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nature reviews
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https://doi.org/10.1038/s44287-024-00136-4
Review article
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The second optical metasurface
revolution: moving from
science to technology
Mark L. Brongersma
1
, Ragip A. Pala
2
, Hatice Altug
3
, Federico Capasso
4
, Wei Ting Chen
4
, Arka Majumdar
5,6
& Harry A. Atwater
7
Abstract
Optical metasurfaces are judiciously nanostructured thin films capable
of manipulating the flow of light in a myriad of new ways. During the
past two decades, we have witnessed a true revolution in the basic
science that underlies their operation. As a result, these powerful
optical elements can now deliver never-seen-before optical functions
and transformed the way we think about light–matter interaction at
the nanoscale. They also offer a favourable size, weight, power and cost
metric compared to bulky optical elements such as lenses and prisms
based on polished pieces of glass or moulded plastics. These valuable
traits are especially relevant for use in many emerging applications,
including wearable displays and sensors, autonomous navigation
(robotics, automotive and a er
os pa ce), c
om pu ta ti onal imaging, solar
energy harvesting and radiative cooling. With the advent of advanced
software and h ig h-
vo lume m a n uf a c
tu ring p r
o ce s
s es, the promise of
metasurfaces is becoming a practical reality and has already generated
tremendous interest from industry. This Review discusses the rapid,
recent advances towards transitioning metasurface science into real
technologies, propelling the second metasurface revolution.
Sections
Introduction
Moving metasurface
fabrication to square
kilometres
Manipulation of structural
colour, photodetection and
light emission processes
Biosensing and biomedical
applications
Metasurfaces for 3D imaging
Computational imaging and
hardware–software co-design
Solar energy harvesting
and radiative cooling
Outlook
1
Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA, USA.
2
Meta Materials Inc.,
Pleasanton, CA, USA.
3
School of Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne,
Switzerland.
4
Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University,
Cambridge, MA, USA.
5
Department of Physics, University of Washington, Seattle, WA, USA.
6
Electrical and
Computer Engineering, University of Washington, Seattle, WA, USA.
7
Thomas J. Watson Laboratories of Applied
Physics, California Institute of Technology, Pasadena, CA, USA.
e-mail:
brongersma@stanford.edu
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Review article
image sensors
61
, and solid-state light emitters such as vertical-cavity
surface-emitting lasers (VCSELs)
74
,
75
, solid-state light-emitting diodes
(LEDs)
70
, and displays
76
,
77
. Here, performance benefits and new func-
tions can be achieved through the application of a sub-wavelength
patterning step to the typically continuous metal and semiconduc
-
tor layers in these devices. The patterning of metal films can afford
control over the reflection phase of light waves. This action can be
used to achieve enhanced light–matter interaction by spectrally tun
-
ing Fabry–Pérot resonances and spatially manipulating the standing
optical waves in layered devices
53
,
76
. Nanopatterning of semiconduc-
tors can be used to notably reengineer their spectral absorption and
emission properties
54
,
65
. Typically the reflection, transmission and
absorption properties are considered to be intrinsic material proper-
ties that are directly linked to their electronic band structure. However,
nanopatterning provides substantial freedom in engineering these
properties and can broaden the pallet of usable materials. This is an
area wherein commercialization of metasurface concepts can pos
-
sibly be fast as the device infrastructure for realizing nanostructured
optoelectronic devices is already very mature. Existing and commer
-
cially viable technologies can be re-envisioned by exploring whether
minor additional patterning steps in a process can reduce their power
consumption and speed of operation while possibly also bringing
entirely new features.
The metasurface field has seen incredible scientific advances, and
by now, we have a solid fundamental understanding of the way these
optical elements can manipulate light at the subwavelength scale.
Together with the recent availability of advanced optical simulation
software, large-scale nanofabrication technologies, and emergence
of a large number of application areas such as smart glass, sensors,
wearables, robotics, automotive, smart home, security and internet of
things, this is an opportune moment to launch a second metasurface
revolution. This revolution will be centred on leveraging our fundamen
-
tal understanding of metasurface behaviours to create a wide range
of real-life, optical technologies in which metasurface concepts and
elements are at the operational core. Already, many startup and large
public companies are actively pursuing metasurface technologies in
real applications. Various US government agencies have also funded a
plurality of small companies on developing metasurface technologies
through
SBIR and STTR programmes
by more than 10 million between
2020 and 2022. The technological developments are extremely rapid,
and for this reason, it is a good moment to describe where some of the
first commercial applications are emerging and what the major road-
blocks to their successful implementation are. Rather than providing
an exhaustive description of all the commercial opportunities, we
will highlight several key areas that are leading some of the develop
-
ment of commercial metasurface technologies. For each technology,
we explain why metasurfaces can provide key advantages over the
state-of-the-art and valuable new opportunities. We will then discuss
the most important and recent developments. We will also describe
the large-area nanofabrication strategies that are critical to bringing
metasurface technologies to scale and to democratize the technology.
We will conclude with an assessment of the most promising future
directions and current trends.
Moving metasurface fabrication to
square kilometres
The development of scalable nanofabrication techniques is essential
for the practical realization of nanostructured metasurface materials
and devices. In the past decade, phase-shift lithography
78
has been
Introduction
In optics, we are used to controlling the flow of light using bulky opti-
cal components. Metasurfaces aim to manipulate light in new ways
and with much more compact and desirable form factors. Many of
us are familiar with diffractive optical elements such as the Fresnel
lens, which are predecessors to metasurfaces. Diffractive optical ele-
ments reshape the wavefront of light by manipulating its propagation
phase. They were created by judiciously structuring the surface of
transparent materials to create wavelength-scale height variations.
These designer topographic features directly translate into changes
of the local phase shift as light propagates through the optical ele
-
ment. Binary blazed gratings evolved from such echelette components,
and they manipulate the propagation phase by nanostructuring thin
films into dense forests of nanostructures that display a binary height
profile
1
4
. When the refractive index of the pillars is sufficiently high,
they can be considered as tiny, truncated waveguides whose physical
dimensions determine the locally incurred phase lag upon propagation
from one end to the other. By carefully choosing all of the pillar cross
sections in an array, it is again possible to imprint spatially varying phase
changes onto incident light waves. These elements offered a more con
-
venient fabrication process and higher diffraction efficiencies. In 2001,
geometric-phase, flat optical elements provided yet one more pathway
to shape wavefronts by varying the orientation, as opposed to the size
and shape of subwavelength structures
5
8
. They offer complete 0 to
2π phase control by simply rotating structures, whose geometry and
spacing are optimized to achieve high efficiencies
9
. Another important
conceptual step in the development of flat optics was the realization
that optical resonances in metallic
10
,
11
and high-index semiconductor
12
,
13
nanostructures could also be harnessed to create flat optical elements.
Resonances can notably enhance light scattering cross sections of
small particles and effectively control the phase
14
(Box
1
). Their design
closely follows that of radiofrequency antennas and were consequently
termed optical antennas
15
. As metal and high-index structures reso
-
nate light waves with strongly reduced wavelengths, their size can
be much smaller than the free-space wavelength
16
19
. For this reason,
optical antennas can be used to further thin optical elements that rely
on manipulating propagation phase. The resulting flat optical element
can be treated as essentially two-dimensional, and this appears to cause
a discontinuity in the phase of transmitted and reflected light waves.
This brought in the important notion that the optical properties of a
nanopatterned surface of a material can be as important as their bulk
optical properties, that is, the refractive index, and led to formulation
of generalized laws of refraction
10
,
20
and the term metasurface
21
,
22
. With
new, computational inverse and topological design techniques
23
25
, the
dispersive nature of scattering elements and their optical coupling can
be harnessed, and as a result, we can now create very high efficiency
and largely aberration-free metasurfaces
26
. Such metasurfaces can be
used to create small-form-factor optics with multi-functionality
27
33
,
lenses with very high-numerical apertures
34
,
35
, straightforward integra
-
tion with conventional optics
36
, minimal aberrations
37
40
, non-linear
optics
41
,
42
, 3D holograms
43
46
, and control over the light field
47
49
. For
applications, such optical spectrometry and imaging of the cosmos,
large-area (≥10 cm) optical elements are required, and it has been possi
-
ble to fabricate them with high accuracy by electron beam lithography
50
and more recently deep ultraviolet (UV) lithography
51
.
Metasurfaces are not only used to control the flow of light,
but they can also enhance and manipulate light absorption
52
65
and
emission
66
70
processes. For this reason, they have been applied in
solar cells
71
73
, complementary metal–oxide–semiconductor (CMOS)
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extensively utilized for the fabrication of subwavelength-scale pat
-
terns in the microelectronic industry. In this technique, UV light passes
through a transparent phase mask which modulates its phase. The
resulting interference of light can create a subwavelength exposure
pattern in a photoresist. More recently, deep UV and extreme UV lithog
-
raphy technologies have been developed to provide the deep sub
-
wavelength resolution required for the semiconductor chip industry.
Nevertheless, production using these processes has so far been limited
to wafer-scale manufacturing, and the cost of the equipment is too high
for applications that require very-large-area printing.
In recent years, a range of emerging printing technologies based
on soft lithography
79
,
80
, including nanoimprint lithography
81
,
82
and
transfer printing, have demonstrated their potential for enabling
large-area, high-throughput production of nanostructured materials
83
.
These breakthroughs have led to the demonstration of diverse metasur
-
face applications in solar energy
84
,
85
, photonic devices
86
and biosensor
technology
87
,
88
.
Despite the remarkable progress, substantial challenges persist,
particularly concerning the attainment of the necessary resolution,
accuracy and repeatability on a larger scale. In this section, we will
review these techniques and their associated limitations, and we will
discuss the essential steps required to scale them up effectively for full
production. Industry-specific examples will be provided to illustrate
their practical implementation.
Early applications of metasurfaces
89
have emerged in consumer
electronics
90
and automotive
91
, wherein semiconductor foundries,
originally designed for chip manufacturing, have been utilized for
implementing metasurfaces on full glass wafers. Notably, Metalenz
Inc. is an early company that leverages existing semiconductor manu
-
facturing technology to develop 3D sensing solutions for face recog
-
nition, as discussed in a following section. The company partnered
with United Microelectronics Corporation, one of the three major
semiconductor foundries, for the high-volume manufacturing of their
metasurfaces
92
.
Box 1 | Educational box on optical resonances in nanostructures
In his famous Bakerian Lecture
105
in 1857, Michael Faraday for the first
time presented systematic studies of the vibrant colours produced
by metallic nanoparticles, and these were explained mathematically
by Maxwell Garnett in 1904 (ref.
254
). Physically, these colours
arise from the excitation of collective electron oscillations known
as surface plasmons. Here, the electric field of light waves exerts a
force on the conduction electrons in a metal nanostructure, causing
a charge displacement. The redistribution of the negative charges
results in an exposure of positive charges from fixed ion cores. These
positive charges in turn produce a strong Coulombic restoring force
on the displaced electrons. Their subsequent oscillatory motion
can be modelled in analogy to a mass–spring system. On the basis
of this analogy, it can be understood why metal nanostructures
display optical resonances that facilitate an efficient conversion of
light energy to a collective electron motion. Because of the light
mass of the electron and the strong Coulombic restoring force (that
is, a stiff spring) in nanoscale systems, these resonances appear in
the visible spectrum
107
. By engineering the size and the shape of
these structures, we can engineer the motion of the light-driven
charges and alter the restoring forces and resonant frequencies
across the visible spectrum. This is shown in the scanning electron
microscopy (SEM) and darkfield white-light scattering images in the
figure, part
a
255
. At the resonant frequencies, the oscillating charges
can effectively concentrate, absorb and scatter optical fields. By
bringing nanoparticles together, one can further manipulate the
resonances and achieve even larger field concentration and stronger
light–matter interaction. This is shown in the SEM image and optical
simulation in the figure, part
b
256
. For a long time, it was thought that
effective subwavelength control over the flow, visible light would
have to involve the use of metallic nanostructures. However, in 2010,
it was shown that high-index semiconductor nanoparticles and
wires down to ~10 nm sizes can also display geometry-dependent
and size-dependent visible colours that are attributable to the
excitation of optical Mie-style resonances (see the figure, part
c
)
108
,
257
.
As with plasmonic nanostructures, the design and implementation
of high-index dielectric antennas in the visible and near-infrared
spectrum could greatly benefit from earlier pioneering works on
dielectric antenna structures and metamaterials in the microwave
regime
258
,
259
. High-index nanostructures bring a wealth of optical
resonances that can be harnessed to achieve new levels of control
over light absorption
260
, scattering
119
,
261
263
, and emission properties
of light
264
267
but without the losses that are intrinsic to manipulating
light with metals
109
. For high-index structures, the resonances can
be manipulated by bringing particles together in pairs or into large
arrangements to form metasurfaces. This is shown in the darkfield
white-light scattering and SEM images in the figure, part
d
268
.
These days, we can leverage very advanced computational
design tools to harness the resonances in nanostructures for the
design and optimization of practical metasurface technologies.
Having access to resonances in semiconductor nanostructures
further allowed for the realization of deep-subwavelength,
high-performance optoelectronic devices and opened up the use of
standard semiconductor manufacturing to scale up the technology.
Part
a
reprinted with permission from ref.
255
, Wiley. Part
b
from
ref.
256
. Reprinted with permission from AAAS. Part
c
reprinted
with permission from ref.
108
. Copyright 2010 American Chemical
Society. Part
d
reprinted with permission from ref.
268
. Copyright
2011 American Chemical Society.
200
0
30 nm
185 nm
a
b
c
d
300 nm
200 nm
|
E
|
2
|
E
o
|
2
5
μ
m
1
2
50
n
m
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Soft lithography
79
,
80
, a manufacturing technology refined over
the past three decades, provides an alternative, very-high-throughput
production process for metasurfaces through simple processes such
as moulding and stamping, using cost-effective organic materials
and reusable stamps. Leveraging soft-lithography principles, innova-
tive roll-to-roll (R2R)
81
,
82
,
93
and roll-to-plate
94
nanoimprinting tech
-
niques have been introduced for large-area fabrication on flexible
substrates. In these processes, a prefabricated mould with the inverse
of the desired pattern is mechanically pressed onto a resist-coated
substrate using a roller press. This method has facilitated the successful
creation of polymer patterns with sub-30-nm feature sizes
95
. Although
these techniques offer precise patterning capabilities, their integration
into a production line requires multiple independent manufacturing
steps to be implemented before and after the nanoimprint step to
realize practical applications.
To illustrate how multiple steps are arranged in practice, con
-
sider the mass-production of nanopatterned optical coatings using
R2R nanoimprint lithography. In a first step (mask origination),
a pre-originated hard stamp is produced by electron beam (e-beam)
lithography, which is replicated onto a soft stamp via an initial nano
-
imprint lithography process. To create the large-area mould for R2R
embossing, the soft stamp is used as a template in a step-and-repeat
nanoimprint lithography process, that is, recombination. The recom
-
bined stamp then needs to be further converted to an equally large,
hard stamp to endure the mechanically harsh R2R casting process. The
end product can be a nanoimprinted coating or plasmonic nanostruc
-
tures with a further R2R metallization step. The high-quality transfer of
metasurface nanostructures from the mould to the recombined hard
stamp is crucial to preserve the optical resonance properties of the
nanostructures in a finalized device.
We show the R2R nanoimprint fabrication steps in an industrial-
ized production line, from e-beam origination to a finished product
(Fig.
1
). In the final step, nanoimprinted surfaces can be used as vari
-
ous coatings such as optical coatings for couplers and anti-reflection
coatings. Nanoimprinted films can be also further metallized to create
structural colour filters, wherein colour is locally tuned by control
-
ling the critical dimensions of the pattern and metal thickness. Nota-
bly, the end product demonstrated in Fig.
1
is a security feature used
in banknotes (
KolourOptik Stripe
, presented with permission from
META), which consists of plasmonic colour pixel elements created by
metalizing the nanoimprinted polymer. Combining different sizes of
plasmonic nanostructures with microstructures, different functionali
-
ties can be embedded into the film, such as colour shift, motion and
three-dimensional depth.
Applications of optical metasurfaces utilizing nanoimprint lithog
-
raphy techniques has also emerged in consumer electronics, particu-
larly in 3D light field displays
96
. Leia Inc. has used nanostructured pillars
to create a diffractive light field backlighting, transforming regular
liquid crystal displays to lenticular 3D displays. Additionally, Moxtek
and NILT have demonstrated high-quality fabrication of meta-optics
in the visible and near-infrared region.
Despite these advancements, R2R and roll-to-plate nanoimprint
lithography still lack the necessary resolution and precision for some
optical applications. Plate-to-plate nanoimprint lithography offers
improved capabilities achieving finer resolution. For instance, Canon
recently announced the successful replication of chip manufacturing
processes using nanoimprint technology with an impressive 5 nm
spatial resolution
97
.
One limitation of the nanoimprint process is the continuous resid
-
ual layer
94
in the imprinted polymer resist, restricting its application
Conventional microlens
META’s KolourOptik
200 nm
200 nm
e
a
b
d
c
Electron-
beam
lithography
R2R casting
UV NIL
Roll-to-roll
metallization
Demetallization
Slitting and
trimming
quality control
Finished stripe
product
UV step-and-
repeat
nanoimprint
lithography
Silver seed layer
deposition
Electroforming
UV resin as
microlens
UV resin
Micro-embossed ink
Shallow microstructure
Focal
distance
20–50
μ
m
8–30
μ
m
3–5
μ
m
150
n
m
10–100
μ
m
1–2
μ
m
Plasmonic
nanopixels
Focal distance
PET spacer
4
μ
m
Bare
microstructure
Nanostructures conformed
to microstructure
Fig. 1 | Industrialized nanofabrication steps and examples of printed
optical metasurface structures using R2R nanoimprint.
a
, Roll-to-roll
(R2R) nanoimprint fabrication steps from e-beam origination to finished
product.
b
, META’s KolourOptik Stripe on a banknote, demonstrating 3D
depth.
c
, Comparison of traditional micro-lens (left) and KolourOptik (right)
technology.
d
, Nanostructures of different dimensions create colour pixels,
which are combined with microstructured elements to create 3D depth.
e
, Plasmonic nanostructures created on the contoured surfaces and interstitial
bases of dome-shaped microstructures. PET, polyethylene terephthalate;
UV, ultraviolet. Images are courtesy of META.
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to certain use cases wherein the patterned polymer serves as the final
product. Transfer printing
98
presents an alternative technique that
circumvents this limitation. It involves the transfer of nanostructured
materials from a donor substrate onto a target substrate. In this tech-
nique, the desired pattern is fabricated using a stamp by tuning the
adhesion between the donor and stamp substrates or via direct depo
-
sition onto the stamp. In the transfer process, an adhesive (sacrificial)
layer is applied to facilitate the transfer. This technique offers versatility
in material choice, whether metallic
99
or high-index dielectric
100
, and
allows for the integration of nanostructures into various applications,
including those in sensing, imaging and energy harvesting discussed
in this Review.
Another emerging soft-lithography technique for creating micro
-
structures and nanostructures is capillary force lithography
101
,
102
, which
is suitable for low-cost R2R processing. The method relies on capillary
action and utilizes a liquid precursor that evaporates after pattern for
-
mation. Similar to transfer printing, capillary force lithography draws
the liquid resist to form the pattern on the stamp. Post-processing steps
such as curing, etching or electroplating can be utilized to finalize the
patterned structures. Although certain limitations exist, such as mate
-
rials selection and dimension compatibility with capillary action, it is
compatible with R2R processing and industrial applications.
From these examples, it is clear that metasurfaces have already
made their way into consumer products. However, further advance
-
ments in scalable lithography techniques are essential to meet the
demand for high-volume applications, especially at shorter wave
-
lengths. Various soft-lithography techniques have demonstrated their
ability to facilitate cost-effective and large-scale production. Although
these techniques might fall short in terms of resolution and preci
-
sion for certain optical applications, the field is rapidly evolving with
improved material formulations and innovative fabrication processes.
We anticipate that these advancements in scalable manufacturing
techniques will have a pivotal role in making optical metasurface appli
-
cations widely accessible and are essential to realize their full potential
in high-volume applications.
Manipulation of structural colour, photodetection
and light emission processes
Metasurfaces provide a myriad of new opportunities to control light
emission, scattering and detection processes. This section discusses
some of the most elementary applications of such light control. We start
by describing how a large gamut of structural colours and appearances
can be created and then analyse how colour filtering or generation can
be utilized in new display and imaging functions.
Colours have a vital role in determining how we perceive and
analyse the world around us. Hence, they are used in a wide variety of
applications ranging from art, tagging and identification, chemical
analysis, sensing, displays for entertainment, and sources of informa
-
tion to security features and paints. Different colours naturally emerge
from optical scattering and interference effects when materials are
structured at length scales corresponding to the wavelength of visible
light (380 nm <
λ
< 760 nm). In creating new colours and appearances,
we can benefit from a long history of using nanostructures to produce
colourful pottery and glass artifacts
103
. Nature has also provided much
inspiration for the many ways we can structure materials to achieve
desired colours
104
. Fundamental research has shown that metallic
and semiconductor nanostructures display particularly efficient scat
-
tering and vivid colours as they support strong, highly engineerable
plasmonic
105
107
and Mie resonances
108
110
(Box
1
). Recently, there has
been a flurry of activity
111
,
112
on the application of nanostructures in
structural colours with the advances in nanotechnology that have
delivered a plurality of approaches to scale metasurface production
to square kilometres, as discussed in the previous section.
Metasurface-based structural colours bring several valuable
advantages over pigments and dyes and, thus, can open new applica-
tion areas. As this type of colour is derived from the geometry of struc
-
tures as opposed to their chemical or electronic structure, decorations
can be made to be more robust against environmental and chemical
degradation. Metasurfaces deliver additional benefits in the way that
they can be patterned with a high spatial definition and uniformity
over large areas. Current approaches allow for the creation of colour
pixels that are deep-subwavelength in size and, thus, make it possible
to print colours at an impressive 10
5
dots per inch (dpi), well above that
of commercial laser printers at a few thousand dpi
113
115
(Fig.
2a
,
b
). In
realizing high-definition metasurface colour scenes, it is of great value
that nano-sized metal and semiconductor nanostructures can harness
optical resonances to deliver very large absorption and scattering
cross section that can even exceed their physical size. This also means
that a single, ultrathin (<100 nm) nanostructured layer is sufficient to
produce vivid colours, bringing potential reductions in materials and
fabrication cost.
The use of nanostructures also allows for new levels of control over
the spectral scattering and emission properties of light. Recently, it was
shown that within a single semiconductor materials system, it is pos-
sible to produce a large gamut of highly saturated and high-brightness
colours
116
118
(Fig.
2c
) and that it is even possible to mix colours at the
subwavelength scale
65
. It is important to note that such capabilities
are distinct from the behaviour of gratings and photonic crystals that
are structured at the wavelength scale and display iridescence, which
means that the observed colours gradually change with the viewing
angle. Such optical behaviour is well known from many examples in
nature, such as opals, seashells, soap bubbles, butterfly wings and bird
feathers, and brings distinct applications. For example, the company
Morphotonix
has developed scalable ways to mould the surface of
chocolate to form a range of colourful, iridescent microstructures
(Fig.
2d
). Control over the angular
119
and polarization properties
120
of
scattering and emission
121
of light provides additional opportunities,
including the creation of 3D full-colour metaholograms
46
and ways to
manipulate the glossy or matte appearance of objects by controlling
the relative importance of specular and diffuse scattering
122
(Fig.
2e
).
Such control has opened up new ways to realize critical components
in augmented and virtual-reality headsets
123
,
124
and to compactify the
optics and empty space in such systems
125
128
.
Given the many advantages of metasurface-based structural col-
ours, it is exciting to speculate about new applications that might
come with recent demonstrations of dynamically tunable metasurface
properties
129
,
130
, structural colours
131
and complex light fields
132
,
133
.
With the advancement of large-volume manufacturing techniques,
the application of metasurfaces for structural colour is rapidly devel-
oping. One example is META’s KolourOptik Stripe discussed in Fig.
1b
,
wherein nanoscale and microscale structures can be realized over large
areas to display colourful features with movement and 3D stereoscopic
depth. These types of technologies can be applied in next-generation
security, authentication, steganography and data storage. Here, it is of
value to think of individual optical nanostructures as pixels that can
store multi-bit information depending on the size, shape and orienta
-
tion that can be read out and programmed optically
134
,
135
. Given the
plurality of degrees of freedom of light (frequency, phase, polarization,