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Multilayer Graphene Broadband Terahertz Modulators withFlexible SubstrateDOI:10.1007/s10762-018-0480-8
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Citation for published version (APA):Kaya, E., Kakenov, N., Altan, H., Kocabas, C., & Esenturk, O. (2018). Multilayer Graphene Broadband TerahertzModulators with Flexible Substrate. Journal of Infrared, Millimeter, and Terahertz Waves, 39(5), 483-491.https://doi.org/10.1007/s10762-018-0480-8
Published in:Journal of Infrared, Millimeter, and Terahertz Waves
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Download date:10. Dec. 2020
Broadband THz Modulators Based on Multilayer Graphene
Emine Kaya,1 Nurbek Kakenov,2 Hakan Altan,3 Coskun Kocabas,2,4 and Okan Esenturk1,a) 1 Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
2 Department of Physics, Bilkent University, 06800 Ankara, Turkey 3 Department of Physics, Middle East Technical University, 06531 Ankara, Turkey
4UNAM-National Nanotechnology Research Center, Bilkent University, 06800 Ankara, Turkey
ABSTRACT
THz modulators are key components for the improvement of THz technology. However, it has been proved to be
challenging to fabricate simple devices to obtain high modulation depth across a broad bandwidth. In this study four
different CVD grown multilayer graphene (MLG) modulators based on MLG/ionic liquid/gold sandwich structures have
been investigated. Flexible substrates (PVC and PE) were chosen as host materials, and devices were fabricated at three
different thicknesses: 30, 60 and 100 layers. The resultant MLG devices can be operated at preferentially low voltages
ranging from 0 to 3.5 V and provided nearly complete modulation between 0.2 THz and 1.5 THz at ca. 3.5 V with low
insertion losses. Even at such low gate voltages the devices have been doped significantly inducing an enormous
improvement in their sheet conductivities ranging between 7 to 11 times depending on the thickness of the device. In
addition, sheet conductivity has been improved more than 3 times with change in the layer number from 30 to 100. With
the demonstration of promising device performances, the proposed modulators can be potential candidates for
applications in THz and related optoelectronic technologies.
Since its discovery in 2004, graphene has attracted
intense attention in many fundamental areas due to its
remarkable electronic and mechanical properties.1 With its
gapless nature and symmetrical band structure, graphene
has extraordinary physical properties such as room-
temperature quantum Hall effect and micrometer long
mean free path.2,3 Its massless carriers result in an
extremely high carrier mobility exceeding
200000 cm2/V·s 4 and graphene becomes a unique
material in applications of high speed electronics. Single
layer graphene’s fairly low (ca. %2.3) absorption of visible
and IR radiation5 makes it utilizable in the application of
transparent electrodes, photodetectors, and broadband
infrared electro-optical modulators.6 Exploration of
graphene carrier dynamics has shown that electronic
structure of graphene is more sensitive to the THz region
of the electromagnetic spectrum rather than IR and optical
range.7 THz beams allow characterization of carrier
dynamics near the Fermi level.8,9 Therefore, graphene is
recognized as a potentially active material for
photosensitive THz devices in the application of active
filters, switches and modulators. These optical devices are
urgently needed by THz technology in order to advance a
diverse range of applications such as nondestructive
imaging,10,11 spectroscopy,12 biomedical diagnosis,13
ultrahigh wireless communication,14 and security.15
Conventional THz modulators that are based on
semiconductor materials 16 and hetero-structure containing
2D electron gas17 showed low modulation depths. Metal
gates used in the structures can limit the working range of
carrier density and Fermi energy tuning.18 Compared to
those, single layer graphene based THz modulators have
higher carrier mobilities with an electrically tunable carrier
density and offer very low insertion loss (0.2-0.5dB).6,19
However, theoretically expected high modulation depth
a) Author to whom correspondence should be addressed. Electronic mail: eokan@metu.edu.tr
and broadband performance is difficult to achieve due to
its strong dependence on quality of graphene 6 and
unforeseen component effects of the devices such as
substrate effects 20. In order to improve THz modulation
by single layer graphene different methods such as
integrating graphene with photonic cavities 21 and
metamaterials 22,23 have been reported. In their study
Kakenov et al. have demonstrated a flexible active THz
surface constructed with a large-area single graphene
layer, a metallic reflective electrode, and an electrolytic
medium in between that provides complete modulation in
the THz reflectivity at 2.8 THz. 21 50% amplitude
modulation at low voltages is reported by Gao et al using
a gated single-layer graphene modulator with metallic ring
aperture.22 However, the modulation is limited to a quite
narrow bandwidth. Increased modulation depth can be obtained by use of
multilayer graphene (MLG) alone or MLG with ionic
liquids.10,24–28 Shen et al. presented a metamaterial based
modulator with a multilayer stack of alternating patterned
graphene sheets with 75% modulation depth.23 However,
the narrowband operational range and polarization
dependent response of metamaterial based modulator may
limit their future applications.29 In their study Baek et al.
has shown improvement in THz modulation with
production of high quality MLG.26 In that study the optical
sheet conductivity increase has also been demonstrated as
the layer number increase from 1 to 12. The dielectric
substrates can cause change in the fermi level of a single
layer graphene due to band gap opening and this situation
could mislead optical results.30 Whereas in MLG, optical
response is dominated by the layers that do not interact
with substrate. In their study Wu et al. investigated a
graphene/ionic liquid/graphene device where ionic liquid
forms interfaces with the graphene electrodes.25 As the
FIG. 1. (a) A schematic of the THz set-up and MLG structure (inset) consisting of multilayer graphene, electrolyte medium, and gold electrode. Drawings showing (b) No doping case at zero applied voltage and (c) Intercalation of ions through the graphene layers by gating.
layer number of the graphene electrodes increased an
increased modulation is observed, which is explained by
elimination of boundary defects during multilayer
formation. Kakenov et al. presented another ionic liquid
based THz amplitude modulator.31 Due to efficient mutual
gating of graphene electrodes and ionic liquid more than
50 % modulation depth was obtained. Furthermore, Liu et
al used ionic liquid in their THz modulator device and
achieved a modulation depth of 22 %.32 In this study, high
flexibility of THz modulator has been demonstrated by the
great flexibility of graphene, ionic gel and also the host
material, polyethylene terephthalate.
A compromise between modulation depth, polarization
dependence, ease of fabrication, design flexibility, large
area production, and operational bandwidth exist in most
of the studies reported in literature. In this study we present
large area MLG devices on flexible substrates that do not
compromise on the modulation performance. The study
experimentally demonstrates an excellent performance on
THz amplitude modulation by devices made from ionic
liquid doped MLG structures on Polyvinyl chloride (PVC)
and Polyethylene (PE) substrates. The modulation depths
were investigated at a broadband frequency range from 0.2
to 1.5 THz with application of very low voltages ranging
between 0 V and 3.5 V. To our knowledge, this is one of
the highest modulation depth achieved by graphene based
THz modulators with such a broad THz range at such low
gate voltages.
A sketch of THz-TDS system is given in Figure 1(a).
The spectrometer has an effective working range of 0.2-
1.5 THz with the sample. An amplified femtosecond laser
is the light source that is centered at 800 nm and has 180
fs pulsewidth and a repetition rate of 1 kHz. A <110> ZnTe
crystal was used to generate coherent THz radiation via
optical rectification. Through the Pockells effect, phase of
the detection pulse is retarded by the oscillating electric
field of the THz radiation. Change in the polarization is
monitored by quarter wave plate and Wollaston prism.
Voltage from the balanced detector is synchronously
detected using a lock-in amplifier. The water vapor
attenuation effect is minimized by enclosing the system in
an atmosphere controlled box with dry air.
Multi-layer graphene samples were grown on nickel foils
using chemical vapor deposition method. Due to high
solubility of carbon atoms on Ni surface, highly crystalline
MLG with varying layer numbers can be grown on nickel
foils. The growth process takes place in quartz chamber at
the presence of argon, hydrogen and methane gases. The
temperature in the chamber determines the layer number
of synthesized graphene samples. Our samples were grown
at 850 °C, 900 °C and 1000 °C corresponds to nearly 30,
60 and 100 layers of MLG, respectively. The layer
numbers are estimated from optical measurements.33 After
the growth, MLG samples with 30, 60 and 100 layers were
transferred on PVC (labelled as MLG850, MLG900 and
MLG1000) and 100 layers on PE (MLG1000PE) by
lamination, and nickel was removed with iron chloride
(FeCl3.6H2O) solution.
Inset of Figure 1(a) demonstrates the fabricated MLG
structure. The device consists of MLG and gold electrodes
sandwiching ionic liquid [deme][Tf2N] (Diethylmethyl(2-
methoxyethyl) ammoniumbis (trifluoromethylsulfonyl)
imide). The gold electrode has a circular opening of 5 mm
through which terahertz transmittance was measured. At
zero bias the transmittance through the MLG device is
maximum (Figure 1(b)) suggesting very low doping level,
ZnTe
Crystal
PM
QWP
WP
Detector
Sample
Laser
Purge Box
(a) (b)
(c)
if any. Upon application of bias voltage, the ions of the
same polarity intercalate through layers of graphene by
inducing charge carriers in layers of graphene resulting in
attenuation of THz transmission33,34 (Fig. 1(c)).
Figure 2(a) presents change in terahertz waveforms
around the peak amplitude for MLG850 as the applied gate
voltage is increased. The inset of Figure 2(a) presents a full
profile at 0 V as an example. Corresponding frequency
domain data of the device is given in Figure 2(b). Full
waveform in time domain for all devices and their
corresponding frequency domain data are given in
Supplementary Material, Fig. S1 and S2. The gate
voltage applied appears to be low enough that no
observable phase change was noticed neither in time
domain nor in phase data. No observed change in phase
might be because of graphene’s robust nature under
electrolyte gating25 and having a thickness much less than
the THz wavelength. Figure 2(c) shows the change in THz
peak amplitude of all four devices as the bias voltage was
varied from 0 V to 3.4 V. The maximum transmission was
at 0 V while the minimum is at 3.4V. As the gate voltage
increased almost linear decrease in the amplitude was
observed up to ca. 2 V. Beyond 2 V, a sharp nonlinear
decrease with the voltage was observed. Approximately
2.5 V appeared to be turning point where the decrease
slowed down and reached to a minimum at 3 V. The
observed change in THz field amplitude is controlled by
mobile carrier density that is being tuned by doping level
or by the applied potential in effect. The voltage dependent
behavior of the MLG devices is very similar to each other
except the thinnest device which requires slightly higher
voltage (ca. 0.5 V), which might be due to thickness
dependent diffusion limit of the ions. Voltage dependent
sheet resistance behavior of MLG850 device measured
with four probe method is given in inset of Figure 2(c) and
shows a very similar behavior. In Figure 2(d) THz
transmission of MLG850 is given between 0.2 THz and
1.5 THz at all applied voltages. (For other thicknesses
please see supporting material) The observed modulation
with the set voltage appears to be independent of the THz
frequency and, thus, limited by the instrument response.
Up to 1.5 V less than 20 % modulation is observed. A
modulation between 20 and 30% at ca 2 V can be achieved
depending on the MLG thickness (Supplementary
Material, Fig. S3). While the thinnest layer showed the
lowest modulation at 2.6 V as approximately 50%, the
remaining MLG devices had more than 80% modulation.
Almost full power modulation has been achieved with all
MLG devices at voltages beyond 3 V. The modulation
depth is significantly improved compared to single7 and
multilayer26 devices over a very broad range. Among all
devices MLG850 provided a more controllable modulation
change with increased voltage.
Optical conductivity of graphene appears to follow its
electrical conductivity at the THz frequencies and, thus,
follows the Drude model.35–39 With its high quantum
efficiency and electron-phonon interaction beyond 7 THz 37 all the changes observed in sheet conductivity is
-16.8 -16.5 -16.2
2
4
6
(d)(c)
Sig
na
l (a
.u.)
Time (ps)
0 V 0.5 V 1 V 1.5 V 2 V 2.6 V 2.8 V 3 V 3.2 V 3.4 V
(a)
0.6 1.2 1.8
0
2
4
6
Am
pli
tud
e (
a.u
.)
Frequency (THz)
(b)
0.6 1.20
50
100
Tra
ns
mit
tan
ce
Frequency (THz)0 3
0
1
E.
Fie
ld (
No
rm.)
Voltage (V)
1 2 3
0
300
V
RS
h
FIG. 2. (a) Voltage dependent THz field around the peak amplitude of the
waveform. The inset shows an example of time domain THz profile. (b)
Corresponding frequency domain amplitudes between 0.2 THz and 1.5 THz for MLG850. (c) Voltage dependent modulation of THz peak
amplitude of the electric field (MLG850, black square; MLG900, red
circle; MLG1000, green triangle; MLG1000PE, blue pentagon). Inset: Four probe measurement of sheet resistance change of MLG850. (d) THz
transmittance of MLG850 at all applied voltages relative to 0V. The
others are given in Supplementary material (S3).
expected to be due to a change in carrier density and/or
change in scattering time.40 Therefore, THz sheet
conductivities (𝜎𝑠ℎ) of MLG devices is proportional to the
amplitude ratio of reference (PVC or PE) substrate and
MLG sample as given in equation (1) 41
𝜎𝑠ℎ = (𝑛1 + 𝑛2) (𝐴𝑠𝑢𝑏𝑠𝑟𝑎𝑡𝑒
𝐴𝑀𝐿𝐺 − 1) /𝑍0 (1)
here, Z0 is the impedance of free-space, n1 and n2 is the
refractive index of air and substrate, respectively. The
sheet conductivities of the MLG850 device at all voltages
are given in Figure 3(a). The sheet conductivities of MLG
devices at the Dirac point are featureless and inherently
broadband between 0.3 to 1.5 THz and increase with
increase in layer number as expected (Supplementary
Material, Fig. S4). The conductivity values of MLG850,
MLG900, MLG1000 and MLG1000PE are derived to be
4.4 mS, 7.5 mS, 17.8 mS and 10.3 mS respectively. The
device with PE substrate appears to be lower than the PVC
counterpart. The observed difference in conductivity is
most likely due to the quality of the graphene, crystalline
size,42 since it will affect the scattering time of the carriers.
The calculated DC conductivities of the devices as inverse
of the sheet resistance, which is measured with four probe
method, are 3.3 mS for MLG850, 5.8 mS for MLG900,
31.2 mS for MLG1000. Our conductivities measured with
THz are close to the DC conductivities measured and are
consistent with the ones reported for the similar devices in
literature.19,26,32,39,40
Recent studies have shown quite high scattering time
for well grown multilayer graphene ranging from 100fs to
300fs.43 Considering an average value of 200 fs and using
experimental sheet resistance values carrier densities are
estimated as ca. 1.5x1012 cm-2 for MLG850, ca. 4.5x1012
cm-2 for MLG900, and ca. 1.3x1014 cm-2 for MLG1000.
The carrier densities are comparable to ones reported in
literature.26,40,44,45
As the gate voltage is varied, change in sheet
conductivity (Δ𝜎𝑠ℎ) with respect to 0 V were calculated
using equation 2, which is derived from equation 1.46
Δ𝜎𝑠ℎ =𝑛+1
𝑍0[
𝐴0
𝐴𝑔𝑎𝑡𝑒(1 + 𝜎𝑠ℎ,0
𝑍0
𝑛+1) − 1] (2)
where A0, Agate is the transmitted THz field amplitude of
the MLG device at 0 V and gate voltages, respectively, and n is the refractive index of substrate. The results are given
in Supplementary Material Fig. S4 for all the devices.
As expected, sheet conductivities of the devices increased
as the gate voltage increased and showed a saturation
behavior at 3.4 V for all four samples. Figure 3(b) presents
a comparison of the conductivities at representative
voltages of 1.5 V, 2.8 V and 3.4 V calculated at 0.8 THz.
Large increase in conductivity with doping can be seen
when compared to their conductivities at 0V by (𝜎3.4𝑉−𝜎0𝑉)/𝜎0𝑉. The conductivities were increased by
997% for MLG850, 716% for MLG900, 705% for
MLG1000, and 1153% for MLG1000PE with an
application of a comparably low gate voltage of 3.4 V.
Similarly, Qi et al observed increase in conductivity with
doping due to increase in hole carrier concentration. 40
Besides doping effect, growth temperature can also affect
the conductivity drastically. When the thinnest and the
thickest devices are compared to each other at 0 V
[𝜎=(𝜎1000 ˚C −𝜎850 ˚C) /𝜎850 ˚𝐶] an enhancement of 304%
was observed. The difference is still significant but less
pronounced with an increase of 197% at 3.4 V for the same
samples. The increase in layer numbers should not affect
the THz conductivity of MLG devices or their carrier
momentum scattering time 40 since the sheet conductivity
is defined as σsheet = 𝜎THz x dN−layer where dN‑layer is
the thickness of N-layer (N = 33, 64, or 98) graphene. Thus
only sheet conductivity should increase due to increased
carrier density with thickness. Similarly, an increase in the
sheet conductivity amounting to a 73% improvement was
noted as the layer number increased from mono to 12
layers of graphene in the study by Baek et al.26 In addition,
Wu et al has also shown that the sheet resistance (and
hence sheet conductivity) change with increase in layer
number.28
In addition to being a preferential host material for
flexible photonic devices PVC and PE are also preferred
for their very low insertion losses; less than 1 dB at all
frequencies (Supplementary Material, Fig. S5). The
thinnest device had an average of 3 dB insertion loss
though the loss increased with the layer thickness. The
highest loss is observed for the thickest devices ranging
from 8 dB to 12 dB. Figure 3(c) presents modulation of
THz amplitude versus the insertion loss at 0.8 THz for
1.5 V, 2.8 V and 3.4 V. Here the insertion loss is the initial
loss of the THz power when the device is inserted in the
beam path and modulation represents the further change in
the transmission of THz wave. The best performance is
achieved with the thinnest MLG device with its much
lower insertion loses and almost 100 % modulation depth
with an application of 3.4 V. Depending on the application
type, devices with optimal numbers of graphene layers can
be designed considering the trade-off between modulation
depth and insertion loss at the preferred operational
voltage.
In conclusion, THz modulators were fabricated using
CVD-grown MLGs and change in the THz transmission
with applied voltage due to the change in carrier density
was investigated by THz time-domain spectroscopy. The
thinnest device with 33 layers provided almost complete
modulation of THz waves with an operation voltage of less
than 3.5 V. The effect of increased number of layers on
THz modulation was also investigated. Complete
modulation was demonstrated at all the thicknesses;
however, as the layer thickness increased an increase in the
insertion loss was also observed. With the strong gating
effect of dopant molecules, it was possible to achieve an
extraordinary modulation depth with a very low operation
voltage and broadband response over 0.2 and 1.5 THz.
Here, bandwidth response appears to be limited only by
the instrument. Sheet conductivities were also improved
with doping and with increasing thickness, which is
promising for new THz devices. Ionic liquid doped MLG
devices are a promising platform to produce THz active
filters with controlled modulation and expected to have a
strong impact on improving THz optoelectronic devices in
wide application areas.
-3 -6 -9 -120
20
40
60
80
100
Mo
du
lati
on
%
(c)(b)
MLG850
MLG900
MLG1000
MLG1000PE
Layer Number Insertion Loss (dB)
(a)
40 60 80 1000.00
0.08
0.16
Co
nd
ucti
vit
y (
S)
0.4 0.8 1.2
0.00
0.05
MLG850
2.6 V
2.8 V
3 V
3.2 V
3.4 V
0 V
0.5 V
1 V
1.5 V
2 V
Co
nd
ucti
vit
y (
S)
Frequency (THz)
FIG. 3. (a) THz sheet conductivity of MLG850. (b) THz sheet
conductivity of MLG devices for 1.5 V, 2.8 V, and 3.4 V at 0.8THz. (c) Modulation vs insertion loss at 0.8 THz at selected voltages of 1.5 V,
2.8 V, and 3.4 V.
See Supplementary Material for the results of all the
devices.
The authors acknowledge the helpful discussions with Dr.
Bulend Ortac. This research was supported by Scientific
and Technological Research Council of Turkey
(TUBITAK). E.K. and O.E. acknowledge funding from
TUBITAK grant 111T393; H.A. and C.K. acknowledge
funding from TUBITAK grant 114F379.
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