Application of a multilayer Molybdenum Disulfide (MoS2) thin film as a saturable absorber was experimentally demonstrated by realizing a stable and robust passive mode-locked fiber laser via the evanescent field interaction between the light and the film. The MoS2 film was grown by chemical vapor deposition, and was then transferred to a side polished fiber by a lift-off method. Intensity-dependent optical transmission through the MoS2 thin film on side polished fiber was experimentally observed showing efficient saturable absorption characteristics. Using erbium doped fiber as an optical gain medium, we built an all-fiber ring cavity, where the MoS2 film on the side polished fiber was inserted as a saturable absorber. Stable dissipative soliton pulse trains were successfully generated in the normal dispersion regime with a spectral bandwidth of 23.2 nm and the pulse width of 4.98 ps. By adjusting the total dispersion in the cavity, we also obtained soliton pulses with a width of 637 fs in the anomalous dispersion regime near the lasing wavelength λ = 1.55 μm. Detailed and systematic experimental comparisons were made for stable mode locking of an all-fiber laser cavity in both the normal and anomalous regimes.
© 2014 Optical Society of America
In recent years, 2-dimensional nano-sheet materials have been intensively studied since remarkable research based on graphene has been reported . Molybdenum disulfide (MoS2) crystals consist of alternating hexagonal planes of Mo and S atoms bound by a weak van der Waals interaction, which allows formation of nano-sheet structures. In contrast to graphene, MoS2 bulk crystal is a semiconductor whose band changes from an indirect gap (1.29 eV) to a direct gap in the monolayer structure allowing novel functionalities [2–4] in electronic devices [5,6]. MoS2 films have been applied in various research fields such as in the development of catalysis, in nano-tribology, microelectronics, batteries, hydrogen storage, and in optoelectronics [7–15]. Monolayer MoS2 has also shown potential in light emitting devices due to its strong optical emission and extraordinary photoluminescence by the direct band-gap at the K point in the Brillouin zone, unlike the gapless structure of graphene [16–19]. Despite these scientific efforts, investigation and exploitation of the nonlinear optical properties of MoS2 in an optical fiber laser cavity has been very limited thus far [20–22].
Recently, notable optical properties have been observed from MoS2 nano-sheets [23–29] including a high third order susceptibility on the order of 10−15 esu, and saturable absorption . The reported nonlinear optical absorption of MoS2 through Z-scan measurements [20,26] confirmed that MoS2 not only has significant saturable absorber performance in broadband wavelengths but also demonstrates a higher saturable absorption response than graphene. The mode-locked fiber lasers and Q-switched fiber laser based on MoS2 ferrule-type SA [20–22] has comparatively simple fabrication and is vulnerable to destruction by mechanical damage or high power operation. A saturable absorber (SA) shows an optical intensity dependent absorption such that absorbance saturates at a higher intensity of light. The SA characteristics of single wall carbon nano-tube (SWCNT) and graphene have been successfully applied to passive mode locking of fiber laser cavities [1,30–39]. Various methods have been used to implement these carbon SAs in fiber lasers, such as depositing material on fiber ferrules [40,41], putting them into hollow optical fibers (HOF) or photonic crystal fibers (PCF) [30,31,42,43], and using evanescent field interaction in a side polished fiber (SPF) [44–47]. In the case of graphene and CNT, it has been a common practice to use a polymer binder such as polymethylmethacrylate (PMMA) or polyvinyl alcohol (PVA) to maintain the thermal stability and optical nonlinearity in the fiber laser cavities [32,33,48–57]. In the case of SWCNTs, polymer bind had to be used to reduce the optical loss, which resulted from the residual bundled nanotubes and catalyst particles, as well as clumps SWCNTs. Recently, topological isolator (TI) materials have attracted great attention due to their remarkable saturable absorption characteristics as well [58,59]. The TIs are known as three dimensional materials which have a Dirac band structure like graphene. However, these materials do not have a band-gap structure property like MoS2.
In this study, we experimentally demonstrated application of multilayer MoS2 grown by chemical vapor deposition (CVD) as a SA in a passively mode-locked Er-doped fiber laser, successfully achieving both soliton and dissipative soliton pulses, for the first time, to the best knowledge of the authors. In particular, we removed the polymer binder in preparing our MoS2 film on SPF, which has not been attempted in prior reports for CNTs, graphene, or MoS2. The main disadvantage of polymer buffer in prior reports was a low damage threshold, which limited the laser pulse energy. Our attempt to remove any polymer matrix in the MoS2 thin film can potentially increases the damage threshold of the SA and increase output power. The polymer binders usually incurred additional optical losses in the laser wavelength, especially in the IR range, and removal of polymer binder can decrease the laser cavity loss and contribute to a lower laser threshold. The hygroscopic nature of binding polymers such as PVA has also caused the laser instability in a humid environment. Moreover, the optical interface between the SA and the polymer might form due to the high index difference between two materials, which served as scattering sites to increase the cavity loss. These shortcomings and disadvantages of polymer binder could be fundamentally removed in our polymer-free MoS2 SA structure. We have confirmed the stable SA characteristics of the multilayer MoS2 film on SPF by measuring the nonlinear optical transmission. The prepared MoS2 SA was inserted into two fiber ring cavities. In the normal dispersion cavity, whose dispersion was optimized by adding a segment of dispersion compensating fiber, we obtained stable dissipative soliton pulses with 4.98 ps pulse width at the repetition rate of 26.02 MHz showing a broad spectral width of 23.2 nm. In anomalous dispersion regime, we obtained soliton pulses with pulse duration of 637 fs at the repetition rate of 33.48 MHz, and with a spectral width of 12.38 nm.
2. Fabrication process and characterization of the multilayer MoS2
Side-polished fiber (SPF) was prepared to provide efficient evanescent wave interaction (EWI) between the MoS2 film and light in the core while increasing the damage threshold of the SA. The SPF was fabricated by polishing a single mode optical fiber buried in a curved groove in a quartz block. The cladding thickness was optimized by balancing between the insertion loss (IL) and the efficiency of EWI. Here we optimally set the index-oil drop loss as 25 dB, which is reasonably used value in fabrication of side-polished fiber type directional coupler . The IL was found to be 0.15 dB in this condition. We observed that transmission of the fabricated SPF exhibit nearly polarization-independent.
The multilayer MoS2 thin film was produced by using a chemical vapor deposition (CVD) method . The MoS2 thin film was prepared on SiO2 substrates using MoO3 and S precursors similar to prior method. Polymethylmethacrylate (PMMA) was spin-coated over the CVD-grown MoS2 thin film on SiO2 substrate (step 1: 500 rpm for 10 s; step 2: 2500 rpm for 40 s), followed by baking at 80°C for 5 minutes.
An etchant solution to remove SiO2 substrate was prepared by completely dissolving 7 g of potassium hydroxide (KOH) pellets in 19 mL deionized (DI) water along with 4 mL isopropyl alcohol . The sample was immersed in a prepared solution to remove the SiO2 substrate. The PMMA/MoS2 film floating on the solution was transferred to DI water. The substrate and PMMA/MoS2 thin film are shown in Fig. 1(a). The PMMA/MoS2 film was then transferred onto the surface of the SPF, as depicted in Fig. 1(b). We then further removed the PMMA layer by immersing the PMMA/MoS2 thin film SPF device in acetone. Here the output power was continuously monitored, as depicted in Fig. 1(c), to confirm the efficient removal of the PMMA layer. Figure 1(d) summarizes the experimental measurements of the output power as a function of process time during the PMMA removal process. The PMMA has a higher refractive index than silica, and its removal increased the transmission through SPF about 30% while the insertion loss was reduced to 1.1dB. The inset of Fig. 1(d) shows the optical microscopic image of the final MoS2 film on SPF, with the red light signifying the core of the optical fiber. This image confirms that the MoS2 thin film covered the whole evanescent wave interaction length of the SPF of ~3 mm. This procedure was followed by rinsing the sample with isopropyl alcohol and DI water to remove any residue. Repeating these processes, we successfully achieved MoS2 film on SPF without any organic binder, which has not been attempted in prior reports.
Raman spectroscopy has been widely used to characterize the layer structure of nano-sheet materials . Raman spectra from two random spots on the prepared CVD MoS2 film on SPF were obtained and the results are shown in Fig. 2(a). Two typical Raman active modes of bulk MoS2 are E12g at 383 cm−1 and A1g at 409 cm−1 , which are related to the planar vibration (E12g) and the out-of-plane vibration of sulfides (A1g). The E12g peak blue-shifts while the A1g peak red-shifts as the film gets thinner . In Fig. 2(a), we found that the spacing between the two peaks was ~21 cm−1 for two arbitrary spots, which indicates formation of uniform multilayer MoS2. The Raman spectrum of a pristine CVD MoS2 film on SiO2 substrate is shown in the inset of Fig. 2(a), where we could confirm the identical peaks.
Figure 2(b) displays an optical microscopic image of the MoS2 layer’s boundary on the SiO2 substrate and how the triangle-shaped monolayers of MoS2 merge to form a continuous MoS2 thin film with a lateral size up to 2 mm. Morphological properties of the MoS2 film were investigated with field emission scanning electron microscopy (SEM), using JEOL JSM-6701F as shown in the inset of Fig. 2(b). The image revealed that CVD growth of MoS2 on SiO2 substrates (with 530 µm thickness) produces uniform atomic layers. Using an atomic force microscope (AFM), the thickness of MoS2/PMMA layer and MoS2 thin film after PMMA removal process were measured which are approximately 90 nm and 7 nm, respectively (see Fig. 2(c)). Furthermore, another CVD multilayer MoS2 has been grown with the same condition and transferred on a sapphire substrate. Figure 2(d) demonstrates the linear absorption spectrum of the prepared thin film on the sapphire substrate through a spectrophotometer. The four transition peaks (between 400 and 450 nm and 600-700 nm) of MoS2 are corresponded to the four different electronic transitions of the MoS2 band structure . The MoS2 has a linearly flat absorption in the near-infrared wavelength band which confirms a broadband optical material, as it has been experimentally proved in Ref .
For the PMMA-free CVD MoS2 film on SPF, the nonlinear transmission characteristics were investigated using an Er-doped mode-locked fiber laser as schematically shown in Fig. 3(a). The laser was operated at a central wavelength of 1563 nm with a pulse duration of 800 fs. The laser output power was changed using a variable optical attenuator (VOA) and its polarization state was varied using a polarization controller (PC). The MoS2 layer on SPF increased the polarization-dependent loss (PDL) to 1.72 dB.
By changing both the VOA and the PC, we measured the optical power-dependent transmission and the results are summarized in Figs. 3(b)–3(d). These plots correspond to the polarization states to give the minimum, intermediate and maximum output power, respectively. The measurement results were well-fitted to the two-level saturable absorber model [1,48]. In Fig. 3(b), we estimated the modulation depth to be 0.3% and the non-saturable loss as 79%. The transmission does not reach the full saturation level due to the limited power of the laser currently available. In Figs. 3(c) and 3(d) we estimate that the modulation depth is more than 2.5%. We confirmed the characteristics of SA for the MoS2 film on SPF by changing the measurement conditions, decreasing the incident light and observing that the inferred nonlinear absorption parameters remained almost constant.
A bulk material of MoS2 has an indirect band-gap of 1.29 eV  and the few-layer MoS2 has much smaller band-gap than bulk MoS2. The band-gap of the few-layer MoS2 can be reduced to 0.08 eV depends on the structure and ratio between Mo and S ions which has less electron-phonon interaction as it is more close to direct band-gap and it has been proved by Wang’s group . When few-layer MoS2 with semiconducting properties is excited by photons with higher energy than the band-gap, electrons will be transferred from the valance band to the conduction band. Under strong excitation, the final states will be fully occupied and exhibited saturable absorption properties as it has been confirmed via Z-scan measurements . We also experimentally demonstrated the saturable absorption characteristics of CVD multilayer MoS2 through nonlinear transmission measurements. In comparison with the chemically exfoliated MoS2 nano-sheets which consist of mono- and few-layer MoS2 , the advantage of CVD multilayer MoS2 is that the CVD thin film does not contain any monolayer nano-sheets which has a band-gap of 1.8 eV. Monolayer MoS2 can be excited by multi photon absorption results in decrease of the transmission and leading to a valley centered at the focal point in the Z-scan curve [20,26]. In contrary, the CVD MoS2 is a uniform multilayer thin film which can be excited with one photon absorption and the strong SA originates from the multilayer nano-sheets. Furthermore, the CVD thin film is highly uniform, unlike the deposited surface with chemically exfoliated nano-sheets which are randomly stacked.
3. Mode-locked fiber laser experiments
We constructed a ring fiber laser cavity by inserting the MoS2 side-polished fiber saturable absorber (MoS2 SPF-SA) to achieve passive mode-locking, and the experimental setup is presented in Fig. 4. The total length of the single mode fiber in the resonator was approximately 7.95 m. The normal dispersion regime was achieved by adding 1.39 m of dispersion compensating fiber (DCF) for a total net cavity dispersion of + 0.095 ps2. An erbium doped optical fiber (EDF) with a length of 1.4 m provided optical gain in the C-band. A ring cavity was pumped by a 980 nm laser diode (LD) through a polarization insensitive hybrid component which included a 980/1550 nm wavelength division multiplexing (WDM) coupler, a directional coupler with 10% output coupling ratio and a polarization independent optical isolator to eliminate back-reflection. A polarization controller (PC) was inserted to control the polarization state inside the laser cavity. The optical spectra through the 10% output port was monitored using an optical spectrum analyzer (Yokogawa AQ6370 B), autocorrelator (Femtochrome; FR-103HS), a digital oscilloscope (Tektronix TDS 784D), and a radio-frequency (RF) spectrum analyzer (Agilent technologies N9000A).
Self-starting mode-locked pulses were successfully generated and we summarized the output characteristics of the fiber laser in Fig. 5. The typical flat-top shape of the dissipative soliton pulse spectrum  was measured as in Fig. 5(a) at LD power of 469 mW. The spectrum had its center wavelength at 1568 nm and 3-dB bandwidth of 23.2 nm. As shown in Figs. 5(b) and 5(c), the repetition rate of the fiber laser was 26.02 MHz, which corresponded to the laser cavity length and was verified by the time interval of the output pulse train with a signal-to-noise ratio of 63 dB. In Fig. 5(d), the wide span measurement of the RF spectrum showed a stable operation of the mode-locked laser without Q-switching instability. The measured pulse duration was well-fitted with a Gaussian pulse profile and the full width half maximum (FWHM) of the autocorrelation trace was 4.98 ps, as shown in Fig. 5(e). In Fig. 5(f), the average output power of the dissipative soliton laser was measured as a function of the input pump LD power. An input pump power of about 50 mW started the continuous wave (CW) operation of the laser. As the pump was further increased to 322 mW, a stable and self-starting mode-locking was observed and maintained up to the pump power of 482 mW without changing the polarization controller state. Multi-pulsing was observed for a higher pump power. These experimental results confirmed that the SA nature of MoS2 thin film enabled highly stable dissipative soliton mode-locking operation of the fiber laser by evanescent wave interaction.
Furthermore, we changed the intra-cavity dispersion to the anomalous regime to confirm the potential of MoS2 thin film for soliton pulse shaping. We removed the DCF and added SMF of 2 m to change the total dispersion of the cavity into the anomalous regime. We also changed the EDF with the length of 1 m to adjust the dispersion further. The total length of the laser cavity was 6.18 m, with a net cavity dispersion of −0.041 ps2. When the pump LD power increased over 56 mW, a stable soliton pulses were self-started and the laser output characteristics at a pump LD power of 155 mW are summarized in Fig. 6.
The fiber laser was operating in the soliton regime and Kelly sidebands, the narrow peaks in the optical spectrum, were also observed, which are a typical feature of solution pulses originated from the resonant coupling between solitons and weak dispersive waves . The laser showed an output spectrum with a 3-dB spectral width of 12.38 nm at the center wavelength of λ = 1568 nm (Fig. 6(a)). The time trace of the oscilloscope is shown in Fig. 6(b) showing the pulse repetition rate of 33.48 MHz. In Fig. 6(c), a transform-limited pulse with full-width half-maximum of 637 fs was measured using an intensity autocorrelator, which was fitted by a Sech2-shaped pulse. Figure 6(d) represents the radio frequency (RF) spectra of the generated pulse train with different spans. The fundamental peak exhibited a cavity repetition rate of 33.48 MHz, and the background noise was well suppressed by 61 dB from the main peak, corresponding to a laser cavity length of 6.18 m. The wide span RF spectrum clarifies this stable operation of the soliton mode locking without exhibiting Q-switching instability (inset of Fig. 6(d)).
We experimentally confirmed that the passive mode-locking is self-started. In addition, the mode-locking state was stably maintained under slight perturbation of polarization state in the intra-cavity. Thus, we believe that the main mode-locking mechanism is originating from saturable absorption by MoS2 layer rather than nonlinear polarization rotation by the polarization dependent loss in our SA. Our experimental demonstration of successful and efficient mode-locking is consistent with recent Z-scan measurements of MoS2 layers showing an improved saturable absorption response with a higher saturable carrier density than that of graphene [26,27]. Recently dissipative soliton pulses at 1.05 μm were generated using a mode-locked ytterbium-doped fiber laser with a MoS2 fiber ferrule saturable absorber . We believe our study is the first report of stable mode-locking of erbium-doped fiber laser in the region around 1.55 μm, using multilayer CVD MoS2 thin film on a side polished fiber without any organic polymer binder. By removing the polymer matrix, we could increase the damage threshold thus there was no degradation on the saturable absorber properties even with the maximum pump power of 640 mW. Moreover, due to the polymer removal the insertion loss of the saturable absorber was decreased; subsequently the mode locker could start pulse generation in a lower pump power. Further investigation on high power laser based on MoS2-SA is ongoing. Prior fiber ferrule-type SAs are known to be vulnerable to optical and mechanical damage , and the inherently short nonlinear interaction length in the ferrule type SA could restrict the pulse shaping ability of SA, as well as the performance of the lasers. We used side polished fiber (SPF) to overcome these shortcomings of the ferrule-type SA by providing a higher damage threshold and a long interaction length. Previous mode-locked fiber lasers based on SAs such as graphene and CNT all used polymer binders such as PMMA or PVA, which have been thought to play a role in preserving the optical nonlinearity [32,33,48–57]. In contrast, we used MoS2 thin film itself as a SA without any organic binder, which has a high potential to further increase the peak power and energy of a mode-locked laser. With these new features of pure CVD MoS2 film on SPF, we could increase the efficiency of the nonlinear interaction and thereby facilitate laser mode locking. For instance, our Er-doped fiber laser output dissipative solitons which had a spectral bandwidth more than 10 times wider, and pulse duration was almost 160 times shorter, than a prior report .
In summary, we have successfully transferred CVD-grown multilayer MoS2 film onto a side polished fiber (SPF) without using any organic binder. Uniform multilayer MoS2 film formation was confirmed by both Raman spectroscopy and scanning electron microscopy. Saturable absorption characteristics through the MoS2 on SPF were experimentally observed for a pulsed Er-doped fiber laser at the wavelength of λ = 1563 nm. In the nonlinear optical transmission measurements, we found a modulation depth of more than 2.5%. We experimentally studied the application of MoS2 thin film as a saturable absorber (SA) in an erbium ring fiber laser cavity operating in both the normal and anomalous regimes of mode locking. In the normal dispersion regime, stable dissipative soliton pulse trains were obtained with a pulse width of 4.98 ps, a bandwidth of 23.2 nm at the center laser wavelength of 1568 nm, with a repetition rate of 26.02 MHz. In the anomalous regime, soliton-like shaped pulses were generated with a pulse width of 637 fs, a bandwidth of 12.38 nm at the center laser wavelength of 1568 nm, with a 33.48 MHz repetition rate. The mode locked pulses in both regimes maintained stable operation without Q-switching instability. We found that the CVD-grown multilayer MoS2 thin film without any organic binder showed highly reliable SA functionality to enable efficient and stable pulse formation generating both dissipative soliton and soliton-like pulses.
This work was supported in part by the Human Frontier Science Program (HFSP 2013-11-2042), in part by the National Research Foundation of Korea (NRF), by a grant funded by the Korea government (MSIP) (2012-11-0936, 2012M3A7B4049800). H. Jeong and D.-I. Yeom have been supported by NRF of Korea (NRF-2013R1A1A2A10005230).
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