Large-scale monolayer molybdenum disulfide (MoS2) for mid-infrared photonics

Han Pan 1 , Hongwei Chu 1 , Zhongben Pan 2 , Shengzhi Zhao 1 , Ming Yang 3 , Jianwei Chai 3 , Shijie Wang 3 , Dongzhi Chi 3 , and Dechun Li 1
  • 1 School of Information Science and Engineering, Shandong University, 266237, Qingdao, China
  • 2 Institute of Chemical Materials, China Academy of Engineering Physics, 621900, Mianyang, China
  • 3 Institute of Materials Research and Engineering, A*STAR, 138634, Singapore, Singapore
Han Pan
  • School of Information Science and Engineering, Shandong University, Qingdao, 266237, China
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, Hongwei ChuORCID iD: https://orcid.org/0000-0002-1231-2381, Zhongben Pan
  • Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang, 621900, China
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, Shengzhi Zhao
  • School of Information Science and Engineering, Shandong University, Qingdao, 266237, China
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, Ming Yang
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  • Institute of Materials Research and Engineering, A*STAR, Singapore, 138634, Singapore
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, Jianwei Chai
  • Institute of Materials Research and Engineering, A*STAR, Singapore, 138634, Singapore
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, Shijie Wang
  • Institute of Materials Research and Engineering, A*STAR, Singapore, 138634, Singapore
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, Dongzhi Chi
  • Institute of Materials Research and Engineering, A*STAR, Singapore, 138634, Singapore
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and Dechun Li
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  • School of Information Science and Engineering, Shandong University, Qingdao, 266237, China
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Abstract

Mid-infrared (MIR) photonics has attracted tremendous interest because of its broad applications at atmospheric windows. In this work, we report high-performance MIR photonics based on large-scale and good-quality monolayer molybdenum disulfide (MoS2). The open-aperture Z-scan measurement on the nonlinear saturable absorption features shows that the as-grown monolayer MoS2 possesses a modulation depth of 26% and a low saturable intensity of 271 kW/cm2, enabling its application as an excellent saturable absorber for the MIR pulse generation. This is further evident by the measured high effective nonlinear absorption coefficient βeff (−16 cm/MW). In addition, the two-photon absorption coefficient and the nonlinear refractive index of monolayer MoS2 are also determined by the closed-aperture Z-scan technology. As an application, we demonstrate a passively Q-switched Tm,Ho:CaLu0.1Gd0.9AlO4 (Tm,Ho:CLGA) disordered crystal laser at 2.1 μm by using the monolayer MoS2 as the saturable absorber for the first time, producing a minimum pulse width of 765 ns and a pulse repetition rate of 36 kHz. Our results demonstrate that large-scale monolayer MoS2 is a promising candidate for the MIR photonic applications.

1 Introduction

The mid-infrared (MIR) spectrum ranging from 2 to 20 µm has diverse applications such as bio-photonics and medical photonics, absorption spectroscopy, environmental monitoring and security, and free-space communications [1]. Among these applications, owing to the important atmospheric windows and strong fundamental absorptions of molecular species, the MIR absorption spectroscopy is of great interest in civilian and military domains [2]. In such a spectroscopy, because the fundamental absorption and vibration bands of most molecules appear in the MIR region, detecting with the MIR sources is of more sensitivity and precision [2]. For real applications, MIR lasers with high power and high energy are in urgent need to achieve a sensitive and precise detection. Thus, the advent of the MIR optoelectronic and electronic devices is highly desirable for next-generation photonic applications.

It is well known that the nonlinear optical materials play an essential role in the advanced photonics as the fundamental components [3], [4], [5], [6], [7], [8], [9], [10]. In recent years, the two-dimensional (2D) materials have emerged as promising candidates for the applications in the MIR region [11], [12], [13], [14], [15] because of their excellent electronic and optoelectronic properties and broadband nonlinear optical absorption. Among these 2D materials, transition metal dichalcogenides have drawn plenty of attention [16], [17], [18], [19], as they are stable and have a direct band gap in the visible light range in a monolayer form. In particular, monolayer molybdenum disulfide (MoS2) has demonstrated promising application in transistors, photocatalysis, solar cells, and other photonic applications [20], [21], [22], [23]. For example, MoS2 2D layers have been successfully used as the nonlinear photonic devices to Q-switch and mode-lock the MIR lasers at 2–3 µm [24], [25], [26], [27], [28], [29], [30], [31], [32]. In these photonic applications, the device performance is strongly dependent on the quality and size of the MoS2 layers [33], [, 34]. The large-scale MoS2 monolayer with good quality is highly desired as the nonlinear saturable absorber to integrate with the optical devices, which, however, is still a challenging issue.

In the present work, we report nonlinear properties and modulation characteristics in the MIR spectral band by using the large-scale monolayer MoS2. The nonlinear linear optical characteristics were studied by exploiting the open aperture Z-scan techniques, which show a modulation depth of 26%, a low saturable intensity of 271 kW/cm2, and a high effective nonlinear absorption coefficient βeff of −16 cm/MW. All these results reveal that the monolayer MoS2 is an excellent saturable absorber for the MIR pulse generation and an appealing nonlinear linear optical photonic device to modulate the optical pulses. Using the monolayer MoS2 as the nonlinear optical saturable absorber, a passively Q-switched Tm,Ho:CLGA disordered crystal laser was realized operating at 2.1 µm in the MIR region with a recorded shortest pulse duration (765 ns) for the first time.

2 Monolayer MoS2 preparations, characterizations, and simulation

The good-quality monolayer MoS2 thin film was grown by physical vapor deposition (PVD) on the c-plane sapphire substrate (2-inch in diameter) using the sputtering process as documented in our previous works [34], [35], [36], [37]. Figure 1 displays the characterization of the sputtering deposited 2-inch monolayer MoS2. From the atomic force microscopy image (Figure 1(A)), we can see that the MoS2 film is continuous and smooth in a large scale with a small root mean square roughness of ∼0.21 nm. In addition, the thickness of the as-grown MoS2 films was also measured. To expose the substrate, we used a knife to scrape off the MoS2 film. The raised parts marked by two dashed lines in Figure 1(B) are caused by the MoS2 stacked on the edge during the scratching process. Figure 1(C) shows the height profile of the MoS2 film on the sapphire substrate. The unusual height bump is caused by the stacking of materials on the scratched edge. It can be seen that the thickness of the as-grown MoS2 film was about 0.8 nm, suggesting the single layer of MoS2. The X-ray photoelectron spectroscopy in Figure 1(D) shows the core-level peaks of Mo 3d3/2 (∼233 eV), Mo 3d5/2 (∼229.6 eV), and S 2s (∼227 eV), which are the typical peaks of tetravalent Mo4+ and divalent S2− in 2H-MoS2 semiconductor [38]. We also do not observe the Mo suboxidation states from the Mo 3d core-level spectra. The thickness of the sputtered MoS2 film can be accessed by using Raman spectra. As shown in Figure 1(E), the relative shift between A1g and E2g is about 20.1 cm−1, indicating the dominant monolayer MoS2 on the sapphire substrate [22]. The sputtered monolayer MoS2 shows strong photoluminescence (PL) emission, as shown in Figure 1(F). As the PL emission is usually determined by exciton for the low-dimensional materials, the direct band gap of monolayer MoS2 should be smaller than the dominant emission peak of 1.87 eV. The SEM images of the monolayer MoS2 thin film is also shown in Figure 1. The white straight line in Figure 1(G) is the substrate scratched with a knife, which is beneficial to observing the morphology of the MoS2 film more clearly through comparison. Figure 1(H) is the SEM image after further increasing the magnification. It can be seen from both SEM images that the as-grown monolayer MoS2 film features the excellent uniformity. All these characterizations confirm the good quality of the deposited large-scale monolayer MoS2 on the sapphire substrate.

Figure 1:
Figure 1:

Characterizations of the deposited monolayer molybdenum disulfide (MoS2) on the sapphire substrate.

(A) Atomic force microscopy (AFM) image, (B) AFM height image of the as-grown MoS2 thin film, (C) line scan across the MoS2 thin film and exposed substrate interface, (D) X-ray photoelectron spectroscopy (XPS) core-level spectra of Mo 3d, (E) Raman spectra, (F) Photoluminescence spectra, and (G and H) SEM images.

Citation: Nanophotonics 9, 16; 10.1515/nanoph-2020-0331

To further investigate the band gap of monolayer MoS2, we also simulated the band structure of monolayer MoS2 with density functional theory calculation based on Vienna ab initio Simulation Package (VASP) code. The projected augmented wave pseudopotentials are used, as implemented in the VASP. In the calculations, Perdew–Burke–Ernzerhof of generalized gradient approximation function is used for exchange–correlation interaction. The cut-off energy of planewave basis for calculation was set as 400 eV. Optimization of geometric structure was relaxed until the total energy and maximum residual force are 10−6 eV and 0.02 eV/A. K-point meshes of 14 × 14 × 1 have been used. Figure 2(A) displays the model of monolayer MoS2. The result is shown in Figure 2(B), which indicates that the direct band gap of monolayer MoS2 is about 1.8 eV. The simulated result is in good agreement with the PL spectrum.

Figure 2:
Figure 2:

(A) Schematic of the model of MoS2 for density functional theory (DFT) calculation. Brown and yellow dots represent molybdenum atoms and sulfur atoms, respectively. (B) Band structure of monolayer MoS2 with valence band maximum and the conduction band minimum at the K point of the MoS2 Brillouin zone.

Citation: Nanophotonics 9, 16; 10.1515/nanoph-2020-0331

An open-aperture Z-san technology was exploited to measure the nonlinear saturable absorption (SA) features of the monolayer MoS2 at 2 µm. The laser source used in nonlinear absorption measurements is a home-made Q-switched laser with a pulse width of 50 ns at a repetition rate of 3 kHz. A lens with a focal length of 200 mm is used as a focusing device. Then the sample is put behind the lens, moving along the z-axis. With the different location of the sample on the z-axis, the pump laser intensity on the sample will change accordingly. To avoid the thermal effect on the sample, a pulsed laser with a low frequency and a low power is used in our case, which can efficiently reduce the heat accumulation on the MoS2 film [39], and the low power density incident on the MoS2 also decreases the influence of thermal-induced nonlinearity. In addition, the focal length of the used lens is large, which can cause the slow beam caustic to reduce the heat accumulation. Moreover, the large size of the uniform monolayer MoS2 film benefits the thermal diffusion.

As Figure 3(A) shows, the normalized transmission curve exhibits a symmetrical peak. The maximum peak presents a strong nonlinear SA in the MoS2 at the focus (Z = 0). An effective nonlinear absorption coefficient βeff was introduced to characterize the nonlinear absorption, which can be expressed as follows [10], [, 40]:
T=m=0[q0(z,0)] m(m+1)1.5 ,mNq0(z,0)=βeffLeffI0(1+z2/z02)
where Leff is the effective length, I0 represents the on-axis peak intensity, and z0 is the Rayleigh range. Therefore, the nonlinear absorption coefficient βeff was estimated as −16 ± 0.5 cm/MW, which is comparable with the previous works [41]. The relationship between the transmission of MoS2 saturable absorber and incident laser intensity was recorded as displayed in Figure 3(B). The measurement data were fitted by the equation as following [9], [10], [11]:
T=1ΔTexp(I/IS)Tns,
in which, ΔT represents the modulation depth, and Is and Tns represent the SA intensity and the nonsaturable losses, respectively. The modulation depth, SA intensity, and nonsaturable losses of 26%, 271 kW/cm2, and 7% were obtained, respectively. Note that the monolayer MoS2 materials feature a direct band gap of 1.87 eV, which is much larger than the photon energy at 2 µm. Therefore, the SA response of monolayer MoS2 at 2 µm comes from the defect energy levels and subbandgap generated by the edge effect [28], [41], [42]. In comparison with the previous results shown in Table 1, the monolayer MoS2 thin film used in our work exhibits stronger nonlinear optical response, such as larger modulation depth and lower saturation intensity. This indicates that the as-grown monolayer MoS2 by the PVD method could be a potential saturable absorber for Q-switched lasers around 2 µm. In comparison with the few-layered MoS2, the monolayer MoS2 possesses more point defects because of the stronger quantum confinement effect caused by size reduction and more nonstoichiometric ratio. These point defects including atomic vacancy defects and antisite defects will form a large number of defect energy levels, which may result in a strong nonlinear SA to mid-infrared lasers. In addition, compared with few layers, the in-band relaxation rate in monolayer is increased because of defect-assisted scattering [43], [, 44], and the defects of the monolayer MoS2 will cause a direct state with a local band gap, reducing the influence of the phonon effect [45]. All abovementioned factors will benefit the modulation performance of monolayer MoS2 as a saturable absorber for MIR lasers.
Figure 3:
Figure 3:

(A) Normalized transmission versus the position at the light peak intensity of 1.25 MW/cm2 and (B) the intensity-dependent nonlinear saturable absorption of single-crystalline monolayer molybdenum disulfide (MoS2) at 2 μm.

Citation: Nanophotonics 9, 16; 10.1515/nanoph-2020-0331

Table 1:

Comparisons of saturable absorption properties of molybdenum disulfide (MoS2) saturable absorber in the mid-infrared (MIR) band.

λ (nm)Saturable absorberModulation depth (%)Saturable absorption intensity (MW/cm2)Ref.
2840Few-layered MoS24.2[27]
2021Few-layered MoS2210.8[28]
1977Few-layered MoS27.2[30]
2090Large-sized monolayer MoS2260.27This work
To further investigate the nonlinear optical property of monolayer MoS2, we increased the laser intensity to observe the optical limiting phenomenon. The decreased transmittance at high intensities is due to the two-photon absorption (TPA) effect, which leads to the reversed SA (RSA) prevailing over the SA behavior. Considering the effects of both SA and RSA, the experimental data are fitted by the equation as follows [46]:
T=[1α0LISIS+I0/(1+z2/z02)βTPALI0/(1+z2/z02)]/(1α0L)
here, Is is the SA intensity, z0 is the diffraction length of the beam, and βTPA is the TPA coefficient. The normalized transmittance curve of monolayer MoS2 is shown in Figure 4(A). The TPA coefficient of monolayer MoS2 at 2 μm was also obtained as −31 ± 0.8 cm/MW.
Figure 4:
Figure 4:

(A) Reversed saturable absorption (RSA) phenomenon at the light peak intensity of 2.35 MW/cm2 (B) The results of closed-aperture (CA) Z-scan measurements. Lines: fitting curves; dots: normalized transmission data.

Citation: Nanophotonics 9, 16; 10.1515/nanoph-2020-0331

Furthermore, a closed-aperture (CA) Z-scan measurement was also implemented to determine the nonlinear refractive index. For the CA Z-scan, an aperture is placed in front of the sample to confine the transmitted beam incident on the detector. The nonlinear refractive index (n2) is estimated by fitting the normalized transmittance from the measurement data with the following equation as follows [46]:
Tclose/open=1+4(z/z0)Δϕ((z/z0)2+9)((z/z0)2+1)
where Δϕn2I0kd represents the nonlinear phase shift induced in the samples with n2, I0, k, and d, denoting the nonlinear refractive index, peak intensity, wave-vector, and the thickness of the sample, respectively. Figure 4(B) displays the normalized transmittance obtained by CA Z-scan measurement. The nonlinear refractive index n2 of monolayer MoS2 at 2 μm is −25 ± 0.6 × 10−4 cm2 MW−1.

3 MIR photonic device: saturable absorber

In this section, we would like to address the MIR photonic application of the monolayer MoS2 as a saturable absorber to produce short MIR laser pulses. The laser cavity was a compact plane-concave resonator with a length of 15 mm, as shown in Figure 5. The input mirror M1 (radius of curvature: 200 mm) was coated for high reflectance at 2.1 µm and antireflectively coated for the pump wavelength at 794 nm. The gain medium was a polished c-cut Tm,Ho:CaLu0.1Gd0.9AlO4 (Tm,Ho:CLGA) disordered crystal. In the experiment, the laser crystal was mounted into a water-cooled copper sink cooled at 15 °C. The large-scale monolayer MoS2 was placed near the output coupler (OC) as the saturable absorber to generate the short MIR pulses. The OC was a plane mirror with a T = 1% at 2.1 µm. The pump source was a commercial fiber-coupled diode laser emitting the radiation at 794 nm (Coherent Inc., USA). The diameter and the numerical aperture of the coupled fiber were 400 μm and 0.22, respectively. The pump beam was focused into the gain crystal with a waist radius of 100 μm by a 1:1 optical imaging system. A longpass filter was put behind the OC to block the residual pump beam. The pulse temporal behavior was recorded by a DPO 7104C digital phosphor oscilloscope with a high-speed photodetector. A laser power meter was used to measure the average output power (Figure 5).

Figure 5:
Figure 5:

An illustration of the passively Q-switched Tm,Ho:CLGA laser with large-size monolayer MoS2 as a saturable absorber.

Citation: Nanophotonics 9, 16; 10.1515/nanoph-2020-0331

First, we investigated the free running of the Tm,Ho:CLGA disordered crystal laser without the monolayer MoS2 as the saturable absorber. The threshold absorbed pump power for the lasing was 2.38 W. The maximum output power was 86 mW, with a slope efficiency of 4.3%, as shown in Figure 6(A). When the monolayer MoS2 was inserted into the laser resonator working as the saturable absorber, the stable passive Q-switching operation can be achieved by slightly aligning the mirrors. In this case, the threshold power for the laser operation is slightly increased to 2.7 W, while the stable Q-switching operation starts at 2.98 W. Under the highest absorbed pump power of 4.26 W, the maximum output power for the passive Q-switching operation is 56 mW, corresponding to a slope efficiency of 3.3%. As Figure 6(B) shows, the pulse duration decreases monotonically from 4 µs to 765 ns, while the pulse repetition rate increases from 15 to 36 kHz. Once the output power, the pulse duration, and the pulse repetition rate were given, the pulse energy and the peak power can be estimated. The corresponding highest pulse energy and the maximum peak power are 1.55 µJ and 2.03 W at an absorbed pump power of 4.26 W, respectively. The temporal pulse profile and stable pulse train with the pulse width of 765 ns at the repetition rate of 36 kHz are displayed in Figure 6(C) and (D), respectively. From the recorded pulse train, the pulse peak-to-peak fluctuation is estimated as 4.6% in the RMS error method, demonstrating a stable Q-switching operation.

Figure 6:
Figure 6:

(A) The output power and (B) the pulse duration and the pulse repetition rate versus the absorbed pump power; (C) typical temporal pulse profile and (D) the stable pulse train at an absorbed pump power of 4.26 W.

Citation: Nanophotonics 9, 16; 10.1515/nanoph-2020-0331

The comparisons of the Q-switched laser performance at around 2 μm between our work and previous reported works have also been summarized in Table 2. The good laser performance is mainly attributed to the strong SA properties of monolayer MoS2, such as large modulation depth and low saturable intensity. The resonator and saturable absorber can be further optimized to have a better performance, such as a shorter pulse duration and higher pulse energy. In addition, during the laser experiments, the monolayer MoS2–based SA worked stably always even at a higher pump power, which indicated its relative high damage threshold.

Table 2:

Comparisons of the Q-switched laser performance at 2 μm with previous reported works.

Crystalλ (nm)Saturable absorberPulse width (ns)Single pulse energy (μJ)Ref.
Tm:GdVO41902Few-layered MoS28002.08[26]
Tm,Ho:YAP2129Few-layered MoS24355[28]
TDFL2032Few-layered MoS217601[29]
Tm:CLNGG1979Few-layered MoS248400.72[30]
Tm,Ho:CLGA2090Large-sized monolayer MoS27651.55This work

MoS2, molybdenum disulfide.

To make sure the laser is working in the MIR region, the oscillating wavelength of the monolayer MoS2 saturable absorber Q-switched Tm,Ho:CLGA disordered crystal laser was recorded by an IR spectroscope meter (Avantes, the Netherlands). As Figure 7(A) shows, the peak of oscillating wavelength locates at ∼2090 nm with a full-width at half-maximum (FWHM) of 20 nm. Compared with the broad spontaneous emission spectrum of Tm,Ho:CLGA disordered crystal (∼300 nm) [47], the linewidth of MoS2-based Q-switched laser is much narrower, which indicates that the stable laser operates in the constructed solid-state laser device [48]. Moreover, the beam caustic versus the position is shown in Figure 7(B). From the experimental data, the beam quality M2 from the monolayer MoS2 saturable absorber Q-switched Tm,Ho:CLGA laser at 2.1 µm can be estimated as <1.1 under the highest absorbed pump power of 4.26 W, indicating good optical properties of the large-size monolayer MoS2–based saturable absorber.

Figure 7:
Figure 7:

(A) The working spectrum of the single-crystalline monolayer molybdenum disulfide (MoS2) Q-switched Tm,Ho:CLGA laser and inset: spontaneous emission spectrum of Tm,Ho:CLGA crystal; (B) the beam quality M2 factor at the absorbed pump power of 4.26 W.

Citation: Nanophotonics 9, 16; 10.1515/nanoph-2020-0331

It is worth mentioning that the monolayer MoS2 possesses higher thermal conductivity, which benefits the thermal diffusion [49]. Meanwhile, owing to the horizontal extension of materials in large-sized MoS2, the MoS2 around the beam will help reduce the heat accumulation of the MoS2 illuminated by the beam. Both factors make the large-sized monolayer MoS2 good thermal stability, which ensures good modulation performance. On the other hand, the excellent uniformity of large-sized MoS2 thin-films ensures the continuity and repeatability of laser experiment. What’s more, under the premise of ensuring uniformity, large-size MoS2 thin films can break through the limitations of the manufacturing process to achieve large-scale preparation and tunable thickness, which is also very important for promoting the application of MoS2 nanofilms in optoelectronic fields.

4 Conclusions

In conclusion, we have demonstrated the application of large-scale monolayer MoS2 for the MIR photonic applications. The PVD monolayer MoS2 shows a large modulation depth of 26% and a low saturable intensity of 271 kW/cm2, indicating that the monolayer MoS2 is an excellent saturable absorber for the MIR pulse generation. The effective nonlinear absorption coefficient βeff is measured to be −16 cm/MW. Furthermore, the TPA coefficient and the nonlinear refractive index of monolayer MoS2 are also determined as −31 ± 0.8 cm/MW and −25 ± 0.6 × 10−4 cm2 MW−1, respectively. Based on the large nonlinear absorption coefficient and modulation depth, we demonstrate a passively Q-switched Tm,Ho:CaLu0.1Gd0.9AlO4 (Tm,Ho:CLGA) disordered crystal laser at 2.1 μm by using the monolayer MoS2 as the saturable absorber for the first time, which shows remarkable performance such as a short pulse width of 765 ns and pulse repetition rate of 36 kHz. Our results unravel that sputtered large-scale monolayer MoS2 is a promising candidate for the MIR photonic devices.

Acknowledgments

The authors would like to thank Dr. Na Qi and Mr. Daozhi Li from Shandong University for their help in the morphology characterization and the DFT simulation.

Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

Research funding: This work is partially supported by National Natural Science Foundation of China (NSFC) (61575109, 12004213), Fundamental Research Fund of Shandong University (2018TB044), and Foundation of President of China Academy of Engineering Physics (YZJJLX2018005). D.C., S.W., J.C., and M. Y. acknowledge the funding support from A*STAR Science and Engineering Research Council PHAROS 2D Program (SERC Grant No. 152-70-00012).

Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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  • View in gallery

    Characterizations of the deposited monolayer molybdenum disulfide (MoS2) on the sapphire substrate.

    (A) Atomic force microscopy (AFM) image, (B) AFM height image of the as-grown MoS2 thin film, (C) line scan across the MoS2 thin film and exposed substrate interface, (D) X-ray photoelectron spectroscopy (XPS) core-level spectra of Mo 3d, (E) Raman spectra, (F) Photoluminescence spectra, and (G and H) SEM images.

  • View in gallery

    (A) Schematic of the model of MoS2 for density functional theory (DFT) calculation. Brown and yellow dots represent molybdenum atoms and sulfur atoms, respectively. (B) Band structure of monolayer MoS2 with valence band maximum and the conduction band minimum at the K point of the MoS2 Brillouin zone.

  • View in gallery

    (A) Normalized transmission versus the position at the light peak intensity of 1.25 MW/cm2 and (B) the intensity-dependent nonlinear saturable absorption of single-crystalline monolayer molybdenum disulfide (MoS2) at 2 μm.

  • View in gallery

    (A) Reversed saturable absorption (RSA) phenomenon at the light peak intensity of 2.35 MW/cm2 (B) The results of closed-aperture (CA) Z-scan measurements. Lines: fitting curves; dots: normalized transmission data.

  • View in gallery

    An illustration of the passively Q-switched Tm,Ho:CLGA laser with large-size monolayer MoS2 as a saturable absorber.

  • View in gallery

    (A) The output power and (B) the pulse duration and the pulse repetition rate versus the absorbed pump power; (C) typical temporal pulse profile and (D) the stable pulse train at an absorbed pump power of 4.26 W.

  • View in gallery

    (A) The working spectrum of the single-crystalline monolayer molybdenum disulfide (MoS2) Q-switched Tm,Ho:CLGA laser and inset: spontaneous emission spectrum of Tm,Ho:CLGA crystal; (B) the beam quality M2 factor at the absorbed pump power of 4.26 W.