Plasmonics

https://doi.org/10.1007/s11468-024-02427-w

RESEARCH

Si3N4 Dielectric Hemi‑sphere Arrayed Plasmonic Metasurface With Top

Metal Coating for Multiresonant Absorption in NIR Regime
Prasanta Mandal1

Received: 24 May 2024 / Accepted: 12 July 2024

© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2024

Abstract
Present report focuses on the design and optical perfect absorption/reflection properties of novel plasmonic metasurface
made of square array of ­Si3N4 hemi-spheres on flat ­Si3N4 surface. The whole structure is sandwiched between flat gold layer
and top gold coating. Theoretical study using Finite Difference Time Domain (FDTD) computation shows multiple near
perfect absorptions (80–100%) with narrow line width (~ 50 nm) between 550 to 1500 nm. Four distinct absorption peaks
(or reflection dips) are observed at 1020 nm (A1), 888 nm (A2), 614 nm (A3), 740 nm (A4) which can be manipulated by
varying structural parameters such as period, hemi-sphere diameter and top gold coating thickness. These multiple absorp-
tions arise due to electric dipolar resonance, magnetic resonance, excitation of various surface plasmon modes (such as
(1,0); (2,0); (1,1)) and cavity mode, as evident from near-field analysis. With appropriate structural parameters, multiband
well resolved near perfect absorptions are achieved at desired wavelengths. The proposed metasurface is insensitive to the
polarization of excitation beam, and has relatively large launch angle tolerance (~ 20°), making it suitable for optical and
optoelectronic device integration.

Keywords Plasmonic metasurface · Si3N4 hemi-spheres in square lattice · visible-near-infrared (Vis-NIR) wavelengths ·
multiband absorber · FDTD computation

Introduction Generally, absorbing metasurfaces are designed using sand-

wich structure (metal-dielectric-metal) [40–44]. However,
Absorptive metasurfaces are important class of materials all-metal-layer or all-dielectric-layer (structured) can also
having potential implications in optical and optoelectronic be used to realize highly absorption (or perfect absorption)
devices, such as, for light manipulation [1–6], energy har- characteristics [45–49].
vesting [7–11], chiral detection [12–16], and molecular sens- In a sandwich structure, various dielectrics, such as, ­SiO2
ing [17–21]. Usually, depending upon application interests, (silicon dioxide), ZnO (zinc oxide)/ZnS (zinc sulphide),
single or multi-resonant absorption characteristics with ­VO2 (vanadium dioxide), ­Si3N4 (silicon nitride), GeSbTe
narrow or broad line widths are looked at in a wide wave- (germanium-antimony-tellurium) etc. are being used.
length range spanning over UV (ultra-violet) to IR (infra- ­Si3N4 is an attractive high band gap dielectric material that
red) regime [22–30]. In most of the cases, metasurfaces are has optical transparency in wide spectral range (Visible-
fabricated with optimized structural parameters for obtain- near IR) [50]. Furthermore, ­Si3N4 can be implemented
ing specific optical response where dynamical control- in various microelectronic devices as electrical/thermal
ling of optical response is not essential. To control optical insulating material [51, 52], as antireflective coating in
property dynamically (such as, Q-switching) one needs to solar energy harvesting [53, 54], perfect absorber [55,
design metasurface with material whose properties can be 56], waveguides [57, 58]. Perfect absorbers are made of a
controlled externally using temperature and voltage [31–39]. variety of structured surfaces, such as, vertical split-ring,
ancient coin-like structure, cross-bar-patch structure, and a
* Prasanta Mandal polyimide dielectric spacing layer, multiple-channel metal-
pmandal@ddn.upes.ac.in shell rod-shaped structure etc., and studied widely to achieve
multiband perfect or near-perfect absorption [25–27, 41, 46,
1
Department of Physics, Applied Science Cluster, School 59, 60]. However, absorbing metasurfaces made of ­Si3N4
of Advanced Engineering, UPES, Dehradun−248007, India

Vol.:(0123456789)
 Plasmonics

hemi-spheres array and strong multi-resonant absorption resonances, ωo = plasma frequency, Γ = collision frequency,
characteristics in the visible-NIR regime have not been ωn = resonance frequency, S = strength, εr,∞ = dielectric
investigated. Nevertheless, simpler designed structures constants at infinite frequency, and εr = relative dielectric
are favoured over complex geometry. Therefore, design of constant) [61]. The resonance details for S ­ i3N4 (Characteristics
simpler absorbing metasurface made of ­Si3N4 and study of and Influenceve) are as follows: n = 1, ωo = 14.5808 × ­1015 rad/s,
their multi-resonant absorption characteristics seem to be not S = strength = 2.954315, Γ = 7.292 × ­1 0 13 rad/s,
only attractive but also have fundamental and technological ω1 = 14.5808 × ­1015 rad/s, εr,∞ = 1 (as available in optiwave
importance. The present study focuses on the design of FDTD master library (OPTIWAVE is used for computation)).
plasmonic metasurface made of ­Si3N4 hemi-spheres array Excitation source is a Gaussian modulated continuous wave (for
on flat S
­ i3N4 surface, and study of its optical absorption/ broad band excitation) with peak wavelength of 750 nm, time
reflection characteristics in the visible-NIR regime using off-set of 4 × ­10–15 s and half width of 1 × ­10–15 s [61]. Linear-x
FDTD computation. Multiple absorption peaks between polarization is set for excitation beam (due to structural
500 to 1500 nm are achieved whose strengths approach symmetry, linear-y polarization excitation will give same results
to ~100%. as that for linear-x). For angle dependent study launch angle is
varied from 0 to 30 degrees. Periods (­ Px and P ­ y) of 600 nm are
set along X − Y − directions. PBC (periodic boundary
FDTD Computation Details of the Absorptive conditions) along X-Y and APML (anisotropic perfectly
Metasurface matched layers) along Z direction are applied. Time step of
5000 and appropriate mesh size (10 cells per wavelength) are
Schematic representation (three-dimensional (3D) and two- set [61]. To detect reflection, a monitor is placed at a distance
dimensional (2D)) of Au coated square array of hemi-sphere (away from the top surface) greater than Fresnel’s limit.
plasmonic absorptive metasurface (unit cell) is shown in Fig. 1. Similarly, transmission is detected using a monitor placed at the
The proposed metasurface consists of (from bottom side): (a) a back side of the metasurface.
­SiO2 substrate (refractive index (μ) 1.5 (fixed)) of thickness 200
nm at the bottom ­(h1), (b) a 100 nm thick gold (Au) layer (­ h2) on
the supportive S ­ iO2 layer, (c) a 150 nm thick flat S
­ i3N4 (≡ SiN) Results and Discussions
layer ­(h3), (d) S
­ i3N4 hemi-sphere square array on the top of the
flat ­Si3N4 layer, (e) a top gold coating (25 nm (or 50 nm)) (­ h4), Absorption Characteristics and Influence of Au
and (f) top SiN coating (if required). A suggestive approach to Coating and Hemi‑sphere Size
fabricate the proposed metasurface is mentioned after the
conclusion section (note-1). Hemi-sphere has radius ­Rh Absorption/reflection spectra of the hemi-sphere arrayed
typically in the range of 100 nm to 175 nm. Unit cell period is metasurface are shown in the Fig. 2. Structural parameters
set to 600 nm (along X: ­Px = 600 nm, and along Y: ­Py = 600 of the metasurface are as set: period = 600 nm, h­ 4 = 25 nm,
nm). Gold (Au) is considered to be Lorentz − Drude (LD) and hemi-sphere diameter = 200 nm, 250 nm, 300 nm and
s 𝜔2o
dispersive (εr = 𝜀r,∞ + n 𝜔2 −𝜔n2 +j𝜔Γ , here n = number of 350 nm. Absorption spectrum (A(λ)) is obtained from the
∑
n n

Fig. 1  3D and 2D schematic

view of hemi-sphere metasur-
face unit cell. Unit cell periods
along x and y are ­Px and ­Py,
respectively. Electric and mag-
netic fields are denoted by E
and H, respectively. Propagation
(propagation vector K) of light
is in the z direction. Incident
and reflected light beams are
represented by colour arrays as
indicated
Plasmonics

Fig. 2  Optical absorption/reflec-

tion spectra of the metasurface
excited by linear-x polarized
light: hemi-sphere diameter
a = 200 nm, b 250 nm, c 300
nm, d 350nm. Top gold coating
thickness = 25 nm. Metasurface
period is fixed at 600 nm

relation, A(λ) = 1 − R(λ). Here, T(λ) = 0 is considered due absorption spectra of metasurface with hemi-sphere diam-
to the fact that 100 nm thick gold layer is thick enough to eter of 200 nm, 250 nm, 300 nm and 350 nm are shown.
block the transmission. For the case of hemi-sphere diam- For 200 nm sphere diameter, three distinct absorption peaks
eter of 200 nm, three absorption peaks are observed at 932 are observed at 895 nm (A1), 807 nm (A2) and 642 nm
nm (A1), 864 nm (A2), and 630 nm (A3) with absorption (A3). The absorption peaks, A1 and A2 are blue-shifted
strength of 80%, 100% and 55%, respectively (Fig. 2(a)). by 37 nm and 57 nm, respectively in comparison to the
When diameter changes to 250 nm, the absorption peaks are absorption peak A1 and A2 for the case of 25 nm top gold
red-shifted to 963 nm, 860 nm, and 625 nm, respectively. coating. Moreover, A1 and A2 peak absorption strengths
While absorption strengths of A1 and A2 remain same but significantly reduce to 25% and 37% from 80 and 100%,
with lesser overlap, the A3 absorption strength increases respectively. Absorption strength of A3 peak at 642 nm is
to 85%. Further increase in diameter (300 nm) shows more significantly increased to 91%. Similar trend is observed
red-shifts in the peak position. The characteristic peaks for the other cases (hemi-sphere diameter: 250 nm, 300 nm
are now observed at 996 nm, 868 nm and 630 nm. A new and 350 nm). Absorption peak positions and strengths in
peak is appeared at 730 nm (A4) with absorption strength all these cases are summarized in Table 1. It is noticeable
of ~ 73%. A1, A2 and A3 peak absorption strengths increase that numbers of resonant peaks are less in this case (50 nm
to 80%, 99% and 98%. Widths of A2 and A3 peaks increase to gold) compared thinner coating (25 nm). This is espe-
too. When diameter changes to 350 nm red-shifted resonant cially observed for metasurface with higher hemi-sphere
peaks are observed at 1020 nm (A1), 888 nm (A2), 614 nm diameter. In general, to achieve multiple resonances with
(A3), 740 nm (A4). A newly appeared peak at 1157 nm stronger absorption characteristics thinner top gold coat-
shows absorption strength of ~ 40%. Absorption strengths ing (less than 30 nm (skin depth of gold)) is more effec-
of A2, A3, A4 are > 90% and comparable to each other. tive than thicker one. Due to thin coating, there is a chance
It is noteworthy that with higher hemi-sphere diameter, of surface plasmon excitation on both the dielectric sides
the absorption peaks are observed to be stronger and well of gold. Furthermore, due to thinner metal layer plasmon
resolved. To understand absorption characteristics and fab- modes can overlap and make resonance broad. For thicker
rication tolerance, top gold thickness dependence is studied gold coating top surface reflection will be more, and only
in details. Top gold thickness is increased from 25 to 50 SPR related to gold-air interface can be excited (without
nm keeping all other parameters unchanged. In the Fig. 3, further top SiN coating).
 Plasmonics

Fig. 3  Optical absorption/reflec-

tion spectra of the metasurface
excited by linear-x polarized
light: hemi-sphere diameter
a = 200 nm, b 250 nm, c 300
nm, d 350nm. Top gold coating
thickness = 50 nm. Metasurface
period is fixed at 600 nm

Double layer top coating with Au followed by SiN is at 1200 nm and 1156 nm), 1052 nm (A2), 971 nm (A4)
employed to study further influence on the absorption/ and 855 nm (A3). The respective absorption strength
reflection characteristics. On the top of the gold (25 nm) reaches to 90% (doublet absorption strength: 100% and
layer, SiN coating is applied. Thickness is varied, as if, 92%), 92%, 92% and 100%. At 250 nm SiN coating, fur-
gold coated SiN hemi-sphere is embedded in SiN layer ther red-shift is seen. It is noticeable here that the dou-
(either side) (see Fig. 1: metasurface in 2D after top SiN blet becomes broad and appeared to be single peak at
coating). Metasurface period and hemi-sphere diam- 1240 nm with absorption strength ~ 99%. Peak at 1070
eter are kept fixed at 600 nm and 300 nm, respectively. nm shows absorption of about 82%. Two peaks at 975 nm
These are typically set due to the fact that distinct multi- (absorption: 85%) and 925 nm (absorption 100%) overlap
resonant absorptions are seen in this case. The spectrum making resonance very broad. The SiN-Au-SiN struc-
with zero (0) top SiN coating (only 25 nm top gold coat- ture makes the resonant peak to split into symmetric and
ing case) are re-plotted in Fig. 4(a) for easy comparison. antisymmetric modes (doublet). These peaks are closely
When 200 nm top SiN coating is applied, a significant separated making the resonant absorption broad. Finally,
enhancement in the absorption (> 90%) is seen, espe- for very thick SiN coating (300 nm thick), absorption
cially for A1 and A4 peak (Fig. 4(b)). All the absorption peaks show reduction in absorption strength. Absorption
peaks are red-shifted to 1175 nm (a doublet (A1): peaks contrast reduces too.

Table 1  Resonant peak wavelength (nm) and absorption strength (% absorption) for the metasurface with 600 nm period
Hemi-sphere Absorption peak wavelength (λ (nm)) and strength (%A)
diameter (nm)
A1 A2 A3 A4
h4 = 25 nm h4 = 50 nm h4 = 25 nm h4 = 50 nm h4 = 25 nm h4 = 50 nm h4 = 25 nm h4 = 50 nm
λ (%A)

200 932 (80) 895 (25) 864 (100) 807 (37) 630 (55) 642 (91) - -
250 963 (80) 930 (25) 860 (100) 807 (50) 625 (85) 650 (98) - -
300 996 (80) 960 (25) 868 (99) 807 (68) 630 (98) 675 (90) 730 (73) -
350 1020 (91) 1000 (20) 888 (99) 807 (85) 614 (91) 700 (88) 740 (75) -
Plasmonics

Fig. 4  Top ­Si3N4 coating thick-

ness dependent optical absorp-
tion/reflection spectra of the
metasurface excited by linear-x
polarized light. Hemi-sphere
diameter = 300 nm, period = 600
nm, top gold coating thick-
ness = 25 nm. SiN thickness is
varied as: 0 nm a, 200 nm b,
250 nm c and 300 nm d

Fig. 5  Period dependent

resonant absorption/reflection
spectra. Periods: a 600 nm,
b 700 nm, c 800 nm, and d
500 nm. Top gold coating and
subsequent SiN coating are kept
fixed at 25 nm and 200 nm,
respectively. Fixed hemi-sphere
diameter = 300 nm
 Plasmonics

Table 2  Peak wavelength, Period (nm) Hemi-sphere Resonant peak wavelength (nm)
absorption strength for diameter (nm) (Au coating of 25 nm + final top layer SiN coating of 200 nm)
different periods. Hemi-sphere
diameter = 300 nm (fixed) A1 (%A) A2 (%A) A3 (%A) A4 (%A)

600 300 1211 (90), 1050 (90) 975 (90) 852 (100)
1158 (100)
(doublet)
700 300 1263 (100) 1160 (90) 897 (100), 852 (85) (doublet) -
800 300 1372 (85) 1242 (90) 961 (82), 930 (80) 859 (100)
(doublet)
500 300 1116 (95) 1004 (95), 646 (99) -
919 (80)
(doublet)

Absorption/Reflection Characteristics: Metasurface period is fixed at 800 nm. Other parameters

Period Dependence are kept as follows: top gold coating = 25 nm, subsequent
top SiN coating: 200 nm, hemi-sphere diameter: 300 nm.
It is essential to optimize structural parameters for obtain- Launch angle dependent absorption spectra are shown in
ing optimized absorption/reflection characteristics and the Fig. 6. When launch angle changes to 10°, in compari-
design flexibility for real fabrication. One of the key son to the spectrum at 0°, the spectrum shows no changes
aspects is to study influence of change in structural period. in the peak wavelength as well as in the absorption
In the present study, period is varied from 500 to 800 nm strength. Similar absorption characteristics are observed
in a step of 100 nm. Top gold coating and subsequent SiN for the excitation at 20° and 30°, and all the absorption
coating are kept fixed at 25 nm and 200 nm, respectively. peaks still remain well resolved. However, absorption
Absorption/reflection spectra for four different periods are contrast (ratio of absorption strength of resonant peak
shown in the Fig. 5. As discussed previously, well resolved (say, at 1242 nm) to the strength at non-resonant wave-
multiple resonances with absorption strength > 90% are length (1068 nm)) decreases as launch angle increases. A
seen in the wavelength range from 800 to 1300 nm for the decrease in absorption contrast from ~ 7 at 0° to 1.6 at 30°
600 nm period (Fig. 5(a)). When period changes to 700 nm is observed. Angle dependent absorption in metasurface
(Fig. 5(b)), three distinct peaks are seen at 1263 nm, 1160 is being studied previously [10, 11, 65]. The present study
nm, and a closely spaced doublet at 897 nm (with a side shows, for an effective operation, launch angle may be
hump at 852 nm). At 800 nm period, the spectral absorp- varied till absorption contrast remains > 2 (non-resonant
tion peaks are red-shifted to 1372 nm, 1242 nm and 961 nm absorption is half the peak strength).
(doublet: side hump at 930 nm) (Fig. 5(c)). Fourth peak is
observed at 859 nm. All the absorption peaks have strong
absorption (~ 90% or more) with good contrast. Red-shift
in resonant wavelength with increasing period is due to
plasmon assisted excitation of various modes (as discussed
in details in the "Absorption/Reflection Characteristics:
Near‑field Analysis" section) [62–64]. To study the influ-
ence of lower period, we have reduced the structural period
to 500 nm. All the absorption peaks (A1 at 1116 nm, A2
(doublet) at 1004 nm and 919 nm, and A3 at 646 nm) are
well resolved but broad (Fig. 5(d)). Absorption strength
approaches to 100% in most of the cases. Details of absorp-
tion strength and peak wavelengths for different periods are
summarized in Table 2.

Absorption Characteristics: Launch Angle Dependence

For an incident beam with fixed polarization (linear-x),

incident angle dependent optical absorption is studied by Fig. 6  Launch angle (θ) dependent absorption spectra. Launch angle
varying launch angle from 0° to 30° at a step of 10°. is varied from 0° to 30° at a step of 10°
Plasmonics

Fig. 7  Electric ­(Ex) and magnetic ­(Hy) near-field distributions (upper lengths are 855 nm, 972 nm, 1052 nm and 1154 nm. Various field
and lower panels) of hemi-spheres square arrayed metasurface excited components are as labelled in the Figure. Scale bars represent relative
by linear-x polarized light. Upper panel: excitation wavelengths are field strengths
632 nm, 732 nm, 866 nm and 997 nm. Lower panel: excitation wave-

Absorption/Reflection Characteristics: absorption peak wavelengths. For the first case, there are
Near‑field Analysis four strong absorption peaks at 632 nm, 732 nm, 866 nm,
997 nm, while, in the second case, peaks are at 855 nm, 972
Near-field distribution is studied to unveil mechanism of nm, 1052 nm, 1154 nm. At lower wavelengths the electric
multi-resonant absorption characteristics observed for the near-fields ­(Ex, X–Z plane) are predominately confined at
hemi-spheres arrayed metasurface. Two cases are consid- the gold-air interface of the hemi-sphere dome as shown in
ered: (a) top gold coating = 25 nm, and (b) top gold coating the upper panel of Fig. 7. Field confinement is also seen
(25 nm thick) followed by SiN coating (200 nm thick). within the S
­ i3N4 dielectric region, especially for 632 nm and
Structural period is fixed at 600 nm, and hemi-sphere diam- 866 nm excitations. At 997 nm excitation, entire field is
eter is set to 300 nm. Single wavelength excitation is carried confined within the S ­ i3N4 region under hemi-sphere dome.
out for both the cases (normal incidence and linear-x polar- Magnetic near-field ­(Hy) is confined weakly except for 997
ized light). Wavelengths are set according to resonant nm excitation (upper panel: lower row). The near-field
 Plasmonics

confinement is caused by the electric dipole resonance with strength of ~ 80–100% and narrow line width of ~ 50 nm.
effective resonating wavelength, λres = 2μLeff = 600 nm. Distinct absorption peak position and strength can be
Here, μ = refractive index of air, ­Leff = dipolar length = diam- manipulated by varying structural parameters, such as,
eter of the hemi-sphere. However, plasmonic excitation period, hemi-sphere diameter and top gold coating thick-
(grating coupling: square array of hemi-sphere) at the ness. These multiple absorptions arise due to electric dipo-
­Si3N4-gold interface can not be ruled out completely. For lar resonance, magnetic resonance, excitation of various
plasmon assisted absorption signature, we need to calculate surface plasmon modes (such as (1,0); (2,0); (1,1)) and
resonating wavelength
� 𝜀which can be estimated from the rela- cavity mode, as evident from near-field analysis. With
tion, 𝜆res = √ 2 2 𝜀 +𝜀 , where, εd, εm are dielectric per-
P d 𝜀m
appropriate structural parameters, well resolved near perfect
m +n
absorption peaks can be obtained at desired range. Moreo-
d m

mittivities of air (or S­ i3N4) and metal (gold), respectively,

P = period = 600 nm, m and n are integers [66]. Looking at ver, the designed structure is insensitive to incident beam
the near-field distribution (upper panel: E ­ x field due to 632 polarization and launch angle, therefore, may suitably be
nm excitation), ­Ex near-field is confined at the unit cell used for optoelectronic devices.
edges within S ­ i3N4 regime. This is attributed to the (2,0)
mode of plasmon excitation at ­Si3N4-gold interface. Peak at Note‑1 Suggestive approach to fabricate the proposed
732 nm is purely due to dipolar resonance as near field is metasurface: on a supportive substrate, flat gold layer
confined at the air-metal interface. The strong peak at 866 and SiN layer (of appropriate thickness) can be deposited
nm is caused by plasmon excitation of (1, 1) mode at the using sputtering technique [Surface study of RF magne-
­Si3N4-gold interface. Peak at 997 nm is attributed to the tron sputtered silicon nitride thin films by Majeeda et al.
cavity mode (λres = 2μLeff ≈ 1000 nm, where, L ­ eff ~ 250 nm, J. of Optoelectronic and Biomedical Materials 15 (2023)
μ of ­Si3N4 ~ 2 [67]). The 997 nm absorption peak may also 55-64]. On the top of SiN layer, hemi-sphere patterning
be contributed by magnetic resonance (confinement of H ­ y can be achieved using standard nanopatterning tools, such
magnetic at the regime between hemi-sphere and flat dielec- as, focused ion beam (FIB) technique or lithography (such
tric layer). as, optical lithography or electron beam lithography) [For
Subsequent top covering by SiN shifts the resonant peak references: Implementing infrared metamaterial perfect
wavelength towards higher wavelength. Red-shifts can be absorbers using dispersive dielectric spacers by Zhao
explained on the basis of near-field distribution of ­Ex and ­Hy et al. Optics Express 27 (2019) 1727-1739]. Associated
components as presented in the lower panel of Fig. 7. Near- steps, such as, lift-off and/or selective ion etching (RIE:
field distribution at 855 nm excitation clearly shows the field reactive ion etching) may be employed if required. For a
confinement on the top of the hemi-sphere (either side of the general approach of micro/nanopatterning interested read-
interface). The strong peak at 855 nm is due to plasmon exci- ers referred to article, ‘Shape-modification of patterned
tation of (1, 1) mode at the S ­ i3N4-gold interface. This reso- nanoparticles by an ion beam treatment’ by Heo et al. Sci-
nant mode gets split into antisymmetric (lower wavelength) entific Reports 5 (2015) 8523. Finally, on the top, gold
symmetric (higher wavelength) mode, and the overlapping and SiN (if required) may further be deposited (sputtering
of these peaks makes the resonant absorption broad. Cavity technique/thermal evaporation/e-beam evaporation). As
mode is visible at 972 nm. Peak at 1052 nm is caused by the the present work is based on the ideal theoretical concept
excitation of (1, 0) antisymmetric mode at the ­Si3N4-gold and computation; the actual fabrication procedure may
interface. Symmetric mode (1, 0) appears at 1154 nm. A side change depending upon the fabrication complexities fac-
peak at ~ 1220 nm is closely spaced from 1154 nm peak. The ing at each level.
1220 nm peak is caused by the magnetic resonance.
Acknowledgements P. Mandal acknowledges the Science and Engi-
neering Research Board (SERB), Department of Science and Technol-
Conclusions ogy (DST), Government of India (GOI), for the research grant (file:
CRG/2019/000701).

A plasmonic metasurface made of square array of S ­ i 3N 4

Authors’ Contribution P. Mandal is sole author of this work.
hemi-spheres on flat S­ i3N4 surface is proposed and inves-
tigated numerically using FDTD computation to achieve Funding Financial support (grant no.: CRG/2019/000701) from SERB,
multiband optical perfect or near perfect absorption. The DST, Government of India is thankfully acknowledged.
whole structure is sandwiched between flat gold layer and
Availability of Data and Materials No datasets were generated or ana-
top gold coating. The study shows multiple near perfect lysed during the current study.
absorptions between 550 to 1500 nm of spectral regime.
Resonant absorption peaks are very strong with absorption Code Availability Not Applicable.
Plasmonics

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