0.6
P
0.4
, and InGaP were set as a constant of 3,
3.39
+
0.004i, 3.3
+
0.004i, 3.2
+
0.004i, respectively. The effective refractive index
of MQW can be found in Supplementary Fig. 18.
Sample fabrication
. First, the bottom Ge/Ni/Au Ohmic contact of thickness 43
nm/30 nm/87 nm was deposited on the n-doped GaAs substrate of the MQW wafer
by electron beam evaporation. Next, a 1.5-
μ
m-thick 950 PMMA A9 layer was spin
coated at 4000 rpm on the front side of the prepared MQW wafer for 60 s. The
MQW sample with the PMMA layer on top was then baked on a hot plate for
3 min at 180 °C. Subsequently, we de
fi
ned the top Ohmic contacts and alignment
markers by using the development, metal deposition and lift-off processes where
the patterning was done via an electron beam direct write lithography system
[VISTEC electron beam pattern generator (EBPG) 5000
+
] at an acceleration
voltage of 100 keV with a current of 5 nA. After de
fi
ning the top Ohmic contacts
and alignment markers, ZEP 520A was spin coated at 4000 rpm for 60 s, and the
sample was then baked on a hot plate for 3 min at 180 °C. The double-slit struc-
tures were de
fi
ned via the electron beam writing system at an acceleration voltage
of 100 keV with a current of 0.3 nA. The sample was then baked for 2 min at 110 °C
and developed at about 10 °C for 90 s. The structured ZEP 520 A resist was used as
a mask for dry etch process, which was employed for fabrication of double slits.
The etching was performed using a III
–
V compound semiconductor etcher (ICP-
RIE, Oxford Instruments System) with gas
fl
ow rates of Cl
2
:Ar
=
5 sccm: 30 sccm
under 5 mTorr chamber pressure for 80 s. The double slits were obtained after the
removal of ZEP 520A using remover PG. The resonators were
fi
nally de
fi
ned by the
third electron-beam writing process, followed by the same recipe for development,
1 cm
10
μ
m
4
μ
m
100
μ
m
b
a
c
d
e
1
μ
m
–15 –10
–5
0
5
10
15
0.0
0.5
1.0
0.5
1.0
0.5
1.0
Angle (
°
)
Normalized intensity
V
a
0
V
a
0
V
a
0
Fig. 5
Tunable beam steering with MQW-based all-dielectric metasurface.
a
Photographic image of the fabricated gate-tunable metasurface for the
demonstration of dynamic beam steering. The metasurface sample is mounted on a voltage-deriving PCB, which has 64 contact pads for individually
applying bias on each metasurface unit element.
b
–
d
Scanning electron microscopy images of the gate-tunable metasurface. To electrically isolate every
unit structure, portions of the sample are fully etched. Inset in (
d
) shows the top view of the fabricated metasurface.
e
Measured results of dynamic beam
steering by electrically changing the periodicity of metasurface. Black arrows indicate the position of the
fi
rst diffraction order in each case. Right column
illustrates how the spatial arrangement of electrical bias changes the periodicity of metasurface. The incident wavelength is
fi
xed at 917 nm. Scale
bars: 3
μ
m
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11598-8
ARTICLE
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
7
and dry etched as before with a chamber pressure of 3 mTorr for 8 min. The
fi
nal
removal of ZEP 520A was performed using remover PG.
Electrical connection to printed circuit boards (PCBs)
. We design two printed
circuit boards (PCBs) to individually apply bias to each metasurface element across
the antenna array. The sample is mounted on the
fi
rst PCB, and 63 individual
metasurface elements as well as the bottom contact are wire-bonded to 64 con-
ducting pads on the PCB. Each conducting pad of the
fi
rst PCB is then connected
to an external pin on the second board. This PCB is capable of providing 64
independent voltages that can be individually controlled through the reference
voltages derived by an external power supply (Keithley 2400). The second board
has different con
fi
gurations of voltage paths. By switching between different con-
fi
gurations, one can electrically change the grating periodicity of the metasurface.
Data availability
The data that support the
fi
ndings of this study are available from the authors on
reasonable request; see author contributions for speci
fi
c data sets.
Received: 16 February 2019 Accepted: 15 July 2019
References
1. Hsiao, H.-H., Chu, C. H. & Tsai, D. P. Fundamentals and applications of
metasurfaces.
Small Methods
1
, 1600064 (2017).
2. Yu, N. & Capasso, F. Flat optics with designer metasurfaces.
Nat. Mater.
13
,
139
–
150 (2014).
3. Minovich, A. E. et al. Functional and nonlinear optical metasurfaces.
Laser
Photonics Rev.
9
, 195
–
213 (2015).
4. Haffner, C. et al. All-plasmonic Mach
–
Zehnder modulator enabling
optical high-speed communication at the microscale.
Nat. Photon.
9
, 525
–
528
(2015).
5. Lee, H. W. et al. Nanoscale conducting oxide PlasMOStor.
Nano Lett.
14
,
6463
–
6468 (2014).
6. Wang, S. et al. A broadband achromatic metalens in the visible.
Nat.
Nanotechnol.
13
, 227
–
232 (2018).
7. Chen, B. H. et al. GaN metalens for pixel-level full-color routing at visible
light.
Nano Lett.
17
, 6345
–
6352 (2017).
8. Aieta, F. et al. Aberration-free ultrathin
fl
at lenses and axicons at telecom
wavelengths based on plasmonic metasurfaces.
Nano Lett.
12
, 4932
–
4936
(2012).
9. Chen, W. T. et al. A broadband achromatic metalens for focusing and imaging
in the visible.
Nat. Nanotechnol.
13
, 220
–
226 (2018).
10. Wu, P. C. et al. Visible metasurfaces for on-chip polarimetry.
ACS Photonics
5
,
2568
–
2573 (2018).
11. Rubin, N. A. et al. Polarization state generation and measurement with a
single metasurface.
Opt. Express
26
, 21455
–
21478 (2018).
12. Chen, W. T. et al. Integrated plasmonic metasurfaces for spectropolarimetry.
Nanotechnology
27
, 224002 (2016).
13. Lin, J., Genevet, P., Kats, M. A., Antoniou, N. & Capasso, F. Nanostructured
holograms for broadband manipulation of vector beams.
Nano Lett.
13
,
4269
–
4274 (2013).
14. Huang, Y.-W. et al. Aluminum plasmonic multicolor meta-hologram.
Nano
Lett.
15
, 3122
–
3127 (2015).
15. Kamali, S. M. et al. Angle-multiplexed metasurfaces: encoding independent
wavefronts in a single metasurface under different illumination angles.
Phys.
Rev. X
7
, 041056 (2017).
16. Jha, P. K., Ni, X., Wu, C., Wang, Y. & Zhang, X. Metasurface-enabled remote
quantum interference.
Phys. Rev. Lett.
115
, 025501 (2015).
17. Stav, T. et al. Quantum entanglement of the spin and orbital angular
momentum of photons using metamaterials.
Science
361
, 1101
–
1104
(2018).
18. Wu, P. C. et al. Broadband wide-angle multifunctional polarization
converter via liquid-metal-based metasurface.
Adv. Opt. Mater.
5
, 1600938
(2017).
19. Dong, K. et al. A Lithography-free and
fi
eld-programmable photonic
metacanvas.
Adv. Mater.
30
, 1703878 (2017).
20. Arash, N., Qian, W., Minghui, H. & Jinghua, T. Tunable and recon
fi
gurable
metasurfaces and metadevices.
Opto-Electron. Adv.
1
, 180009 (2018).
21. Thyagarajan, K., Sokhoyan, R., Zornberg, L. & Atwater, H. A. Metasurfaces:
millivolt modulation of plasmonic metasurface optical response via ionic
conductance.
Adv. Mater.
29
, 1701044 (2017).
22. Yao, Y. et al. Electrically tunable metasurface perfect absorbers for
ultrathin mid-Infrared optical modulators.
Nano Lett.
14
, 6526
–
6532
(2014).
23. Shcherbakov, M. R. et al. Ultrafast all-optical tuning of direct-gap
semiconductor metasurfaces.
Nat. Commun.
8
, 17 (2017).
24. Dicken, M. J. et al. Frequency tunable near-infrared metamaterials based on
VO2 phase transition.
Opt. Express
17
, 18330
–
18339 (2009).
25. Miao, Z. et al. Widely tunable terahertz phase modulation with gate-controlled
graphene metasurfaces.
Phys. Rev. X
5
, 041027 (2015).
26. Mousavi, S. H. et al. Inductive tuning of Fano-resonant metasurfaces using
plasmonic response of graphene in the mid-infrared.
Nano Lett.
13
,
1111
–
1117 (2013).
27. Sherrott, M. C. et al. Experimental demonstration of >230° phase modulation
in gate-tunable graphene
–
gold recon
fi
gurable mid-infrared metasurfaces.
Nano Lett.
17
, 3027
–
3034 (2017).
28. Benz, A., Montaño, I., Klem, J. F. & Brener, I. Tunable metamaterials
based on voltage controlled strong coupling.
Appl. Phys. Lett.
103
, 263116
(2013).
29. Sarma, R. et al. A metasurface optical modulator using voltage-
controlled population of quantum well states.
Appl. Phys. Lett.
113
, 201101
(2018).
30. Lee, J. et al. Ultrafast electrically tunable polaritonic metasurfaces.
Adv. Opt.
Mater.
2
, 1057
–
1063 (2014).
31. Kim, Y. et al. Phase modulation with electrically tunable vanadium
dioxide phase-change metasurfaces.
Nano Lett.
19
, 3961
–
3968 (2019).
32. Komar, A. et al. Dynamic beam switching by liquid crystal tunable dielectric
metasurfaces.
ACS Photonics
5
, 1742
–
1748 (2018).
33. Yin, X. et al. Beam switching and bifocal zoom lensing using active plasmonic
metasurfaces.
Light Sci. Appl.
6
, e17016 (2017).
34. Jun, Y. C. et al. Epsilon-near-zero strong coupling in metamaterial-
semiconductor hybrid structures.
Nano Lett.
13
, 5391
–
5396 (2013).
35. Kafaie Shirmanesh, G., Sokhoyan, R., Pala, R. A. & Atwater, H. A. Dual-gated
active metasurface at 1550 nm with wide (>300°) phase tunability.
Nano Lett.
18
, 2957
–
2963 (2018).
36. Howes, A., Wang, W., Kravchenko, I. & Valentine, J. Dynamic transmission
control based on all-dielectric Huygens metasurfaces.
Optica
5
, 787
–
792
(2018).
37. Huang, Y.-W. et al. Gate-tunable conducting oxide metasurfaces.
Nano Lett.
16
, 5319
–
5325 (2016).
38. Lu, Y.-J. et al. Dynamically controlled Purcell enhancement of visible
spontaneous emission in a gated plasmonic heterostructure.
Nat. Commun.
8
,
1631 (2017).
39. Arbabi, E. et al. MEMS-tunable dielectric metasurface lens.
Nat. Commun.
9
,
812 (2018).
40. She, A., Zhang, S., Shian, S., Clarke, D. R. & Capasso, F. Adaptive metalenses
with simultaneous electrical control of focal length, astigmatism, and shift.
Sci.
Adv.
4
, eaap9957 (2018).
41. Malek, S. C., Ee, H.-S. & Agarwal, R. Strain multiplexed metasurface
holograms on a stretchable substrate.
Nano Lett.
17
, 3641
–
3645
(2017).
42. Tseng, M. L. et al. Two-dimensional active tuning of an aluminum
plasmonic array for full-spectrum response.
Nano Lett.
17
, 6034
–
6039
(2017).
43. Ou, J.-Y., Plum, E., Zhang, J. & Zheludev, N. I. An electromechanically
recon
fi
gurable plasmonic metamaterial operating in the near-infrared.
Nat.
Nanotechnol.
8
, 252
–
255 (2013).
44. Zhu, W. M. et al. Microelectromechanical Maltese-cross metamaterial with
tunable terahertz anisotropy.
Nat. Commun.
3
, 1274 (2012).
45. Chu, C. H. et al. Active dielectric metasurface based on phase-change medium.
Laser Photonics Rev.
10
, 986
–
994 (2016).
46. Wang, Q. et al. Optically recon
fi
gurable metasurfaces and photonic
devices based on phase change materials.
Nat. Photon
10
,60
–
65
(2016).
47. Wu, P. C., Papasimakis, N. & Tsai, D. P. Self-af
fi
ne graphene metasurfaces for
tunable broadband absorption.
Phys. Rev. Appl.
6
, 044019 (2016).
48. Brar, V. W., Jang, M. S., Sherrott, M., Lopez, J. J. & Atwater, H. A. Highly
con
fi
ned tunable mid-infrared plasmonics in graphene nanoresonators.
Nano
Lett.
13
, 2541
–
2547 (2013).
49. Kuo, Y.-H. et al. Strong quantum-con
fi
ned Stark effect in germanium
quantum-well structures on silicon.
Nature
437
, 1334 (2005).
50. Papichaya, C. et al. Recent progress in GeSi electro-absorption modulators.
Sci. Technol. Adv. Mater.
15
, 014601 (2014).
51. Cho, Y.-C. et al. Optical device including three coupled quantum well
structure. Google Patents, assignee, US20150286078A1 (2015).
52. Savinov, V., Fedotov, V. A. & Zheludev, N. I. Toroidal dipolar excitation and
macroscopic electromagnetic properties of metamaterials.
Phys. Rev. B
89
,
205112 (2014).
53. Wu, P. C. et al. Optical anapole metamaterial.
ACS Nano
12
, 1920
–
1927
(2018).
54. Zhu, A. Y. et al. Giant intrinsic chiro-optical activity in planar dielectric
nanostructures.
Light Sci. Appl.
7
, 17158 (2018).
ARTICLE
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11598-8
8
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
55. Xu, Z., Wang, C., Qi, W. & Yuan, Z. Electro-optical effects in strain-
compensated InGaAs/InAlAs coupled quantum wells with modi
fi
ed potential.
Opt. Lett.
35
, 736
–
738 (2010).
56. Hao, F., Pang, J. P., Sugiyama, M., Tada, K. & Nakano, Y. Field-induced
optical effect in a
fi
ve-step asymmetric coupled quantum well with modi
fi
ed
potential.
IEEE J. Quantum Elect.
34
, 1197
–
1208 (1998).
Acknowledgements
This work was supported by Samsung Electronics (P.C.W.), and NASA Early
Stage Innovations (ESI) Grant 80NSSC19K0213 (H.A.A. & G.K.S.). The authors
used facilities supported by the Kavli Nanoscience Institute (KNI). P.C.W.
acknowledges the support from Minist
ry of Science and Technology, Taiwan
(Grant numbers: 108-2112-M-006-021-MY3; 107-2923-M-001-010-MY3; 107-
2923-M-006-004-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). The authors deeply
appreciate help in the form of the close reading of the manuscript and review
responses by Rebecca Glaudell, Phil Jahelka, Kelly Mauser, Michael Kelzenberg,
Joseph DuChene, and Haley Bauser. The authors also thank Artur Davoyan for
useful discussions.
Author contributions
P.C.W, R.A.P., and H.A.A. conceived the original idea. P.C.W. performed the numerical
design, device fabrication, built up the optical setup, and performed the optical as well as
high-speed measurements, analyzed numerical and experimental data, and wrote the
paper; R.A.P. performed the numerical design, developed the dry etching process, and
helped with the build-up of optical setup for measurement; G.K.S. helped with the
sample fabrication, designed and build-up the PCB for individually electrical control of
metasurface elements; W.-H.C. helped with the sample fabrication, data analysis, and
optical measurement; R.S. developed the theoretical model for MQWs, performed cal-
culations, and wrote the paper; M.G. helped with the high-speed measurement and data
analysis; M.A. and D.L. helped with discussions; H.A.A. organized the project, designed
experiments, analyzed the results, and prepare the papers. All authors discussed the
results and commented on the paper.
Additional information
Supplementary Information
accompanies this paper at
https://doi.org/10.1038/s41467-
019-11598-8
.
Competing interests:
The authors declare no competing interests.
Reprints and permission
information is available online at
http://npg.nature.com/
reprintsandpermissions/
Peer review information:
Nature Communications
thanks Andrey Fedyanin and the
other anonymous reviewer(s) for their contribution to the peer review of this work. Peer
reviewer reports are available.
Publisher
’
s note:
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional af
fi
liations.
Open Access
This article is licensed under a Creative Commons
Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative
Commons license, and indicate if changes were made. The images or other third party
material in this article are included in the article
’
s Creative Commons license, unless
indicated otherwise in a credit line to the material. If material is not included in the
article
’
s Creative Commons license and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from
the copyright holder. To view a copy of this license, visit
http://creativecommons.org/
licenses/by/4.0/
.
© The Author(s) 2019
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11598-8
ARTICLE
NATURE COMMUNICATIONS
| (2019) 10:3654 | https://doi.org/10.1038/s41467-019-11598-8 | www.nature.com/naturecommunications
9