# Enhanced Meta-Displays Using Advanced Phase-Change Materials

Omid Hemmatyar1,† Sajjad Abdollahramezani1,† Ioannis Zeimpekis2 Sergey
Lepeshov3 Alex Krasnok4 Asir Intisar Khan5 Kathryn M. Neilson5 Christian
Teichrib6 Tyler Brown1 Eric Pop5 Daniel W. Hewak2 Matthias Wuttig6 Andrea
Alù4,7 Otto L. Muskens8 Ali Adibi1 1School of Electrical and Computer
Engineering, Georgia Institute of Technology, 778 Atlantic Drive NW, Atlanta,
Georgia 30332-0250, US  2Zepler Institute, Faculty of Engineering and Physical
Sciences, University of Southampton, SO17 1BJ Southampton, United Kingdom
3ITMO University, St. Petersburg 197101, Russia  4Photonics Initiative,
Advanced Science Research Center, City University of New York, New York, New
York 10031, United States  5Department of Electrical Engineering, Department
of Materials Science and Engineering, Precourt Institute for Energy, Stanford
University, Stanford, California 94305, United States  6Physikalisches
Institut IA, RWTH Aachen, Sommerfeldstrasse 14, 52074 Aachen, Germany
7Physics Program, Graduate Center, City University of New York, New York, New
York 10016, United States  8Physics and Astronomy, Faculty of Engineering and
Physical Sciences, University of Southampton, SO17 1BJ Southampton, United
Kingdom,  †These authors contributed equally to this work.

(August 27, 2024)

###### Abstract

Structural colors generated due to light scattering from static all-dielectric
metasurfaces have successfully enabled high-resolution, high-saturation, and
wide-gamut color printing applications. Despite recent advances, most
demonstrations of these structure-dependent colors lack post-fabrication
tunability. This hinders their applicability for front-end dynamic display
technologies. Phase-change materials (PCMs), with significant contrast of
their optical properties between their amorphous and crystalline states, have
demonstrated promising potentials in reconfigurable nanophotonics. Herein, we
leverage tunable all-dielectric reflective metasurfaces made of newly emerged
classes of low-loss optical PCMs, i.e., antimony trisulphide (Sb2S3) and
antimony triselenide (Sb2Se3), with superb characteristics to realize
switchable, high-saturation, high-efficiency and high-resolution dynamic meta-
pixels. Exploiting polarization-sensitive building blocks, the presented meta-
pixel can generate two different colors when illuminated by either one of two
orthogonally polarized incident beams. Such degrees of freedom (i.e., material
phase and polarization state) enable a single reconfigurable metasurface with
fixed geometrical parameters to generate four distinct wide-gamut colors. We
experimentally demonstrate, for the first time, an electrically-driven micro-
scale display through the integration of phase-change metasurfaces with an on-
chip heater formed by transparent conductive oxide. Our experimental findings
enable a versatile platform suitable for a wide range of applications,
including tunable full-color printing, enhanced dynamic displays, information
encryption, and anti-counterfeiting.

dynamic metasurfaces, phase-change materials, nanophotonics, structural colors

## Introduction

In the past decades, absorption and emission of light from organic dyes and
chemical pigments have been the most common color generation mechanisms in
color-imaging and display devices [1]. Nevertheless, there are still several
challenges with the developed technologies, such as environmental hazards,
vulnerability to high-intensity light, and limited scalability to smaller
pixel sizes. In order to address these issues, structural colors have emerged
as compelling alternatives. Structural colors are observed in numerous natural
species, whose bright features arise from light scattering and interference in
micro/nanostructured patterns of their skins or scales [2]. Inspired by nature
and enabled by recent advancement in nanofabrication, artificial structural
colors generated via a resonant interaction between incident white light and
miniaturized building blocks in optical metasurfaces [3, 4, 5, 6], i.e.,
arrays of subwavelength patterned nanostructures, have gained great attention
in recent years. In this context, plasmonic metasurfaces made of gold, silver
and aluminum nanostructures have been extensively used to generate structural
colors based on plasmon resonances [7]. Despite their versatility, the broad
and weak plasmon resonances, imposed by the significant inherent ohmic loss of
the constituent metallic materials, result in low color saturation and purity
[8].

Figure 1: Working principle of a polarization-encoded dynamic display composed
of phase-change meta-pixels. a,b, Schematic representation of a reflective
display consisting of phase-change meta-pixels. Each meta-pixel is a
metasurface formed by a periodic arrangement of Sb2S3 nanopillars shown in
(b), which can generate four different colors; two colors for each
polarization attributed to the amorphous and crystalline phases of Sb2S3 (i.e.
A-Sb2S3 and A-Sb2S3, respectively) nanopillars or two colors for each Sb2S3
phase (corresponding to x-polarized and y-polarized incident white light). For
all metasurfaces, the height ($h$) of the nanopillars is fixed while their
periodicity in x- and y-directions (i.e., $p_{x}$ and $p_{y}$, respectively)
change to generate different colors. The major and minor axes of the
nanopillars in x- and y-directions are proportional to the corresponding
periodicities in those directions with a constant aspect ratio, i.e.
$d_{x,y}=\alpha\,p_{x,y}$, in which $\alpha$ is constant. The colors shown in
(a) correspond to Sb2S3 metasurfaces with (i) $p_{x}=p_{y}=310$ nm (for green)
and $p_{x}=p_{y}=390$ nm (for red), (ii) $p_{x}=310$ nm, $p_{y}=390$ nm, and
(iii) $p_{x}=390$ nm and $p_{y}=310$ nm, with $\alpha=0.6$ and $h=120$ nm.
c-f, Multipolar decomposition analysis: c,e, Calculated normalized scattering
cross-sections and simulated reflectance (R) spectrum of a Sb2S3 metasurface
with geometrical parameters of $p_{x,y}=310$ nm, $d_{x,y}=0.6\,p_{x,y}$, and
$h=120$ nm for the (c) amorphous and (e) crystalline phases. The constructive
interference between the electric dipole (ED) and magnetic dipole (MD) modes
at $\lambda_{a}=560$ nm ($\lambda_{c}=652$ nm) boosts the backward scattering
intensity, and in turn, results in a reflectance peak in the case of A-Sb2S3
(C-Sb2S3). d,f, Normalized magnetic field intensity with arrow surface of
electric field (top panel), and normalized electric field intensity with arrow
surface of magnetic field (bottom panel) for the metasurfaces in b(i) at (d)
$\lambda_{a}=560$ nm and (f) $\lambda_{c}=652$ nm, respectively.

To meet the challenges associated with plasmonic metasurfaces, recently, all-
dielectric metasurfaces made of high-refractive-index materials supporting
Mie-type resonances with electric dipole (ED) and magnetic dipole (MD) modes
have been used for generating a full range of vivid and highly saturated
structural colors desired for high-resolution display technologies [9, 10, 11,
12]. However, these colors are fixed-by-design and cannot be tuned since the
geometrical parameters of passive all-dielectric metasurfaces cannot be
changed after fabrication. In order to enable active display applications, a
real-time color tunability is essential.

To realize high-resolution structural color tunability in metasurfaces,
several modulation techniques have been proposed. Some examples are liquid
crystals in conjunction with plasmonic nanoantennae [13, 14], utilizing
mechanically stretchable substrates integrated with plasmonic [15] and
dielectric [16] nanoscatterers, changing the refractive index of the medium
surrounded nanostructures [17], modifying the optical properties of the
constituent magnesium-based nano-cavities of a hybrid plasmonic-dielectric
platform via a chemical reaction [18], and changing the polarization state of
incident light [19]. Despite impressive advancements, these approaches can
hardly meet the requirements for lightweight, flexible, durable, high-
resolution, high-speed, and cost-effective dynamic color displays with high
color contrast and saturation, multiple stable colors, and high refreshing
rates.

To overcome the existing shortcomings, chalcogenide phase-change materials
(PCMs) [20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34], with
optical properties (e.g., refractive index) that can be strongly modified upon
applying an external stimulus (optical, electrical, or thermal), have been
successfully used as tunable materials for color switching [35, 36, 37, 38,
39]. The advantages of PCM-based color-switching techniques over other
counterparts originate from unique electrical and optical features of PCMs
including nonvolatility, high index contrast, fast reversible switching speeds
(10s-100s nanoseconds) between two stable phases, high durability (up to
$10^{12}$ cycles), notable scalability (down to nm-scale sizes), good thermal
stability (up to several hundred degrees), and adaptability with the CMOS
fabrication technology [22]. Considering these unique features, single [35,
36] or multiple ultrathin films [37] made of germanium antimony telluride (GST
in short) and germanium telluride (GeTe) alloys in a multistack configuration
with other dielectric and/or metallic films have been utilized for color
switching [38]. In spite of the unparalleled properties of PCMs, these
demonstrations suffer from the high absorption loss of GST and GeTe within the
visible wavelength range, which results in low-quality-factor (low-Q)
reflectance resonances. This, in turn, yields colors with low saturation, low
color value (i.e., the reflectance value at the resonance peak) and purity in
both amorphous and crystalline states of these PCMs.

To address these challenges, here we systematically design and experimentally
demonstrate an actively tunable platform for color displays comprising all-
dielectric metasurfaces formed by a geometrical arrangement of phase-change
nanoellipsoids. We leverage a less explored class of PCMs, i.e., antimony
trisulphide (Sb2S3) and antimony triselenide (Sb2Se3), exhibiting low-loss
property in the visible spectral range [40, 41, 42, 43, 44, 45]. Due to their
high refractive indices, these materials support strong Mie-type ED and MD
resonances. The sensitivity of these modes on refractive index enables high-
resolution (up to $\sim$80,000 dots per inch (dpi)) phase-transition-based
color switching with high saturation and purity [11]. Moreover, owing to the
polarization-sensitivity of the constituent asymmetric PCM nanopillars, we can
encode two different colors into two mutually orthogonal polarization states
of the incident light. This results in realization of a display with fixed
geometrical parameters that can generate four different colors upon transition
in the structural state of the contributed PCM. Finally, the integration of an
electrically controlled transparent heater with the polarization-encoded
phase-change meta-pixels, reported for the first time in this work, enables
real-time reconfiguration for applications ranging from tunable full-color
printing and displays, information encryption, and anticounterfeiting to
wearable screens and electronic papers.

## Results and Discussion

Figure 1a demonstrates the operation principle of a dynamic display formed by
phase-change meta-pixels. Each meta-pixel is composed of a periodic array of
rectangular unit cells, with different periodicity along x- and y-directions
(i.e., $p_{x}$ and $p_{y}$ in Fig. 1b (ii)), containing asymmetric elliptical
Sb2S3 nanopillars on top of a glass substrate. The major and minor axes of the
Sb2S3 nanopillars are proportional to the periodicity of the unit cell in the
corresponding directions, i.e., $d_{x,y}=\alpha\,p_{x,y}$, in which $\alpha$
is fixed between 0 and 1. The height of the nanopillars ($h$) is constant for
the fabrication preference. The reflected color upon normally incident
x-polarized white light can change by varying the geometrical parameters of
the elliptical amorphous-Sb2S3 (A-Sb2S3) nanopillars or equivalently those of
the unit cell (see the bottom-left display in Fig. 1a). Upon phase transition,
crystalline-Sb2S3 (C-Sb2S3) nanopillars with the same geometrical parameters
and under the same illumination conditions generate colors that are different
from those generated by their A-Sb2S3 counterparts (compare the top and bottom
displays in Fig. 1a). This phase-change color switching is attributed to the
refractive index change of the constituent Sb2S3 meta-pixels upon transition
between amorphous and crystalline phases.

To reveal the switching mechanism of, we performed the multipole decomposition
analysis of the scattering spectrum of a Sb2S3 meta-pixel under white light
illumination, as shown in Figs. 1c-f (see Supporting Information Note I and
Fig. S1 for more details). The analysis shows negligible contribution of the
higher-order moments so that the optical response of the unit cell is governed
by the electric dipole (ED) and magnetic dipole (MD) moments. In fact, the
strong coupling between these refractive index-dependent ED and MD moments
excited inside the Sb2S3 nanopillars with the directly reflected light yields
the resonances in the reflectance spectra shown in Figs. 1c,e. Therefore, the
spectral position of these resonances, or equivalently the generated color by
a meta-pixel under a specific polarized light, is determined by the refractive
index of the Sb2S3 nanopillars. In addition to the color switching mechanism
described above, the asymmetric nature of nanopillars can enable a
polarization-based color switching in which one meta-pixel can generate
different colors upon white light illumination with different polarization
states (i.e., x- to y-polarization) in each phase of Sb2S3 (compare the left
and right displays in Fig. 1a). Therefore, one meta-pixel with fixed
geometrical parameters can generate four different colors owing to the phase-
change-tunability and polarization-sensitivity of the constituent Sb2S3
nanopillars.

Figure 2: Simulation results and experimental characterization of
polarization-encoded dynamic phase-change meta-pixels. a,b, Simulated (a) and
experimental (b) reflectance spectra for the polarization-insensitive A-Sb2S3
(solid lines) and C-Sb2S3 (dashed lines) metasurfaces as well as their
corresponding colors and SEM images for different periodicity
($p_{x}=p_{y}=p$). The curves are displaced vertically for better visibility
and comparison. The diameter of Sb2S3 nanopillars varies as $d=0.65\,p$. The
sharp resonances observed in (a) and (b) are attributed to the interference
between ED and MD modes inside the Sb2S3 nanopillars as shown in Figs.
1(c)-(f) causing the spectral position of these resonances become refractive-
index-dependent. Therefore, red-shifting is observed upon phase transition of
Sb2S3. c, Corresponding CIE 1931 chromaticity coordinates of the reflectance
spectra shown in (a,b) for A-Sb2S3 (black circles) and C-Sb2S3 (white
squares). d,e, The color palettes for the fabricated (d) A-Sb2S3 and (e)
C-Sb2S3 meta-pixels considering different periodicity in x- and y-directions
($p_{x}$ and $p_{y}$, respectively) varying with 20 nm increments. f, SEM
images showing magnified bird’s eye views of three meta-pixels indicated by
dashed boxes in (d,e). The scale bar in (f) is 500 nm. The height of the Sb2S3
nanopillars is fixed at $h=120$ nm. The images of color pixels are captured
through a $2.5\times$ objective lens with numerical aperture (NA) of 0.075.

To study the effect of material phase on the generated color, we first
consider polarization-insensitive meta-pixels with a square lattice
($p_{x}=p_{y}=p$) and circular ($d_{x}=d_{y}=d$) Sb2S3 nanopillars. We set the
geometrical parameters as $h=120$ nm and $d=0.65\,p$ while we vary the
periodicity of the unit cell from $p=290$ nm to $p=450$ nm, with a step of 20
nm, to cover a wide range of possible colors. The corresponding reflectance
spectra obtained from full-wave simulations (see Method), and in turn, the
generated colors are shown in Fig. 4a. The method used for obtaining the
associated colors with each reflectance spectrum is detailed in Supporting
Information Note II and Fig. S3. As shown in Fig. 4a, by increasing $p$, the
spectral position of the resonance peaks for both amorphous (solid lines) and
crystalline (dashed lines) states red-shift. Moreover, switching the phase of
the material shifts the resonance peak to the longer wavelengths. This is due
to positive refractive index contrast ($\Delta
n_{\textrm{Sb}_{2}\textrm{S}_{3}}=n_{\textrm{C-Sb}_{2}\textrm{S}_{3}}-n_{\textrm{A-Sb}_{2}\textrm{S}_{3}}>0$)
within the visible wavelength range (see Supplementary Fig. S2c). The higher
absorption loss of C-Sb2S3 mean that the nanopillars in the crystalline state
(dashed lines in Fig. 2a) do not support equally strong and sharp resonances
as seen for A-Sb2S3 (solid lines).

To demonstrate the validity of our approach, we fabricated and characterized
$50\times 50$ $\mu$m2 Sb2S3 meta-pixels with the same design parameters as
those in Fig. 4a (see Methods for fabrication and characterization details).
The measured reflectance spectra, associated colors observed under the
microscope as well as magnified top-view scanning electron micrographs (SEMs)
of fabricated meta-pixels are shown in Fig. 4b demonstrating an overall good
agreement with the results obtained from simulations. To qualitatively analyze
the performance of the presented color generation/switching mechanism in terms
of saturation maintenance and hue variation, we display the generated colors
in the amorphous (black circles) and crystalline (white squares) phases in the
same International Commission on Illumination (CIE) 1931 chromaticity
coordinates in Fig. 4c. While for greenish and reddish colors, both simulation
and experimental results demonstrate high saturation values (i.e., those
markers close to the edge of the gamut), the purplish colors cannot be
produced in the experiments. We attribute this to the presence of undesired
secondary peaks observed in the reflectance spectra in Fig. 4b for $p>390$ nm
due to fabrication imperfections. A thorough quantitative study on the color
gamut coverage, saturation, and hue for the case of Sb2S3 meta-pixels and an
other low-loss PCM (i.e., Sb2Se3) is presented in Supplementary Note III and
Figs. S4,5.

Figure 3: Dynamic displays enabled by polarization-sensitive Sb2S3 meta-
pixels. a, Reproduction of the image of The Cheshire Cat by A-Sb2S3 and
C-Sb2S3 meta-pixels. For the case of A-Sb2S3, switching the polarization of
incident white light changes the generated color throughout the image. This
phenomenon is also observed for incident y-polarized light upon
crystallization of A-Sb2S3. Under x-polarization, however, all parts of The
Cheshire Cat body in the amorphous phase vanish except its teeth and eyes upon
switching to the crystalline phase. This is also the case when changing the
polarization of the incident white light from y- to the x-direction in the
crystalline phase. b, The SEM image of the fabricated array of Sb2S3
nanopillars associated with the face of The Cheshire Cat indicated by the blue
dashed box shown in (a). The magnified SEM image shown in the inset
demonstrates that a meta-pixel containing only four Sb2S3 nanopillars is
capable of generating the desired color justifying the high-resolution nature
of the presented color-printing approach. c, Encryption of two images (i.e.,
Georgia Tech logo and symbol) into a display containing an engineered
arrangement of Sb2S3 meta-pixels. One image can be switched to another either
by changing the polarization of incident light in each phase, or by changing
the phase of the Sb2S3 meta-pixels under the same polarization. The latter is
the first experimental demonstration of encryption of two totally different
images into the phase of the constituent material of meta-pixels. d, The SEM
image of the fabricated Sb2S3 meta-pixels corresponding to the blue dashed box
shown in (c). The design strategy and geometrical parameters of different
parts of the images shown in (a-d) is explained in Supplementary Figs. S15-17.
The images in (a) and (c) are captured through $10\times$ (NA = 0.3) and
$2.5\times$ (NA = 0.075) objective lenses, respectively. Figure 4:
Electrically driven dynamic color display device integrating a transparent
heater. a,b, The bright-field microscope images of the electrically tunable
color palettes comprising $50\times 50$ $\mu$m2 (a) A-Sb2S3 and (b) C-Sb2S3
meta-pixels fabricated on a glass substrate and encapsulated by a 150 nm-thick
film of SiO2. The transparent heater is formed by fabrication of a 50 nm-thick
ITO bridge connecting Au probing pads at the two ends on top of the SiO2 film.
The geometrical parameters of the Sb2S3 meta-pixels shown in (a) and (b) are
similar to those used in Fig. 4 (d,e). The images are captured using a
$2.5\times$ objective (NA = 0.075). c, Simulated stationary temperature map in
the cross section of Sb2S3 meta-pixels in the course of applying a 27 V
electrical signal to the Au probing pads. The uniform heat distribution across
the palettes ensures realization of large-scale displays with selective
controllability of the material phase of all meta-pixels. The scale bars are
100 $\mu$m.

In order to add polarization sensitivity to our color-switching approach, from
now on, we also consider elliptical nanopillars in asymmetric unit cells with
different periodicity in the x- and y-directions, i.e., $p_{x}$ and $p_{y}$,
as shown in Fig. 1b. By varying $p_{x}$ and $p_{y}$ from 290 nm to 510 nm with
a 20-nm increment and a fixed ratio with respect to the major and minor axes
of the nanopillars (i.e., $d_{x,y}=0.65\,p_{x,y}$), we fabricate the color
palettes shown in Figs. 4d,e captured under x-polarized illumination.
Measurement under y-polarization yields the same pattern flipped in $p_{x}$
and $p_{y}$ (results not shown here). The magnified bird’s eye view of three
meta-pixels indicated by dashed boxes in Fig. 4d,e are displayed in Fig. 4f.
The simulated palettes as well as a detailed analysis on the polarization-
based and phase-change-based color switching approaches in the presented
platform considering both Sb2S3 and Sb2Se3 meta-pixels are provided in
Supplementary Note IV and Figs. S6-9. In addition to the ratio used in Fig.
4d,e (i.e., $\alpha=0.65$), we fabricate palettes of meta-pixels with other
ratios of 0.45 and 0.55 and plot the captured images in Supplementary Fig.
S10. Moreover, based on the simulation results in Supplementary Fig. S8, we
design and fabricate palettes of Sb2Se3 meta-pixels with different ratios and
display their microscopic images in Fig. S11. The sensitivity of the generated
colors to the polarization angle, incident angle, and different design
parameters of the meta-pixels are analyzed in Supplementary Notes V (Fig.
S12), VI (Fig. S13), and VII (Fig. S14), respectively.

The color switching enabled by the phase transition of Sb2S3 and polarization
of the incident light can be employed for implementation of a dynamic display.
According to the color palettes in Figs. 4d,e, under x- (y-) polarization, a
column (row) of different colors in the amorphous phase can be mapped onto a
column (row) of relatively similar colors in the crystalline phase. We
leverage this unique feature of the Sb2S3 meta-pixels for switching off some
parts of an image while maintaining the colors of the remaining parts. As an
illustrative example, the image of The Cheshire Cat is generated by A-Sb2S3
meta-pixels illuminated with x-polarized white light as shown in Fig. 3a (i).
Upon phase-transition to the C-Sb2S3, all parts of the body vanish, but the
grinning and eyes remain (see Fig. 3a (ii)). It is also the case when changing
the polarization state from y to x in the crystalline phase. On the other
hand, altering the polarization in amorphous phase as well as switching the
material phase under y-polarization result only in a variation of colors in
the components of the image. The SEM image of the fabricated array of Sb2S3
nanopillars associated with the face of The Cheshire Cat indicated by the blue
dashed box shown in Fig. 3a is displayed in Fig. 3b. The magnified SEM image
shown in the inset demonstrates that a meta-pixel containing only four Sb2S3
nanopillars can generate the desired color showing the high-resolution nature
of the presented approach. The geometrical parameters of the meta-pixels used
for the generation of Figs. 3a,b are tabulated in Supplementary Fig. S15.

Another interesting characteristic of our platform is its two degrees of
freedom, i.e., phase-change and polarization-based color switching, at the
same time. In fact, it is possible to darken (brighten) some parts of an image
using the polarization-based control, while brightening (darkening) other
parts using phase-change control of the meta-pixels. We benefit from this
capability to encrypt two different images (i.e., Georgia Tech logo and
symbol) into a display containing an array of Sb2S3 meta-pixels as shown in
Fig. 3c. One image can be switched to another one either by altering the
polarization in each material phase, or by changing the phase of the Sb2S3
meta-pixels under a fixed polarization. While the former has been reported in
previous works, the latter, to the best of our knowledge, is the first
demonstration of encryption of two totally different images into the phase of
the constituent materials of a meta-pixel. The SEM image of the fabricated
Sb2S3 meta-pixels corresponding to the blue dashed box in Fig. 3c is
demonstrated in Fig. 3d. The design strategy of the meta-pixels used for
generating Figs. 3c,d is provided in Supplementary Fig. S16. Moreover, we
demonstrate other examples of dynamic displays using Sb2Se3 meta-pixels in
Supplementary Fig. S17. For the case of Sb2Se3, we demonstrate the encoding
and decoding of four different images into the A-Sb2Se3 (ON-state) and
C-Sb2Se3 (OFF-state), respectively, under x- and y-polarizations. These
capabilities can be used in many applications such as information coding,
cryptography, high-density optical data storage, security encryption, and 3D
displays.

In all experiments shown in Figs. 1-3, the phase-transition in our Sb2S3 meta-
pixels is performed by using a bulky heater for a relatively long annealing
time (see Methods for details). Though laser pulses can be used to expedite
the conversion process [45], the on-chip integration of high-power fast lasers
is challenging if not impossible. This hinders the applicability of our
approach for on-demand compact, high-resolution, fast, and on-chip display. To
promote the presented approach to a practical paradigm, we must electrically
convert the Sb2S3 meta-pixels. Recently, electrical switching of PCMs based on
Joule heating has been successfully demonstrated using metal micro-heaters
[27, 33, 34]. However, none of these platforms is suitable for structural
color generation due to the excessive loss of their constituent material in
the visible range. Thanks to their reduced optical loss, micro-heater formed
in transparent conductive oxides hold the promise to enable next-generation
dynamic structural colors.

As a proof-of-concept demonstration, we leverage an indium tin oxide (ITO)
heater to electrically reconfigurequality if the the phase-change meta-pixels
without compromising the quality of the generated colors. To this end, the
fabricated palettes in Figs. 4d,e are first encapsulated by a SiO2 layer
followed by fabrication of a 50 nm-thick indium tin oxide (ITO) bridge
connecting two gold (Au) probing pads at the two ends on top of the SiO2 film
(see Figs. 4a,b and Methods for fabrication details). The electro-thermal
simulation in Fig. 4c illustrates that a fairly uniform heat distribution can
be realized across the whole area of the display upon applying the voltage
pulse ensuring simultaneous and uniform conversion of all palettes. Such a
Joule heating platform offers the precise electrical control of the
intermediate phases of PCMs (beyond amorphous and crystalline) which is
critical for realization of multicolor displays, a key attribute of our
approach. We further investigate the potential of ITO-based micro-heater for
reversible switching of colors in Supplementary Information Figs. S19-22.

## Conclusion

In summary, we demonstrated here a new platform for generating and switching
high-efficiency, high-saturation, and wide-gamut structural colors using
switchable meta-pixels by employing PCM-based metasurfaces made of low-loss
and less explored Sb2S3 and Sb2Se3 nanopillars. Upon the nonvolatile phase-
transition of the constituent PCM, the generated color in the amorphous phase
switches to a distinctive stable color in the crystalline phase. In addition,
the properly designed asymmetric characteristics of elliptical nanopillars
enable polarization-based color switching. Combining these two tuning
mechanisms, we systematically designed a single-layer meta-pixel capable of
producing four different colors. This can be extended to the realization of
multi-color artificial images by gradually changing the crystallinity of the
constituent PCMs and/or the incident polarization angle. We also showed that
by engineering the arrangement of PCM-based nanopillars, features like image
switching, ON/OFF switching, and color shading can be realized. More
interestingly, we experimentally demonstrate, for the first time, an
electrically driven micro-scale display by integration of an optically-
transparent heater to our color without compromising the color quality. We
believe that this research provides a significant step towards the realization
and commercialization of compact metaphotonic devices for applications like
full-color dynamic displays, information storage, image encryption, and anti-
counterfeiting.

## Acknowledgements

The work was primarily funded by the Office of Naval Research (ONR)
(N00014-18-1-2055, Dr. B. Bennett) and by the Air Force Office of Scientific
Research MURI program. The support of the UK’s Engineering and Physical
Science Research Centre is gratefully acknowledged, through ChAMP–Chalcogenide
Advanced Manufacturing Partnership (EP/M015130/1). The Stanford authors
acknowledge partial support from the Stanford Graduate Fellowship, from the
Nonvolatile Memory Technology Research Initiative (NMTRI), and from Draper
Labs. This work was performed in part at the Georgia Tech Institute for
Electronics and Nanotechnology (IEN), a member of the National Nanotechnology
Coordinated Infrastructure (NNCI), which is supported by NSF (ECCS1542174).

## Disclosures

The authors declare no conflicts of interest.

## Methods

Sample fabrication. The fabrication flow for the Sb2S3 metasurface and
integrated transparent heater of the meta-display is illustrated in Fig. S23.
A Sb2S3 film of nominally 130 nm thickness is first sputtered on a cleaned
fused silica substrate from a stoichiometric target followed by the deposition
of a 15-nm thick ZnS:SiO2 film serving as a protective layer to prevent
oxidation and elemental loss of Sb2S3 undergoing the heating process. Next,
the sample is coated with a layer of hydrogen silsesquioxane (HSQ) negative
e-beam resist and a thin water-soluble conductive layer of ESpacer to hamper
the charge accumulation during the writing process. E-beam lithography is them
performed to define the nanopillar pattern in each 50$\times$50 $\mu$m2 meta-
pixel. After washing out ESpacer using DI water, the exposed photoresist is
developed by subsequently immersing it in a bath of 25% tetramethylammonium
hydroxide (TMAH) and rinsing with gently flowing DI water. Inductively couple
plasma reactive ion etching (ICP-RIE) is performed with a gas mixture of
Ar:CF4 with the etching rate of $\sim$ 75 nm/min to form nanostructure
patterns. The etching process is conducted through two 1-min cycles with a
long-enough cooldown break in between. Right after the etching, a 15 nm
protective layer of SiO2 is grown on the sample using atomic layer depostion
(ALD) at 100 ∘C, which is low enough to prevent the crystallization of Sb2S3.
To convert the material state of Sb2S3 to the crystalline phase, the sample is
annealed at 270 ∘C for 10 mins in a chamber filled with an ultrahigh pure Ar
gas.

To realize the electrically-driven display, the fabricated sample (excluded
from the annealing process) is first transferred to the ALD system to deposit
a 200-nm thick layer of thermal SiO2 as a supporting substrate for the
integrated heater. After defining the pattern of the ITO bridge in the
polymethyl methacrylate (PMMA)-coated sample using e-beam lithography, a 50
nm-thick layer of ITO is deposited by the RF-magnetron sputtering from an
indium oxide/tin oxide (In2O3/SnO2 with 90/10 wt %) target in an argon/oxygen
plasma atmosphere. The prolonged nature of the deposition facilitates the
crystallization of ITO necessary for the formation of a uniform conductive
layer enabling spatially consistent heat generation. After the lift-off
process, to further enhance the electrical conductivity of the ITO film, post-
deposition annealing under a mild flow of oxygen, which also reduces the
optical loss of ITO, is conducted at 200 ∘C for 30 mins. This temperature is
low enough to preclude crystallization of as-deposited Sb2S3, . Two Au/Ti
(250/20 nm) electrodes are formed at the two ends of the ITO bridge through
subsequent e-beam lithography and e-beam evaporation processes. After the
lift-off process, in the final step, a 100 nm layer of SiO2 is grown to
prevent the failure of the heater caused by the electric breakdown of the air
at the sharp corners of the device. To fully transform the Sb2S3 phase from
amorphous to crystalline based on the Joule heating process, a 32 V long-
enough (1 min) pulse is applied to the integrated heater using a source
measurement unit (Keithley 2614B).

Optical measurements. To investigate the optical response of the fabricated
meta-displays, bright-field optical imaging and reflection spectra
measurements of the color palettes are conducted. Optical images are captured
using a conventional upright bright-field reflection microscope (Nikon ECLIPSE
L200, Nikon Inc.) equipped with a high-definition color camera head (DS-Fi2)
and a 50 W halogen lamp light source. To observe different colors of The
Cheshire Cat and Georgia Tech logo and symbol images under different
polarization states of incident white light, the corresponding images are
magnified with a 10$\times$ objective lens (NA = 0.3) and a 2.5$\times$
objective lens (NA = 0.075), respectively, under illumination of polarized
light in both orthogonal directions. The optical spectra ($\lambda$ = 450-850
nm) are measured in reflection mode using a home-built microscope set-up
equipped with a 75 W broadband xenon source (Newport) and a UV-visible-near
infrared (NIR) spectrometer (USB 2000+, Ocean Optics Inc.). The polarized
light illuminates a colour palette at normal incidence through an achromatic
10$\times$ objective lens (NA = 0.25) and is collected through the same
objective and back into the spectrometer and a CCD camera. The measured
reflectance spectra are normalized to the reflected light from an aluminium-
coated mirror. All measurements are carried out at room temperature ($\sim$ 25
∘C).

Numerical simulations. The full-wave simulations of the reflectance spectra of
the metasurfaces are performed using the commercial software Lumerical
Solutions based on the finite-different time-domain (FDTD) technique. The
periodic boundary condition is used in the x- and y-directions to mimic the
periodicity, while perfectly matched layers are used in the z-direction (top
and bottom layers) to model the free space. The refractive index of the glass
substrate is set at 1.46 for the entire wavelength range. The dispersive
optical constants of PCMs obtained from spectroscopic ellipsometry
measurements shown in Supplementary Fig. S2 are incorporated into simulations.

Electro-thermal simulations. A three-dimensional finite element method (FEM)
simulation is performed in the software package COMSOL Multiphysics to
simulate the Joule heating and heat dissipation effects in the electrified
hybrid display. In our simulations, we consider certain assumptions and
boundary conditions to mimic the experimental conditions. The multiphysics
problem is solved through coupling of an electric currents (ec) module to a
heat transfer in solid (ht) physics model. Material properties used for fused
silica, Ti, Au, and ITO are adopted from the available references [46, 47].
The electrical conductivity of ITO obtained from the four-point probe
measurement is set at 1.42$\times$104 S/m. The thermal conductivity, density,
and heat capacity of Sb2S3 are 1.16 W/m·K, 4600 kg/m3, and 120 J/mol.K,
respectively [45]. The ec module is applied to the ITO bridge and electrodes.
Electric insulation are assigned to all boundaries except for the two endfaces
of the bridge where normal current density and electric ground are applied.
The ht physics model is assigned to all domains. The convective cooling
boundary condition with an ambient temperature of 20 ∘C and the heat transfer
coefficient of 5 W/m2.K is used at the top and bottom surfaces. Open boundary
condition is applied to the walls of the substrate in the lateral directions.

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## Supplementary Information

##  I Multipolar decomposition

Electromagnetic properties of the nanoparticles in the arrays are numerically
studied by using the commercial software CST Microwave StudioTM. In the
canonical basis we perform a multipole expansion of the scattered field of the
hybrid nanoparticles into vector spherical harmonics, which form a complete
and orthogonal basis allowing the unique expansion of any vectorial field. To
calculate electric (aE(l,m)) and magnetic (aM(l,m)) spherical multipole
coefficients, we project the scattered electric field $\mathbf{E}_{sca}$ on a
spherical surface, enclosing the nanoparticles centered at the symmetric point
of the nanodisc, onto vector spherical harmonics based on the following
relations [48, 49]:

$\begin{split}a_{E}(l,m)=&\frac{(-i)^{l+1}kR}{h_{l}^{(1)}(kR)E_{0}\sqrt{\pi(2l+1)(l+1)l}}\\\
&\int_{0}^{2\pi}\int_{0}^{\pi}Y^{*}_{lm}(\theta,\phi)\mathbf{r}\mathbf{E}_{sca}(\mathbf{r})\sin\theta
d\theta d\phi,\end{split}$ (S.1)
$\begin{split}a_{M}(l,m)=&\frac{(-i)^{l}kR}{h_{l}^{(1)}(kR)E_{0}\sqrt{\pi(2l+1)}}\\\
&\int_{0}^{2\pi}\int_{0}^{\pi}\mathbf{X}^{*}_{lm}(\theta,\phi)\mathbf{E}_{sca}(\mathbf{r})\sin\theta
d\theta d\phi,\end{split}$ (S.2)

where $R$ is the radius of the enclosing sphere, $k$ is the wavenumber,
$h_{l}^{(1)}$ is the Hankel function with the asymptotic of the outgoing
spherical wave, $E_{0}$ is the amplitude of the incident wave, $Y^{*}_{lm}$
and $\mathbf{X}^{*}_{lm}$ are scalar and vector spherical harmonics. The
integers $l$ and $m$ describe the order of the multipole (dipole, quadrupole,
…) and the amount of the z-component of angular momentum that is carried per
photon, respectively. Due to the azimuthal symmetry of the nanoparticles under
normal excitation, the amplitude of the scattering coefficients with opposite
$m$ indices are identical, i.e., $a_{E,M}(l,m)=a_{E,M}(l,-m)$.

##  II Color generation

To achieve generated colors, the International Commission on Illumination
(CIE) XYZ tristimulus values corresponding to the reflection spectra are
calculated as [11]:

$\displaystyle
X=\frac{1}{k}\int{I(\lambda)R(\lambda)\bar{x}(\lambda)d\lambda},$
$\displaystyle
Y=\frac{1}{k}\int{I(\lambda)R(\lambda)\bar{y}(\lambda)d\lambda},$ (S.3)
$\displaystyle
Z=\frac{1}{k}\int{I(\lambda)R(\lambda)\bar{z}(\lambda)d\lambda}$

where $k$ is the normalization factor, $I(\lambda)$ is energy distribution of
the reference light; $R(\lambda)$ is the reflection spectrum obtained from the
designed mestasurface under illumination; and $\bar{x}(\lambda)$,
$\bar{y}(\lambda)$, and $\bar{z}(\lambda$) are the CIE 1931 standard color-
matching functions (see Figure S2a). These chromaticity functions are then
normalized as $x=X/(X+Y+Z)$ and $y=Y/(X+Y+Z)$, which fall between 0 to 1, to
represent the colors in the CIE 1931 chromaticity diagram.

##  III Quantitative analysis on color gamut coverage, saturation maintenance
and hue variation

As shown in Fig. S4d-f, by increasing $p$, the spectral position of the
reflectance resonances for both amorphous (solid lines) and crystalline
(dashed lines) states red-shifts. The spectral position of each of these
resonances is dependent on the refractive index of the constituent phase-
change material (PCM) owing to the interference between ED and MD modes inside
the PCM nanopillars as will be discussed later. Therefore, by switching the
state of the nanopillars from amorphous to crystalline, the central
wavelengths of the resonances red-shift in the cases of Sb2S3 and Sb2Se3 (see
Fig. S4d,e), and blue-shift in the case of GeSe3 (see Fig. S4f) because
$\Delta n_{\textrm{Sb}_{2}\textrm{S}_{3}}$, $\Delta
n_{\textrm{Sb}_{2}\textrm{Se}_{3}}>0$, while $\Delta n_{\textrm{GeSe}_{3}}<0$
(with $\Delta n=n_{\textrm{C-PCM}}-n_{\textrm{A-PCM}}$) within the visible
wavelength range (see the refractive indices in Fig. S2a,b). The actual shift
of the resonance wavelengths are
$|\Delta\lambda_{\textrm{Sb}_{2}\textrm{S}_{3}}|<180$ nm,
$|\Delta\lambda_{\textrm{Sb}_{2}\textrm{Se}_{3}}|<200$ nm, and
$|\Delta\lambda_{\textrm{GeSe}_{3}}|<70$ nm (see Fig. S4d-f). The relative
strength of the wavelength shifts in these PCMs, i.e.
$|\Delta\lambda_{\textrm{Sb}_{2}\textrm{Se}_{3}}|>|\Delta\lambda_{\textrm{Sb}_{2}\textrm{S}_{3}}|>|\Delta\lambda_{\textrm{GeSe}_{3}}|$,
is attributed to the relative strength of the change in the real part of their
refractive indices upon the phase transition between amorphous and
crystalline, i.e. $|\Delta n_{\textrm{Sb}_{2}\textrm{Se}_{3}}|>|\Delta
n_{\textrm{Sb}_{2}\textrm{S}_{3}}|>|\Delta n_{\textrm{GeSe}_{3}}|$, as shown
in Supplementary Fig. S1. On the other hand, the sharpness of the reflectance
resonances in Fig S4d-f is mainly dependent on the PCM extinction coefficient
shown as the dashed curves in Fig. S4e,f. In the case of Sb2S3 nanopillars,
the high-efficiency resonances (i.e., those with high reflectance value at the
resonance peak) in the low-loss amorphous phase are damped upon the transition
to the crystalline phase with higher absorption loss (compare solid and dashed
curves in Fig. S4d). This high absorption loss arises for both amorphous and
crystalline Sb2Se3 nanopillars, resulting in relatively low-efficiency
reflectance resonances (see Fig. S4f). In contrast, GeSe3 nanopillars remain
very low-loss across the entire visible range for both the amorphous and
crystalline phases, yielding high-efficiency resonances in both cases (see
Fig. S4f).

For a quantitative comparison between the presented three PCMs in terms of
color generation/switching, Figs. 4g-i show the generated colors in the
amorphous (black circles) and crystalline (white squares) phases in the same
International Commission on Illumination (CIE) 1931 chromaticity coordinates
for the three PCMs in top panels, and their corresponding hue and saturation
values for amorphous (solid-circle lines) and crystalline (dashed-square
curves) phases in the bottom panels. The approach of calculating the CIE XYZ
tristimulus of the reflectance spectra and their corresponding hue and
saturation values are given in the Supporting Information Note I. In terms of
the color gamut coverage, the calculated color gamut area for A-Sb2Se3
(C-Sb2Se3) is around 98.3% (43.4%) of the standard RGB (sRGB) and 72.9%
(32.2%) of the Adobe RGB, from Fig. 4g. The color gamut area for the case of
A-Sb2Se3 (C-Sb2Se3) is around 70.1% (33.3%) of the sRGB, and 52% (24.7%) of
the Adobe RGB (from Figure 4h). For the case of A-GeSe3 (C-GeSe3), a full-
range of colors with gamut area of 57.8% (90.8%) of the sRGB, and 42.9%
(67.3%) of the Adobe RGB can be obtained (from Figure 4i). Therefore, in terms
of color gamut area, Sb2S3 and GeSe3 have almost the same performance, yet
better than Sb2Se3. Moreover, these results show that our all-dielectric PCM-
based metasurfaces can generate a wide color gamut larger than the state-of-
the-art plasmonic colors ($\sim 45\%$ of sRGB [50]) for A-Sb2S3, A-Sb2Se3 and
A/C-GeSe3 cases.

In the RGB color-mixing model, the hue (H) is defined as the proportion of the
dominant wavelength (resonance wavelength in this case) with respect to other
wavelengths in the reflected light and is independent of the intensity of the
light. It simply indicates the ”perceived color” by the human eyes and ranges
from $0^{\circ}$ to $360^{\circ}$, in which $0^{\circ}$ (and $360^{\circ}$),
$120^{\circ}$ and $240^{\circ}$ represent pure red, pure green, and pure blue,
respectively (See Supplementary Fig. S3 for more details). The saturation, on
the other hand, is defined as the ratio of the intensity of the reflected
light at the resonance wavelength (associated to the perceived color) to that
of the incident white light, simply indicating the purity of a color and
ranging from 0% to 100%. Considering this definition, the narrower the
bandwidth of the reflectance resonance, the higher the saturation of the
generated color. In the content of color switching between two phases, the
performance measure is achieving two highly-saturated colors in both phases
with a maximum hue variation
($\Delta\textrm{H}=\textrm{H}_{\textrm{C-PCM}}-\textrm{H}_{\textrm{A-PCM}}$)
upon switching. To analyze the performance of the presented phase-transition-
based color-switching approach, the hue and saturation values of the simulated
colors in Fig. S4d-f are plotted in the bottom panels in Fig. S4g-i for both
amorphous (solid-circle lines) and crystalline (dashed-square curves) phases
of the PCMs. In terms of saturation preservation upon phase transition, GeSe3
shows high-saturation values for both amorphous and crystalline cases (due to
sharp reflectance resonances), while Sb2S3 shows highly saturated colors only
in the amorphous phase. Sb2Se3, however, demonstrates a median level of
saturation values in both amorphous and crystalline phases due to the wide
reflectance resonances. With regards to hue variation, the hues of the
generated colors in Sb2S3 and GeSe3 cases change by varying $p$ in both
amorphous and crystalline states while maintaining
$\Delta\textrm{H}<80^{\circ}$ upon phase transition. One may use this feature
to switch the coloration of pixels of an image individually with each pixel
being a Sb2S3 or GeSe3 metasurface formed by of an array of down to 5$\times$5
or 6$\times$6 nanopillars [11]. In the case of Sb2Se3, however, by changing
$p$, all the varying hue values in the amorphous phase switch to an almost
fixed hue in the crystalline phase. Using this property, one can turn off all
the pixels of an image on a display comprising Sb2Se3 metasurfaces (i.e.,
pixels) by switching the phase of the Sb2Se3 nanopillars from the amorphous
state (ON-state) to the crystalline state (OFF-state). This is a unique
feature that is absent in other approaches in previous works, e.g., the
polarization-sensitive color-switching approach [19].

To provide a comparison between Sb2S3, Sb2Se3, and GeSe3 metasurfaces for
color switching applications, a spider chart is shown in Fig. S5. The figure
of merit (FOM) is defined as the maximum variation of the hue over the
refractive index change open phase transition between amorphous and
crystalline, i.e. $|\Delta\textrm{Hue}|/|\Delta n|$ in which $\Delta
n=n_{\textrm{A}}(\lambda_{\textrm{A}})-n_{\textrm{C}}(\lambda_{\textrm{C}})$),
with $n_{\textrm{A}}(\lambda_{\textrm{A}})$ and
$n_{\textrm{C}}(\lambda_{\textrm{C}})$ being the index of refraction in the
amorphous and crystalline phases and at the corresponding resonance
wavelengths ($\lambda_{\textrm{A}},\lambda_{\textrm{C}}$), respectively. While
high FOM is desirable, the saturation and value (i.e. the reflectance value at
the resonance peak) of the colors in both amorphous and crystalline phases
should be as high as possible. Considering all these performance measures,
GeSe3 demonstrates superior properties over Sb2S3 and Sb2Se3 when switching
from a color associated with a reflectance spectrum with a resonance peak at
$\lambda=600$ nm (chosen as the middle wavelength in the visible range from
400 nm to 800 nm) in the amorphous phase, to another color in the crystalline
phase.

##  IV Polarization-sensitive dynamic color generation

To add the polarization-sensitivity to our color-switching approach, we also
consider elliptical nanopillars in asymmetric unit cells with different
periodicities in the x- and y-directions, i.e. $p_{x}$ and $p_{y}$, Fig 1b. By
varying $p_{x}$ and $p_{y}$ with a fixed ratio with respect to the major and
minor axes of the nanopillars (i.e., $d_{x,y}=\alpha\,p_{x,y}$), we generate
the color palettes shown in Fig. S6a,d,g, for the case of Sb2S3 ($p_{x,y}$
range from 310 nm to 470 nm with 40-nm increments), Sb2Se3 ($p_{x,y}$ range
from 200 nm to 400 nm with 50-nm increments), and GeSe3 ($p_{x,y}$ range from
270 nm to 430 nm with 40-nm increments), respectively (see Supplementary Fig.
S4-S6 in the for full color palettes). In each figure, the top (bottom) panels
show the colors generated by the x-polarized (y-polarized) incident white
light for amorphous (left panels) and crystalline (right panels) cases. While
Sb2S3 and GeSe3 metasurfaces can generate a full palette considering both
amorphous and crystalline phases (see Fig. S6a,g), respectively, Sb2Se3
metasurfaces cannot generate bluish colors (see Fig. S6d). This stems from the
high optical loss of Sb2Se3 within the blue range of the visible wavelengths
(see Fig. S2a,b). It is also clear that the y-polarization palettes can be
obtained by transposing the x-polarization palette, i.e., replacing each (j,i)
element with corresponding (i,j) element. However, this is not the case for
amorphous and crystalline palettes in Fig. S6a,d,g since the crystalline
palettes contain completely different colors from those in the amorphous
palettes. This shows the advantage of using PCMs as the number of colors in
the phase-transition-based color-switching approach is twice as many as those
in the polarization-based approach.

To analyze the effect of polarization-sensitivity in both amorphous and
crystalline cases on the reflected colors, we select five metasurfaces for
each PCM with geometrical parameters in the dashed boxes in Fig. S6a,d,g, and
plot the corresponding simulated reflectance spectra with their hue and
saturation values in the inset in Fig. S6b,e,h, respectively. It is seen that
by increasing $p_{y}$ in each box, the central wavelength of the reflectance
resonances does not experience a considerable shift for the x-polarization
(see the top panels in Fig. S6b,e,h). This leads to almost unchanged hue
values for the corresponding colors, which in turn results in a limited
trajectory in the corresponding color gamuts shown in the top panels of Fig.
S6c,f,i in which black circles (white squares) represent the colors in
amorphous (crystalline) phase. In contrast, it is observed that increasing
$p_{y}$ results in a tangible redshift in the reflectance spectra for the
y-polarization for all PCMs (see the bottom panels in Fig. S6b,e,h). This
redshift results in a relatively large hue change in all cases, except
C-Sb2Se3, as the corresponding color gamuts in the bottom panels of Fig.
S6c,f,i demonstrate. The simulated full color palettes as well as their
corresponding gamuts are provided in Figs. S7-9. Based on these simulation
results, we designed and fabricated palettes of Sb2S3 and Sb2Se3 meta-pixels
with different ratios and display their corresponding microscopic images in
Fig. S12 and 13, respectively.

Finally, in the Supplementary Note V, we show that by continuously varying the
incident polarization angle ($\varphi$) one can enable dynamic color tuning
(See Figure S12).

##  V Sensitivity to the incident polarization angle

To analyze the effect of the variation of the incident polarization angle
($\varphi$) on the reflected colors, we select one metasurface for each type
of PCMs with geometrical parameters shown in Fig. S12a,d,g and change
$\varphi$ from $0^{\circ}$ (y-polarization) to $90^{\circ}$ (x-polarization).
The reflectance spectra of these metasurfaces for $\varphi=0^{\circ}$ and
$\varphi=90^{\circ}$ for both amorphous and crystalline states are plotted in
Fig. S12c,f,i. In both amorphous and crystalline states, a resonance shift of
at least 100 nm is observed, which enables us to dynamically tune the
reflected colors by varying the incident polarization angle. This
polarization-based color tunability is demonstrated in the colors in Fig.
S12b,e,h, which are generated through varying $\varphi$ from $0^{\circ}$ to
$90^{\circ}$ in a step of $15^{\circ}$. The colors in Fig. S12b,e,h and their
corresponding CIE diagrams show that using Sb2S3 and GeSe3 metasurfaces, one
can tune the colors from green to reddish purple to blue, while Sb2Se3 can
enable color tuning from dark green to red to purple.

##  VI Sensitivity to the incident angle

To analyze the effect of the incident angle ($\theta$) on the reflectance
spectrum of a metasurface (Fig. 1b), we select a metasurface with Sb2S3
nanopillars, as shown in Fig. S13a,b, and vary the angle of the incident light
from $\theta=0^{\circ}$ to $\theta=30^{\circ}$. Fig. S13c,d show the
reflection spectra for amorphous Sb2S3, and Fig. S13e,f show the results of
crystalline Sb2S3, with TE- and TM-polarized light, respectively. In the case
of TE-polarized light incident on amorphous Sb2S3 (Fig. S13c), the incident
angle has a small impact on the reflection spectrum. The intensity of the
reflected is reduced by 20% when $\theta$ approaches $5^{\circ}$, but the
reflection spectra does not suffer any redshift. The spectra resulted from the
crystalline Sb2S3 experiences a redshift of more than 100 nm and is less
intense and is less intense compared to the amorphous case, but these spectra
remain largely unaffected by the incident angle variation.

In the case of TM-polarized light on amorphous Sb2S3, a much greater
dependence on $\theta$ is observed from Fig. S13d,f. As $\theta$ increases,
two effects can be seen from these figures: 1) the initial peak at
$\theta=0^{\circ}$ seen begins to lose intensity and experiences a redshift,
and 2) a new peak forms and becomes more pronounced, both as $\theta$ goes
beyond $5^{\circ}$. When Sb2S3 is crystalline in this case, no considerable
changes are observed for $0^{\circ}<\theta<20^{\circ}$ after which the peak
redshifts by about 100 nm at $\theta=30^{\circ}$. In addition, for
$\theta>20^{\circ}$, the second peak that was observable in the amorphous case
is not seen in the crystalline case. These results are not surprising; the
reflectance of these metasurfaces is largely due to ED and MD resonances that
are supported by the nanopillar structures, and the ED resonances are the
dominating resonances seen in the reflectance spectra. Since the component of
the electric field parallel to the top surface of the Sb2S3 nanopillars does
not change in the case of the obliquely incident TE-polarized light (See Fig.
S9a), the incident angle should not have a major effect on the output spectra.
Likewise, since the this component of the electric field changes in the case
of obliquely incident TM-polarized light (See Fig. S13b), we should see a
greater impact of varying $\theta$ on the resulting spectra.

##  VII Influence of different design parameters

Analysis must also be done to determine the effects that the physical
dimensions of the nanopillars have on the reflection spectrum. Fig. S14a,b,c
show the reflectivity spectrum of a Sb2S3 array with nanopillars Fig 1b of
varying heights (h), periods (p), and diameters (d). A control case is picked
with $h=120$ nm, $p=390$ nm, and $d=0.6\,p$. Fig. S14a shows the effect of
varying $h$ from 100 nm to 400 nm in the control case. This figure shows that
few values of $h$ give sharp reflections. Increasing the height past 100 nm
causes a redshift in the reflection and a severe broadening of the reflection
spectrum, until it decreases around $h=300$ nm and ultimately disappears
around $h=400$ nm. Also, around $h=200$ nm, another reflection appears in the
spectrum. Increasing $h$ beyond this point causes a redshift without the same
severe broadening.

Fig. S14b shows the effect of varying $p$ from 200 nm to 500 nm in the control
case. Fig. S14b shows that increasing $p$ causes a redshift in the reflection
spectrum throughout this test case. Also, the reflected spectrum narrows by
increasing $p$ from around $p=200$ nm to around $p=400$ nm. Fig. 14c shows the
effect of varying $d$ from $d=150$ nm to $d=350$ nm. Figure S8c shows that
increasing $d$ from 150 nm causes a redshift in the resulting spectrum. This
peak decreases for $d>250$ nm. However, another peak starts to appear around
$d>250$ nm and remains at larger values if $d$ in this range. This new peak
does not experience a red shift with an increase in $d$, but another,
narrower, peak starts to appear with the increase in $d$. The change from
amorphous to crystalline Sb2S3 has a nearly uniform effect in all these cases.
The phase change to crystalline severely decreases the reflectivity of the
metasurface and causes a redshift at the same time.

Figure S1: Multipolar decomposition analysis. a,b, Multipolar decomposition of
scattering cross-section in terms of electric dipole (ED, the dotted lines)
and magnetic dipole (MD, the dashed lines) for the case of an periodic array
of (a) amorphous and (b) crystalline Sb2S3 nanopillars with $h=120$ nm,
$d=0.6\,p$ in a lattice with varying periodicity of $p$ on top of a SiO2
substrate. The reflectance (R) response for each case is plotted in solid
lines. Figure S2: Optical characteristics of low-loss phase-change materials.
a,b, Real (solid lines) and imaginary (dashed lines) parts of the refractive
index of Sb2S3, Sb2Se3, and GeSe3 for (a) amorphous (A) and (b) crystalline
(C) phases. c, The absolute value of the change in the refractive index (solid
lines, $\Delta n=|n_{\textrm{C-PCM}}-n_{\textrm{A-PCM}}|$) and the extinction
coefficient (dashed lines, $\Delta k=|k_{\textrm{C-PCM}}-k_{\textrm{A-PCM}}|$)
versus the wavelength upon the transition between amorphous and crystalline
phase-states for Sb2S3, Sb2Se3 and GeSe3. Figure S3: Color generation and
characteristics. a, CIE 1931 standard color-matching functions. b, HSV color
solid cylinder saturation gray [11]. Figure S4: Color switching enabled by
phase-transition of the PCM nanopillars. a-c, Schematic and geometrical
parameters of a unit cell of a polarization-insensitive PCM metasurface made
of (a) Sb2S3, (b) Sb2Se3 and (c) GeSe3 circular nanopillars with a fixed heigh
$h$. The periodicity of the unit cell in both x and y directions is $p$, and
the diameter of the nanopillars is $d=\alpha\,p$ with $\alpha$ being a
constant. d-f, Simulated reflectance spectra for the amorphous (solid lines)
and crystalline (dashed lines) phases and their corresponding colors for
different periodicities ($p$). The PCM is (d), (e), and (f) is Sb2S3, Sb2Se3,
and GeSe3, respectively. The curves for different $p$s are diplaced vertically
for better visibility and comparison. The sharp resonances observed in (d-f)
are attributed to the interference between ED and MD modes inside the PCM
nanopillars. Upon the PCM phase transition, a red-shift of
$|\Delta\lambda_{\textrm{Sb}_{2}\textrm{S}_{3}}|>180$ nm and
$|\Delta\lambda_{\textrm{Sb}_{2}\textrm{Se}_{3}}|>200$ nm is observed for the
case of (d) Sb2S3 and (e) Sb2Se3, respectively, while a blue-shift of
$|\Delta\lambda_{\textrm{Ge}\textrm{Se}_{3}}|<70$ nm is observed for the case
of (f) GeSe3. g-i, Corresponding CIE 1931 chromaticity coordinates of the
reflectance spectra, and the hue and saturation values of the colors shown in
(d-f) for amorphous (black circles in the top panel and circle-solid line in
bottom panel) and crystalline (white squares in the top panel and square-
dashed line in the bottom panel) phases of the corresponding PCMs in (d-f).
Figure S5: Comparison of low-loss PCMs for color switching applications. A
spider chart that compares Sb2S3, Sb2Se3 and GeSe3 in terms of FOM (defined as
the maximum (max) of $|\Delta\textrm{Hue}|/|\Delta n|$ in which $\Delta
n=n_{\textrm{A}}(\lambda_{\textrm{A}})-n_{\textrm{C}}(\lambda_{\textrm{C}})$),
maximum saturation and maximum value (i.e. the reflectance value at the
resonance peak) in amorphous and crystalline phases at
$\lambda_{\textrm{A}}=600$ nm. Figure S6: Multiple color generation enabled by
phase-transition-based and polarization-based color switching mechanisms.
a,d,g, Generated color palettes considering different periodicities in x- and
y-directions ($p_{x}$ and $p_{y}$, respectively) for (a) Sb2S3 ($\alpha=0.6$
and $h=120$ nm), (d) Sb2Se3 ($\alpha=0.55$ and $h=120$ nm), and (g) GeSe3
($\alpha=0.55$ and $h=250$ nm). $p_{x}$ and $p_{y}$ in (a), (d) and (g) vary
with 40 nm, 50 nm, and 40 nm increments, respectively. b,e,h, Reflectance
spectra of the colors indicated by the dashed rectangular boxes shown in the
corresponding color palette in (a,d,g), respectively, with the values of hue
and saturation (sat.) in the inset. c,f,i, Corresponding color gamuts for
amorphous (black circles) and crystalline (white squares) phases of the
corresponding PCM in (a,d,g), respectively. In each figure, the upper (lower)
panel represents the results related to x-polarization (y-polarization).
Figure S7: Dynamic color generation by Sb2S3 meta-pixels. a,b, The color
palettes and c, d, corresponding CIE 1931 chromaticity diagrams generated by
Sb2S3 metasurfaces in (a, c) amorphous and (b, d) crystalline phase-states
under x-polarized normally incident white light. The lattice periodicities in
x- and y-directions vary from $p_{\textrm{x,y}}=310$ nm to
$p_{\textrm{x,y}}=470$ nm with a step of 20 nm while the diameter of the
nanopillars changes as $d_{\textrm{x,y}}=0.6\,p_{\textrm{x,y}}$, and the
height of the nanopillars is fixed at $h=120$ nm. Figure S8: Dynamic color
generation by Sb2Se3 meta-pixels. a,b, The color palettes and c, d,
corresponding CIE 1931 chromaticity diagrams generated by Sb2Se3 metasurfaces
in (a, c) amorphous and (b, d) crystalline phase-states under x-polarized
normally incident white light. The lattice periodicities in x- and
y-directions vary from $p_{\textrm{x,y}}=200$ nm to $p_{\textrm{x,y}}=400$ nm
with a step of 25 nm while the diameter of the nanopillars changes as
$d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, and the height of the nanopillars
is fixed at $h=120$ nm. Figure S9: Dynamic color generation by GeSe3 meta-
pixels. a,b, The color palettes and c, d, corresponding CIE 1931 chromaticity
diagrams generated by GeSe3 metasurfaces in (a, c) amorphous and (b, d)
crystalline phase-states under x-polarized normally incident white light. The
lattice periodicities in x- and y-directions vary from $p_{\textrm{x,y}}=270$
nm to $p_{\textrm{x,y}}=430$ nm with a step of 20 nm while the diameter of the
nanopillars changes as $d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, and the
height of the nanopillars is fixed at $h=250$ nm. Figure S10: Experimental
color palettes of Sb2S3 meta-pixels a,b,c, A-Sb2S3 (left) and C-Sb2S3 (right)
meta-pixels considering different periodicities in x- and y- directions
($p_{x}$ and $p_{y}$, respectively) varying with 20 nm increments while the
diameter of the nanopillars changes as (a)
$d_{\textrm{x,y}}=0.65\,p_{\textrm{x,y}}$, (b)
$d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, (c)
$d_{\textrm{x,y}}=0.45\,p_{\textrm{x,y}}$, and the height of the nanopillars
is fixed at $h=120$ nm.  Figure S11: Experimental color palettes of Sb2Se3
meta-pixels a,b, A-Sb2Se3 (left) and C-Sb2Se3 (right) meta-pixels considering
different periodicities in x- and y- directions ($p_{x}$ and $p_{y}$,
respectively) varying with 20 nm increments while the diameter of the
nanopillars changes as (a) $d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, (b)
$d_{\textrm{x,y}}=0.65\,p_{\textrm{x,y}}$, and the height of the nanopillars
is fixed at $h=120$ nm. Sb2Se3 is sputtered in a magnetron sputtering system
using 30 W radio frequency (RF) power at a deposition pressure of 4 mTorr and
Ar flow of 30 sccm. The deposition rate for Sb2Se3 is $\sim$1 nm/min. Before
deposition, the chamber base pressure is maintained at $\sim$10-7 Torr.
Additionally, the samples are capped with 15 nm of SiO2 sputtered in situ, to
prevent oxidation during later characterization. As an aside, several pre- and
post-deposition treatments of the sputtering chamber are performed for
selenide deposition. These include cleaning the chamber followed by annealing
and O2 plasma cleaning. Figure S12: Polarization-based continuous color-
switching enabled by rotating the incident polarization angle. The asymmetric
unit cells of the polarization-sensitive metasurface with the optimized design
parameters are shown in a, d, and g, respectively, with their corresponding
variation of colors with polarization angle $\varphi$ and their color gamts
shown in b, e, and h, respectively. The simulated reflectance spectra from c,
Sb2S3, f, Sb2Se3, and i, GeSe3 metasurfaces for x-polarization
($\varphi=90^{\circ}$) and y-polarization ($\varphi=0^{\circ}$). The
reflection-mode color response varies from reddish purple to the yellowish
green for A-Sb2S3, bluish purple to the reddish orange for C-Sb2S3, red to
dark green for A-Sb2Se3, red purple to brown for C-Sb2Se3, purple to green for
A-GeSe3, and red purple to blue for C-GeSe3. Figure S13: Analysis of the
sensitivity to the angle of incidence. The structure used in the study of the
angle sensitivity of a Sb2S3 metasurface for the case of obliquely incident
plane waves of white light for a, TE and b, TM polarizations, respectively.
c,d,e,f, The simulated reflection spectra of the metasurface, showing the
incident angle (degrees) versus wavelength (nm) for: (c) amorphous phase and
TE polarization, (d) amorphous phase and TM polarization, (e) crystalline
phase and TE polarization, (f) crystalline phase and TM polarization. Figure
S14: Analysis of the effect of different design parameters. Simulated
reflection spectrum of the Sb2S3 metasurface in Figure 2a versus; a, The
height of the constituents nanopillars, i.e., $h$, while other parameters are
fixed at $p=390$ nm and $d=0.6\,p$ for (top) amorphous and (bottom)
crystalline phases; b, period of the unit cell, i.e., $p$, with $d=0.6\,p$ and
$h=120$ nm for (top) amorphous and (bottom) crystalline phases; c, diameter of
the constituent nanopillars, i.e., $d$, with $p=390$ nm and $h=120$ nm for
(top) amorphous and (bottom) crystalline phases. Figure S15: Design strategy
for generating the dynamic image of Cheshire The Cat. The geometrical
parameters of the Sb2S3 metasurfaces used for producing each pixel of the
image of Cheshire The Cat shown in Fig. 3a in the main text
($d_{\textrm{x,y}}=0.65\,p_{\textrm{x,y}}$ and $h=120$ nm in Fig. 1b). Figure
S16: Design strategy for encryption of four different images into the phase
and polarization of Sb2S3 meta-pixels. a,c, Phase-transition-based switching
between two different images. The colors are generated by four different
metasurfaces consisting of Sb2S3 nanopillars (Fig. 1b) with periodicities
reported in the table, diameters $d_{\textrm{x,y}}=0.65\,p_{\textrm{x,y}}$,
and a fixed height $h=120$ nm. b, The definition of different zones in each
image. Figure S17: Design strategy for encryption of four different images
into the phase and polarization of Sb2Se3 and GeSe3 meta-pixels. a,
Polarization-based switching between two different images. The colors are
generated by four different metasurfaces consisting of GeSe3 nanopillars (Fig.
S3b) with periodicities reported in the table, diameters of
$d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, and a fixed height $h=250$ nm. b,
The definition of different zones in each image. c, Phase-transition-based
switching between the ON-state (amorphous) and the OFF-state (crystalline).
The colors are generated by four different metasurfaces consisting of Sb2Se3
nanopillars with periodicities reported in the table, diameters
$d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, and a fixed height $h=120$ nm.
Figure S18: Design strategy for encryption of four different images into the
phase and polarization of Sb2Se3 and GeSe3 meta-pixels. a, Polarization-based
switching between two different images. The colors are generated by four
different metasurfaces consisting of GeSe3 nanopillars (Fig. S3b) with
periodicities reported in the table, diameters
$d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, and a fixed height $h=250$ nm. b,
The definition of different zones in each image. c, Phase-transition-based
switching between the ON-state (amorphous) and the OFF-state (crystalline).
The colors are generated by four different metasurfaces consisting of Sb2Se3
nanopillars with periodicities reported in the table, diameters
$d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, and a fixed height $h=120$ nm.
Figure S19: Electrical conversion of color palettes of Sb2S3 meta-pixels using
ITO heater. a,b, A-Sb2S3 (left) and C-Sb2S3 (right) meta-pixels observed from
(a) top and (b) bottom of the sample considering different periodicities in x-
and y- directions ($p_{x}$ and $p_{y}$, respectively) varying with 20 nm
increments while the diameter of the Sb2S3 nanopillars changes as
$d_{\textrm{x,y}}=0.45\,p_{\textrm{x,y}}$, and the height of the nanopillars
is fixed at $h=120$ nm. The scale bars are 100 $\mu$m. Figure S20: Electrical
conversion of color palettes of Sb2S3 meta-pixels using ITO heater. a,b,
A-Sb2S3 (left) and C-Sb2S3 (right) meta-pixels observed from (a) top and (b)
bottom of the sample considering different periodicities in x- and y-
directions ($p_{x}$ and $p_{y}$, respectively) varying with 20 nm increments
while the diameter of the Sb2S3 nanopillars changes as
$d_{\textrm{x,y}}=0.55\,p_{\textrm{x,y}}$, and the height of the nanopillars
is fixed at $h=120$ nm. The scale bars are 100 $\mu$m. Figure S21: Electrical
conversion of color palettes of Sb2S3 meta-pixels using ITO heater. a,b,
A-Sb2S3 (left) and C-Sb2S3 (right) meta-pixels observed from (a) top and (b)
bottom of the sample considering different periodicities in x- and y-
directions ($p_{x}$ and $p_{y}$, respectively) varying with 20 nm increments
while the diameter of the Sb2S3 nanopillars changes as
$d_{\textrm{x,y}}=0.65\,p_{\textrm{x,y}}$, and the height of the nanopillars
is fixed at $h=120$ nm. The scale bars are 100 $\mu$m. Figure S22: Electrical
conversion of a Sb2S3 meta-pixel using ITO micro-heater. a-d, Microscope
images of (a,b) 100$\times$100 $\mu$m2, and (c,d) 10$\times$10 $\mu$m2 micro-
heaters with 50$\times$50 $\mu$m2 and 5$\times$5 $\mu$m2 meta-pixels at the
center, respectively. e, Simulated temperature distribution in the cross-
section of the meta-pixel in (d) at the end of a 7 V pulse with 15 $\mu$s
duration. d, Real-time temperature profile at the center of the meta-pixel
upon applying the re-amorphization pulse to the microheater. Figure S23:
Fabrication process. Figure S24: Optical characterization setup. Figure S25:
Characterization of the anisotropic C-Sb2S3 crystals. a-d, Optical images a
film of (a) A-Sb2S3 and (b-d) C-Sb2S3 under microscope with (b) unpolarized,
(c) x- and (d) y-polarized incident white light. The crystalized regions at
(i) and (ii) switches from greenish colors to brownish ones going from x- to
y-polarization, while colors in regions (iii) and (iv) changes from brownish
to greenish, and colors in areas (v) and (vi) remains the almost unchanged.
The dark particle at the center of the images is used as the marker for
positioning. Figure S26: Characterization of the anisotropic C-Sb2Se3
crystals. a-d, Optical images a film of (a) A-Sb2Se3 and (b-d) C-Sb2Se3 under
microscope with (b) unpolarized, (c) x- and (d) y-polarized incident white
light. The dark particle at the center of the images is used as the marker for
positioning.