of 9
Citation:
Jeon, T.; Myung, J.; Choi, C.;
Shayegan, K.; Lewis, S.M.; Scherer, A.
Novel Deposition Method of
Crosslinked Polyethylene Thin Film
for Low-Refractive-Index
Mid-Infrared Optical Coatings.
Sensors
2023
,
23
, 9810. https://
doi.org/10.3390/s23249810
Academic Editors: Wilmar
Hernandez, Guillermo Villanueva,
Sabina Merlo, Faisal Mohd-Yasin,
Aldo Minardo, Nunzio Cennamo,
Cosimo Trono and Andreas Savakis
Received: 19 October 2023
Revised: 6 December 2023
Accepted: 12 December 2023
Published: 14 December 2023
Copyright:
© 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
sensors
Communication
Novel Deposition Method of Crosslinked Polyethylene Thin
Film for Low-Refractive-Index Mid-Infrared Optical Coatings
Taeyoon Jeon *
, Jieun Myung, Changsoon Choi
, Komron Shayegan
, Scott M. Lewis and Axel Scherer
Applied Physics and Materials Science, California Institute of Technology, 1200 East California Boulevard,
MC 200-79, Pasadena, CA 91125, USA
*
Correspondence: jty209@caltech.edu
Abstract:
Mid-infrared optics require optical coatings composed of high- and low-refractive-index
dielectric layers for the design of optical mirrors, filters, and anti-reflection coatings. However, there
are not many technologies for depositing a material with a refractive index of less than 2 and a
low loss in the mid-infrared region. Here, we present a unique deposition method of crosslinked
polyethylene thin film for mid-IR optical filter design. Polyethylene has a refractive index of 1.52 in the
mid-infrared region and a small number of absorption peaks, so it is useful for making optical filters
in the mid-infrared region. Only 1 keV of energy is required to crosslink the entire film by irradiating
an electron beam while depositing polyethylene. In addition, crosslinked polyethylene thin film has
high mechanical strength, so there is no cracking or peeling when used with germanium. This allows
for the use of crosslinked polyethylene as a low refractive index for mid-infrared optical coating.
Keywords:
mid-infrared optical coating; polyethylene; crosslinking; low-refractive-index materials
1. Introduction
The mid-infrared (mid-IR) region is a molecular fingerprint region which is widely
used for molecular detection [
1
]. In this mid-IR region, various optical devices such as
Fourier transform infrared spectroscopy (FTIR) [
2
] and quantum cascade laser (QCL) [
3
]
are used for molecular detection. In addition, there have been studies to miniaturize
molecular sensors in the mid-infrared region using resonators such as the Fabry–Perot
filter [
4
], frequency comb [
5
,
6
], and metasurface [
7
,
8
]. One of the major limitations in
designing mid-IR optical systems is that there are few low-refractive-index materials with
low absorption in the mid-IR region. The commonly used high-refractive-index materials
in the mid-infrared region are germanium and silicon [
9
], and the refractive indices of each
material are about 4 and 3.4, respectively [
10
,
11
]. However, low-refractive-index materials
used in the mid-infrared region, such as zinc sulfide (ZnS) [
12
] and zinc selenide (ZnSe) [
13
],
have a refractive index greater than 2 in the mid-infrared region, making it difficult to
maximize the difference in refractive index when making an optical device. This results in a
narrow bandwidth of the optical filter, and many layers need to be deposited to improve the
refractive index contrast in the optical filter design [
14
]. Recently, studies on chalcogenide
glasses have been extensively researched to find other infrared transparent alternative
materials due to its transparency in the infrared region. However, these materials are not
easy to prepare, and the refractive index of the chalcogenide glasses is also over 2 [
15
,
16
].
A fluoride material such as calcium fluoride (CaF
2
) can be used as a substrate for a low-
refractive-index material [
17
]. However, in order to make an optical filter, a technique for
depositing as a thin film is required. In addition, since most fluoride materials such as
CaF
2
, yttrium fluoride (YF
3
), and cerium fluoride (CeF
3
) begin to absorb light over 10
μ
m,
there is a limit to use them over 10
μ
m region [18].
Here, we propose the use of polyethylene (PE) as a low-refractive-index material
in the mid-IR region because PE has low absorption in the mid-infrared region and a
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https://www.mdpi.com/journal/sensors
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refractive index of around 1.53 [
19
]. However, in order to manufacture an optical filter, it
is essential to deposit a robust PE thin film. There have been studies on the preparation
of a polyethylene thin film using a solution process and thermal evaporation by using
a vacuum chamber. However, the solution process (spin-coating) has the problem that
the sample and solution must be heated at a high temperature (180
C) to dissolve the
PE due to the low solubility of PE in the solvent at room temperature, and the resulting
film has some voids in the film [
20
]. This is not sufficient for optical filter design. In
addition, thermal evaporation of the polymer usually decomposes the polymer during
heating, resulting in low mechanical stability of the deposited film [
21
]. Research on
crosslinking in polyethylene using electron beam irradiation to enhance the mechanical
stability of polyethylene film has been conducted [
22
,
23
]. Here, we incorporated electron
beam irradiation into the process of evaporating polyethylene via thermal evaporation,
aiming to achieve both highly controllable thickness of smooth polyethylene film and
improve the mechanical stability of the deposited film.
We developed the thin film deposition method of crosslinked polyethylene (XPE) by
using an electron beam-assisted thermal evaporation system. By irradiating electrons while
depositing PE, it is possible to deposit XPE thin film that is mechanically stable and has a
low refractive index in the mid-infrared range. Polyethylene is one of the polymers with
the fewest number of absorption peaks in the mid-infrared region (Figure 1) because of its
simple chemical structure [
24
]. In the mid-IR range (from 5
μ
m to 20
μ
m or from 2000 cm
1
to 500 cm
1
), PE has an absorption peak of 1466 cm
1
(6.82
μ
m) and 723 cm
1
(13.83
μ
m).
The peak at 1466 cm
1
is C–H deformation vibrations in –(CH
2
)
n
–, and the peak at 723 cm
1
is from C–C rocking vibrations in –(CH
2
)
n
– [
25
]. These peak positions have little overlap in
the long-wave infrared (LWIR) range, which is 7.5–14
μ
m (wavelength scale) or 1333.3 cm
1
to 714.3 cm
1
(wavenumber scale). This LWIR range is the area outside the absorption
peaks of moisture in the air, and it is the bandpass filter transmittance range used in mid-IR
optical devices such as thermal imaging cameras [
26
]. Therefore, when polyethylene is
used in this LWIR region, the absorption peaks of polyethylene hardly overlap with the
LWIR region, so it is possible to manufacture an optical device with polyethylene in this
region. The refractive index of PE in the mid-infrared region is around 1.53, which is
much smaller than the conventional low-refractive-index materials (ZnS and ZnSe). The
extinction coefficient value of the PE film in the transparent region is on the order of 10
3
,
and at the peak position, the extinction coefficient value is on the order of 10
2
[19].
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Here, we propose the use of polyethylene (P
E) as a low-refractive-index material in
the mid-IR region because PE has low absorption in the mid-infrared region and a refrac-
tive index of around 1.53 [19]. Howeve
r, in order to manufacture an optical
fi
lter, it is
essential to deposit a robust PE thin
fi
lm. There have been studies on the preparation of a
polyethylene thin
fi
lm using a solution process and thermal evaporation by using a vac-
uum chamber. However, the solution process
(spin-coating) has the problem that the sam-
ple and solution must be heated at a high te
mperature (180 °C) to dissolve the PE due to
the low solubility of PE in the solvent at room temperature, and the resulting
fi
lm has
some voids in the
fi
lm [20]. This is not su
ffi
cient for optical
fi
lter design. In addition, ther-
mal evaporation of the polymer usually decomposes the polymer during heating, result-
ing in low mechanical stability of the deposited
fi
lm [21]. Research on crosslinking in pol-
yethylene using electron beam irradiation to
enhance the mechanical
stability of polyeth-
ylene
fi
lm has been conducted [22,23]. Here, we incorporated electron beam irradiation
into the process of evaporating polyethylene
via thermal evaporation, aiming to achieve
both highly controllable thickness of smooth polyethylene
fi
lm and improve the mechan-
ical stability of the deposited
fi
lm.
We developed the thin
fi
lm deposition method of crosslinked polyethylene (XPE) by
using an electron beam-assisted thermal ev
aporation system. By irradiating electrons
while depositing PE, it is possible to deposit XPE thin
fi
lm that is mechanically stable and
has a low refractive index in the mid-infrared range. Polyethylene is one of the polymers
with the fewest number of absorption peaks
in the mid-infrared region (Figure 1) because
of its simple chemical structure [24]. In th
e mid-IR range (from 5 μm to 20 μm or from
2000 cm
1
to 500 cm
1
), PE has an absorption peak of 1466 cm
1
(6.82 μm) and 723 cm
1
(13.83 μm). The peak at 1466 cm
1
is C–H deformation vibrations in –(CH
2
)
n
–, and the peak
at 723 cm
1
is from C–C rocking vibrations in –(CH
2
)
n
– [25]. These peak positions have
li
tt
le overlap in the long-wave infrared (LWIR) range, which is 7.5–14 μm (wavelength
scale) or 1333.3 cm
1
to 714.3 cm
1
(wavenumber scale). This LWIR range is the area outside
the absorption peaks of moisture in
the air, and it is the bandpass
fi
lter transmi
tt
ance range
used in mid-IR optical devices such as th
ermal imaging cameras [26]. Therefore, when
polyethylene is used in this LWIR region, the absorption peaks of polyethylene hardly
overlap with the LWIR region, so it is possible to manufacture an optical device with pol-
yethylene in this region. The refractive index of PE in the mid-infrared region is around
1.53, which is much smaller than the conventional low-refractive-index materials (ZnS and
ZnSe). The extinction coe
ffi
cient value of the PE
fi
lm in the transparent region is on the
order of 10
3
, and at the peak position, the extinction coe
ffi
cient value is on the order of
10
2
[19].
Figure 1.
Transmi
tt
ance spectrum of PE in the mid-IR re
gion. The absorption peaks are at 1466 cm
1
and 723 cm
1
. The transparent region of PE in the mid-IR
is indicated by green arrows, and the LWIR
region is indicate
d by red arrows.
2. Deposition Method of XPE Thin Film
Figure 1.
Transmittance spectrum of PE in the mid-IR region. The absorption peaks are at 1466 cm
1
and 723 cm
1
. The transparent region of PE in the mid-IR is indicated by green arrows, and the LWIR
region is indicated by red arrows.
2. Deposition Method of XPE Thin Film
The XPE thin film deposition system is designed as shown in Figure 2A. PE molecules
are heated in a tungsten boat, evaporated, and deposited on the sample substrate. We
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purchased polyethylene raw material from Sigma-Aldrich, and its molecular weight is
about 4000. To evaporate the polyethylene, the boat temperature should be increased up to
350
C. The deposition rate is 1 nm/s. An accelerated electron beam is irradiated to the
substrate for the crosslinking of the deposited PE molecules. An electron beam is generated
in a tungsten coil heated via an applied current. This thermionic emission enables electron
emission by adding thermal energy to the electron to overcome the work function of the
tungsten [
27
]. Then, generated electrons are accelerated by an electric potential (1 kV). A
sample wafer is placed on the surface of a positively charged electrode, and an insulator is
placed between the electrode and the sample holder, which can accelerate the electron beam
to the sample surface. With this technique, PE can be crosslinked at the same time as PE
molecules are deposited. In order to check whether the deposited PE film is crosslinked, the
PE film deposited on the silicon wafer was immersed in a heated toluene solution. The PE
film deposited without e-beam radiation is soluble in toluene, as shown in Figure 2B, but
PE film deposited with e-beam radiation is not soluble in toluene, as shown in Figure 2C,
due to crosslinking of PE. To measure the crosslinking density more precisely, the thickness
of crosslinked PE film was measured using an interferometry method, and the thickness
was checked after dipping in the toluene for 30 min. The interferometry measurement
showed the same thickness of film after dipping it
©
n the toluene, indicating that it did not
dissolve and polyethylene film completely transformed into a gel form (Figure S1). This
crosslinking can also increase the mechanical properties of film. By depositing a thin film
of Ge on uncross-linked PE and XPE films, the mechanical properties of the films can be
checked. As can be seen in Figure 2D, the deposition of germanium on an uncross-linked
PE film causes a lot of cracking in the film. This is because the uncross-linked PE film
cannot withstand the internal stress of the film. On the other hand, when Ge is deposited
on the XPE film, cracks do not occur in the film.
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The XPE thin
fi
lm deposition system is designed as shown in Figure 2A. PE molecules
are heated in a tungsten boat, evaporated,
and deposited on the sample substrate. We
purchased polyethylene raw material from Sigma-Aldrich, and its molecular weight is
about 4000. To evaporate the polyethylene, th
e boat temperature should be increased up
to 350-degree C. The deposition rate is 1 nm/s
. An accelerated electron beam is irradiated
to the substrate for the crosslinking of the deposited PE molecules. An electron beam is
generated in a tungsten coil heated via an a
pplied current. This ther
mionic emission ena-
bles electron emission by adding thermal en
ergy to the electron to overcome the work
function of the tungsten [27]. Then, generated
electrons are accelerated by an electric po-
tential (1 kV). A sample wafer is placed on the surface of a positively charged electrode,
and an insulator is placed between the electrode and the sample holder, which can accel-
erate the electron beam to the sample surface.
With this technique, PE can be crosslinked
at the same time as PE mole
cules are deposited. In order to check whether the deposited
PE
fi
lm is crosslinked, the PE
fi
lm deposited on the silicon wafer was immersed in a heated
toluene solution. The PE
fi
lm deposited without e-beam radiat
ion is soluble in toluene, as
shown in Figure 2B, but PE
fi
lm deposited with e-beam radiation is not soluble in toluene,
as shown in Figure 2C, due to crosslinking
of PE. To measure the crosslinking density
more precisely, the thickness of crosslinked PE
fi
lm was measured using an interferome-
try method, and the thickness was checked after dipping in the toluene for 30 min. The
interferometry measurement showed the same thickness of
fi
lm after dipping it ©n the
toluene, indicating that it did not dissolve and polyethylene
fi
lm completely transformed
into a gel form (Figure S1). This crosslinking can also increase the mechanical properties
of
fi
lm. By depositing a thin
fi
lm of Ge on uncross-linked PE and XPE
fi
lms, the mechani-
cal properties of the
fi
lms can be checked. As can be seen in Figure 2D, the deposition of
germanium on an uncross-linked PE
fi
lm causes a lot of cracking in the
fi
lm. This is be-
cause the uncross-linked PE
fi
lm cannot withstand the internal stress of the
fi
lm. On the
other hand, when Ge is deposited on the XPE
fi
lm, cracks do not occur in the
fi
lm.
Figure 2.
(
A
) Schematic showing a vacuum chamber for depositing crosslinked polyethylene thin
fi
lms. Electrons generated in the tu
ngsten coil are accelerated to the sample wafer due to the electric
potential. Polyethylene monomers are crosslinked as
soon as they are deposited on the sample by
accelerated electrons. (
B
) Digital camera image of PE thin
fi
lm. On the left is a PE thin
fi
lm on a
silicon wafer that has not been immersed in toluen
e. The image on the right is after immersion in
toluene. (
C
) Digital camera image of the XPE thin
fi
lm. On the left is a PE thin
fi
lm on a silicon wafer
that has not been immersed in toluene. The imag
e on the right is after immersion in toluene. (
D
)
Digital camera image when Ge was deposited on PE
(left) and XPE (right). When Ge is deposited
on PE, cracks occur, whereas, in the ca
se of XPE, there is no crack on the
fi
lm.
The transmi
tt
ance of the XPE thin
fi
lm is shown in Figure 3. The spectrum exhibits
an absorption peak at 1720 cm
1
area, which corresponds to the C=O stretch bond. This is
Figure 2.
(
A
) Schematic showing a vacuum chamber for depositing crosslinked polyethylene thin
films. Electrons generated in the tungsten coil are accelerated to the sample wafer due to the electric
potential. Polyethylene monomers are crosslinked as soon as they are deposited on the sample by
accelerated electrons. (
B
) Digital camera image of PE thin film. On the left is a PE thin film on a
silicon wafer that has not been immersed in toluene. The image on the right is after immersion in
toluene. (
C
) Digital camera image of the XPE thin film. On the left is a PE thin film on a silicon
wafer that has not been immersed in toluene. The image on the right is after immersion in toluene.
(
D
) Digital camera image when Ge was deposited on PE (left) and XPE (right). When Ge is deposited
on PE, cracks occur, whereas, in the case of XPE, there is no crack on the film.
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The transmittance of the XPE thin film is shown in Figure 3. The spectrum exhibits an
absorption peak at 1720 cm
1
area, which corresponds to the C=O stretch bond. This is
due to the outgassing of oxygen during the thermal evaporation of polyethylene inside the
vacuum chamber and can be solved by performing a degassing process before deposition.
The energy dispersive x-ray spectroscopy (EDX) measurements for both the raw material
and evaporated PE film are presented in Figure S2. The results indicate that the raw
material is 100% carbon, while the evaporated XPE contains oxygen. Nevertheless, the XPE
remains transparent in the LWIR region.
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due to the outgassing of oxygen during the
thermal evaporation of polyethylene inside
the vacuum chamber and can be solved by pe
rforming a degassing process before depo-
sition. The energy dispersive x-ray spectros
copy (EDX) measurements for both the raw
material and evaporated PE
fi
lm are presented in Figure S2. The results indicate that the
raw material is 100% carbon, while the evaporated XPE contains oxygen. Nevertheless,
the XPE remains transparent in the LWIR region.
Figure 3.
Transmi
tt
ance spectrum of XPE in the mid-IR re
gion shows an absorption peak around
1720 cm
1
area. This peak corresponds to the C=O stretch bond, and it is a
tt
ributed to the outgassing
of oxygen during the evaporation. The XPE remains transparent in the LWIR region.
3. Monte Carlo Simulation
The electron beam penetration depth ca
n be calculated using the CASINO Monte-
Carlo program [28]. The 200-electron trajectory with 1 keV energy inside the PE
fi
lm is
shown in Figure 4A. The blue trajectories show absorbed energy inside of PE
fi
lm, while
the red trajectory shows backsca
tt
ered electrons. The probability of backsca
tt
ering is about
20%. To calculate the absorbed
energy distribution according to the PE polymer depth,
100,000 electrons with 1 keV are irradiated to the PE
fi
lm in the simulation. The absorbed
energy density along the penetration depth from
the average of 100,000 electrons’ trajec-
tory is shown in Figure 4B. The integration of all data along the depth of the PE
fi
lm is
about 0.8 keV, meaning that 80% of the irradiated electrons are absorbed, and 20% are
backsca
tt
ered. The peak position of the electron
absorption energy density is around 21.1
nm in depth, and the absorption energy is almo
st zero in the 50 nm depth in the case of 1
keV electrons. Since the deposition rate of PE molecules in our system is 1 nm/s and PE
molecules are exposed to an electron beam as
soon as they are deposited on the substrate,
this 1 keV is su
ffi
cient to crosslink the
fi
lm. This is a huge advantage in terms of the re-
quired electron beam energy and
crosslinking uniformity of the
fi
lm. For example, irradi-
ating an electron beam after
fi
nishing PE
fi
lm deposition requires 10 keV to penetrate a 1
μm thick PE polymer, as shown orange curve in Figure 4B, and this required energy will
be much bigger for deeper penetration. Compar
ed to this energy, a system with simulta-
neous deposition and irradiation requires mu
ch less electron energy because PE deposi-
tion and crosslinking are performed simultaneously. Also, this results in uniform cross-
linking density along the
fi
lm depth.
Figure 3.
Transmittance spectrum of XPE in the mid-IR region shows an absorption peak around
1720 cm
1
area. This peak corresponds to the C=O stretch bond, and it is attributed to the outgassing
of oxygen during the evaporation. The XPE remains transparent in the LWIR region.
3. Monte Carlo Simulation
The electron beam penetration depth can be calculated using the CASINO Monte-
Carlo program [
28
]. The 200-electron trajectory with 1 keV energy inside the PE film is
shown in Figure 4A. The blue trajectories show absorbed energy inside of PE film, while
the red trajectory shows backscattered electrons. The probability of backscattering is
about 20%. To calculate the absorbed energy distribution according to the PE polymer
depth, 100,000 electrons with 1 keV are irradiated to the PE film in the simulation. The
absorbed energy density along the penetration depth from the average of 100,000 electrons’
trajectory is shown in Figure 4B. The integration of all data along the depth of the PE
film is about 0.8 keV, meaning that 80% of the irradiated electrons are absorbed, and 20%
are backscattered. The peak position of the electron absorption energy density is around
21.1 nm in depth, and the absorption energy is almost zero in the 50 nm depth in the case
of 1 keV electrons. Since the deposition rate of PE molecules in our system is 1 nm/s
and PE molecules are exposed to an electron beam as soon as they are deposited on the
substrate, this 1 keV is sufficient to crosslink the film. This is a huge advantage in terms of
the required electron beam energy and crosslinking uniformity of the film. For example,
irradiating an electron beam after finishing PE film deposition requires 10 keV to penetrate
a 1
μ
m thick PE polymer, as shown orange curve in Figure 4B, and this required energy
will be much bigger for deeper penetration. Compared to this energy, a system with
simultaneous deposition and irradiation requires much less electron energy because PE
deposition and crosslinking are performed simultaneously. Also, this results in uniform
crosslinking density along the film depth.
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Figure 4.
(
A
) Monte-Carlo simulation of trajectories of 200 electrons with 1 keV energy in a polyeth-
ylene layer. Blue trajectories show absorbed
electrons, while red ones indicate backsca
tt
ered elec-
trons. (
B
) Absorbed energy density at each depth in a polyethylene layer calculated by using 100,000
number of electrons. The absorbed energy density
curves have their maxima at 21.1 nm for 1 keV
electrons and 1.35 μm for 10 keV electrons, respectively.
4. Mid-IR Optical Filter Fabrication
A mid-IR optical
fi
lter can be fabricated using XPE thin
fi
lm deposition technology.
The overall design of the fabricated optical
fi
lter is shown in Figure 5A. The optical
fi
lter
has a structure in which two Distributed Bragg re
fl
ectors (DBR) face each other with a
spacer thickness apart. The spacer thickne
ss determines the cavity thickness of the
fi
lter
and the resonance frequency of the
fi
lter. The DBR is composed of Ge (as a high-refractive-
index material) and XPE (as a low-refractive-i
ndex material). The target wavelength of
DBR is 9 μm, so the thickness of each layer is 9/4n, where
n
is the refractive index of each
layer. So, the thickness of Ge is about 560 nm
, and XPE is about 1480 nm. A cross-sectional
view of one deposited DBR is shown in Figure 5B, and the transmi
tt
ance spectrum of one
DBR is shown in Figure 5C. The bandwidth of DBR was designed to target the LWIR
range. To create a Fabry–Perot
fi
lter, two DBRs are combined, creating a cavity between
them. For an optical
fi
lter, a thin
fi
lm of polydimethylsiloxane (PDMS) is placed between
the two DBRs to form an air cavity. The transmi
tt
ance spectrum of the optical
fi
lter is
shown in Figure 5D. The resonance peak is at 1085 cm
1
(9.2 μm). The resonant wavelength
of light is enhanced in the cavity region (Figure 5E). This
fi
lter has a huge advantage over
other low-index materials as it can produce sharp transmission peaks with only four pairs
of layers due to the large refractive index contrast.
Figure 4.
(
A
) Monte-Carlo simulation of trajectories of 200 electrons with 1 keV energy in a polyethy-
lene layer. Blue trajectories show absorbed electrons, while red ones indicate backscattered electrons.
(
B
) Absorbed energy density at each depth in a polyethylene layer calculated by using 100,000 number
of electrons. The absorbed energy density curves have their maxima at 21.1 nm for 1 keV electrons
and 1.35
μ
m for 10 keV electrons, respectively.
4. Mid-IR Optical Filter Fabrication
A mid-IR optical filter can be fabricated using XPE thin film deposition technology.
The overall design of the fabricated optical filter is shown in Figure 5A. The optical filter has
a structure in which two Distributed Bragg reflectors (DBR) face each other with a spacer
thickness apart. The spacer thickness determines the cavity thickness of the filter and the
resonance frequency of the filter. The DBR is composed of Ge (as a high-refractive-index
material) and XPE (as a low-refractive-index material). The target wavelength of DBR is
9
μ
m, so the thickness of each layer is 9/4n, where
n
is the refractive index of each layer.
So, the thickness of Ge is about 560 nm, and XPE is about 1480 nm. A cross-sectional view
of one deposited DBR is shown in Figure 5B, and the transmittance spectrum of one DBR is
shown in Figure 5C. The bandwidth of DBR was designed to target the LWIR range. To
create a Fabry–Perot filter, two DBRs are combined, creating a cavity between them. For
an optical filter, a thin film of polydimethylsiloxane (PDMS) is placed between the two
DBRs to form an air cavity. The transmittance spectrum of the optical filter is shown in
Figure 5D. The resonance peak is at 1085 cm
1
(9.2
μ
m). The resonant wavelength of light
is enhanced in the cavity region (Figure 5E). This filter has a huge advantage over other
low-index materials as it can produce sharp transmission peaks with only four pairs of
layers due to the large refractive index contrast.
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Figure 5.
(
A
) Schematic diagram showing the design of a mid-IR optical
fi
lter composed of two
DBRs made of Ge and XPE. (
B
) SEM image of a DBR consisting of two pairs of Ge and XPE layers.
(
C
) Transmission spectrum of the Ge/XPE mirror in (
B
). (
D
) Transmission spectra of a Ge/XPE
fi
lter
when two DBRs are combined with an ai
r cavity in between. Inset is a magni
fi
ed spectrum at trans-
mi
tt
ance peak position. (
E
) Calculated electric
fi
eld intensity distribution inside the cavity when
resonant wavelength light passes through the
fi
lter.
5. Discussion
Because we developed a thin
fi
lm deposition method for robust polyethylene suitable
for mid-IR optics, it can be utilized as a low-refractive-index material in mid-IR optical
fi
lms. When compared to conventional mid-IR
low-refractive-index materials, polyeth-
ylene o
ff
ers distinct advantages.
One advantage is its low refractive index. In comparison to other commonly used
low-refractive-index materials in the mid-IR region, such as ZnS and ZnSe, which have
refractive indices over 2, polyethylene possesses a refractive index of about 1.53 in the
mid-IR region.
The second advantage is that polyethylene
is transparent at wavelengths exceeding
15 μm. This is a signi
fi
cant bene
fi
t when contrasted with other
fl
uoride-based materials
like CaF
2
, as many
fl
uoride materials begin to absorb light beyond 12 and 13 μm. There-
fore, if a refractive index lower than 2 is re
quired over the 15 μm wavelength region, pol-
yethylene can be an ideal material. The refractive indices of conventional low-refractive-
index materials are compared with polyethylene, along with their transparent regions, in
Ta b l e 1 .
Ta b l e 1.
Comparison of refractive index and transpar
ent region between polyethylene and conven-
tional low-refractive-index materials at mid-IR re
gion. The refractive index values of ZnS, ZnSe,
Chalcogenide glass, and CaF
2
are sourced from references [12,13,16,17] respectively.
Materials
Refractive index at 10 μm
Transparent on between 5 μm to 20 μm
Polyethylene 1.53
Transparent up to 20 μm, except for two
peaks positioned near 6.8 μm and 13.8 μm
Chalcogenide
Glass
Over 2.5 (change depend-
ing on the composition)
Transparent up to 20 μm
ZnS
2.2
Transparent up to 14 μm
ZnSe 2.4
Transparent up to 14 μm and transmi
tt
ance
gradually decrease up to 20 μm
Figure 5.
(
A
) Schematic diagram showing the design of a mid-IR optical filter composed of two
DBRs made of Ge and XPE. (
B
) SEM image of a DBR consisting of two pairs of Ge and XPE layers.
(
C
) Transmission spectrum of the Ge/XPE mirror in (
B
). (
D
) Transmission spectra of a Ge/XPE
filter when two DBRs are combined with an air cavity in between. Inset is a magnified spectrum at
transmittance peak position. (
E
) Calculated electric field intensity distribution inside the cavity when
resonant wavelength light passes through the filter.
5. Discussion
Because we developed a thin film deposition method for robust polyethylene suitable
for mid-IR optics, it can be utilized as a low-refractive-index material in mid-IR optical
films. When compared to conventional mid-IR low-refractive-index materials, polyethylene
offers distinct advantages.
One advantage is its low refractive index. In comparison to other commonly used
low-refractive-index materials in the mid-IR region, such as ZnS and ZnSe, which have
refractive indices over 2, polyethylene possesses a refractive index of about 1.53 in the
mid-IR region.
The second advantage is that polyethylene is transparent at wavelengths exceeding
15
μ
m. This is a significant benefit when contrasted with other fluoride-based materials like
CaF
2
, as many fluoride materials begin to absorb light beyond 12 and 13
μ
m. Therefore, if
a refractive index lower than 2 is required over the 15
μ
m wavelength region, polyethy-
lene can be an ideal material. The refractive indices of conventional low-refractive-index
materials are compared with polyethylene, along with their transparent regions, in Table 1.
Table 1.
Comparison of refractive index and transparent region between polyethylene and conven-
tional low-refractive-index materials at mid-IR region. The refractive index values of ZnS, ZnSe,
Chalcogenide glass, and CaF
2
are sourced from references [12,13,16,17] respectively.
Materials
Refractive Index at 10
μ
m
Transparent on between 5
μ
m to 20
μ
m
Polyethylene
1.53
Transparent up to 20
μ
m, except for two peaks
positioned near 6.8
μ
m and 13.8
μ
m
Chalcogenide Glass
Over 2.5 (change depending on the
composition)
Transparent up to 20
μ
m
ZnS
2.2
Transparent up to 14
μ
m
ZnSe
2.4
Transparent up to 14
μ
m and transmittance gradually
decrease up to 20
μ
m
CaF
2
1.3
Transparent up to 8
μ
m and transmittance gradually
decrease and become zero at about 12
μ
m
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2023
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This low refractive index gives XPE a significant advantage in terms of wide bandwidth
when employed in DBR designs. Furthermore, fewer layers are needed to achieve low
transmittance (or high reflectance) for the mirror.
To verify this effect, the transmission spectra of DBR using two different low-index
materials, XPE and ZnSe, are calculated. The target wavelength of DBR is 9
μ
m. The thick-
ness of each layer is determined by 9/4
n
, where
n
is the refractive index of the respective
material. For germanium (Ge),
n
is 4; for XPE, it is 1.53; and for ZnSe, it is 2.4. As shown in
Figure 6A,B, the bandwidth at the half-maximum of transmittance spans from 657 cm
1
(15.2
μ
m) to 1565 cm
1
(6.4
μ
m) for the two pairs of Ge/XPE. The transmittance value at
the center wavelength (9
μ
m) is 2.4%. However, the center wavelength transmittance for
the two pairs of Ge/ZnSe is 13.9% (Figure 6C,D), which is substantially higher than the
two pairs of Ge/XPE.
Sensors
2023
,
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, x FOR PEER REVIEW
7 of 9
CaF
2
1.3
Transparent up to 8 μm and transmi
tt
ance
gradually decrease and become zero at
about 12 μm
This low refractive index gives XPE a signi
fi
cant advantage in terms of wide band-
width when employed in DBR designs. Furthe
rmore, fewer layers are needed to achieve
low transmi
tt
ance (or high re
fl
ectance) for the mirror.
To verify this e
ff
ect, the transmission spectra of DBR using two di
ff
erent low-index
materials, XPE and ZnSe, are calculated. The ta
rget wavelength of DBR is 9 μm. The thick-
ness of each layer is determined by 9/4
n
, where
n
is the refractive index of the respective
material. For germanium (Ge),
n
is 4; for XPE, it is 1.53; and for ZnSe, it is 2.4. As shown
in Figure 6A,B, the bandwidth
at the half-maximum of transmi
tt
ance spans from 657 cm
1
(15.2 μm) to 1565 cm
1
(6.4 μm) for the two pairs of Ge/XPE. The transmi
tt
ance value at
the center wavelength (9 μm) is 2.4%. However, the center wavelength transmi
tt
ance for
the two pairs of Ge/ZnSe is 13.9% (Figure 6C,D
), which is substantially higher than the
two pairs of Ge/XPE.
To achieve a transmi
tt
ance comparable to two pairs of Ge/XPE (about 2%), four pairs
of Ge/ZnSe are required, as demonstrated in
Figure 6E,F. The bandwidth at the half-max-
imum of transmi
tt
ance for these four pairs of Ge/ZnSe ranges from 866 cm
1
(11.5 μm) to
1356 cm
1
(7.4 μm), and transmi
tt
ance at the center wavelength is 1.9%. Therefore, by using
XPE for low-refractive-index materials instead of using ZnSe, the number of multilayers
needed to achieve approximately 98% re
fl
ectance at the center wavelength is halved, and
the free spectral range is considerably expanded.
Figure 6.
(
A
,
C
,
E
) Schematic diagrams of tw
o pairs of Ge/XPE DBR (
A
), two pairs of Ge/ZnSe (
C
),
and four pairs of Ge/ZnSe (
E
). (
B
,
D
,
F
) Calculated transmi
tt
ance spectra of two pairs of Ge/XPE DBR
(
B
), two pairs of Ge/ZnSe (
D
), and four pairs of Ge/ZnSe (
F
). Red arrows in (
B
,
F
) indicate the band-
width of the free spectral range.
6. Conclusions
This
fi
rst result using XPE as mid-IR optics
shows that polymers can be used to de-
sign optical
fi
lters in the mid-IR region. Electron be
am irradiation during the evaporation
of PE molecules allowed the preparation of crosslinked PE thin
fi
lms with high mechani-
cal stability and high smoothness. Because the
e-beam irradiates the PE molecule as it
deposits onto the substrate, the resulting crosslinking density can be uniform throughout
the entire
fi
lm. And it has been shown that this XPE
fi
lm deposition technique can be used
Figure 6.
(
A
,
C
,
E
) Schematic diagrams of two pairs of Ge/XPE DBR (
A
), two pairs of Ge/ZnSe (
C
),
and four pairs of Ge/ZnSe (
E
). (
B
,
D
,
F
) Calculated transmittance spectra of two pairs of Ge/XPE
DBR (
B
), two pairs of Ge/ZnSe (
D
), and four pairs of Ge/ZnSe (
F
). Red arrows in (
B
,
F
) indicate the
bandwidth of the free spectral range.
To achieve a transmittance comparable to two pairs of Ge/XPE (about 2%), four
pairs of Ge/ZnSe are required, as demonstrated in Figure 6E,F. The bandwidth at the half-
maximum of transmittance for these four pairs of Ge/ZnSe ranges from 866 cm
1
(11.5
μ
m)
to 1356 cm
1
(7.4
μ
m), and transmittance at the center wavelength is 1.9%. Therefore,
by using XPE for low-refractive-index materials instead of using ZnSe, the number of
multilayers needed to achieve approximately 98% reflectance at the center wavelength is
halved, and the free spectral range is considerably expanded.
6. Conclusions
This first result using XPE as mid-IR optics shows that polymers can be used to design
optical filters in the mid-IR region. Electron beam irradiation during the evaporation of
PE molecules allowed the preparation of crosslinked PE thin films with high mechanical
stability and high smoothness. Because the e-beam irradiates the PE molecule as it deposits
onto the substrate, the resulting crosslinking density can be uniform throughout the entire
film. And it has been shown that this XPE film deposition technique can be used to
fabricate a mid-IR optical filter. Compared to other low-index materials used in mid-
infrared optics, the refractive index of XPE is much lower, so the bandwidth is wider, and
fewer layers are required to design high-Q filters. This technique shows the potential