of 13
NEMS-tunable dielectric chiral metasurfaces
Hyounghan Kwon
1,2
,
Andrei Faraon
1,2,*
1
T. J. Watson Laboratory of Applied Physics and Kavli Nanoscience Institute, California Institute of
Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
2
Department of Electrical Engineering, California Institute of Technology, 1200 E. California Blvd.,
Pasadena, CA 91125, USA
Abstract
Active control of strong chiroptical responses in metasurfaces can offer new opportunities for
optical polarization engineering. Plasmonic active chiral metasurfaces have been investigated
before, but their tunable chiroptical responses is limited due to inherent loss of plasmonic
resonances, thus stimulating research in low loss active dielectric chiral metasurfaces. Among
diverse tuning methods, electrically tunable dielectric chiral metasurfaces are promising
thanks to their potential for on-chip integration. Here, we experimentally demonstrate nano-
electromechanically tunable dielectric chiral metasurfaces with reflective circular dichroism (CD).
We show a difference between absolute reflection under circulary polarized incident light with
orthogonal polarization of over 0.85 in simulation and over 0.45 experimentally. The devices
enable continuous control of CD by induced electrostatic forces from 0.45 to 0.01 with an
electrical bias of 3V. This work highlights the potential of nano-electromechanically tunable
metasurfaces for scalable optical polarization modulators.
Keywords
Chiral metasurfaces; NEMS; Polarization modulator; Photonic crystal; Guided mode resonance
I. INTRODUCTION
Chirality is an asymmetric feature describing structures that are not superimposed onto
their mirror images. Chiral structures are known to interact differently with light polarized
with different handedness and the optical responses that are sensitive to handedness are
called chiroptical responses. Although chirality is ubiquitous in various molecules, the
*
Corresponding author: A.F.: faraon@caltech.edu.
AUTHOR CONTRIBUTIONS
H.K. and A.F. conceived the project. H.K. designed the device, fabricated the samples, performed the measurements, and analyzed the
data under the supervision of A.F.. H.K. and A.F. co-wrote the manuscript.
Supporting Information
This material is available free of charge via the internet at
https://pubs.acs.org/
.
Measured design parameters of the devices, schematic illustration of the experimental setup, numerical investigation on fabrication
errors, measured reflection spectra with electrical biases.
CONFLICT OF INTERESTS
The authors declare no competing financial interests.
HHS Public Access
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Published in final edited form as:
ACS Photonics
. 2021 October 20; 8(10): 2980–2986. doi:10.1021/acsphotonics.1c00898.
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chiroptical effects in natural materials are generally very weak, requiring considerable
propagation distances to observe the chiroptical effects such as circular dichroism (CD)
or circular birefringence. To overcome the limited amount of the natural chiroptical effects,
chiral metamaterials or metasurfaces have been investigated in the past two decades [
1
3
]. First, plasmonic 3D-printed metamaterials or multilayers of patterned metasurfaces
have been explored [
4
7
]. Despite their strong and broadband chiroptical properties,
complicated fabrication procedures limit practical applications and extensions to tunable
devices. Planar plasmonic chiral metasurfaces with a single patterned layer have also been
investigated. In particular, patterned plasmonic chiral metasurface on top of a flat back
metallic mirror has achieved high circular dichroism [
8
10
]. Unlike conventional metallic
mirrors which reflect circular polarized light with reversal of handedness, these plasmonic
mirrors selectively reflect one circularly polarized light without changing the handedness
while the other circular polarization is absorbed [
8
10
]. These polarization selective
mirrors are often called chiral spin-preserving mirrors and have potential applications in
valley exciton-polaritonics [
11
]. Moreover, the plasmonic chiral metasurfaces can support
broadband chiroptcial responses and large CD in absorption. Complementary to plasmonic
metastructures, all-dielectric chiral metasurfaces also have shown strong chiroptical effects
[
12
14
]. In particular, it has been recently demonstrated that a single-layer dielectric
metasurface is able to realize near-unity CD in reflection [
14
]. Unlike the aforementioned
plasmonic chiral structures with back metallic mirrors [
8
10
], the single-layer dielectric
chiral metasurface selectively reflects one circular polarized light with preserved handedness
and transmits the other circular polarized light with flip of handedness [
14
].
Dynamical control of the chiroptical effects in metamaterials can boost development of
devices for novel polarization control. During the last decade, reconfigurable plasmonic
chiral metastructures have been extensively investigated in THz and microwave domain
[
15
18
]. For example, diverse active platforms using optical tuning [
15
], global mechanical
deformation [
16
], micro-electromechanical systems [
17
], and electrical gating of graphene
[
18
] have been proposed. In addition to devices working in THz or microwave domain,
dynamic plasmonic chiral metasurfaces have been also extensively studied in the optical
domain. For example, all-optical tuning of phase change materials [
19
] or DNA structures
[
20
] has enabled switchable chiroptical responses. Also, global environmental tuning of
liquid [
21
], pH [
22
], strain [
23
], and magnetic field [
24
] have been explored. However,
these all-optical or global tuning methods often require complicated setups hindering the
on-chip integration. Thus, electrical control of the chirality can be more attractive than other
methods for practical applications. Recently, the nano-electromechanically tunable chiral
plasmonic metasurfaces have been demonstrated, thus enabling active control of polarization
and chiroptical responses through nano-electromechanical actuation in vertical direction
[
25
,
26
]. Nevertheless, all of the aforementioned tunable plasmonic chiral metasurfaces are
inherently lossy which limit optical performance.
In contrast to the extensive works on tunable plasmonic chiral metasurfaces, investigations
related to tunable dielectric chiral metasurfaces have been limited. For instance, optothermal
moving of nanoparticles [
27
] and all-optical tuning of Si nonlinearity [
28
] have enabled
tunable chiroptical responses. However, both systems still require additional optical systems
to modulate the device optically, imposing considerable limits on compact integration [
27
,
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28
]. More broadly, optomechanical GaAs chiral metasurfaces have been demonstrated using
excitation of global mechanical oscillation of the membrane through a bulky piezoelectric
actuator [
29
]. However, the system has not just shown weak tunable response mainly due
to the limited movement of the membrane, but also only enabled the oscillation of the
output signal [
29
]. Therefore, to the best of our knowledge, electrical tuning of dielectric
chiral metasurfaces has not been explored and can be a promising alternative to that of the
plasmonic chiral metasurfaces thanks to the low-loss optical property of dielectric materials.
Furthermore, from a perspective of devices, scalable material platforms such as silicon and
low electrical bias within CMOS logic level can be important for on-chip integration. In
this work, we experimentally demonstrate nano-electromechanically tunable dielectric chiral
metasurfaces in the telecom wavelength, fabricated by using standard silicon-on-insulator
platforms. The metasurfaces resonantly work as chiral spin-preserving mirrors, exhibiting
selective reflection for one circular polarization without external bias. Furthermore, the
resonances hosted by the metasurfaces can be continuously tuned by electrostatic forces
using voltages less than 3V. In particular, the devices experimentally achieve transition of
CD in reflection from 0.45 to 0.01 at the resonant wavelength.
II. MAIN RESULTS
Figure 1 shows the conceptual illustrations of the proposed nano-electromechanically
tunable chiral metasurface. In Fig. 1a, the illustrative schematic of the top view of the
suspended silicon chiral metasurface is displayed. The metasurface consists of two sets of
pairs of doped silicon nanostructures and an electrode is deposited on each set for electrical
bias. In Figs. 1a and 1b, different colors are used to explicitly visualize the two sets of
the nanostructures. Throughout this paper, all devices are composed of 675 nm thick and
45
μ
m long silicon nanostructures. The design parameters are shown in Fig. 1b. Period,
p
,
is chosen to be 700 nm so that the period of the pair is smaller than the wavelength of
interest to avoid unwanted diffraction under normal incidence. As shown in Fig. 1b, we
intentionally break n-fold rotational symmetry for n>2 and any in-plane mirror symmetry
to achieve strong chiroptical effects in reflection. It is worth noting here that this design
approach has been extensively used for single layer dielectric chiral metasurfaces [
14
] and
plasmonic chiral metasurfaces having back metallic reflectors [
8
10
]. When electrical bias
is applied, all pairs of the neighboring nanostructures that are connected to the different
electrodes have voltage difference and become capacitors. Thus, the induced electrostatic
forces between the nanostructures enable continuous mechanical actuation as a function of
the external bias. In other words,
g
1
(
g
2
) decreases (increases) by applying the bias, where
g
1
(
g
2
) is the gap size between the two neighboring bars in the different (same) set. In Fig.
1c and Fig. 1d, optical functions of the metasurface are schematically illustrated. In Fig. 1c,
the structure without the actuation reflects right circular polarized (RCP) light without flip
of the handedness while left circular polarized (LCP) light is transmitted with reversal of
handedness. When the bias is applied, the suspended nanostructures are actuated and their
chiroptical properties are continuously modulated. As shown in Fig. 1d, the metasurface
can exhibit negligible chiroptical responses with the actuation so its co- and cross-polarized
reflection and transmission become identical for LCP and RCP illuminations at the target
wavelength.
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First, the proposed metasurfaces are numerically investigated using a commercial software
based on the finite element method, COMSOL
®
(see Methods for details). The polarization
analysis of reflection from the metasurface is plotted in Fig 2a. The reflection coefficient
R
L,L
(
R
R,R
) is defined as the reflection of the LCP (RCP) component from the metasurfaces
to the LCP (RCP) input light. Similarly,
R
R,L
(
R
L,R
) is defined as the reflection of the
RCP (LCP) component from the metasurfaces to the LCP (RCP) input light. In Fig. 2a,
the polarization-sensitive reflection is observed near the resonant wavelength of 1478 nm.
The metasurface selectively reflects RCP light without flip of the handedness, while the
LCP light is mostly transmitted. Specifically,
R
L,L
,
R
R,R
, and CD in reflection, defined by
|
R
L,L
-
R
R,R
| in this paper, are 0.04, 0.89, and 0.85, respectively. Moreover,
R
R,L
and
R
L,R
are identical due to the symmetry of the unit cell [
30
] and it is confirmed by the grey
curve plotted in Fig.2a. In Fig. 2b, electric field profiles in the
x
y
plane at the middle of
the nanostructures (i.e. 337.5 nm above from the bottom) are plotted under LCP and RCP
illuminations, showing that the metasurfaces interact with LCP and RCP lights differently at
the resonance. The chiroptical responses in Fig. 2a and the two distinct electric field profiles
in Fig. 2b can be qualitatively explained by the spectral overlap of two leaky guided mode
resonances hosted by the dielectric metasurfaces [
14
]. For example, the input polarization
state determines the amplitude and phase of two leaky guided modes and the interference
between the resonant modes results in the two distinct field profiles shown in Fig. 2b. The
input polarization state also affects the phase and amplitude of the radiations from the two
leaky guided modes, so the radiations from the guided modes interfere differently with
directly transmitted or reflected light. Therefore, the reflection spectra shown in Fig. 2a
highly depend on the handedness of the input polarization.
To simulate the mechanical tuning of the chiroptical effects, the spectra of
R
L,L
and
R
R,R
are calculated by varying the nanobeam displacement,
g
2 −
g
1
2
, from 0 to 80 nm and plotted
in Fig. 2c. In Fig. 2c, an increase of
g
2 −
g
1
2
causes red shift of peaks in the spectra of
R
R,R
and blue shift of dips in the spectra of
R
L,L
. Furthermore, the amount of the spectral shift of
the
R
L,L
is larger than that of
R
R,R
. To visualize the change of the chiroptical effects clearly,
|
R
L,L
R
R,R
| is evaluated from Fig. 2c and plotted in Fig. 2d. In Fig. 2d, it is clearly shown
that the mechanical displacement in lateral direction causes the strong change of |
R
L,L
R
R,R
| around the resonant wavelength of 1478 nm in Fig.2d. The 80 nm displacement leads
to a change of |
R
L,L
R
R,R
| from 0.85 to 8×10
−4
at 1478 nm in Fig. 2d. Namely, the
mechanical actuation in the lateral direction results in the transition from strong to negligible
chiroptical response even with the presence of chirality in the structure.
To experimentally verify nano-electromechanically tunable chiroptical responses, the
devices are fabricated using a conventional nanofabrication process for silicon-on-insulator
substrates (see Methods for details). We should mention here that
g
1
and
g
2
are adjusted in
the fabrication process such that
g
1
is 60 nm smaller than
g
2
. This adjustment allows for a
large shift of the resonance with nano-electromechanical tuning of the gaps. Figure 3a shows
scanning electron microscopy images, confirming good agreement with the illustrative
schematics in Figs. 1a and 1b. To characterize chiroptical responses of the devices, spectra
of
R
L,L
and
R
R,R
are measured using the setup in Supporting Fig. 1. The measured spectra
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shown in this paper are normalized by the reflection from 65 nm gold electrode in order to
estimate the absolute reflection efficiency and remove fluctuations resulting from variations
in polarization states of the input tunable laser (see Methods and Supporting Figure 1
for details about measurement procedures). Two different devices are measured and the
corresponding spectra of
R
L,L
and
R
R,R
are plotted in Figs. 3b and 3c (see Supporting Table
1 for the measured design parameters of the devices). In Fig. 3b, the dip in the spectrum
of
R
L,L
and the peak in the spectrum of
R
R,R
are overlapped at ~1475 nm, showing good
agreement with the shape of the simulated spectra in Fig. 2a. However, the dips of the
spectra of
R
L,L
and
R
R,R
in Fig. 3c are overlapped at ~1492 nm. This deviation mainly
results from the non-perfect match of the design parameters. Specifically, the value of
l
p
is
slightly larger than its optimal value (see Supporting Figure 2 for details). The spectra in
Figs. 3b and 3c show maximal |
R
L,L
R
R,R
| of 0.45 and 0.37 at the resonance wavelengths
of 1475 nm and 1493 nm, respectively. Even with a few percentage reflection loss of
the gold electrode used for the normalization, the maximal values calculated from Figs.
3b and 3c are still higher than 0.44 and 0.36, respectively. On the other hand, the ratios
between
R
L,L
and
R
R,R
reach 3.58:1 and 8.19:1 in Figs. 3b and 3c, respectively. The large
ratio between
R
L,L
and
R
R,R
directly indicates the potential for electromechanically tunable
circular polarization filters. The measured values of |
R
L,L
R
R,R
| are smaller than the
simulated value shown in Fig. 2a. We believe that the deviation results from the finite length
of the resonators, which may cause limited coupling between the resonance and the input
light. Furthermore, the imperfect fabrication and the high sensitivity of the design shown
in Fig. S2 possibly lead to the difference between the measurement and the simulation.
However, we should note here that optimized single-layer dielectric metasurfaces are able to
reach near unity CD in reflection [
14
].
To demonstrate nano-electromechanical tuning of the chiroptical responses, static electrical
bias is applied to the electrodes and the induced changes in the optical reflection are
characterized. The spectra of |
R
L,L
R
R,R
| are measured under several electrical biases and
plotted in Figs. 4a and 4b (see Supporting Figure 3 for the measured spectra of
R
L,L
and
R
R,R
). The devices used in Figs. 3b and 3c are used for Figs. 4a and 4b, respectively. In
both spectra, the CD in reflection is varied from the maximum value to nearly zero. For
example, an external bias of 2.75 V (2.8 V) causes a change in CD from 0.45 (0.37) to 0.01
(1×10
−4
) in Fig. 4a (Fig. 4b). The required electrical bias for the maximal change of the
chiroptical response is smaller than 3V, which is already within CMOS logic level. Also, the
static bias causes blue-shifts of the main peaks of the |
R
L,L
R
R,R
|, which mainly results
from the dominant blue-shift of
R
L,L
shown in Fig. 2c. The peak shifts of |
R
L,L
R
R,R
|
shown in Fig. 4a and Fig. 4b are as large as −2 nm and −6 nm under the bias of 2.75 V
and 2.8 V, respectively. The large spectral shifts up to 6 nm indicate that the low-Q leaky
guided resonances are sufficient to achieve considerable chiroptical tunablity. The large
spectral shift is advantageous in terms of bandwidth and robustness, which are important
from practical considerations. Furthermore, the measured spectral shift is the lower bound of
the limit, as the induced voltage up to 2.8V is smaller than the pull-in voltage.
Finally, the dynamic response of the chiral metasurfaces is investigated in air. A periodic
square-wave signal with a modulation frequency of 100 Hz, amplitude of 2V, and duty cycle
of 50% is applied (see Methods for details). The device used for Fig. 4b is measured with
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the input light at the wavelength of 1493 nm. The input electrical signals and the measured
raw signals of
R
L,L
are plotted in Fig. 4c. It clearly demonstrates that the optical responses
are fully reconfigurable. In Fig. 4c, the measured rise time (up to 90% power) and fall time
(down to 10% power) are 1.48 ms and 460
μ
s, respectively. We believe that the low speed
mainly results from the low-doping density of the Si layer of the silicon-on-insulator wafer.
III. DISCUSSION AND SUMMARY
Many aspects of the proposed devices can be improved with further investigations. First of
all, the modulation depths of CD in reflection shown in Figs. 4a and 4b are not limited
by the spectral shifts but by the measured maximal CD in Figs. 3b and 3c. Considering
the low loss of the dielectric metasurface, a large sweep of the design parameter spaces
may result in near unity CD in reflection [
14
]. Furthermore, inverse-design of the tunable
chiral metasurfaces can be of interest [
31
]. The inverse-design may not only improve CD
in reflection, but also find other guided modes that are more robust against imperfect
fabrication or more efficient for nano-electromechanical tuning. Specifically, the inverse
design may find two leaky guided mode resonances which similarly react to variations
in design parameters. Thus, the large chiroptical responses resulting from the interference
between two modes possibly become tolerant against imperfect fabrication. For efficient
nano-electromechanical tuning in lateral direction, the optical modes can be optimized to
store considerable electromagnetic energy at the gap between the nanostructures instead
of inside of the nanostructures. In addition to the optical aspects, considering the scale of
the devices, we envision that high switching speed up to a few MHz is achievable with
a highly-doped Si layer, co-optimization from both mechanical and optical perspectives,
and proper packaging [
32
,
33
]. With mechanical resonances supported by the metasurfaces,
the proposed devices might provide efficient electromechanical platforms for polarization
controlled optomechanical transduction [
29
].
In conclusion, we demonstrate nano-electromechanical tuning of the suspended silicon
chiral metasurfaces. With an external bias below 3V, the devices experimentally achieve
continuous tuning of CD in reflection from 0.45 to 0. This work paves the way of
nano-electromechanically tunable dielectric chiral metasurfaces towards scalable and novel
optical modulators, which can be used in diverse applications such as dynamic polarization
engineering, stereoscopy, valleytronics, polarization optomechanics, and chiral sensing.
IV. METHODS
Simulation and design
The reflected spectra of the metasurface were calculated using a commercial software based
on the finite element method, COMSOL
®
. Assuming an infinite periodic array, the 3D
silicon structure is simulated in air with normal incident light. The design parameters used in
the simulation are shown in Fig. 2.
Device fabrication
The devices are fabricated using a silicon-on-insulator SOI wafer. The detailed fabrication
process can be found in Ref. [
34
]. We use a wafer having a device layer of 675 nm and a
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buffered oxide layer of 3
μ
m on a 1 mm thick silicon wafer. The fabrication includes two
sequential e-beam lithography steps. First, the metasurfaces were fabricated with e-beam
lithography and reactive ion etching. Subsequently, the electrodes are defined by the e-beam
lithography, e-beam deposition of 65 nm gold/5 nm chromium layers, and lift-off process.
Buffered hydrofluoric acid was used to etch the buffered oxide layer under the silicon layer.
The time of the under-cut process is carefully controlled so that the metasurfaces are fully
suspended while the anchors are supported by the SiO
2
. In other words, one end of every
suspended nanostructure is connected to anchors and the other end is connected to the
silicon layer. That is because the connections on both sides prevent breaking during the
under-cut. To prevent destruction of the suspended device, the device is dried by a critical
point dryer after the under-cut process. The device is bonded to a custom printed circuit
board using a wire bonder (WestBond 7476D).
Measurement procedure
All of the reflection spectra presented in this paper are characterized using the set-ups shown
schematically in Supporting Fig. 1. We use a tunable laser (Photonetics, TUNICS-Plus) as
the light source and the wavelength of the light is tuned from 1450 nm to 1580 nm. A
beam splitter is placed in front of the fiber collimator (Thorlabs, F260FC-1550) to capture
the power from the source and send the light to the sample. For reference, the power from
the source is captured by a InGaAs detector (Thorlabs, PDA10CS). A linear polarizer and
a quarter waveplate (QWP) are inserted between a polarization beam splitter and a 20×
infinity-corrected objective lens (Mitutoyo, M Plan Apo NIR) to set the polarized state of
the incident light. The QWP is mounted on a rotation stage to set the input polarization
state to LCP or RCP. The sample at the object plane is imaged by the objective lens and
a tube lens with a focal length of 200 mm. At the image plane, a pinhole with a diameter
of 400
μ
m is inserted to select a region of interest with a diameter of 20
μ
m in the object
plane. The spatially filtered light was simultaneously focused onto another InGaAs detector
for the measurement of the spectra, or imaged on an InGaAs SWIR camera (Goodrich,
SU320HX-1.7RT) using relay optics. All spectra in this paper were obtained by dividing
the signal from the sample by the signal from the sources. To estimate absolute reflection
and remove fluctuation resulting from variation in polarization states of the input laser, the
spectra are normalized by the reflection from the 65 nm gold electrode. Considering that
the reflection of the 65 nm of gold layer is ~98% in simulation, the actual reflection can
be a few percentages smaller than the values presented in Figs. 3 and 4. For the measured
dynamic responses shown in Fig. 4, we use a function generator (FeelTech, FY6600-60M).
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
ACKNOWLEDGEMENTS
We thank Tianzhe Zheng for helpful discussion and preparation of printed circuit boards. This work was supported
by the National Institutes of Health (NIH) brain initiative program, grant NIH 1R21EY029460-01. The device
nanofabrication was performed at the Kavli Nanoscience Institute at Caltech. H.K. acknowledges a fellowship from
Ilju organization.
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DATA AVAILABILITY
The data that support the findings of this study are available from the corresponding author
upon request.
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Kwon and Faraon
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FIG. 1. Nano-electromechanically tunable all-dielectric chiral metasurfaces.
a
Schematic illustration of a top view of the metasurface. The metasurface is composed
of two sets of doped silicon nanostructures. Anchors and the gold electrodes are marked.
Electrodes are deposited on each silicon layer for mechanical actuation.
b
Schematic
illustration of two pairs of the nanostructures constituting the metasurface. Geometric
parameter definitions are shown in the illustration.
c
and
d
Illustrations of reflection
behavior of the metasurfaces without and with external bias.
c
: Without external bias, the
metasurface selectively reflects RCP light by keeping the handedness and transmits LCP
light with change of handedness.
d
With actuation, the metasurface becomes achiral. The
amounts of co- and cross-polarized reflection and transmission become symmetric for both
RCP and LCP input lights. In
a-d
, two different colors are employed to distinguish two sets
of the nanostructures and visualize voltage difference between two sets. As shown in
b
, pink
and red colors represent ground, GND, and applied bias,
V
0
, respectively.
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FIG. 2. Simulated mechanically tunable chiroptical responses.
a
Simulated reflection spectra of co- and cross-polarized components under RCP and LCP
illuminations. The spectra of
R
L,L
and
R
R,R
are plotted by dashed and solid black lines,
respectively.
R
R,L
and
R
L,R
are identical and plotted together by a grey line. Geometric
parameters used in the simulation:
p
= 700 nm,
w
= 505nm,
g
1
=
g
2
= 195 nm,
w
p
= 85nm, and
l
p
= 265nm.
b
Electric field distribution cuts from a middle plane of the
nanostructure. The magnitude of the field profiles are plotted under LCP (left) and RCP
(right) illuminations at the resonance wavelength of 1478 nm.
c
Calculated reflection spectra
of
R
L,L
and
R
R,R
with mechanical movements. The spectra of
R
L,L
and
R
R,R
are plotted
in dashed and solid lines, respectively. The mechanical displacement, expressed by
g
1 −
g
2
2
,
varies from 0 nm to 80 nm. The value of
g
1 −
g
2
2
for each color is shown in the legend.
d
Spectra of circular dichroism in reflection, |
R
L,L
R
R,R
|, for the different mechanical
displacement. The spectra are calculated from
c
. The value of
g
1 −
g
2
2
for each color is shown
in the legend.
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FIG. 3. Dielectric chiral metasurfaces and measurements of chiroptical responses in reflection.
a
Scanning electron microscope images of the fabricated metasurfaces. Left: An array of
the nanostructures. Right: Zoom-in scanning electron microscope image of the 2 pairs of
the nanostructures. Scale bars in left and right denote 5
μ
m and 1
μ
m, respectively.
b
and
c
Measured reflection spectra of
R
L,L
, and
R
R,R
for two different structures (see Supporting
Table 1 for the measured design parameters). The spectra of
R
L,L
and
R
R,R
are plotted in
dashed and solid lines, respectively.
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FIG. 4. Nano-electromechanical tuning of chiroptical responses.
a
and
b
Measured circular dichroism in reflection, |
R
L,L
R
R,R
|, under different external
biases. The devices exploited in Fig. 3b and Fig. 3c are measured and plotted in
a
and
b
, respectively. The applied bias for each color is shown in legends.
c
Measured temporal
response of the metasurfaces. Top: Input square wave signal of which duty cycle, frequency,
and amplitude are 0.5, 100 Hz, and 2V, respectively. Bottom: Measured output signals of
R
L,L
by a photodetector. Raw and filtered reflection signals are plotted by grey and black
curves, respectively.
Kwon and Faraon
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