Optical Materials 84 (2018) 447–452

Contents lists available at ScienceDirect

Optical Materials
journal homepage: www.elsevier.com/locate/optmat

Highly polarization and wide-angle insensitive metamaterial absorber for T

terahertz applications
Salman Daniela,∗, Prince Bawuahb,c
a
Institute of Photonics, University of Eastern Finland, P.O. Box 111, FI-80101, Joensuu, Finland
b
School of Pharmacy, Promis Centre, University of Eastern Finland, P.O. Box 1617, FI-70211, Kuopio, Finland
c
Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, CB3 0AS, Cambridge, United Kingdom

A R T I C LE I N FO A B S T R A C T

Keywords: Metamaterial absorbers, due to their potential applications in the field of photonics, are vastly studied in the last
Metamaterials decade. Herein, we present a novel design of a resonant metamaterial absorber in the terahertz (THz) region
Terahertz based on numerical simulations. This perfect THz absorber, due to its symmetry, is highly independent on the
Absorbance polarization state of an incident plane wave. Moreover, the absorber is insensitive to a wide range of incident
Photonic devices
angle from 0 to 70°. The absorbance approaches unity at a resonance frequency of 3.74 THz. The absorber shows
remarkable results even at the two extreme incident angles, e.g. a minimum absorbance value of ∼80% was
recorded at an incident angle of 70°. With this promising performance, the proposed THz absorber can serve as a
possible sensing device for transparent analytes adsorbed on it. For this structure, aluminum and TiO2 are used.
Both materials are commonly available for manufacturing photonic devices.

1. Introduction [7]. Recently, numerous highly polarization insensitive absorbers con-

taining different shapes or designs have been studied. Typical examples
Metamaterials have gained a tremendous amount of importance of these structures are bilayer structure at 1.3 THz [8], crossed-shaped
during the last decade due to their peculiar electromagnetic (EM) be- for bio sensing [9], thin wire-crossed [10], dual-band [11], circular-
havior [1]. These sub-wavelength structures can produce remarkable sectors [12], Via arrays [13], modified electric rings resonators [14],
and interesting properties due to their negative index of refraction that metal groove features [15], symmetric slotted sectors [16] and ultrathin
is impossible to observe in naturally existing materials [2]. The nega- absorbers [17] etc. Moreover, THz-based metamaterial absorbers with
tive values of the permittivity and permeability are the key catching tunable absorbance [18], varying thickness of dielectric layers [19,20]
points of the metamaterials that make their use possible for manu- and tunable frequency [21] have been reported in the literature. High
facturing superlenses, cloacks and spoof plasmons just to name a few performance absorbers in the infrared [22], visible [23] and microwave
[3–5]. [24] regions have also been investigated. However, the resonant fre-
In recent years, various resonant metamaterial structures with high quency region of interest for RMAs is usually within the terahertz gap
absorbance properties are introduced. The principle of resonant meta- since it is difficult to find naturally occurring resonant absorbers in this
material absorber (RMA) is to make the values of transmission and part of the spectrum [25,26]. Furthermore, the relative long wave-
reflection equal to zero at a frequency of interest. The reflectivity is length of the terahertz waves requires micrometer-size metamaterials,
minimized if the impedance of a free space Z0 becomes equal to the which is an advantage from the fabrication point of view. These re-
impedance Z of the metamaterial structures. Impedance is a ratio of sonant absorbers have potential applications in both civilian and mili-
permeability (μ) to permittivity (ε). The transmission can be made tary products. For examples, a THz absorber can serve as a thermal
equal to zero by enhancing the absorption of the structured material detector or as a coating layer to prevent light reflections [8,27–30] in
[6]. Both conditions are achievable by designing a metamaterial with order to reveal or conceal military items.
resonant at a certain frequency. From a designing or manufacturing Herein, we present a novel design of a THz metamaterial absorber
point of view, the most efficient metamaterial absorbers are the ones based on a numerical simulation performed using the finite element
that generate a maximum absorption for a wide range of incidence method (Comsol Multiphysics). The ability to design a perfect absorber
angles irrespective of the polarization state of an incidence EM wave to perform at maximum efficiency (a unity absorbance) irrespective of

∗
Corresponding author.
E-mail address: salman.daniel@uef.fi (S. Daniel).

https://doi.org/10.1016/j.optmat.2018.07.053
Received 16 April 2018; Received in revised form 10 July 2018; Accepted 18 July 2018
0925-3467/ © 2018 Elsevier B.V. All rights reserved.
S. Daniel, P. Bawuah Optical Materials 84 (2018) 447–452

Fig. 2. Relative impedance of the proposed metamaterial structure as a function

of frequency. Re(Z) and Im(Z) are the respective real and imaginary parts of the
impedance. The dotted black line represents the resonant frequency of 3.74
[THz].
Fig. 1. A schematic diagram of a metamaterial absorber (split ring resonator).
The optimized parameters of the structure are the following: L = 38 μm,
W = 38 μm, a = 4 μm, b = 10 μm, c = 13 μm, r = 15 μm and d = 1 μm. The analogy, the two pairs of plates in the y-direction, C3 and C4, and the
structure is made of aluminum with a layer thickness of 50 nm. C1 and C2 sides of the CSD facing C1 and C2 are only activated when interacted
represent the two pairs of capacitor plates along the x-axis while C3 and C4 are with a polarized electric field in the x-direction. The dimension (length,
the two pairs along the y-axis. CSD is a centered-square disc of Al. An incident L × width, W) of the device in Fig. 1 is ∼38 × 38 μm. The thickness of
plane wave in the xz-plane is demonstrated wherein E and B represent electric the TiO2 layer is 2 μm. The bottom layer of the Al is added to eliminate
and magnetic fields, respectively, which are perpendicular to the wave vector k.
the possible transmission of the incident signal through the device. The
The plane wave is incident with an angle θ with respect to the normal (black
thickness of the Al layer, 200 nm, is thicker than the skin depth
dotted line) of the surface. The thickness of the TiO2 and Al layers are 2 μm and
200 nm, respectively.
(∼35 nm) of the incident THz radiation. The thickness values for the
various layers are wisely chosen to ensure optimum performance as
well as ease of fabrication of the device.
the polarization state of the incidence wave is quite challenging. In principle, to maximize the absorption of the structure, the
However, the designed THz absorber of this study gives approximately transmission and the reflection should be minimized according to the
a unity absorbance for any polarization state of an incidence EM-wave relationship A = 1 – T – R, where A, T and R respectively represent
compared to previously reported works [24,30–32]. Moreover, the absorption, transmission and reflection. The metamaterial structure is
absorbance remains ∼80% for a wide incident range (0–70°) of a plane designed in such a way that it achieves a near perfect impedance that
THz wave for both transverse electric (TE) and transverse magnetic matches with the free space impedance. The impedance of the structure
(TM) modes. To the best of our knowledge, this design is one of the is determinable from the calculated scattering parameters as demon-
most efficient THz absorbers with an incredible performance at a wide strated in Ref. [36]. In Fig. 2, a relative impedance of the structure is
range of incident angle. With this structure, the absorption approaches plotted as a function of frequency. At the resonant frequency 3.74 THz
unity at a resonance frequency of 3.74 THz. The absorber consists of (a black dotted line), the real Re(Z) and the imaginary Im(Z) parts of
two metallic layers of aluminum (Al), a split ring resonator (SRR) the impedance approach unity and zero, respectively. This indicates the
structure [33–35] on top and a metallic layer at the bottom. The me- perfect match between the impedance of the medium Z and that of the
tallic parts are separated by a layer of a dielectric TiO2. The thickness of free space Z0 at 3.74 THz. At this impedance matching condition, the
the device is only ∼2 μm. The Al and TiO2 are commonly used mate- value of reflection R approaches zero whilst that of the imaginary part
rials in nanofabrication processes and therefore provide an easy route of of the complex refractive index (ñ) becomes large. The large values of ñ
fabrication for the designed structure. at the impedance matching condition increases the wave attenuation
within the medium [37] and hence, maximum absorption is achieved.
2. Concept and design Also, the transmission T is set to zero due to the presence of the bottom
metallic layer of the structure, therefore all energy is absorbed by the
The schematic diagram of the structure (SRR) with its optimized structure. The imaginary part of the refractive index can be deduced
parameters is shown in Fig. 1. In the figure, the SRR (circular as well as from ε(ω) and μ(ω) where ω is an angular frequency. Thus, by opti-
square parts at the top) is made of an Al with thickness of 50 nm. The mizing ε and μ, and achieving the impedance matching condition, the
symmetric nature of the designed metamaterial makes it polarization value of ñ can be made as large as possible to ultimately enhance the
insensitive since its absorption efficiency is not affected by the polar- absorption [6]. Herein, for the metamaterial absorber, we calculate the
ization state of an incident THz field. For instance, when the field is S-parameters S11 and S21, which are reflection and transmission coef-
polarized in an y-direction the two pairs of capacitor plates, C1 and C2, ficients, respectively [38].
along x-axis direction and sides of a centered-square disc (CSD) facing Aside being commercially available and low cost, TiO2 and Al are
C3 and C4 contain high-energy region in-between. Based on a similar used as the major materials of the proposed structure due to the

448
S. Daniel, P. Bawuah Optical Materials 84 (2018) 447–452

following merits. Aluminum is a conductive material due to the abun- phenomenon is shown in Fig. 4. In Fig. 4(a) and (b), electric field in-
dance of free conduction electrons. This property makes it suitable for tensity |E|2 in a xy-plane is plotted for the frequency of 3.74 THz at an
many potential optical applications especially, in the field of plas- incidence angle of 0°. White double headed arrow represents the or-
monics [2]. For this analysis, we used a conductivity value of ientation of the incidence electric field E with respect to x- or y-axis. In
37 × 106 S/m for the used Al. The imaginary part of the refractive Fig. 4(a), the electric field component E is polarized in the y-direction
index has large negative values in the terahertz range that indicates the (TE) and therefore, the sides of CSD facing C3 and C4 as well as the
high inside losses of the metal [39]. And, as a dielectric medium, TiO2 is plates (C1 and C2) contain maximum energy. It is noticeable that al-
used [40]. most no energy accumulates in-between C3 and C4 for a TE mode. The
opposite case is shown in Fig. 4(b) where the electric field E is polarized
along the x-axis (TM). Similarly, the sides of CSD facing C1 and C2 and
3. Simulations and results plates, C3 and C4, are activated. In general, the energy or |E|2 dis-
tribution is the same in Fig. 4(a) and (b), which manifests the in-
In this study, a whole unit cell, as shown in Fig. 1, is simulated by sensitivity of the designed metamaterial absorber to both TE or TM
applying perfect electric and magnetic boundary conditions using finite modes at normal incidence of the EM-wave.
element method (FEM). A linearly polarized EM plane wave in x or y- To observe the charge distribution in the near surroundings of the
direction illuminates the structure from the top in the negative z-di- structure, current density is plotted for both the TE and the TM modes.
rection (see axis in Fig. 1). Therefore, the electric field component E The results are shown in Fig. 4(c) and (d). It is interesting to notice the
that is polarized in the y-direction (TE) is incident in a perpendicular arrangement patterns of the charges, for example, in the case of the TE
direction regarding the pair of capacitor plates (C1 and C2), which are mode. The sidewalls of the upper half of the Al-squared disc contain
present along the x-axis and top-bottom sides of CSD. In contrast, the negative charges whereas that of the bottom half are charged positively.
electric field E that is polarized in the x-direction (TM) incidents per- The reason could be that the centered metallic element preserves an
pendicularly to the plates along y-axis, i.e. C3 and C4, and left-right overall charge neutrality condition. In addition, for the TE mode,
sides of CSD. The angle of incidence with respect to the normal of the charges assemble only in the gap between plate sets C1 and C2. There is
surface is θ. a tiny amount of charges in-between C3 and C4 that are considerably
The amplitudes of the reflection S11 and transmission S21 are cal- low compared to the generated charges in C1 and C2. The opposite case
culated to extract the absorbance, i.e. A = 1 – T – R or A = 1 – S112 – is studied in Fig. 4(d), where E field is oriented along x-axis. The right
S212. The used frequency range is 3 THz to 4.5 THz. The presence of Al and left sides of CSD are charged positively and negatively, respec-
in the bottom part of the design (see Fig. 1) cancels any possible tively. In summary, the response of the metamaterial absorber is
transmission i.e. S21 = 0. In Fig. 3, reflection (dark blue solid line) and identical for both the TE and the TM modes.
transmission (red solid line) as a function of the frequency are plotted. Now, in order to assess the performance of the designed structure
It is clearly seen in the figure that the reflection (S112) approaches zero with regard to the incident angle, the absorption was calculated for a
at 3.74 THz as it was indicated by the impedance matching condition in range of incidence angles from 0 to 70° with 10° interval. The results are
Fig. 2. In the same vein, the transmission is zero in the frequency range shown in Fig. 5. The θ is the angle between incidence wave vector k and
of 3–4.5 THz. The angle of incidence for the plane wave is 0°. the normal to the surface in the xz-plane of incidence. In Fig. 5(a), the
As a typical phenomenon of a split ring resonator, high amount of electric field E is polarized along the y-direction, which is indicated in
energy or charges are assembled in-between the capacitor plates and the figure as TE. The absorbance is near unity for a range of angles e.g.
CSD exterior sides. A clear manifestation of the occurrence of this from 0 to 30°. The absorbance begins to gradually decrease beyond 30°
and drops to ∼80% at an incident angle of 70°. The observed decrease
in absorbance can be attributed to the occurrence of impedance mis-
match (details will be discussed later) beyond 30° angle of incidence.
Similar observation was made in the case of the TM mode (Fig. 5(b))
where the absorbance, again, drops to 80% at 70° angle of incidence.
The presence of the CSD increases the charge storing ability of the
structure by causing more charges to be aligned in the direction of the
field and hence induces high absorption within the structure [see
Fig. 4(c) and (d)]. In other words, almost all the field entering the
structure is used in the driving and perfectly alignment of charges
within the structure. The incident energy is therefore lost in a form of
heat generation due to the perfect alignment of charges within the
structure. Now, due to the symmetric nature of the device, the electric
and magnetic fields at the selected angular range remains almost un-
changed [41], which explains the physical mechanism behind the ob-
served high absorbance for the wide angular range of the incident
wave. Further evidence of the mechanism is verified by Fig. 5 wherein
the absorbance is high for a range of incidence angles for either TE or
TM modes.
For an ideal absorber, the values of absorbance and resonant fre-
quency remain unchanged for any state of polarization and angle of
incidence of an EM-wave [7,29]. However, it is a challenging task to
have control over minor variation in the performance of the absorber
[31]. In Fig. 6(a), the absorbance values at the resonant frequency
Fig. 3. Reflection S112 (dark blue solid line) and transmission S212 (red solid 3.74 THz are plotted as a function of an incidence angle θ. The ab-
line) of the resonant metamaterial absorber. The black-dotted line represents sorption values are within ∼1–0.8 (black dotted horizontal lines) for
the resonance frequency of 3.74 THz. An incident angle of 0° for the plane wave both the TE (in dark blue) and the TM mode (in red) of the used range
is used for the above simulations. (For interpretation of the references to color of incidence angle. At relatively large angles 40°–70°, there is a slight
in this figure legend, the reader is referred to the Web version of this article.) difference in the absorbance values. For instance, the difference in

449
S. Daniel, P. Bawuah Optical Materials 84 (2018) 447–452

Fig. 4. Electric field intensity |E|2 and charge distribution

around the resonant ring structure. In panels (a) and (b), |E|2
is plotted for a TE and a TM mode, respectively. In panels (c)
and (d), current or charge distribution in near surroundings
of the structure are shown. The results are obtained at an
incidence angle of 0° and a resonant frequency of 3.74 THz.

Fig. 5. Absorbance of the designed metamaterial structure in the terahertz frequency range of 3 THz – 4.5 THz. In Fig. 5(a), the electric field E is polarized along y-
direction (TE) while in Fig. 5(b) E is oscillating along x-axis (TM). The inset values of θ represent the angle of incidence of a plane wave.

absorbance is 0.02 at the incident angle of 70° for both TE and TM shift in the opposite directions until at 70° where frequency shifts of
modes. The observed fluctuations in the absorbance at a given incident 0.02 THz and 0.01 THz were recorded for the TE and the TM modes,
angle might be due to the occurrence of impedance mismatch, which respectively. However, the magnitude of the spectral shifts are not
increases the reflectance of the incident wave [11]. However, in gen- significant considering the spectral resolution (c.a. 0.03 THz or 1 cm−1)
eral, the metamaterial absorber depicts a great response for a wide of a typical terahertz time domain spectrometer [42]. The insignificant
range of incident angles. Moreover, above in Fig. 5, it is interesting to shift observed for both TE and TM modes with respect to the incident
notice that a slight shift in the resonant frequency appears as the in- angle further proves the promising results (see the high absorbance
cident angle increases. In the TE case, resonant frequency shifts towards values in Fig. 5) and the robustness of the designed THz absorber.
higher frequencies while for the TM mode it shifts towards lower fre- Generally, absorbers show variations in the magnitude of absorption
quencies. The shifts in frequency as a function of the incident angle are as well as in resonant frequency with respect to the polarization state of
plotted in Fig. 6(b). It can be seen that the resonant frequency remains an incidence EM-wave [29]. However, these fluctuations should not be
unchanged at angles 0°–20° for the TE mode and 0°–40° for the TM offset too much and must provide a relatively high absorbance [31].
modes (Fig. 6(b)). Beyond these angular ranges, the resonance begins to Herein, we calculated the absorbance for different polarization angles

450
S. Daniel, P. Bawuah Optical Materials 84 (2018) 447–452

Fig. 6. In (a), absorbance as a function of the incidence angle θ at the resonance frequency 3.74 THz is plotted for the TE and the TM modes of the incident THz wave.
Black dotted horizontal lines represent the absorbance range of 80–100%. In (b), the shift in resonant frequency with respect to the incidence angle is shown.

Fig. 7. In (a), absorbance as a function of frequency

for polarization angle α is plotted. The incidence
angle θ is 0°. The vertical lines in green and red color
represent a slight shift of the individual resonant
frequencies from the selected frequency 3.74 THz
(dark blue color). The inset depicts a demonstration
of the polarization angle α with respect to x-axis. In
(b), the shift in the resonant frequency with respect
to α is plotted. The green points are the calculated
values whereas the red dotted curve is a fitted data.
(For interpretation of the references to color in this
figure legend, the reader is referred to the Web ver-
sion of this article.)

(α) of an incidence plane wave (see Fig. 7(a)). The α is an angle be- the bottom of device.
tween the x-axis and the electric field E as demonstrated by the insert of
Fig. 7(a). Due to the symmetric nature of the structure, the absorbance 5. Conclusions
for angles α = 0°, 20°, 45°, 70° and 90° is plotted. Our proposed me-
tamaterial structure displays a great response in terms of absorbance for In summary, we presented and theoretically demonstrated, in terms
all values of α. The absorbance remains 100%, however, resonant fre- of absorbance, a highly polarization insensitive and wide-angle in-
quency shifts for angles 20°, 45°and 70°. The shifts are plotted with dependent metamaterial absorber in the terahertz regime. The designed
respect to α in Fig. 7(b). At α = 0° and 90°, no shift was observed in structure is investigated by analyzing a distribution of electric field
resonant frequency. However, a shift of 0.02 THz was recorded at intensity and charge density for both TE and TM modes of an incident
α = 20°, which decreases to 0.1 THz at α = 45°, and back to 0.02 THz at electromagnetic wave. Minor variations in absorbance and resonant
α = 70°. The shift in resonant frequency corresponding to the polar- frequency are studied for a wide range of angles. A relatively high
ization angle indicates the symmetric nature of the structure. None- absorption (∼80%) was obtained for incident angle range of 0–70° at a
theless, the difference in frequency is relatively small and insignificant, resonance frequency of 3.74 THz. Based on the availability and prop-
which testifies the highly polarization insensitivity of the designed erties of the used materials (Al and TiO2), we have proposed possible
metamaterial absorber. Adding to the above merits, the designed me- experimental approaches for the fabrication of the designed structure.
tamaterial absorber has shown a spectacular performance by keeping
the absorbance at a maximum value for all polarization angles Author contributions
(Fig. 7(a)).
S. D. conceived the idea and performed the simulations. S. D and P.
4. Towards fabrication B analyzed the results and both contributed substantially to write the
paper.
Although fabrication of the sample and experimental results are not
presented in this work, the commercial availability of the materials Conflicts of interest
(TiO2 and Al) as well as the micrometer size range of the device,
practically, show that it is quite feasible to manufacture this device by The authors declare no conflict of interest. The funding sponsors
applying commonly used nanofabrication processes. The structure can had no role in the design of the study; in the collection, analyses, or
be built up on a silicon-wafer which is further coated by 200 nm and interpretation of data; in the writing of the manuscript, and in the de-
2 μm layers of Al and TiO2 [43], respectively. The layers can be applied cision to publish the results.
by a thermal evaporation and atomic layer deposition (ALD) methods.
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