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science.sciencemag.org/content/364/6445/1087/suppl/DC1
Supplementary Materials for
Phase-only transmissive spatial light modulator based on tunable
dielectric metasurface
Shi-Qiang Li, Xuewu Xu, Rasna Maruthiyodan Veetil, Vytautas Valuckas,
Ramón Paniagua-Domínguez, Arseniy I. Kuznetsov*
*Corresponding author. Email: [email protected]
Published 14 June 2019, Science 364, 1087 (2019)
DOI: 10.1126/science.aaw6747
This PDF file includes:
Materials and Methods
Figs. S1 to S9
Table S1
References
1
Materials and Methods
Fabrication of the nanoantenna-based SLM device
We start with a commercial ITO-coated conductive glass slide (Latech Scientific Supply Pte.
Ltd, Singapore) with ITO film thickness of 23 ± 5 nm (Fig. S9A). Measured transmittance of the
glass slide is above 87% in the whole visible spectral range and resistivity is ~100 Ω/. Amorphous
TiO2 film is then deposited onto the ITO glass slides with Oxford Optofab 3000 ion assisted
deposition system (Fig. S9B). The deposition rate is 0.28 Å/s to ensure the smoothness of the films.
The surface roughness after the TiO2 film deposition is characterized by Vecco DI 3100 Atomic
Force Microscope and the root mean square value obtained is around 1.0 nm from several random
areas with size of 10 by 10 microns. Following that, a 30 nm Chromium (Cr) film is evaporated
on top of the TiO2 film by an electron-beam evaporator (Angstrom EcoVac), as shown in Fig. S9C.
Electron-beam lithography (EBL) is then performed to define the masks for the nanoantennas
using Elionix ELS-7000 EBL system. A standard procedure is conducted using a negative tone
electron-beam resist, Hydrogen silsesquioxane (HSQ, Fox-22, Dow-Corning Inc.), to define the
mask pattern of the nanoantennas (Fig. S9D and S9E). Following that, the pattern is transferred to
the Cr layer by a reactive ion etching process in Oxford PlasmaLab 100. Processes with Cl2 and
O2 gases are used to ensure the fidelity of the pattern transfer due to their etch selectivity of Cr
over HSQ (Fig. S9F). With Cr as a hard mask, formation of the TiO2 nanoantennas is achieved
through dry etching with CHF3 gas (Fig. S9G) in the same etcher. Chromium mask is then removed
by immersing the sample in a chromium etchant solution (Sigma-Aldrich) for 4 minutes (Fig.
S9H). A second EBL process is then performed on the sample. The standard process for the
positive tone electron-beam resist (ZEP520A, ZEONREX Electronic Chemicals) is used for the
second layer. Electrode pattern is directly transferred from ZEP to the ITO layer by another
reactive ion etching process in the same etcher. A recipe with CH4 and Ar gases is used to achieve
good etching selectivity of ITO over ZEP resist and the structure after etching is shown
schematically in Fig. S9I.
Scanning electron microscopy
Scanning electron microscopy (SEM) imaging of the nanofabricated samples is performed on
Hitachi SU8200 Ultra-high resolution scanning electron microscope. Electron acceleration voltage
used is 1 kV with a nominal current of 10 nA. The low voltage is used in order to prevent the
charging effect on the samples fabricated on insulating glass substrates.
Assembly of the nanoantenna-based SLM device
ITO conductive glass slide with the same specification as the device substrate is used as the
top covering slide to form the liquid crystal cell. Firstly, a layer of polyimide is spin coated on the
covering slide followed by curing at 180 °C for 1 hour on a hot plate. The cured slides are then
rubbed unidirectionally with a piece of soft velvet clothes to define a preferred liquid crystal
alignment direction of the cell. The rubbing strength and duration are controlled by a commercially
available rubbing machine (Holmarc Opto-Mechatronics - HO-IAD-BTR-03). Following that, the
cover slide is pressed onto the device structure, with the spacing defined by a layer of glue (Norland
optical adhesive NOA81). After the desired gap thickness between the two cover slides is reached,
the glue layer is cured via ultra-violet light irradiation. The spacing and flatness between the top
2
covering slide and the bottom substrate is precisely controlled through a home-made press
equipped with a multi-point optical profilometer. Lastly, nematic liquid crystal molecules (E7 from
Merck) are introduced into the gap formed between the top covering slide and the bottom substrate
by capillary forces and vacuum pumping. After the infiltration the nematic liquid crystal directors
are aligned parallel with the alignment direction introduced by the rubbing process on the covering
slide. Fig. S9J shows a schematic of the assembled device.
Setup for optical characterization and beam bending measurements.
The optical characterization setup used in the experiments has been described in details
elsewhere (8). In brief, the incident beam from a halogen lamp is linearly polarized along the
shorter axis of the bottom electrodes of the SLM. A pupil diaphragm on the optical condenser is
closed to its minimum to limit the angular spread of the incident light to the smallest limit (2° from
the normal to the device surface). Light transmits through the device and is collected through a
bright field objective lens (Nikon 50X, NA = 0.45). The back focal plane of the objective is then
imaged to the entrance slit of Andor Spectrograph (SR-303i). The direction of the slit is aligned
with the short axis of the electrodes so that the beam deflection along that direction is spatially
resolved. Along the perpendicular direction, light is dispersed by a blazed grating so that we obtain
the spectral information of the deflected beam. The image with an angular resolution of 0.38° and
a spectral resolution of 0.34 nm is collected by an EMCCD (Andor Newton Series) sensor array
and stored in a computer for further analysis. The active modulation of the bias voltage on the
electrodes is provided by a power supply with a voltage resolution of 10 mV. (Keysight
Technology U8031A Triple Output DC Power Supply)
Finite-element-method Simulation
The simulations were performed using COMSOL Multiphysics software package. The
detailed procedure is a typical electromagnetic wave simulation in the frequency domain with one
unit cell of the nanoantenna array as the simulation cell. An example is provided by COMSOL Inc
at the following link (https://comsol.com/model/plasmonic-wire-grating-wave-optics-14705;
accessed on December 29, 2018)
Electronics to address the electrodes
Device with individually addressable electrodes was fabricated with the procedure discussed
in the first section above. In addition, a custom designed and fabricated printed circuit board is
used to interface with the device and a driving circuit is used to address each electrode
independently. The number of electrodes of the device is limited to 28 due to the fabrication
constraint, which can be extended further using different technologies, such as active matrix circuit
integration. The bias voltage for each electrode is provided by a digital to analog (DAC) output
port with range from 0 volts to 10 volts and 12-bit resolution (provided by MAXIM 11300 Chip).
3
Fig. S1.
(A) Real and imaginary parts of refractive index (n and k) of our TiO2 films extracted from
ellipsometry measurements; (B) Real parts of the extraordinary (solid line) and ordinary (dashed
line) refractive indices of E7 liquid crystal obtained from literature data. (30)
A
B
4
Fig. S2.
Numerically calculated (A) amplitude and (B) phase shift of the transmitted light for nanoantenna
arrays with a height of 195 nm.
A B
5
Fig. S3.
Numerically calculated (A) amplitude and (B) phase shift of the transmitted light for nanoantenna
arrays with a height of 215 nm.
A B
6
Fig. S4
Electric field distribution (Ex, corresponding to the incident polarization) in the unit cell for three
LC director orientations at the operating wavelength (665 nm). When the field distribution for
these three selected configurations are plotted side by side, one can see that the wavefronts of the
incident plane wave remain almost similar during propagation through the LC layer, indicating
little phase difference accumulated. On the contrary, upon interaction with the nanoantennas the
wavefronts of transmitted light experience significantly different phase retardations, forming three
adjacent phase steps. Consequently, light transmitted through a system consisting of a periodic
repetition of such unit cells would be deflected into the perpendicular direction to the tilted
wavefront, outlined by the yellow dotted lines.
7
Fig. S5.
Optical transmission measurement of a nanoantenna array device in air, before infiltration of the
liquid crystal, and its comparison to the simulations. Parameters of the simulated unit cell are
similar to the nanoantenna array dimensions measured by SEM. Height of TiO2 nanodisks h = 205
nm; radius of TiO2 nanodisks R = 135 nm; period of the unit cell p = 360 nm. A 60 nm gap is
included in the direction of the polarization of the incident electric field between every three unit-
cells along the same direction (to simulate the introduced gap between individual electrodes). The
wave-vector of the incident light is normal to the surface of the glass substrate. ITO layer is ignored
in the simulation as its contribution was verified to be negligible but computationally demanding.
8
Fig. S6. (A) Simulated beam deflection angles with different numbers of pixels in the unit cell.
(B) The corresponding efficiencies.
9
Fig. S7.
(A) Measured far field intensities as a function of deflection angle under different bias voltage
applied to the bottom left electrode. The top and bottom right electrodes remain grounded. (B)
Same as (A) but bias voltage is applied to the bottom right electrode while top and bottom left ones
remain grounded. In both (A) and (B), the first five diffraction orders are shown (from -2 to +2) at
the wavelength of 663 nm. At zero bias, electrodes are in a homogeneous LC environment and the
incident light goes straight into the zeroth order (as in Fig. 3A, middle panel). When we start to
ramp up the electrical bias applied to the set of electrodes on the left side (Fig. 3A, left panel) from
0V to values above 6V, the beam starts to get deflected into the first diffraction orders. With
relatively low voltages (see (A) at 6V and 8V), the zeroth order intensity decreases and both -1
and +1 diffraction orders increase simultaneously. In this case, the device functions as a symmetric
grating. This implies that the unconnected central electrode has not yet gained enough potential
and the LC on top retains in-plane orientation. With further increase of the bias on the left
electrode, the central electrode starts picking up an increasing electric potential. This rotates the
LC directors on top of it, the phase front starts to tilt and, as a result, the beam starts to be deflected.
We found that the highest efficiency of beam deflection (at 11° angle) occurs at 12 V, as shown in
(A). We test the beam deflection into the opposite direction by reverting the configuration, i.e.
grounding the top and bottom left electrodes and biasing the bottom right electrodes (as in Fig.
3A, right panel). The results are shown in (B). As expected, in this case the beam at an optimum
voltage is deflected to the right. It should be noted, however, that now the zeroth order is more
suppressed than in the preceding case and the optimum deflection occurs at a slightly higher
voltage (13V). We attribute these differences to imperfections of the fabricated device. We note
that, for higher bias levels, the electric field and, consequently, LC rotation at the unconnected
electrode raises above the optimum value and the efficiency drops (see Fig. S8).
10
Fig. S8.
Measured far field intensities as a function of deflection angle when the device operates beyond
the optimum voltage. As seen in the plot, beyond 13V, the intensity into diffraction angle
corresponding to the beam steering (~ -11º) drops and that of the zero order increases (resulting in
a drop of the steering efficiency).
11
Fig. S9.
Process Flow of the nanoantenna-based SLM device fabrication
A
B
C
D
E
F
G
H
I
J
12
Table S1.
Light bending efficiency for 3-phase level SLM for different numbers of TiO2 nanoantennas per
pixel (Design “3LnP” corresponds to n nanoantennas per pixel). w: Width of each electrode;
Wavelength corresponding to optimum diffraction efficiency; Ttotal: Total light transmission; T0,
T+1 and T-1: transmission into 0, +1 and -1 diffraction orders respectively; -1: First order diffraction
angle.
Design w
(m)
(nm) T
0 T
+1 T
-1 T
total -1 ()
3L1P 0.36 666.0 0.002 0.155 0.288 0.520 38.07
3L2P 0.72 665.5 0.0002 0.134 0.506 0.789 17.94
3L3P 1.08 662.5 0.001 0.093 0.537 0.834 11.80
3L5P 1.80 663.0 0.006 0.052 0.539 0.854 7.05
3L7P 2.52 664.0 0.006 0.0520 0.533 0.869 5.04
3L9P 3.24 664.0 0.007 0.0512 0.545 0.890 3.92
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