Enhanced nonlinear optical response of Se-doped MoS2 nanosheets for passively Q-switched fiber laser application

Rongfei Wei; Tian Qiao; Xiangling Tian; Hang Zhang; Xin He; Zhongliang Hu; Qiuqun Chen; Jianrong Qiu

doi:10.1088/1361-6528/aa6bc8

Introduction

Ultrafast fiber lasers have widespread applications in optical communication, optical sensing, industrial material processing, scientific research and biomedical diagnostics [1, 2]. The generation of ultrafast lasers is mainly based on the passive mode-locking or Q-switching techniques, the key component of which is the saturable absorber. Currently, the semiconductor saturable absorber mirror (SESAM) is the dominant scheme. However, the SESAM is not only very expensive, but also suffers from a low damage threshold and a narrow wavelength range. Hence, it is urgent that we seek new saturable absorbers that can be applied to achieve ultrafast pulse lasers. In recent years, it has been verified that graphene has significant saturable absorption (SA). Excellent passive Q-switching and mode-locking generations at different dispersion regimes have been reported with graphene-based saturable absorbers [3–6]. Inspired by graphene, two-dimensional (2D) layered molybdenum disulfide (MoS₂), a semiconducting analogue of graphene, has sparked extensive research in nonlinear photonics [7–30]. Wang et al first demonstrated that MoS₂ dispersions display better SA than the graphene dispersions in the same excitation conditions [21]. Subsequently, Zhang et al studied the nonlinearity of MoS₂ nanosheets and confirmed a Y-doped laser passively mode locked with a layered MoS₂ saturable absorber [28]. Applying a few-layer MoS₂-polymer composite, Woodward et al achieved stable passively Q-switched pulses at 1068 nm in a Yb-doped fiber laser [27]. By introducing suitable defects, Wang et al constructed a broadband MoS₂ saturable absorber, which can be employed in fiber lasers to realize stable passive Q-switching operations at wavelengths of 1.06, 1.42 and 2.1 μm [24]. Huang et al obtained widely-tunable Q-switched fiber lasers with a few-layer MoS₂ saturable absorber [22]. These reports imply that layered MoS₂ is a promising saturable absorber for generating ultra-fast pulsed lasers.

However, for the pristine MoS₂, the intrinsic shortcomings mean that layered MoS₂ is not always the most suitable candidate for real utilizations. Appropriate design and construction engineering is significant and crucial to obtain capable materials and devices with superior nonlinear optical (NLO) properties. Recently, it has been reported that combining layered MoS₂ with other semiconductor materials is a useful strategy [31, 32]. Compared with the individual MoS₂, MoS₂ hybrid nanomaterials often display an improved NLO response and/or flexible processability, attributed to the exotic structures and synergistic coupling effects between the constituents. Most recently, it has been discovered that tailoring the chemical composition of MoS₂ by introducing a dopant can effectively enhance the catalytic performances. For instance, nickel, vanadium and selenium (Se) doping can all improve the hydrogen evolution reaction activity of MoS₂ electrocatalysts [18, 19]. The incorporation of atoms into the MoS₂ forms more defects and disorders of atomic arrangements and reduces the average grain size, leading to the structural discontinuity and more active edge sites. In particular, strong second-order NLO susceptibilities at the boundaries and edges of the 2D MoS₂ crystal have been confirmed owing to the abundant edge states induced by the structural discontinuity [33]. The superior Q-switching behavior for the hierarchical MoS₂, which was better than that of liquid-phase exfoliated MoS₂ and compared to that of CVD MoS₂, was observed by Zhang et al most recently, probably due to the large SA induced by the hierarchical structure with a group of exposed edge states [29]. These factors motivate our curiosity and lead us to explore the third-order NLO property and the ultrafast pulse emission of MoS₂ after introducing dopant.

Therefore, in this work, we have successfully synthesized Se-doped MoS₂ nanosheets through a facile annealing method using diphenyl diselenide (DDS) as the Se dopant. After Se doping, an enhanced NLO response of the MoS₂ was observed by an open-aperture (OA) z-scan measurement. Applying the Se-doped MoS₂ nanosheets as a saturable absorber, an excellent passive Q-switching performance was realized in the fiber laser, at 1559 nm, with minimum pulse duration of 1.502 μs, a large output power of 17.2 mW, and a high pulse energy of 133.07 nJ, which is superior to that of the pristine MoS₂ nanosheets-based passively Q-switched operation. This work will broaden our vision to promote the NLO performance of various 2D nanomaterials by tailoring the chemical composition.

Experimental section

Preparation of 2D MoS₂ nanosheets

Typically, 1 mmol ammonium molybdate tetrahydrate and 30 mmol thiourea were dispersed in 35 ml deionized water under vigorous stirring to form a homogeneous solution, then transferred into a 50 ml Teflon-lined stainless steel autoclave and maintained at 210 °C for 24 h. After cooling down to room temperature naturally, the resulting precipitate was rinsed with deionized water and absolute ethanol several times to remove any possible ions, and finally dried in a vacuum oven at 60 °C for 6 h. Hence, the 2D MoS₂ nanosheets were attained.

Synthesis of Se-doped MoS₂ nanosheets

0.2 g MoS₂ nanosheets and 0.078 g DDS were ultrasonically dissolved in ethanol for 1 h. Subsequently, the suspension was spread onto an evaporating dish and dried, obtaining a uniform solid mixture. The mixture was placed into a tube furnace and heated at 800 °C for 2 h under Ar atmosphere. At last, the products were cooled to room temperature and collected. For comparison, the pure MoS₂ was treated under the same conditions.

Fabrication of nanoscaled Se-doped MoS₂ /polyvinyl alcohol (PVA) composite film

To obtain uniform dispersion and study the NLO response and the corresponding application, the nanoscaled Se-doped MoS₂/PVA composite film was fabricated according to previous work [26]. 0.6 g PVA was dissolved in 10 ml deionized water under mild stirring for 2 h at 85 °C. Then the as-prepared Se-doped MoS₂ nanosheets were homogeneously dispersed in the above transparent PVA solution. The solid film was fabricated after the resulting solution was cast on the SiO₂ substrate and dried at 30 °C for 6 h. The Se-doped MoS₂/PVA composite film can be completely removed from the SiO₂ substrate to form a flexible film. For comparison, the MoS₂/PVA composite film was also prepared.

Material characterization

The morphology and size of the products were identified using a field-emission scanning electron microscope (FESEM, Nova NanoSEM430) and a transmission electron microscope (TEM, JEOL-2100F). The high-resolution TEM (HRTEM) and corresponding energy electron microscopy (EDS) mapping analyses were carried out on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. X-ray diffraction (XRD) patterns were performed on a Bruker diffractometer applying Cu Kα radiation (λ = 1.5418 Å). Raman spectra were conducted by a Raman spectrometer (RenishawinVia, Gloucestershire, UK) and a 785 nm laser as the excitation source. X-ray photoelectron spectra (XPS) were acquired on a physical electronics 5600 multi-technique system with achromatic 200 W Al Kα x-rays as the excitation source and the resolution of 0.05 eV. An open-aperture (OA) z-scan system was applied to study the NLO response of the products, which was conducted by employing the 130 femtosecond (fs) laser pulses at 800 nm with a repetition rate of 1 kHz. To more precisely verify the measured data, CS₂ solution contained in a cuvette with a thickness of 1 mm was applied to calibrate. All the measurements were carried out at room temperature.

Results and discussion

The morphology and structure of the products were investigated by SEM and TEM in figure 1. It reveals that the basic morphology of the nanosheets was almost unchanged apart from the reduction in size after Se doping. Figure 1(c) shows an HRTEM image taken from the edge of the Se-doped MoS₂ nanosheets. Compared to those of pristine MoS₂ nanosheets, in the Se-doped MoS₂ nanosheets: (1) more disordered and defective lattice fringes were observed, suggesting the disordering of atomic arrangements and the depressed crystallinity, confirmed by the selected area electron diffraction (SAED) in the inset of figure 1(c); (2) slightly different interlayer spacing between neighboring (002) planes was presented. As displayed in figure 1(d), the Se doping can lead to the enlargement of the lattice spacing from 0.619 nm to over 0.829 nm, as confirmed by the XRD results.

To study the structures of the products, XRD of the pure MoS₂ and the Se-doped MoS₂ nanosheets were conducted, as shown in figure 2(a). All diffraction peaks match well with the standard pattern of hexagonal structure MoS₂ (JCPDS Card No. 65-1951) with no distinguishable impurities phase, indicating the high purity of the products. It is worth noting that all main diffraction peaks slightly shifted toward smaller diffraction angles after Se doping, which implies that the interplanar distance was expanded due to the Se atom being larger than sulfur (S). In addition, we also observe that the XRD peaks of the Se-doped MoS₂ nanosheets are broader and weaker in intensity, demonstrating the decrease in the crystallinity and average grain size and an increase in disordered and defective structures in the Se-doped MoS₂ nanosheets owing to their open edges after Se doping.

Figure 2(b) presents the Raman spectra of the pure MoS₂ and the Se-doped MoS₂ nanosheets. Under laser excitation at 532 nm, two characteristic peaks at 380.9 and 405.9 cm⁻¹ resulting from the in-plane ${{{\rm{E}}}^{1}}_{2{\rm{g}}}$ and out-of-plane A_1g vibrational modes of the hexagonal MoS₂ are found [26]. After Se doping, both the peaks shift to lower frequency and the intensity is decreased. The shift is attributed to the interaction between Se and S atoms, giving rise to soft Mo-S vibration and reduced frequency [18, 34]. The decreased intensity is likely ascribed to the change of lattice symmetry that determines the matrix elements and selection rules for Raman active vibrational modes [8]. Besides, by introducing Se, the value of I (E¹_2g/A_1g) is increased, which is associated with the abundant edge structures [35, 36].

XPS analyses were conducted to examine the chemical states and the Se content in the Se-doped MoS₂ nanosheets. As shown in figure 3(a), the peaks of Mo 3d, S 2p, Se 3d, C 1 s and O1s signals are observed in the XPS survey spectrum of the Se-doped MoS₂ nanosheets. Two characteristic peaks located at 229.3 and 232.4 eV in figure 3(b) are assigned to Mo 3d_5/2 and Mo 3d_3/2 orbitals, indicating the dominance of Mo⁴⁺ in the products [20]. Whereas the S 2p region (figure 3(c)) shows the peaks at 162.3 and 163.5 eV corresponding to S 2p_3/2 and S 2p_1/2, which coincides with the −2 oxidation state of S [20]. Besides, the Se 3d_5/2 and Se 3d_3/2 peaks (figure 3(d)) are located at around 54.8 and 55.5 eV [17, 18], respectively; a well-known characteristic of substitutional doping of Se to S, in comparison with S occupying the interstitial sites of MoS₂. Detailed compositional analysis reveals the Se doping concentration of ∼6.3% in the Se-doped MoS₂ nanosheets. The energy dispersive x-ray spectrometry (EDX) analysis in figure 3(e) further shows the existence of Se in the Se-doped MoS₂ nanosheets. Moreover, typical scanning TEM (STEM) and elemental mapping images of the Se-doped MoS₂ nanosheets were shown in figures 3(f)–(h), suggesting that Mo, S and Se are uniformly distributed in the product.

NLO properties of the Se-doped MoS₂ nanosheets were investigated employing the OA z-scan method. As presented in figure 4(a), a sharp and narrow peak shows at the beam focus and increases gradually as the incident pump power is increased, which belongs to the characteristic of the nonlinear SA performance [26, 31]. At the same pump power, Se-doped MoS₂ nanosheets exhibit a much stronger peak than that of the pristine MoS₂ nanosheets, as shown in figure 4(b). Fitting the z-scan curves (figure 4(b)) with an incident pump power of 38 μW exhibit that the nonlinear SA coefficient (β) for the Se-doped MoS₂ nanosheets is higher than that of the pristine MoS₂ nanosheets (table 1), indicating an enhanced SA response of the Se-doped MoS₂ nanosheets. Firstly, such enhancement is due to the increased edge states associated with increased defects and disorders after Se doping. Defects and disorders induce edge states which result in deep trap emission, trapping the charge carriers. The increased edge-states may increase the density of charge carriers, leading to the excellent optical nonlinearity [26, 29]. In addition, owing to the more metallic nature of Se, the electrical conductivity is improved after the Se atom doping into the frameworks of MoS₂. Se-doped MoS₂ nanosheets with superior electrical conductivity could efficiently transfer the photo-generated electrons when excited by the fs pulsed laser at 800 nm, which could suppress the charge recombination and produce a charge-separated excited state, giving rise to strong and ultrafast NLO performance [37, 38]. The imaginary part of the nonlinear third-order susceptibility Imχ⁽³⁾ is linked with β by Imχ⁽³⁾ = [10⁻⁷cλn²/96π²]β (cm W⁻¹) [21], where c stands for light speed in vacuum, λ and n represent the wavelength of the laser pulse and the refractive index, respectively. To exclude the influence arising from the linear absorption α₀, which can be evaluated according to the UV–vis absorption spectra presented in figure 2(c), the third-order nonlinear figure of merit (FOM = ∣Im χ⁽³⁾/α₀∣) was applied to evaluate the NLO behavior of a material and help us to compare it with the other nanomaterials reported [21]. Hence, Im χ⁽³⁾ and FOM of the Se-doped MoS₂ and the pristine MoS₂ were calculated and shown in table 1. The modulation depth and saturation intensity of the Se-doped MoS₂ nanosheets and the pristine MoS₂ nanosheets were also investigated by fitting the data in figure 4(c) using the equation T = 1−α_s/(1 + I/I_sat)−α_ns [26], where, I, α_s, α_ns and I_sat are the input pulse intensity, the modulation depth, the nonsaturable loss and the saturation intensity, respectively. The obtained values were compared in table 1. Such high modulation depth and low nonsaturable loss imply that the Se-doped MoS₂ nanosheets are very promising for application in a Q-switcher to generate stable Q-switched pulses.

Table 1. NLO parameters of the pure MoS₂ and the Se-doped MoS₂ nanosheets. α₀: linear absorption coefficient (cm⁻¹); β: nonlinear absorption coefficient (cm/GW); Imχ⁽³⁾: the imaginary part of the nonlinear third-order susceptibility (×10⁻¹² esu); FOM: the third-order nonlinear figure of merit (×10⁻¹⁵ esu cm); α_s: modulation depth (%); α_ns: nonsaturable loss (%); I_s: saturation intensity (GW cm⁻²).

Material	α₀	β	Imχ⁽³⁾	FOM	α_s	α_ns	I_s
Pristine MoS₂	379.8	2.78	1.59	4.19	10.2	33.8	14.5
Se-doped MoS₂	579.8	7.96	4.55	7.85	19.1	24.6	16.5

A ring-cavity erbium-doped fiber (EDF) laser was used to verify the functionality of the as-prepared Se-doped MoS₂ nanosheets as a saturable absorber. The schematic diagram was presented in figure 5(a). The cavity mainly comprised 30 m EDF (Liekki ER110-4/125) with a peak core absorption of 110 ± 10 dB m⁻¹ at 1530 nm as the gain medium. The 980 nm laser diode pump light of 600 mW maximum power was injected into the gain fiber via a wavelength division multiplexer (WDM). The other component in the cavity was the standard single-mode-fiber (SMF). The 30% intra-cavity laser was output from the 70:30 fiber coupler. The adjustment of the polarization state in the laser cavity was conducted by the polarization controller (PC). A polarization insensitive isolator (PI-ISO) was employed to assure the unidirectional operation of the laser cavity. The Se-doped MoS₂/PVA film was embedded between two fiber connectors in the cavity. A stable and self-starting Q-switching pulse was observed at a relatively low threshold pump power of 50 mW and witnessed to be independent of the adjustment of the polarization controller. Figures 5(b)–(d) summarize the output performances of the Q-switching operation for the Se-doped MoS₂ nanosheets. An oscilloscope trace with no pulse modulation was attained in figure 5(b), indicating the stable Q-switching operation. Even in a wider span of 220 kHz, no other frequency component other than the fundamental and harmonic frequency is observed in the radio-frequency (RF) output spectrum (the inset of figure 5(c)), further confirming the stability of the passive Q-switching operation. The signal-to-noise ratio (SNR) is about 56 dB. A central wavelength of the pulse at 1559 nm was described in the inset of figure 3(c). To further understand the behaviors of the output Q-switched pulses, the evolution of the Q-switching parameters as increasing the pump power was studied, as presented in figures 5(c), (d). The initial Q-switching pulses have a pulse width of 8.56 μs, a pulse repetition rate of 28.97 kHz, an output average power of 0.894 mW and a pulse energy of 30.87 nJ. Upon boosting the pump power, the pulse duration reduced firstly, and then continued almost unchanged once the pump power was over 200 mW, implying that the saturable absorber was saturated. For a passively Q-switched laser, the pulse duration t can be calculated via the formula: t = 3.52T_R/ΔT [39], where T_R and ΔT represent the cavity round-trip time and the modulation depth of the saturable absorber, respectively. Hence, the minimum pulse duration could be further narrowed by shortening the cavity length and (or) increasing the modulation depth of the as-prepared materials. Tthe repetition rate and output power increased nearly linearly with the pump power elevated. The high damage threshold of the Se-doped MoS₂ nanosheets was verified as the Q-switching operation could be maintained even if the incident pump power exceeded 400 mW. In order to avert unpredictable thermal damage of the organically natured PVA at higher pump power, the Q-switching operations were performed at pump strengths of under 400 mW. The maximum output power was 17.2 mW, with a repetition rate of 130 kHz and a minimum pulse width of 1.502 μs, and the maximum pulse energy was approximately 133.07 nJ. The pristine MoS₂ nanosheets-based passively Q-switched fiber laser experiment was also conducted under the same experimental conditions for comparison. As illustrated in table 2, the results reveal that the laser utilizing the Se-doped MoS₂ nanosheets as a saturable absorber displayed a lower Q-switching threshold, a wider range of repetition rates, a higher SNR, a shorter pulse duration, a larger output power and a higher pulse energy of the laser. Such a superior Q-switching operation for the Se-doped MoS₂ nanosheets can probably be ascribed to the large nonlinear absorption induced by Se-doped MoS₂ nanosheets with a large amount of exposed edge states and the superior electrical conductivity. This demonstrates that the Se-doped MoS₂ nanosheets are a potential saturable absorber for Q-switched fiber lasers.

**Figure 5.** (a) Schematic diagram of a passively Q-switched fiber laser based on the saturable absorber of the Se-doped MoS₂ nanosheets. (b) Oscilloscope trace. (c) The pulse duration and repetition rate. (d) Output average power and pulse energy versus the pump power of the Q-switching pulses. The insets of (c) and (d) are RF optical spectra with the span of 220 kHz and single Q-switching pulse profile, respectively.
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Table 2. Output performances comparison of the pristine MoS₂-based and Se-doped MoS₂-based passively Q-switched fiber lasers. λ: central wavelength (nm); f_rrr: repetition rate range (kHz); t: shortest pulse duration (μs); SNR: single-to-noise ratio (dB); P_max: max. Output power (mW); Q_max: max. pulse energy (nJ).

Material	λ	f_rrr	t	SNR	P_max	Q_max
Pristine MoS₂	1559	79.95–124.94	1.756	51.7	13.2	114.25
Se-doped MoS₂	1559	28.97–130	1.502	56	17.2	133.07

Conclusions

We have successfully prepared Se-doped MoS₂ nanosheets via a facile annealing approach. The Se-doped MoS₂ nanosheets exhibited enhanced NLO properties and outstanding passive Q-switching performance. It was found that the Q-switched fiber laser utilizing Se-doped MoS₂ nanosheets displayed a lower Q-switching threshold, a wider range of repetition rates and a higher SNR compared with the pristine MoS₂ nanosheets-based passively Q-switching operation. More importantly, the shortest pulse duration of 1.502 μs, the output power of 17.2 mW and the pulse energy of 133.07 nJ could be realized in the Se-doped MoS₂ nanosheets-based passively Q-switched fiber laser. Such work paves a new pathway for improving the NLO property and obtaining excellent ultrafast pulse emission from MoS₂, which can then be expanded to other 2D nanomaterials.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grants no. 51132004, 51102096, 11404114), Guangdong Natural Science Foundation (Grants no. S2011030001349, 1045106410104887), Open Fund from the State Key Laboratory of Precision Spectroscopy (East China Normal University) and Open Fund from the State Key Laboratory of High Field Laser Physics of the Shanghai Institute of Optics and Fine Mechanics of Chinese Academy of Science, China.

Enhanced nonlinear optical response of Se-doped MoS₂ nanosheets for passively Q-switched fiber laser application

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Abstract

Introduction