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Wet-Chemical Etching: a Novel Nanofabrication Route to Prepare
Broadband Random Plasmonic Metasurfaces
Piragash Kumar R. M.
1
&Venkatesh A.
1
&Moorthy V. H. S.
1
Received: 15 April 2018 /Accepted: 11 July 2018
#Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Broadband optical metasurfaces are gaining enormous attention owing to their potential applications in optoelectronic devices,
sensors, and flat optics. Here, we demonstrate for the first time a single-step, novel wet-chemical etching-based nanofabrication
method to produce broadband random plasmonic metasurfaces (RPMS). The nanofabrication method is inexpensive, simple,
versatile, and compatible with semiconductor processing technologies. The RPMS is made of a single-layer optically thick Ag
thin film nanostructured with random nanoholes and nanocavities. The building block of the RPMS is a multi-resonant meta-cell
composed of disordered nanoholes with variety of sizes, shapes, and aspect ratios. The composition of the multi-resonant meta-
cell can be modified by varying the duration of immersion (DoI) of the Ag thin films in the etchant solution. The RPMS exhibits
broadband extraordinary transmission in the 550–800 nm wavelength range with an efficiency of transmission of 2.3. Broadband
absorption of light is observed in the entire visible region; incident light is strongly absorbed (~70%) in the nanocavities via
localized surface plasmons (LSPs) in the 400–550 nm wavelength range. Further, 40–50% of the light is absorbed in the metal
film via surface plasmon polaritons (SPPs) excited by the multi-resonant meta-cells, elsewhere on the spectrum. The RPMS
exhibits Lambertian type scattering with nearly 50% efficiency in the entire visible wavelength range. The RPMS with these
broadband optical properties can find useful applications in plasmonic solar cells, surface-enhanced Raman spectroscopy
(SERS), thermoplasmonic devices, and plasmoelectric potentials based all-metal optoelectronic devices.
Keywords Wet-chemical etching .Random plasmonic metasurfaces .Optical metasurfaces .Broadband absorption .Scattering
and plasmonic solar cells
Introduction
Broadband optical metasurfaces have garnered interests ow-
ing to their broadband optical properties that find potential
applications in flat optics [1,2] and optoelectronic devices
such as solar cells [3,4] and sensors [5]. An optical
metasurface is the two-dimensional counterpart of metamate-
rials that engineer the properties of light such as amplitude,
phase, and polarization [6,7]. It is constructed by
nanostructuring a metal thin film as proposed by J. B.
Pendryetal.in2005[8] and is also called a plasmonic
metasurface. Plasmonic metasurfaces are typically composed
of periodic nanostructures such as nanoholes, V-shaped anten-
nas, and gap plasmon resonators [9].Thesemetasurfacesma-
nipulate a narrow-band of the incident light due to the period-
icity and homogeneity of their constituent nanostructures. For
example, an array of periodic identical nanoholes in a plas-
monic metal thin film selectively transmits the incident light
with a narrow wavelength dispersion [10]. The artificially
engineered plasmonic metasurfaces have enabled the realiza-
tion of a new class of flat optical devices such as metalens
(imaging below the diffraction limit), spatial light modulators
(SLM), and holograms [11]. The narrow-band optical
metasurfaces have also found useful applications in wavefront
shaping [12], optical chirality [13], plasmonic coloring [14],
and surface-enhanced spectroscopy-based sensors [15].
Despite its wide range of applications, these metasurfaces
fail to deliver the broadband optical response demanded by
plasmonic solar cells [3], universal plasmonic sensors [5], and
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s11468-018-0813-4) contains supplementary
material, which is available to authorized users.
*Moorthy V. H. S.
vhs.moorthy@manipal.edu
1
Research Laboratory for Plasmonics, Department of Electronics and
Communication, Manipal Institute of Technology, Manipal
Academy of Higher Education, Manipal 576 104, India
Plasmonics
https://doi.org/10.1007/s11468-018-0813-4
the nascent thermoplasmonic [16] and all-metal optoelectron-
ic devices [17]. There are a few simulation studies which
brings forth importance of disorder nanostructuring of the ma-
terial which produce broadband far field optical properties
[18]. A broadband optical metasurface can trap and fold light
of the entire visible spectrum within thin active layers of the
plasmonic solar cells. Such broadband light trapping has the
potential to enhance the efficiency by manifolds and reduce
the cost/watt of the solar cells [19]. There are attempts, based
on transformation optics [20,21], to design non-reflecting
surfaces from inhomogeneous and anisotropic materials and
also to fabricate perfect matched layers through anisotropic
loss media. In these techniques, the electromagnetic properties
of metasurfaces depend on subwavelength structural details
but not on chemical composition. Further, the material can
be constructed whose permittivity and permeability values
may be designed to vary independently and arbitrarily
throughout the material. The main idea of nanostructuring
the film is to render the impedance offered by the 2D medium
minimal to the free space light coupling. Moreover, the nano-
structured surfaces can also produce multiple electric-field
hotspots at different wavelengths of incident light, enabling
the development of a universal plasmonic sensor to detect
multiple analytes on a single platform [5]. Broadband absorp-
tion of light in a metasurface also finds useful applications in
perfect absorbers, SERS, and enhanced fluorescence.
The bandwidth of a narrow-band optical metasurface is typ-
ically increased by either multiplexing identical nanostructures
in at least more than one periodicity [22]orbydesigninga
metasurface with multi-resonant meta-cells [23]; they are com-
posed of various nanostructures designed to resonate at multiple
wavelengths. It requires sophisticated nanofabrication tech-
niques such as electron beam or focused-ion beam lithography
to fabricate a metasurface with precisely tuned geometry of the
nanostructures [22,23]. Disordered nanostructures on a plas-
monic metasurface have recently exhibited fairly broad optical
response than its engineered periodic counterparts [24,25]. The
essential physics of free space coupling of light with random
nanostructured thin films is explained by Kevin Vynck et al.
Random nanostructuring—holeing going through the entire
film thickness—of 2D thin films enables index guiding of light,
each hole acting as a scattering center. Thus facilitating light
confinement in 2D. However, these EM modes are leaky and
can be manipulated by the free space coupling phenomenon
into the 2D structure through the third dimension, i.e., perpen-
dicular to the plane of the film [18]. Examples of disordered
plasmonic metasurfaces are disordered nanoholes in silver thin
films [26], disordered silver nanoparticles on silver thin films
[27], and disordered silver nanocubes on a metal thin film sep-
arated by spacer [28]. Owing to the broad effective periodicity
of the disordered nanostructures and their interactions, the dis-
ordered metasurfaces exhibit broad optical response. The ad-
vantage of the disordered metasurfaces, besides their broadband
optical properties, is that they can be fabricated with inexpen-
sive and non-lithographic techniques. The optical response of
the disordered metasurfaces, albeit broader than the periodic
metasurfaces, is still lesser than the requirement to cover the
entire visible spectrum. This is because the nanostructures were
only disordered in their arrangements but identical in their mor-
phology—in each case, the nanoholes, nanoparticles, and
nanocubes were of same size, shape, and aspect ratio.
In addition to disorder, nanostructures with different fea-
tures can be multiplexed into multi-resonant meta-cells to pro-
duce the required broadband optical metasurface. To the best
of our knowledge, experimental efforts to realize random
nanostructure meta-surfaces with disorder in size, shape, and
periodicity are not explored. In this article, for the first time,
we demonstrate a universal nanofabrication technique [29]—
novel wet-chemical etching—to prepare random plasmonic
metasurfaces (RPMS). The RPMS is composed of disordered
multi-resonant meta-cells in optically thick Ag thin films. The
multi-resonant meta-cells comprise disordered nanoholes with
random size, shape, and aspect ratio. Such an optical
metasurface exhibits broadband optical response in the entire
visible spectrum with extraordinary optical transmission, en-
hanced absorption, and efficient scattering of light. The
nanofabrication technique is inexpensive, lithography free,
simple, and versatile. It is an intrinsically large area technique
that is compatible with the semiconductor processing
technologies.
Materials and Methods
Fabrication of RPMS Optically thick Ag thin films—150 nm
thickness—were prepared on freshly cleaned glass coverslips
(Corning Inc.) using a DC magnetron sputtering (12′′ MSPT,
HHV Pvt. Ltd., India). The surface of the Ag thin film was
nanostructured when immersed into an etchant solution
contained in a petri-dish. After a given duration of immersion,
the Ag thin film was removed from the etchant bath and the
excess solution sticking onto the surface was gently blow
dried using N
2
gas. The Ag thin films were nanostructured
with disordered nanoholes and nanocavities to form random
plasmonic metasurfaces (RPMS). It is a single-step process of
nanofabrication. The duration of immersion was varied from
544 to 1900 s to prepare RPMS with different surface mor-
phologies. All the experiments were performed in normal lab-
oratory conditions.
The etchant solution is a mixture of H
2
O
2
(50%, Merck),
NH
4
(28–30%, Merck), and deionized water (ELGA) in
1:1.5:100 ratio by volume, respectively. A similar combina-
tion of chemicals is used in the MEMS industry as an etchant
but at higher concentrations [30]. The etchant solution is novel
in its proportion and concentration of chemicals used to fab-
ricate RPMS. It is the first instance where it has been used to
Plasmonics
nanostructure the Ag thin films to prepare optical
metasurfaces.
Surface Morphology of RPMS A Zeiss EVO-18 Special
Edition SEM and a Bruker INNOVA AFM was used for sur-
face morphology characterization. The AFM was used in con-
tact mode. All the SEM micrographs were acquired in 1 μm
scale from multiple spots on a sample. The gray scale SEM
images were processed using ImageJ software to extract and
analyze the morphology of nanoholes and nanocavities in
RPMS. The radial distribution function (RDF) plugin in
ImageJ was used to analyze the nearest neighbor distance
between the disordered nanoholes.
Optical Microscopy Imaging A Nikon Eclipse LV-100 micro-
scope equipped with Clemex color CCD camera was operated
in transmission and reflection mode to image the nanoholes in
RPMS using a 100x objective. A 100x dark-field (DF) objec-
tive was used to collect DF scattered-light images of
nanoholes and nanocavities in RPMS. All the images were
taken with 20 μm scale under white light illumination. A
halogen lamp (50 W) was used as a source of illumination.
Optical Spectroscopy Measurements A Perkin Elmer Lambda
750 UV–Vis–NIR spectrophotometer was used in integrating
sphere (LabSphere) geometry for the optical transmission and
total reflection measurements. The transmission and reflection
spectra of RPMS were acquired with air and standard BaSO
4
white plate as reference, respectively, in 400–800 nm wave-
length range. The RPMS were illuminated on the air-metal
interface for all the measurements. The absorbance of RPMS
was calculated from the measured transmittance and reflec-
tance using the formula, A=1 −T−R. Dark-field scattered
light spectra of RPMS were acquired with reference to stan-
dard BaSO
4
white plate using the Nikon microscope coupled
spectrometer (MAYA 2000 PRO, Ocean Optics). The
scattered light from RPMS was coupled into an optical fiber
with 200 μm core diameter and directed into the spectrometer.
Results
An optically thick Ag thin film was immersed into the etchant
solution to nanostructure its surface. The duration of immer-
sion (DoI) of the Ag thin films in the etchant solution—the
independent variable of the study—was varied to study its
effect on the nanostructuring of Ag thin films. Figure 1shows
the SEM micrographs of nanostructured Ag thin films for
various durations of immersion: 544 s (Fig. 1a), 990 s (Fig.
1b), and 1900 s (Fig. 1c). Figure 1d shows an AFM image of
the surface of the nanostructured Ag thin film and a corre-
sponding depth profile from the AFM image. The AFM image
and the depth profile revealed that the Ag thin films were
nanostructured with random nanoholes and nanocavities. A
nanohole is an indentation on the surface whose depth is equal
to the thickness of Ag thin film, i.e., 150 nm—a feature with
~150 nm depth was observed in Fig. 1e. On the other hand, a
nanocavity is an indentation with its depth shorter than the
thickness of the thin film, i.e., less than 150 nm—afeature
with 80 nm depth was observed in Fig. 1e. Figure 1a–c
showed the increase in density of nanostructures—number
of nanostructures per unit area—on the Ag thin films with
Fig. 1 Surface morphology of
RPMS showing the
nanostructures on Ag thin films at
various durations of immersion.
Panels a–cshow the SEM
micrographs of RPMS at 544 s
(a), 990 s (b), and 1900s (c)
duration of immersion. Panels d,e
are the AFM micrograph and its
corresponding depth profile along
the line shown in (d),
respectively, of a typical RPMS
Plasmonics
the duration of immersion. Figure 1conclusively proved that
the optically thick Ag thin films were nanostructured with
random nanoholes and nanocavities; the randomly nanostruc-
tured Ag thin films were termed as random plasmonic
metasurfaces (RPMS).
ImageJ—open source image editing software—was used
to analyze the structural characteristics of RPMS such as
nanoholes area coverage (NAC), size, shape, aspect ratio,
and the spatial distribution of nanoholes and their dependence
on DoI. SEM micrographs from different spots on the samples
were used as sources for the ImageJ analysis.
The NAC increased with DoI; from 1.9% at 544 s DoI, it
increased to 2.7% at 990 s DoI and to 4.7% at 1900 s DoI. The
number of nanoholes per unit area, hereafter referred as den-
sity of nanoholes, increased slowly with DoI than NAC; 29%
increase was observed as the DoI increased from 544 to 990 s
and 59% for increase in DoI from 990 to 1900 s. The RPMS
was fabricated with nanoholes with a distribution of size,
shape and aspect ratio (Fig. 1). Majority of the nanoholes in
RPMS were smaller in area (equivalent to circular nanoholes
with a diameter range of 100–150 nm) at any duration of
immersion. Area of nanohole was chosen as an appropriate
measure of the size of nanoholes. Increase in DoI raised the
number of nanoholes with larger area (equivalent to circular
nanoholes with a diameter range of 200–300 nm); 48% in-
crease was observed from 544 to 990 s DoI and 81% for
increase in DoI from 990 to 1900 s. Figure 2a is a collage of
several magnified SEM micrographs that portrayed different
sizes, shapes, aspect ratios, and patterns of nanoholes in
RPMS for various durations of immersion. Well-defined
shapes such as circle, ellipse, and rectangle—with aspect ratio
of one and two—were observed at 544 s DoI. The nanoholes
grew into irregular shapes as the DoI was increased. The as-
pect ratios of larger nanoholes were two, three, and four at
longer durations of immersion, whereas the smaller nanoholes
had aspect ratio of one and two at any duration of immersion.
Different nanohole pair formations such as dimers and trimers
were observed in RPMS at all durations of immersions.
Peculiar nanoholes cluster formations were also observed
(Fig. 2a).
Radial distribution function (RDF) of random nanoholes in
RPMS is shown in Fig. 2b for various durations of immersion.
A peak in RDF spectrum is related to a characteristic distance
of separation between neighboring nanoholes. It is called an
effective periodicity and characterizes a short-range ordered
(SRO) nanoholes, i.e., the nanoholes have no long-range or-
dered arrangements [31]. The multiple peaks observed in Fig.
2b represented multiple effective periodicities of nanoholes in
RPMS. Increasing the DoI raised the density of nanoholes and
as a consequence, reduced the range of effective periodicities
from 200 to 1000 nm at 544 s DoI to 100–700 nm at 990 s DoI
and to 100–500 nm at 1900 s DoI. Oscillating features ob-
served at longerdistances implied some ordering of nanoholes
even at those longer distances [31].
The RPMS was composed of clusters of nanoholes at vary-
ing length scales as shown in Fig. 3. All the clusters were
made of nanoholes with different sizes, shapes, aspect ratios,
and arrangements. These clusters were termed as multi-
resonant meta-cells because they would resonate with incident
light at multiple wavelengths due to the variety of nanoholes
that constituted them. The length scale of these meta-cells
varied from 485 to 1750 nm along the long axis (Fig. 3)and
from 260 to 1510 nm along the short axis of the meta-cells at
1900 s DoI; the length scale covers the entire visible region of
the electromagnetic spectrum.
In summary, RPMS were fabricated by etching the Ag thin
films. The degree of nanostructuring was modified by varying
the DoI. Increasing the DoI increased the NAC, density of
Fig. 2 Structural characteristics of RPMS at different durations of
immersion. aMagnified SEM micrographs portraying individual
nanoholes with various sizes, shapes, and aspect ratios and nanohole
pair formations. Duration of immersion is mentioned on the left side of
the figure. bRadial distribution function (RDF) of random nanoholes in
RPMS
Plasmonics
nanoholes, size, and aspect ratio of nanoholes and reduced the
range of effective periodicities. Multi-resonant meta-cells with
length scales in the optical wavelength range were formed at
longer durations of immersion.
The random plasmonic metasurfaces (RPMS) were studied
for their optical characteristics such as transmission, reflec-
tion, absorption, and scattering of light. The opaque Ag thin
films exhibited colorful transmission of light—under white
light illumination—when nanostructured with random
nanoholes, as shown in Fig. 4a–c. The incident white light
was selectively transmitted through the RPMS by the multi-
resonant meta-cells (shown in Fig. 3). Colors of the transmit-
ted light covered the entire visible spectrum, i.e., from blue to
red. Majority of the transmitted light occupied the blue-green
region of the optical frequencies at 544 s DoI (Fig. 4a). Color
of the transmitted light red shifted from blue-green region to
green-red region when the DoI was increased from 544 to
900 s (Fig. 4b) and 1900 s (Fig. 4c). The number of colored
dots in the transmission micrographs increased with DoI; this
was due to the increase in density of nanoholes with DoI.
Figure 4d–f shows the reflection micrographs of the RPMS
for various durations of immersion. The reflection of light from
the RPMS diminished with increasing DoI. Increased NAC and
density of nanoholes rendered a black tone to the surface of
RPMS at 1900 s DoI (Fig. 4f). True color dark-field (DF) scat-
tering micrographs, shown in Fig. 4g–i, revealed the scattering
of light by nanostructures on RPMS. The nanoholes—respon-
sible for colored dots in transmission micrographs—and
nanocavities effectively scattered light. Color of the scattered
light red shifted from blue-green range at 544 s DoI to green-red
range at 1900 s DoI as the density of nanostructures increased
with DoI. The increased density of colored dots in DF scattering
micrographs, at a given DoI, was related to the presence of
nanocavities on RPMS (as observed in Fig. 1d); the comparison
was made with the transmission micrographs acquired at the
same spot on the sample.
The RPMS displayed a broadband optical transmission in
the visible region of the electromagnetic spectrum, as shown
in Fig. 5. The RPMS exhibited an optical transmission in the
400–550 and 650–800 nm wavelength ranges at 544 s DoI
with a transmission minima in the 550–650 nm wavelength
range; the flat, i.e., non-nanostructured, Ag thin film was
opaque in the 400–800 nm wavelength range. The un-etched
regions between multi-resonant meta-cells on RPMS ap-
peared black in the optical transmission micrographs (Fig.
4a–c) owing to the opaqueness of the Ag thin films in the
visible region. The intensity of transmission maxima was
low—0.02%—at 544 s DoI due to less NAC and density of
nanoholes. However, vivid colors corresponding to the trans-
mission maxima wavelengths were observed in the transmis-
sion micrographs (Fig. 4a). The intensity of transmission max-
ima increased manifolds with DoI; from the barely measurable
0.02% at 544 s DoI, it increased by 48-folds to 0.96% at 990 s
DoI. The intensity of transmission further increased by 3.3-
folds, from 0.96 to 3.2% at 1900 s DoI. The RPMS at 990 and
1900 s DoI exhibited extraordinary transmission of light with
1.4 and 2.3 efficiency of transmission (see S. I). The transmis-
sion minima blue shifted from 550 to 650 nm at 544 s DoI to
400–500 nm at 990 s DoI and 400–530 nm at 1900 s DoI. On
the other hand, the broadband transmission maxima red
shifted from 400 to 500 nm wavelength range at 544 s DoI
to 530–800 nm at 990 s DoI and 550–800 nm wavelength
range at 1900 s DoI.
The measured total reflection spectra of RPMS for var-
iousDoIareshowninFig.6a. The reflection spectrum of
RPMSat544sDoIwassimilartothatoftheflatAgthin
film. Intensity of the reflected light decreased considerably
at 990 s DoI; from 90% at 800 nm wavelength, it reduced
gradually to 65% at 400 nm. On the other hand, RPMS at
1900 s DoI displayed a steep reduction in the intensity of
reflected light when compared to RPMS at 544 s; from
only 60% at 800 nm wavelength, it reduced linearly to
30% at 400 nm with a small broad dip in the 400–
Fig. 3 Magnified SEM micrographs portraying the formation of multi-
resonant meta-cells at different length scales. The scale bar is 100 nm
Plasmonics
500 nm wavelength range. The absorption spectra shown
in Fig. 6b were calculated from the measured transmission
and reflection spectra using the relation, A=1−T−R.The
absorption spectra exhibited characteristics that were
Fig. 4 Optical micrographs of RPMS for various durations of immersion.
a–cTransmission micrographs exhibiting extraordinary transmission of
light through RPMS for various durations of immersion: 544 s (a), 990 s
(b), and 1900s (c). Panels d–f,g–ishow the reflection and dark-field
scattering micrographs, respectively, in the same order of duration of
immersion as shown in (a–c)
Fig. 5 Transmission spectra of RPMS for various durations of immersion: 544 s (red line; inset), 990 s (blue line), and 1900s (black line). The colored
arrows are guide to axes of respective colored spectrum
Plasmonics
similar to the inverted reflection spectra; intensity of ab-
sorption increased with DoI. The absorption of light in
RPMS increased from longer wavelengths to shorter wave-
lengths; from 35% absorption at 800 nm, it increased lin-
early to nearly 70% at 400 nm for RPMS fabricated at
1900 s DoI. A small broad hump in the 400–500 nm wave-
length range—similartothesmallbroaddipinreflection
spectrum—was observed in the absorption spectrum of
RPMS at 1900 s. Figure 7shows the dark-field (DF) scat-
tering spectrum of RPMS fabricated at 1900 s DoI. The
RPMS exhibited a Lambertian type scattering of light in
the entire visible region. The scattering efficiency reached
nearly 50% in the entire visible wavelength range when
normalized to the area of nanostructures.
Discussion
The multi-resonant meta-cells, shown in Fig. 3, provide the nec-
essary impedance matching conditions for the free-space propa-
gating photons to interact with the RPMS; the interaction excites
surface plasmons on the nanostructured Ag thin films. Each
multi-resonant meta-cell resonates at multiple wavelengths and
together with their wide length scales that range from 260 to
1750 nm, it facilitates the RPMS to interact with broad range
of photons that span the entire visible wavelength region.
The metasurface, at 1900 s DoI, exhibits broadband ex-
traordinary transmission of light through the multi-resonant
meta-cells in the 550–800 nm wavelength range with an effi-
ciency of 2.3, i.e., more than twice the light is transmitted than
Fig. 6 aReflection and babsorption spectra ofRPMSfor various durations of immersion: 544 s (red line), 990 s (blue line), and 1900s (black line). The
arrows are guide to the axes of the respective spectrum
Fig. 7 Dark-field scattering
spectra of RPMS at 1900 s
duration of immersion
Plasmonics
that is incident on the nanoholes. The extraordinary transmis-
sion of light is a condition that occurs when the efficiency of
transmission is greater than unity [32]. The efficiency of trans-
mission is defined as the ratio of the amount of light transmit-
ted through nanoholes in RPMS to the amount of light trans-
mitted through a macroscopic hole of area same as that occu-
pied by nanoholes in RPMS [32]. The steep raise in intensity
of transmission with DoI is related to the aspect ratio of
nanoholes and the number of multi-resonant meta-cells with
high aspect ratio nanoholes. We attribute the manifold in-
crease in the intensity of transmission to the aspect ratio of
nanoholes for two reasons: (i) nanoholes with higher aspect
ratio (and same area) increase the intensity of transmission
maximum by manifolds [33] and (ii) the raise in NAC and
density of nanoholes with DoI is feeble when compared to the
astronomical raise in the intensity of transmission. Therefore,
the intensity of transmission affects the efficiency of optical
transmission in RPMS.
Besides increasing the intensity, the nanoholes with
high aspect ratio—greater than one—red shift the opti-
cal transmission of RPMS [33]. Increase in DoI in-
creases the aspect ratio of nanoholes and the number
of nanoholes with large aspect ratios. As a result, it
red shifts the transmission maxima; from 400 to
500 nm at 544 s DoI, the transmission maxima is red
shifted to 500–800 nm at 990 s DoI and 550–800 nm at
1900 s DoI. On the other hand, the transmission minima
is blue shifted with increase in DoI. In the periodic
nanohole arrays, the periodicity affects the transmission
minimum, i.e., the transmission of photons with wave-
lengths corresponding to the periodicity of the nanohole
array is cut-off [10]. Similar phenomenon has been ob-
served in the short-range ordered (SRO) nanoholes
where the transmission minimum is related to its effec-
tive periodicity [31]. We too find that the broadband
transmission minima (observed in Fig. 5) occur at wave-
lengths corresponding to the multiple effective periodic-
ities shown in the RDF spectra (Fig. 2b). The transmis-
sion minimum of RPMS at 544, 990, and 1900 s DoI
are positioned at 580, 440, and 465 nm, respectively,
with a bandwidth of 120–150 nm. The corresponding
effective periodicities are observed in the 200–800 nm
range and reduced with DoI.
The reflection of light from a metasurface is governed
by the generalized law of reflection [12]: sinθ
r
–
sinθ
i
=(Δφ/n
i
k
o
), where θ
i
and θ
r
are the angle of inci-
dence and reflection, respectively. n
i
is the refractive in-
dex of the light incident medium and k
o
is the wave vector
of the incident photon. Δφ is the gradient in phase dis-
continuity and relates to the measure of phase changes
experienced by SPPs while traveling across a meta-cell.
When Δφ =0—no phase change is experienced by
SPPs—the generalized law of reflection reduces to
Snell’s law of reflection; Δφ characterizes the reflection
of light from a metasurface. In RPMS, the multi-resonant
meta-cells affect the reflected light by imparting random
phase changes to the SPPs traveling across them.
Therefore, the reduction in reflected light intensity with
increasingDoIcouldbeattributedtotheformationof
increased number of multi-resonant meta-cells.
We observed the number of nanocavities to increase
with DoI. The nanocavities are visualized in the SEM mi-
crographs as pale gray features alongside dark gray fea-
tures that represent the nanoholes. Unlike nanoholes that
are responsible for optical transmission, the nanocavities
strongly absorb light via localized surface plasmons
(LSPs) [34]. The resonance frequency of a LSP depends
on the effective cavity length of the nanocavity [35]. The
size of a nanocavity is always smaller than the smallest
nanohole fabricated using this method of nanofabrication;
the etching process starts with a nanocavity and ends with
a nanohole over a period of time. The strong broadband
absorption (70%) high energy photons in the 400–550 nm
wavelength range is due to the rich varieties of
nanocavities on the RPMS. As a result, the reflection of
light is lowest in this wavelength range. SPPs are excited
on the RPMS in the 550–800 nm wavelength range at
1900 s DoI. Three percent of these SPPs propagate out
through the nanoholes as transmitted light (Fig. 5)and
50–60% are out-coupled in to the surrounding dielectric
medium and propagate as reflected light. Then, 40–50%
of the SPPs, that travel in-plane, are absorbed in the metal
film. Kevin Vynck et al. have performed the 3D FDTD
simulations of a random pattern of holes in a thin absorb-
ing film using periodic boundary conditions, which indi-
cates the large broadband absorption enhancement [18].
This result is sought to be corroborated [36]withincrease
of energy density distribution in the nanostructured thin
film as compared to free space and thus higher level of
light trapping. In that work, understanding of light cou-
pling process is attempted by calculating so-called spectral
function—which is the total number of optical modes (den-
sity of states per unit volume) that can be excited in a given
frequency range and wave vectors, by taking into consid-
eration of the underlying disorderness.
On the other hand, RPMS at 1900 s DoI exhibited
broadband scattering of light (Fig. 7)withnearly50%
scattering efficiency. The scattering has two contributions:
from the LSPR of the individual nanoholes and
nanocavities and from the SPPs—excited by the multi-
resonant meta-cells—that propagate along the interface
on the metal film [27]. The Lambertian type scattering
from RPMS can potentially increase the absorption of light
in the surrounding semiconductor absorber medium by (i)
effectively increasing the optical thickness of the medium
by efficient scattering of light and (ii) out-coupling of SPPs
Plasmonics
into various optical modes supported by the absorber layer
[19,27]. Optimization of the structural parameters in the
RPMS paves the way to increase the aforementioned
Lambertian scattering efficiency. Fabricated RPMS sam-
ples consist, apart from nano cavities, nano holes of differ-
ent size, shape, and having different effective periodicities.
Thus, there is a requirement of performing electromagnetic
simulation studies on RPMS system to elucidate the far
field properties. However, these simulation studies are be-
yond the scope of this paper.
Conclusions
In conclusion, we have demonstrated a single-step universal
nanofabrication method to fabricate broadband random plas-
monic metasurfaces (RPMS). An optically thick Ag thin film
was nanostructured with multi-resonant meta-cells to produce
the metasurface. The method of nanofabrication is inexpen-
sive, simple, and versatile. Ag thin films, ranging from ultra-
thin to optically thick, can be nanostructured to prepare RPMS
for variety of applications. The intrinsically large area fabrica-
tion and parallel nature of processing of the technique together
with its compatibility with semiconductor processing technol-
ogies would facilitate mass manufacturing of RPMS for com-
mercial devices. The RPMS with Lambertian type scattering
of light can be used as a potential back contact in plasmonic
solar cells to enhance the absorption of light in the adjacent
active layers. The strong absorption of light in RPMS can be
harnessed for a host of applications that include surface-
enhanced spectroscopies such as SERS and enhanced
photoluminescence, thermoplasmonic devices, perfect ab-
sorbers, and all-metal optoelectronic devices based on
plasmoelectric potentials in metal nanostructures. We propose,
in our future work, to improve the methodology of
nanostructuring the optically thick Ag thin films to minimize
the un-etched Ag thin film regions, such that better density
and uniformity of different multi-resonant meta-cells can be
achieved. Thus reduction of impedance to the free space light
coupling into meta-material surface can be achieved. As this is
the first report on the fabrication of RPMS by the proposed
universal nanofabrication technique—wet-chemical etch-
ing—it facilitates the room to optimize the experimental pa-
rameters for various thicknesses of silver thin film, which
gives understanding of the effect of film thickness on the
optical properties of RPMS.
Acknowledgements The authors would like to acknowledge the support
of Dr. M. G. Sreenivasan (Technical Manager, Hind High Vacuum
Company Pvt. Ltd. India) in performing the optical spectroscopy mea-
surements for the present study. Dr. V. H. S wants to acknowledge Dr. R.
Bhattacharya, Honorary Adjunct Professor, IIEST, Shibpur, India for in-
troducing him to the exciting field of plasmonics.
Funding The authors would like to thank the Department of Science and
Technology (DST), India (Grant no: DST/TM/SERI/2K10/63(G)) and
Department of Biotechnology (DBT), India (Grant no: BT/PR12874/
NNT/28/452/2009)) for financially supporting the research work.
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