Electrically tunable all-dielectric optical metasurfaces based on liquid crystals
Andrei Komar, Zheng Fang, Justus Bohn, Jürgen Sautter, Manuel Decker, Andrey Miroshnichenko, Thomas
Pertsch, Igal Brener, Yuri S. Kivshar, Isabelle Staude, and Dragomir N. Neshev
Citation: Appl. Phys. Lett. 110, 071109 (2017); doi: 10.1063/1.4976504
View online: http://dx.doi.org/10.1063/1.4976504
View Table of Contents: http://aip.scitation.org/toc/apl/110/7
Published by the American Institute of Physics
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Electrically tunable all-dielectric optical metasurfaces based on liquid
crystals
Andrei Komar,
1
Zheng Fang,
1
Justus Bohn,
2
J
€
urgen Sautter,
1
Manuel Decker,
1
Andrey Miroshnichenko,
1
Thomas Pertsch,
2
Igal Brener,
3
Yuri S. Kivshar,
1
Isabelle Staude,
1,2
and Dragomir N. Neshev
1,a)
1
Nonlinear Physics Centre and Centre for Ultrahigh bandwidth Devices for Optical Systems (CUDOS),
Research School of Physics and Engineering, The Australian National University, Canberra, 2601 ACT,
Australia
2
Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, 07745 Jena,
Germany
3
Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185,
USA
(Received 12 October 2016; accepted 25 January 2017; published online 15 February 2017)
We demonstrate electrical tuning of the spectral response of a Mie-resonant dielectric metasurface
consisting of silico n nanodisks embe dded into liquid crystals. We use the reorientation of nematic
liquid crystals in a moderate applied electric field to alter the anisotr opic permittivity tensor
around the met asurface. By switching a control voltage “on” and “off,” we induce a large spect ral
shift of the metasurface resonances, r esulting in an absolute transm ission modulation of up to
75%. Our experiment al demonstration of voltage control of dielectric metasurfaces paves the way
for new types of electrically tunable metadevices, including dynamic displays and holograms.
Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4976504]
All-dielectric metasurfaces have proven to be a versatile
tool for lossless manipulation of optical wavefronts,
1–8
spec-
trum,
9–13
and polarization.
6,14
These high transmission effi-
ciencies, derived from the lossless nature of the optical
resonances in high refractive index dielectric nanoparticles,
have spurred research into a wide range of applications,
15,16
including lenses, beam deflectors, and holograms. However,
these application prospects would broaden a lot further if the
optical properties of the metasurface could be dynamically
altered by an external control parameter, enabling applica-
tions such as flat lenses with dynamic focus, active beam
steering, and holographic optical elements. Dielectric meta-
surfaces deriving their functionality from strong localized
electric and magnetic Mie-type
17–19
resonances of designed
high-refractive index nanoparticles are particularly suited
for achieving dynamic behaviour as their resonant optical
properties are associated with a strong spectral dispersion,
which can be tailored at will by the metasurface design.
Several realizations of active tuning of the resonance posi-
tions of Mie-resonant dielectric metasurfaces were presented
recently.
20–23
The first demonstration used the temperature
dependent change of the refractive index tensor of a nematic
liquid crystal (LC) covering the silicon nanoresonators.
20
Another work proposed to use the electric-field induced
reorientation of LCs for tuning of all-dielectric metasurfa-
ces;
24
however, this effect was only theoretically studied.
More recently, resonance tuning was demonstrated based on
mechanical stretching of a flexible matrix into which the
dielectric nanoresonators are embedded.
21
This method was
also employed to realize a dielectric metasurface lens with
adjustable focal length.
22
Tuning of a dielectric metasurface
using the amor phous-crystalline transi tion in a chalcogenide
phase-change material was also achieved.
23
While all these
methods can allow for large tuning ranges, they empl oy
slow control mechanisms, namely, temperature change,
20
mechanical stretch,
21
and raster-scanni ng of a laser beam;
23
then they are rather cumbersome and unlikely to find their
way into practical applications. Ult rafast all-optical switch-
ing of Mie-resonant silicon nanostructures was also demon-
strated;
25–27
however, these effects are weak even at high
opti cal intensit ies.
Here, we demonstrate an active control of the spectral
response of Mie-resonant silicon metasurfaces using voltage-
induced reorientation of the anisotropic nematic LC in a
moderate electric field. We achieve a large tuning range of
50 nm and 75% absolute transmission change at k 1550 nm
by application of an AC bias voltage of 70 V at 1 kHz. In
addition, while a dynamic phase shift of a reflecting metasur-
face consisting of a metal and a conducting oxide was pre vi-
ously measured by Huang et al .,
28
here we directly retrieve
the tuning of the transmittance phase across the Mie-type
resonance of a dielectric metasurface.
In our work, we use an all-dielectric metasurface com-
posed of silicon nanodisks integrated into an LC cell, as
shown in Figs. 1(a) and 1(b). Being composed of crystalline
silicon, the nanodisks exhibit very low absorption losses at
wavelengths above the silicon electronic bandgap around
1.1 lm and support strong electric and magnetic dipolar Mie-
type resonances in the telecommunication spectral range,
which can be tailored by adjusting the disk height, diameter,
or lattice constant of the metasurface.
4,9
Furthermore, such
metasurfaces can be designed to operate as highly transparent
Huygens’ metasurfaces by bringing their electric and mag-
netic dipole resonances into spectral overlap.
3,4
In this work,
however, we choose the parameters such that the electric and
a)
Author to whom correspondence should be addressed. Electronic mail:
Dragomir.Neshev@anu.edu.au
0003-6951/2017/110(7)/071109/4/$30.00 Published by AIP Publishing.110, 071109-1
APPLIED PHYSICS LETTERS 110, 071109 (2017)
the magnetic resonances are well separated in order to clearly
reveal and evaluate the resonance shifts induced by the volt-
age controlled LC realignment.
The fabrication of the metasurfaces is performed using
electron-beam lithography on silicon-on-insulator wafers,
followed by reactive ion etching and residual resist removal
with an oxygen plasma, as described in detail in our previous
work.
8,9
Fig. 1(b) shows a scanning electron microscopy
(SEM) image of a typical nanodisk metasurface. In our
experiments, the nanodisks have a height of h ¼ 220 nm and
a diameter of d ¼ 606 nm. They are arranged in a square-
array with a lattice constant of a ¼ 909 nm. The dimensions
of the disks are chosen such that the electric and magnetic
dipolar resonances occur in the telecommunication wave-
length range. The fabricated metasurface is then integrated
into an LC cell in such a way that the silicon nanodisks are
fully embedded into the LC. We employ the nematic LC
Merck Licristal E7, which is widely used in display technol-
ogies and exhibi ts high birefringence (0.2), with n
e
¼ 1.7
and n
0
¼ 1.51 at room temperature.
29
The LC cell is con-
structed by sandwiching the LC between the metasurface
wafer and a glass substrate coated with indium-tin-oxide
(ITO) and with a brushed layer of Nylon-6 in 2,2,2-trichlor-
oethanol. A suitable spacer material is employed to fix the
(inside) thickness of the LC cell to 5 lm. The brushed
Nylon-6 layer induces a preferred alignment direction of the
LC. The ITO layer renders the substrate conductive for use
as an electrode. In this way, a bias voltage can be applied
between the ITO electrode and the silicon handle wafer of
the metasurface sample. Although we do not additionally
control the alignment of the LC on the metasurface, without
an applied voltage, the LC alignment induced by the brushed
Nylon-6 layer is approximately sustained throughout the LC
cell (see Fig. 1(c), voltage “off”). This was verified by a
strong optical anisotropy of the metasurface embedded into
the LC without applied voltage. The application of a bias
voltage between the two electrodes reorientates the LC mole-
cules perpendicular to the metasurface (see Fig. 1(c), voltage
“on”). As common for LC cells, we choose an AC bias volt-
age (1 kHz), which is common in the modern LC industry
standards and allows to avoid current flows, liquid crystal
orientation instabilities, heating, and other detrimental
effects associated with dc fields.
30,31
The small thickness of
the LC cell allows for strong reorientation of the LC in the
vicinity of silicon disks.
In order to investigate the effect of LC reorientation on
the optical properties of the metasurface, we numerically cal-
culate the transmittance spectra of the silicon nanodisk meta-
surface covered by an ideal uniformly distributed nematic
LC. We use the software package COMSOL Multiphys ics
and represent the LC as a homogeneous anisotropic dielec-
tric medium with n
e
¼ 1.7 and n
0
¼ 1.51.
29
The refractive
index of silicon disks is 3.5 and silica substrate is 1.45. The
angle between the metasurface plane and the LC anisotropy
axis is denoted with H
LC
, with H
LC
¼ 0
being equivalent to
the voltage “off” case and H
LC
¼ 90
to the voltage “on”
case (see also Fig. 2(a)). The polarization of the incoming
field is parallel to the anisotropy axis of the LC in the “off”
case. Thereby, the extraordinary wave is excited during the
“off” state and the ordinary wave during the “on” state. To
model the reorientation of the LC in the externally applied
electric field, we tune the angle H
LC
from 0
to 90
in steps
of 10
. These results are shown in Fig. 2(a). The white lines
highlight the spectral positions of the two resonances of inter-
est. We observe a red-shift of the magnetic dipole resonance
and a blue-shift of the electric dipole resonance with increas-
ing angle H
LC
. Thus, the two resonances, having a clear spec-
tral separation in the “off” case, move closer together for the
LC realignment. Figure 2(b) shows the transmittance spectra
for the ideal “on” and “off” cases for comparison with experi-
mental data.
Next, using a home-built white-light spectroscopy setup,
we measure the linear-optical transmittance spectra of the
metasurface integrated into the LC cell. We use an unstruc-
tured etched area of the sample inside the same LC cell as a
reference. A separate reference spectrum was taken for each
applied voltage. Other Fabry-Perot artifacts induced by the
layered structure of the system are avoided using the proce-
dure described by Chong et al.
4
generalised to the layer
structure of the LC cell. The results for the “off” case and for
an applied AC bias voltage of 70 V at 1 kHz are shown in
Fig. 3(a). In the “off” case, we can clearly identify two pro-
nounced transmission minima corresponding to the electric
and magnetic dipole resonances of the silicon metasurface.
Upon application of the bias voltage, the predicted red-shift
of the magnetic dipole resonance and the blue-shift of the
electric dipole resonance of the silicon metasurface are
clearly observed and in good qualitative agreement with
numerical expectations. Quantitatively, the observed shifts
of 29 nm for the electric and 55 nm for the magnetic reso-
nance are smaller in the experiment compared with numeri-
cal spectra (69 nm for the electric and 61 nm for the
magnetic resonance). Furthermore, we find deviations in the
exact positions of the resonances both for the “off” case and
FIG. 1. (a) Sketch of a silicon nanodisk metasurface integrated into the LC
cell. (b) SEM image of a fabricated silicon nanodisk metasurface. (c)
Schematic of the LC alignment for no applied voltage (“off” case) and for
the case when a moderate voltage is applied between the two electrodes of
the LC cell (“on” case). The red arrow indicates the polarization of the inci-
dent light.
071109-2 Komar et al. Appl. Phys. Lett. 110, 071109 (2017)
for the “on” case. This can be explained by the imperfect
alignment of the LC molecules in the vicinity of the metasur-
face. The measured absolute change in transmittance DT
¼ T(70 V) – T(0 V) for a transition from the “off” state to the
“on” state is shown in Fig. 3(b) and reaches a maximum of
75%. One can clearly see that the transmittance is switched
from opaque to transparent at around 1540 nm and vice versa
in the spectral region from 1590 nm to 1620 nm. Note that in
the literature instead of absolute transmittance (or reflec-
tance) changes, some authors prefer to specify the achieved
tuning or switching contrast DT/T
0
, which reaches up to
700% in our case for T
0
¼ T(0 V). This high value can be
attributed not only to the strong transmittance change but
also to low transmission at resonance.
Finally, we directly measured the phase change introduced
by the metasurface for different applied voltages at the
telecommunication wavelength of 1550 nm. Using a home-
built imaging Mach-Zehnder interferometer and the phase
retrieval method previously described by Chong et al.,
4
we
imaged the phase imprinted onto the incident light field upon
transmission through the metasurface for a variation of the
applied voltage from 0 V to 70 V in steps of 5 V. The wave-
length of the employed laser source is k ¼ 1550 nm, which is
close to the spectral position of the magnetic dipole resonance
in the “off” case and in an off-resonant spectral regime of the
metasurface for the “on” case. For a quantitative evaluation of
the induced phase change, we average the phase values over
the spatial extent of the metasurface as a function of applied
voltage. These results are shown in Fig. 4(b). As expected
for complete detuning of a single resonance away from
the laser wavelength, w e observe a phase change with a
magnit ude of up to p. The switching dynamics was mainly
observed between 10 V a nd 30 V corresponding to the reori-
entation of the liquid c rystals. While for a single resonance,
the observed phase change is connected to a change in
transmission intensity, the concept of silicon Huygens’
metasu rfaces employing overlapping electric and magnetic
dipoleresonancesmayopenawaytoachieveaphase
change without changing the transmission intensity.
3
In conclusion, we have demonstrated voltage tuning of
the spectral positions of the electric and magnetic dipolar
Mie-type resonances of an all-dielectric metasurface using the
reorientation of the nematic liquid crystals by an external
FIG. 2. (a) Simulation of the spectral resonance shifts expected for a com-
plete reorientation of the LC surrounding the metasurface. H
LC
denotes the
angle between the metasurface plane and the LC anisotropy axis, with volt-
age “off” corresponding to H
LC
¼ 0
and voltage “on” corresponding to
H
LC
¼ 90
. White lines denote the spectral positions of the electric (solid,
ED) and magnetic (dashed, MD) resonance positions. The additional spec-
tral features at small wavelengths correspond to lattice-type resonances. (b)
Calculated transmittance spectra for H
LC
¼ 0
(blue) and H
LC
¼ 90
(red).
The resonance positions of the electric (ED) and magnetic dipole (MD) reso-
nances are indicated by the solid and dashed vertical lines, respectively.
FIG. 3. (a) Measured transmittance spectra for 0 V (blue) and 70 V (red)
bias voltages. The spectral positions of the electric (ED) and magnetic dipole
(MD) resonances are indicated by the solid and dashed vertical lines, respec-
tively. (b) Measured change in transmittance DT ¼ T(70 V) T(0 V) upon
application of the bias voltage.
071109-3 Komar et al. Appl. Phys. Lett. 110, 071109 (2017)
electric field. We achieved experimental transmission modula-
tions of up to 75% and a phase change of up to approximately
p. Our results pave the way for realizing dynamically recon-
figurabl e metasurfaces with broad application prospects,
including dynamic holography, tunable imaging, and active
beam steering.
The authors acknowledge the financial support from the
Australian Research Council and the Thuringian State
Government within its ProExcellence initiative (ACP
2020
).
This work was performed, in part, at the Center for Integrated
Nanotechnologies, an Office of Science User Facility
operated for the U.S. Department of Energy (DOE) Office of
Science. Sandia National Laboratories is a multiprogram
laboratory managed and operated by Sandia Corporation, a
wholly owned subsidiary of Lockheed Martin Corporation,
for the U.S. Department of Energy’s National Nuclear
Security Administration under Contract No. DE-AC04-
94AL85000. The authors acknowledge their participation in
the Erasmus Mundus NANOPHI project under Contract No.
2013 5659/002-001.
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FIG. 4. (a) Measured transmission phase across the array of the LC metasur-
face for different biasing voltages at k ¼ 1550 nm. (b) Phase accumulated by
the light transmitted through the metasurface. The error bars indicate the
spatial phase variation observed over the extent of the metasurface.
071109-4 Komar et al. Appl. Phys. Lett. 110, 071109 (2017)
