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Tunable Ultrafast Nonlinear Optical Properties of Graphene/MoS2 van der Waals Heterostructures and Their Application in Solid-State Bulk Lasers Xiaoli Sun,† Baitao Zhang,*,† Yanlu Li,*,† Xingyun Luo,† Guoru Li,† Yanxue Chen,†,‡ Chengqian Zhang,† and Jingliang He† †
State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China School of Physics, Shandong University, Jinan, Shandong 250100, China
‡
S Supporting Information *
ABSTRACT: For van der Waals (vdW) heterostructures, optical and electrical properties (e.g., saturable absorption and carrier dynamics) are strongly modulated by interlayer coupling, which may be due to effective charge transfer and band structure recombination. General theoretical studies have shown that the complementary properties of graphene and MoS2 enable the graphene/MoS2 (G/MoS2) heterostructure to be used as an important building block for various optoelectronic devices. Here, density functional theory was used to calculate the work function values of G/MoS2 with different thicknesses of MoS2, and its relaxation dynamic mechanism was illustrated. The results reveal that the G/MoS2 heterostructure interlayer coupling can be tuned by changing the thickness of MoS2, furthering the understanding of the fundamental charge-transfer mechanism in few-layer G/MoS2 heterostructures. The tunable carrier dynamics and saturable absorption were investigated by pump−probe spectroscopy and open-aperture Z-scan technique, respectively. In the experiments, we compared the performances of Q-switched lasers based on G/MoS2 heterostructures with different MoS2 layers. Taking advantage of ultrafast recovery time and good saturable absorption properties, a femtosecond solid-state laser at 1.0 μm with G/MoS2 heterostructure saturable absorber was successfully achieved. This study on interlayer coupling in G/MoS2 may allow various vdW heterostructures with controllable stacking to be fabricated and shows the promising applications of vdW heterostructures for ultrafast photonic devices. KEYWORDS: G/MoS2 heterostructure, nonlinear optical response, femtosecond solid-state bulk laser, saturable absorption, charge-transfer process ince the first discovery of graphene in 2004, 1 corresponding research has become one of the hottest topics in pure and applied physics, chemistry, and materials science.2,3 Due to Pauli blocking of electron states, graphene possesses saturable absorption properties and has been successfully used as a saturable absorber (SA) in the visible to mid-infrared region.4−6 Graphene has an intraband relaxation time of less than 150 fs and should be a fast SA.7 However, the limited light absorption of one atomic layer is an obstacle to graphene applications for high-modulation-depth SA. Thus, other alternative two-dimensional (2D) materials, such as black phosphorus (BP), topological insulators (TIs), MXene, and transition metal dichalcogenides (TMDs; e.g., MoS2, WS2, and MoSe2),8−10 have attracted attention. Black phosphorus has a high on/of ratio and hole mobility and emerged as a potential material in optoelectronic devices.
S
© XXXX American Chemical Society
However, it is unstable and easily oxidized in the ambient conditions.9 It is worth mentioning that TIs have a strong and broadband nonlinear optical response; regretfully, the extremely low saturable intensity of TIs on the order of W/cm2 makes it difficult for continuous-wave mode-locked (CWML) operation.11 Broad optical response and large effective nonlinear absorption coefficient of MXene have been investigated systematically.10 Nevertheless, a complicated preparation method limits its application at present. MoS2, a layered TMD with hexagons of covalently bonded Mo and S atoms, shows strong light−matter interactions because of Van Received: August 16, 2018 Accepted: October 17, 2018 Published: October 17, 2018 A
DOI: 10.1021/acsnano.8b06236 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Photograph of (a) G/1MoS2 (monolayer graphene covered by monolayer MoS2), G/4MoS2 (monolayer graphene covered by 4 layers of MoS2), and G/10MoS2 (monolayer graphene covered by 10 layers of MoS2) on a sapphire substrate. (b) G/10MoS2 on a flat mirror substrate with a high-reflection coating at 1000−1100 nm. Elemental analysis for G/MoS2 heterostructure: (c) SEM image of the mapping area, (d−f) EDX elementary mapping of C, S, and Mo in the G/MoS2 heterostructure.
Hove singularities in the density of states.12,13 Strong covalent bonding exists between the atoms within each layer, and predominantly weak van der Waals bonding exists between adjacent layers.13 The relaxation time of MoS2 is quite long (approximately 1.8−2.1 ps),14,15 which does not allow ultrashort pulses to be generated. The band gap of MoS2 is greater than 1.2 eV, limiting its application as a broadband SA unless defects are introduced.16 Heterostructures, which are assembled by stacking different layers of structured materials on top of each other, are an important tool for engineering the electronic and optical properties of semiconductors.17,18 In these vdW heterostructures, each material can maintain its individual properties because of the weak interactions between the layers. Recently, researchers have demonstrated that the advantages of broadband response and ultrafast carrier transport performance from graphene, and the strong light−matter interaction from MoS2 can be integrated in G/MoS2 heterostructures.19,20 High environmental stability, high modulation depth, broadband absorption, and opportune saturable intensity make G/MoS2 a promising SA (optical modulator) in nanosecond and ultrafast laser fields. As known, saturable absorption and carrier dynamic processes are crucial for SA to modulate intracavity loss and turn continuous-wave (CW) laser operation into pulses. These two characteristics are strongly affected by interaction coupling for heterostructures. Given the thickness sensitivity of MoS2,21,22 the charge distribution in each MoS2 layer and the layer-to-layer resistance must be considered when analyzing the interlayer coupling of graphene/MoS2. Furthermore, the valence band edge of MoS2 at the Γ point is the most sensitive to variations in thickness, resulting in the calculated change in the MoS2 Fermi level with the graphene orientation. Therefore, explicitly studying how the chargetransport mechanisms of G/MoS2 vary with the number of MoS2 layers is essential for assessing the potential of G/MoS2 heterostructures for different potential optoelectronic applications. In this work, we have observed that the carrier relaxation time and saturable absorption properties of G/MoS 2 heterostructures are tunable depending on the thickness of MoS 2 . Based on the theoretical analysis, a G/MoS 2
heterostructure with different thickness of MoS2 is constructed, and its work function and electron-transfer mechanism are characterized systematically. Using the G/MoS2 SA with different MoS2 layers in solid-state lasers, passively Q-switched lasers at 1.06 μm are demonstrated. To further study its ability to generate ultrafast pulses, its ultrafast photonics application in a femtosecond solid-state bulk laser was investigated successfully, and pulses as short as 236 fs were obtained at 1.03 μm with an output power of 0.55 W. Our work provides an insight for tailoring 2D vdW heterostructures so as to develop desired photonic applications.
RESULTS AND DISCUSSION Material Characterizations. To date, significant attempts have been made to fabricate 2D-layered materials by different routes such as mechanical cleavage,23 liquid-phase exfoliation (LPE), chemical vapor deposition (CVD),24 and pulsed laser deposition (PLD). Although the mechanical cleavage method is widely used for fundamental studies of 2D materials, its poor scalability and low yield make this method unsuitable for realistic large-scale applications. LPE is a simple but effective way to synthesize scalable samples; however, the lack of precise control of the compound ratio and consistency among the number of layers limit its application to high-quality material preparation. Compared with the other fabrication methods, CVD and PLD are more sophisticated methods used to produce scalable and uniform graphene or other 2D materials.25 In our work, monolayer graphene was grown on a sapphire or high-reflective mirror (K9) substrate using the CVD method first. Then, MoS2 layers are synthesized on a graphene surface by PLD because this method can control, to some extent, the film composition and structure.26 The MoS2 thickness was controlled by changing the growth kinetics via manipulating the repetition rate and pulse energy of the laser as well as the deposition pressure. In order to eliminate the effect of different defects of MoS2 on the nonlinear optical response of G/MoS2 heterostructures, three samples had a similar ratio of molybdenum (Mo) to sulfur (S). It was due to the two ions having different evaporation rates in the PLD technique.26,27 By changing the pulse number of the laser, G/ MoS2 heterostructures with monolayer graphene covered by B
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Figure 2. AFM image of a 25 × 25 μm2 area and typical height profiles for (a,b) G/1MoS2, (c,b) G/4MoS2, and (d,e) G/10MoS2. (g) TEM images of MoS2 nanoflakes (10 layers) on a graphene surface. (h) Typical HRTEM image taken from the center of a G/10MoS2 flake. The lattice spacing of 0.271 nm matches the (100) spacing of MoS2. (i) Raman spectra of the G/MoS2 heterostructures. (j) Magnified view of the 350−650 cm−1 region in the Raman spectra.
were ∼2, 4.5, and 8.5 nm. Based on the thickness of graphene (∼0.24 nm), MoS2 (∼0.7 nm),28 and the average interlayer distance (∼1.1 nm),29 the number of MoS2 layers was determined to be ∼1, 4, and 10. Figure 2g shows the transmission electron microscopy (TEM) image of the G/ MoS2 nanoflakes. The nanoflakes are stacked together. The inplane high-resolution transmission electron microscopy (HRTEM) image shown in Figure 2h shows a clear lattice fringe, and the lattice spacing was 2.71 Å, corresponding to the (100) planes of MoS2.30 The left part of the orange borderline is the G/MoS2 heterostructure, and the right one is pure graphene because the graphene was located at the bottom of heterostructure. Figure 2i shows a comparison of the Raman signals obtained from three G/MoS2 heterostructures and pure graphene by Raman spectroscopy with 532 nm laser excitation. The D peak is located at 1345 cm−1 due to the presence of structural disorders or defects in G/MoS2. These defects may relate to stress, strain, and lattice disorder induced by compression.31 We think that the G band intensity relates to the doping in graphene (p-type) to some extent.32 However, the main contribution to the G peak enhancement arises from
different layers of MoS2 were successfully fabricated. Figure 1a shows monolayer graphene covered by 1, 4, and 10 layers of MoS2 on a sapphire substrate with dimensions of 10 × 10 × 0.3 mm3. A high-reflection mirror at a wavelength of 1.0−1.1 μm was chosen as the substrate to prepare a G/10MoS2 saturable absorption mirror (SAM) for a mode-locked laser. As shown in Figure 1b, monolayer graphene is transferred within the red dashed box, and MoS2 is deposited on the left part of the mirror. Therefore, the G/10MoS2 heterostructure exists in the pink dashed box. Figure 1c shows the SEM image of G/ MoS2 heterostructure. To confirm the existence of MoS2 and graphene, we carried out element mapping of the nanocomposites. Considerable quantities of molybdenum and sulfur were detected along with carbon, implying the existence of MoS2 on the surface of graphene. Atomic force microscopy (AFM) was performed to characterize the morphology of the G/MoS2 samples and to determine the number of MoS2 layers. Figure 2a−f shows that the distributions of the samples are uniform, which is helpful for applications as an SA in solid-state bulk lasers. The crosssectional analysis indicated that the thicknesses of the flakes C
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Figure 3. C 1s core levels of (a) G/1MoS2, (b) G/4MoS2, and (c) G/10MoS2. (d) Mo 3d and S 2p spectra of the three samples. The measured EDS image of (e) G/1MoS2, (f) G/4MoS2, and (g) G/10MoS2.
Figure 4. Bandgap energies of (a) G/1MoS2, (b) G/4MoS2, and (c) G/10MoS2 derived from UV−vis-NIR transmission spectra. (d) Transient absorption dynamics of G/MoS2 heterostructure samples with a pump wavelength of 400 nm and probed at a wavelength of 532 nm. The dots are the measured data, and the curves are the data fits obtained using exponential functions.
bonding energies located at 284.7, 286.2, and 287.8 eV, respectively, as shown in Figures 3a−c. The peak located at 284.7 eV arises from sp2 bonding in graphene nanosheets.35 The emergence of a significant signal for the C−S bond clearly indicates that the MoS2 and graphene layers form vdW heterostructures with strong interaction coupling rather than simple stacking with vdW forces. The formation of C−S bonds also has an effect on the degree of defects in G/MoS2, which may cause a change in the band gap.16 The intensity of the C− S bonding energy increased as the number of MoS2 layers increased. Thus, when the number of MoS2 layers was controlled, the interaction coupling could be tuned. Figure 3d shows the well-defined spin-coupled Mo (3d5/2, 3d3/2, and S2s) and S (2p3/2 and 2p1/2) doublets at the same binding energies observed for MoS2.36 A small peak for Mo6+ is present at 235.8 eV due to slight oxidation of MoS2. An analysis of the Mo (3d) and S (2p) peak intensities (corrected with sensitivity factors based on Scofield cross sections) shows that the ratios (R) between S and Mo are 2.31:1, 2.35:1, and 2.37:1 as the MoS2 thickness increases, and this result is in good agreement with the values obtained from energy-dispersive X-ray spectroscopy (EDS), as shown as Figure 3e−g. The R value
electron−hole excitations at regions of strong interlayer coupling (i.e., near the Van Hove singularities).33 The position and intensity of the 2D peaks are nearly constant for the G/ MoS2 heterostructures with different layers. The intensity ratio of the G/2D peaks in pure graphene is ∼1.4, indicating the nature of monolayer graphene. To analyze the changes in the Raman signals associated with the heterostructures more clearly, the fine structures within 350−650 nm were examined and are shown in the inset of Figure 2j. According to previous reports, the gap between the in-plane (1E2g) and out-of-plane (A1g) vibration mode peaks (the two prominent Raman-active modes of MoS2) decreases in a stepwise manner as the number of MoS2 layers decreases. Furthermore, the peaks located at 491 and 635 cm−1 can be seen clearly, which may be due to the interaction coupling of graphene and MoS2.20,34 The crucial issue related to the heterostructure is whether chemical bonds (C−S bonds) formed at the G/MoS2 interface. Therefore, a surface analysis of the G/MoS2 heterostructures was necessarily carried out by using X-ray photoelectron spectroscopy (XPS) to further confirm C−S bonding and determine the chemical composition. The C 1s spectra exhibit three peaks after curve fitting, that is, C−C, C−O, and C−S D
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Figure 5. Typical open-aperture Z-scans and nonlinear transmittance curves under excitation by 15 ps pulses at 1064 nm (a) for pure monolayer graphene, 1MoS2, 4MoS2 and 10MoS2. (b) for G/1MoS2, G/4MoS2, and G/10MoS2.
are relatively fast compared to that of pure MoS2 (∼2.1 ps). The fast charge-transfer and relaxation channels in graphene speed up the carrier dynamics in G/MoS2.38,39 Moreover, the delay time decreases and the signal magnitude becomes stronger as the MoS2 thickness increases. The results imply that the ultrafast recovery time is variable by changing the MoS2 thickness, which can be attributed to the stronger and faster charge-transfer effect in the G/MoS2 heterostructure. Nonlinear Optical Properties. The open-aperture Z-scan technique is used to determine the nonlinear optical (NLO) response of G/MoS2 heterostructures and their relationship to the thickness. A mode-locked Yb fiber laser (wavelength = 1064 nm, pulse width = 15 ps, repetition rate = 600 kHz) was selected as the pump source. Figure S2 shows the measurement setup. The open-aperture Z-scan measurement results for pure monolayer graphene, 1MoS2, 4MoS2, 10MoS2, G/1MoS2, G/4MoS2, and G/10MoS2 are shown in Figure 5. Based on the analysis model of a two-level SA, it shows that the heterostructure has a stronger nonlinear optical response, indicating an enhanced light−matter interaction compared to that of pure graphene and MoS2. Using eqs S3 and S4, the modulation depth, saturable intensity, and nonsaturable loss were fitted and are listed in Table 1. A G/MoS 2
is fairly constant as the number of MoS2 layers increases because the defects can be controlled using the PLD technique. A U-3500 Hitachi UV/vis/NIR spectrophotometer was used to measure the transmission spectra of the pure MoS2 and G/ MoS2 heterostructures to study their linear optical properties. Considering Fresnel reflection loss of about 15% on both sides of the sapphire substrate, the optical transmission spectrum of pure MoS2 and G/MoS2 film was plotted in Figure S1. For pure MoS2, small absorption peaks are located around the 1215 nm wavelength, corresponding to a band gap of 1.02 eV. The 1.02 eV of band gap is smaller than the theoretical value (1.29 eV) of MoS2. It is mainly due to introduction of defects.16 Wang et al. have reported that the band gap of MoS2 can be reduced from 1.08 to 0.08 eV by the introduction of defects.16 After the van der Waals G/MoS2 heterostructure is formed, the band gap of materials would be widely changed because of the C−S chemical bond formation. The advantage of broadband absorption from graphene also can be assembled into the G/MoS2 heterostructure, and the orientation of graphene with respect to MoS2 strongly influences the value of the electronic band gap in MoS2.37 With Tauc’s semiempirical equation (eq S1), the relationship between the optical absorption coefficient (α) and the photon energy (hν) was identified in Figure 4a−c. Extrapolating the linear portion to the x abscissa intercept shows that the band gaps of the three samples are reduced to ∼0.29 (G/1MoS2), ∼0.25 (G/4MoS2), and ∼0.2 eV (G/10MoS2). Considering the semiconducting saturable absorption properties, light with photon energy larger than the band gap energy can be absorbed by SA; thus, the G/ MoS2 heterostructure can be used as a broadband SA. Carrier Dynamics. For photoelectronic applications, the carrier dynamic process of SA plays a crucial role in determining the duration of a pulse generated by a passively mode-locked laser. Figure 4d shows the time-dependent change, ΔT, in the normalized transmissivity through the pump−probe technique. After photoexcitation, the transmissivity sharply increased. The carrier dynamic relaxation process can be described by two distinct time scales: fast relaxation times (τ1) related to carrier−carrier intraband scattering rates and slower relaxation times (τ2) attributed to carrier−phonon intraband scattering. A biexponential decay formula is described in the Supporting Information. Fitting the pump−probe data with eq S2 shows that the intraband relaxation time (τ1) of the heterostructures is modulated, τ1 = 1.8, 1.1, and 0.6 ps for the G/1MoS2, G/4MoS2, and G/ 10MoS2 heterostructures, respectively. These relaxation times
Table 1. NLO Parameters Obtained by Theoretically Fitting the Experimental Data for Pure Graphene, MoS2, and G/ MoS2 samples monolayer graphene 1MoS2 4MoS2 10MoS2 G/1MoS2 G/4MoS2 G/10MoS2
modulation depth (%)
saturable intensity (MW/cm2)
nonsaturable loss (%)
4
470
2.6
6 8.5 12.7 8.8 13.4 16.7
410 373 160 350 196 72
5.2 7.3 9.1 8.3 12.3 13.1
heterostructure possesses a low saturable intensity and large modulation depth compared to that of pure graphene and MoS2, which may due to strong charge transfer. Furthermore, the saturable intensity is 350, 196, and 72 MW/cm2 for the G/ 1MoS2, G/4MoS2, and G/10MoS2 SAs, respectively, as shown in Figure 5b. The corresponding modulation depth was 8.8, 13.4, and 16.7%. The G/MoS2 heterostructure has a great increasing/decreasing of the modulation depth/saturation intensity with the thickness of MoS2 on graphene increases. E
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Figure 6. Calculated work functions of (a) G/1MoS2, (b) G/2MoS2, (c) G/3MoS2, and (d) G/4MoS2. (e,f) Diagram of the charge-transfer process in a G/MoS2 heterostructure.
Figure 7. (a,b) Recorded pulses trains of pure monolayer graphene, monolayer MoS2, and G/MoS2-based Q-switched lasers on the time scale of 5 μs/div.
graphene shows p-type character with a small band gap. Meanwhile, the Fermi level of the G/MoS2 heterostructure is close to the conduction band minimum of MoS2, leading to the n-type character of MoS2. Due to charge redistribution, the potential of graphene increases and its bands move down, while the potential of MoS2 decreases and its bands move up, causing the work function of G/MoS2 heterostructures to fall between those of graphene and MoS2. It illustrates the strong interlayer interaction between graphene and MoS2. Electron transfer from graphene to MoS2 leads to the accumulation of negative charge in MoS2 and positive charge in graphene, forming an internal electric field from graphene to MoS2. Under the internal electric field, charge separation occurs by injecting photogenerated electrons from the conduction band (CB) of MoS2 to graphene, while photogenerated holes in the valence band (VB) of MoS2 cannot diffuse to graphene due to the large Schottky barrier. The calculated work functions of the G/MoS2 heterostructures exhibit a strong dependence on the MoS2 thickness. As shown in Figure 6a−d, the calculated work functions of G/ 1MoS2, G/2MoS2, G/3MoS2, and G/4MoS2 are 4.63, 4.74, 4.84, and 5.00 eV, respectively, and these values increase as the thickness increases. Such a dependent relationship has not been observed for the work function of pure MoS2 and demonstrates that charge transfer between graphene and MoS2 will be enhanced by increasing the MoS2 thickness. Therefore, the strong interlayer coupling of a G/MoS2 heterostructure
This variation of nonlinear optical properties is mainly attributed to the enhanced interlayer coupling.29 All results offer a meaningful strategy to tune the NLO response of heterostructure materials by controlling the thickness. Theoretical Analysis. In order to gain deeper insight into the electron-transfer mechanism between graphene and MoS2 and variations in the thickness-dependent interlayer coupling in G/MoS2 heterostructures, we constructed crystal models of monolayer graphene, MoS2 with 1−4 layers, and G/MoS2 heterostructures to investigate the interface charge transfer and band alignment by theoretically calculating the work function. Using density functional theory (DFT), we obtained a work function value of WG = 4.25 eV for monolayer graphene, which is close to the commonly measured values in the 4.3−4.6 eV range,40 and work functions of WMoS2 = 5.07−5.11 eV (depending on the thickness), which are in agreement with the values reported in the literature.28 The formation of a G/ MoS2 heterostructure changes the electric and optical characteristics of graphene and MoS2 due to interface interactions and charge redistribution. Figure 6e,f illustrates the charge-transfer process after graphene and MoS2 come into contact. Because graphene has a work function lower than that of MoS2, the electrons in graphene can partially transfer to MoS2 when the two are in contact to form a heterostructure. The Fermi level of the resultant G/MoS2 heterostructure can thus move below the Dirac point of graphene (≈0.38−0.75 eV depending on the thickness of MoS2 in our calculation), and F
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decreasing pulse widths were related to the reduced saturable intensity and increased modulation depth that occur as the thickness of MoS2 in G/MoS2 increases, and these results were consistent with the open Z-scan results. We observed no optical damage or degradation of the laser performance during the experiments. To characterize the ultrafast saturable absorption of G/ MoS2, a plane mirror with a high-reflectance coating at 1000− 1100 nm was used as the substrate for the G/10MoS2 SAM (the SAM photograph is shown in Figure 1b). A schematic of the mode-locked laser with the G/MoS2 heterostructure-based SA is shown in Figure 8.
would be further enhanced as the thickness of MoS2 on graphene increases. As shown in previous reports, strong interlayer coupling will lead to a great increasing/decreasing of the modulation depth/saturation intensity in the vdW heterostructure,29 which coincided with the open-aperture Zscan measurements very well. When a G/MoS2 heterostructure is irradiated by light with a particular wavelength, more electrons in the VB of MoS2 can be excited to the CB of MoS2 as the MoS2 thickness increases. After photogenerated electrons are excited into the CB, they relax in MoS2 and then transfer to graphene, and such transfer process could serve as a fast decay channel for photogenerated carrier recombination in MoS2. A stronger internal electric field at the G/MoS2 heterostructure interface will accelerate the transfer process of photogenerated carriers. Thus, the delay time will be shorter with enhanced interface coupling of graphene with thicker MoS2, which agrees with the pump−probe experimental results in Figure 4d. Pulsed Laser Applications. The Z-scan data indicated the variation of the saturable absorption of G/MoS2 as a function of the MoS2 thickness, which is significant for investigating pulsed laser performance based on a G/MoS2 SA. A compact 2.8 cm two-mirror cavity was used to achieve a passively Qswitched solid-state laser with Nd:GdVO4 as the gain medium. The input mirror was a plane-concave mirror with a radius of 200 mm, and a flat mirror with a transmission of 8% was employed as the output coupler (OC), as shown in Figure S3. Using pure monolayer graphene, monolayer MoS2, and G/ 1MoS2 as the SA, passively Q-switched lasers were obtained, as shown in Figure 7a. The pulse trains were recorded by a digital oscilloscope (Tektronics DPO 7104) and an InGaAs fast photodetector (New focus 1611). When the absorbed pump power increased to 1.8 W, the pulse trains became unstable. The pulse train of the Q-switched laser based on pure monolayer graphene was unstable, and its fluctuations were larger because of the low modulation depth and limited optical absorption coefficient. The detailed output parameters listed in Table 2. The highest repetition rate with pure monolayer
Figure 8. Schematic of the mode-locked Yb:KYW laser setup based on G/MoS2 heterostructures.
A CW operation was first investigated using a highreflectance flat mirror to replace the G/MoS2 SAM. As shown in Figure 9a, the laser threshold pump power was 3.15 W, and the slope efficiency was approximately 17.1%. A maximum average output power of 0.79 W was obtained at an absorbed pump power of 9.14 W. During the experiment, no signs of Kerr lensing mode-locked (self-mode-locking) phenomenon were observed. With G/MoS2 SA employed in the resonator, a stable CWML operation was achieved. Figure 9a shows the relationship between the absorbed pump power and the average output power of the mode-locked laser with the G/MoS2 SA. Figure 9b illustrates the QML operation at an absorbed pump power of 6.2 W. A QML pulse envelope was obtained within a 700 ns long Q-switched envelope under a repetition rate of 312 kHz. When the absorbed pump power reached 6.98 W, a stable CWML operation was achieved. The CWML regime can be sustained while the absorbed pump power increased to 8.8 W, corresponding to a slope efficiency of 10.2%. At this point, the maximum average output power of the CWML operation was 0.55 W, which was the maximum output power from the G/MoS2-based mode-locked lasers. The stability at the maximum average output power was measured for 3 h with a fluctuation of 2%. We also recorded the CWML pulse trains at 8.8 W absorbed pump power with time spans of 50 ns and 1 ms, as plotted in Figure 9c, and the results demonstrated good amplitude stability with a pulse repetition rate of 41.84 MHz, which matched the cavity roundtrip time. The maximum pulse energy was calculated to be 19 nJ. Using a noncollinear autocorrelator (APE, Pulse Check 150), the pulse duration was measured to be ∼236 fs, assuming
Table 2. Results of Passively Q-Switched Lasers Based on Different Samples at 1.06 μm samples monolayer graphene monolayer MoS2 G/1MoS2 G/4MoS2 G/10MoS2
repetition rate (kHz)
pulse width (ns)
output power (mW)
224
871
84
130 161 194 398
1543 1051 434 117
65 77 70 56
graphene, monolayer MoS2, and G/1MoS2 was 224, 130, and 161 kHz, respectively. The shortest pulse duration was 871, 1543, and 1051 ns, respectively. Figure 7a and Table 2 show that the Q-switched laser based on the G/1MoS2 heterostructure has a better performance with relatively stable pulse trains and higher repetition rates compared with those of the lasers based on pure monolayer graphene and MoS2. Then, we compared the performance of lasers with different thicknesses of G/MoS2. Figure 7b shows the pulse trains of the Q-switched operation based on G/1MoS2, G/4MoS2, and G/10MoS2, corresponding to highest repetition rates of 161, 194, and 398 kHz, respectively. The shortest pulse width was 1051, 434, and 117 ns, respectively. The increasing repetition rates and G
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Figure 9. (a) Average output power of CW and CWML versus absorbed pump power. (b) QML pulse trains recorded at 6.2 W. (c) CWML pulse trains recorded at the maximum pump power. (d) Autocorrelation race for 236 fs duration. Inset: Corresponding spectrum centered at 1037.2 nm. (e) Recorded frequency spectrum of the mode-locked laser with an RBW of 50 Hz. Inset: 800 MHz wide-span spectrum.
a Sech2 pulse shape, as shown in Figure 9d. This is the shortest pulse width generated from a mode-locked bulk laser with a G/ MoS2 SA thus far. Considering the ultrafast recovery time of G/MoS2 is 0.6 ps, which is shorter than that of MoS2, a much shorter pulse can be generated in a G/MoS2 SA-based modelocked laser than that in the MoS2 case. The corresponding laser spectrum centered at 1037.2 nm is shown in the inset of Figure 9d. The fwhm of 5 nm results in a time bandwidth product of 0.329, which is somewhat longer than the Fourier transform-limited value (0.315) for Sech2-shaped pulses. Table 3 lists the femtosecond pulsed laser generation performance of
bandwidth). No spurious modulation was observed, demonstrating clean CWML operation of a G/MoS2 femtosecond laser was obtained.
CONCLUSION In G/MoS2 heterostructures, the covalent donor−acceptor structure plays a key role in saturable absorption mechanisms. The effect of MoS2 thickness on the nonlinear optical response of G/MoS 2 has been demonstrated theoretically and experimentally. The work functions of G/MoS2 heterostructures with 1−4 layers of MoS2 range from 4.63 to 5.00 eV, as calculated by DFT. Moreover, as the number of MoS2 layers increase, the saturable intensity of G/MoS2 decreases from 350 to 72 GW/cm2, and the intraband relaxation time changes from 1.8 to 0.6 ps. Knowledge of the dependence of the interlayer coupling, nonlinear optical response, and carrier dynamics on the number of layers is essential for tuning G/ MoS2 SAs. In combination with the ultrafast carrier recovery time, G/MoS2 heterostructures exhibit excellent saturable absorption properties in terms of a low saturable absorption intensity and broadband optical response. Nanosecond and femtosecond pulsed lasers based on G/MoS2 were achieved. Pulses as short as 236 fs and an average output power of 0.55 W are the best results ever achieved with a heterostructurebased mode-locked laser. Our work may be helpful for future vdW heterostructure-based device fabrication.
Table 3. Results of Femtosecond Pulsed Laser Performance Based on Different 2D Materials materials
pulse duration (fs)
output power (mW)
ref
BP WS2 Ti2C3 graphene CWNT G/MoS2
277 736 316 30 168 236
820 270 770 26.2 47 550
41 42 43 44 45 this work
different 2D materials. Compared with other 2D materials based SA used in the mode-locked lasers, the pulse width is relatively short and the output power is relatively high as obtained by G/MoS2 SA in our work. The fundamental repetition rate was 41.84 MHz, as measured by a spectrum analyzer (Agilent N9000A). A clean and stable single-pulse femtosecond laser was observed without spurious frequency components or modulations over the entire 0.8 GHz span, as shown as Figure 9e. Moreover, the measured extinction ratio was well above 65 dB (with a 50 Hz resolution
METHODS Synthesis of G/MoS2 Heterostructures. A standard CVD process was used to grow graphene on 1 mm thick copper foils (Nanjing 2DNANO Tech. Co., Ltd.). The graphene film was then wet-transferred onto a sapphire substrate and K9 mirror to grow MoS2 nanoplatelets on top of the film. The MoS2 nanoplatelets were H
DOI: 10.1021/acsnano.8b06236 ACS Nano XXXX, XXX, XXX−XXX
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ACS Nano prepared by a PLD technique. The MoS2 target was synthesized by cold pressing MoS2 powder into a 40 mm diameter pellet at approximately 70 MPa. A complex Pro 201 KrF excimer laser (provided by Coherent Inc.) was operated at a wavelength of 248 nm and pulse width of 20 ns to provide radiation and ablation on the target in a spot with an area of 5 mm2. During the deposition process, energy of 600 mJ and a repetition rate of 5 Hz were employed. The base pressure of the vacuum chamber system was 8.9 × 10−5 Pa, and the pressure increased to approximately 5 × 10−4 Pa during the deposition. The target and graphene substrate were both rotated to enhance the uniformity of the film. Computational Details. All the calculations were carried out based on DFT implemented in the Vienna ab initio simulation (VASP) package.46 The Perdew Burke Ernzerhof (PBE) form of generalized gradient approximation (GGA) for the exchangecorrelation term was used with the projector-augmented wave (PAW) method.47,48 A plane wave cutoff energy of 400 eV was set with forces converging to 0.01 eV/Å. K-points were sampled by 10 × 10 × 1 for the relaxation and total energy calculations.49 The optimized lattice parameters are a = b = 2.46 Å for graphene, a = b = 3.18 Å and c = 39.12 Å for bulk MoS2, and a = b = 3.18 Å for monolayer MoS2, which are in line with experimental observations.50,51 A series of models, including graphene, nMoS2 (n = 1−4), and G/nMoS2 (n = 1−4), were constructed using a supercell method. A vacuum slab of 15 Å was added to each model to prevent periodic interaction. The G/nMoS2 heterostructure models were constructed using 3 × 3 MoS2 supercells and 4 × 4 graphene supercells to obtain the best lattice structure matching. Femtosecond Laser Based on G/10MoS2 SA. A Yb3+ iondoped (3 atom %) Yb:KYW crystal with dimensions of 3 × 3 × 4 mm3 was used as the gain medium and was wrapped with indium foil, mounted in a copper heatsink, and cooled by running water at a temperature of 18 °C. A 25 W fiber-coupled laser diode (core diameter 105 μm, numerical aperture 0.22) emitting at 976 nm was used as the pump source. The pump beams from the fiber output were collimated and focused into the laser crystal to a spot with a diameter of approximately 58 μm, which was close to the diameter of the TEM00 cavity mode (55 μm) calculated by ABCD propagation matrix theory. The laser mode size focused on the G/MoS2 SA was 78 μm in diameter. A plane OC with a transmittance of 1% for the spectral range of 1000−1100 nm was used. To reduce the intracavity loss and to compensate for the normal group delay dispersion (GDD) introduced by the gain medium and G/MoS2 SA, we employed a plane Gires-Tournois interferometer mirror with a GDD of −1000 fs2 per round.
ACKNOWLEDGMENTS This research was supported by the Natural Science Foundation of China (NSFC) (Grant Nos. 61675116, 61575110, and 51502158), the Key Research and Development Program of Shandong Province (2017GGX20134, 2017CXCC0808), and Young Scholars Program of Shandong University (Grant No. 2017WLJH48). REFERENCES (1) Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The Structure of Suspended Graphene Sheets. Nature 2007, 446, 60−63. (2) Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Experimental Observation of the Quantum Hall Effect and Berry’s Phase in Graphene. Nature 2005, 438, 201−204. (3) Ponraj, J. S.; Xu, Z. Q.; Dhanabalan, S. C.; Mu, H. R.; Wang, Y. S.; Yuan, J.; Li, P. F.; Thakur, S.; Ashrafi, M.; Mccoubrey, K.; Zhang, Y. P.; Li, S. J.; Zhang, H.; Bao, Q. L. Photonics and Optoelectronics of Two-Dimensional Materials Beyond Graphene. Nanotechnology 2016, 27, 462001. (4) Song, Y. F.; Zhang, H.; Tang, D. Y.; Shen, Y. Polarization Rotation Vector Solitons in a Graphene Mode-Locked Fiber Laser. Opt. Express 2012, 20, 27283. (5) Yu, H. H.; Chen, X. F.; Zhang, H. J.; Xu, X. G.; Hu, X. B.; Wang, Z. P.; Wang, J. Y.; Zhuang, S. D.; Jiang, M. H. Large Energy Pulse Generation Modulated by Graphene Epitaxially Grown on Silicon Carbide. ACS Nano 2010, 4, 7582−7586. (6) Wang, Z. T.; Chen, Y.; Zhao, C. J.; Zhang, H.; Wen, S. C. Switchable Dual-Wavelength Synchronously Q-Switched ErbiumDoped Fiber Laser Based on Graphene Saturable Absorber. IEEE Photonics J. 2012, 4, 869. (7) Sun, D.; Wu, K. Z.; Divin, C.; Li, X. B.; Berger, C.; de Heer, W. A.; First, P. N.; Norris, T. B. Ultrafast Relaxation of Excited Dirac Fermions in Epitaxial Graphene Using Optical Differential Transmission Spectroscopy. Phys. Rev. Lett. 2008, 101, 157402. (8) Chen, B.; Zhang, X.; Wu, K.; Wang, H.; Wang, J.; Chen, J. QSwitched Fiber Laser Based on Transition Metal Dichalcogenides MoS2, MoSe2, WS2 and WSe2. Opt. Express 2015, 23, 26723. (9) Chen, Y.; Jiang, G. B.; Chen, S. Q.; Guo, Z. N.; Yu, X. F.; Zhao, C. J.; Zhang, H.; Bao, Q. L.; Wen, S. C.; Tang, D. Y.; Fan, D. Y. Mechanically Exfoliated Black Phosphorus as a New Saturable Absorber for Both Q-Switching and Mode-Locking Laser Operation. Opt. Express 2015, 23, 12823. (10) Jiang, X. T.; Liu, S. X.; Liang, W. Y.; Luo, S. J.; He, Z. L.; Ge, Y. Q.; Wang, H. D.; Cao, R.; Zhang, F.; Wen, Q.; Li, J. Q.; Bao, Q. L.; Fan, D. Y.; Zhang, H. Broadband Nonlinear Photonics in Few-Layer MXene Ti3C2Tx (T = F, O, or OH). Laser Photonics. Rev. 2018, 12, 1700229. (11) Wang, Y. R.; Lee, P.; Zhang, B. T.; Sang, Y. H.; He, J. L.; Liu, H.; Lee, C. K. Optical Nonlinearity Engineering of Bismuth Telluride Saturable Absorber and Application of Pulsed Solid State Laser Therein. Nanoscale 2017, 9, 19100. (12) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497−501. (13) Britnell, L. R.; Ribeiro, M.; Eckmann, A.; Jalil, R.; Belle, B. D.; Mishchenko, A.; Kim, Y. J.; Gorbachev, R. V.; Georgiou, T.; Morozov, S. V.; Grigorenko, A. N.; Geim, A. K.; Casiraghi, C.; Neto, A. H. C.; Novoselov, K. S. Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films. Science 2013, 340, 1311−1314. (14) Wang, Y. W.; Huang, G. H.; Mu, H. R.; Lin, S. H.; Chen, J. Z.; Xiao, X.; Bao, Q. L.; He, J. Ultrafast Recovery Time and Broadband Saturable Absorption Properties of Black Phosphorus Suspension. Appl. Phys. Lett. 2015, 107, 091905. (15) Wang, Q.; Ge, S.; Li, X.; Qiu, J.; Ji, Y.; Feng, J.; Sun, D. Valley Carrier Dynamics in Monolayer Molybdenum Disulfide from Helicity-Resolved Ultrafast Pump-Probe Spectroscopy. ACS Nano 2013, 7, 11087−11093.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b06236. Additional details about the nonlinear optical measurements; work function calculation details and the Qswitched laser experimental setup (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Xiaoli Sun: 0000-0001-9320-4173 Yanlu Li: 0000-0002-1983-0368 Notes
The authors declare no competing financial interest. I
DOI: 10.1021/acsnano.8b06236 ACS Nano XXXX, XXX, XXX−XXX
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