Large-area highly crystalline WSe2 atomic layers for ultrafast pulsed lasers

: Large-area and highly crystalline transition metal dichalcogenides (TMDs) films possess superior saturable absorption compared to the TMDs nanosheet counterparts, which make them more suitable as excellent saturable absorbers (SA) for ultrafast laser technology. Thus far, the nonlinear optical properties of large-scale WSe 2 and its applications in ultrafast photonics have not yet been fully investigated. In this work, the saturable absorption of chemical vapor deposition (CVD) grown WSe 2 films with large-scale and high quality are studied and the use of WSe 2 films as a broadband SA for passively mode-locked fiber lasers at both 1.5 and 2 μ m ranges is demonstrated. To enhance the light-material interaction, large-area WSe 2 film is tightly transferred onto


Introduction
Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have attracted worldwide attentions due to their unique electronic and photonic properties [1][2][3][4]. Recent reports have demonstrated that the energy band structures of the semiconducting TMDs show a unique transition from indirect to direct band gap as the thickness is reduced to monolayer [5][6][7]. These monolayer TMDs with direct band gap can offer exciting opportunities for potential applications in both electronic and optoelectronic devices [4, [8][9][10][11][12][13][14][15][16][17]. Among all TMD materials, WSe 2 has recently received huge interest due to their distinctive electro-optical properties and remarkable electronic characteristics [18][19][20][21][22][23][24]. For example, it has been recently reported that mechanically exfoliated monolayer WSe 2 was adopted to fabricate high performance n-type and p-type field-effect transistors [18][19][20]. Monolayer WSe 2 p-n junctions and heterostructure p-n junctions composed of p-type WSe 2 and n-type other TMDs (WSe 2 , MoS 2 , or WS 2 ) were demonstrated for optoelectronic devices [21,22]. Meanwhile, for the ultrafast photonic applications, it has also been recently reported that liquid-phase exfoliation method prepared WSe 2 nanosheets were employed as saturable absorbers (SAs) for passively Q-switched and mode-locked pulse generation due to their excellent nonlinear optical absorption and ultrafast carrier dynamics [23,24]. However, the lateral size and thickness of WSe 2 nanosheets fabricated by both mechanical exfoliation and liquid-phase exfoliation are limited in a few to a few tens of micrometers. As the outstanding optical properties are closely related to the size and morphology of the materials [25], the uncontrollable and nonuniform WSe 2 nanosheets usually result in a low surface-to-volume ratio and then renovate their properties. Compared to the TMDs nanosheets, one can expect that large-area TMDs films can possess better optical properties. For example, recent demonstrations have indicated that the chemical vapor deposition (CVD) grown large-area MoS 2 films have shown remakable modulation depth of 35.4% [26], which is over one order of magnitude larger than that of the MoS 2 nanosheets [23]. Unlike MoS 2 , large-area atomic-layered WSe 2 for optoelectronic devices yet remains a significant challenge as the chemical reactivity of selenium is much lower than that of sulfur. The CVD method has been recently presented for synthesis of large-scale WSe 2 films by introducing hydrogen as a strong reducer during the synthesis reaction [19,27,28]. The excellent electronic characteristics of large-area WSe 2 films such as mobility of 350 cm 2 v −1 s −1 and on/off ratio of 10 8 have been studied in electronic devices [19], while their nonlinear optical properties and the potential applications in ultrafast photonics have remained elusive.
Large-area atomically thin WSe 2 films integrated with microfiber structures can offer tight optical confinement for enhancing the light-material interaction, and further provide an ideal platform for studying their nonlinear optical properties and their applications with superior mode-locking performance. In this contribution, large-area atomically thin WSe 2 film was grown on a sapphire substrate by a CVD method. The material characterization confirmed the high quality of the as-grown WSe 2 film. The large-area WSe 2 shows remarkable nonlinear optical performance, such as a large modulation depth (~54.5%) and low saturable intensity (2.97 MW·cm −2 ). Based on the microfiber WSe 2 SA with strong evanescent wave coupling effect, a stable soliton mode-locked erbium-doped fiber (EDF) laser was obtained with pulse duration of 477 fs, which is the shortest mode-locked laser based on WSe 2 SAs. Moreover, we also demonstrated the mode-locked thulium-doped fiber (TDF) laser based on WSe 2 SA at 2 μm regime. Large-area and highly crystalline WSe 2 film was grown on a sapphire substrate by using a CVD method, and the experimental setup was illustrated in Fig. 1(a). The growth processes were performed in a quartz tube furnace using WO 3 and Selenium (Se) powders as starting materials. The WO 3 powders (~0.1g) were placed in a ceramic boat located in the heating zone center. The Se powders (~0.5 g) in a separate ceramic boat were placed at a distance ~50 cm at the upper stream of WO 3 powders. The sapphire substrates were put at the downstream side at ~20 cm away from the heating zone center. During the reaction, the Se and WO 3 vapors were brought on the targeting sapphire substrates by Ar/H 2 flow gas (Ar = 100 sccm, H 2 = 10 sccm, chamber pressure = 1 Torr). The temperature distribution along quartz tube is approximatively Gaussian. The growth temperature is an important factor, which directly determines the morphology of WSe 2 . The optical microscope (OM) image in Fig. 1(b) shows the monolayer WSe 2 triangles grown at substrate temperature of 790 °C, and the lateral size of triangle is typically larger than 10 μm with a maximum size up to 50 μm. With further growth, additional layers were grown on the top of WSe 2 triangles, instead of a continuous monolayer film. Such additional layer growth result at high temperature during CVD synthesis is consistent with previous reports [19]. The lateral size of triangles might be limited by the lattice mismatch between WSe 2 and sapphire substrates. In our experiment, we expected the WSe 2 triangles merge together to form a large-area continuous film for ultrafast photonic applications with superior performance. As we decrease the substrate temperature to 750 °C, the small WSe 2 domains merge to form a continuous film on sapphire substrate, since a lower growth temperature typically results in a higher nucleation density [27]. The continuous film with large-scale can be clearly seen in Fig. 1(c) due to the high contrast of color display between WSe 2 layers and sapphire substrate. The morphology and height profile of WSe 2 film are investigated by using atomic force microscope (AFM, Bruker Dimension Icon). The AFM image in Fig. 1(d) shows the morphology of few-layer WSe 2 . We observe the CVD grown monolayer, bilayer and few-layer small domains on the top of continuous monolayer. The height profile in Fig. 1(f) shows the thickness of the as-grown film along the line 1 in Fig. 1(d), proving the layer-by-layer structure of the film. The thickness of monolayer is around 0.7 nm, which is in a good agreement with the exfoliated monolayer WSe 2 [19]. The magnified AFM image in Fig. 1(e) for the area squared by dashed lines in Fig. 1(d) reveals the monolayer (1L), bilayer (2L) and trilayer (3L) of the as-grown film. The AFM cross-sectional height profiles at lines 2 and 3 in Fig. 1(e) further confirms the layers numbers according to the step height of ~0.7 nm, as shown in Fig. 1(g). The additional WSe 2 triangles with different layers bonded on the top of the monolayer WSe 2 continuous film, which can strengthen the mechanical strength of the WSe 2 film for the benefit of further large-scale transfer process.

Growth and characterization of WSe 2 atomic layers
The crystal structure of WSe 2 film was probed by a transmission electron microscopy (TEM, JEM-2010) operated at an electron accelerating voltage of 200 kV. The WSe 2 samples were transferred onto a carbon grid using polymethyl methacrylate (PMMA) transfer technique [29]. Low magnification TEM image in Fig. 2(a) shows the film-like structure of the transferred WSe 2 sample. From Fig. 2(b), the high-resolution TEM (HRTEM) image at the cross area in Fig. 2(a) shows the periodic atom arrangement of the WSe 2 atomic layer, demonstrating that the CVD-grown WSe 2 film is highly crystalline. The inset shows the selected area electron diffraction (SAED) pattern with clear six-fold symmetry diffraction spots, revealing the hexagonal lattice of WSe 2 film. Figure 2(c) shows the magnified HRTEM image for the area squared by dashed lines in Fig. 2(b), and the lattice constant was measured to be around 0.33nm, consistent with that of previously reported WSe 2 [27]. The characteristic peaks of energy dispersive spectroscopy (EDS) spectrum in Fig. 2(d) are in a good agreement with the previously reported results [19,30]. The presence of Cu is probably induced from the carbon grid. The EDS spectrum reveals that the atomic ratio between W and Se is about 1:2.1, which suggest tungsten defects in the WSe 2 film. Figure 2(e) shows the Raman spectra of hexagonal WSe 2 with different thickness, which were collected on a Raman system (HORIBA LabRAM HR800) using a laser excitation of 532nm. All WSe 2 layers reveal two characteristic peaks located at 252 cm −1 and 262 cm −1 , which are assignable to the in-plane E 2g 1 mode and out-of-plane A 1g mode (insert of Fig. 2(e)), respectively [28,31]. Note that the Raman peak B 2g 1 mode at 309 cm −1 is attributed to the interlayer interactions [31]. The B 2g 1 mode is absent in 1L WSe 2 , which further confirms the monolayer film. The B 2g 1 mode is active in 2L and few-layer WSe 2 film reflecting the presence of the interlayer interaction [18,19]. The strong Raman peaks suggest the high quality of WSe 2 film grown by CVD method. The optical properties of WSe 2 film were further investigated using photoluminescence (PL). Figure 2(f) shows that the monolayer WSe 2 film exhibits strong emission at approximately 762 nm (1.63 eV) arising from direct gap transitions at Κ point, in a good agreement with previous reports [19,27,32]. From bilayer PL spectrum, the emission intensity is dramatically reduced. We can observe two PL peaks attributed to direct and indirect band gap emission at Κ Κ → and Κ → Γ points, respectively [6,7]. Both emission spectra were fitted with Gaussian peaks located at 766 nm (1.62eV) and 812 nm (1.53eV). The layerdependent PL spectra confirm the transition from direct to indirect bandgap with the increasing of thickness. Note that the absence of indirect gap emission peak in the PL spectrum of 1L WSe 2 further suggests that the monolayer WSe 2 film has direct band gap. The PL spectra suggest the unique bandgap structure and excellent optical properties of the asgrown WSe 2 film.

Nonlinear optical properties
We transferred the WSe 2 films onto microfibers to fabricate WSe 2 -based fiber SAs. Different from the previous WSe 2 -polymer composite [23,24], the microfiber WSe 2 SA cannot only enhance the nonlinear optical absorption through the strong evanescent wave interaction within a long fused waist zone, but also increase the capability to thermal damage resistance and compatibility to fiber components. Herein, we employed a polymethyl methacrylate (PMMA) assisted method to transfer the WSe 2 film from sapphire substrate onto the waist of microfiber. Figure 3(a) schematically shows the preparation process of the SAs fabrication. A thin PMMA film was firstly spin coated on the upper surface of WSe 2 film. The WSe 2 -PMMA composite was then stripped from the sapphire substrate after drying up. The PMMA in composite was etched off by acetone to obtain pure WSe 2 film. After that, the pure WSe 2 film was moved to deionized (DI) water. Due to the strong surface tension of water, the WSe 2 would float on the top surface of the DI water smoothly. Note that the DI water is used as not only the residues cleaning fluid, but also the supporting medium of WSe 2 film. A pieces of WSe 2 film with the size of ~2 × 4mm can be observed on water in Fig. 3(a). The free-standing WSe 2 film was finally transferred onto the waist of microfiber by bailing process with the micromechanical control under a microscope. The microfiber is fabricated by fiber-tapering equipment with single mode fiber (SMF-28e). The waist diameter of the microfiber is around 11 μm and the length of WSe 2 covered section is about 2 mm. After the residual DI water on WSe 2 volatilized at room temperature, the WSe 2 film tightly attached on the side wall of the microfiber by Van der Waals force, as shown in the scanning electron microscopy (SEM, HITACHI SU8000) image in Fig. 3(b). The magnified SEM image in Fig. 3(c) reveals largearea WSe 2 film integrated with microfiber closely and smoothly. The few wrinkles on WSe 2 film visually confirm that WSe 2 film was successfully transferred onto the waist of microfiber. The OM image in Fig. 3(d) shows the transferred WSe 2 film near edge area. The WSe 2 film can be clearly seen according to the metallic reflected light. Once a 630 nm laser beam was launched into the microfiber, scattered light from WSe 2 film can be observed in the OM image of Fig. 3(e), which suggests the strong light-WSe 2 interaction through evanescent wave coupling effect. The linear absorption of few-layer WSe 2 film from 1 to 2 um was measured by using a UV/VIS/NIR spectrophotometer (Cary 5000). The experimental results are shown in Fig.  4(a), the absorbance of few-layer WSe 2 film at 1550 and 1990 nm are measured to be 8.22% and 4.87%, respectively. The nonlinear optical absorption of the microfiber WSe 2 SA was studied by a simple two-terminal power-dependent measurement at 1550 nm. The representative result from the microfiber WSe 2 SA is shown in Fig. 4(b). It is found that the transmission is closely related to the pump intensity due to saturable absorption. From Fig.  4(b), we note low saturation intensity of 2.97 MW·cm −2 is obtained. The modulation depth is measured to be ~54.5% at 1550 nm, which is one order of magnitude larger than that of previous WSe 2 nanoflakes [23,24]. Moreover, the modulation depth at 1864 nm is measured to be ~1.83% by using the similar method, as shown in Fig. 4(c), which is the same order magnitude as that of graphene, black phosphorus, WS 2 and MoSe 2 at that range [33][34][35][36]. Note that the modulation depth at 1.9 μm is much smaller than that at 1.5 μm, which is mainly limited by the high transmission loss (~90%) of microfiber at this region. We believe that the relatively large modulation depth of WSe 2 SA is originated from the enhanced light-materials interaction in large-scale WSe 2 film during optical absorption process. Compared to the discrete light-materials interaction in the SAs based on nanosheets, continuous light-materials interaction in a long section (~2mm) of WSe 2 film covered region provides much more photogenerated carriers for band filling, which has contributed significantly to increasing of the modulation depth. The large-area WSe 2 film with promising saturable absorption was expected to achieve stable mode-locking operation with ultrashort pulse duration in all fiber laser system. Surprisingly, it is worth to note that the photon energies (0.8 eV and 0.67 eV) of WSe 2 broadband saturable absorption are lower than the material direct bandgap (1.63 eV). The mechanism of saturable absorption at 0.8 eV and 0.67 eV originates from the subbandgap absorption due to the edges or defects in the small triangular shapes of WSe 2 , which also has been experimentally and theoretically demonstrated in other TMDs nanoflakes [37,38]. For example, recent reports have demonstrated the broadband saturable absorption of MoS 2 due to the introduction of both Mo and S defects [11,37]. In our work, the W defects of WSe 2 films identified from the EDS spectrum and the edges of small WSe 2 triangles observed from the AFM image may make significant contribution to the broadband saturable absorption, which is analogous to that of previously reported MoS 2 [11,37,38].

Ultrafast fiber laser application
As the microfiber WSe 2 SA was incorporated into the fiber laser cavity, passively modelocked fiber lasers at 1.5 and 2 μm spectral regions were operated using EDF (Liekki 110-4/125) and TDF (Nufern SM-TSF-9/125) as gain materials, respectively. Figure 4(d) shows the experimental setup of the WSe 2 mode-locked fiber laser. In mode-locked EDF laser cavity, a section of 0.7 m EDF with GVD of 12 ps 2 ·km −1 as gain medium was pumped by a 976 nm LD through a 980/1550 nm WDM. A PI-ISO at 1550 nm was adopted to ensure the unidirectional laser operation. The cavity polarization state and intra-cavity birefringence were adjusted by a PC. A 20/80 OC was employed as 20% output. The output optical spectrum, pulse duration, and RF spectrum were simultaneously monitored by optical spectrum analyzer (Yokogawa AQ6370C), autocorrelator (APE Pulsecheck) and RF spectrum analyzer (ROHDE & SCHWARZ FSV13) integrated with an InGaAs PIN photodetector (EOT ET-3500FEXT). In this laser ring cavity, no polarization-dependent components were used to avoid self-starting mode-locking operation arising from the nonlinear polarization evolution (NPE). Furthermore, as we change the polarization of input light passing through microfiber-based WSe 2 SA, the polarization dependent loss (PDL) maintains in a small range of 60.72%~62.84% at 1550 nm with 3.21 mW input power, which confirms the PDL could be ignored in the microfiber based WSe 2 SA to avoid undesired NPE effect. Moreover, it is well known that the stable and ultrashort pulse trains can be generated in a passively mode-locked laser after achieving the balance between the self-phase modulation (SPM) and negative group velocity dispersion (GVD) in laser cavity [39]. The WSe 2 SA has normal dispersion, but the accurate value could not be calculated yet because of the hybrid structure of microfiber and WSe 2 film. In order to achieve stable soliton mode-locking operation, we tried to add extra SMF-28e fiber with GVD of −21 ps 2 ·km −1 at 1.5μm into the ring cavity, which was considered as a simple and effective approach to optimize the net cavity dispersion to be anomalous. The self-starting mode-locking operation was realized in the EDF laser cavity with a total cavity length of ~14 m, when the pump power reached the threshold of 160 mW. Figure 5(a) shows the optical spectrum of the mode-locked pulses with central wavelength of 1556.42 nm and a bandwidth of 6.06 nm. The Kelly sidebands are symmetrically distributed at two sides of spectrum, attributed to the periodic soliton perturbation in the laser cavity, verified that the mode-locked laser operates at the soliton state. The autocorrelation (AC) trace of the modelocked pulses is shown in Fig. 5(b), well fitted by a sech 2 profile with 734.6 fs full width at half maximum (FWHM). After multiplying the AC trace width with the de-correlation factor of 0.648 for the sech 2 pulses, the pulse duration is calculated to be 477 fs, which is the shortest for WSe 2 mode-locked fiber lasers [24]. From the measured 6.06 nm spectral bandwidth, the transform limit is calculated around 420 fs, which indicates the output pulses were slightly chirped. Figure 5(c) shows the radio-frequency (RF) spectrum of the modelocked pulses at a fundamental repetition rate of 14.02 MHz. The signal-to-noise ratio (SNR) is larger than 80 dB (10 8 contrast), which is much better than that of the previously reported mode-locked fiber laser based on WSe 2 nanosheets [24]. The wideband RF spectrum recorded within a 1 GHz span is presented in Fig. 5(d), which indicates the good stability of the modelocked pulses. Self-starting mode-locking was also achieved with different cavity lengths. As we increase the cavity length up to 80 m, the pulse duration was in this case 1.23 ps. Worth noting that shorter pulse width can be realized by further optimizing the cavity length with approximate zero dispersion in cavity [39]. Recent demonstrations indicated that several kind of 2D materials, like graphene [33], black phosphorus [34], WS 2 [35], and MoSe 2 [36], show broadband nonlinear optical absorption, which have been used as SA for mode-locking operation. However, thus far, mode-locked laser at 2 μm region based on WSe 2 has not been fully investigated. Here, we demonstrated the WSe 2 mode-locked fiber laser at this region. The 2 μm mode-locked laser cavity was shown in Fig. 4(d). A piece of 2.3 m TDF with −20 ps 2 ·km −1 dispersion at 2.0 μm was used as the gain medium, which was reversely pumped by a 1550 nm LD through a 1550/2000 nm WDM. Other fiber components, including PI-ISO, PC, OC (20/80), work at 1.9 μm. The connect fiber between each components was SMF-28e fiber with −71 ps 2 ·km −1 dispersion at 2.0 μm. The total cavity length of TDF laser cavity was around 18 m. The output optical spectrum was recorded by an optical spectrum analyzer (Yokogawa AQ6375B). Stable mode-locking operation was realized after the pump power exceeds the threshold value of 650 mW. We believe that the high mode-locking threshold mainly results from the bending loss of the SMF-28e fiber and large transmission loss (~90%) of the microfiber at 2 μm region. As the pump power increased up to 1310 mW, the laser remained in the mode-locked state with 32.5 mW output power. The high output power reveals that the microfiber WSe 2 SA possesses high thermal damage threshold (0.137 GW·cm −2 ). The mode-locking performance was characterized at the maximum pump power of 1310 mW. Figure 6(a) shows the output optical spectrum with 1886.22 nm central wavelength and 3.29 nm FWHM bandwidth. The Kelly sidebands on optical spectrum confirm the soliton mode-locked state in the anomalous dispersion regime. The dips on optical spectrum are probably originated from strong water absorption lines located in the 1.9 μm region [34]. The AC trace of the modelocked pulses with a sech 2 fitting is depicted in Fig. 6(b). The pulse duration is 1.18 ps after deconvolution. The RF spectrum of mode-locked pulses is shown in Fig. 6(c). The repetition rate is equal to 11.36 MHz matching well with the cavity roundtrip time (18 m cavity length), and the SNR is larger than 80 dB, comparable to that of our mode-locked 1.55 μm laser based on WSe 2 . The RF spectrum in the 500 MHz span confirms the stability and uniformity of the output pulse train. In addition, we measured the PDL within a small range of 9.52%~9.71% at 1990 nm with 5.84 mW input power, which confirms that the undesired NPE effect could be avoided at 1.9 μm region in this cavity. 1886.2/3.29 ~1.83 80 a CVD, chemical vapor deposition; LPE, liquid phase exfoliation; ME, mechanical exfoliation; τ, pulse duration (fs); λ/FWHM, central wavelength (nm)/ full width at half maximum (nm); α s , modulation depth (%); SNR, signal-to-noise ratio (dB).
As we remove the microfiber WSe 2 SA from laser cavity, no mode-locked pulses can be observed in both EDF and TDF laser systems, no matter how we increase the pump power or turn the angle of the PC in a wide range. This verifies that the microfiber WSe 2 SA plays a critical role in mode-locking operation. The performance comparison of mode-locked lasers based on different 2D materials is listed in Table 1. The mode-locked operations are realized at both 1.5 and 2.0 μm, and the ultrashort pulse duration is achieved up to hundreds of femtosecond. Compared to the extensively studied 2D materials SA such as graphene, black phosphorus, MoS 2 , and WS 2 [33][34][35][36][37][38][40][41][42][43][44][45], large-area WSe 2 has less been reported in ultrashort pulse generation. This contribution provides the compelling evidence of WSe 2 as excellent SAs for ultrafast photonic applications. Moreover, the optical properties and photonic applications of WSe 2 have not yet been fully explored up to now. For example, the promising WSe 2 p-n junction and heterojunction contacted with other 2D materials possessing unique energy band structure and charge transport characteristics have attracted much interest in electronic and optoelectronic devices [21,22], while their potential applications in ultrafast photonic are worth to be further investigated.

Summary
Summarily, we have demonstrated CVD-grown large-area WSe 2 films as SAs for passively mode-locked fiber lasers at 1.5 and 2 μm region. The CVD growth process was presented in details, and material characterizations indicated the high quality of the as-grown large-area WSe 2 film. By employing a PMMA assisted transfer method, the large-area WSe 2 film was successfully transferred onto the side wall of a microfiber to fabricate a WSe 2 SA with enhanced light-WSe 2 interaction through the strong evanescent wave coupling effect. As a result, the WSe 2 SA shown an extraordinarily large modulation depth of 54.5%. For the EDF laser system, highly stable mode-locked pulses were obtained with the pulse duration of 477 fs. The stable mode-locking operation at both 1.5 and 2 μm regions suggests that the largearea WSe 2 film was an excellent broadband SA for ultrafast photonics.