Article
https://doi.org/10.1038/s41467-024-45544-0
Dynamic light manipulation via
silicon-organic slot metasurfaces
Tianzhe Zheng
1,6
,YiranGu
2,6
, Hyounghan Kwon
1,3,4
, Gregory Roberts
1,5
&
Andrei Faraon
1,3
Active metasurfaces provide the opportun
ity for fast spatio-temporal control
of light. Among various tuning methods,
organicelectro-opticmaterialspro-
vide some unique advantages due to thei
r fast speed and large nonlinearity,
along with the possibility of using f
abrication techniques based on in
fi
ltration.
In this letter, we report a silicon-organic
platform where organic electro-optic
material is in
fi
ltrated into the narrow gaps of slot-mode metasurfaces with high
quality factors. The mode con
fi
nement into the slot enables the placement of
metallic electrodes in close proximity
, thus enabling tunability at lower vol-
tages. We demonstrate the maximum t
uning sensitivity of 0.16nm/V, the
maximum extinction ratio of 38% withi
n ± 17V voltage at telecommunication
wavelength. The device has 3dB band
width of 3MHz. These results provide a
path towards tunable silicon-organi
c hybrid metasurfaces at CMOS-level
voltages.
Relying on sub-wavelength nanostructures, metasurfaces have been
shown as promising candidates for replacing conventional free-space
optical components by arbitrarily manipulating the amplitude, phase,
and polarization of optical wavefronts in certain applications
1
–
3
.In
recent years, the scope of their applications has been expanded
towards complete spatio-temporal control through the introduction
of active metasurfaces. These developments open up exciting new
possibilities for dynamic holography
4
, faster spatial light modulators
5
,
and fast optical beam steering for LiDAR
6
. Large efforts have been
channeled into various modulation mechanisms
7
. Microelec-
tromechanical and nanoelectromechanical systems (MEMS and
NEMS)
8
–
11
have the advantages of low-cost and CMOS-compatibility,
but the speed is limited up to MHz. Phase-change materials
12
–
14
have
fast, drastic, and non-volatile refractive index change, but lack con-
tinuous refractive index tuning and have a limited number of cycles
constraining applicability to recon
fi
gurable devices. Through mole-
cule reorientation, liquid crystal can have index modulation over 10%,
while under relatively low applied voltages Tunable liquid crystal
metasurfaces, U.S. patent number 10,665,953 [Application Number
16/505,687]
15
. Techniques of liquid crystal integration have also
advanced after decades of development. However, the tuning speeds
are limited to kHz range
16
. Thermal-optic effects can induce relatively
large refractive index changes
17
,
18
, but the speed is inherently limited
and the on-chip thermal management can be challenging. The co-
integration of transparent conductive oxide and metallic plasmonic
structures
5
,
6
has been demonstrated in epsilon-near-zero (ENZ) regime
to control the wavefront of re
fl
ected light, but the low re
fl
ection
amplitude induced by the optical loss of the materials and the ENZ
regime is unavoidable.
In modern photonics, a multitude of technologies for tunable
optics and frequency conversion
19
,
20
are realized with nonlinear
materials that have low loss and a strong
χ
(2)
effect, such as lithium
niobate
21
,
22
, aluminum nitride
23
, and organic electro-optic (OEO)
materials
24
. Their ultrafast responses make it possible to use RF or
millimeter-wave control
25
. Developments in computational chemistry
have also led to arti
fi
cially engineered organic molecules that have
Received: 30 May 2023
Accepted: 25 January 2024
Check for updates
1
T. J. Watson Laboratory of Applied Physics and Kavli Nanoscience Institute, California Institute of Technology, 1200 E. California Blvd., Pasadena
,CA91125,
USA.
2
Department of Applied Physics and Material Science, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA.
3
Department
of Electrical Engineering, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA.
4
Present address: Center for Quantum
Information at Korea Institute of Science and Technology, 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul, Republic of Korea.
5
Present address: Tech4Health
Institute, New York University Langone Health, New York, NY 10016, USA.
6
These authors contributed equally: Tianzhe Zheng and Yiran Gu.
e-mail:
faraon@caltech.edu
Nature Communications
| (2024) 15:1557
1
1234567890():,;
1234567890():,;
record-high nonlinear coef
fi
cients with long-term and high-
temperature stability
26
,
27
. However, their potential in modifying free-
space light has been relatively unexplored until recently. Several OEO
material-hybrid designs have demonstrated improved tunability of
metasurfaces
28
–
30
. Utilizing dielectric resonant structures and RF-
compatible coplanar waveguides, a free-space silicon-organic mod-
ulator has recently accomplished GHz modulation speed
31
.However,
all demonstrations to date require high operating voltages ± 60V, due
to low resonance tuning capability (frequency shift / voltage), which
hinders their integration with electronic chips.
In this work, we propose combining high-Q metasurfaces based
on slot-mode resonances with the unique nano-fabrication techniques
enabled by OEO materials, which drastically reduces the operating
voltage. The low voltage is mainly achieved from the ability to place the
electrodes in close proximity to each other while hosting high-Q
modes in between and the large overlap of the optical and RF
fi
elds in
OEO materials. In the following sections, we
fi
rst provide the design
concepts and considerations for achieving a reduced operating vol-
tage. Next, we numerically demonstrate the advantage of a particular
selected mode compared to other supported modes in the structure.
Finally, we experimentally realize our concepts and characterize the
performance of the electro-optic metasurface.
Results
The reported device and its operation scheme are depicted in Fig.
1
.
Light polarized along
x
(
E
x
) is incident onto the device along
−
z
direction, and then couples into the slot mode hosted in between the
silicon nano-bars. Gold electrodes are placed on top of the nano-bars
and doped silicon is used to maximize the voltage drop across the slots
fi
lled with the OEO material. The active OEO material regions have
nonlinear coef
fi
cients
r
33
with each two adjacent slots exhibiting
opposite signs of nonlinear coef
fi
cients due to the poling
fi
eld
direction. When the operating signal is applied, the active layer indu-
ces a refractive index change
Δ
n
ð
t
Þ
=
1
2
n
3
mat
r
33
E
ext
ð
t
Þ
ð
1
Þ
where
E
ext
is the external electric
fi
eld in the OEO material. Notice that
due to the geometry of the electrodes, the signal electric
fi
elds
E
ext
(
t
)
also have opposite signs in adjacent gaps, as shown in Fig.
1
.Therefore,
the overall responses from any two adjacent slots are the same. Besides
the operating voltage or the
fi
eld in slots, the perturbation strength to
the optical mode also depends on the overlap factor
Γ
c
between the
electric
fi
eld pro
fi
le and the optical mode pro
fi
le.
Γ
c
is irrelevant to the
external voltages. To calculate this overlap factor, we need to treat
E
ext
,
r
33
as spatial dependent variables. Based on cavity perturbation
theory
32
(See the full derivation in supplementary section 2), we could
extract the formula to calculate the shift of the resonant frequency:
Δ
ω
=
Δ
n
avg
n
mat
ω
Γ
c
ð
2
Þ
where
Δ
n
avg
=
1
2
n
3
mat
r
33
V
ext
w
g
denotes the refractive index change
upon applying a constant
fi
eld of
V
ext
w
g
across the gap.
By examining Equation (
2
), we can gain valuable insights into two
distinct methods for reducing the voltage of the device: reducing the
distance between electrodes and increasing the overlap factor. How-
ever, introducing closer metallic electrodes leads to inevitable losses,
thus reducing the quality factors and limiting the sensitivity to
refractive index changes. The
refore, transmitting electric
fi
elds
through conductive dielectrics is preferred. In our reported device,
doped silicon acts as the electrodes with gap width (
w
g
)downto
100nm. At the same time, this gap between the silicon nano-bars will
+
-
E
pol
E
pol
t
t
II
V
x
y
z
GND
V
V
r
33
−r
33
Fig. 1 | Conceptual schematic of silicon-organic electro-optic tunable
metasurfaces.
A beam of light is incident on the metasurface, which consists of
silicon nano-bars. The light is coupled into the slot mode inside the metasurface,
which is sensitive to any refractive index perturbation in the slots. The OEO material
is coated on top of the metasurface and
fi
lls the slot waveguide between the silicon
nano-bars. The organic molecules inside the slots are aligned with the DC/RF
fi
eld
generated by the electrodes. When the RF bias voltage is applied on the electrodes,
the electro-optic (Pockels) effect will generate refractive index modulation. As a
result, the intensity of the re
fl
ected beam will be modulated accordingly.
Article
https://doi.org/10.1038/s41467-024-45544-0
Nature Communications
| (2024) 15:1557
2
host slot modes
33
,
34
whose demonstrated high overlap with the OEO
material has been utilized in many integrated silicon-organic
modulators
35
–
37
. However, the slot waveguide is intrinsically decoupled
from the free-space light due to momentum unmatching. To enable
coupling with normally incident light, we create periodic notches
along every slot
38
,
39
. The notch periodicity and the notch size dom-
inantly determine the resonance wavelength and the coupling strength
of the slot resonance, respectively. As a result, both the quality factor
and the resonant wavelength could be judiciously engineered (see
supplementary sections 1
–
2). Although a similar structure has been
proposed for sensing
40
, the design strategy and target applications
here are completely different.
To show the advantage of the slot m
odes while shrinking the dis-
tance between electrodes down to 100nm, it is worth discussing the
possible optical modes in such a structure. The detailed schematic view,
topview,andcross-sectionview
of the example device are shown in
Fig.
2
a, where three different colors (gray, blue, green) indicate different
materials (silicon, silica, OEO material or HLD). In the numerical simu-
lations shown in Fig.
2
b
–
h, HLD has a refractive index of 1.85
27
.Figure
2
b
shows the poling
fi
eld pro
fi
le upon applying a DC bias across the elec-
trodes. The
x
-
z
cross-section is cut at the center of a notch pair shown as
dashedrectangleintheschematicviewofFig.
2
a. Neglecting any
interface effect
41
, we can treat the relative amplitude of
r
33
at each spatial
point as following this pictured poling
fi
eld pro
fi
le. The geometry of the
electrodes results in a high
E
x
fi
eld along the slot and a rapid
fi
eld decay
above and below the slot. Therefore, to simplify the simulation, we
assume that only the OEO ma
terial inside the slot i
snonlinearlyactive.
The structure will host various optical modes, many of which have a
signi
fi
cant
E
x
fi
eld component such that it could strongly overlap with
the incident
E
x
beam. Figure
2
c
–
e show three cross-sectional optical
mode pro
fi
les (Mode I, II, III) originating from different parts of the
device. The cross-sections are cut at the same
y
position as in Fig.
2
b.
Mode I is the slot mode, which has the
fi
eld highly con
fi
ned inside the
slot, as discussed above, even upon applying the notch perturbation.
Mode II is the bound state guided within the slab. Notice that besides
the slot mode, periodic notches also unlock the free-space radiation for
this guided mode in the slab
42
.ModeIIIisaguidedmodeinOEO
material, which has also been reported in
29
. Unlike the other optical
modes, the
fi
eld in slot mode is well aligned with the poling
fi
eld and
thus has the highest overlap factor
Γ
c
.Asaresult,underthesamebias
voltage, the slot mode has the largest resonance shift, as demonstrated
through the simulation results in Fig.
2
g
–
i. With the same amount of
index change in the active region, Mode I, II, and III have resonance
shifts of 2.73nm, 0.29nm, and 0.46nm, respectively. A more accurate
model estimates the overlap factor by considering the orientation of the
nonlinearity
29
(see supplementary material section 2). The calculated
Γ
based on this model for modes I, II, a
nd III are 0.156, 0.017, and 0.015,
respectively. The slot mode shows an order of magnitude higher
Γ
c
,
compared to the others. Therefore, the slot mode is crucial for low-
voltage modulation in silicon-organic metasurfaces.
We experimentally realize the concept discussed above, using a
silicon-on-insulator (SOI) wafer. The device
’
s cross-section, top view,
and voltage setting are schematically illustrated in Fig.
3
a, b. The
detailed device parameters are shown in supplementary material sec-
tion 7. The nanostructures are fabricated with conventional nanofab-
rication techniques (see Methods section for details on the fabrication
procedure). The step-by-step zoom-out scanning electron micro-
scopy (SEM) images (prior to spin-coating of the OEO material) are
shown in Fig.
3
c
–
f. Doped silicon nano-bars have a resistance of
1~10
Ω
⋅
cm, and ~100nm wide gold strips along the nano-bars are
added to further reduce the voltage drop across the silicon. In Fig.
3
c,
the gold strips are deliberately aligned at the center of the silicon rail
so that only minimal absorption is introduced (See supplementary
section 3 and Fig. S3). Figure
3
eshowsan80×100
μ
m
2
device. Multi-
ple devices are fabricated on a chip as shown in Fig.
3
f for increasing
the tolerance of the fabrication errors and testing multiple geometric
parameters. After the coating of the OEO material, the device is wire
bonded to a customized printed circuit board for poling and operat-
ing, shown in Fig.
3
g.
To experimentally verify the relationship between resonant opti-
cal characteristics of the slot modes and the geometry of the device,
we fabricated several devices having different design parameters and
compared their measured optical properties with corresponding
numerical calculations (see methods, supplementary section 6, and
Fig. S7 for details in optical measurement setup). Figure
4
shows the
calculated and measured spectra of the slot mode resonances with
different geometries. We characterize the slot resonances by varying
the notch period and notch size in Fig.
4
a, b and c, d, respectively. In
Fig.
4
a, b, we observe that a 20 nm increment in notch periodicity leads
to ~21.6 nm and 22.2 nm average redshift of the resonance in simula-
tion and experiment, respectively. Also, with respect to the resonant
wavelengths, the resonance amplitudes, and the spectral shapes of the
resonance, the measured spectra in Fig.
4
c show good agreement with
the calculated spectra in Fig.
4
d. In particular, the quality factor
increases with the decrease of the resonance amplitude. This trade-off
is mainly due to the decreasing radiation rate to the top port (to +
z
direction), which results in the under-coupling between the slot mode
and the illuminated light from the top
18
.Speci
fi
cally, the amplitude of
the resonance is determined by the ratio of the mode coupling rate
between the input light and the slot mode to the sum of other unde-
sired decay rates
43
. The undesired decays include absorption in the
gold layer, scattering from the rough sidewalls or
fi
nite edges of the
chip, and radiation to the oxide layer or the silicon substrate. As the
absorption in the gold layer and the radiation to the silicon substrate
are nearly inevitable in the proposed planar structures, the trade-off
between the resonance amplitude and the quality factor is inevitable,
especially when the coupling rate decreases. The proposed devices in
Fig.
4
e can achieve modulation amplitude over 10% and Q-factor over
1000 in the experiment even with the absorption in the gold.
Operation results under DC bias are shown in Fig.
5
.Figure
5
a
–
c
show the results from 3 different devices under maximum bias vol-
tages before any dielectric breakdown. The variation in the breakdown
voltages results from the quality of the OEO material preparation and
the device fabrication. The maximum absolute frequency shift of
5.5nm is achieved under ±17V in Fig.
5
a, and the spectral shift per unit
external DC bias is
S
abs
=
Δ
λ
/
Δ
V
= 0.161nm/V, which is ~1.6 × higher than
that of the previously reported tunable free-space optical
modulators
31
. Using Eqs. (
1
)and(
2
), the in-device
r
33
is calculated as
45.7 pm/V at 1495nm. In Fig.
5
b and c, 2.3 nm and 2.6 nm resonance
shifts are observed with ±11V and ±12V bias voltages, respectively.
Figure
5
b and c show high Q-factors over 1000. The increase in
Q-factor also improves the normalized modulation
fi
gure-of-merit
(
S
n
=
Δ
λ
/(
FWHM
⋅
Δ
V
)
44
. In our best-performing device shown in Fig.
5
b,
S
n
is 0.09/
V
, which is an order of magnitude higher than other reported
devices
44
.There
fl
ection spectra are plotted in Fig.
5
dasafunctionof
different bias voltages. The spectra clearly show the bidirectional lin-
ear relationship between the bias voltage and the resonance shift,
con
fi
rming that the spectral shift results from the electro-optic
effect
29
.InFig.
5
e, the relative modulation ratio,
Δ
R
/
R
,fromthe
device in Fig.
5
c is plotted. The maximum modulation ratio is over 40%.
It is worth noting that the asymmetry of the modulation is due to the
Fano shape of the resonance
45
. The inset in Fig.
5
e, shows the re
fl
ection
intensity as a function of the bias voltage when the input light wave-
length is 1486.5nm. From
−
12V to + 12V, the re
fl
ection amplitude is
gradually increasing.
The AC modulation characteristic is tested with the devices in
Fig.
5
a and plotted in Fig.
6
a. A sine wave with peak-to-peak value,
V
pp
, of 20V is applied into the device while the wavelength of the
incident light is set at 1490nm where the device achieves the highest
modulation depth(estimated as 6.8% from Fig.
5
a). The sine wave
Article
https://doi.org/10.1038/s41467-024-45544-0
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| (2024) 15:1557
3
Fig. 2 | The advantage of slot mode resonance in organic electro-optic
modulators. a
The schematic view (top left), top view (bottom left) and cross-
section (right) of the device that supports the slot mode. In the schematic view, the
OEO material is plotted transparent to show the slot structure underneath. The
slots are formed in the device layer of the silicon-on-insulator (SOI) substrate, which
is covered by the OEO material HLD. To show the essence of the problem, only the
slot is considered as the active region. The dashed rectangles in the schematic view
represent the top view across the device layer and the cross-section of a period cell.
b
The poling
fi
eld pro
fi
le when the left and right silicon rail have bias voltages
V(V > 0) and 0, respectively.
c
–
e
Normalized electric
fi
eld pro
fi
les for three optical
modes that could couple to
E
x
incident light.
c
the slot mode.
d
the guided mode in
the silicon bar.
e
the guided mode in the OEO material.
f
–
h
. the tuning performance
of the three optical modes. Figures
f
,
g
,and
h
match with the
fi
eld pro
fi
le in
fi
gures
c
,
d
,and
e
, respectively. The inset in
h
is a zoom-in spectrum between 1649 nm and
1652.5 nm.
Article
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4
frequency is swept from 50kHz to 5.8MHz. The cutoff 3dB band-
width is at 3MHz. The insets in Fig.
6
a show the normalized mod-
ulation signal when driving with frequency
f
=80kHz and
f
= 2.8MHz, respectively. To investigate the AC response, we use a
simpli
fi
ed model to predict the AC response shown in Fig.
6
b. The
model collectively considers the contributing factors to the
response speed within and outside the devices. In the devices, we
model the nano-bars as a resistance, including the contribution
from the gold strip
R
Au
and the silicon nano-bar
R
Si
(see supple-
mentary section 4). The slot is modeled as the parallel connected
resistance
R
OEO
and capacitance
C
OEO
. Outside the devices, we
assume that the major electrical components are the stray capaci-
tance in the circuits, which are split into the capacitance due to the
SOI wafer
46
C
SOI
and other factors
C
load
. The parameters in the model
are determined by both the geometry of the devices and the AC
response of similar devices with different substrates or electrode
layouts (See supplementary section 4). The AC response from the
prediction of the model is shown as the green line in Fig.
6
a, which
agrees with the experimental result. Based on the model estimation,
the capacitance from the silicon-organic platform and the stray
capacitance along the whole circuits are the main factors
preventing the speed increase. The capacitance from the SOI wafer
could be solved by advanced CMOS technology used in integrated
electro-optic modulators
47
. Judicious material and structural engi-
neering of these circuits have already achieved gigahertz operation
of electro-optic modulators
31
,
48
–
50
. As a result, there is no funda-
mental limit in increasing the operation speed up to GHz in our
platform.
Discussion
In this work, we propose a silicon-organic metasurface for free-space
modulation with reduced operation voltage less than ± 17V. We
experimentally observed the resonance with quality factor 330
–
1310
and up to ~5.5nm shift with nonlinear coef
fi
cient
r
33
= 45.7pm/V at
1495 nm. The proposed slot mode combines the advantages of a short
distance between two electrodes and a large overlap with the OEO
material, achieving a tuning sensitivity
S
abs
= 0.161nm/V, which shows
an improvement with a factor of 1.6 in sensitivity compared to the
state-of-art
31
,
44
. Finally, the metasurface has up to 3MHz bandwidth.
The use of the slot mode is not limited to electro-optic systems. The
proposed design approach can be applied to any system where sen-
sitivity to perturbations in low-index media is critical. For example, in
b
a
x
z
SiO
2
Si
HLD
V
GND
V
GND
x
y
p
l
d
V
GND
V
GND
c
de
gf
Fig. 3 | The electro-optic free-space modulator. a
–
b
The cross-section and top
view of the experimentally fabricated device.
c
–
g
The step-by-step zoom-out image
of the device and setup.
c
–
f
are the SEM images. The scale bars are 500 nm, 3
μ
m,
50
μ
m, and 1 mm, respectively.
g
is the optical image of the device. Multiple devices
are fabricated within a chip, and they are wire-bonded to the printed circuit board
for parallel testing.
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5
NEMS systems, the slot mode resonance could potentially improve
sensitivity to the mechanical movement, compared to conventional
guided mode resonances
51
.
The currently demonstrated sensitivity is limited by the quality
factor and the nonlinear coef
fi
cient
r
33
. The quality factor could be
improved by using smaller notches or a re
fi
ned fabrication process.
The relatively low nonlinear coef
fi
cient compared to what we
expected
27
is partly due to the surface state of doped silicon used as
the electrode
52
and the small width of the slot
41
. Barrier layer
protection
53
has the potential to increase the nonlinear coef
fi
cient
r
33
by 4-5 times
54
, reducing the tuning voltage down to CMOS-level.
Judicious doping level adjustments and the microwave coplanar
waveguide design could enable GHz speed operation
31
,
36
. Therefore,
with the increase of electro-optic coef
fi
cient and operation bandwidth,
our platform is a potential solution for GHz free-space modulation at
the CMOS level voltage.
This study primarily focuses on structures which are periodic in
both
x
and
y
dimensions. However, it is not a necessary condition for
preserving the slot mode. As a mode propagating along the
y
direction,
theslotmodedoesn
’
t necessitate periodic conditions in
x
direction
55
.
By varying the geometry of the slots, individual slot modulation could
be potentially achieved. In the
y
-dimension, by introducing high con-
trast index variations or photonic crystal mirrors, the footprint of the
resonant device could be reduced considerably
56
,
57
. Furthermore, the
proposed devices expect to achieve phase modulation if the over-
coupling condition is satis
fi
ed by the out-of-plane asymmetry in
nanostructures
10
or a bottom mirror
5
,
6
.
In summary, this report presents a low-voltage amplitude mod-
ulator using a silicon-organic platform. The slot mode metasurface has
the potential to enable high-speed an
d low-voltage optical switching,
sensing, and tuning, for numerous applications such as LiFi, LiDAR,
spatial light modulators, and quantum optical communication.
Methods
Fabrication and poling methods
The device is fabricated from an SOI wafer, which consists of 300nm
p-doped silicon (1
−
10
Ω
⋅
cm), 300nm BOX (buffered silicon oxide),
and 500
μ
m
silicon substrate. The detailed fabrication work
fl
ow is
shown in supplementary section 6. The device requires two
sequential nanofabrication steps for the silicon rails and metallic
strips, respectively. Both E-beam lithography steps utilize ZEP-520A
(Zion Corporation) as the resist, 100kV electron beam (EBPG-5200,
Raith GmbH) to expose, and ZED-N50 (Zion Corporation) as the
developer. After the
fi
rst E-beam lithography, we use the resist as
the soft mask and the pattern is transferred to silicon by ICP-RIE
etching (PlasmaLab System 100, Oxford Instrument). Next, the
resist is removed by Remover PG. The second E-beam lithography
writes the liftoff mask for the electrodes, following which 5nm Ti
Fig. 4 | Slot mode resonance characterization. a
–
d
The simulated (
a, c
)and
experimentally measured (
b, d
)re
fl
ection spectra when sweeping different sets of
perturbation parameters.
a
–
b
Sweep the periodicity of the notches. All curves have
the same notch size
l
= 160 nm,
d
= 80 nm. Blue:
p
= 720 nm. Orange:
p
= 740 nm.
Green:
p
= 760nm. The resonance shifts due to periodicity changes are labelled in
experiment and simulation curves. c-d. Sweep the notch sizes. All curves have the
same notch periodicity
p
= 740 nm. Red:
l
=200nm,
d
=120nm.Purple:
l
= 160 nm,
d
=80nm.Brown:
l
= 150 nm,
d
=50nm.Pink:
l
= 140nm,
d
=25nm.Thequality
factor of the resonances are labelled for experimental and simulated plots.
Article
https://doi.org/10.1038/s41467-024-45544-0
Nature Communications
| (2024) 15:1557
6
and 60nm Au are deposited sequentially using an E-beam eva-
porator (Kurt J. Lesker E-beam evaporator). Liftoff is then per-
formed in Remover PG. Finally, a layer of OEO material (HLD, NLM
Photonics) is spin-coated on top of the device, followed by a 3-hr
solvent removal in a vacuum oven at 65° C. The detailed work
fl
ow is
shown in supplementary section 6.
Poling of the HLD material is performed by heating the device
under the nitrogen environment while applying a poling voltage. This
voltage creates a poling
fi
eld around 100V/
μ
m across the slot. The
heating process consists of a 6° C/s temperature ramping, 5 to 10
minutes of holding at 95°C, and rapid cooling.
Simulation methods
The simulations in Fig.
2
,andFigs.S1
–
S3 use COMSOL Multiphysics
software. The periodic condition is applied in both x and y directions.
Refractive indices of the silicon, silicon oxide, and OEO material are
assumed to be 3.52, 1.44, and 1.85. Figure
4
a, c are simulated with
a FDTD simulation software (Ansys Lumerical FDTD) with periodic
boundary conditions applied in
x
and
y
.
Measurement
The measurements were conducted using the experimental setup
depicted in Fig. S7. The light source utilized was a tunable external-cavity
Fig. 5 | DC tuning characteristics. a
–
c
Re
fl
ection spectra of three different devices
under DC tuning. The applied biases are denoted in the legend.
d
The re
fl
ection
spectra of the device in
b
with bias voltages ranging from -11V to 11V.
e
The max-
imum modulation ratio (
Δ
R
=
R
=
ð
R
max
R
min
Þ
=
R
V
=0
) for each wavelength in device
shown in
c
. The inset depicts the absolute re
fl
ection as the DC bias voltage is swept
from -12V to 12V for a
fi
xed wavelength of incident light of 1486nm. The absolute
re
fl
ection changes over 10%.
Article
https://doi.org/10.1038/s41467-024-45544-0
Nature Communications
| (2024) 15:1557
7