RESEARCH ARTICLE

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Design of Planar Reconfigurable, Tunable, and Wide Angle

Resonant Absorbers for Applications in the IR Spectrum
Igor Leonardo Gomes de Souza,* Vitaly Felix Rodriguez-Esquerre,* and Israel Alves
Oliveira*

transmission, and reflection of the electro-

A hybrid resonant absorber Fabry Perot—FP tunable from a simple stack of magnetic wave, in various regions of the
planar layers of ultra-thin films of phase change material is proposed, electromagnetic spectrum. MMs is a type
designed, and demonstrated numerically. This work shows that properly of artificial material usually not found in
designed metal–dielectric–metal) structures comprising thin films of titanium nature[1] their electromagnetic properties
are defined mainly by the infinitely orga-
(Ti), germanium telluride (GeTe) on a silver (Ag) substrate produce a dynamic
nized periodic unit cells as for example in
modulation of light in the near IR with almost perfect absorption (A > 93%). It "meta-atoms",[2] with a feature size much
is demonstrated that the resonant peaks of the hybrid absorber can be tuned smaller than the operational wavelength.[3]
in two different ways for any wavelength, with the crystallization fractions and Absorbers based on MMs have attracted lot
the thickness of the dielectric. The influence of the metallic top layer thickness of interest in the last decades due to the vast
amount of applications in photo-detection,
is also analyzed and demonstrated the fabrication error tolerance of the
image generation, thermal emission, sen-
proposed absorber is demonstrated. Also analyzed is the influence of the sors and shielding.[4] Basically there are two
thickness of the metallic top layer and tolerance to fabrication error of the types of MMs absorbers, the broadband that
proposed absorber is demonstrated. Finally, the physical mechanisms for the has high/maximum absorption for all wave-
coupling of the electromagnetic field and the absorbed optical power in the lengths of a region of the electromagnetic
structure proposed are presented and discussed. This study allows the spectrum and the narrow-band that has
maximum absorption for a given resonant
beginning of the development of hybrid devices to control the absorption of
wavelength.[5] Perfect ultra-narrow band ab-
light in the region of the electromagnetic spectrum used in optical sorbers have also been studied in Refs.
communications. [6]
and [7] . This resonant structures com-
posed by metals and dielectrics with sub-
wavelength dimensions have been used
as mirrors to suppress transmission by
1. Introduction increasing the reflection and inducing destructive interfer-
ence from reflected light at resonant wavelengths that are
Metamaterials (MMs) and plasmonics nanostructures provide absorbed within the structure.[8] The design of planar struc-
a high capacity for controlling and manipulating light-matter tures, efficient, easy to fabricate, and compact optical devices
interactions at the nanoscale and allow a great advance in the has attracted much interest from the scientific community
engineering of many optical processes, such as absorption, due to their possible potential applications in many devices
capable of modulating electromagnetic waves. Perfect planar
absorber (PA) with three metal–insulator–metal (MIM) layers
Prof. I. L. Gomes de Souza
Institute of Science
were analyzed in,[9] it was possible to obtain a narrowband
Technology and Innovation at the Federal University of Bahia (ICTI-UFBA) narrow band super absorber (≈17 nm) and absorption of ≈97%.
Camaçari, Bahia 42802-721, Brazil A broadband PA was demonstrated experimentally by,[10] the
E-mail: ilgsouza@ufba.br structure showed a high absorption in the visible and near IR
Prof. V. F. Rodriguez-Esquerre range (400–900 nm). A resonant MIM absorber that is insen-
Graduate School of Electrical Engineering, Federal University of Bahia sitive to the angle of incidence (0° to 80°) was proposed and
(UFBA)
Salvador, Bahia 40210-630, Brazil analyzed in,[11] the structure showed a high absorption for all
E-mail: vitaly.esquerre@ufba.br colors of the visible spectrum region. Recent work has demon-
I. Alves Oliveira strated the great potential of phase change materials (PCMs) for
Department of Electrical Engineering Federal University of Bahia (UFBA) nanophotonics, and nanoplasmonics that can be tuned.[12] This
Salvador, Bahia 40210-630, Brazil class of materials allows for optical modulation in micro and
E-mail: israel.alves@ufba.br
nanoscale by means of a rapid phase change in the crystalline
The ORCID identification number(s) for the author(s) of this article structure of a dielectric material. The high optical modulation
can be found under https://doi.org/10.1002/adts.202100002 capability of PCMs has been used in many nanostructure op-
DOI: 10.1002/adts.202100002 tical devices designed to light transmission,[13] absorbers,[14]

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Figure 1. Proposed FP color absorber based on a nano-resonator integrated with a dielectric overlay

emissivity,[15] among others. A multi-layered planar optical 2. Results and Discussions

cavity using PCM germanium telluride (GeTe) to produce highly
adjustable reflective colors was analyzed in,[16] it was possible to Figure 1 shows the proposed planar hybrid absorber based on
observe the temperature of the GeTe was dependent on the thick- PCM, the structure is composed of three planar layers, consist-
ness of the film. A tunable hybrid planar PA based on three lay- ing of a thin metallic film at the top, below a layer of PCM and
ers PCM–Dielectric–Metal was demonstrated experimentally in a metallic substrate. The top metal film was Titanium due to the
Ref. [17] a large optical modulation of a strong reflection ≈80% low loss of optical absorption in the short IR of the electromag-
and a perfect absorption. A hybrid planar reconfigurable using netic spectrum, below the PCM GeTe, and the metallic substrate
PCM was analyzed in Ref. [18]. The structure analyzed theoret- of Silver (Ag). dTi and dGeTe are the thicknesses of the Ti and GeTe
ically and experimentally allowed the control of absorption in all layer of the absorber, respectively. The numerical simulation was
visible spectrum, the proposed study opens a wide path for the performed using the finite element method (FEM) with the com-
development of non-volatile display technologies and material se- mercial software COMSOL version 5.2.[19]
lection. In this work we propose a hybrid resonant absorber based The refractive index (n) and the extinction coefficient (k) of the
on MIM using GeTe as an insulator. The proposed structure is materials used in the absorber were taken from the experimental
tunable in the IR of 1000–2200 nm for the range of wavelengths results: i) Ti and Ag;[20] and ii) for GeTe.[21] This optical response
used in optical communications. We used the Lorentz–Lorentz that occurs in PCMs, can be gradually adjusted through the use
theory of the partial fraction to tune the resonance peak of the of different energies in GeTe, which can occur to the formation
absorber with the crystallization fraction of GeTe. We also eval- of nucleation in a-GeTe (amorphous phase) for the c-GeTe state
uated the behavior of absorbers for oblique angles of incidence. (crystalline phase), or an intermediate state between amorphous
In summary, we present the proposal of resonant absorbers and crystalline.[22] To describe the effective permittivity of GeTe
exploring the properties of a PF where the combined effects of with various fractions of crystallization, we used Equation 1 the
the dispersion properties of the dielectrics and metals used. The Lorentz–Lorenz relation.[23]
structure presented a perfect absorption A (𝜆) = 1−R (𝜆)−T (𝜆)
𝜀eff (𝜆) − 1 𝜀c−GeTe (𝜆) − 1 𝜀 (𝜆) − 1
> 93% for all fractions of crystallization. The design results in a = m. + (1 − m). a−GeTe (1)
wide-angle resonator for incident angles of ≈45°. We propose two 𝜀eff (𝜆) + 2 𝜀c−GeTe (𝜆) + 2 𝜀a−GeTe (𝜆) + 2
possibilities to tune the resonance of the hybrid absorber, with
the crystallization fraction, and with the thickness of the GeTe. In where Ɛa-GeTe (𝜆) e Ɛc-GeTe (𝜆)are the permittivity as a function of
addition, we also evaluated the effects of fabrication errors on the the wavelength of a-GeTec-GeTe,[24] respectively. m is defined as
deposition of the top layer of Titanium (Ti) and the physical ab- the crystallization fraction of GeTe, ranging from 0 to 1.[25] We
sorption mechanisms through the analysis of the distribution of use the relations 𝜀(𝜆) = n2 (𝜆) − k2 (𝜆) (2) and 𝜀(𝜆)
̃ = −2nk (3), to
electromagnetic fields, and the power absorbed in the proposed calculate the real and imaginary permittivity of GeTe, and using
structure. (1) we determine the Ɛeff (𝜆) for each fraction of crystallization.

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Table 1. Technical data and absorption results.

Crystallization fraction [m] Peak/Abs FWHM[nm]

m = 0%—Amorphous Phase 1268 nm—95.3% 340

m = 20%—Semi crystalline phase 1356 nm—95.2% 350
m = 40%—Semi crystalline phase 1495 nm—95.1% 402
m = 60%—Semi crystalline phase 1597 nm—94.4% 290
m = 80%—Semi crystalline phase 1756 nm—93.4% 275
m = 100%—Amorphous Phase 1963 nm—95.5% 332

property modulation in these classical PCMs systems stems from

a change in chemical bonding configuration accompanied by a
metal–insulator transition.[14] In addition, this structural reorga-
nization of the phase change can be conducted thermally, electri-
cally, or optically.[26] We also numerically analyzed the behavior
of the resonant peak with the GeTe creative fraction (Figure 2b).
The yellow curve shows the absorption coefficient at each peak,
it is possible to observe that the structure has absorption above
90% for all resonant peaks. The blue curve shows that the first
peak is shifted to the right with the increase in the crystallization
fraction of GeTe. Using precise statistical methods we were able
to predict a function that can obtain the peak of resonance as a
function of the crystallization fraction of GeTe (Equation 2).

𝜆(m% ) = 1267.34 + 4.27m + 0.009m2 + 1.76.10−4 m3 (2)

where, m is the crystallization fraction of GeTe. This equation can

be used to modulate any resonance peak as a function of m in
that interval of 1000 to 2200 nm. Table 1 shows a summary of the
Figure 2. a) The evolution of the absorption spectrum for different crystal- theoretical parameters of the Crystallization fraction (m), the val-
lization levels. By varying the crystallization level of the GeTe the transmis- ues of the absorption peaks and the full width at half-maximum
sion peak is shifted in the analyzed spectrum and can be approximated b) (FWHM) of the proposed hybrid absorber.
by a polynomial of degree 3. The percentages shown in the legend corre- In order to calculate the absorption coefficients of the angle
spond to the GeTe crystallization levels.
insensitive Fabry Perot—FP structure we used FEM.[19] As
expected, the simulated dispersion curves exhibit that the res-
onance remains at the surface plasmon resonance wavelength
For the proposed hybrid absorber the thicknesses of the tita- over a large incident angular range for TM polarization as exhib-
nium and GeTe layers were respectively dTi = 5 nm and dGeTe = ited in Figure 3a–f. The numerical results show near one-unit
200 nm. Figure 2a shows the evolution of the absorption spec- absorption for all the analyzed phases of the normal incidence
trum as GeTe crystallization level increases for a normal inci- (𝜃 = 0°) up to the 45° oblique incidence angles, confirming the
dence angle. It is possible to observe that the first 1267 nm peak theory insensitive to the angle of PA of the type FP.[29–31] Angular
is shifted to the right with the increase in the level of insensitivity absorbers are promising for various applications,
crystallization of GeTe. The possibility of being able to control the including, photodetectors, photovoltaics, thermophotovoltaics,
resonance peak with a crystallization fraction in PCMs is pos- heat-assisted magnetic recording, hot-electron collection, bio-
sible because these materials have unique chemical bonding chemical sensing, and thermal emitting.[28]
properties, sometimes referred to as resonant or metavalent We also analyzed the resonance peak capacity of this hybrid
bonding.[17,26] Because of the strong electronic polarizabilities of planar resonator to be sintonyzedwith the thickness of the GeTe
the material in the crystalline state that produce a high value in the amorphous, semi-crystalline (m = 50%) and crystalline
of very large refractive index over a wide wavelength range, phases. Figure 4 shows the numerical results of the analysis. We
n≈6–7 (Figure 2a) in the region of the infrared electromagnetic analyze the ΔdGeTe thickness 6 nm above and below the ideal
spectrum.[27] The metavalent bond requires a long-range order value (dGeTe = 200 nm). Figure 4a–c shows a high sensitivity of
between the atoms and is therefore lost when the PCM is in the structure with this geometric parameter, we observed a shift
an amorphous state.[28] This fact causes the PCM refractive in- of the resonance peaks to smaller wavelengths when we reduced
dex in the amorphous state to return to the values of a usual the thickness of the GeTe and an opposite behavior when we in-
optical semiconductor n≈3.0–4.5 (Figure 2a).The phase shift of creased the thickness. Although there is a small decrease in ab-
the GeTe from amorphous to crystalline (or vice versa), produces sorption for lower values of dGeTe (Figure 4a,b), and in all cases
an exceptionally large modulation of the refractive index. Optical analyzed, the absorption of the hybrid resonator was above 90%.

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Figure 3. Dependence of the absorption for incident angles from 0° to 45° in the 1000 to 2200 nm range a) Amorphous Phase (m = 0%), b) m = 20%,
c) m = 40%, d) m = 60%, e) m = 80%, and f) crystalline phase (m = 100%).

The results show the high capacity to be able to tune the PA with linear and angular coefficients to tune any resonance peak with
the thickness of the PCM used (GeTe). In Figure 4d we evalu- the PCM thickness, the results are shown in Table 2. "The inter-
ated the behavior of the resonance peak with the thickness of val of variation of the peak absorption wavelengths as a function
the GeTe for the three cases Amorphous Phase (black curve), of the dielectric thickness and crystallization states are from 1240
Semi-Crystalline Phase—m = 50% (red curve) and crystalline to 1298 nm for m = 0; from 1493 to 1561 nm, for m = 50% and
phase (green curve), a linear behavior can be clearly observed in from 1909 to 2018 nm for m = 100%."
the three phases analyzed. Additionally and using mathematical To analyze the fabrication tolerance of the hybrid resonant
methods we fitted the curves and determined a function with the absorber we analyzed the behavior of the metallic thickness of

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Figure 4. Dependence of the absorption with GeTe thickness in the 1000 to 2200 nm range a) amorphous phase, b) semi-crystalline phase, and c)
crystalline phase, d) linear dependence of the peaks of a, b, and c with the thickness of the GeTe layer (dGeTe ).

Table 2. Linearization of the resonance peak of the hybrid absorber with the Ti top, the numerical results can be seen in Figure 5a–c.
the thickness of the GeTe. The results show a high absorption for all phases of GeTe from
1 to 5 nm, for higher values the absorption decreases and the
GeTe phase Function resonance peak is shifted to the right. Increasing the Ti thickness
Amorphous Phase 𝜆(da−GeTe ) = 301.93 + 4.83da−GeTe (5) results in pronounced absorption peaks and dips throughout the
Semi-Crystalline 𝜆(dm=50%−GeTe ) = 393.06 + 5.67dm=50%−GeTe (6) wavelength of interest, due to the Fabry–Perot resonant cavity.
Phase—m = 50% This fact was analyzed numerically and experimentally in.[26]
Crystalline Phase 𝜆(dc−GeTe ) = 137.13 + 9.13dc−GeTe (7) For a thicknesses greater than 10 nm thick Ti film, transmission
through Ti layer is significantly reduced, with increased reflec-
tivity therefore Fabry–Perot resonance becomes weaker and the
absorption intensity reduces.

Figure 5. Absorption map with respect to top Ti thickness (dTi ) sweep for a) amorphous phase, b) semi-crystalline phase, and c) crystalline phase

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where nGeTe is the GeTe refractive index, dGeTe the thickness of the
layer, 𝜃 the angle of incidence and m = 0, ±1, ±2…
To better understand and visualize the physical coupling
mechanism of the FP hybrid resonant absorber, we calculated
the normalized electric field (E), the normalized magnetic field
(H) and the absorbed optical power (Pabs ) at the absorption peak
of GeTe in the crystalline phase, 𝜆 = 1964 nm, Figure 7a–c. Pabs
were determined by multiplying the electrical intensity and the
imaginary part of the metal’s permittivity:[9,35]

Pabs = 𝛼.Im(𝜀).|E|2 (9)

where 𝛼 is the normalized coefficient and E is the amplitude of

the electric field calculated numerically using FEM. Figure 7a
shows the strong concentration of the E field in the dielectric re-
gion between the metal film and the substrate, due to construc-
tive interference and the formation of a stationary wave in the
region. This constructive interference was caused by the choice
of the physical and geometrical parameters of the structure for
Figure 6. Physical coupling mechanism of the hybrid resonant absorber phase compensation, resulting in total absorption at the reso-
FP, incorporating a phase compensation overlap, 𝜑1 and 𝜑2 are the refec- nant peak. Figure 7b shows the spatial distribution of the nor-
tion phase shift at the two metal–spacer interfaces, respectively malized magnetic field H, in this case it is possible to observe a
strong intensity at the metal/GeTe interface. This fact can be at-
Figure 6 shows the illustration of the physical coupling mech- tributed to the excitation of surface plasmons polaritons that oc-
anism of the hybrid resonant absorber FP, according to[32] when cur for TM polarization. Due to the enhanced electric field result-
the physical and geometric parameters of the structure are sat- ing from the cavity effect, most of the optical power is absorbed
isfied, the aggregate phase shift in the metal / dielectric inter- inside the top and bottommetallic film (Figure 7c). Spatial distri-
faces (𝜑ref 1 + 𝜑ref 2 ) is canceled with the phase shift in the di- bution of the absorption reveals that absorption is higher at the
rection perpendicular to the interfaces in the dielectric region metal−dielectric interfaces.[35] It is rather interesting to achieve
𝜑⊥ = 4𝜋nGeTe dGeTe cos 𝜃∕𝜆. almost perfect absorption using continuous metallic films.
When using a thick metallic substrate,[33] the transmission will
be null and the perfect condition of constructive interference be-
tween the incident wave and the reflected wave is given by Equa- 3. Conclusion
tion 8.[26,34]
In conclusion, a hybrid resonant absorber FP was demon-
𝜑ref 1 + 𝜑ref 2 + 4𝜋nGeTe dGeTe cos 𝜃∕𝜆 = 2m𝜋 (8) strated using a resonator tres layers MDM with a PCM (GeTe)

Figure 7. Spatial distribution of the a) normalized Electric field, b) normalized magnetic field, and c) absorbed power for the hybrid resonant absorber
FP at the peak absorption wavelength of 1964 nm of the crystalline phase.

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sandwiched between Ti films and substrate Ag. We have

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