Ultrafast All-Optical Switching Incorporating in Situ Graphene Grown

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Ultrafast All-Optical Switching Incorporating in Situ Graphene Grown Along Optical Fiber by Evanescent Field of Laser Pulak Chandra Debnath, Siam Uddin, and Yong-Won Song ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00925 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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Ultrafast All-Optical Switching Incorporating in Situ Graphene Grown Along Optical Fiber by Evanescent Field of Laser Pulak C. Debnath,†,‡ Siam Uddin,†,‡ Yong-Won Song†,‡,* †

Center for Opto-Electronic Materials and Devices, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea ‡

Division of Nano & Information Technology, Korea University of Science and Technology, Daejeon 34113, Republic of Korea *Corresponding Author: E-mail: [email protected]

ABSTRACT Graphene, with its high optical nonlinearity and unique dispersionless nonlinear optical response over a broad-wavelength-range, has been studied extensively to implement optical devices such as fiber laser, broadband modulator, polarizer, and optical switches. Conventionally synthesized graphene relying on high temperature and vacuum equipment suffers from deleterious transfer steps that degrade the graphene quality, thereby affecting the efficiency of nonlinear optical operation and lacking the customized patterning with minimized footprint as well as missing the facilitated fabrication process. Here, a laser-aided in situ synthesis of multilayered graphene directly onto the flat surface of a side polished optical fiber in ambient condition is demonstrated for absolute investigation of as-grown graphene crystal in optical domain. The evanescent field of an amplified continuous wave (CW) laser, propagating through optical fiber, provides activation energy for carbon atoms to diffuse through the nickel catalyst and grow graphene directly on the polished side of optical fiber. Ultrafast all-optical switching near 1550 nm is elucidated by exploiting four-wave mixing (FWM) with the grown graphene to confirm that the nonlinear response improvement of 1 ACS Paragon Plus Environment

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58.5 % is originated from the graphene. The incident signal is modulated at the ultrafast speed of up to 20 GHz, and the modulation information is successfully copied in the newly generated signals at different wavelengths. KEYWORDS: in situ graphene, ultrafast optical switch, evanescent field interaction, four-wave mixing, optical nonlinearity

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In recent decades, Graphene, a two dimensional (2D) honeycomb lattice constituted of sp2−hybridized carbon-carbon bonded atom pairs, with its outstanding physical, electronic and nonlinear optical properties, has received a ubiquitous research interest in the field of condensed matter physics and nanoscale devices including electronic, thermal, mechanical, and photonic applications.1−6 With its remarkable optical properties especially high third order optical nonlinearity (χ(3)) and linear dispersion of massless Dirac fermions in graphene, it is enabling an ever increasing number of optical, optoelectronic and electro-optic applications.3−11 In the optical domain, graphene enhanced applications include photodetectors, ultrafast mode-locking, optical modulators, and wavelength conversion.12−25 It is indicated in the earlier reports that, the nonlinear optical response in graphene is fundamentally dispersionless over an extensive range of wavelength and is considerably higher compared to that of conventional bulk semiconductors. The nonlinear refractive index of graphene has been theoretically calculated to be n2 ≈ 10−15 m2 W−1 by Cheng and associates considering only the two-photon absorption mechanism.26 Such high optical nonlinearity from graphene crystal can be exploited in order to comprehend numerous optically nonlinear efficient devices for ultra-high speed telecommunications likewise, signal regenerators, tunable wavelength converters, and ultra-fast optical switches. By using graphene–silicon composite optoelectronic devices, some nonlinear optical phenomena such as optical bistability, four-wave mixing (FWM), and self-induced regenerative oscillations have been successively demonstrated.19−25 It is acclaimed that, advanced optical modulation techniques have become imperative in assisting ultrafast optical networks where tunable wavelength conversion as well as ultrafast optical switching is vastly anticipated.27 In this case, a valuable objective is to discover the ultrafast optical switching enabling advanced modulation formats based on FWM in graphene. FWM has been realized with graphene in various arrangements, such as, graphene−silicon composite photonic crystal waveguides,24 optically deposited graphene onto the optical fiber end facet,28 and graphene-based micro fibers.25 Although graphene, in the family of 2D materials, dominates the pioneering work of all-optical signal processing, similar works has been 3 ACS Paragon Plus Environment

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demonstrated by other 2D materials such as black phosphorus and MoS2, followed by transition metal dichalcogenides (TMDs) and topological insulators (TI).29−33 In a recent work reported by Xu et al., mechanically exfoliated thin graphene film was employed to experimentally observe optical wavelength conversion based on FWM originated from that graphene film while operating at high speed, up to 10 Gb/s non-return-to-zero (NRZ) signal.34 Thus far, the most established method to prepare graphene-based optical devices for FWM is acquired by the synthesis of graphene on a specific substrate in a high temperature CVD process under vacuum condition, which requires external carbon sources to grow graphene. Moreover, the pre-synthesized CVD graphene needs to be transferred to a waveguide or optical fiber to complete the optical device fabrication. Most renowned and reported techniques for integrating pre-synthesized graphene or other optical materials into optical systems for experimental purposes involve insertion of the optical materials between the end facet of two optical fibers,15, 17, 35−43 or attachment to either the polished and flat Dshaped fiber surface,44−45 or depositing around the midriff of a tapered fiber.46−47 Yet, these conventional techniques are not out of disadvantages of having to transfer the optical materials to the appropriate location; more particularly, these transfer procedures can potentially incorporate unexpected contaminations into the nonlinear materials. According to graphene films, these techniques have the chances to damage the graphene crystal, which may result in degraded performance of graphene as an optical material. Utilizing the high optical nonlinearity of presynthesized graphene for ultrafast optical manipulation without degrading the quality of the graphene crystal during transfer steps is one of the keys to make graphene into a real-world optical material. Other key obstacles on that route also include the limited control on the chip-scale integration of graphene patterns with minimum footprint and the complicated fabrication process. Customized pattering of graphene on optical devices is not yet possible due to the limitations harnessed by transfer steps. In situ synthesis of graphene on optical fiber will bring the opportunity of in-fiber customized patterning of graphene with chip-scale integration of graphene based optical and photonic devices. To resolve these problems, an in situ technique can be adapted to evade these 4 ACS Paragon Plus Environment

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existing conventional harmful techniques and to synthesize graphene film directly onto the waveguide or the optical fiber modules. In the recent past, interfacial synthesis of multilayer graphene has been demonstrated directly at the substrate and metal catalyst interface,38−39, 48−51 where, diffusion of C atoms took place through pre-deposited metal thin film, from an external C source to grow graphene interfacially in an elevated thermal condition. In addition, lasers functioning in the range from 248 nm to 1060 nm have been effectively used as thermal sources for both synthesizing or depositing and patterning graphene, graphite, and graphene oxide materials on arbitrary substrates from external carbon atom rich sources for example, CH4:H2 gas, graphite nanoparticles, S-1805 photoresist, SiC, graphene/polyvinyl acetate (PVAc) solution, and poly(methyl methacrylate) (PMMA).37, 47, 52−61 Recently, a telecommunication laser set to the conventional wavelength at 1550 nm has been used to demonstrate in situ formation of multilayer graphene as a saturable absorber (SA) directly onto optical fiber end facets to establish mode-locked fiber laser.62 However, the formed graphene with limited area on optical fiber core (diameter: ~10 µm) end facet exhibits low sp2 bond density in the graphene, which limits the obtained third order optical nonlinearity as well as the performance as an ultra-fast optical device. The interaction length among the laser and the graphene is also limited with the graphene formed onto the optical fiber end facet. Thus, a large area synthesis of graphene on the optical fiber surface should be considered where both the density of sp2 bonds and the light−matter interaction can be enhanced. Polishing the side surface of a single mode fiber 'extracts' evanescent optical field from propagating light to enhance interaction with the optical nanomaterials synthesized on the polished surface. Since the interaction occurs along the axial direction of an optical fiber, the interaction length can be increased dramatically. In this study, we exhibit a laser evanescent field induced in situ technique to synthesize multilayered graphene directly on the flat surface of side polished single mode optical fiber in normal atmospheric condition, without any external carbon sources. This in situ synthesis is accomplished by laser irradiation on a metal catalyst coated onto the flat surface of D-shaped fiber 5 ACS Paragon Plus Environment

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employing evanescent field of an amplified continuous wave (CW) laser operating at 1550 nm. The quality of the grown graphene is examined using precision analysis tools. Quantitative analysis by X-ray photoelectron spectroscopy (XPS) shows that the percentage of sp2 bonding in the synthesized graphene is 71 %. Analysis by transmission electron microscopy (TEM) confirms the synthesis of graphene and ensures in-plane atomic structure of carbon in grown graphene. We demonstrate FWM-based wavelength conversion with the in situ grown graphene for incident signals modulated up to 20 GHz ensuring the effect of 3rd order optical nonlinearity (χ(3)) in the graphene. We compare the FWM output for graphene-grown D-shaped fiber to that of bare Dshaped fiber to find a 58.5 % enhancement of the optical nonlinearity from the grown graphene. Moreover, the tunability of wavelength conversion by wavelength detuning experiment and the conversion efficiency (CE) of FWM is also investigated.

RESULTS AND DISCUSSION Laser-aided in Situ Approach for Graphene Synthesis. Figure 1 elucidates the procedure of the laser aided in situ graphene synthesis directly along the polished side of waveguide (polished region of prepared D-shaped fiber device in this work) and the mechanism of FWM in the as-grown graphene on D-shaped fiber. At the beginning, a layer of Ni polycrystalline thin film with depth of 100-nm deposited on the flat polished region of the sample fiber by using electron beam evaporation as illustrated in Figure 1a. Carbon atoms are highly soluble in Ni, which is one of the reasons to choose Ni as the catalyst to grow graphene. As Ni pellets are put into the graphite crucible during the deposition procedure, The C atoms adjacent to the surface of the Ni pellets are also evaporated as impurities, thereby, a layer of polycrystalline Ni with abundant amount of C atoms are also deposited along the flat polished region of the D-shaped optical fiber. The close-up view of the C atoms enriched polycrystalline Ni is schematically shown in the inset of Figure 1a. Figure 1b shows the optical micrograph of the Ni deposited D-shaped polished fiber, which is employed in this work to synthesize graphene by laser irradiation. Figure 1c illustrates the laser


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irradiation process used to synthesize graphene at the interface of deposited Ni and the flat polished fiber surface. An optically amplified, telecommunication-band, continuous wave laser operating at ~1550 nm is joined to one end of D-shaped fiber, which provides required thermal energy in order to synthesize graphene. Due to the appearance of asymmetry in its cross section, the side polished fiber is highly sensitive to the state of polarization (SOP) of the input laser light. Hence, SOP of the incident laser is manipulated with a polarization controller (PC) to ensure maximum light−matter interaction at the interface by setting minimum power at the output (Figure S1, Supporting Information). Because the D-shaped fiber is asymmetric in its cross section after polishing to remove a portion of the cladding, y polarized light components (along with many other components with different SOP) are not able to propagate through the fiber but are scattered through the flat polished surface fiber and interact with deposited Ni. In that way, the temperature at the interface among Ni film and polished fiber increases as a result of strong light-matter interaction of the incident laser evanescent field with the deposited Ni film. Consequently, the neighboring carbon atoms residing within Ni grains start diffusing through the Ni grains and precipitate towards the interface of Ni/fiber that empowers to synthesize graphene. Normally the net diffusion of atoms is determined through the interdiffusion coefficient in the composite materials structure.63 Under laser irradiation, the carbon atoms residing Ni grain matrix exhibited a predictable net diffusivity which is adequately high enough for permitting their advent into Ni/fiber interface. Therefore, consideration on the elemental concentration distribution of existing elements leads to finding the diffusion procedure, which can be interpreted according to Fick’s second law (∂φ/∂t)=D(∂2φ/∂x2),64 where, D, φ, t, and x are the diffusivity, elemental concentration, time, and distance, respectively. As reported earlier, carbon diffusion through polycrystalline nickel grain is the proposed mechanism for the synthesis of multilayered graphene along the interface of Ni film deposited on the polished fiber under laser irradiation.62 A description and the model of this mechanism is presented in the Supporting Information (Figure S2, Supporting Information). Since the side polished fiber is made by manually polishing the cladding of the single mode fiber, the length of the 7 ACS Paragon Plus Environment

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interaction between laser and the Ni on the polished surface is determined by controlling the length of the polished surface. In that way, large area graphene synthesis can be achieved by evanescent field laser irradiation on Ni over long length of the flat polished region of fiber. The laser irradiation is run for one hour to synthesize multilayered graphene on the polished fiber surface. As soon as the irradiation is finished, the sample is immersed in a 0.2 M Iron (III) Chloride (FeCl3) solution for 30 minutes to etch out the Ni film. At the end of etching process, the D-shaped fiber device is left with the graphene synthesized directly on its flat surface. The polished surface of prepared D-shaped fiber sample device at different steps of the experiment is shown in Figure S3 in the Supporting Information. Figure 1d is an optical photograph of the flat surface of D-shaped after Ni etching. A graphene pattern is seen on the core surface after the synthesis and etching process. The length of the graphene formed surface is measured to be ~150 µm, which is later mentioned as interaction length during optical operation. This device can be easily employed to acquire FWM-based ultrafast all-optical switching or wavelength conversion. Figure 1e illustrates a schematic of the FWM phenomenon originated from synthesized graphene along the flat polished region of the prepared fiber by polishing. A continuous wave laser pump of wavelength at ω1 and a high frequency modulated signal probe of wavelength at ω2 are combined to launch together through the graphene synthesized nonlinear polished fiber device having third order high optical nonlinearity (χ(3)). This configuration utilizes evanescent fields of both propagating signals which interact with the graphene to exploit its nonlinear properties. Evanescent field exhibits strong electric field, long interaction length and low-loss transmission through the graphene/optical fiber surface which offers high third order nonlinearity as well as high optical damage threshold eventually. At the output, additional new signals are generated at the wavelength of ω3 and ω4 by FWM in the graphene-based nonlinear device. Since the third order nonlinearity is a ultrafast process, highly nonlinear behavior of graphene on the optical fiber surface can identify the ultrafast modulation of the incident signal and the modulation pattern of the incident signal is exactly followed by the newly generated signals by FWM. Mimicking the modulation pattern of the incident signal by newly generated signals is the 8 ACS Paragon Plus Environment

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standard of ultrafast all-optical switching. As long as the energy is conserved in the FWM procedure, an expression 1/λgenerated signal = 2/λpump－ 1/λsignal can be utilized to carry out the wavelength of the newly generated signal. Therefore, the wavelength of the newly generated signal (λgenerated signal) can be adjusted easily by attuning the pump (λpump) and signal (λsignal) wavelengths. Figure 1f explains the principle of FWM induced in graphene. When a pump light and a signal light with wavelengths of ω1 and ω2, respectively, are incident on the graphene crystal simultaneously, because of photon absorption and emission, electronic transitions in graphene band structure occur, hence generating new frequencies ω3 = 2ω1－ω2 and ω4 = 2ω2－ω1. Characterization of in Situ Grown Graphene. The graphene film synthesized by in situ technique along the polished region of the fiber is analyzed by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM), as depicted in Figure 2. Figure 2a illustrates the Raman spectrum taken from the polished flat region of fiber after graphene synthesis followed by Ni etching. This spectrum shows D peak, G peak and 2D peak at ~1340 cm-1, ~1600 cm-1, and ~2840 cm-1, respectively, which indicate the projected peak positions to confirm the graphene formation. The peak at 1600 cm-1, denoted by G, originates due to E2g phonon vibration mode at the doubly degenerate zero-center, corresponds to the stretching motion of the sp2 hybridized C-C bonded atom pairs.65 The intensity ratio of D peak and G peak (ID/IG) shows the degree of lattice crystallinity of the graphene film,65−66 is calculated as ~0.32, which indicates that the graphene is of good quality with few defects. The defects in the graphene may have arisen because of symmetry breaking in the basal plane of grown graphene.67−68 The polished region of fiber had been prepared by manually polishing the cladding of the SMF, which results in rough surface on the side polished fiber. The roughness of this surface may induce strain in the grown graphene that leads symmetry breaking. In the basal plane of grown graphene, symmetry breaking may arise due to either uniaxial strain or doping. In this work, both can be occurred, which is unavoidable. Moreover, in the basal plane of grown graphene, spontaneous Ni doping might occur, which results in defects in grown graphene 9 ACS Paragon Plus Environment

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film as well. Figure 2b shows the detailed structural and compositional analysis done by using XPS on the grown graphene films. The XPS spectrum in long span is presented in detail within the Supporting Information Figure S4. In Figure 2b the C1s spectrum portrays a single peak having supreme intensity, which provides sufficient information of existence of abundant amount of sp2 hybridized carbon atoms with binding energy of 284.53 eV. Deconvolution of the peak area gives a clear indication that, 74 % of the carbon atoms are sp2 hybridized, 14.44 % are related to the sp3 hybridization, having a binding energy of 286.3 eV, and rest of the 11.47 % are associated with C=O with binding energy of 288.3 eV. Further analysis is done using EDS on the graphene grown D-shaped fiber sample. Figure 2c displays a comparison between the chemical elements present in the core portion and cladding portion on the flat surface of D-shaped fiber, which is done by using EDS analysis on the sample. The EDS result from fiber core region confirms regarding existence of both germanium (Ge) as well as carbon, which ensures that, graphene formation was successful on the polished region of D-shaped fiber, especially on the core region, which is originally doped with Ge. In contrast, only silicon (Si) and oxygen (O2) are detected on the cladding region of the Dshaped fiber by utilizing the EDS analysis, suggesting no graphene formation on the cladding region. Note that the result of EDS analysis from our in situ grown graphene samples are comparable and analogous enough to the references of laser induced graphene and CVD grown graphene samples.62, 69 TEM analysis is carried out by manually transferring the synthesized graphene from the surface of the D-shaped fiber to a holey C film on a Cu grid. Figure 2d shows a TEM image of multilayered graphene on the Cu grid confirming the two dimensional graphene morphology. The corresponding selected area electron diffraction (SAED) pattern of the TEM image is depicted in the inset, which indicates the hexagonal crystal structure of the grown graphene. Two bright rings in the pattern correspond to the graphene crystal planes.49 The interplanar spacing of the C atoms arranged in the graphene lattice fringes measured from the SAED rings is 0.218 nm (corresponding to the graphene crystal plane (0110)) and 0.123 nm (corresponding to the graphene crystal plane (1210)) (Figure S5, Supporting Information).49, 70 Figure 2e shows an HRTEM image 10 ACS Paragon Plus Environment

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exhibiting lattice fringes with an interplanar distance measured at 0.218 nm, which corresponds to the graphene crystal.49, 70 The inset of Figure 2(e) shows the corresponding fast Fourier transform (FFT) illustrating the six-fold symmetry of the lattice fringes in the image. Multiple sets of the hexagonal pattern are originated from the multilayered graphene.71 Figure 2f shows an HRTEM image (left) exhibiting lattice fringes of graphene and the corresponding filtered image (right), which shows the hexagonal lattice of C atoms in the two dimensional graphene crystal plane. Both the SAED patterns and the FFT patterns exhibit multiple sets of hexagonally arranged spots that comprise a given ring in the patterns (Figure S6 and Figure S7, Supporting Information). This originates from the disoriented stacking order in the multilayered graphene due to the tearing, folding and ripples that occurs during imperfect transfer of graphene to the TEM grid.72 The asgrown graphene on D-shaped fiber is employed to perform ultrafast all-optical switching based on the FWM technique. High speed modulation pattern of the input signal is transferred/copied to the generated signals by FWM with another amplified CW pump in the graphene/D-shaped fiber device. Ultrafast All-Optical Switching Based on Four-Wave Mixing. An experiment setup consisting of in situ grown graphene along the polished flat region of a D-shaped fiber device in order to detect FWM-based wavelength conversion is illustrated in Figure 3a. A distributed feedback CW laser source operating at a wavelength of 1552.4 nm and a tunable CW laser source operating at a wavelength of 1559 nm serve as a pump and a signal, respectively for FWM. The signal is modulated up to 20 GHz to ensure the ultrafast nonlinear response in the as-grown graphene. A regular erbium-doped fiber amplifier (EDFA) is connected following to the pump laser in order to amplify the pump signal, while amplification of the modulated signal light is performed by a low noise high power erbium-doped fiber amplifier (HP-EDFA). Both of the amplified signals are filtered to cut off the non-required amplified spontaneous emission (ASE) mode. Polarization states of the lights are controlled by two separate polarization controllers (PC) before combining them through a 3 dB coupler to the graphene/D-shaped fiber sample device. The injected pump power through the graphene/D-shaped fiber sample device is measured to be ~19 dBm while the power of 11 ACS Paragon Plus Environment

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modulated signal is set to comparatively lower value. Figure 3b shows the output optical spectrum originated from FWM phenomenon obtained from an optical spectrum analyzer (OSA) after graphene/D-shaped fiber device. It is seen in the spectrum that, the incident pump and signal lights generate new signals at different wavelengths through FWM in the graphene. As the Graphene/Dshaped fiber device is highly polarization sensitive, it is important to note that, the state of polarization (SOP) for both pump and signal is required to be controlled and set to a specific SOP to acquire the FWM in the graphene device. The intensities of evanescent fields from both the input lasers which interact with in situ grown graphene on fiber are responsible for the FWM. Since the interaction length is ~150 µm which is obtained during graphene synthesis on D-shaped fiber, the evanescent fields with specific SOPs of both inputs have the higher degree of interaction with graphene. Thereby, enhanced third order nonlinearity of graphene is achieved to generate new signals through FWM. The modulation frequency of the input signal is tuned up to 20 GHz to distinguish whether the FWM based wavelength conversion is from the thermal effect or from highspeed third order nonlinearity in the graphene crystal. The thermal effect induced by the intensified evanescent field can be responsible for wavelength conversion due to the material properties changes with temperature, but this effect is unable to response to high speed modulation of the input signal. Whereas, the nonlinear effect is an ultrafast process, therefore, the third order nonlinear effect in graphene response to the incident 20-GHz signal and generate the new signals with the same modulation frequency. The converted signal appears as two generated signals at 1566 nm and 1545 nm according to the FWM standard. The conversion efficiency (CE), which can be defined as a ratio of measured power of the generated signal to power of the incident original modulated signal, is measured as -71.8 dB. Figure 3c shows the close up view of the original input signal at 1558.9 nm with the modulation frequency of 20 GHz. Modulation information carried by the input signal is clearly seen in this spectrum with spectral broadening (sideband generation) in the graph. Figure 3d shows the close up view of the generated signal at 1566 nm. It is seen that, the generated signal at 1566 nm also exhibits spectral broadening with sideband generation corresponds to the input signal. 12 ACS Paragon Plus Environment

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The distances of the generated sidebands from the fundamental peak in the generated signal are same as those in the original input signal. Therefore, the modulation format of the original signal at ~1558.9 nm is exactly followed by the generated signal at ~1566 nm and the information carried by the input modulated signal is successfully copied by the newly generated signal. In this way, 20GHz signals at different wavelengths are generated by FWM of a pump channel and 20-GHz input signal channel at different wavelengths. This confirms that the ultrafast all-optical switching, based on FWM of two input signals in grown graphene, is originated from the high third order optical nonlinearity of graphene, which is required for high speed optical signal processing. The extinction ratio of the generated signal is measured as 6 dB for 20-GHz case. The same experiment is done for a bare D-shaped fiber (before graphene synthesis) to compare the effects under same experimental conditions with the same input pump and signal channel and the noise level at the output for both experiment are maintained at the same level. There are also generated signals (wavelength conversion) by employing FWM in the bare fiber, but it is due to only the pristine third order optical nonlinearity in the silica fiber. However, the extinction ratio (ER) of the generated signal is smaller than that of the D-shaped fiber with as-grown graphene. Both samples are examined at different modulation frequency ranges up to 20 GHz under same experimental conditions. Figure 3e represents the difference between the ER of the generated signal from as-grown graphene on Dshaped fiber and the ER of the generated signal from the bare, just polished fiber. Therefore, the difference of the ER (∆ER) can be measured by the numerical differences between the ER of generated signals created from these two cases. The ∆ER values also can be calculated by the ratio of the peak intensities of the generated signals in two cases using the logarithmic formula of ‘∆ER = 10 х log10(Pwith graphene/Pwithout graphene)’, where, Pwith graphene and Pwithout graphene are the peak intensities (mW scale) of the generated signals through FWM process with graphene on polished area of Dshaped fiber device and that of without graphene on just polished surface of fiber, respectively. Indubitably, it is seen that, the ER from the D-shaped fiber device with as-grown graphene is enhanced up to ~2 dB than that from the bare D-shaped polished fiber under the same experimental 13 ACS Paragon Plus Environment

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conditions. This points out that, the peak intensity of the newly generated signal without grown graphene is 58.5% lower than the one with grown graphene. This means that 58.5 % of the nonlinearity required for FWM is originated from as-grown graphene. Therefore, the in situ synthesized graphene on the optical fiber plays the major role to provide the required third order optical nonlinearity for FWM while interacting with the evanescent fields of the pump and the input signal at ultrafast speed of up to 20 GHz and generate the new signals at the corresponding speed. Figure 4a shows the graph for the generated signal at ~1566 nm by FWM of CW pump and signal with different modulation frequency ranges from 200 MHz to 20 GHz. The modulation pattern of the generated signal also gradually changes like the input signal in the applied modulation frequency range. The sidebands move apart from the fundamental peak of the generated signal as the modulation frequency is increased, which is also seen in the original signal at 1559 nm in the same frequency range. Figure 4b is a plot of the ER of the generated signal and the spectral broadening (the separation of first sideband from fundamental peak) for both input signal and generated signal as a function of modulation frequency. The ER of the generated signal is stable at ~6 dB over the large modulation frequency range. The separation of first sideband from the fundamental peak of the generated signal increases linearly with the modulation frequency, which is the same as in the input signal as shown in the graph. Thus, grown graphene along the polished flat region of the D-shaped fiber device by in situ technique provides third order optical nonlinearity in a considerable margin to generate new signals at different wavelengths by the mean of FWM and transfers the modulation information of input signal to generated signals at the ultrafast speed of up to 20 GHz. It is important to ensure the versatility of the nonlinear device by introducing it into a different optical setup. A simpler setup would be preferred to result in the FWM effect. Moreover, in the above-mentioned setup, it is not possible to check wavelength detuning in the FWM effect since the bandwidth of the filters used in that setup is too high. For that reason, it is not possible to put the pump and signal channel close to each other to check the FWM effect. Figure 5a shows the updated 14 ACS Paragon Plus Environment

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setup for the FWM experiment to address the wavelength detuning issues that may appear in the previous setup. The simpler set up is done using only one EDFA for both pump and modulated signal probe. In this way, it is possible to demonstrate degenerate FWM and wavelength detuning. Moreover, a single filter is enough for the implementation, thus the use of additional equipment can be reduced. In this turn, both the pump channel at 1553.05 nm and the signal channel at 1552.05 nm are passed through a 3.2-nm bandpass filter. The signal channel is modulated up to 20 GHz with peak intensity of 15 dBm. The CW pump channel has the comparatively higher peak intensity of 16.8 dBm to enhance the optical nonlinearity in as-grown graphene, therefore, the wavelength conversion based on FWM is achieved. Figure 5b shows the OSA spectrum of wavelength conversion for a 130-MHz signal channel. New signals (generated signal 1 and generated signal 2 in Figure 5b) are generated at 1551.05 nm and 1554.05 nm, which satisfy the standard of the FWM process. Figure 5c shows the OSA spectrum of wavelength conversion for a 20-GHz signal channel. The generated signals (both 1 and 2) with corresponding modulation frequency are also appeared as expected. It is again seen that, the modulation information of the input signal at 20 GHz is transferred/copied to the newly generated signals, which again ensures the FWM exploiting high third order optical nonlinearity originated from directly grown graphene on polished surface of Dshaped fiber by in situ technique. For this setup, the CE is measured which is raised up to -35.391 dB as both the pump and modulated signal channels are amplified by a single HP-EDFA. The tunability of the generated signal is investigated in this experiment setup by tuning the input pump wavelength in a range of around 2 nm. For this investigation, the input signal channel is fixed at the wavelength of 1552.05 nm, while the pump channel wavelength is gradually tuned in a range from 1552.05 nm to 1553.95 nm. Initially, both the pump and the signal channels are at the same wavelength, and it is not possible to see the new signal generation as seen in Figure 5b. As the pump channel moves apart from the signal channel and the channel distance is increased from 0 nm to 1.9 nm by 0.1 nm steps, there is a new signal generated at the same distance from the signal channel. As seen in the graph, the generated signal moves apart from the pump channel as the signal 15 ACS Paragon Plus Environment

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channel moves apart from the pump signal, which is another strong evidence of FWM generation and confirms the wavelength detuning ability of the devices with in situ graphene. The ultimate power conversion efficiency in this work can be enhanced by generating more third order optical nonlinearity which is originated from the grown graphene along the polished surface of D-shaped fiber device. Length of the grown graphene along the polished surface of the D-shaped fiber is one of the key parameters which directly effects the optical nonlinearity hence the conversion efficiency consequently. The length of grown graphene in longitudinal direction (of D-shaped fiber) can be considered as interaction length. The interaction length of the device can be controlled as per the requirement for the better device performances. During polishing the D-shaped fiber, there is opportunity to keep the polished area as long as possible, where large area graphene can be formed via strong evanescent light−mater interaction along the polished area, thus interaction length can be increased. As large as the interaction length, more nonlinearity can be generated thus the conversion efficiency. In that way the limitations regarding the device length can be overcome. The polished area of D-shaped fiber also plays important role as well in this in situ approach, which may limit the device performances. Because, D-shaped fibers in this technique are prepared manually by polishing the clad portion of the single-mode optical fiber. Therefore, there is high possibility that, the surface of the polished area will be extremely rough which may lead the uneven light−mater interaction at the interface and therefore, irregular formation of graphene on the D-shaped fiber which may lead the bad performance. However, this limitation can be overcome by an acceptable margin if the polishing is done very gently following a proper method. CONCLUSION Firstly, multilayered graphene has been synthesized directly along the polished surface of Dshaped fiber using laser irradiation in ambient conditions. The in situ synthesis of graphene on Dshaped has been confirmed by a wide range of characterization techniques. This laser-induced in situ grown graphene exhibits a high density of sp2 bonds thus enhances the functionality of chipscale integration for graphene based optical devices with minimum footprint. It is guaranteed that, 16 ACS Paragon Plus Environment

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in situ approach is possible without any transfer steps to implement graphene based optical devices, which can perform as an ultrafast all-optical switch and/or wavelength converter based on FWM. This work can be further improvised, modified and repeated to acquire better device performances. An important perspective of this work is to confirm the validity and enhancement of third order optical nonlinearity in the device after in situ approach to grow graphene. Secondly, we have demonstrated the FWM-based wavelength conversion of ultrafast signal with frequency modulation up to 20 GHz utilizing the long interaction length of as-grown graphene along the polished flat surface of the D-shaped fiber in this experiment. Results have been compared with those from the bare (without graphene) D-shaped fiber. The power of the generated signal without graphene has been seen to be ~2 dB lower than that of the signal with graphene. Finally, we have ensured the tunability of the ultrafast switching by wavelength detuning experiment with significant conversion efficiency of -35.391 dB. We expect that, in situ synthesized graphene along the polished flat surface of D-shaped fibers may find additional remarkable applications in optical signal processing beyond FWM based wavelength conversion. This in situ technique of graphene synthesis is not limited to synthesize graphene only on the D-shaped fiber; excellent graphene films can be grown and patterned directly on many other arbitrary surfaces and materials for realizing ultrafast optoelectronic and photonic devices, which can improve the current technology in the fields of photonics, optical communications, optical computing, and optoelectronics. This in situ technique pave the way for future on-chip optical signal processing and optical interconnect along with the chip-scale integration of the electro-optic hybrid devices and circuits for future ‘data-hungry society’. METHODS Ni Deposition on D-shaped Fiber. A 100 nm thick polycrystalline film of Ni (99.999 %) was deposited via electron beam (E-beam) evaporation technique onto the flat surface of D-shaped fiber samples. D-shaped polished fiber sample devices were put in the evaporation chamber, and the pressure was lessened below to 1.6×10-6 Torr of the chamber. The first layer of Ni with a depth of 17 ACS Paragon Plus Environment

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10 nm was deposited at a slow rate of 0.2–0.3 angstroms/s while the remaining 90-nm deposition was accomplished at a rate of 2–3 angstroms/s. During Ni deposition by evaporation, the temperature inside the chamber reached to ~40 °C due to heat generation by electron beam; nonetheless, while inevitable, this was not detrimental to both the D-shaped fiber device samples and the Ni film. Laser Induced Synthesis. D-shaped fibers were made by polishing the single-mode fiber (SMF) cladding. The polarization dependent loss and the minimum insertion loss of prepared Dshaped fibers were measured and calculated to be ~1 dB and 3 dB, respectively. First, the deposition of a 100-nm nickel film on the flat surface of D-shaped fiber from 99.999 % polycrystalline Ni sources was performed employing electron beam evaporation technique. Meanwhile, the C atoms as impurities on the surface of Ni pellets were also vaporized together with Ni to form C-incorporating Ni layer. An amplified CW laser is input to the end of the fiber, and the guided radiation becomes a thermal source for the graphene synthesis. The experiment setup for graphene synthesis is shown in Figure S1a in the Supporting Information. A CW laser output from a DFB laser diode (LD), operating at 1552 nm and amplified in the following step by a high-power erbium-doped fiber amplifier (HP-EDFA), was joined to the input end (port 1) of an optical circulator, which facilitates to the unidirectional laser radiation incident to the Ni-deposited Dshaped fiber sample via output end (port 2) of the circulator component. Since the D-shaped fiber is highly polarization sensitive, the light−Ni layer interaction at the interface strongly depends on the SOP of the incident light. At the starting of laser treatment at the Ni/fiber interface for in situ synthesis, the SOP of the incident laser light was optimized in a state to ascertain the maximum interaction by properly arranging the polarization controller in the setup, so that, minimum output power is found at the other end of the fiber which ensures maximum light−Ni interaction at the interface as seen in Supporting Information Figure S1b. The strong interaction between the evanescent field, generated from amplified CW lasers, and the Ni deposited on the polished surface of the fiber results in the rise of the temperature at the Ni/D-shaped fiber interface, and the C atoms 18 ACS Paragon Plus Environment

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start to diffuse out of Ni layer. The incident laser power was 20 dBm (100 mW), while the average minimum output power set by the polarization controller was 0 dBm (1 mW). Therefore, the incident power at the interface to grow graphene was estimated to be 19.96 dBm (99 mW). Once the radiation was removed, the Ni layer cooled rapidly, and the C atoms precipitated towards the grain boundaries and the interface to grow graphene. As soon as the irradiation for 60 minutes was finished, the sample was dipped in a bath of Ni etchant (FeCl3 in DI water) for 30 minutes to etch out the Ni film. At the end of etching process, the D-shaped fiber device was left with the graphene synthesized directly on its flat surface. Analysis. Raman spectrum was recorded by a Horiba Jobin Yvon LabRAM HR Evolution equipped with a laser of excitation wavelength at 514 nm, XPS was performed with an Ulvac-PHI 5000 Versa Probe. Both the Raman and XPS were taken on the graphene as-grown on the flat area of the D-shaped side polished fiber. TEM analysis was done by manually transferring the graphene from the grown region of polished area to a Cu grid. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website. Experiment setup for graphene synthesis, Graphene growth mechanism, Optical images, XPS analysis, TEM images of graphene flakes with SAED patterns are briefly explained with Figure S1, Figure S2, Figure S3, Figure S4 and Figure S5 through Figure S7 respectively. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS This research was supported by the Bio & Medical Technology Development Program (Grant No. NRF-2015M3A9E2030105), and the Basic Science Research Program (Grant No. NRF2015R1A2A2A04006979) of the National Research Foundation (NRF) funded by the Ministry of 19 ACS Paragon Plus Environment

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Science, ICT and Future Planning, South Korea. Also, supported by the Institutional Program funded by the Korea Institute of Science and Technology (KIST), South Korea (Project No. 2E27150).


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FIGURE CAPTIONS: Figure 1. Schematic illustrations and optical images. (a) Polycrystalline Ni enriched with C atoms deposited on flat surface of D-shaped fiber. (b) Corresponding optical image of the Ni-deposited Dshaped fiber. (c) Laser-induced in situ synthesis of graphene onto flat surface of a D-shaped fiber. (d) Corresponding image of the device after laser irradiation and after Ni etching. (e) FWM with asgrown graphene on D-shaped fiber; showing two newly generated signals (ω3 and ω4) arising from the FWM effect with two interacting channels (pump and modulated signal at ω1 and ω2 respectively) propagating through graphene grown D-shaped fiber. (f) Schematic of the FWM principle with graphene band structure.

Figure 2. Characteristics of synthesized graphene. (a) Raman spectra (with 514-nm Raman laser) on the flat surface of D-shaped fiber after graphene synthesis. (b) High resolution XPS scan for C1s spectrum of grown graphene with de-convoluted peaks. (c) EDS analysis of both core and cladding area after Ni etching. (d) TEM image of graphene with corresponding hexagonal SAED pattern in inset (scale bar in inset is 5 nm-1). (e) HRTEM image showing graphene lattice fringes with the measurement of interplanar distance (inset: FFT hexagonal pattern). (f) HRTEM image (left) exhibiting lattice fringes of graphene and corresponding filtered image (right) with enhanced visibility of fringes.

Figure 3. Ultrafast all-optical switching based on FWM mixing in in situ synthesized graphene. (a) Experiment setup for FWM, (b) FWM spectrum (with 20-GHz input signal) obtained from asgrown graphene. (c) Close-up view of original signal with modulation frequency of 20 GHz. (d) Close-up view of generated signal exhibiting modulation frequency of 20 GHz (same as original signal). (e) Comparison of the extinction ratio (∆ER) of generated signal from FWM with and without graphene under same experimental conditions.


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Figure 4. Characteristics of generated signal. (a) Generated signal spectra from FWM with various modulation frequencies (ranging from 200 MHz to 20 GHz) correspond to the modulation frequencies of the original input signal. (b) Characteristics of generated signal as a function of the modulation frequency of original input signal. The ER of the generated signal is recorded as 6 dB over long modulation frequency range. The distance of 1st sideband from fundamental peak (which represents the spectral broadening) in both the input signal and the generated signal linearly increases as the modulation frequency of input signal increases.

Figure 5. Ultrafast optical switching and wavelength detuning from degenerate FWM in a simplified setup. (a) Simplified experiment setup. FWM spectrum (b) with input signal channel having modulation frequency of 130 MHz, (c) with input signal channel having modulation frequency of 20 GHz. (d) Wavelength detuning exhibition of generated signal by tuning the wavelength of input pump channel. Each time pump channel shifts apart from input signal channel by 0.1 nm, the converted/generated signal shifts accordingly.


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For Table of Contents Use Only

Ultrafast All-Optical Switching Incorporating in Situ Graphene Grown Along Optical Fiber by Evanescent Field of Laser Pulak C. Debnath,†,‡ Siam Uddin,†,‡ Yong-Won Song†,‡,*

Graphene is synthesized directly onto the flat surface of D-shaped fiber without any external Carbon source in ambient condition by evanescent field laser irradiation. Nickel acts both as catalyst and Carbon-host for interfacial growth of graphene under laser irradiation. Entire fibergraphene device is used as a ultrafast all-optical switch based on four-wave mixing (FWM) operating and transfering the modulation information to the generated signals at the ultrafast speed of up to 20 GHz. This in situ synthesis technique enables the customized patterning of graphene on optical fibers and chip-scale integration of the graphene based optical devices.


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Figure 1. Schematic illustrations and optical images. (a) Polycrystalline Ni enriched with C atoms deposited on flat surface of D-shaped fiber. (b) Corresponding optical image of the Ni-deposited D-shaped fiber. (c) Laser-induced in situ synthesis of graphene onto flat surface of a D-shaped fiber. (d) Corresponding image of the device after laser irradiation and after Ni etching. (e) FWM with as-grown graphene on D-shaped fiber; showing two newly generated signals (ω3 and ω4) arising from the FWM effect with two interacting channels (pump and modulated signal at ω1 and ω2 respectively) propagating through graphene grown Dshaped fiber. (f) Schematic of the FWM principle with graphene band structure. 322x211mm (300 x 300 DPI)

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Figure 2. Characteristics of synthesized graphene. (a) Raman spectra (with 514-nm Raman laser) on the flat surface of D-shaped fiber after graphene synthesis. (b) High resolution XPS scan for C1s spectrum of grown graphene with de-convoluted peaks. (c) EDS analysis of both core and cladding area after Ni etching. (d) TEM image of graphene with corresponding hexagonal SAED pattern in inset (scale bar in inset is 5 nm1 ). (e) HRTEM image showing graphene lattice fringes with the measurement of interplanar distance (inset: FFT hexagonal pattern). (f) HRTEM image (left) exhibiting lattice fringes of graphene and corresponding filtered image (right) with enhanced visibility of fringes. 398x206mm (300 x 300 DPI)


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Figure 3. Ultrafast all-optical switching based on FWM mixing in in situ synthesized graphene. (a) Experiment setup for FWM, (b) FWM spectrum (with 20-GHz input signal) obtained from as-grown graphene. (c) Close-up view of original signal with modulation frequency of 20 GHz. (d) Close-up view of generated signal exhibiting modulation frequency of 20 GHz (same as original signal). (e) Comparison of the extinction ratio (∆ER) of generated signal from FWM with and without graphene under same experimental conditions. 289x351mm (300 x 300 DPI)


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Figure 4. Characteristics of generated signal. (a) Generated signal spectra from FWM with various modulation frequencies (ranging from 200 MHz to 20 GHz) correspond to the modulation frequencies of the original input signal. (b) Characteristics of generated signal as a function of the modulation frequency of original input signal. The ER of the generated signal is recorded as 6 dB over long modulation frequency range. The distance of 1st sideband from fundamental peak (which represents the spectral broadening) in both the input signal and the generated signal linearly increases as the modulation frequency of input signal increases. 452x153mm (300 x 300 DPI)


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Figure 5. Ultrafast optical switching and wavelength detuning from degenerate FWM in a simplified setup. (a) Simplified experiment setup. FWM spectrum (b) with input signal channel having modulation frequency of 130 MHz, (c) with input signal channel having modulation frequency of 20 GHz. (d) Wavelength detuning exhibition of generated signal by tuning the wavelength of input pump channel. Each time pump channel shifts apart from input signal channel by 0.1 nm, the converted/generated signal shifts accordingly. 303x321mm (300 x 300 DPI)


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Ultrafast All-Optical Switching Incorporating in Situ Graphene Grown

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