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Abstract
Graphene/WS2 (G/WS2) van der Waals (vdW) heterostructures are utilized as saturable absorbers (SAs) in compact mode-locked fiber lasers operating in the telecommunication L-band for the first time. The interlayer coupling is confirmed by Raman and photoluminescence spectra. In comparison with pure WS2, the heterostructure exhibits excellent nonlinear optical properties in terms of larger modulation depth and lower saturation intensity due to the strong interlayer coupling. By incorporating the G/WS2-based SA into an all-anomalous-dispersion fiber laser, stable conventional-soliton pulses with a pulse duration down to 660 fs can be realized at 1601.9 nm, manifesting better output performance compared to pure WS2. In addition, through shifting the cavity dispersion to the net-normal dispersion, the G/WS2 SA can also be applied for dissipative-soliton generation. Resultant output pulses feature the central wavelength of 1593.5 nm and the pulse duration of 55.6 ps. Our results indicate that the G/WS2 vdW heterostructure is a promising candidate as SA for pulsed laser applications, which pave the way for the development of novel ultrafast photonic devices with desirable performance.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast fiber lasers have attracted tremendous interest in both industry and scientific research [1]. Passive mode-locking techniques employing saturable absorbers (SAs) are the most popular schemes for ultrashort-pulse generation. The SA is a nonlinear optical device, which affords the capability to turn the continuous wave (CW) into short optical pulse trains. As a highly successful saturable absorption material, semiconductor saturable absorber mirrors (SESAMs) have been routinely used in commercial laser systems due to their remarkable optical properties. By virtue of the well-developed semiconductor techniques, the SA parameters of SESAMs can be flexibly customized. However, SESAMs also suffer from some drawbacks in terms of narrow operation bandwidth, slow recovery time and complicated manufacturing procedures [2]. A relatively simple alternative is to use carbon nanotubes (CNTs), which have the advantages of sub-picosecond response time, low cost and easy integration with optical fibers [3]. Nevertheless, wide diameter distributions are often required for broadband operation, which introduce large non-saturable loss and compromise the laser performance. This motivates the ongoing exploration on novel saturable absorption materials for further developing ultrafast fiber lasers.
Recently, two-dimensional (2D) materials have been considered as promising candidates for applications in ultrafast photonics due to their intriguing electronic and optical properties. As the most representative 2D material, graphene has been investigated extensively since it was mechanically isolated in 2004 [4]. To date, a number of publications regarding the use of graphene in ultrafast fiber lasers have been presented, unambiguously verifying the potential of graphene as an eximious saturable absorption material for ultrashort-pulse generation [5–11]. The successful application of graphene stimulates the search for other 2D materials, such as topological insulators (TIs), transition metal dichalcogenides (TMDs, e.g., MoS2, WS2 and TiS2), and group VA monolayer materials (e.g., black phosphorus (BP) and antimonene). Thus far, these 2D materials have also been developed as excellent SAs to obtain ultrashort pulses in fiber lasers, where the central wavelengths are covered from the visible to mid-infrared region [12–25]. Nevertheless, the aforementioned 2D materials still have some inherent shortcomings, which hinder their practical exploitation in ultrafast laser systems. For instance, although graphene possesses the fascinating properties of ultrafast recovery time and wideband operation, the limited light absorption of monolayer graphene presents a challenge for its use as a large-modulation-depth SA [3]. TIs are recognized as a new state of quantum matter with metallic properties on the surface. Analogous to graphene, the band structure of their surface states exhibits the Dirac-like linear dispersion relation, enabling them to serve as broadband SAs. However, high quantities of intrinsic defects introduced during synthesis could result in a decrease in the modulation depth and saturation intensity of TIs-based SAs [26]. In addition, their thermal damage threshold should also be improved [27]. 2D TMDs possess advantages of strong light-matter interaction, high third-order nonlinear susceptibility, as well as ultrafast carrier dynamics [28]. Nevertheless, the band gap of 2D TMDs is relatively large, which remains challenging for their applications in the long-wavelength range unless defects are introduced. BP has a moderately narrow direct band gap, i.e., from 0.3 eV (bulk) to 2 eV (monolayer), which bridges the gap between graphene and 2D TMDs [29]. This feature makes it extremely useful for ultrafast photonic applications, especially in the mid-infrared region. Nonetheless, efforts are needed to improve the stability of BP under ambient conditions. As a member of group VA material, antimonene has been recently demonstrated as a wide band-gap 2D semiconductor with enhanced stability [22]. However, it was found that free standing antimonene is of indirect type, which might delimit its application in ultrafast photonics. In parallel with research on these individual 2D materials, a combination of distinct nanomaterials into heterostructures through the van der Waals (vdW) forces has recently sparked an upsurge of attention [30]. The properties of such hybrids can be efficiently tuned through the interlayer coupling, which point up new opportunities to develop novel functional devices with desirable performance. Till now, 2D heterostructure materials have been implemented in the fabrication of various electronic and optoelectronic devices, such as field-effect transistors, photodetectors and light-emitting diodes [31,32]. In the field of ultrafast photonics, mode-locked pulse generation using the heterostructure-based SAs has been demonstrated [33–44]. As a kind of 2D vdW heterostructure, graphene/WS2 (G/WS2) shows many interesting properties caused by the interlayer coupling, such as broadband photocarrier generation, fast interlayer charge transfer, as well as excellent photodetection performance [45–47]. It is possible to utilize the interlayer coupling interaction between graphene and WS2 to realize saturated light absorption below the band gap of WS2 [45]. Thus, G/WS2 heterostructures can be used as SAs in infrared (IR) pulsed lasers. However, ultrafast fiber laser operating in the telecommunication long wavelength band (L-band, i.e., 1565-1625 nm) with G/WS2 SA has not yet been reported.
In this paper, compact L-band fiber lasers mode-locked by G/WS2 SAs are demonstrated for the first time. Stable conventional-soliton (CS) pulses centered at 1601.9 nm with 660-fs pulse duration can be generated in an all-anomalous-dispersion fiber laser using the G/WS2 SA, which is superior to the output performance of the pulsed laser based on the WS2 SA. Furthermore, the G/WS2 SA are utilized in the net-normal-dispersion fiber laser, where dissipative-soliton (DS) pulses with a pulse duration of 55.6 ps are obtained at 1593.5 nm. The results suggest that the G/WS2 vdW heterostructure can serve as a promising material for applications in ultrafast photonics.
2. Experiment
Monolayer graphene and WS2 were synthesized by the chemical vapour deposition method, which can be found in our previous reports [48,49]. For the fabrication of G/WS2 vdW heterostructure, a step-by-step transfer method was used. First, monolayer WS2 on sapphire was transferred to the SiO2(270 nm)/Si substrate by surface energy assisted technique using polystyrene as protection layer [50]. Second, graphene on copper foil was transferred on the WS2/SiO2/Si substrate using polymethyl methacrylate (PMMA) as protection layer. Finally, the PMMA/G/WS2/SiO2/Si substrate was baked at 120 °C for 10 min. For Raman and Photoluminescence (PL) characterizations, the PMMA film was dissolved in acetone before measurements. The Raman and PL spectra were obtained by using Raman spectrometer (Andor SR-5001-A-R) with a 532-nm excitation laser. For SA preparation, the underlying SiO2 was etched by HF solution (10%) to lift off the PMMA/G/WS2 layer from the substrate. The PMMA/G/WS2 film was transferred to deionized water to remove the etchant and residues. Then, the PMMA/G/WS2 film was transferred to the end-face of a fiber connector (FC), followed by drying in air for 1 h. The G/WS2 coated FC was connected to a clean FC with a fiber adapter to form the G/WS2-based mode locker. To characterize the nonlinear transmittance of the fabricated WS2 and G/WS2 SAs, a balanced twin-detector measurement system was employed. Detailed experimental procedures and setup have been presented in our previous work [15]. Herein, the input pulsed source is a homemade ultrafast fiber laser with a central wavelength of 1596 nm, a repetition rate of 10.8 MHz, and a pulse duration of 710 fs.
A schematic of the mode-locked fiber laser used in our experiments is shown in Fig. 1. A segment of erbium-doped fiber (EDF) served as the gain medium. Two in-line polarization controllers (PCs) were engaged to adjust the polarization state for mode-locking optimization. The as-prepared SA was inserted between the two PCs. A polarization-independent optical integrated device (PI-OIC) was employed to construct the cavity, which possesses the hybrid functions of a wavelength-division multiplexer (WDM), a polarization-insensitive isolator (PI-ISO), and a 10% output coupler (OC). More details of this PI-OIC have been covered in our published paper [15]. The PI-OIC contained two sections of pigtails, i.e., 0.5-m HI 1060 Flex fiber and 0.5-m standard single-mode fiber (SMF) with dispersion parameters of −10 ps2/km and −23 ps2/km, respectively. It is worth noting that the usage of the PI-OIC greatly simplifies the cavity configuration. For CS operation in an all-anomalous-dispersion fiber laser, a 1.5-m EDF (Liekki Er80-8/125) with dispersion parameter of −20 ps2/km was adopted and a 6.8-m SMF was added into the cavity. The total length of the cavity was about 9.5 m and the net-cavity dispersion was estimated to be −0.21 ps2. For the generation of DS pulses, the anomalous-dispersion gain medium was replaced by a 2-m normal-dispersion EDF (Liekki Er110-4/125) and the 6.8-m SMF was also changed by a section of 54-m dispersion compensation fiber (DCF). The dispersion parameters of the normal-dispersion EDF and DCF are 66 ps2/km and 3.3 ps2/km, respectively. The total cavity length was about 57.2 m with a net-cavity dispersion of 0.29 ps2. The laser performance is monitored by a commercial optical spectrum analyzer (Yokogawa AQ6370C), a 500-MHz digital oscilloscope (Rigol DS4054) together with a 1.2-GHz photodetector (Thorlabs DET01CFC), a 50-GHz radio frequency (RF) spectrum analyzer (Rohde & Schwarz FSU50), and an autocorrelator (Femtochrome FR-103XL).
3. Results and discussion
3.1 Material characterization
The photograph of the as-prepared G/WS2 heterostructure film on SiO2/Si substrate is presented in Fig. 2(a), which clearly shows individual WS2 and graphene areas as well as the overlapped G/WS2 heterostructure area. Figure 2(b) depicts the Raman spectrum collected from the graphene area. The typical peaks of D, G, and 2D are located at 1342.3, 1587.8, and 2681.4 cm−1, respectively. The intensity ratio between 2D and G peaks is about 1.9, confirming the single layer characteristic of the graphene. Due to the strong luminescence of the WS2, it is difficult to distinguish the Raman signal of graphene in the heterostructure area. On the contrary, the Raman signals of WS2 can be observed in both WS2 and G/WS2 heterostructure areas, as presented in Fig. 2(c). It is shown that the Raman peak near 350 cm−1 can be resolved into several distinct peaks. The 60.2-cm−1 frequency difference between A1g(Γ) and E12g(Γ) peaks in the WS2 area indicates the presence of monolayer WS2. There are some changes in the positions of Raman peaks between WS2 and G/WS2 heterostructure areas, which is related to the interlayer coupling between graphene and WS2. Furthermore, PL spectra emerged from WS2 and G/WS2 heterostructure areas are shown in Fig. 2(d). The PL intensity in the heterostructure area is only 12% of that in the WS2 area, suggesting a strong electronic interaction between graphene and WS2. To illustrate the reduced PL intensity in the heterostructure area, a schematic of the band structure based on literature [45] is shown in the Fig. 2(e). Due to the unique band alignment, the photogenerated electrons transfer to WS2 while holes transfer to graphene, inhibiting the direct recombination of electron-hole pairs. As a result, the PL intensity is significantly decreased. In addition, one can see from the inset of Fig. 2(d) that the PL emission in the heterostructure area shows a red shift of peak position. As demonstrated in Fig. 2(f), the PL spectra can be decomposed into three peaks labelled as X, X-, and D. The X emission is ascribed to neutral exciton emission, X- belongs to trion emission, and D corresponds to defect-related emission. The emission ratio between X and X- peaks is reduced from 1.78 to 0.19 after the formation of G/WS2 heterostructure, which accounts for the red shift of the PL peak shown in the inset of Fig. 2(d). The increased X- emission suggests the increased electron density in WS2, which is consistent with the charge transfer picture. Moreover, the D emission is increased, which may be caused by the increase of defects during the transfer process. The unique band alignment of G/WS2 heterostructure permits the possible transition from graphene to WS2 under a low photon-energy irradiation, which can give rise to strong light absorption in IR region, benefiting for its use as SA in L-band pulsed lasers.
Fig. 2. Optical characterizations of the G/WS2 heterostructure. (a) Photograph of the G/WS2 heterostructure on SiO2/Si substrate. (b) Raman spectrum from graphene area. (c) Raman spectra from WS2 and G/WS2 heterostructure areas. (d) PL spectra from WS2 and G/WS2 heterostructure areas. Inset: normalized PL spectra. (e) Band alignment of the heterostructure. (f) Fitting curves of the PL spectra.
In order to check the possibility of G/WS2 heterostructure as SA, its nonlinear transmittance at 1596 nm was measured, as shown in Fig. 3. For comparison, the nonlinear transmittance of WS2 was also recorded. The experimental data can be well fitted by a simple two-level SA model:
where ΔT denotes the modulation depth, Tns represents the non-saturable loss, I is the instantaneous intensity, and Isat depicts the saturation intensity. As shown in Fig. 3(a), the obtained parameters of ΔT, Isat and Tns for the WS2 SA are 2.9%, 8.1 MW/cm2 and 17%, respectively. As for the G/WS2 SA, the three parameters are fitted to be 16.8%, 4.3 MW/cm2 and 23%, as presented in Fig. 3(b). In comparison with WS2, the modulation depth of the G/WS2 SA increases while its saturation intensity decreases, which may be ascribed to the strong interlayer coupling between graphene and WS2 [38].It should be noted that the saturable absorption properties of monolayer graphene have been well investigated in the previous literatures. In general, the optical modulation depth is around 1% for monolayer graphene [3], which hinders its practical exploitation in ultrafast laser systems. Our results indicate that the G/WS2 heterostructure has a stronger nonlinear optical response compared to that of pure WS2 and graphene.
3.2 L-band conventional soliton mode locking
The saturable absorption properties of G/WS2 heterostructure make it suitable for applications as SAs to construct L-band mode-locked fiber lasers. The as-prepared G/WS2 SA was first fused inside the all-anomalous-dispersion cavity. CW operation of the proposed laser started at a pump power of 54 mW. When the pump power was beyond 92 mW, self-started mode locking could be achieved by adjusting the intra-cavity PCs. The performance of the output pulses at the pump power of 134 mW is illustrated in Fig. 4. Figure 4(a) demonstrates the oscilloscope trace of the output pulse train. The adjacent-pulse interval is 45.9 ns, matching well with the cavity length of 9.5 m. As shown in Fig. 4(b), the optical spectrum with a 3-dB spectral width of 4.3 nm is centered at 1601.9 nm. Several pairs of Kelly sidebands are observed in the spectrum, which confirms that the laser operates in the CS mode-locking regime. Figure 4(c) depicts the autocorrelation (AC) trace of the resultant mode-locked pulses. Assuming a sech2 pulse intensity profile, the actual pulse duration is 660 fs. The calculated time-bandwidth product (TBP) of 0.33 implies that the pulses are slightly chirped. As presented in the RF spectrum of Fig. 4(d), the repetition rate of the output pulses is 21.78 MHz. The signal-to-noise ratio (SNR) is 68 dB, which indicates a good stability in the CS operation. The strong capability for noise suppression is related to the ultrafast carrier dynamics of the G/WS2 SA [51]. At a pump power of 167 mW, the maximum output power was 5.1 mW, corresponding to single pulse energy of 0.23 nJ. With higher pump power, stable fundamental mode-locking was destroyed and the laser operated in multiple-soliton state. When the pump power was beyond 350 mW, the G/WS2 SA was damaged. In this case, the mode-locked pulse train became unstable and eventually disappeared.
Fig. 4. Output characteristics of the all-anomalous-dispersion fiber laser with a G/WS2 SA. (a) Mode-locked pulse train. (b) Optical spectrum. (c) AC trace. (d) RF spectrum.
To evaluate the long-term stability of the G/WS2-based mode-locked fiber lasers, the optical spectrum of output pulses at the pump power of 134 mW was continuously monitored for 8 hours under an ambient environment. It is shown that the central wavelength and 3-dB spectral bandwidth remain stable during the period, which confirms that G/WS2 heterostructure is an effective SA delivering stable mode-locking operation.
Furthermore, the G/WS2 SA was substituted by the WS2-based SA and the corresponding laser performance at the pump power of 134 mW is shown in Fig. 5. The pulse-to-pulse separation is 45.6 ns from the mode-locked pulse train [Fig. 5(a)]. The slight difference in the pulse interval between G/WS2- and WS2-based pulsed lasers is attributed to the small change of cavity length during splicing the SAs. The central wavelength of the optical spectrum and the 3-dB bandwidth are 1604.5 nm and 2.9 nm, respectively [Fig. 5(b)]. The AC trace of the pulse can be well fitted by the sech2 function and the pulse duration is 1.04 ps [Fig. 5(c)]. The calculated TBP is 0.35, which demonstrates that the output pulses are near transform limited. The RF spectrum [Fig. 5(d)] depicts that the SNR and the repetition rate of the pulses are 52 dB and 21.93 MHz, respectively. It should be noted that the pulse duration of 1.04 ps is about 1.6 times larger than that obtained in the fiber laser with G/WS2 SA, which can be explained as follows. On the one hand, the carrier dynamics of SA play a crucial role in determining the pulse duration [52]. Considering that graphene is a fast SA while WS2 is a relatively slow one, the formation of G/WS2 heterostructure speeds up the carrier recombination in WS2 due to fast charge transfer and relaxation channel in graphene. In this regard, a shorter pulse is prone to achieving in the G/WS2-based mode-locked fiber laser. On the other hand, the light absorption of G/WS2 heterostructure in the IR region can be greatly enhanced due to the interlayer transition, which results in an increase in its modulation depth, as demonstrated in Fig. 3. In general, the pulse duration is related to the net-cavity dispersion and the modulation depth of SA. Since these two pulsed lasers have similar configurations, their averaged dispersion values are almost the same. Therefore, it is reasonable to generate a narrower pulse in the fiber laser based on G/WS2 SA with a larger modulation depth. The results suggest that the G/WS2 heterostructure is a promising SA for pulsed laser applications. The available pulse duration can be further shortened by optimizing the parameters of G/WS2 SA and the net-cavity dispersion.
Fig. 5. Output characteristics of the all-anomalous-dispersion fiber laser with a WS2 SA. (a) Mode-locked pulse train. (b) Optical spectrum. (c) AC trace. (d) RF spectrum.
3.3 L-band dissipative soliton mode locking
In order to confirm the potential of G/WS2 heterostructure for DS pulse generation, the intra-cavity net dispersion was changed from the all-anomalous-dispersion to the large-normal-dispersion regime. When the pump power was beyond 110 mW, stable mode-locked pulses were achieved in the cavity by fine tuning the PCs. The laser performance at the pump power of 123 mW is presented in Fig. 6. The oscilloscope trace of the output pulse train indicates a pulse interval of 275.5 ns, as presented in Fig. 6(a). Figure 6(b) illustrates that the optical spectrum with a 3-dB bandwidth of 13.1 nm is centered at 1593.5 nm. The spectrum exhibits steep edges, which is a characteristic of DS pulses. As depicted in Fig. 6(c), the single pulse profile can be well fitted by the Gaussian function. The actual pulse duration is 55.6 ps, indicating that the resultant pulses are highly chirped. The corresponding RF spectrum of Fig. 6(d) shows that the repetition rate of the output pulses is 3.63 MHz and the SNR is 63 dB, which implies high stability in the DS operation state. In this case, the output power of DS pulses was 1.9 mW, corresponding to the single pulse energy of 0.52 nJ.
Fig. 6. Output characteristics of the net-normal-dispersion fiber laser with a G/WS2 SA. (a) Mode-locked pulse train. (b) Optical spectrum. (c) AC trace. (d) RF spectrum.
To eliminate the possibility of mode-locking operation caused by the nonlinear-polarization-rotation effect, the G/WS2 SA was deliberately removed from the cavity. No matter how to adjust the pump power and PCs within a wide range, no pulse train could be observed on the oscilloscope, indicating that mode-locked pulse generation is actually attributed to the saturable absorption of G/WS2 heterostructure. The proposed laser was turned on over 8 hours in the laboratory environment to test its long-term stability. The relative fluctuations of the output power were measured to be ∼3.6%, manifesting a good stability of the laser operation.
4. Conclusion
We have demonstrated mode-locked pulse generation in compact L-band fiber lasers with G/WS2 vdW heterostructure SAs. The strong interlayer coupling between graphene and WS2 has been confirmed by Raman and PL spectra. Compared to WS2, the G/WS2 heterostructure has shown superior saturable absorption properties in terms of larger modulation depth and lower saturation intensity. Based on the G/WS2 SA, 660-fs CS pulses operating at 1601.9 nm have been realized in the all-anomalous-dispersion fiber laser, exhibiting better output performance in comparison with pure WS2. The G/WS2 SA has also been employed in the net-normal-dispersion fiber laser, where DS pulses with a central wavelength of 1593.5 nm and a pulse duration of 55.6 ps have been obtained. Our results suggest that the G/WS2 heterostructure is beneficial for applications in ultrafast photonics.
Funding
National Natural Science Foundation of China (NSFC) (61775031, 61421002, 61875033).
Disclosures
The authors declare no conflicts of interest.
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