Thickness-Dependent Ultrafast Photonics of SnS2 Nanolayers for

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Thickness-Dependent Ultrafast Photonics of SnS Nanolayers for Optimizing Fiber Lasers 2

Wenjun Liu, mengli liu, Xiaoting Wang, Shen Tao, Guoqing Chang, Ming Lei, Hui-Xiong Deng, Zhongming Wei, and Zhi-Yi Wei ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00190 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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ACS Applied Nano Materials

Thickness-Dependent Ultrafast Photonics of SnS2 Nanolayers for Optimizing Fiber Lasers Wenjun Liu1,2, Mengli Liu1, Xiaoting Wang1,3, Tao Shen3, Guoqing Chang2, Ming Lei1,*, Huixiong Deng3,*, Zhongming Wei3, and Zhiyi Wei2,*

1

State Key Laboratory of Information Photonics and Optical Communications, School of

Science, P. O. Box 91, Beijing University of Posts and Telecommunications, Beijing 100876, China 2

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,

Chinese Academy of Sciences, Beijing 100190, China 3

State

Key

Laboratory

of

Superlattices

and

Microstructures,

Institute

of

Semiconductors, Chinese Academy of Sciences & Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100083, China

KEYWORDS: nonlinear optical materials, ultra-fast photonic device, fiber laser, modelocked laser, transition metal dichalcogenides, chemical vapor transport

ACS Paragon Plus Environment

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ABSTRACT: Transition metal dichalcogenides (TMDCs) with different thickness can greatly influence the performance of photonic devices. However, how to accurately control the layers of TMDCs and realize the application of TMDCs in ultrafast photonics remains challenging. Here, we study the thickness dependence of ultrafast photonics of SnS2, which is one of newly emerging TMDCs. The SnS2 crystals are synthesized by the chemical vapor transport technique, and are confirmed as the n-type material by firstprinciples calculations. As a potential application, SnS2 samples with three different thickness are successfully applied in fiber lasers. The importance and effectiveness of thickness dependence of SnS2 are demonstrated by different laser performance. Results indicate that SnS2 has excellent optical properties with controllable thickness, and may be beneficial for the applications of electronics, optics and sensors.

INTRODUCTION Since the first isolation of graphene from bulk crystals by mechanical exfoliation, twodimensional (2D) materials have become the research hotspot in the world [1]. As an outstanding material, graphene not only provides access to obtain new physics in reduced dimensions [2-4], but also exhibits unique mechanical, optical and sensing performances [5-8]. However, the lack of a sizable bandgap restricts its application in electricity and optics [9]. Although the engineering bandgap can optimize this drawback to a certain extent, the resulting fabrication complexity and additional costs make them unsuitable for mass production [10]. In order to look for alternatives with better performance, more and more materials are gradually coming into view [11-21].


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Transition metal dichalcogenides (TMDCs), which are comprised of chemically inert layers with internal covalent bonding held by weak van der Waals (vdW) interactions, represent a rising class of 2D materials alternative for graphene [22-24]. The layer-dependent structural, electronic, and vibrational properties of TMDCs tend to endow them with unique properties, such as second/third harmonic generations [25-26], high carrier mobility [16] and ultrafast nonlinear optical absorption [28-30]. Up to now, some common TMDCs, such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), have attracted extensive interest due to their diverse electronic, optical, and thermal properties [31-36]. Their high charge carrier mobility and on-off current ratios make them advantageous in field-effect transistors and sensitive photodetectors [37-38]; their high nonlinearity enables them to act as excellent optical modulators and facilitate the generation of ultrashort pulses [39-46]. Moreover, the bandgap of MoS2 can be modulated by defects, thus breaking through theoretical application bandwidth and realizing broadband applications [47]. In recent years, researchers are starting to focus on the newly emerging TMDCs, such as SnSe2, ReS2 and SnS2 [48-50]. The study of their applications on optics is still in its infancy. Among them, SnS2 is a potential emerging TMDCs, which has aroused extensive research upsurge [51-52]. Generally speaking, the absence of a sizable reserves in raw material will limit widespread use in devices. Tin and sulfur are earth-abundant elements. Moreover, SnS2 is of high chemical stability, environmentally sound and superior photoelectric properties. But different from MoS2 whose bandgap type change with thickness variation, SnS2 always maintains indirect bandgap over the entire thickness range from bulk to single-layer. The bandgap value of SnS2 changes between 2.033 eV and 2.4 eV, and this wider bandgap plays a crucial role in some electronic applications [53]. Moreover, according to previous studies, SnS2 exhibits absorption characteristics at 1550 nm, which indicates that SnS2 is expected to have broadband absorption properties similar to MoS2 [47]. In this paper, the thickness dependence of optical properties of SnS2 is investigated in fiber lasers. The intrinsic defect properties of SnS2 crystals are investigated by first-principle calculations, and the Fermi energy in n-type SnS2 bulk is studied under the growing environment close to the experiment. We prepare SnS2 samples with three different thickness via the chemical vapor transport (CVT) technique. Under the same fabrication process and technology, SnS2 with


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different thickness exhibits great difference in nonlinear absorption characteristics, specifically in the modulation depth. The difference in the nonlinear optical response of SnS2 saturable absorber (SA) also further affects the performance of the corresponding fiber lasers. After successfully applying three SnS2 samples to mode-locked erbium-doped fiber (EDF) lasers, we find that SnS2 with a thickness of 108.7 nm has the best performance in achieving ultrashort pulses. The shortest pulse duration is 132.9 fs. This thickness dependence reaction can not only fully reflect the nonlinear properties of the SnS2 SA, but also play a pioneering role in obtaining ultrashort pulses.

RESULT AND DISCUSSION Characterization of SnS2 SA. As shown in Figure 1, the SnS2 samples were transferred to the fiber end facet by the mechanical transfer method (more details are described in the METHODS). The detailed crystal structures of SnS2 crystals were characterized by X-ray diffraction (XRD) patterns in Figure 2(a), and can be well indexed to the hexagonal unit cell (JCPDS No. 23-0677) without other impurity phases. Figure 2(b) displays the high resolution TEM (HRTEM) image and corresponding selected-area electron diffraction (SAED) pattern of the exfoliated SnS2 nanosheets. The lattice spacing of 3.18 Å corresponds to (100) or (010) plane with the surface oriented along the [001] crystal axis, identical to the [001] orientation observed by XRD. The Raman spectrum of SnS2 is shown in Figure 2(c) with characteristic A1g mode at 314.5 cm-1, as with previous reports [54-56]. The chemical composition was further investigated by X-ray photoelectron spectroscopy (XPS). The Sn peaks (3d5/2 at 485.88 eV, 3d3/2 at 494.30 eV) and S peaks (2p3/2 at 161.83.11 eV, 2p1/2 at 163.02 eV) have been presented in Figure 2(d), and the ratio of Sn to S is 0.96:2.04. These results confirm the high-quality crystalline structure of the assynthesized SnS2. Saturable absorption characteristics of SnS2 SA. The spectral absorbance of as-prepared SnS2 sample was measured from 250 nm to 2000 nm by Jasco MSV-5200 microscopic spectrophotometer, as in Figure 2(e), confirming the existence of light absorbance in the whole spectral range. The exfoliated 2D SnS2 nanosheets commonly have sizes over 10 μm, and can be easily transferred onto the end-facet of optical fiber as SAs (details in Methods) for further measurements. The thicknesses of the SnS2 samples on fiber ends were measured by atomic


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force microscopy (AFM), as shown in Figure 2(f), respectively with values of 2.8, 9.1 and 108.7nm (the thickness of monolayer SnS2 is about 0.6 nm). With the balanced twin-detector method, the nonlinear absorption of SnS2 SAs with three different thickness are investigated in Figure 3. The pump source is a homemade ultrafast fiber laser (pulse duration is 600 fs, and the repetition rate is 120 MHz) operating at 1.55 μm. The experimental data are well fitted via the simplified two-level saturable absorption model,  (I ) 

s 1  I / I sat

  ns

,

the saturable absorption parameter αs, nonsaturable absorption parameter αns and saturation intensity Isat are listed in Figure 3. The modulation depth corresponding to SnS2 with the thickness of 108.7 nm, 9.1 nm and 2.8 nm are 40.5%, 36.5% and 31.4%, respectively. When the thickness changes, the different effects of light-matter interaction and relaxation time on the modulation depth indicate that there must be a balance point among them. That is to say, the suitable thickness is beneficial to the large modulation depth. For the SnS2 SA with the thickness of 108.7 nm, the calculated optical to optical efficiency of this SA is 85.545%. Application of SnS2 SA in lasers. The set-up diagram of adopted ring laser cavity is demonstrated in Figure 4. In order to verify the effect of three different SAs on laser performance, the laser cavity maintains the same configuration whether it's length or device type. The laser diode (LD) operating at 980 nm with the maximum output power of 680 mW serves as the pump source. Through 980/1550 wavelength-division multiplexer (WDM), the pump light is delivered into the cavity. The EDF (0.5 m) with a dispersion parameter of 12 fs2/mm is used as the gain medium. The polarization controller (PC), as an intra-cavity device mainly used to adjust the polarization state of light, is able to optimize the operation state of lasers. Isolator (ISO) is able to block the transmission of the backward light, and maintain the unidirectional and orderliness of light. As the only light outlet, the 20:80 optical coupler (OC) provides convenience for monitoring and measuring the operation state of lasers. Thickness dependent optical properties of SnS2. When the SnS2 SA with a thickness of 108.7 nm is coupled into the cavity as an optical modulator, the mode-locked waveform output is obtained by properly increasing the pump power and fine-tuning the PC. The start threshold of the mode-locked regime is recorded as 200 mW. When the power continues to increase to the pumping limit, the laser keeps stable pulse trains as shown in Figure 5(a). Multi time spectrum


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monitoring maps are provided in Figure 5(b). The mode-locked regime operates at 1561 nm with 3 dB spectrum of 28 nm. From the measured autocorrelation trace shown in Figure 5(c), the pulse duration is considered to be 132.9 fs. Thus, the time-bandwidth product (TBP) is calculated to be 0.514 (the theoretical limit value is 0.315), we attributed the disparity of TBP to the slight chirp of optical pulses. With the resolution of 3 Hz and the span of 70 kHz, the fundamental repetition rate of realized mode-locked regime locates at 79.1 MHz as shown in Figure 5(d). The signal-to-noise ratio (SNR) is up to 61 dB. Moreover, the variation tendency of the RF spectrum with a wide bandwidth of 1000 MHz is uniform. All of these indicate the stability of the laser system. The maximum average output power is 24.8 mW, and the maximum pulse energy is 0.35 nJ. The damage threshold of the prepared SnS2 SA with a thickness of 108.7 nm is 127 mJ/cm2. As shown in Figure 6, we have continuously monitored the output power of mode-locked lasers for up to 13 hours. After measurement, the standard deviation of average output power is 0.603, which indicates that the operation is stable. Subsequently, the primary SA is replaced by the SnS2 with a thickness of 9.1 nm. On the premise of maintaining the cavity length and intracavity devices, we also succeeded in achieving mode-locking pulses. As shown in Figure 7(a), the fundamental repetition rate of 79.066 MHz indicates the length of the cavity is almost the same. However, the pulse duration of 149.5 fs in Figure 7(b) is a little wider than the previous one we just mentioned. It seems that there is some connection between thickness and pulse duration, but the evidence is not enough. Therefore, we continue to reduce the thickness of SnS2, trying to find the change regulation. The SnS2 with a thickness of 2.8 nm has also been successfully employed to the mode-locked laser. The RF spectrum with the fundamental repetition rate of 79.0317 MHz is shown in Figure 7(c). The corresponding pulse duration is 159.9 fs as shown in Figure 7(d). To compare the performances of three EDF lasers in detail, the main parameters of each laser are summarized in Table 1. By comparing, we find that the modulation depth of SAs are significantly different when the thickness of the materials change. And it seems that the thick material shows greater modulation depth. Furthermore, by comparing the performances of the three lasers, we find that the modulation depth has a great influence on the pulse duration of the


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laser. In the same case, the SA with large modulation depth has more advantages in achieving ultrashort pulse duration. Furthermore, the pulse duration we demonstrate is impressive compared to the same kind of material or other commonly used SAs in Table 2. Compared with similar SnS2 material, the modulation depth of the SnS2 SA in this work has been greatly improved. Moreover, the obtained pulses at 1561 nm have the shortest pulse duration among the reported EDF lasers mode locked by TMDCs. What is worth mentioning is that compared with other materials, the performance of SnS2 is no worse. Furthermore, the broadband saturable absorption of the SnS2 SA at 1561 nm indicates that it can cover important communication band. Theoretical calculations. To compare the conductive properties of SnS2, the band structure, intrinsic defect properties and carrier concentration of intrinsic SnS2 bulk are calculated by using VASP coding based on first principle [68]. The unit cell of SnS2 bulk is shown in Figure 7(a). As shown in Figure 8(b), the formation energies of intrinsic defects in SnS2 are calculated in S-rich and Sn-rich growth conditions. Results show both in S-rich and S-poor conditions, the n-type intrinsic defects have lower formation energy than that of p-type intrinsic defects, demonstrating the intrinsic defects are easier to form n-type SnS2. For example, the Sn interstitial has a very low formation energy for the S-poor condition, and also its transition energy is above the CBM, which indicates that it very easy releases the electron to form n-type SnS2. With the low formation energy and transition energy of donor, the Sn interstitial defects and S vacancy defect cause that the defect density of donor is far greater than acceptors. Thus, the Fermi energy is close to CBM. With the μS increases, the formation energy of VS and Sni increases, which results in the decrease of the Fermi level and the


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electron concentration. For the S-poor condition, the electron concentration of SnS2 can reach about 1019 cm-3 at the room temperature, as shown in the Figure 7(c).On the other side, the Sn interstitial has the highest defect density under S-poor condition. From the calculated result in Figure 7(d), heavy doping of Sn interstitial may leads to a higher Fermi level, which is 0.16 eV higher than CBM. This explains the absorption of SnS2 at 1561 nm, as shown in Figure 7(d).

CONCLUSIONS Collectively, the thickness of SnS2 has been accurately controlled, and the application of SnS2 has been realized in fiber lasers. The SnS2 crystals have been calculated as the n-type semiconductor by first principle, and the results of defect density and defect energy level have well explained the absorption of SnS2 at 1561 nm. SnS2 samples with three different thickness have been successfully prepared and applied in mode-locked EDF lasers. By comparing the performance of three different SnS2, the modulation depth corresponding to SnS2 with the thickness of 108.7 nm, 9.1 nm and 2.8 nm are measured to be 40.5%, 36.5% and 31.4%, respectively. The experiment has proved that the EDF laser based on the SnS2 SA with a thickness of 108.7 nm has the best performance in achieving ultrashort pulses. Moreover, the obtained pulses of 132.9 fs at 1561 nm have the shortest pulse duration among the reported EDF lasers based on TMDCs. The thickness dependence of SnS2 obtained in this paper is beneficial to the design and research of ultrafast photonic devices.

METHODS Materials preparation. The bulk SnS2 crystals were synthesized by using chemical vapor transport (CVT) technique with commercially available Sn and S powder (Sigma Aldrich, purity 99.99%) as the precursors and iodine as the transport agent. The mechanical transfer method was used to transfer exfoliated 2D SnS2 flakes onto the end-facet of optical fiber. With a spin


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processor running at 3000 r/s, a few drops of self-made polymethyl methacrylate (PMMA) solution was dripped on the SiO2 substrate with exfoliated SnS2 samples. After that, the substrate was placed on the hot plate, heating for 30 min at 150 °C, and then immersed in the NaOH solution at 80 °C until the PMMA thin film could be taken off from the substrate. Finally, the PMMA film with almost all SnS2 nanoflakes was shifted over the fiber end facet. We employed acetone as the solubilizer to dissolve the organic PMMA, leaving the inorganic SnS2 flakes on the core area of fiber end-facet. Calculation details. In this paper, all the first-principles calculations are performed by using VASP code [68] within the density functional theory (DFT) [69]. To approach the experimental band gap, the HSE method [70] is used to calculate the band structure with the exchange parameters α = 0.25. The fully relaxed SnS2 lattice constant is a = 3.70 Å and c = 13.52 Å, and the band gap is 2.32 eV, which are close to the experimental value of 3.64 Å [71] and 2.2 ~ 2.4 eV [72]. Based on the defect calculation method in Ref. [73], the defect properties are calculated in a full relaxed 96-atom host supercell. The formation energy of defect α with charge state q can be calculated as [73],

Hf ( , q )  E ( , q )  E (host )   ni ( Ei   i )  q[ VBM (host )  EF ] ，

(1)

where E(α,q) is the total energy of defect α with charge state q, E(host) is the total energy of host, ni is the number of elements, μi and Ei are the chemical potential and energy of element i, and EF is the Fermi level referenced to the valence band maximum (VBM). The carrier density and defect density are calculated by the scheme in Refs. [74, 75]. Under the thermodynamic equilibrium growth conditions, the density of defect α with charge state q at the growth temperature T can be given by n( , q )  Nsitegqe

H f ( , q )/ k BT

，

(2)

where Nsite is the density of possible site of defect α, gq is the degeneracy factor which indicates possible electron occupations. The concentrations of thermally excited electron and hole at a given temperature can be calculated as, n0  N C e  EF / kBT ,

p0  NV e

( EF  Eg )/ k BT

(3) ,

(4)

where the effective density of state, NC and NV, are defined as


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

N C   d  [1  e

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(   Eg )/ k BT 1

] D( ) ,

Eg

(5)

0

NV   d  [1  e(  )/ kBT ]1 D( ) .

(6)



Here, D(ε) is the electron density of state at energy level ε with zero energy at VBM. According to the charge neutrality condition, the concentrations of defect ionizations need to satisfied

p0   i qi nDqii   n0   j q j nAjj

q 

(7)

q 

where nDqii  and nAjj are the density of donor Di and acceptor Aj with charge state q. By solving Eqs. (1)-(7), the Fermi energy, carrier concentration and defect density can be get as functions of chemical potential. When considering the concentration of carrier after quenching rapidly, the total density of defect Nα should remain unchanged. Thus, the density of defect α with charge q at cooling temperature T can be calculated as

n( , q )  N

gqe



H f ( , q )/ k BT

ge q q

H f ( , q )/ k BT

.

(8)

The new Fermi energy, carrier density and defect density after quenching can be got by solving Eqs. (2)-(8).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Author Contributions W. L., X. W. and T. S. contributed equally. W. L., M. L., G. C. and Z. W. performed the measurements and designed the project. X. W., M. L. and Z. W. fabricated the samples and performed the TEM results. T. S. and H. D. developed the theory. All authors


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discussed the result, and contributed to the writing of the manuscript. Notes The authors declare no competing financial interest.

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Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (NSFC) (Grant nos. 11674036, 11875008 and 61622406); Beijing Youth TopNotch Talent Support Program (Grant no. 2017000026833ZK08); Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications, Grant no. IPOC2017ZZ05).


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Figure 1. Schematic diagram of SnS2 samples transfer to the fiber end facet.

Figure 2. (a) XRD pattern (b) HR-TEM and SAED (inset) images (c) Raman spectrum (d) HRXPS peaks of Sn 3d and S 2p of the SnS2 crystal; (e) Absorption spectrum of 2D SnS2 SA on SiO2 substrate. Inset: corresponding optical image. Scale bar: 50 μm; (f) AFM images of 2D SnS2 SA and typical height profiles. Scale bar: 5 μm.


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Figure 3. The nonlinear response of SnS2 SAs with different thickness. (a) 108.7 nm; (b) 9.1 nm; (c) 2.8 nm.


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Figure 4. The experimental scheme of SnS2-based EDF laser.


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Figure 5. (a) The mode-locked sequence; (b) The spectral; (c) The autocorrelation trace of pulses; (d)The RF spectrum.


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Figure 6. Continuous monitoring of average output power.


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Figure 7. (a) The RF spectrum of SnS2 with a thickness of 9.1 nm; (b) The pulse duration of SnS2 with a thickness of 9.1 nm; (c) The RF spectrum of SnS2 with a thickness of 2.8 nm; (d) The pulse duration of SnS2 with a thickness of 2.8 nm.


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Figure 8. (a) Crystal structure of SnS2. (b) The calculated formation energy of intrinsic defects in SnS2 as a function of the Fermi energy under S-rich conditions and Sn-rich conditions. (c) The Fermi energy, carrier density and defect density as functions of S chemical potential under thermodynamic equilibrium growth conditions at 1200 K (left), and quenched to 300 K (right). (d) Conduction band structure of SnS2 supercell with S interstitial defects at Γ-kpoint.


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Table 1. The mode-locked lasers performance based on SnS2 with different thickness. Material

MD (%)

RF(MHz)

λc/△λ(nm)

τ(fs)

SNR(dB) P(mW)

SnS2 (108.7nm)

40.5

79.133

1561/28

132.9 61.13

24.8

SnS2 (9.1nm)

36.5

79.066

1559.9/26

149.5 61.7

22.2

SnS2 (2.8nm)

31.4

79.0317

1560.5/25.5

159.9 62.48

24.5

Note: BPs, black phosphorus; MD, Modulation depth; τ, pulse duration; P, average output power.

Table 2. Comparison of mode-locked EDF lasers with SAs based on different materials. Material

MD (%)

τ (fs)

Spectral width (nm)

SNR (dB)

P(mW)

Refs.

CNT

12.05

5900

0.49

~70

-

57

graphene

4.8

88

48

65

1.5

58

BP

4.6

272

10.2

65

0.5

59

Bi2Se3

3.9

660

4.3

55

1.8

60

Sb2Te3

-

1800

1.8

60

0.5

61

WS2

1.8

1320

2.3

50

-

62

MoS2

35.4

1280

2.6

62

5.1

31

WSe2

0.5

1250

2.1

50

0.84

63

MoSe2

0.63

1450

1.76

61.5

0.44

64

WTe2

2.85

770

4.14

67

0.04

65

MoTe2

1.8

1200

2.4

50

-

66


27


SnS2

Page 28 of 36

4.6

623

6.09

45

1.2

67

5

1630

1.6

60

-

50

40.5

132.9

28

61.13

24.8

This work

Table of Contents Graphic

Thickness dependent nonlinear absorption properties of SnS2 are studied for the first time. The modulation depth corresponding to SnS2 SAs with the thickness of 108.7 nm, 9.1 nm and 2.8 nm are 40.53%, 36.51% and 31.37%, respectively. Those samples are successfully applied in the EDF lasers to achieve mode-locked operation. The experiment prove that the EDF laser based on the SnS2 SA with the 40.53% modulation depth has the best performance in achieving ultrashort pulses.


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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28


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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9


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ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21


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Thickness-Dependent Ultrafast Photonics of SnS2 Nanolayers for

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