Nonlinear optical properties of MoS 2-WS 2 heterostructure in fiber lasers

As a saturable absorption material, the heterostructure with the van der Waals structure has been paid much attention in material science. In general, the heterogeneous combination is able to neutralize, or even exceed, the individual material’s advantages in some aspects. In this paper, which describes the magnetron sputtering deposition method, the tapered fiber is coated by the MoS2-WS2 heterostructure, and the MoS2-WS2 heterostructure saturable absorber (SA) is fabricated. The modulation depth of the prepared MoS2-WS2 heterostructure SA is measured to be 19.12%. Besides, the theoretical calculations for the band gap and carrier mobility of the MoS2-WS2 heterostructure are provided. By employing the prepared SA, a stable and passively erbium-doped fiber laser is implemented. The generated pulse duration of 154 fs is certified to be the shortest among all fiber lasers based on transition mental dichalcogenides. Results in this paper provide the new direction for the fabrication of ultrafast photon modulation devices. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Transition mental dichalcogenides (TMDs), which are considered to be the supplements or even the substitutes of graphene in some cases, have also been investigated in the field effect transistors, photocatalysts, photodetectors and optical modulators [16][17][18][19][20][21][22][23][24][25][26][27][28][29].WS 2 and MoS 2 , as two representative materials of TMDs, have been fully explored from the physical to optoelectronic properties.The nonlinear optical response of MoS 2 which is stronger than graphene at 800 nm has been investigated by using Z-scan technique [30].Then, the broadband saturable absorption of MoS 2 has been illustrated by theoretical arithmetic and further successfully demonstrated at 1.06 μm, 1.42 μm and 2.1 μm [31].As the analogue of MoS 2 , WS 2 not only has good performance in ultra-wide response, but also exhibits a large second-order nonlinear susceptibility [32].Therefore, WS 2 has great potential in realizing ultrashort pulses.Both MoS 2 and WS 2 perform well as optical modulation materials in fiber lasers.
As far as we know, molecular layers of TMDs are connected by van der Waals (vdW) forces.The weak vdW forces between two adjacent TMD layers not only enable them easier to be stripped into monolayer nanosheets, but also provide a chance to manufacture optoelectronic devices by stacking different TMDs without considering the problem of mismatch [33,34].The structure of heterostructure opens up a different avenue for the fabrication of better photoelectric devices.From previous reports, the MoS 2 -WS 2 heterostructure exhibits remarkable performances.The type-II semiconductor heterostructures came from stacked MoS 2 -WS 2 heterostructure facilitate the transfer of holes [35,36].Moreover, the absorption of MoS 2 -WS 2 heterostructure is larger than the simple superposition of the respective absorptions of MoS 2 and WS 2 [37].Therefore, the heterostructure consisting of two different materials with various geometric composition and electronic energy exhibits unique electrical and optical properties [38][39][40].However, the MoS 2 -WS 2 heterostructure SA is rarely used in the fiber lasers for ultrafast photonics.As far as we know, only Chen et al. has made an attempt at the application of the WS 2 -MoS 2 -WS 2 heterostructure in the fiber laser so far [41].
Among the various preparation methods, the magnetron sputtering deposition (MSD) method with the simple operation is considered to be the most suitable method for the preparation of MoS 2 -WS 2 heterostructure with high quality in this paper.MoS 2 and WS 2 are coated on the tapered fibers to manufacture the MoS 2 -WS 2 heterostructure SA.Herein, the fiber laser is implemented to investigate the related nonlinear optical performance of the proposed MoS 2 -WS 2 heterostructure SA.After the application of SA in fiber laser, a stable mode-locking system is obtained.The obtained pulse duration of 154 fs is proved to be the shortest in the congeneric fiber lasers.The theoretical and experimental results illustrate that the MoS 2 -WS 2 heterostructure provides reference value for the innovation and development of fiber lasers.

Theoretical calculation of MoS 2 -WS 2 heterostructure SA
By the density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP), the theoretical calculations were performed.The interactions between valence electrons and core electrons were described with the projector augmented wave (PAW) pseudopotential.A cutoff energy was chosen to be 420 eV for the plane-wave expansion of wave functions.For the integration over the first Brillouin zone, the Monkhorst-Pack scheme of the k-point sampling was adopted.In order to avoid the interactions between two slabs in the nearest-neighbor unit cells, the periodic boundary conditions and a vacuum space of 30 Å along the z direction were applied.A 7 × 7 × 1 grid for the k-point sampling was used for the geometry optimization, while 37 × 37 × 1 was used for the static total energy calculations.When the change of the total energy was less than 10 −4 eV, the forces became smaller than 0.01 eV/ The mobility μ of the MoS 2 -WS 2 heterstructure was calculated using the deformation potential (DP) theory on the basis of the effective mass approximation: where T is the temperature, and C is the elastic modulus.For the 2D system, the in-plane value is defined as 2 , where E, δ, and 0 S are the total energy, applied uniaxial strain and area of the investigated system, respectively.The DP constant E along a certain direction is obtained by , where edge E is the energy of the band edges (valence-band maximum for holes, and conduction-band minimum for electrons).
The heterostructure was modeled by a MoS 2 -WS 2 bilayer structure in Fig. 1(a).To get the most stable structure, the heterostructure of the strain WS 2 monolayer was used, and the corresponding lattice mismatch is 0.15%.The band gap of the MoS 2 -WS 2 heterostructure is 1.26 eV, which is smaller than the band gap of the monolayer MoS 2 (1.77 eV) shown in Fig. 1.The effective mass of electrons and the effective mass of holes in K are 0.46 m 0 and 0. 72 m 0 , respectively.The effective mass of holes is smaller than that of the monolayer MoS 2 , and the effective mass of electrons is almost equal to the monolayer MoS 2 .The effective mass and carrier mobility of the MoS 2 -WS 2 heterostructure, monolayer MoS 2 and WS 2 are shown in Table 1.The carrier mobility of the MoS 2 -WS 2 heterostructure is larger than that of the monolayer MoS 2 .These results may be related to the type-II heterostructure from the stacked MoS 2 -WS 2 heterostructure.Specifically, upon optical excitation, the electron tends to stay in the MoS 2 layer which reside with the minimum conduction band, and the holes prefer to stay in the WS 2 layer which reside with the maximum valence band.Therefore, the type-II heterostructure effectively assist electrons and holes separate quickly.The relationship between relaxation time ( τ < > ) and electron , where m is the effective mass.As far as we know, the large relaxation time can bring about greater modulation depth.Therefore, in the same case, the greater the product of effective mass ( m ) and electron mobility ( 2D μ ), the higher the modulation depth.

Fabrication and characterization of MoS 2 -WS 2 heterostructure SA
The MoS 2 -WS 2 heterostructure SA was prepared by the MSD method.Before deposition, the surface impurities of the WS 2 and MoS 2 raw materials were removed, which guaranteed the purity of materials produced.During deposition, the MoS 2 target and taper fiber were placed in the vacuum chamber at the same time, and the pressure of the cavity was set to be 1.7 × 10 −3 pa.The Ar gas was continuously excited under the 0.4 A alternating current (AC) with voltage of 6 V for 70 s.Following, WS 2 nanosheets were deposited on the surface of MoS 2 in the same way under the 0.5 A with voltage of 6 V for 180 s.Finally, a dense layer of gold film, which prevents the material from being damaged oxidized, was deposited on the surface of the MoS 2 -WS 2 heterostructure.
To characterize the surface and lateral properties of the MoS 2 -WS 2 heterostructure SA, the scanning electron microscope (SEM) was employed.Figure 2(a) exhibits the surface morphology of the SA.We can observe that the particles are arranged in a compact and uniform manner.As shown in Fig. 2(b), the lateral surface illustrates the thickness of the heterostructure.To ensure accuracy, the thickness of the sample is measured in three different places.The average value of three different places which is calculated as 63 nm illustrates the thickness of MoS 2 -WS 2 heterostructure.According to measuring results, the thickness of MoS 2 is 21 nm and the thickness of WS 2 is 42 nm.The thickness of the SiO 2 substrate is measured to be 59 nm.Those results prove that the obtained MoS 2 -WS 2 has layered structure.Raman analysis is a common method to distinguish the types of materials through different vibration modes.In order to confirm the successful manufacture of the MoS 2 -WS 2 heterostructure SA, the corresponding Raman spectra are measured in Fig. 3(a).The measurements show that the Raman spectra of the pure MoS 2 sample have peaks at 378 cm −1 and 404 cm −1 , which correspond to the E 2g and A 1g , respectively [42].The peaks at 355 cm −1 and 419 cm −1 in Raman spectra of WS 2 is corresponding to E 2 1g and A 1g modes [34].The four peaks shown in the Raman spectra of the MoS 2 -WS 2 heterostructure are in good agreement with the four vibration modes of MoS 2 and WS 2 .The nonlinear absorption properties of the MoS 2 -WS 2 heterostructure SA are investigated by the balanced twin-detector method.In the measurement, the mode-locked fiber laser with the repetition rate of 135 MHz and pulse duration of 100 fs is used as the exciting source.The measurement results in Fig. 3(b) demonstrate that the prepared MoS 2 -WS 2 heterostructure SA has the modulation depth of 19.12%.Moreover, the saturation absorption intensity of 1.361 MW/cm 2 is relatively small, which is beneficial to the low self-starting threshold for the fiber laser.The optical damage threshold of the current absorber device is calculated to be 4.17 mJ/cm 2 , which is higher than the commercial available semiconductor saturable absorber mirrors (SESAMs) (500 μJ/cm 2 ).

Nonlinear optical performance
To investigate the nonlinear absorption characteristics of the MoS 2 -WS 2 heterostructure SA, it has been integrated into the erbium-doped fiber (EDF) laser in Fig. 4. The wavelength division multiplexer (WDM) (980/1550 nm) coupled the pump light into the ring cavity.With the excitation of the pump which operated at 976 nm, a piece of EDF of 42 cm amplified the pulse through the energy level transition.The polarization controller (PC) was used to fine tune the polarization state and birefringence in the cavity.In order to avoid reflecting light damage to the device, isolator (ISO) enforced light to transmit in a fixed direction.The starting threshold of the laser was 180 mW.When the pump power was greater than this value, the laser maintained the stable mode-locking operation.Through the 20:80 optical coupler (OC), the experimental results were displayed and recorded.The main test instruments we adopted were a 250 MHz oscilloscope and spectrum analyzer.Figure 5 summarizes the laser performance at the maximum pump power of 630 mW.As indicated in Fig. 5(a), the central optical spectrum is located at 1560 nm with 3 dB spectral width of 24.4 nm.From the symmetrical Kelly sidebands of the spectrum, it can be inferred that this is the soliton mode locking system.The regular array of the pulse train in Fig. 5(b) indicates that the mode-locked system is in a stable operative condition.The time interval of two neighboring pulses is 13.4 ns, which is corresponding to the fundamental repetition rate 74.6 MHz of mode-locked pulses.The resolution bandwidth (RBW) and span adopted in the measurement of the fundamental frequency are 10 Hz and 10 kHz, respectively.Under high resolution and small span, there is no obvious frequency interference signals appear, and the signal-to-noise ratio (SNR) is 91.2 dB in Fig. 5(c), which further illustrate that the modelocked operation is relatively stable.Moreover, the pulses in the illustration of Fig. 5(c) are in the uniform arrangement.The symmetrical autocorrelation trace is fitted by the Sech 2 function in Fig. 5(d), which indicates that pulse duration of the mode-locked fiber laser is 154 fs.The corresponding time-bandwidth product (TBP) is 0.4403, indicating that the output mode-locked pulses are slightly chirped.The maximum output power of the laser is 19.8 mW.Results demonstrate that the MoS 2 -WS 2 heterostructure SA showns the saturable absorption property around 1.5 μm.According to previous reports, the direct bandgap of the monolayer MoS 2 is 1.8 eV, and the indirect bandgap of the bulk MoS 2 is 1.29 eV.Obviously, the photon energy is below the optical (excitonic) bandgap of MoS 2 at 1560 nm in our work.Although the governing mechanism is still unclear, there have been several convincing theories put forward by researchers, such as multiphoton absorption, edge state saturable absorption and defect state saturable absorption [43].In the theory of defect state, the imperfection of the 2D material is inevitable in the production process, which has an impact both on its electronic and optical properties [44].Wang et al. has demonstrated that the MoS 2 bandgap can be reduced from 1.08 to 0.08 eV by introducing the defects in a suitable range.By the introduction of S defects, the MoS 2 has been successfully applied in fiber lasers at the operating wavelength of 1.06, 1.42, and 2.1 μm [45].Therefore, it is justified to believe that there are unavoidable defects in the material, which result in the decrease of the band gap and broadband absorption beyond expectation.To prove the long-term stability of new SA device, the continuous monitoring for output power of the fiber laser is implemented in Fig. 6.The total monitoring time was up to 16 hours, and the output power of the fiber laser was recorded every second.As shown in Fig. 6, the standard deviation of the output power is only 0.123.This result indicates that the system has good stability.Limited by the experimental conditions, we only monitored the output power within 16 hours, but it can be observed that the trend of data remained stable in the later period, which indicated that the system could maintain stability for a longer time.Therefore, the new SA device can withstand long-term illumination.To illustrate the relative advantages and improvements of the proposed mode-locked EDF laser based on the MoS 2 -WS 2 heterostructure SA, the performance of mode-locked fiber lasers employed with pure TMDs materials are compared in Table 2.Although there have been some researches on the electronic properties and nonlinear characteristics of the monolayer MoS 2 (or WS 2 ), almost all MoS 2 (or WS 2 ) that applied in mode-locked fiber lasers are multilayered according to the previous reports.Because the preparation of the highly efficient SA based on monolayer material is complicated and difficult.The pulse duration obtained is 154 fs, which is almost the shortest among the similar fiber lasers.Considering the structural particularity of the MoS 2 -WS 2 heterostructure, we think there are two reasons for the remarkable performance of the fiber laser: Firstly, the ultrafast transfer time of carriers from the type-II semiconductor heterostructures may beneficial to the generation of ultrashort pulses.For the monolayer MoS 2 , the intra-layer carrier recombination time is 2 ps [41].However, it has reported that the electronic transfer in the MoS 2 -WS 2 heterostructure occur within 50 fs upon photo-excitation [35].Compared with MoS 2 itself, the carrier recombination of the MoS 2 -WS 2 heterostructure is much faster.Moreover, we can see that no matter what the thickness of the material is, the pulse duration of the corresponding laser is not as short as that of the fiber laser based on the MoS 2 -WS 2 heterostructure in Table 2.The above results give us reason to believe that the vertically stacking of WS 2 and MoS 2 may cause ultrafast carrier recombination, which further benefit to the generation of ultrashort pulses.Secondly, the large modulation depth of the MoS 2 -WS 2 heterostructure SA is also propitious to the generation of ultrashort pulses.On the one hand, compared with the pure TMDs materials, the combination of the nonlinear absorption properties of two materials make it more advantageous in the modulation of light.It has been reported that the absorption of the MoS 2 -WS 2 heterostructure is larger than the simple superposition of the respective absorptions of MoS 2 and WS 2 in previous works [40].On the other hand, the tapered fiber structure of the MoS 2 -WS 2 heterostructure SA enhance its nonlinearity.The tapered fiber allows a very long interaction length.In commonly used sandwich structures, the interaction length of the material and light is limited by the thickness of the material, often in the nanoscale.However, in the tapered fiber, the interaction length can be extended to centimeter magnitude order by means of the evanescent field effect.This sufficient reaction of the material and light enables the material to fully exhibit its nonlinearity.Besides, SAs based on tapered fibers with different specifications show differences in the modulation depth, and SAs owning small waist diameter tend to show larger modulation depth.Therefore, we believe that the tapered fiber structure of the MoS 2 -WS 2 heterostructure SA can enhance its nonlinearity.

Conclusions
In this paper, we have prepared the MoS 2 -WS 2 heterostructure SA using the MSD method.
The modulation depth and saturation absorption intensity of the SA is 19.12% and 1.361 MW/cm 2 , respectively.The corresponding band gap and electron mobility have been theoretically calculated.In order to investigate the nonlinear absorption characteristics of the MoS 2 -WS 2 heterostructure SA, it has been integrated into the EDF laser.In addition, the stable mode-locked fiber laser operating at 1560 nm with SNR of 91.2 dB and output power of 19.8 mW has been implemented.The obtained pulse duration of 154 fs has been proved to be the shortest in the congeneric fiber lasers.Results in this paper not only reveal the impressive optical nonlinearity of the MoS 2 -WS 2 heterostructure SA, but also provide reference value for the application and development of TMDs heterostructures.
o A , the structure began to relax.The calculated lattice constant of monolayer MoS 2 , monolayer WS 2 and MoS 2 -WS 2 heterostructures are 3

Fig. 1 .
Fig. 1.Atomic and electronic structure of the MoS 2 -WS 2 heterostructure.(a) Top and side views of the MoS 2 -WS 2 heterostructure, the dashed rectangle denotes the primitive cell.(b), (c) and (d) are band structures of MoS 2 -WS 2 heterostructure, monolayer MoS 2 and monolayer WS 2 , respectively.Here, the fermi level is set to be zero, and the orange line denotes valenceband maximum for the holes and conduction-band minimum for the electrons.Besides, red points project the contribution from the MoS 2 in (b).(e) is the band alignment of the MoS 2 -WS 2 heterostructure.The energy levels of MoS 2 and monolayer WS 2 slabs are shown in both sides.Here, the monolayer MoS 2 , monolayer WS 2 , and MoS 2 -WS 2 heterostructure are considered.

Fig. 2 .
Fig. 2. SEM of the MoS 2 -WS 2 heterostructure SA.(a) The surface morphology; (b) The thickness of heterostructure indicated by the lateral.

Fig. 5 .
Fig. 5.The performance of the passively mode-locked EDF laser employed with the MoS 2 -WS 2 heterostructure SA.(a) The optical spectrum located at 1560 nm with 3 dB spectral width of 24.4 nm; (b) The mode-locked pulse train; (c) The radio-frequency spectrum; (d) The symmetrical autocorrelation trace of mode-locked pulses.

Fig. 6 .
Fig. 6.The continuous monitoring of output power of the fiber laser.