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: arseniy_kuznetsov@imre.a-star.edu.sg

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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