Controlling the Colour of Metals: Intaglio and Bas-Relief Metamaterials

Abstract

The fabrication of indented ('intaglio') or raised ('bas-relief')
sub-wavelength metamaterial patterns on a metal surface provides a mechanism
for changing and controlling the colour of the metal without employing any form
of chemical surface modification, thin-film coating or diffraction effects. We
show that a broad range of colours can be achieved by varying the structural
parameters of metamaterial designs to tune absorption resonances. This novel
approach to the 'structural colouring' of pure metals offers great versatility
and scalability for both aesthetic (e.g. jewellery design) and functional (e.g.
sensors, optical modulators) applications. We focus here on visible colour but
the concept can equally be applied to the engineering of metallic spectral
response in other electromagnetic domains.

Full text

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arXiv:1011.1977v1 [physics.optics] 9 Nov 2010
Controlling the Colour of Metals: Intaglio and Bas-Relief Metamaterials
Jianfa Zhang1, Jun-Yu Ou1, Nikitas Papasimakis1, Yifang Chen2, Kevin F. MacDonald1,∗and Nikolay I. Zheludev1
1Optoelectronics Research Centre & Centre for Photonic Metamaterials,
University of Southampton, Highfield, Southampton, Hampshire, SO17 1BJ, UK
2Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0QX, UK
(Dated: November 10, 2010)
The fabrication of indented (‘intaglio’) or raised (‘bas-relief’) sub-wavelength metamaterial patterns
on a metal surface provides a mechanism for changing and controlling the colour of the metal without
employing any form of chemical surface modification, thin-film coating or diffraction effects. We show
that a broad range of colours can be achieved by varying the structural parameters of metamaterial
designs to tune absorption resonances. This novel approach to the ‘structural colouring’ of pure
metals offers great versatility and scalability for both aesthetic (e.g. jewellery design) and functional
(e.g. sensors, optical modulators) applications. We focus here on visible colour but the concept can
equally be applied to the engineering of metallic spectral response in other electromagnetic domains.
Human visual perception of surface colour is de-
termined by its properties of reflection, transmission,
and absorption at wavelengths between about 400 and
700 nm.Most pure metals are essentially colourless
because, with plasma frequencies in the ultraviolet do-
main, their valence electrons are able to absorb and re-
emit photons efficiently over the entire visible spectral
range. Gold and copper - obvious exceptions to this rule
- have lower plasma frequencies and consequently absorb
blue and blue/green light respectively, thereby achiev-
ing their characteristic yellow and red/orange colours1.
This limited palette of metallic colours is typically ex-
tended or replaced through the application of coatings
(such as paint and multilayer dielectrics) or controlled
chemical modification (e.g. oxide formation by anodiza-
tion). In the natural world, many plants and animals
display dramatic ‘structural colours’ derived from aston-
ishingly intricate three-dimensional assemblies of intrin-
sically colourless bio-materials2.
these colours is in many cases well understood, replicat-
ing them remains a significant challenge3–5and typically
requires complex (multi- or atomic) layer deposition and
etching fabrication procedures.
While the physics of
Here, we report on a form of structural colouring for
pure metals that relies only on the formation of arrays
of sub-wavelength elements inscribed into or raised above
the surface to a depth/height of the order 100 nm. These
intaglio and bas-relief patterns constitute a new fam-
ily of planar metamaterials: Conventional forms com-
prise discrete plasmonic ‘meta-molecules’ distributed on
a dielectric substrate or meta-molecule voids perforat-
ing a metallic thin film6. As such they present a dis-
continuous metallic structure to incident electromag-
netic radiation. In contrast, the meta-surfaces consid-
ered here (wherein a patterned ‘layer’ effectively sits on
a ‘substrate’ of the same metal, as shown in Fig. 1a)
present a continuous metallic profile to incident light.
These designs can be engineered to selectively provide
highly efficient resonant absorption, thereby dramatically
changing the metal’s reflection spectrum and perceived
colour (Fig. 1b). The colours produced can, by design,
be polarization-dependent or -independent and are rela-
a
y
z
x
<λ
b
Increasing etch depth
10 m
µ
1 m
µ
FIG. 1: Metallic structural colour. a, Artistic impression
of an intaglio metamaterial array of sub-wavelength single
ring meta-molecules inscribed into a metal surface. b, The
realization of this concept in gold: The words ‘NANO META’
seen under an optical microscope on the right are formed from
arrays of 170 nm diameter rings (as shown in the electron
microscope image, left) milled to a depth that increases in six
steps from 60 to 200 nm across the sample.†
tively insensitive to viewing angle.
A microspectrophotometer was employed to measure
normal incidence reflection spectra for a variety of gold
and aluminium metamaterial designs.
metamaterial patterns were fabricated by focused ion
beam milling on 250 nm thick gold films evaporated on
glass substrates. Fig. 2a shows spectra for square arrays
of 170 nm single rings milled to four different depths into
a gold film, alongside the reflection spectrum for the ad-
jacent unstructured gold surface. The red shift of ab-
sorption resonance with increasing etch depth is clearly
seen and the associated changes in the colour of gold
are strikingly illustrated by the inset optical microscope
image. Aluminium bas-relief structures were fabricated
at an interface between the metal and an optically pol-
ished fused silica substrate using electron beam lithogra-
phy and anisotropic reactive ion etching: Patterns were
Gold intaglio

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2
A
B
C
D
a
Reflection, %
20
60
100
Unstructured
gold
170 nm
A: 85 nm B: 120C: 180D: 205
b
x-pol.
y-polarization
1 m
µ
x
y
Reflection, %
Wavelength, nm
400600800 1000
20
60
100
Unstructured
aluminium
FIG. 2: Changing the colour of gold and aluminium.
a, Reflection spectra for an unstructured gold surface and
for the same surface patterned with 170 nm diameter rings
cut to depths ranging from 85 to 205 nm (as labelled). The
insets show electron microscope images of the intaglio meta-
material design (at oblique incidence and in plan view) and
optical microscope images of the different patterned domains.
b, Reflection spectra for an unstructured aluminium/silica in-
terface and for regions of the same sample patterned with a
raised (bas-relief) metallic pattern of asymmetric split rings
for incident polarizations parallel and perpendicular to the
split. The insets show an electron microscope image of the
split ring design etched in silica prior to aluminium deposi-
tion and optical microscope images of the patterned domain
for the two orthogonal incident light polarizations.†
etched into the silica to a nominal depth of 70 nm then
coated by evaporation with a 250 nm layer of aluminium.
Fig. 2b shows spectra for an anisotropic aluminium bas-
relief design of asymmetric split rings. Again the change
in colour from that of the unstructured metal is dramatic,
and in this case the effect is, by design, dependent on the
polarization of incident light.
The sub-wavelength periodicity of the metamateri-
als designs preclude diffraction effects and the colours
achieved by metallic surfaces structured in this way
are related to the plasmonic resonances of the meta-
molecules. Fig. 3 presents a numerical analysis of a
single-ring aluminium intaglio design: the unstructured
H @ z = 5 nm
z
b
E @ x = 0
y
y
z
x
y
0
4
8
12
8
0
-8
Al
Air
105
75
100 nm
Patterned Al
Reflection
Absorption
Unstructured Al reflection
Wavelength, nm
Reflection, Absorption, %
0
20
40
60
80
100
400500 600700
Reflected
colours
a
FIG. 3: Colour by design. a, Numerically simulated re-
flection and absorption spectra for an aluminium surface pat-
terned with a square array 100 nm deep intaglio metamaterial
design of 210 nm single rings alongside the reflection spectrum
for unstructured aluminium and inset blocks showing the per-
ceived colours of these surfaces. b, Maps of surface-normal
magnetic field intensity in a z-plane 5 nm above the top sur-
face of the metal (left) and electric field intensity parallel to
the metal surface in an x-plane that diametrically bisects a
ring (right). The structure is illuminated at normal incidence
with y-polarized plane waves.†
metal uniformly reflects more than 80% of light across the
range from 400 to 800 nm and is consequently seen to be
light grey in colour (Fig. 3a). An array of 210 nm rings
milled to a depth of 100 nm below the surface endows
the metal with a strong absorption resonance centred in
the green part of the spectrum at 510 nm. The associ-
ated substantial modification of its reflection spectrum
gives the aluminium a vivid magenta colour. The elec-
tromagnetic field maps shown in Fig. 3b illustrate how
this absorption is linked, in this particular case, to a cir-
culating slot mode of the ring structure7with azimuthal
mode number m=1.
By adjusting meta-molecule geometries one can
achieve a wide palette of colours. Fig. 4 illustrates how
variations in the simple single ring intaglio design on alu-
minium and gold can provide access to a significant pro-
portion of colour space. The parameter space for meta-

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r1
r2
d
(120,90,120)
(120,90,100)
(120,90,150)
(105,75,100)
(120,80,280)
(90,60,60)
(100,0,100)
Al
Au
CIE1931 colour space
(120,60,80)
(r , r , d)
1
2
0.0
0.2
0.4
x
0.6
0.0
0.2
0.4
0.6
0.8
y
FIG. 4: Accessible metallic colours. a, CIE1931 chro-
maticity diagram overlaid with points corresponding to the
simulated reflected colours of single ring intaglio metamaterial
designs in aluminium (circular symbols) and gold (triangles),
labelled according to their structural parameters (external ra-
dius r1, internal radius r2, depth d, array period = 300 nm
in all cases). Points for the unstructured metals are indicated
by open symbols.†
molecule design and distribution is obviously almost un-
limited and different geometries will undoubtedly extend
the accessible colour range. That said, the ‘pure’ colours
found at the boundaries of the CIE1931 standard colour
space presented in Fig. 4 are likely to present a significant
challenge (they would require suppression of reflectivity
across all but a narrow band of the visible range).
The intaglio/bas-relief metamaterial approach pro-
vides a mechanism for dramatically changing and
controlling the colour, or more broadly the spectral
response, of metal surfaces whilst retaining other
properties (e.g. smoothness, hardness, lustre, electrical
conductivity) that would be lost in the use of materially
different coatings. In being composed of ‘pattern’ and
‘substrate’ layers of the same medium, such structures
offer considerable advantages in ease of fabrication
and application to bulk (as opposed to thin-film)
media and/or non-planar surface profiles.
be produced with minimal adaptations to standard
metal-forming process (e.g.
and applied to anything from an item of jewellery to a
piece of automotive bodywork. At the same time, such
structures may provide optical properties that are ex-
tremely difficult to imitate, thereby facilitating security
applications (e.g.banknote anti-forgery features) and
providing high-value exclusivity in aesthetic applications.
They may
pressing, rolling, casting)
Acknowledgements:
the UK’s Engineering and Physical Sciences Research
Council (all authors), The Royal Society (NIZ) and the
China Scholarship Council (JZ).
This work was supported by
†Colour presentation: The balance of optical microscope
images is referenced to a test pattern of 24 RGB-specified
sample colours (ColourChecker Mini by X-Rite). Per-
ceived colours are derived from experimentally measured
and numerically simulated reflection spectra using Judd-
Vos-modified CIE 2-deg colour matching functions8–10
assuming an illuminating light source with the spectral
radiant power distribution of a 6500 K black body. Read-
ers should be aware that their monitor and/or printer set-
tings may affect colour rendering (Fig. 5 shows reference
colours with specified RGB values for comparison).
R,G,B:
0, 0, 2550, 255, 0 255, 0, 0 0, 0, 0255, 255, 0
FIG. 5: Reference colours.
∗kfm@orc.soton.ac.uk; www.nanophotonics.org.uk
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