1. Introduction

Novel two-dimensional (2D) materials bring up a new area of 2D nano-systems and have attracted intense interests in the recent years. 2D semiconducting transition metal dichalcogenides (TMDs), including molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), have in particularly received significant attention because of their semiconducting property with tunable bandgaps and abundance in nature [1–4]. 2D atomically thin WS₂ nanosheets exfoliated from bulk counterparts have shown exotic electronic and optical properties, such as indirect-to-direct bandgap transition with reducing number of layers, high carrier mobility and strong spin-orbit coupling due to their broken inversion symmetry [5–7], which have enabled widely potential applications in viable photonic and optoelectronic devices [3, 8]. Similar to graphene, 2D WS₂ can be fabricated by the methods including micro-mechanical cleavage, liquid-phase exfoliation (LPE) and chemical vapor deposition [2]. Among these methods, liquid-phase preparation is scalable and permitting fabrication of wafer-scale thin films and coatings, showing good prospects for making flexible electronics and composite materials [1, 5].

In various applications of 2D materials, saturable absorbers (SAs) for mode-locked lasers and Q-switched lasers play a key role for the ultrafast photonic applications. Graphene is the first discovered 2D material and its saturable absorption has been widely investigated for the mode locked lasers and Q-switched lasers [9–11]. Triggered by the study of graphene, saturable absorption has been discovered in other 2D materials, which opens up a door to a promising area that a wide class of 2D materials may all have saturable absorption and the choice for SAs can be significantly extended. Graphene oxide [12, 13] and topological insulators [14–16] based SAs have been demonstrated for their mode locking and Q-switching operations. Very recently, MoS₂ has been reported for its saturable absorption in a wide band from 400 nm to 2 μm [17–20]. Z-scan studies have revealed the high optical nonlinearity of MoS₂ which is comparable to that of graphene [19]. Mode locking and Q-switching operations based on MoS₂ SA have been achieved near 1 μm, 1.55 μm and 2.1 μm [21–23]. Femtosecond pulse output [24], harmonic mode locking [25, 26] and wide tunability [22, 23, 27] have also been reported. It is interesting to find that although the direct bandgap of monolayer MoS₂ is ~1.8eV (688 nm) and the indirect bandgap is 0.86 - 1.29 eV (1443 - 962 nm) [20], the saturable absorption property was observed in a wide band beyond this limitation, i.e., the sub-bandgap absorption. A few discussions have been put for the origin of this sub-bandgap absorption, including absorption induced by the edge modes of the nanosheets [22, 23], defects in the materials [20] and two-photon absorption (TPA) [19]. Further investigation is still required to understand this phenomenon.

Moreover, it is still unclear whether other TMD materials with similar atomic structures have similar nonlinear properties as MoS₂? If yes, a wide class of TMDs can then be potential SAs for ultrafast photonic applications which may significantly extend the choice of SAs. Based on this incentive, we investigated the saturable absorption of another TMD material WS₂ and its applications in ultrafast lasers. In this work, the saturable absorption of WS₂ nanosheets in the telecommunication wavelength near 1550 nm was discovered and characterized. By embedding WS₂ nanosheets into PVA thin film, WS₂-PVA SA was obtained with a modulation depth of 2.9% and a saturation intensity of 370 MW/cm² for ultrafast photonic applications. Incorporating WS₂-PVA SA into a fiber laser cavity, both mode locking and Q-switching operations near 1550 nm were demonstrated. Femtosecond pulse width of 595 fs and high RF spectral purity of 75 dB extinction ratio were obtained in the mode locking operation whereas tunable repetition rate from 90 kHz to 125 kHz and pulse energy of tens of nano joule were obtained in the Q-switching operation. These findings indicate that saturable absorption indeed exists in WS₂ near 1550 nm and WS₂-PVA SA is a promising mode locker and Q switcher for the ultrafast photonic applications. Furthermore, WS₂ based mode locking and Q-switching operation at a sub-bandgap wavelength suggests that there may be some unknown physical phenomenon leading to the sub-bandgap absorption in the TMDs, which has never been reported in other saturable absorber materials such as graphene and topological insulators. Our observation of sub-bandgap absorption in WS₂, together with other similar sub-bandgap phenomenon reported in MoS₂, may help to trigger the further research on the physical properties of TMDs to better understand the origin of this sub-bandgap absorption.

Note: during the revision of this manuscript, two works of WS₂ based mode locking and Q-switching operations have been reported [28, 29].

2. Material preparation and characterization

A high-quality WS₂-based saturable absorber with thin-film form provides significant flexibility for the photonic applications such as mode-locked and Q-switched fiber lasers. Liquid-phase exfoliation method was applied to prepare dispersions with large populations of monolayer and few-layer WS₂ using sodium cholate (SC) as surfactant. Typically, 5 mg of WS₂ powders were dispersed in 1.5 mg/ml SC aqueous solution and sonicated for 1 hour using a horn probe sonic tip (VibraCell CVX; 750 W) with 38% output power. The dispersions were then centrifuged at 3000 rpm for 90 minutes to remove the unexfoliated ones. The top 2/3 of the dispersions was collected by pipette. The prepared WS₂/SC dispersions had a concentration of ~0.015 mg/ml. Transmission electron microscope (TEM) was utilized to confirm the existence of WS₂ nanosheets in the dispersions, shown in Fig. 1(a). WS₂ nanosheets with the size of a few hundreds of nanometers can be clearly observed in the TEM image due to the effective exfoliation of the LPE method.

 

Fig. 1 (a) TEM image of WS₂ nanosheets, (b) WS₂-PVA thin film on a glass plate, (c) WS₂-PVA SA on fiber end, (d) transmission spectra of WS₂-PVA thin film and pure PVA thin film and (e) Raman spectra of WS₂ dispersions without PVA and WS₂-PVA thin film.

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The WS₂ dispersions were then mixed with PVA aqueous solution, sonicated, dropped on a slide glass and dried in the room temperature to form the thin film, shown in Fig. 1(b). The thin film was cut into small pieces and transferred onto the fiber end. To enhance the effect of saturable absorption, 5 pieces of WS₂-PVA thin film were transferred and stacked on the fiber end as WS₂-PVA SA, shown in Fig. 1(c). The total thickness of WS₂-PVA SA is ~100 μm. To confirm the incorporation of WS₂ nanosheets into the PVA thin film, the transmission spectra of WS₂-PVA thin film and pure PVA thin film were characterized with a spectrometer (PerkinElmer Lambda 750 instrument), shown in Fig. 1(d). The pure PVA thin film has a transmission of ~98% near 1550 nm and the WS₂-PVA thin film has a transmission of ~96% near 1550 nm. The dip near 632 nm (1.96 eV) in the transmission spectrum of WS₂-PVA thin film is a typical fingerprint of WS₂ nanosheets due to the direct bandgap transition. A Raman spectroscopy system (Renishaw invia) with an excitation wavelength of 488 nm was utilized to confirm the existence of WS₂ nanosheets in the PVA thin film, as shown in Fig. 1(e). The in-plane vibrational mode E¹_2g at 355.9 cm⁻¹ and the out-of-plane vibrational mode A_1g at 420 cm⁻¹ can be clearly observed. The frequency difference between the E¹_2g mode and A_1g mode reveals the number of layers of WS₂ nanosheets [5]. Therefore, the frequency difference of ~64.1 cm⁻¹ in this work implied the number of layers to be ~3. For comparison, the Raman spectrum of the WS₂ nanosheets in the dispersions without PVA was also measured. It can be seen that the Raman spectra of WS₂ dispersions and WS₂-PVA thin film were nearly identical, which indicates that the WS₂ nanosheets were embedded in the PVA thin film with nearly intact atomic structures. This finding suggests that PVA is a suitable host for WS₂ nanosheets and filmy WS₂-PVA SA provides significant flexibility in the practical applications due to its compactness compared with aqueous dispersions.

To investigate the saturable absorption of the prepared WS₂-PVA SA, the nonlinear transmission property was characterized at the telecommunication wavelength using a standard 2-arm transmission measurement scheme, shown in Fig. 2(a). A commercial mode-locked laser was applied as a pulsed source. The laser had a center wavelength of 1560 nm, and a pulse width of ~510 fs. The output of the laser was split into two arms with the upper arm for the power-dependent transmission measurement of WS₂-PVA SA and the lower arm for reference. The measurement results were shown in Fig. 2(b). The measurement was performed first by increasing the input power (or intensity) and then by decreasing the input power. The nearly unchanged results from the two measurements confirmed the existence of saturable absorption in WS₂-PVA SA. The modulation depth of the WS₂-PVA SA is ~2.9% with saturation intensity of ~370 MW/cm². The non-saturable loss is ~30.9%. The non-saturable loss is due to the scattering and absorption of the WS₂ flakes and PVA thin film, and the coupling loss between two fiber connectors.

 

Fig. 2 (a) Measurement setup of the nonlinear transmission of WS₂-PVA SA and (b) measured saturable absorption with a modulation depth of 2.9%.

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It should be mentioned that although the direct bandgap of monolayer WS₂ is ~2.0 eV (~630 nm) and the indirect bandgap is ~1.4 eV (~886 nm) [2], the saturable absorption was observed near 1550 nm corresponding to a photon energy of ~0.8 eV which is sub-bandgap absorption. Similar to the analysis for MoS₂ in the introduction, this sub-bandgap absorption may be attributed to the absorption of edge modes, defects and TPA. Among these possible sources of absorption, absorption induced by the edge modes of the WS₂ nanosheets is possibly the main contributor of the sub-bandgap absorption near 1550 nm in our experiments. The typical size of the nanosheets is a few hundreds of nanometer and the beam diameter in optical fiber is 9-10 μm. Therefore there can be many edges in the beam area leading to a relatively high absorption at long wavelength. LPE method is known to produce chemically pristine material dispersions [1] and thus the absorption from the defects should be weak. The contribution of TPA is related to the inverse saturable absorption at high input intensity [19]. Thus the edge modes are considered to be the main reason of sub-bandgap saturable absorption in the experiments. However, no experimental evidence can be given here because the edge modes are dependent on the shape, size and number of layers of the nanosheets and it is very difficult to produce nanosheets with uniform shape, size and number of layers with current technologies. Further investigation on the materials is still required to better understand this phenomenon of sub-bandgap absorption.

3. Mode-locking and Q-switching operations

Passively mode-locked fiber lasers with femtosecond output pulses attract intense interest due to its wide applications in communications, sensing and frequency metrology. We first demonstrated the WS₂-PVA SA as a mode locker for a passively mode-locked fiber laser near 1550 nm. The laser setup is shown in Fig. 3. The laser cavity consisted of ~1 m Erbium-doped fiber with anomalous dispersion and ~7.2 m standard single mode fiber. The net cavity dispersion was estimated to be ~-0.18 ps² and the laser was operating in the soliton mode locking regime. Pump light at 976 nm was injected into the cavity via a wavelength division multiplexer (WDM). Two polarization controllers were used to adjust the cavity birefringence. An isolator was incorporated to guarantee the single-direction operation and a 90:10 coupler extracted 10% intra-cavity power for output. The WS₂-PVA SA was embedded between two FC/PC fiber connectors. The optical spectra were measured by an optical spectrum analyzer (Yokogawa AQ6370C). The autocorrelation trace was measured by an autocorrelator (Femtochrome 103XL). The oscilloscope traces were measured by a 10-GHz photodetector (EOT 3500F) and a 2.5-GHz real-time oscilloscope (Agilent DSO9254A). The RF spectra were measured by a 44-GHz RF spectrum analyzer (Agilent N9010A EXA).

 

Fig. 3 Experimental setup of the fiber laser with WS₂-PVA SA. EDF: Erbium-doped fiber; WDM: wavelength-division multiplexer.

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The laser output properties are summarized in Fig. 4. The laser self-started with harmonic mode locking at a pump power near 350 mW. Fundamental mode locking was obtained when the pump power was decreased to 260 mW and then the fundamental mode locking can be sustained at the pump power ranging from ~180 mW to 310 mW. At the pump power of 280 mW and fundamental mode locking operation, the laser had an optical spectrum centered at 1572 nm with a 3-dB bandwidth of 5.2 nm, shown in Fig. 4(a). Kelly sidebands can be observed which were due to the periodic perturbation in the cavity during the soliton mode locking operation. The autocorrelation trace in Fig. 4(b) had a width of 919 fs corresponding to a pulse width of 595 fs assuming sech² profile. To confirm the stable mode locking operation, a 10-GHz photodetector and a 2.5-GHz real-time oscilloscope were employed to investigate the pulse train in time domain, shown in Fig. 4(c). Very uniform pulse amplitude can be observed which indicates the laser was operating in the continuous wave (cw) mode locking mode. The laser output power with respect to the pump power was shown in Fig. 4(d) and the regions of fundamental and harmonic mode locking were also denoted. RF spectra were also measured to evaluate the spectral quality of the mode locking operation, shown in Fig. 5. In a span of 400 kHz with resolution bandwidth (RBW) of 10 Hz, the extinction ratio is 75 dB. And in a span of 500 MHz with RBW of 10 kHz, no unwanted spurious sideband was observed which indicated the high spectral purity of the mode locking operation.

 

Fig. 4 (a) Optical spectrum, (b) autocorrelation trace with circles as measured data and solid line as sech² fit, (c) oscilloscope trace and (d) output power with respect to pump power of the mode-locked laser based on WS₂-PVA SA.

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Fig. 5 RF spectra of the laser for (a) a span of 400 kHz with resolution bandwidth of 10 Hz and (b) a span of 500 MHz with resolution bandwidth of 10 kHz.

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Besides mode locking, Q-switching operation is another important operation mode in the fiber lasers, which provides tunable repetition rate and relatively high pulse energy. By adjusting the birefringence in the laser cavity with polarization controllers, the wavelength position of the spectral loss induced by the birefringence filter can be tuned so that Q-switching operation experiences lower total loss in the cavity compared with mode locking operation. Therefore, Q-switching operation can also be obtained in the same laser cavity incorporated with WS₂-PVA SA. Figure 6 summarizes the output properties of the Q-switching operation. The Q-switching operation existed at the pump power ranging from 252 mW to 364 mW. Out of this range, continuous wave operation dominated. In the Q-switching operation, the laser had a center wavelength of 1570 nm, shown in Fig. 6(a). Oscilloscope trace in Fig. 6(b) confirmed the stable pulsed operation of the laser and the repetition rate was dependent on the pump power varying from 90 kHz to 125 kHz, shown in Fig. 6(c). The relation between the laser output power and pump power was given in Fig. 6(d) with the region of Q-switching operation denoted. The maximum pulse energy was 46.3 nJ at the pump power of 280 mW.

 

Fig. 6 (a) Optical spectrum, (b) oscilloscope trace, (c) repetition rate and (d) output power with respect to pump power of the Q-switched laser based on WS₂-PVA SA.

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4. Conclusions

In conclusion, the saturable absorption of WS₂ nanosheets near 1550 nm was discovered and investigated. WS₂-PVA saturable absorber was prepared with a modulation depth of 2.9% and a saturation intensity of 370 MW/cm² for ultrafast photonic applications. Both mode locking and Q-switching operations near 1550 nm were demonstrated in a fiber laser incorporated with WS₂-PVA SA. In the mode locking operation, femtosecond pulse width of 595 fs and high RF spectral purity of 75 dB extinction ratio were obtained whereas in the Q-switching operation tunable repetition rate from 90 kHz to 125 kHz and pulse energy with tens of nJ were obtained. These findings demonstrate the potential of WS₂-PVA SA as a promising mode locker and Q switcher for the ultrafast photonic applications. Moreover, the thin-film form provides significant flexibility in the practical use due to its compactness and easy fabrication.