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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

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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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