Ion-Gel-Gated Graphene Optical Modulator with Hysteretic Behavior

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Ion-Gel-Gated Graphene Optical Modulator With Hysteretic Behavior Jin Tae Kim, Hongkyw Choi, Yongsuk Choi, and Jeong Ho Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16600 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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ACS Applied Materials & Interfaces

Ion-Gel-Gated Graphene Optical Modulator With Hysteretic Behavior Jin Tae Kim,1, * Hongkyw Choi,1 Yongsuk Choi,2 and Jeong Ho Cho2,3

1

Creative Future Research Laboratory, Electronics and Telecommunications Research Institute

(ETRI), Daejeon 34129, South Korea 2

SKKU Advanced Institute of Nanotechnology (SAINT), 3School of Chemical Engineering,

Sungkyunkwan University (SKKU), Suwon 16419, South Korea

*Corresponding author: [email protected]

Abstract We propose a graphene-based optical modulator and comprehensively investigate its photonic characteristics by electrically controlling the device with an ion-gel top-gate dielectric. The density of the electrically driven charge-carriers in the ion-gel gate dielectric plays a key role in tuning the optical output power of the device. The charge density at the ion-gel–graphene interface is tuned electrically, and the chemical potential of graphene is then changed to control its light absorption strength. The optical behavior by the ion-gel gate dielectric exhibits a large hysteresis which originates from the inherent nature of the ionic gel and the graphene–ion-gel interface, slow polarization response time of ions. The photonic device is applicable to both TE- and TM-polarized light waves, covering two entire optical communication bands, the O-band (1.26–1.36 µm) and Cband (1.52–1.565 µm). The experimental results are in good agreement with theoretically simulated predictions. The temporal behavior of the ion-gel–graphene-integrated optical modulator reveals a long-term modulation state due to the relatively low mobility of the ions in the ion-gel solution and formation of the electric double layer in the graphene–ion-gel interface. Fast dynamic recovery is

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observed by applying an opposite voltage gate pulse. This study paves the way to understanding the operational principles and future applications of ion-gel-gated graphene optical devices in photonics.

Keywords: Ion-gel, Graphene, Optical modulator, Electro-absorption, Hysteresis, Photonic devices


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INTRODUCTION Graphene has been an exciting optoelectronic material since its appearance in 20041–3. A two-dimensional sheet of sp2-hybridized carbon packed in a honeycomb lattice allows graphene to have a gapless energy structure and to serve as a very thin, flexible, and transparent conducting film consisting of a single atomic layer. Solar cells and touch screens have demonstrated the application of graphene's multifunctional properties such as flexibility and conductivity4,5. Based on the extraordinary electronic, mechanical, and chemical properties of graphene, high-performance field effect transistors, electronic devices, mechanical devices, biosensors, and gas sensors utilizing this material have been investigated6–11. In photonics, the configuration of graphene-based photonic integrated circuits is a current area of investigation. These photonic devices work together with electronic components to provide logic, memory, and interconnected functions on a chip12, 13. To complete proposed on-chip photonic systems, numerous researchers have developed elementary graphene-based

photonic

devices.

Graphene

plasmonic

waveguides14–17,

polarizers18,19,

photodetectors20–23, modulators24–32, and lasers33,34 have all been demonstrated. With the aid of the gapless linear dispersion of Dirac fermions in graphene, most photonic devices using graphene exhibit ultra-broadband and high-speed operation. Optical modulators interconnecting the electric and photonic worlds are essential components. Silicon waveguide-coupled graphene modulators have been experimentally demonstrated, and they possess ultra-small device footprints and large operating bandwidths as well as high-speed modulation.23,24,27,28 Their operating principle is based on the tunable electroabsorption of graphene: the transmission of light propagating along a graphene-integrated waveguide can be dynamically tuned by a suitable electric gating.24 To effectively apply an electrical gating voltage to the graphene integrated on a waveguide, the waveguide must be made of conductive optical material (e.g. doped silicon waveguide),24 or an additional graphene layer or thin metal strip must be transferred onto the bottom graphene layer forming a double electrode capacitor



structure.25,31,32 Satisfying these requirements requires a careful fabrication process, and rigorous, and rather arduous, multiple graphene transfers. In recent years, the availability of ion-gel material as an efficient gating medium has paved a new way to the easy fabrication of graphene-based electronic35–40 and optical devices.41–45 Although ion-gels can be used as a base material for electrochemiluminescent light-emitting devices,46 most ion-gel-based photonic devices are configured with graphene film. A very high gate capacitance of ion-gels has allowed us to dynamically adjusting the chemical potential (Fermi level) of graphene with a very low electric gate voltage. Graphene supercapacitors with an ion-gel electrolyte dielectric are easily fabricated by a simple drop-casting method. They exhibit ultrabroadband optical modulation characteristics. However, most ion-gel-gated graphene modulators have been suitable to the normal incidence light to the graphene surface.41–45 Obtaining large polymer optical waveguide platforms for graphene-based optical devices19 combined with very large-area single-layer graphene growth using the chemical vapor deposition (CVD) method47 has allowed designed graphene devices to modulate horizontally incident beam with an ion-gel dielectric. However, a limited amount of work has been done to demonstrate graphene-based optical devices using an ion-gel or has reported the optical characteristics of these photonic devices. In this work, we designed and fabricated a graphene-integrated polymer waveguide modulator whose optical modulation was controlled by an ion-gel top-gate dielectric. Simple spin coating of a polymer resin and a dry etching process allowed fabricating the photonic integrated circuit platform. Comparatively accurate positioning of transferred graphene and ion-gel on the photonic platform allowed us to develop all-dielectric in-line graphene photonic integrated circuits. The electro-absorption characteristics of the fabricated photonic device were investigated by numerical simulation and experimental measurement. The ion-gel dielectric played a key role in modulating the power of light wave as a transparent upper-cladding medium, increasing the lightgraphene interaction ratio. By applying the gate voltage, the optical output power of the modulator


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was tunable for both TE- and TM-modes. Uniquely, gate-voltage-dependent optical properties of the device exhibit a large hysteresis which originates from the slow polarization response time of ions. The temporal behavior of the device was measured and further applications of the device in photonics were discussed. We believe our investigation into an electrically controlled graphene modulator with an ion-gel gate will make the realization of polymer-based electronic-photonic integrated circuits possible in the future.

RESULTS AND DISCUSSION Figure 1a shows a bird’s-eye view of the proposed graphene-based polymer waveguide modulator that is electrically controlled with an ion-gel top-gate dielectric. The 10 mm-long polymer waveguide consists of an under-cladding, a core, and a middle-cladding surrounding this core. An UV-curable optical polymer resin was employed for the polymer waveguide platform, as described in the Experimental Section. The refractive index difference between the core and cladding material was 0.34% to satisfy the fundamental mode propagation condition for the transverse-electric (TE) and transverse-magnetic (TM) modes in the O-band (1.26–1.36 µm) and Cband (1.530–1.565 µm). The cross-sectional dimensions of the core were also designed to support the fundamental mode propagation conditions. A single-layer graphene film was placed on the photonic device platform parallel to the waveguide core. The Cr/Au electrodes were used for external electrical contacts. Finally, the ion-gel top-gate dielectric was patterned onto the graphene film. The operation of an ion-gel–graphene-integrated optical modulator is based on the Pauli blocking principle: the light absorption coefficient of graphene is tunable by applying an electrical gate voltage to the ion-gel, which tunes the charge carrier density at the graphene surface. The change of the charge density in graphene alters the graphene’s Fermi level (EF), which is nearly equal to its chemical potential (EF ≅ µc). The light absorption coefficient of graphene is then tunable



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and the output optical power is modulated. If the graphene’s chemical potential is lower than half of the energy of a photon (Eph = ωћ > 2|µc|), light waves are absorbed by graphene (lower in Fig. 1b). If µc is larger than half of the energy of a photon (Eph < 2|µc|), the photon absorption by graphene is blocked as there is no available empty state in graphene’s energy band, as depicted in the upper diagram in Figure 1b. The intensity of the output optical power can be predicted by considering graphene’s tunable refractive index, which is derived from its relative permittivity, = + = 1 +

(, )

(1)

∆

where ∆ denotes graphene’s effective thickness. can be changed by adjusting graphene’s chemical potential (µc). Graphene’s optical conductivity (σ) is calculated using the Kubo formalism in a complex form that includes both the intraband and interband parts, σg = σintra + σinter ,48,49 (, ) =

ℏ ("#$)

,

%

+ 2 ln( exp ( ) + 1)- + .

#| |,ℏ("$)

ln %#| |"ℏ("$)ℏ

(2)

where µc is the chemical potential of graphene, 01 is Boltzmann’s constant, T is the temperature, ℏ is the reduced Plank’s constant, ω is the angular frequency, and Γ = 1/τ is the charged particle scattering rate (Γ = 0.833 THz and 100 THz for interband and intraband conductivities, respectively). Graphene was considered to be an anisotropic material since the out-of-plane (normal to the graphene plane) conductivity (σ⊥) is different from its in-plane (parallel to the graphene plane) conductivity (σ∥). Thus, ∥ = 1 +

(, ) ∆

and ε⊥ = 1 or

2.5, 50,51; here, we set ε⊥ = 2.5. Based on graphene’s tunable complex permittivity, we predicted the transmission of the designed graphene-based optical modulator at wavelengths of 1.31 µm and 1.55 µm using a finite element method simulator (PhotonDesign). Fig. 1c displays the cross-sectional view of the device


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and the field distribution of the calculated guided mode. The field of the guided mode interacts with the ion-gel–graphene interface. The results for a 1.31 µm and a 1.55 µm wavelength are presented in Fig. 1d and 1e, respectively. Photons are absorbed by the graphene when their energy is higher than graphene’s chemical potential, Eph = ωћ > 2|µc|. However, the transmission begins to increases as µc increases. In the proposed device, µc can be increased by applying an electric gate voltage to the ion-gel top-gate dielectric. A dramatic decrease in transmission was observed at |µc| = 0.475 eV for the 1.31 µm wavelength, as can be seen in Fig. 1d. The values are half of the photon energy with a 1.31 µm wavelength. As µc increases further (|µc| > 0.5 eV), the photon absorption by graphene is blocked by the Pauli Exclusion Principle. Perfect non-absorbing propagation through graphene is not achieved due to graphene's intrinsic loss factor. The optical transmission behavior is dependent on the polarization of the incident light wave. At µc = 0 eV, transmission loss of the transverse-magnetic (TM)-modes (blue in Fig. 1d and 1e) were comparatively lower than that of the transverse-electric (TE)-modes (red in Fig. 1d and 1e). Interactions with graphene's in-plane permittivity (ε||) mean that the TE mode has a more lossy factor than that of the TM mode, which interacts with the fixed out-of-plane permittivity (ε⊥= 2.5). Thus, the transmission of the TE mode is higher than that of the TM-mode. We defined the off-state and on-state to be when µc = 0 eV and µc = 0.7 eV, respectively. This allowed a periodically modulated optical output power to be obtained when µc alternated from 0 to –0.7 eV. For the 1.31 µm wavelength, the on/off extinction ratios of the TM- and TE-modes were about 0.029 dB/mm and 0.86 dB/mm, respectively. To achieve a 3 dB on/off modulation depth in the output optical power of a 1.31 µm wavelength, we would require a 103-mm and a 3.5-mm long device for the TM- and TEmodes, respectively. For light with a 1.55 µm wavelength, similar optical characteristics were observed, as shown in Fig. 1e. Abrupt photon transmission began when |µc| = 0.4 eV (half of Eph = 0.7999 eV).



For | µc| < 0.3 eV, the transmission loss of for the 1.55 µm wavelength is lower than for light with a 1.31 µm wavelength. The relative field amplitude ratio at the graphene–ion-gel interface with respect to maximum field amplitude (Iinterface/Imax) is 26.9 % and 29.3 % for the TE-polarized 1.31 µm and 1.55 µm wavelength light, respectively. Iinterface/Imax for the TM-polarized 1.31 µm and 1.55 µm wavelength light is 27.0 % and 29.6 %, respectively. Photons with a longer wavelength interact more strongly with graphene film, meaning that they are more intensively absorbed by graphene. The on/off extinction ratios of the TM- and TE-modes were about 0.036 dB/mm and 1.07 dB/mm, respectively. To achieve a 3 dB on/off modulation depth in the optical power of a 1.55 µm wavelength, an 83-mm and a 2.8-mm long device would be required for the TM- and TE-modes, respectively. Fig. 1f displays the on/off extinction ratio as a function of wavelength, covering two entire optical communication bands: the O-band (1.26–1.36 µm) and C-band (1.53–1.565 µm). With the aid of the graphene's zero bandgap structure, graphene-based photonic devices can operate in the ultra-broadband optical spectrum. As the wavelength increases, the extinction ratio increases gradually for both TE- and TM-modes. For the entire C-band, the difference in the extinction ratio between the TE- and TM-mode depending on the wavelength is significant. The TE-mode is more sensitive to changes of graphene's chemical potential. To experimentally verify the proposed photonic device, we fabricated a graphene–ion-gel integrated polymer waveguide modulator. The polymer waveguide platform was fabricated based on a commercially available UV-curable resin, which was processed by spin coating, photolithography, and an O2 plasma dry-etching technique (see Experimental Section and Supplementary Information Fig. S1). Clearly defined polymer waveguide cores on the cladding surface can be observed in Fig. S2, showing a cross-sectional view of the device. The graphene was synthesized using CVD47, and was transferred on the waveguide by the lifting transfer method using a poly(methyl methacrylate) (PMMA) supporting layer, as described in the Experimental Section.


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Obtaining the Raman shift of the graphene on the polymer film was challenging because of the high background signals from the polymer substrate. We obtained the Raman shift (with an excitation wavelength at 514 nm) of the same graphene film as used on the polymer substrate, using a SiO2/Si substrate. The 2D peak and G peak were measured at 2700 cm−1 and at 1580 cm−1, respectively, as shown in Fig. S3. The height of the 2D peak was higher than G peak, suggesting that single layer graphene was transferred to the polymer surface. The D peak at 1350 cm−1 indicates that there were slight defects in the graphene. In order to investigate the optical characteristics, the optical power transmission of the graphene-based polymer waveguide modulator was measured using an optical fiber coupled transmission system (see Experimental Section and Supplementary Information Fig. S4). To excite the guided mode of the waveguide, polarized light was passed through a polarization scrambler and a fiber-pigtailed polarizer was launched at the input port of the waveguide using polarization maintaining fibers (PMFs). After detecting the infrared images of the guided mode using a chargecoupled device (CCD) image sensor, the power of the output light was collected with a PMF and measured by an optical power meter. For the polymer waveguide platform sample without graphene and ion-gel, the optical powers of the TE- and TM-modes were nearly the same because the birefringence (nTE – nTM) of the optical polymer material was less than 0.001. The optical output powers are −7.9 dB and −8.3 dB at a 1.31 µm and a 1.55 µm wavelength, respectively, which includes the loss due to the polymer materials and the coupling loss between the waveguide and the fiber. The optical output power for the 1.51 µm wavelength is slightly higher than that for the 1.31 µm because the polymer material losses at 1.51 µm (0.35 dB/cm) were higher than at 1.31 µm (0.1 dB/cm). The polarization-independent characteristics of the waveguide platform were altered by the presence of the graphene film due to the highly polarization-dependent nature of graphene. Figures 2a and 2b show the measured optical characteristics as a function of polarization angle when



graphene was not covered by the ion-gel gate dielectric. At a 1.31 µm wavelength, the transmission power of the TE- and TM-modes was −8.9 dB and−9.5 dB, respectively, with a 5-mm long graphene. A 1.55 µm wavelength light shows similar optical characteristics: −9.0 dB and−10.8 dB for the TE- and TM-modes, respectively. The CCD images of the guided modes in Fig. 2a make it sure. In a similar manner to previously reported work19, it serves as a TE-pass polarizer providing a TE/TM polarization extinction ratio of 0.6 dB and 1.8 dB for a 1.31 µm and a 1.55 µm wavelength, respectively. The TE/TM polarization extinction ratio of our device without an ion gel is not as large as in Ref. 18. This is attributable to the imperfect transfer process of graphene. Or the difference in the surface of the substrate on which the graphene is transferred (the polymer surface in our study). The degree of doping is different. This can be solved by using a hexagonal boron nitride (hBN) layer under the graphene film. Graphene’s optoelectronic properties are preserved by hBN because it is free of dangling bond and charge trap.52 The presence of ion-gel gate dielectric leads to significant change in the optical characteristics. Figures 2c and 2d present the optical characteristics of the polymer waveguide when the graphene was covered with an ion-gel gate dielectric. The optical power of the TE-mode was greatly suppressed but that of the TM-mode was significantly increased, as shown in the CCD images in Fig. 2c, which show infrared images of the guided TE- and TM-modes. Different polarization-dependent behavior of the graphene–ion-gel integrated polymer waveguide is clearly shown in Fig. 2d and Supplementary Information Fig. S5. For the 1.31 µm wavelength, the output power of the TM-mode increased from −9.5 dB to −8.2dB. On the contrary, that of the TEpolarization surface wave was reduced from −8.9 dB to −11.0 dB with the presence of ion-gel (Supplementary Information Fig. S5). Graphene under an ion-gel gate dielectric was slightly doped.19 Then, graphene’s conductivity increases slightly and supports low loss guidance of the TM-polarization mode.38 For the 1.55 µm wavelength, the optical power of the TM-mode of increased from −10.8 dB to −8.8 dB, while that of the TE-mode decreased from −9.0 to −12.1 dB.


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The waveguide sample with an ion-gel gate dielectric can serve as a broadband TM-pass polarizer that provides the TE/TM extinction ratios of 2.8 dB and 3.3 dB for 1.31 µm and 1.55 µm wavelength light, respectively. The ability of electrically control the graphene-based polymer waveguide modulator was further explored by applying gate voltages to the ion-gel gate dielectric formed on the graphene film. Figures 3a and 3b exhibit the experimental results as a function of gate voltage, Vg, for a wavelength of 1.31 µm and 1.55 µm, respectively. Similar to other graphene modulators based on a field effect transistor, the optical output power of the graphene-based polymer waveguide modulator using an ion-gel is effectively tuned by different bias voltages. Exceptionally, a significant hysteresis in the optical transmission was measured under wide gate voltage sweeping (ranging from −4 V to +4 V) due to slow ion mobility in the ion-gel. For the TM-mode of 1.31 µm wavelength light (the blue line in Fig. 3a), the transmission of the photon is invariable, with a sustained transmission of −8.2 dB despite the application of a gate voltage (ranges from + 4 to −1.4 V) to change graphene's chemical potential (µc). However, the output optical power begins to increase at Vg < −1.5 V. With a decreasing gate voltage, µc becomes to be larger than the photon energy (|µc| > ωћ/2). Light absorption by graphene decreases gradually due to available interband transitions in the graphene decreasing steadily. The transmission increases dramatically at Vg = −2.0 V and reaches a maximum value of −7.7 dB at Vg = −3.2 V, and is then saturated. Further decreases of the gate voltage larger than −3.2 V do not generate an increase of the transmitted optical power. The optical output power is larger than that of the sample without ion-gel gate dielectric (−9.5 dB, Fig. 2a). For the reverse gate voltage sweep, the optical transmission is not instantly changed with the variation of Vg because of the slow polarization response time of ions. However, it begins to decrease at Vg = −1.0 V and reaches the original value at Vg = + 0.5 V. This hysteresis behavior corresponds exactly to the transport behavior of graphene, as shown in Fig. 3c. We may expect that further increases of Vg will lead to increasing of the optical



transmission power again. This was expected due to the ambipolar electrical nature of graphene, which possesses a V-shape drain current curve due to graphene’s extraordinary cone-like zero bandgap6. The transport curves shown in Fig. 3c confirm this attribution, which displays the electrical transport properties of the graphene under an ion-gel gate dielectric. However, the transmission remains unchanged as shown in Fig 3a. This is most likely due to the slow time response of the ion-gel.53 If the off-state and on-state were defined as Vg = 0 V and Vg = −4 V, respectively, and the two voltages are applied periodically, then we can obtain the modulated optical output power at the output facet of the optical device. The measured modulation depth of the TMmodes was 0.5 dB. We fabricated a 5-mm long graphene sample so that the extinction ratio was 0.1 dB/mm. The theoretical prediction was 0.029 dB/mm. This mismatch between theory and experiment was attributable to the presence of loss due to the material in the polymer waveguide’s and losses from the optical power measurements. The response of the TE-mode to the gate bias voltage (red line in Fig. 3a) is noticeably different from that of the TM-mode. This was attributed to graphene’s anisotropic electrical (and hence optical) conductivity. Graphene’s out-of-plane conductivity is different from its in-plane conductivity (σ⊥ ≠ σ||). That is, the out out-of-plane permittivity is not same to the in-plane permittivity: ε|| = 1 +

(, ) ∆

and ε⊥ = 2.5. Thus, the TE-mode whose electric field is parallel to

the plane was very susceptible to changes in graphene’s permittivity caused by the gate bias voltage. On the contrary, the TM-mode whose electric field was vertical to the plane was less affected by graphene’s permittivity since ε⊥ = 2.5. The output optical power of the TE-mode changed from −11 to −8.5 dB, which is slightly larger than that of the sample without the ion-gel gate dielectric (−8.9 dB in Fig. 2a). The modulation depth was about 2.5 dB. The on/off extinction ratio of the TE-mode was about 0.5 dB/mm, which was the half value of the theoretically calculated value of 1.0 dB/mm. For the 1.55 µm wavelength, very similar optical characteristics were observed, as shown


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in Fig. 3b. The optical transmission did not change with a variation of Vg > −1.0 V. It began to increase when Vg < −1.5 V under a forward gate voltage sweep, and reached a maximum value at Vg = −3.2 V. For the reverse sweep, the response of the TE- and TM-modes was invariable due to the slow polarization response time of ions. until Vg reaches at −1.0 V. Further increases of the gate voltage under the reverse sweep led to the transmission returning to the original value. The application of a positive gate voltage does not lead to an ambipolar transmission characteristic as we expected due to the intrinsic p-type doping of the graphene film by the ion-gel top-gate dielectric. Compared with the case of 1.31 µm wavelength light, the dynamic responses to the gate voltage of the TE- and TM-mode were notable because a longer wavelength is more susceptible to the variation of graphene's chemical potential. The modulation depths of the TE- and TM-modes were 2.8 dB and 0.7 dB, respectively. The on/off extinction ratio of the TE- and TM-modes were 0.56 dB/mm and 0.14 dB/mm, respectively. Theoretically predicted extinction ratios were about 1.07 dB/mm and 0.036 dB/mm for the TE- and TM-mode, respectively. The light transmission characteristics of the device composed of various material stacks are summarized in Table 1.

Table 1. The light transmission characteristics of the device composed of various material stacks O-band (at a 1.31 µm)

C-band (at a 1.55 µm)

Material stacks

TE (dBm)

TM (dBm)

TE(dBm)

TM (dBm)

Waveguide (WG) only

– 7.9

– 7.9

– 8.3

– 8.3

WG + graphene

– 8.9

– 9.5

– 9.0

– 10.8

WG + graphene + ion-gel

– 11.0

– 8.2

– 12.1

– 8.8

WG + graphene + ion-gel + Vg

– 8.5

– 7.7

– 9.3

– 8.1

The change of the optical transmission power (on/off extinction ratio) is lower compared to the previously reported values, about 0.1 dB/µm.24 This shortcoming can be improved by replacing



the polymer waveguide with a high index waveguide (e.g. Si or SiN waveguide). The boundary condition for the normal component of the electric field indicates that the field of the guided mode must be continuous with the adjacent material, ε1E1 = ε2E2. Thus, the field amplitude at the graphene–ion-gel interface increases significantly if a high index Si waveguide (nsi = 3.4) is used instead of a low index polymer material (npolymer= 1.455) (see Supplementary Information Fig. S6).32,54 The relative field amplitude ratio at the graphene–ion-gel interface with respect to maximum field amplitude (Iinterface/Imax) is 49 % and 99 % for the TE- and TM-polarized light, respectively. The interaction strength between graphene and light increases and hence, the on/off extinction ratio can be improved further. For an ion-gel-gated optical modulator using a Si waveguide, the theoretically predicted on/off extinction ratios were about 58 dB/mm and 132 dB/mm for the TE- and TM-mode, respectively, at a 1.31 µm wavelength. In this study, we used anionic ion-gel. If we used a cationic ion-gel as a top gate, very similar optical behavior would be observed because graphene is an ambipolar semiconductor without energy gap. Negative dopant set the graphene’s Fermi level (chemical potential) above the Dirac point and positive dopant is the opposite. Optical transmission of the device is based on the Pauli exclusion principle. Thus, regardless of graphene dopant, photons are absorbed by the graphene if Eph = ωћ > 2|µc| and transmits if Eph = ωћ < 2|µc|. The hysteric optical behavior will be measured at a positive gate voltage region if a cationic ion-gel was used. The measurement of the optical modulation depth was also carried out in continuous wavelengths for the entire C-band at 1.26–1.36 µm and O-band at 1.530–1.630 µm, covering the whole optical telecommunication spectrum. Fig. 3d presents the measured on/off extinction ratio of the optical modulator for the entire O- and C-bands, while Vg changed from 0 to −4 V. The dependences of the optical response to the wavelength variation were coincident with theoretical predictions, although the quantitative values did not exactly match with the simulations. For the entire O-band, the extinction ratios of the TE- and TM-modes were 0.1±0.02 dB/mm and 0.5±0.05


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dB/mm, respectively. For the entire C-band, the extinction ratios of the TE- and TM-modes were 0.14±0.03 dB/mm and 0.57±0.03 dB/mm, respectively. These experimental results reveal graphene’s ability to function as an ultra-broadband optical modulator. To obtain further insight in to the fabricated photonic modulator, we measured the temporal behavior of the output optical power while the gate bias voltage of the ion-gel gate dielectric was switched between various combinations of 0, −4 V, and +4 V. Figure 4a exhibits the arbitrary optical power transmission response to application of a gate bias voltage, Vg. The optical modulator exhibited reproducible light modulation. The optical power increased (on-state) when an electric gate of −4 V was applied and then returned to the initial value as the electric gate was turned off (off-state). The fall time was much slower than the rise time. The relatively low mobility of the ions in the ion-gel solution and the formation of the electric double layer in the graphene–ion-gel interface hurdle the fast response to the electrical gating. Ion-gel-gated graphene transistors have showed similar behaviour.35–40 The advantages of the ion-gel gated transistor are both ultrahigh capacitance and long-range polarizability, which enabled both low voltage operation below 4 V and coplanar gate geometry of the optical transistors. The dynamic response is described by W(t) = W0 + D{1−exp[–(t–t0)/τ]} and W(t) = W0 + Dexp[–(t–t0)/τ] for rising and falling, respectively, where W0 is the optical power at ground state, D is the scaling constant, t0 is the gate voltage on/off time, and τ is the time constant. By fitting the experimental data to the above equation (red lines in Fig. 4a), we obtained time constants of 2.4 s and 12.8 s for rising and falling, respectively. Fig. 4b shows the temporal behavior of the optical output power as a function of the gate pulse amplitude. The amplitude gradually decreased and increased with a step by step variation of 1 V at a constant bandwidth of 1 s. As shown in Fig. 4b, the unsatisfactory slower dynamic response in fall time could be improved by applying a positive gate voltage. Based on these results, we measured the dynamic behavior of the device by switching the gate bias voltage from −4 to +4 V at various time intervals (Fig. 4c). The recovery (fall) time could



be reduced to within 1.6 sec by applying a positive gate voltage to the ion-gel gate dielectric (left side in Fig. 4c). However, continuously repeating turning the gating on and off degraded the timedependent behavior of the modulator. The optical modulator exhibited irreproducible light modulation with signal degradation under comparatively fast switching of the gate voltage. Figure 4d shows a zoomed-in section of the right site of Fig. 4c, showing that the time-dependent transmission behavior does not show a full recovery under Vg = + 4 V because the opposite gate voltage of −4 V is applied within the time constant of the fall time (i.e., in an interval shorter than 1.6 sec). The optical transmission gradually increases as the short time pulsed gate voltage switching is applied continuously. If the switching pulse interval is long enough then reversible time-dependent transmission characteristics are obtainable. Very slow electrical control of the graphene–ion-gel integrated polymer optical modulator is not ideal for high-speed optical data communication technology. However, such a long recovery time could be applicable in developing an optical nonvolatile memory device optical memory, optical equalizer, and optical transistor at a low threshold for realizing polymer waveguide-based electronic-photonic integrated circuits on a chip in the near future. The temporal response of the ion gel device can be improved by using ionic liquids with higher ionic conductivity.55 Both operational and environmental stabilities of the ion gel devices were reasonable, compared with other polymeric gate dielectric layers. However, an encapsulation layer may be useful for further stabilization of the ion gel devices.

CONCLUSION We designed and fabricated a graphene-integrated polymer waveguide modulator that is electrically controlled with an ion-gel gate dielectric. The optical modulator device was realized using a conventional semiconductor fabrication processes and careful single layer graphene transfer technology. The experimental results with an electrical top gating of ion-gel are in good agreement with theoretical simulations. The output optical power of the TE- and TM-guided modes were


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electrically controlled by applying proper electrical gate voltage to the ion-gel, based on the Pauli blocking principle of graphene. Optical modulation was possible over the entire O- and C-bands, revealing the suitability of graphene for ultra-broadband applications. The response of the TE-mode to the gate bias voltage was noticeable due to TE-mode's ultra-sensitive susceptibility to the change of the optical properties of graphene and the highly polarization-dependent nature of graphene. The slow polarization response time of ions in the ion-gel interface led to pronounced hysteresis in the electrically controlled optical output power. The slow ion mobility of the gel has a large time constant (~ tens of seconds) and is the cause of the slow operation speed of the fabricated optical modulator. However, the slow polarization response time of ions could be allowable in the development of an optical nonvolatile memory device for completing electronic-photonic integrated circuits on a chip. We conclude that these experimental and theoretical results will play a key role in designing and optimizing the next generation graphene-based polymer waveguide devices and electronic-photonic integrated circuit systems.

EXPERIMENTAL SECTION Fabrication of the polymer waveguide platform: For creating the polymeric waveguide platform, we used Exguide ZPU-RI from ChemOptics (www.chemoptics.co.kr). The fabrication process is described in the Supplementary Information Fig. S1. First, 30 µm-thick under-cladding was spincoated on to a substrate, and then cured with UV light. Subsequently, a 7 µm-thick core layer was formed. Using a photolithography process and O2 dry etching, a 7 µm-wide core structure was defined. Finally, the under-cladding resin was dispensed on the structure to surround the core. A graphene film grown using a thermal CVD method was transferred mechanically onto the core layer, using a PMMA support. After transferring the graphene/PMMA film, the PMMA was removed using acetone. A Cr/Au (5/30 nm) electrode was thermally evaporated using a shadow mask. Finally, UV-curable ion-gel gate dielectric layer consisting of the propylene glycol monomethyl ether



acetate (PEG-DA) monomer, the 2-hydroxy-2-methylpropiophenone (HOMPP) initiator, and the 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]) ionic liquid (weight ratio of 2:1:22) was patterned onto the graphene channel.

Graphene growth and transfer on to the waveguide platform: Single layer graphene was grown using CVD method using 25 µm-thick Cu foil (99.999% Cu foil from Alfa Aesar). After preannealing of the Cu foil for 20 min, graphene was grown at a temperature of 1000 °C with a gas mixture of CH4 (30 sccm) and H2 (10 sccm) under ambient pressure (3.9 × 10−1 Torr) for 20 min. After graphene growth, a rapid cooling step (cooling rate: ≈ 32 °C min−1) followed under an Ar (100 sccm) gas environment. In order to transfer the graphene on a 200 µm thick polyethersulfone (PES) substrate, graphene on a Cu foil was spin coated with a PMMA supporting layer. The backside Cu foil was then etched using 0.1 M ammonium persulfate solution. Finally, the rinsed “PMMA/graphene” films were transferred on to a selected area of the PES substrate and the PMMA was removed by acetone.

Optical measurement: The schematic view of the optical measurement setup is presented in Supplementary Information Fig. S4. Unpolarized light from a fiber-coupled superluminescence diode source (Thorlabs)was scrambled by an all fiber optic polarization scrambler (Fiberpro PS3000 series) and then, the TE- or TM-polarization light was selected by a fiber-pigtailed polarizer (OZ optics). The polarization-selected light was launched at the input facet of the fabricated waveguide using a single-mode polarization maintaining fiber (PMF). The infrared images of the guided mode were measured using a charge-coupled device (CCD) through an objective lens. After capturing the infrared images, the output light were collected by a PMF, and the transmitted power was measured with an optical power meter. The input and output optical fibers were automatically aligned by a motorized x-y-z aligner (Newport) and the maximum optical power was obtained by an optical


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power-meter (Newport 2832). The top gate voltage was applied by a sourcemeter (Keithley 2401).

Acknowledgements: This work was supported by the R&D Program funded by Korea Small and Medium Business Administration in 2016 (Grants No. S2451263), Korea and the Basic Science

Program

through

the

NRF

funded

by

the

Ministry

of

Education

(2017R1A4A1015400), Korea.

Competing Financial Interests The authors declare no competing financial interests.

Supporting Information Additional information and figures related to device fabrication process, Raman shift of graphene, measurement setup, supplementary optical characteristics, calculated field distribution of the device using a Si waveguide.



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Figures

Figure 1. (a) Bird’s-eye view of the designed graphene–ion-gel integrated optical modulator. By applying an electric voltage to the ion-gel top gate, the amplitude of the optical output signal was modulated according to the Pauli blocking principle. (b) Band structure of monolayer graphene; the electro-optic modulation is based on the tunable interband transition in graphene when applying a gate voltage to the ion-gel dielectric. (c) Cross-sectional view of the designed photonic device and the field distribution of the calculated guided mode. (d) and (e) Calculated transmission loss of the graphene-based optical modulator at wavelengths of 1.31 µm and 1.55 µm as a function of graphene's chemical potential µc. (f) Theoretical on/off extinction ratio of the photonic device according to the polarization of the guided modes and the wavelength covering the entire O- and Cbands used in optical telecommunication.


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Figure 2. Optical characteristics of the graphene-based optical modulator. Infrared images were measured at the output port of the fabricated device under (a) the absence and (c) the presence of the ion-gel. Polar images were measured under (b) the absence and (d) the presence of the ion-gel. When graphene was not covered with an ion-gel, the intensity of the TE-mode is brighter than that of the TM-mode. The device serves as a TE-pass polarizer. When graphene was covered with an ion-gel, the intensity of the TM-mode was higher than that of the TE-mode, indicating that the device serves as a TM-pass polarizer.



-7

a

-7

b

λ = 1.55 µm

Transmission (dB)

Transmission (dB)

λ = 1.31 µm

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TM-mode TE-mode

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reverse forward

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0.6

TE mode TM mode

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0.0 1.25

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1.30

1.35

1.54 1.56

Wavelength (µm)

Figure 3. Optical transmission of the graphene–ion-gel integrated optical modulator waveguide modulator according to the top gate electric voltage, Vg, (a) at a wavelength of 1.31 µm and (b) at a wavelength of 1.55 µm. The optical transmission power was dynamically changed with the variation of Vg. The response of the TE-mode is noticeable due to highly polarization-dependent nature of graphene. Hysteresis in the optical transmission was measured under a wide gate voltage sweeping (ranging from −4V to + 4 V). (c) Transfer characteristics of the ion-gel-integrated graphene, showing a significant hysteresis due to charge trapping at the graphene-ion-gel interface. This coincides with the presence of the hysteresis in the optical output power. (d) Measured on/off extinction ratio of the photonic device as a function of wavelength, covering the entire optical telecommunication spectrum (consisting of the O-band at 1.26–1.36 µm and the C-band at 1.530– 1.630 µm).


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b Vg (V)

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Figure 4. Temporal behavior of the fabricated graphene–ion-gel integrated optical modulator. (a) Optical response to the turn on and off of a −4 V gate voltage. Time response of the optical transmission for rising and falling are shown as red lines. (b) Time response to the step increase of the gate voltage. (c) Electric-switching characteristics of the device under irregular gate electric pulse periods. (d) Zoomed-in view of the electric-switching characteristics under comparatively faster gate switching.



Table of contents (TOC) graphic


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Ion-Gel-Gated Graphene Optical Modulator with Hysteretic Behavior

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