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Graphene-Incorporated Soft Capacitor for Mechanically-Adjustable Electro-Optic Modulators Sungjae Lee, Jin Tae Kim, and Yong-Won Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14638 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018
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Graphene-Incorporated Soft Capacitor for Mechanically-Adjustable Electro-Optic Modulators Sungjae Lee a,b, Jin Tae Kim c, and Yong-Won Song *,a,b,d a Center
for Opto-electronic Materials and Devices, Korea Institute of Science and
Technology, Seoul 02792, Republic of Korea b Nanomaterials
Science and Engineering, KIST school, Korea University of Science
and Technology, Seoul 02792, Republic of Korea c Creative
Future Research Laboratory, Electronics and Telecommunications Research
Institute, Daejeon 34129, Republic of Korea d KHU-KIST
Department of Converging Science and Technology, Kyung Hee University,
Seoul 02447, Republic of Korea * Corresponding author, E-mail: [email protected]
ABSTRACT: In addition to ultra-high capacity and speed in data management, future communication networks require enhanced performance via system reconfigurability under limited resources. Extremely high-speed operation renders optical data managing devices as excellent candidates to hybridize with current electronic devices; however, they still need
tunability for system reconfiguration in an integrated scheme. We demonstrate an efficient electro-optic (EO) modulator that is mechanically tunable on a multiple optical waveguide system functioned with a soft capacitor structure incorporating graphene and poly(methyl methacrylate) (PMMA). The flexible capacitor that generates optical signals by temporal light absorption depending on electrical signals can be mechanically detached and reattached from and onto a rigid surface of the waveguide. It provides either the on or off state of the modulating operation, and enables switching of the working waveguides following the reconfigured data routes. Quality-controlled graphene mainly provides the EO operation, and PMMA plays an important role as both the flexible dielectric layer in the capacitor and the passivation layer for graphene protection. The modulation effects of the manually prepared graphene-PMMA capacitor mechanically adjusted onto a sidepolished optical fiber (D-shaped fiber) are investigated in terms of the extinction ratio (ER) of the transmitting light and the operational bandwidth. We successfully display an ER of the modulator up to 19.8 dB with a voltage control ranging from -50 V to 50 V. Its stable operation is verified with a modulation speed up to 2.5 MHz.
Ultra-high capacity and speed in data management systems are critical requirements in future communication networks. Extra efficiency can be incorporated in a system by realizing a system reconfigurability that guarantees a dynamic data management scheme.1-3 In particular, for optical networks with both scalability and reconfigurability, it is essential to develop highly densified and tunable optical devices that are operated by nanomaterials with extremely small footprints, and significant optical nonlinearity and material flexibility.1-3 Thus far, outstanding nanomaterial properties have been successfully demonstrated in various optical devices. Such devices include: ultrafast optical switches, electro-optic modulators, and femtosecond lasers.4-10 However, nanomaterial-based devices, including modulators, do not assure the tunability that could potentially maximize the device and system operation under limited resources.
Among various nanomaterials, graphene, a hexagonal array of carbon atoms in a single-layer, has received great attention owing to its outstanding electrical and optical properties.11-16 It was reported that graphene has a carrier mobility exceeding 200,000
cm2 V-1 s-1 at room temperature, and exhibits constant absorption of light around 2.3% in the broad range of the wavelength, which is wider than few micrometers.17-19 In particular, based on its high third-order nonlinearity and linear distribution of Dirac-Fermions, graphene has been successfully employed in optical, opto-electrical, and EO applications.20-25 Its optical absorption properties can be tuned by shifting the Fermi energy level with the carrier population control in graphene crystals by taking advantage of the unique energy band structure of graphene.20 Liu et al.22 demonstrated the first graphene-based EO modulators with broadband operation in an ultra-compact size. The modulation efficiency was dramatically increased by employing two graphene sheets in the capacitor structure.23 Moreover, graphene EO modulators having an operational speed up to 35 GHz, and stable operation under varying temperature, have been realized by modifying graphene capacitor structures. This modification was done by varying the geometry of the dielectric layers and decreasing the series resistance of the devices.24 Phare et al.25 solved the trade-off problem between the operational speed and efficiency of graphene modulators by employing an interferometric structure such as a ring resonator. However, the graphene EO modulators reported to date still operate within a
conventionally fixed and inefficient operation regime. For a tunable and reconfigurable operation, the EO component in the modulator needs to be separated from the waveguide, and shifted for switching with different waveguides such that the data stream can be embedded onto a newly reconfigured channel. Here, it is important that the EO component be based on soft materials for immunity against the stress and deformation induced by its mechanical movement. Even though flexible devices have been actively developed in diversified fields, such as displays, wearable gears, and energy storages, most of them are being used in the field of electronics. Flexible EO components for use in tunable devices have not yet been investigated.26,27 Moreover, nonlinear interactions with soft and dynamic devices needs to be explored for the propagating light. It is expected that the dynamic harmony of the flexible EO components with the photonic systems
would
ensure
highly
efficient
electronic-optic
hybrid
data-managing
environments.28-30
In our research, we investigated the mechanical tunability of the EO modulators in a multiple waveguide system. We demonstrated a simple but elegant soft-capacitor
structure incorporating graphene and PMMA that can provide an optimized mechanical contact for the capacitor on a rigid surface of the optical waveguide. The hand-made graphene-polymer-graphene capacitor structure was fabricated by transferring the graphene with PMMA, which plays an important role as both the flexible dielectric layer in the capacitor and the passivation layer for graphene protection. In practice, the capacitor structure can be detached and re-attached from and onto a waveguide for achieving system reconfigurability. The quality of the graphene was quantitatively analyzed by transmission electron microscopy (TEM), transmittance analysis, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The structure of the capacitor was confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The modulation effects of the graphene-PMMA capacitor mechanically adjusted onto a D-shaped fiber were investigated in terms of ER of the transmitting light and the operational bandwidth. We successfully demonstrated an adjustable graphene EO modulator that has an extinction ratio up to 19.8 dB with a voltage control ranging from 50 V to 50 V. Its stable operation was verified with a modulation speed up to 2.5 MHz.
Graphene synthesis and transfer: Graphene was synthesized on a Cu foil (Alfa Aesar, 25 um, 99.8%) via chemical vapor deposition (CVD). The graphene on the top of the Cu foil, that had considerably low quality, was eliminated by O2 reactive ion etching (RIE). The PMMA (Sigma Aldrich, MW = 996 k) was dissolved in the chlorobenzene (Sigma aldrich, anhydrous, 99.8%) with a concentration of 46 mg/ml, and was spin coated on the graphene. The Cu foil was etched by immersion in 0.1 M ammonium peroxydisulfate (CAS No. 7727-54-0) solution for 18 hours. The graphene with the PMMA layer was transferred to the substrate after washing the etchant three times in distilled water (for 30 minutes). The transferred graphene with the PMMA layer was blown over by N2 gas, and was heated (above the glass transition temperature of PMMA) to remove the water molecules and to improve the contact between the graphene and the substrate. Fabrication of graphene capacitor structure: To build the capacitor structure of graphene/PMMA/graphene efficiently, the first PMMA/graphene layer was located onto the electrode-patterned SiO2/Si substrate for the electrical connection of graphene to one of the electrodes. The second PMMA/graphene layer was also added partially overlapping the first layer, and was connected to the other electrode. The electrodes in the graphene capacitor structure were connected to the printed circuit board (PCB) that was compatible with the subminiature version A (SMA) cables that could efficiently carry high-frequency electrical signals to a device using Au wires. D-shaped fiber: A single mode fiber (Corning, SMF-28e) (SMF) was fixed by an epoxy resin (EPO-TEK, 353ND) on a curved acryl holder having a curvature radius of 162.5 mm. This
was done to reliably localize the contact point between the capacitor and the fiber whose side was polished by sandpapers (Thorlabs, LF03P). Physical contact method: Graphene capacitor structure was put in physical contact with the D-shaped fiber using a set of precise stages, including the translation stage (SIGMAKOKI, TSD605C), rotation stage (SIGMAKOKI, KSP-606M), and goniometer stage (SIGMAKOKI, GOH60A50). Analysis of the modulation effects: A continuous wave laser (CW laser) with a center wavelength of 1550 nm generated by the distributed feedback laser (Anritsu, MT9810A) (DFB laser) was propagated to the D-shaped fiber that was physically in contact with the graphene capacitor after adjusting the state of polarization (SOP) using a fiber type polarization controller (PC). Prior to measuring the ER of the modulators, on and off functions were checked by switching the DC power supply (KEYSIGHT, E36106A). The modulation depth of the modulated optical signals were analyzed by an optical spectrum analyzer (YOKOGAWA, AQ6370C). The sinusoidal signals generated by a pattern generator (KEYSIGHT, 33120A) were amplified by an electrical amplifier (Mini-circuit, ZHL-6A+) and were voltage-positioned by a bias tee (Mini-circuit, ZFBT-6GW-FT+). These signals were fed into the graphene modulators. The electrical signals were tuned to maximum within a range of 10 V to 40 V with frequency of 100 kHz to 500 MHz. The modulated optical signals were converted to electrical signals by a photodetector (EOT, ET-3500F), and were analyzed by an optical power meter and an RF spectrum analyzer (Keysight, N9000B.26G).
Figure 1 illustrates the control of carrier density in graphene, operational principle of the mechanically-adjustable graphene EO modulators, and its waveguide-tunable operation. The modulators based on the EO properties of graphene can embed signals into an optical channel by tuning the amplitude of light with the controlled voltage applied to the graphene.20 Figure 1a illustrates the tunable EO properties by adjusting the Fermi energy level of graphene with voltage (V) control. When V is applied to graphene, the density of carrier (ns) of graphene is changed as follows.
𝑛𝑠 =
𝜀0𝜀𝑜𝑥 𝑑𝑒
(1)
(𝑉 + 𝑉0)
where ε0 is the dielectric constant of vacuum, εox is the dielectric constant of the dielectric layer, d is the thickness of the dielectric layer, e is the electron charge, and V0 is the offset voltage originating from the natural doping in an actual graphene sheet.31 The Fermi
energy level of graphene (EF) is determined by the density of carrier in the graphene crystal. This can be expressed by the following equation.
(2)
𝐸𝐹 = ℏ𝑣𝐹 𝜋 ∙ 𝑛𝑠
where ℏ is the Dirac constant and vF (~ 1.1 x 106 ms-1) is the Fermi velocity of graphene.32 The adjustment of the Fermi energy level blocks the interband transition (i.e. excitation) of electrons that would occur by absorbing the photon energy of the incident light. The graphene maximally absorbs the light in its natural state and turns transparent with application of the voltage, which is explained by Pauli exclusion principle.33 Consequently, the light propagating through the graphene EO modulator can be modulated with the applied electrical signals. The operational principle of the mechanically-adjustable modulators operated on a waveguide (an optical fiber in this case) is shown in Figure 1b. The field distribution of the light propagating inside the optical fiber is of Gaussian shape displaying a maximum at the center of fiber core.34 When the cladding of the optical fiber
is polished, the evanescent field that is distributed outside the core, is broadened due to a decrease in the effective refractive index in the vicinity of the polished area.35-37 The physically contacted graphene capacitor with the polished surface of the D-shaped fiber can interact with the broadened evanescent field of light propagating inside the optical fiber, and can control the light absorption in graphene allowing light modulation with voltage control. For reconfigurable data systems, tunable operation of the devices is required. In particular, tunability enabled by mechanical movement of the device components can provide a straightforward but effective approach toward realizing a dynamic system. In order to realize a device based on its physical contact operation, the device itself and the employed materials need to be soft and flexible. Due to their inherent flexibility, graphene and polymers can automatically adjust to unexpected surface conditions as well as to misalignments in the optical pathways induced during a mechanical movement. Note that a flexible device placed near the optical waveguide can guarantee a ‘non-blocking operation’ where a weak evanescent field interaction of the light with the device would allow a safe and stable operation.36 The schematic illustration of the waveguide-switchable operation is shown in Figure 1c. Basically the modulation
efficiency depends on the voltage supplied. Since the mechanically-adjustable graphene EO modulators adopt the evanescent field interaction, the modulation operation of the device can be switched from ‘on’ to ‘off’ by taking the graphene capacitor away from the waveguide. It is also possible to tune the number and/or position of the modulated channel by adjusting the location of the graphene capacitor on the multiple waveguide patterns. Moreover, a facilitated management in terms of individual inspection and replacement of the modulator elements is available. Since our modulators can be directly attached to the designed waveguides, additional external optical components such as an interferrometric structure is not required.
Figure 2 represents the fabrication processes of the mechanically-adjustable graphene EO modulators. Graphene was synthesized via CVD and was employed by the modulators by a wet-transfer method.38-40 The high quality of graphene that would guarantee the EO effects was maintained by optimizing the conditions of the graphene transfer processes (see Figure S1). Unlike the conventionally prepared graphene (with the PMMA removed), the PMMA layer can be incorporated into the flexible capacitor
structure as a dielectric layer due to its dielectric and flexible properties. Moreover, the PMMA layer acts as a passivation layer against graphene contamination to guarantee a reliable device operation (see Figure S2).41,42 The capacitor structure of the graphene/PMMA/graphene was formed efficiently by transferring the PMMA/graphene layers onto the electrode-patterned SiO2/Si substrate twice. Considering the physical reliability on the substrate, Ni was selected as the electrodes. The fabricated graphene capacitor structure was physically contacted with the D-shaped fiber using a set of precise translation stages.
Figure 3a shows the cross-sectional image of the capacitor obtained via SEM analysis. The clear boundaries between the PMMA and substrate can be observed, where the two sets of transferred PMMA/graphene layers have formed a capacitor structure. Since each graphene sheet is attached under the PMMA layer respectively, the PMMA layer sandwiched by the graphene sheets acts as a dielectric layer of the capacitor structure. The top PMMA layer functions as the passivation layer that can protect the graphene from the environment and stress induced by the mechanical motion. The
thickness of the PMMA measured by AFM was 300 nm (see Figure 3b). The performance of the modulators based on the graphene capacitor structure depends on the characteristics of the dielectric layers such as, thickness and dielectric constant as expressed by the equations (1) and (2). The thickness of the graphene estimated via AFM analysis was about 1.4 nm (See Figure 3b inset), which is thicker than the theoretical value of 0.345 nm. We concluded that the measured value is from the interlayer voids generated by the imperfect adhesion between the transferred CVD grown graphene and the substrate.43 The characteristics of the graphene were furtherly analyzed by TEM, transmittance analysis, Raman spectroscopy and XPS. For identifying the arrangement of carbon atoms in the graphene crystals, the graphene was transferred onto the Cu grid (Okenshoji, NP-C15) and was analyzed via TEM (see Figure S3). Figure 3c shows the selected area electron diffraction (SAED) patterns of the graphene. The clear hexagonal patterns of the reciprocal lattice ensures a successful formation of the graphene crystals.44 Figure 3d shows the transmittance of graphene in the range of visible to nearinfrared area. The measured transmittance at the telecommunication wavelength of 1550 nm was 97.3%. At the visible wavelength of 550 nm, which is generally used as a standard
wavelength to measure graphene thickness, with a high correlation between the theoretical and experimental values, the transmittance was 96.3%.45 The measured transmittance at 550 nm lies between the theoretical values of mono-layered graphene (97.7%) and the bi-layered graphene (95.4%).46 This can be explained with the multilayered graphene islands that were randomly added onto the graphene surface due to the metal contaminants on the surface of the Cu foil.47 In addition, unwanted wrinkles, impurities and PMMA residues formed during the transfer process reduced the transmittance of the graphene. Figure 3e shows the Raman spectrum of the graphene where the positions of the D peak, G peak, and 2D peak were ~1349, ~1588, and ~2691 cm-1, respectively. The intensity ratio of the 2D peak over G peak was ~4.2, and the D peak over G peak was ~0.06. This indicates that the quality of the synthesized and processed graphene was close to a defectless mono-layered graphene.48,49 Figure 3f shows the C1s spectrum of the graphene obtained via XPS analysis. The single peak with a high-intensity was observed at a binding energy of 284.6 eV. This corresponds to the sp2 bonding of the carbon atoms. The portions of sp2 bonds in graphene measured with and without the PMMA layers were ~51 % and ~75 %, respectively. It has been
reported that the high portion of the sp2 bond in graphene guarantees the electro-optic effects of the modulators.50
Figure 4 illustrates the experimental setup for analyzing the modulation effect of the mechanically-adjustable graphene EO modulators (The photograph of experimental setup is displayed in Figure S4). The CW laser at telecommunication wavelength propagated into the D-shaped fiber in physical contact with the graphene capacitor. Since graphene only absorbs the in-plane electric field of the light, the SOP of CW laser was controlled by a fiber type PC to maximize the absorption. The modulation effect was also maximized by the geometrical alignment between the graphene capacitor and the Dshaped fiber. Considering the required voltage tuning window to achieve a significant optical modulation effect, the electric signals had to be amplified and biased. The modified electric signals (using a set of amplifiers and bias tees) were confirmed by an oscilloscope (Figure S5a). Subsequently, these were applied to both electrodes of the graphene capacitor using SMA cables. The modulated optical signals were converted to electrical signals, and were analyzed by the optical power meter and the RF spectrum analyzer.
Our experimental setup for measuring modulation effect was verified and compared with a commercial EO modulator (See Figure S5b).
Figure 5a shows the transmission changes of the light as switched by the mechanically-adjustable modulators under a varying voltage supply. Here, two types of modulators (with single or double graphene layer) are compared. The maximum ER shown by the modulators with one graphene electrode was ~30 % of the maximum ER of the modulators with two graphene electrodes. It shows the difference in the electro-optic effect, that clearly relies on the number of graphene layers. In addition, the modulation effect was tunable by adjusting the peak-to-peak voltage and the bias voltage of the modulating electrical signals. Figure 5b shows the transmission changes in light as adjusted by the modulators. These modulators had double graphene electrodes in the capacitor structure but different sample sizes. The maximum ERs of the light transmitting into the modulators that have graphene capacitor size of ~60 mm2 and ~120 mm2 were ~19.8 dB and ~16.9 dB, respectively. Two modulators were fabricated in the same process condition except the size of the graphene capacitor. Since the dielectric
constants and thicknesses of the PMMA layers (which determine the change in the carrier density in graphene) were identical, The difference of ERs mainly originated from the different surface condition of the contacted interface as well as the 3-dimensional tilt of graphene capacitor with respect to the surface of D-shaped fiber (See Figure S6). Figure 5c shows the operational speed of the modulators with different sample sizes as explained through Figure 5b. The SOP as well as the physical alignment state between the graphene capacitor and the D-shaped fiber were constant while measuring the ER and the operation speed of modulators. The operation speeds of modulators were analyzed using the S21 parameter that can be explained by the intensity ratio of the RF output to the RF input of the devices (See Figure 5d).51 Since a direct measurement of parameters, such as impedance, current, and voltage of the RF signals at high frequencies is challenging, the S parameter is found to be useful in analyzing the operational characteristics of the devices. The modulated optical signals that have a graphene capacitor size of 60 mm2 were observed up to 2.5 MHz (See Figure S7). The operation of the modulator is mainly limited by the capacitance and the serial resistance of the device. The capacitance of the graphene capacitor having a size of 60 mm2 was
measured as ~193 pF (See Figure S8). The general approach to define the operational speed of the modulator is to measure the f3dB representing the 3 dB bandwidth of the modulators. The f3dB can be defined by the following equation.
1
(3)
𝑓3 𝑑𝐵 = 2𝜋𝜏𝑅𝐶
where τRC is the RC time constant obtained by multiplying the resistance and the capacitance of the capacitor in the modulator.52 The calculated f3dB values of the modulators that had graphene capacitor sizes of ~60 mm2 and ~120 mm2 were ~1.55 MHz and ~0.7 MHz, respectively. Considering that the capacitance is proportional to the size of the capacitor, we can conclude that the 2.2-times difference of the f3dB between the two modulators is analytical. Note that our mechanically-adjustable modulator is targeting low speed signals applicable in data packet guiding and topological reconfiguration in data transmission networks.
Data management is becoming a significant issue for society, which is expected to be hyper-connected. Optical devices with reconfigurable operation as well as ultra-high capacity and speed are strong candidates for solving the underlying technical challenges. We have demonstrated a mechanically-adjustable graphene EO modulator that can be dynamically modified by waveguide selection and switching, allowing a reconfigurable optical network. The graphene capacitor structure was efficiently fabricated by the transfer method, and was analyzed quantitatively. The employed materials in the capacitor structure were flexible, demonstrating their suitability as device components for a
mechanically-tuned
system.
The
fabricated
capacitor
structure
of
graphene/PMMA/graphene was mechanically in contact with the D-shaped fiber to realize an effective adjustable graphene EO modulator. This work experimentally demonstrated the practical aspects of such a reconfigurable optical network. Even though the modulation speed is limited by the size and the electrical properties of the custom-built
devices, we believe that the operation principle can be developed further to achieve ultrahigh efficiency and performance by optimizing the dielectric materials and the structure of the graphene capacitors.
SUPPORTING INFORMATION
The supporting information is available free of charge through ACS publications. Supplemental figures on the fabrication process, mechanical contact method, and the modulation effect analysis are included.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Institutional Program (Grant No. 2E28200) funded by the Korea Institute of Science and Technology (KIST), South Korea. Also supported by the National Research Foundation (NRF) (Grant No. NRF-2015R1A2A2A04006979), and Institute for Information & Communications Technology Promotion (IITP) grant (Grant No. 2018-0-01156) funded by the Ministry of Science and ICT, South Korea.
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Figure 1. (a) The tunable EO effect of graphene with voltage control. (b) The operation principle of the mechanically-adjustable graphene EO modulator. (c) The reconfigurable operation of the mechanically-adjustable optical device.
Figure 2. Fabrication process of the adjustable graphene EO modulators.
Figure 3. (a) SEM image of the graphene capacitor structure. (b) Thickness of the PMMA layer in the graphene capacitor structure and (inset) the thickness of the transferred graphene. (c) SAED pattern of graphene. (d) Transmittance of graphene in the visible to near infrared range. (e) Raman spectrum of graphene. (f) XPS C1s spectrum of graphene.
Figure 4. Experimental setup for measuring the modulation effect and (inset) the photographs of graphene capacitor and D-shaped fiber.
Figure 5. (a) ERs of the light modulated by the two types of the modulators that have either single or double graphene layers. (b) ERs of the light data-embedded by the modulators that have graphene capacitors of different sizes, (c) Operation speed. (d) Schematic illustration of the S parameter.
