Without the need of single-layer graphene, the graphite nano-sheet powder electrochemically exfoliated from graphite foil can also be employed as a stable saturable absorber and mode-locker for fiber lasers. High-quality graphite nano-sheets containing few graphene layers can be obtained by slow electrochemical exfoliation without the need of post annealing procedure. With reducing the electrochemical exfoliation bias of the graphite foil based anode from + 6 and + 3 volts, the electrochemically exfoliated graphite nano-sheets reveals a decreased D-band intensity in Raman scattering spectrum, and the 2D-band intensity is concurrently enlarged by two times to support the improved quality with suppressed oxidation during the exfoliation reaction. The X-ray photoelectron spectroscopy also confirms the suppression of the C-O bonds in the graphite nano-sheets obtained with decreasing the exfoliation bias. After centrifugation, the average diameter of the exfoliated graphite nano-sheets extracted from the acetone solution is shrunk from 7 μm to 100 nm as the anode bias decreases from 6 to 3 volts. Both the quality and size distribution of the graphite nano-sheets are improved with such slow but refined electrochemical exfoliation. In application, the graphite nano-sheets obtained at different exfoliation bias show relatively stable saturable absorption and passive mode-locking performance in Erbium doped fiber lasers (EDFLs). Benefiting from the advantages of high-gain and strong pulse compression in the EDFL, the graphite nano-sheets with different modulation depths only behave as a mode-locking starter and show trivial influence to the pulse shortening in the mode-locked EDFL, indicating that the strong soliton compression mechanism dominates the generation of 430-450 fs pulsewidth in the EDFL passively mode-locked by graphite nano-sheets.
© 2013 Optical Society of America
Since 2004, the single-layer graphene with broadband nonlinear transmission and ultrafast Dirac Fermion transportation has been emerged  and shown great impacts on versatile photonics and optoelectronics device applications , such as transparent conductors , high speed ðeld-effect transistors [4,5], saturable absorbers [6,7], etc. In 2009, the liquid-phase exfoliated graphene-polymer composite was firstly employed as saturable absorber , graphene has been shown its intriguing property of broadband-tunable nonlinear absorption which can be applied to passively mode-lock versatile lasers . The graphene saturable absorber in Erbium-doped fiber laser (EDFL) [10–15] has shown better mode-locking ability than typical carbon nanotube based saturable absorber [16,17]. The absorption loss and modulation depth of graphene are correlated with its layer number [18–21]. In previous works, the mono- or few-layer graphene was mainly fabricated by using mechanical exfoliation . Single-layer graphene was preliminarily obtained by exfoliating the graphite with sticky tapes at early stage; however, this approach was lack of efficiency and reproducibility. Later on, alternative processes including chemical reduction from graphene oxide [22,23] or ultrasonic treatment of graphite foil  were also introduced. The reduction of graphene oxide usually requires high-temperature treatment and exhibits poor electrical characteristics. It is time-consuming to obtain the graphene flake by ultra-sonication of graphite foil. Later on, the chemical vapor deposition (CVD) was proposed as a good candidate to generate high-quality graphene with its layer number rigorously controlled by the precisely parametric tuning of substrate and environmental conditions [19,20]. The CVD method was recently developed to efficiently grow the large-area and high-quality graphene [25–27] which requires rigorous criteria on tuning the processing recipe including the hydrogen contained reaction, the high-temperature environment and the polishing pre-treatment of substrate with metallic host. More recently, the electrochemical exfoliation has also proposed to exfoliate the graphite foil for rapid production of high-quality and few-layer graphene [28,29] During electrochemical exfoliation process, the quality and size of the graphene are strictly affected by the anodic oxidation with a biased electrolysis . Large-scale graphene sheet is applicable to electronic devices [31,32], whereas small-scale graphene can be used for optical applications such as the saturable absorbers and intensity/phase modulators for lasers [33–35]. Hence, the flexibility of the electrochemical exfoliation makes it a potential candidate to produce graphene or graphite nano-sheets with controllable size and tunable layer number by controlling the anode bias. Nevertheless, it was seldom discussed how an unnecessarily high bias affected the size distribution and quality control of the exfoliated graphene or graphite nano-sheets during electrochemical exfoliation. It is thus mandatory to investigate the threshold anodic bias for suppressing the oxidation during the electrochemical exfoliation of the graphite foil.
Without the need of single-layer graphene, the graphite nano-sheet powder electrochemically exfoliated from graphite foil can also be employed as a stable saturable absorber and mode-locker for EDFLs in this work. The graphite nano-sheets containing few graphene layers are obtained by slow electrochemical exfoliation in aqueous sulfuric acid solution at changing anodic bias. Raman scattering spectroscopy and X-ray photoelectron spectroscopy (XPS) are employed to clarify the defect content and structural oxidation of the graphite nano-sheets exfoliated at different anodic biases. The precise control on size uniformity, defect quantity and phase quality of the graphite nano-sheets by reducing anodic bias and additional centrifugation is demonstrated and examined by the scanning electron microscope (SEM) and atomic force microscopy (AFM). Subsequently, the graphite nano-sheet powder is brushed onto a single-mode fiber patchcord and employed as the mode-locker in the EDFL. The sub-picosecond responses of EDFLs passively mode-locked with these exfoliated graphite nano-sheet powders obtained at different exfoliation biases are compared. By constructing the EDFL with high-gain and strong pulse compression ability, the graphite nano-sheets with different modulation depths behave as a stable mode-locking starter and show trivial influence to the pulse shortening of the EDFL.
2. Experimental Setup
The schematic illustration for preparing the electrochemically exfoliated graphite nano-sheet powder is shown in Fig. 1.In step 1, a highly oriented pyrolytic graphite (HOPG) foil was used as an anode, and a Pt wire serves as a cathode. The electrolyte of 5 wt. % diluted sulfuric acid solution was prepared by adding 5.4 ml of 96 wt. % sulfuric acid into 200 ml of deionized water. Both electrodes were dipped into the electrolyte and separated by 5 cm. The DC bias was applied to electrochemically exfoliate graphite flakes from the HOPG electrode. The exfoliation biases were applied from + 3V to + 6V. In principle, the anode reactions during electrolysis are summarized as follows [30,36,37]:Eq. (4) reveals the main procedure for the expansion and exfoliation of HOPG, while reaction Eqs. (2) and (3) relate to the functionalities of oxygen, and reaction Eqs. (5) and (6) cause the irreversible structural defects. Subsequently, the electrolyte solution containing the graphite flakes was extracted by a porous filter. The residues were dried at room temperature and dispersed in acetone for an ultrasonic treatment at 25°C for 10 min. Afterwards, the top part of the solution was extracted for additional centrifugation at 1000 rpm for 120 seconds.
To characterize the electrochemically exfoliated graphite nano-sheet powder, the SEM images were obtained on Elionix ERA-8800FE Field-emission scanning electron microscopy. The Raman scattering spectra were recorded by a JOBIN YVON-SPEX T64000 Microscope Raman spectrometer with a liquid-nitrogen-cooled charged-coupled device detector. The pumping source was Nd-YAG laser at wavelength of 532 nm. The crystalline Si related peak at 520 cm−1 was used for calibration. The XPS was measured with Mg Kα line (1253.6 eV) source at 5 × 10−10 torr. At last, the supernatant was directly dripped on the end-face of a single-mode ðber (SMF, Corning SMF-28TM) patchcord to coat the graphite nano-sheet powder on the patchcord after evaporating the acetone solvent. The SMF patchcord with the graphite nano-sheets was connected with another patchcord in the EDFL cavity.
Figure 2 shows the configuration of the graphite nano-sheet based passively mode-locked EDFL system. The gain medium is a 2-m long Erbium-doped fiber (EDF, Liekki Er80-8/125) with a dispersion coefficient of β2,EDF = −20 ps2/km . The EDFL consisted of two high-power pumping laser diodes with respective central wavelengths of 980 nm (forward pumping) and 1480 nm (backward pumping). The 980 nm/C-band and 1480 nm/C-band wavelength division multiplexers (WDMs) were employed for multiplexing the corresponding pumping light. An isolator was used to determine the direction of light propagated in the EDFL, and the polarization of light prior to the graphite nano-sheet saturable absorber was controlled by a polarization controller. A 1 × 2 optical coupler providing 95% feedback ratio and 5% output ratio was employed. The total length of SMF in the EDFL cavity was 4.7 m with β2,SMF = −21 ps2/km, and the total group delay dispersion (GDD) in the EDFL cavity was calculated to be −0.16 ps2. The pulse shape and the optical spectrum of the graphite nano-sheet based passively mode-locked EDFL were measured by an autocorrelator (Femtochrome, FR-103XL) and an optical spectrum analyzer (Ando, AQ6317B).
3. Results and Discussion
A highly oriented pyrolytic graphite (HOPG) foil was cut into small pieces to serve as the anodic electrode for electrochemical electrolyzation. The electrolysis current under forward and backward reactions was recoded and shown in Fig. 3.The threshold voltage of electrolysis under forward and backward bias was about 2V and 3V, respectively. However, there was no exfoliation of HOPG while suffering backward bias, which implied that the reactions with anions, or sulfate ions for this electrolyte, were important for the exfoliation. While the exfoliation of the HOPG was too slow under threshold voltage of + 2V, the exfoliation biases were selected from + 3V to + 6V.
Although the exfoliation of graphite nano-sheets was accelerated with increasing bias, the exfoliated graphite flakes have shown large dispersions on both size and shape. It was resulted from that the electrolysis reactions occurred more randomly and deeply on the HOPG. In order to purify the size of graphite nano-sheets, the centrifuge process was employed to separate the small graphite nano-sheets from large and thick graphite flakes, in which the centrifugal force is enlarged by raising the rotation speed to accelerate the sinking velocity of insoluble flakes, as illustrated with Fig. 4(a). The graphite nano-sheets and graphite fragments are separated in aqueous solution by their distinguished sinking speed. After centrifuging, large and thick flakes can be removed from the exfoliated product, and the average diameter and size distribution of the graphite nano-sheets are greatly reduced by lengthening the centrifuge duration. The rotation speed and centrifuging time can thus be adjusted to precisely control the desired size distribution of the graphite nano-sheets for different applications. Afterwards, the centrifuged solution with graphite nano-sheets was dropped on silicon substrate, and the SEM image was taken to analyze the size dispersion of the graphite nano-sheets, as shown in Fig. 4(b).
After centrifuging at 1000 rpm for 120 second, average diameters of the graphite nano-sheets exfoliated under + 3V, + 4V, + 5V, and + 6V are 0.1 μm, 2 μm, 3.5 μm, and 7 μm, respectively. Figure 4(c) demonstrates the AFM image of the graphite nano-sheets exfoliated at 3 volt on a silicon substrate as an example, and Fig. 4(d) shows the thickness distributions of the exfoliated graphite nano-sheets with different bias voltages. The average thicknesses of the graphite nano-sheets with the bias voltages of 3, 4, 5, and 6 volts are 3, 6, 8, and 10 nm, respectively. After electrochemical exfoliation, the expanded interlayer distance between graphene layers is about 0.45 nm . Therefore, the average layer number of the graphite nano-sheets vary from 6 to 22 as the bias enlarges from 3 to 6 V.
Figure 5(a) depicts the Raman scattering spectra of the electrochemically exfoliated graphite nano-sheets. The quality of graphite nano-sheets was identified by comparing the Raman scattering peak intensity ratio among D band (1350 cm−1), G band (~1580 cm−1), D’ band (~1620cm−1), and 2D band (~2680 cm−1) [39–41]. The linewidth and intensity of these Raman signals directly reflect the structural defect and phase quality of the electrochemically exfoliated graphite nano-sheets. The G band caused by the sp2 carbon bond mainly represents the crystallinity of graphitic materials. The D band induced by structural defects in the graphite including layer distortion , rotation , and oxygen invasion , etc. The intensity ratio of ID/IG is thus proportional to the defect quantity in the graphite nano-sheets. The D/G band intensity ratio shown in Fig. 5(b) increased from 0.2 to 1.6 with increasing bias from + 3 to + 6V, indicating that a higher defect density is induced by accelerating the oxidation and exfoliation procedure at enlarging the anodic bias. The emergence of D’ band was also an evidence for dense defects . In contrast, the I2D/IG intensity ratio directly correlates with the layer number of the graphite nano-sheets , which decreases from 0.49 to 0.26 with increasing anodic bias to verify the reduced layer number and lower degree of oxidation under an ultraslow electrochemical exfoliation at threshold anodic bias.
Figure 6 shows the C1s XPS spectra of the graphite nano-sheets obtained at different anodic biases. Four main features are located at 284.6 eV, 285.5 eV, 286.6 eV and 289.4 eV, corresponding to C-C, C-O, C = O and C( = O)O bonds, respectively , their relative intensity reveals the degree of oxidation with changing anodic bias. The inset of Fig. 6 shows the peak intensity ratios of the corresponding bonds related to the C-C bond in the graphite nano-sheets. The percentage of oxygen related bonds defined as
The influence of the exfoliation bias to the HOPG electrode is illustrated in Fig. 7. The electrochemical exfoliation rate of HOPG was accelerated with enlarging biased voltage, and the sulfate ions in the electrolyte invade the graphite electrode to violently cause the generation of thicker flakes with heavier surface oxidation or structural destruction. Accelerated reaction rate under high bias leads to faster and rougher exfoliation of the electrode as the HOPG suffers from larger current and denser reaction spots per unit area, which makes the layers of graphite flakes decomposed incompletely. Denser area density of electrochemical reactions causes more intercalation of anions which are responsible for the defects and oxidation observed from the Raman and XPS analyses. The decomposed gases generated around the HOPG electrode also augment with the increasing bias, which result in a larger buoyancy to the HOPG which is being sliced. This force aids the fragments to escape from the HOPG electrode, which results the premature separation of relatively large and thick graphite flakes. In contrast, the graphite nano-sheets exfoliated at lower anodic bias become more like graphene. To fabricate high-quality graphite nano-sheets, the exfoliation under lower bias with fewer gas decomposition and threshold reaction rate at small area is mandatory. Such a stabilized exfoliation further affects the electrical and optical properties of the graphite nano-sheets. The linear and nonlinear optical transmittances of the electrochemically exfoliated graphite nano-sheets are characterized.
In principle, the thickness and quality of the graphite nano-sheets directly affect their electric or optical properties, such as linear and nonlinear transmittance (due to the saturable absorption). Typically, the nonlinear transmission of graphite nano-sheets varies with the transmitted optical intensity. To observe, the graphite nano-sheets are illuminated by a pulsed fiber laser with a pulsewidth of 700 fs at central wavelength of 1560 nm. The transmission with enlarging the pump intensity via an EDFA is recorded by a power meter, as shown in Fig. 8(a). As a result, the nonlinear transmittances of the graphite nano-sheets electrochemically exfoliated at different biases are presented in Fig. 8(b). The linear transmittance is enlarged from 0.77 to 0.925 by stabilizing the electrochemical exfoliation with its bias decreasing from 6V to 3V (the reaction threshold), which is mainly attributed to the higher quality and thinner thickness of the graphite nano-sheets obtained under threshold exfoliation. The variations in the thickness and quality of the graphite nano-sheets change the electronic band structure of the exfoliated graphite nano-sheets, which accordingly influence the linear absorbance (αlin) and nonlinear absorbance (αnon). In order to describe the nonlinear transmission more clearly, the values of linear and nonlinear absorbances (αlin and αnon) and saturation intensity (Isat) are obtained by fitting the nonlinear transmission with equation of T = exp[-αnon/(1 + Iin/Isat)-αlin]. Figure 8(c) shows the normalized absorbance of each corresponding nonlinear transmittance. The modulation depth is defined by normalizing the nonlinear absorbance to the total absorbance (MD = αnon/(αnon + αlin)), which is increased from 17% to 53% with the decreasing bias from 6V to 3V. The linear absorbance, nonlinear absorbance, saturation intensity, and modulation depth of the graphite nano-sheets obtained at different electrochemical exfoliation biases are listed in Table 1.
The electrochemical exfoliation process is highly repeatable as the reaction speed can be precisely controlled by the exfoliation bias. Furthermore, the centrifugation rigorously confines the size and thickness distributions of the graphite nano-sheets dispersed in acetone. The graphite nano-sheets obtained by repeating the electrochemical exfoliations still possess the same optical parameters such as linear and nonlinear absorption. To be the saturable absorber in passively mode-locked fiber lasers, the graphite nano-sheets with small size can provide low absorption loss, which lowers the mode-locking threshold of the laser. In addition, the few-layer graphite nano-sheet also contributes large nonlinear modulation depth which is an important factor to form an intense and sharp pulse [19,46]. Particularly, the graphite nano-sheets with smaller size can be uniformly coated with better dispersion on the end-face of the fiber patchcord. The structural quality of graphite nano-sheet is an important factor to the self-amplitude modulation induced mode-locking at initial stage, as the defect or disorder inside the graphite nano-sheet inevitably enlarges the linear loss and degrades the nonlinear saturable absorption. Therefore, the graphite nano-sheet with small size and high quality is mandatory for the passive mode-locking of fiber lasers. Although both the linear and nonlinear absorption are attenuated for the graphite nano-sheets obtained at a threshold electrochemical exfoliation case, the highest modulation depth are achieved to indicate a best on/off switching or absorption modulation for the incoming ultrafast laser signal. These optical properties, including the lowest absorption loss and the highest modulation depth can be used to form an intense and sharp EDFL pulse.
At last, the electrochemically exfoliated graphite nano-sheets obtained at different biased conditions are employed as the mode-lockers in the EDFL system. Figures 9(a) and 9(b) compare the optical spectra and the autocorrelation traces of the passively mode-locked EDFLs with graphite nano-sheets exfoliated under different exfoliated biases. The passively mode-locked EDFLs with the graphite nano-sheets under different bias show approximately same pulsewidth around 430-450 fs with its corresponding optical spectral linewidth of 6.0-6.5 nm. The time-bandwidth products of all conditions are close to 0.31. The passively mode-locked EDFL pulses generated with the use of different graphite nano-sheets are relatively irrelevant to the layer number of graphite nano-sheets obtained with different exfoliation biases, as listed in Table 2. The threshold pumping powers of two laser diodes (LDs) for the EDFLs mode-locked by graphite nano-sheets exfoliated under different biases are shown in Table 2. The optimized mode-locking performance of EDFLs with different graphite nano-sheets are obtained at pumping powers of 290 mW for 980-nm LD and 140 mW for 1480-nm LD.
Figure 10 shows the RF spectra of the EDFLs passively mode-locked by graphite nano-sheets obtained under different electrochemical exfoliation biases. The repetition rate of the mode-locked pulses remains as 30.61 MHz (measured under a resolution bandwidth of 100 Hz). The sharpest spectral peak with highest signal-to-noise ratio of 74 dB is observed for the mode-locked EDFL with the graphite nano-sheets exfoliated at 3 volt, indicating that there is no parasitic modulation, multi-pulsing or Q-switching phenomenon to provide pedestal components aside the central peak. In contrast, the graphite nano-sheet sample exfoliated at higher bias results in the fundamental longitudinal mode spectrum of the mode-locked EDFL a lower signal-to-noise ratio with smaller spurious peaks which are very close to the central peak. This is attributed to the higher linear loss and a lower modulation depth caused by the larger and thicker graphite nano-sheet, which leaves more spontaneous emission inside the EDFL cavity and is unfavorable for the mode-locked pulse formation at the initial stage. Although the modulation performances of the graphite nano-sheets are distinct from each other, they are only treated as the starting mechanism to make the mode-locked EDFLs deliver the pulse-train with similar pulse duration. This observation indicates that there is another principle mechanism dominating the pulse shortening scheme. The formation of the pulse originates from the self-amplitude modulation caused by the saturable absorbers. Nevertheless, the high-intensity pulse further induces a strong self-phase modulation (SPM) effect to cause the pulse shortening in this high-gain EDFL cavity. The soliton-like pulses dominates the pulsewidth compression performance under a strong interaction between the GDD and SPM effects in the EDFL with slightly negative GDD condition [47,48]. These results reveal that the distinct optical modulation performances of the graphite nano-sheets under different exfoliated biases are less important in this high-gain EDFL with strong SPM and negative GDD effects.
It is found that all kinds of materials which consist of nonlinear saturable absorption property at a desired wavelength can serve as a mode-locker for lasers. The saturable absorption is a second-ordered nonlinear effect that is proportional to the intensity of incoming light. In our previous work, we have demonstrated that the mechanically polished graphite particles can start the mode-locking mechanism in the EDFL. Despite the semi-metallic feature, the graphite nano-sheet constituted by multiple 2D graphene layers can also exhibit the saturable absorption ability that is irrelevant to its band structure and electrical characteristics. In comparison with the single- or few-layer graphene, the graphite particle with comparable saturable absorption property is easy to be obtained by relatively simplified process. At then, the drawback is that those mechanically polished graphite particles are directly brushed onto the end-face of SMF patchcord, neither the size distribution nor the coverage ratio of the graphite particles can be precisely controlled. Our results have shown that the electrochemical exfoliation is the best candidate to obtain the graphite nano-sheet with graphene crystallinity under the bias voltage control. In comparison with the CVD-grown graphene, the electrochemical exfoliation is a room-temperature equipment-less process for fabricating graphite nano-sheet cost-effectively. Moreover, dripping the graphite nano-sheet solution is much simpler and easier than transferring graphene on the end-face of fiber patchcord. This work makes the demonstration on sub-500fs mode-locked EDFL easier with the simplified fabrication of graphite nano-sheet based saturable absorbers.
We have demonstrated the use of the electrochemically exfoliated graphite nano-sheet powder as a saturable absorber and a stabilized starter for the passively mode-locked Erbium-doped fiber lasers (EDFLs) without the need of single-layer graphene. The ultra-low oxidation and slow electrochemical exfoliation of graphite nano-sheet powder from the HOPG foil is precisely controlled by decreasing the exfoliation bias from + 6 to + 3 volts. Thinner graphite nano-sheet with fewer structural defects and smaller layer number can be obtained after such slow electrochemical exfoliation. The C1s orbital electron related XPS spectra also reveal the suppression on oxygen invasion at low-bias exfoliation, which facilitates uniform and less oxidant graphite nano-sheet powder without the need of reduction or annealing treatment. Both the quality and size distribution of the graphite nano-sheets are improved with such slow but refined electrochemical exfoliation. The nonlinear saturable absorption of the graphite nano-sheet induces a loss modulation of the transmitted light at 1560 nm, providing a modulation depth increased from 17% to 53% by using the graphite nano-sheet fabricated with decreasing exfoliation bias from + 6 to + 3 volts. This facilitates the graphite nano-sheet powder from being a stable saturable absorber and mode-locking starter of the EDFL, which successfully generates the mode-locked pulsewidth of 430-450 fs with corresponding linewidth of 6.0-6.5 nm. After starting with the layer number-controllable graphite nano-sheet powder, the mode-locking performance is dominated by the high-gain and strong pulse compression mechanisms in the EDFL. The graphite nano-sheet powders with different modulation depths only behave like a mode-locking starter and show weak influence on the pulse shortening of the passively mode-locked EDFL. These observations have concluded that the electrochemically exfoliated graphite nano-sheet powder can be used as a simple and convenient saturable absorber for starting sub-picosecond EDFLs.
This work was financially supported by National Science Council and National Taiwan University under grants NSC101-2221-E-002-071-MY3 and NSC101-2622-E-002-009-CC2.
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