Ultrathin 2D transition metal carbides for ultrafast pulsed fiber lasers

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Ultrathin 2D transition metal carbides for ultrafast pulsed fiber lasers Mingfen Tuo, Chuan Xu, Haoran Mu, Xiaozhi Bao, Yingwei Wang, Si Xiao, Weiliang Ma, Lei Li, Ding Yuan Tang, Han Zhang, Malin Premaratne, Baoquan Sun, Hui-Ming Cheng, Shaojuan Li, Wencai Ren, and Qiaoliang Bao ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01428 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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Ultrathin 2D transition metal carbides for ultrafast pulsed fiber lasers ∥ ∥ Mingfen Tuo,†, Chuan Xu,‡, Haoran Mu,†,

§

Xiaozhi Bao,# Yingwei Wang,£,

§

£

Si Xiao,

Weiliang Ma,† Lei Li⊥, Dingyuan Tang⊥, Han Zhang#, Malin Premaratne┬, Baoquan Sun†, Hui-Ming Cheng‡, Shaojuan Li †,*, Wencai Ren‡,*, Qiaoliang Bao †,§,* †

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based

Functional Materials and Devices and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China. ‡

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of

Sciences, Shenyang 110016, P. R. China. §

Department of Materials Science and Engineering, and ARC Centre of Excellence in Future Low-Energy

Electronics Technologies (FLEET) Monash University, Clayton, Victoria 3800, Australia. £

Institute of Super-Microstructure and Ultrafast Process in Advanced Materials, School of Physics and

Electronics, Hunan Key Laboratory for Super-Microstructure and Ultrafast Process, Central South University, Changsha 410083, P. R. China. ⊥

Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic

Engineering, Jiangsu Normal University, Xuzhou, Jiangsu 221116, P. R. China. #

SZU-NUS Collaborative Innovation Centre for Optoelectronic Science & Technology, and Key

Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Electronic Science and Technology, and College of Optoelectronics Engineering, Shenzhen University, Shenzhen 518060, P. R. China. ┬

Department of Electrical and Computer Systems Engineering, Monash University, Clayton, Victoria 3800,

Australia ∥

These authors contributed equally to this work.

* Corresponding author(s): Qiaoliang Bao (Q.B.): [email protected], [email protected] and Shaojuan Li (S. L.): [email protected]; Wencai Ren (W. R.): [email protected].

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ABSTRACT Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides and black phosphorus, have attracted intense interests for applications in ultrafast pulsed laser generation, owing to their strong light-matter interactions and large optical nonlinearities. However, due to the mismatch of the bandgap, many of these 2D materials are not suitable for applications at near-infrared (NIR) waveband. Here, we report nonlinear optical properties of 2D α-Mo2C crystals and the usage of 2D α-Mo2C as a new broadband saturable absorber for pulsed laser generation. It was found that 2D α-Mo2C crystals have excellent saturable absorption properties in terms of largely tunable modulation depth and very low saturation intensity. In addition, ultrafast carrier dynamic results of 2D α-Mo2C reveal an ultra-short intraband carrier recovery time of 0.48 ps at 1.55 µm. By incorporating 2D α-Mo2C saturable absorber into either an Er-doped or Yb-doped fiber laser, we are able to generate ultrashort pulses with very stable operation at central wavelengths of 1602.6 and 1061.8 nm, respectively. Our experimental results demonstrate that 2D α-Mo2C can be a promising broadband nonlinear optical media for ultrafast optical applications.

Keywords: 2D α-Mo2C crystal, saturable absorber, fiber laser, mode-locking, broadband


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INTRODUCTION Ultrashort pulsed fiber lasers have widespread applications in the fields of optical communication, material processing, biomedical research, and spectroscopy.1-3 One majority type of ultrashort pulsed fiber lasers employs a nonlinear optical element, called saturable absorbers (SAs), to turn the continuous wave into a train of ultrashort optical pulses, due to their compactness, simplicity and cost-efficiency.4-6 Recently, SAs based on two-dimensional (2D) materials such as graphene,7,8 TMDs (MoS2,9 WS2,10 TiS211) and black phosphorus (BP)12 have experienced rapid development for ultrashort pulse generation, because of their large third order optical nonlinearity,13,14 ultrafast carrier dynamics,13-15 as well as tunable modulation depth.12,16 From point of view of energy structure, zero or small bandgap materials are suitable candidate for broadband operation of those laser applications as they own wavelength insensitive optical response at telecommunication bands. In particular, Dirac materials such as graphene and topological insulators (TIs, including Bi2Te3, Bi2Se3, Sb2Te3, etc) with zero or small bandgap (e.g., 0.15 eV for Bi2Te3 and 0.3 eV for Bi2Se3) have shown intriguing potential for broadband photonics and optoelectronics. Different operation regimes, either mode-locking or Q-switching, by graphene-based SAs have been demonstrated on either fiber laser5,7,17 or solid state lasers18,19 from the visible towards the mid-infrared spectral regime. Furthermore, wavelength-tunable operation20 and multi-wavelength operation21 have also been achieved in fiber lasers. TIs are a new class of materials that have a bulk band gap and gapless Dirac surface/edge state,22 which is protected by the topological symmetry. Since the first report of saturable absorption in Bi2Te3,23 the utilization of different TI SAs for passive mode-locking or Q-switching at 1 µm, 1.55 µm and 3 µm over a very broadband spectral region has been demonstrated.24-26 Nevertheless, there are still some inherent drawbacks which limit further improvement of their performance so as to suit for practical applications. For instance, the limited light absorption in monolayer graphene (πα = 2.3% for monolayer graphene) and increased non-saturable absorption loss in thick graphene films should be balanced. In case of TIs, the heavily populated intrinsic defects lead to electron transport that is dominated by the bulk 3 ACS Paragon Plus Environment

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state instead of the desired massless Dirac surface states, which also implements certain effect on nonlinear optical properties.27 Black phosphorus (BP) is also very suitable for near-infrared (NIR) photonics owing to its direct bandgap from 0.3 (bulk) to 1.5 eV (monolayer),28,29 but BP is unstable in the ambient environment as it is sensitive to oxygen, humidity and light illumination.30-32 Stable pulse generation at 1.55 µm has recently been achieved by encapsulating BP with polymer, which affords a long-term stability in laser operation.12,16 However, when operating at a high incident laser power, the thermal effect on the polymer is inevitable and will give insertion losses, compromising device performance. In addition, BP SAs is usually prepared by mechanically exfoliating BP flakes and then placed onto the end-facet of optical fiber by dry transferring. However, it is very difficult to reproduce BP SAs with identical size and thickness due to the difficulty in producing large area BP thin films with good controllability and repeatability.33 Novel nonlinear materials with strong light absorption, good reproducibility and stability are therefore demanded for the application in broadband laser operation. Recently, it was found that 2D transition metal carbides (TMCs) nanosheet, known as MXenes,34,35 where M is an early transition metal, X is C and/or N, can be produced into thin sheets and shows extraordinary physical properties. The band structure and electron density of states (DOSs) of 2D TMCs have been extensively studied by using the density functional theory (DFT) approach.34 Owing to the incorporation of carbon atoms into the metal lattice, 2D TMCs are normally metallic with a high electron density near the Fermi level,36 which enables 2D TMCs with ultrahigh conductivity, similarly to graphene.37,38 Furthermore, they have very high melting points and good stability in the ambient environment,37 which make them easily preserved and applicable in various applications. Nowadays, large numbers of reports have explored the usage of 2D TMCs for various applications such as 2D superconductor,39-42

electrochemical energy

storage,43,44

electromagnetic

interference

shielding45 and so on. The observation of 2D characteristics of superconducting transitions opens the door for developing more potential electrical and photonic applications that are complementary to existing 2D materials. Enlightened by the zero bandgap,46 ultrahigh 4 ACS Paragon Plus Environment

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electrical conductivity, high melting points and good stability in the ambient environment, it would be naturally interesting to explore whether 2D α-Mo2C can be developed as a new type of nonlinear absorber for broadband pulse reshaping. However, the saturable absorption of 2D α-Mo2C has not yet been experimentally observed. It is noteworthy that another metallic MXene material, Ti3CNTx, has been recently demonstrated to be a broadband saturable absorber for producing Q-switched and mode-locked laser pulses. Ti3CNTx MXene was prepared by selective etching of the ‘A’ layer from layered ternary MAX phases using hydrofuoric acid and further delaminated into monoto few-layer by sonication in tetrabutylammonium hydroxide (TBAOH). The solution sample was deposited on side-polished optical fiber for laser applications.47 Nevertheless, such chemically derived functionalized 2D TMC sheets are very small, with lateral sizes ranging from hundreds of nanometres to ~10 µm, suffering from severe structural defects and fluorine groups on the surface.43,44,48 The chemical structure and preparation technique result in a relatively large non-saturable absorption loss, which further lead to relatively small modulation depth (1.7%) and large saturable absorption threshold power (45 MW/cm2). Considering the recent progress on the synthesis of large size and highly crystalline 2D α-Mo2C,39 it is nontrivial to integrate such material onto optical fiber with improved fabrication approach and further explore the nonlinear absorption properties as well as the application for ultrafast lasers. In this paper, we investigated the nonlinear optical properties of high quality 2D α-Mo2C crystals grown by CVD method. It is found that 2D α-Mo2C has strong absorption from the visible towards the NIR wavelengths. Z-scan experiments were implemented and thickness dependent modulation depth was observed in 2D α-Mo2C. In combination with its ultrafast carrier recovery time, 2D α-Mo2C shows excellent saturable absorption properties in terms of large nonlinear absorption coefficient and low saturable absorption intensity. For the first time, we demonstrated the usage of 2D α-Mo2C SAs in fiber lasers to generate ultrashort mode-locking pulses at 1.55 µm and 1.06 µm with good long-term stability. Our work 5 ACS Paragon Plus Environment

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suggests that 2D α-Mo2C crystals could be a new benchmark material for broadband nonlinear optics and ultrafast photonics.

RESULTS AND DISSCUSSION High quality 2D α-Mo2C crystals were grown by CVD method on a Cu foil sitting on a Mo foil as the substrate (see details in Experimental Section).39 The as-produced 2D α-Mo2C crystals on Cu/Mo substrate have different shapes, as shown by the optical image in Figure 1a. The inset image shows the optical image of the optical fiber end-facet after transferring 2D α-Mo2C samples. A single hexagonal 2D α-Mo2C crystal is fully covering on the fiber core area. Notably, the 2D α-Mo2C crystal preserves intact without obvious wrinkles and defects after the all-drying transfer process49 (see details in Section 1 of the Supporting Information). Atomic force microscopy (AFM) measurements of a hexagonal 2D α-Mo2C crystal reveal that it is very uniform in topography, with a thickness of ~10.3 nm, as shown in Figure 1b. It should be noted that the thickness of 2D α-Mo2C crystals can be tuned in a wide range from 4 nm to 110 nm by changing the growth temperature (see Experimental Section).39 The UV-visible-IR absorption spectrum of a film of multiple 2D α-Mo2C crystals is depicted in Figure 1c. There is a strong and relatively flat absorption across a wide range of wavelength from visible to NIR band, which makes 2D α-Mo2C an excellent broadband optical material. Figure 1d shows a representative transmission electron microscopy (TEM) image of a dodecagonal 2D α-Mo2C crystal, which appears to be ultrathin sheet with the size up to 3 µm. The corresponding selective area diffraction (SAED) pattern shown in the inset of Figure 1d reveals that our sample has an orthorhombic structure.50 The high-resolution TEM (HRTEM) image in Figure 1e clearly resolves the high crystalline quality of 2D α-Mo2C, in which the lattice space of 6.03 Å corresponds to (010) facets. The as-grown 2D α-Mo2C crystals have large size and can be easily transferred to a target substrate and end-facet of optical fiber ferrule as saturable absorber device for further characterizations or measurements. 6 ACS Paragon Plus Environment

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We performed pump-probe experiments to investigate the carrier dynamics of 2D α-Mo2C crystals at 1.5 µm, and the experimental results are shown in Figure 1f. The carrier relaxation

time

can

be

fitted

by

a

bi-exponentially

decaying

function,

∆T T = A1exp(- t τ1 ) + A2exp(- t τ 2 ) , where those two parameters (τ1 and τ2) of decay times correspond to intraband relaxation time (τ1) and interband relaxation time (τ2), respectively. It is concluded that the intraband relaxation constant (τ1) is 0.48 ps. This ultrafast recovery time (intraband relaxation time) makes 2D α-Mo2C a promising nonlinear optical material for ultrafast photonics, particularly as an effective saturable absorber in mode-locked or Q-switched fiber laser. The nonlinear response of 2D α-Mo2C crystals with different thickness was studied by the open aperture Z-scan technique measured at the wavelength of 1.55 µm, as shown in Figure 2. We prepared three samples with different thicknesses by transferring 2D α-Mo2C crystals onto quartz plates. Figure 2a-c shows the statistical distribution of the three samples with Gaussian fits overlaid, which are marked as Sample-1 (S1), Sample-2 (S2) and Sample-3 (S3), respectively. The crystals on Sample S1 have a thickness range from 4 nm to 44 nm in which around ~40% of the crystals have a thickness of 18 nm. The crystals on Sample S2 have a thickness range from 10 nm to 45 nm in which around ~38% of the crystals have a thickness of 23 nm. And the crystals on Sample S3 have a thickness range from 18 nm to 110 nm in which around ~25% of the crystals have a thickness of 60 nm. In order to reduce the experimental error, the nonlinear absorption at different points on the 2D α-Mo2C samples were investigated with similar performance. The corresponding normalized Z-scan curves are shown in Figure 2d-f. It is noted that all the traces have sharp and narrow peaks located at the beam focus, which is a prominent characteristic of saturable absorption. To verify whether the quartz substrate contributes to the nonlinear optics response, pure quartz substrate without sample was used as the control, and there was no response even after irradiation at high laser power (see details in Section 2 of the Supporting Information). 7 ACS Paragon Plus Environment

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To understand the nonlinear optical (NLO) response quantitatively, we extracted the NLO absorption coefficient of the 2D α-Mo2C by fitting the Z-scan data using equation (1),51

T=

1 π q0

∫

+∞

−∞

[

(

)]

In 1 + q0 exp − x 2 dx

(1)

where q0 = β I 0 Leff , β is the NLO absorption coefficient, Leff = [1 − exp(− α s )] (− α s ) . In order to evaluate the saturable absorption intensity (Is) and the modulation depth (αs) of 2D α-Mo2C crystals, we extracted these parameters by fitting the experimental data shown in Figure 2d-e with a saturation numerically model for semiconductors, which is expressed as equation (2).8,49

α

T =1− s −αns 1+ I / Is

(2)

where αns is the nonsaturable component, L is the position in the sample, Is is the saturable

(

absorption intensity, and I = I 0 1 + z 2 z0 2

)

denotes the Gaussian distribution of the incident

laser, where z0 = πω0 λ is the diffraction length of the beam, ω0 is the beam waist at the focus, λ is the wavelength, and I0 is the laser density at the focus. More details of the fitting process were depicted in Section 2 of Supporting Information. Based on the fitting results, the values for β, Is and αs of the 2D α-Mo2C crystals were extracted, and the results are summarized in Table 1. It is found that the modulation depth generally increases with increasing the crystal thickness. For sample S1, sample S2 and sample S3, the modulation depth (αs) are calculated to be about (5.84 ± 2.24)%, (46.25 ± 10.25)% and (58.60 ± 14.60)%, respectively. Similar to graphene, the relatively large modulation depth of the 2D α-Mo2C crystal in this experiment shows an ambiguous dependence on the crystal thickness that is associated with non-saturable loss, which is an intrinsic merit of 2D α-Mo2C crystal arising from its 2D structure.7 In addition, the saturable absorption intensity (Is) of the 2D α-Mo2C crystals exhibits a significant increase when 8 ACS Paragon Plus Environment

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increasing the input laser intensity and sample thickness, and shows a pretty wide tunable 2

range from 0.008 to 128.40 GW/cm. For the thicker sample, more light power is required to saturate it due to stronger absorption; thus, the saturable absorption intensity is higher, and the modulation depth is larger, which affords particular photonic applications, such as laser mode locking to generate ultrashort pulses, in which particular modulation strength by the saturable absorber is needed. Compared to other 2D materials, the values of nonlinear absorption coefficient (β) of the 2D α-Mo2C SAs are approximately several orders of magnitude larger than those of black phosphorus,52 MoS2 nanosheets,53 graphene-Bi2Te3 heterostructure54 and comparable to that of Bi2Se3,55 as shown in Table 2. More importantly, the saturable absorption intensity (Is) of the 2D α-Mo2C crystals is much lower than that of other 2D materials. Such a lower saturable intensity is beneficial to the fabrication of SAs because low saturable intensity would reduce the tendency for Q-switching instabilities induced by thermal effects.56 Additionally, we investigated the nonlinear optical transmission properties of the sample transferred on the optical fiber end facet. The tested sample is the one (Sample-1, S1) that we used in the fiber laser application due to slight thermal effect induced by strong absorption under high input laser energy. From the measured transmission as a function of average pump power at 1596 nm (using a probe laser with pulse width of 10.08 ps, as detailed in Section 3 Supporting Information), the modulation depth (αs) of 2D α-Mo2C were estimated to be 4.29% due to absorption saturation when the peak power is 3.27 W (corresponding to a power density of 5.14 MW/cm2). The feasible tunability of the modulation depth and low saturation intensity of 2D α-Mo2C crystals affords new opportunities for its potential usage as saturable absorber in IR wavelength range. Inspired by these remarkable nonlinear optical properties, we further fabricated 2D α-Mo2C-based saturable absorber and incorporated it into fiber laser cavity so as to evaluate its performance for pulse generation. Moreover, considering its zero bandgap and strong optical absorption from the visible to NIR wavelengths, we applied 2D α-Mo2C as a new


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broadband saturable absorber for both 1.55 µm and 1.06 µm mode-locking pulse generation, which will be elaborated hereinafter. The experimental setup of the laser cavity is schematically depicted in Figure S4. In the mode-locked Er-doped fiber laser for 1.55 µm mode-locking pulse generation, we used a piece of 3 m highly-doped Erbium-doped fiber with a group velocity dispersion (GVD) of 60.96 ps2 km-1 as gain medium pumped by a 980 nm laser diode (LD) through a 980 nm/1550 nm wavelength division multiplexer (980/1550 nm WDM). And the single mode fiber (SMF) with a total length of 106.4 m was incorporated into the cavity to compress optical pulses and improve pulse quality, which is effective to achieve different pulse generation regime. A polarization-independent isolator (PI-ISO) was inserted into the laser cavity to keep the direction of light propagation. The cavity polarization state and intra-cavity birefringence were adjusted by a polarization controller (PC). A 30% output coupler (OC) was employed to direct the output signal to oscilloscope. The 2D α-Mo2C saturable absorber device was spliced inside between PI-ISO and coupler for pulse generation and reshaping. While for the 1.06 µm mode-locking pulse generation in a Yb-doped fiber laser, the active fiber is a piece of 1.39 m long Yb-doped fiber with a GVD of 24.22 ps2 km-1 pumped by 980 nm laser diode using a 980/1064 nm WDM. The PC, PI-ISO and 5% OC with the operation wavelength at 1.06 µm were also spliced into the laser cavity. Next, a 2D α-Mo2C saturable absorber was applied in the 1.55 µm fiber laser and stable mode-locking can be successfully achieved by changing the length of SMF so as to tune the cavity net dispersion. The results are presented in Figure 3. The threshold pump power for stable mode-locking generation is 127.1 mW. The typical optical spectrum of mode-locked pulses is shown in Figure 3a. The central wavelength is around 1602.6 nm with a full width at half maximum (FWHM) bandwidth of 1.66 nm. The symmetrical soliton sidebands are clearly resolved on the spectrum, suggesting a very stable state. Once mode-locking state is achieved, typical pulse train with a repetition rate of 1.88 MHz can be observed (see Figure 3b), which corresponds to the total cavity length of 106.4 m. The autocorrelation (AC) trace 10 ACS Paragon Plus Environment

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of a single soliton pulse with a FWHM of 2.79 ps is shown in Figure 3c. The real pulse width is then obtained by multiplying the AC trace width with the decorrelation factor, which is 0.648 for the sech2 pulses. This gives an actual pulse width of 1.81 ps (= 2.79 ×0.648). The corresponding time-band width product (TBP) is calculated to be 0.351, which is very close to the typical value of sech2 pulse profile (TBP = 0.315). The minor deviation is expected for transform-limited sech2 pulses. This indicates a relatively stable soliton state with little chirped. To further investigate the signal-to-noise ratio, we also measured the radio-frequency (RF) spectrum of the mode-locked pulses. Its fundamental peak locates at the cavity repetition rate (1.88 MHz), as shown in Figure 3d, and a signal-to-noise ratio (SNR) of ~45 dB is achieved. The inset of Figure 3d shows the wideband RF spectrum up to 100 MHz. It is found that there is no extra frequency component except the fundamental and harmonic frequencies, which verifies the low-amplitude fluctuations, single pulse state and good stability of output pulse train. In order to examine the long-term stability of the mode-locked lasers, we continuously monitored the output spectra of the mode-locked pulse for 16 hours. Figure 3e, f show the evolution of the central wavelength and the spectral bandwidth, where the central wavelength only changes from 1062.3 nm to 1062.86 nm while its 3 dB bandwidth fluctuates slightly from 1.51 nm to 1.77 nm, further indicating the stability of our mode-locked fiber laser. For a given wavelength at 1.55 µm, the performance of our 2D α-Mo2C mode-locked laser, that emits 1.81 ps pulses at 1602.6 nm with a 3 dB spectral width of 1.66 nm and a repetition rate of 1.88 MHz, is comparable to that previously achieved with BP SAs (2.18 ps at 1558.14 nm, 3 dB spectral width of 1.25 nm and repetition rate of 15.59 MHz,)12 and MoS2 (1.28 ps at 1568.9 nm, 3 dB spectral width of 2.6 nm and repetition rate of 8.29 MHz).57 Interestingly, we found that a noise-like mode-locked state can also be realized due to significant intrinsic nonlinear optical responses in a high input power condition (see details in Section 5 of the Supporting Information). To demonstrate the broadband saturable absorption property of the 2D α-Mo2C-SA, the mode-locked pulses are also obtained in 1.06 µm ring-cavity Yb-doped fiber laser by incorporating this SA inside the laser cavity. Figure 4 shows the pulse characteristics of the 11 ACS Paragon Plus Environment

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Yb-doped fiber laser at a pump power of 126.6 mW. The central wavelength of the mode-locking pulse is 1061.8 nm with 3 dB bandwidth of 2.62 nm. The steep spectral edges have been observed, which is a typical shape of dissipative solitons (DSs), as shown in Figure 4a. The oscilloscope trace of the DSs pulse train has been plotted in Figure 4b with a pulse-to-pulse interval of 310.83 ns, corresponding to a repetition rate of 3.22 MHz. The mode-locked laser pulse has a FWHM of 418 ps (see Figure 4c), with a TBP up to 128.05. This large frequency chirping is also a feature of DSs, which is a natural result of the balance between the cavity loss, gain, nonlinearity, and dispersion.58 The RF spectrum of the pulse is shown in Figure 4d. The fundamental repetition rate of the mode-locked pulse locates at 3.23 MHz with a SNR of ~64 dB, indicating that the cavity length is 61.92 m. No additional frequency peak has been seen with the wideband RF spectrum up to 300 MHz, as shown in the insert of the Figure 4d. For a given wavelength at 1.06 µm, the output pulse of a FWHM of 418 ps and large SNR over 64 dB, reveals better performance than that previously achieved with 2D CH3NH3PbI3 perovskite nanosheets SA (the output pulse has a FWHM of 931 ps and a SNR of ~53 dB at 1064 nm).49

CONCULUSION In summary, the nonlinear optical properties of 2D α-Mo2C crystals were investigated and the usage of 2D α-Mo2C as a new broadband saturable absorber for pulsed laser generation was demonstrated. By changing the thickness of 2D α-Mo2C crystals or the incident laser intensities, the modulation depth can be adjusted from 4.4% to 73.2%. Furthermore, 2D α-Mo2C crystals show ultrafast carrier dynamics at 1.55 µm with a short intraband relaxation constant of 0.48 ps. By using 2D α-Mo2C crystals as SAs, both passive mode-locking operation in Er-doped fiber laser and in Yb-doped fiber laser with very stable operation at central wavelengths of 1602.6 and 1061.8 nm, respectively, were successfully achieved. The output mode-locked soliton pulses in Er-doped fiber laser have a central wavelength of 1602.6 nm with the pulse duration of 1.81 ps and repetition rate of 1.88 MHz. Based on the Yb-doped 12 ACS Paragon Plus Environment

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fiber laser, mode-locked pulses at central wavelength of 1061.8 nm with the pulse duration of 418 ps and a signal-to-noise ratio of ~64 dB have been obtained. And the long pulse width mainly results from the relatively small modulation depth of the sample as well as the large cavity dispersion (normal or abnormal).59-61 Through intelligently designing the laser cavity and engineering optical gain in the fiber, we believe that much faster pulse could be realized using 2D α-Mo2C-based saturable absorber. Our work reveals the unique nonlinear optical properties of 2D α-Mo2C and provides a novel promising broadband saturable absorber for ultrafast fiber lasers.

MATERIALS AND METHODS Synthesis of 2D α-Mo2C Crystals. 2D α-Mo2C crystals were grown on Cu (Alfa Aesar, 99.9% purity, 12.5 µm thick)/Mo (Alfa Aesar, 99.95% in purity, 100 µm in thickness) substrates at a high temperature above 1085 °C (copper melting point) by CVD process.39 The thickness of 2D α-Mo2C crystals can be controlled by changing the growth temperature with the same growth time. In particular, we used the growth temperature of 1088 °C and long growth time of 10 min to produce large ultrathin 2D α-Mo2C crystals which have a thickness of ~ 6 nm and lateral size over 20 µm. Saturable Absorber Characterization. The as-produced 2D α-Mo2C crystals were then placed onto quartz substrate and cross-section of optical fiber ferrule by all-drying transfer method for optical characterizations and the usage as saturable absorber. The morphology and thickness of 2D α-Mo2C crystal were identified by optical microscopy (Nikon LV 100D) and atomic force microscopy (AFM, Nanoscope IIIa). The micro-structure and crystalline quality were characterized by TEM (FEI Tecnai T12, 120 kV for bright field TEM imaging) and SAED measurements (FEI Tecnai F20, 200 kV for HRTEM measurements). The linear optical absorption spectra were recorded on a UV–vis–NIR spectrometer (Lambda 750, Perkin Elmer). 13 ACS Paragon Plus Environment

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Z-scan Measurements. The femtosecond laser pulse used in the open-aperture Z-scan measurements was generated by an optical parametric amplifier (TOPAS, USF-UV2), which was pumped by the mode-locked Ti: Sapphire regenerative amplifier system (Spectra-Physics, Spitfire ACE-35F-2KXP, Maitai SP and Empower 30) with a repetition rate of 2 kHz. The laser beam was focused by a lens with a focus length of 150 mm. A computer-controlled translation stage was employed to move the sample along the propagation direction (z-axis) of the laser pulses, and the transmitted pulse energy was probed by a IR detector (OPHIR, PD300R-IR). Pump-probe System. The pump-probe system consists of a femto-second pulse laser with a pulse duration of 35 fs and pulse repetition rate of 2 kHz. An optical parametric amplifier (TOPAS, USF-UV2) pumped by a Ti:Sapphire amplifier (Spectra-Physics, Spitfire ACE-35F-2KXP Maitai SP and Empower 30) was used to generate the high energy laser pulse. The laser beam was focused by a lens with the focal distance of 250 mm. The pumped light and probe light at the wavelength of 1.55 µm have an average power of 80 µJ. Laser Performance Characterization. For the fiber laser, an optical spectrum analyzer (OSA, Yokogawa AQ6370C-20) with a resolution of 0.02 nm, a 1 GHz real-time oscilloscope (OSC, Agilent DSO9104A) together with a 2 GHz photodetector (PD, Thorlabs DET01CFC) and a 63 GHz real-time oscilloscope (OSC, Agilent DSAX96204Q) together with a 45 GHz photodetector (PD, New focus 1014) are, respectively, employed to monitor the output optical spectra and pulse train. The radio frequency (RF) spectrum is analyzed by a 3 GHz electrical spectrum analyzer (ESA, Agilent N9320B), and the pulse width is measured by a commercial autocorrelator (Femtochrome FR-103HS).

Acknowledgements


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We acknowledge the support from the National Key Research & Development Program (No. 2016YFA0201900), the National Natural Science Foundation of China (No. 61604102, 51290273, 51325205 and 91433107), the Youth 973 program (2015CB932700), ARC (FT150100450, CE170100039 and IH150100006), the Natural Science Foundation of Jiangsu Province (No. N321465217, BK20150053), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and Collaborative Innovation Center of Suzhou Nano Science and Technology. C. Xu acknowledges the support from SYNL-T.S. K Research Fellowship (Y7N2631161). Q. Bao acknowledges support from the Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET).

ASSOCIATED CONTENT

Supporting Information Supporting images, details on the all-dry transfer process, the details on the fitting process of Z-scan results, the schematic experimental setup for measuring nonlinear power dependent absorption and nonlinear absorption fitting results, the schematical configuration of the fiber laser ring cavity, the results of noise-like mode-locking laser at 1.55 µm and a table of comparison with MXene saturable absorber. .

AUTHOR INFORMATION

Corresponding Author Qiaoliang Bao, Shaojuan Li and Wencai Ren Tel: (+61)-3-99054927; Fax: (+86)-512-65882846; Tel: (+86)-024-23971472


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E-mail: (Q. B.) [email protected], [email protected]; (S. L.) [email protected]; (W. R.) [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Minfen Tuo and Chuan Xu contributed equally to this work.


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REFERENCES (1) Letokhov, V. S., Laser biology and medicine. Nature 1985, 316, 325-330. (2) Okhotnikov, O.; Grudinin, A. B., Ultrafast fiber systems based on SESAM technology: New horizons and applications. New J. Phys. 2004, 6, 211-218. (3) Okhotnikov, O. G.; Gomes, L.; Xiang, N.; Jouhti, T.; Grudinin, A. B., Mode-locked ytterbium fiber laser tunable in the 980–1070-nm spectral range. Opt. Lett. 2003, 28, 1522-1524. (4) He, J.; Dong, H.; Deng, R.; Chen, L., Low-cost bidirectional hybrid fiber-visible laser light communication system based on carrier-less amplitude phase modulation. Opt. Eng. 2016, 55. (5) Ismail, M. A.; Ahmad, F.; Harun, S. W.; Arof, H.; Ahmad, H., A Q-switched erbium-doped fiber laser with a graphene saturable absorber. Laser Phys. Lett. 2013, 10, 025102-025109. (6) Taib, N. A. M.; Bidin, N.; Haris, H.; Adnan, N. N.; Ahmad, M. F. S.; Harun, S. W., Multi-walled carbon nanotubes saturable absorber in Q-switching flashlamp pumped Nd:YAG laser. Opt & Laser Technol. 2016, 79, 193-197. (7) Bao, Q.; Zhang, H.; Wang, Y.; Ni, Z.; Yan, Y.; Shen, Z. X.; Loh, K. P.; Tang, D. Y., Atomic‐Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers. Adv. Funct. Mater. 2009, 19, 3077-3083. (8) Mu, H.; Wang, Z.; Yuan, J.; Xiao, S.; Chen, C.; Chen, Y.; Chen, Y.; Song, J.; Wang, Y.; Xue, Y., Graphene–Bi2Te3 Heterostructure as Saturable Absorber for Short Pulse Generation. ACS Photonics 2015, 2, 832-841. (9) Du, J.; Wang, Q.; Jiang, G.; Xu, C.; Zhao, C.; Xiang, Y.; Chen, Y.; Wen, S.; Zhang, H., Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction. Sci. Rep. 2014, 4, 6346-6353. (10) Zhang, M.; Hu, G.; Hu, G.; Howe, R. C. T.; Chen, L.; Zheng, Z.; Hasan, T., Yb- and Er-doped fiber laser Q-switched with an optically uniform, broadband WS2 saturable absorber. Sci. Rep. 2015, 5, 11453-11461. (11) Zhu, X.; Zhang, M.; Du, J.; Chen, J.; Hu, G.; Zhao, X.; Zheng, Z.; Zhang, H. In Mode-locked Er-doped fiber laser with TiS2 saturable absorber, CLEO: Applications and Technology, 2016. (12) Mu, H.; Lin, S.; Wang, Z.; Xiao, S.; Li, P.; Chen, Y.; Zhang, H.; Bao, H.; Shu, P. L.; Pan, C., Black Phosphorus–Polymer Composites for Pulsed Lasers. Adv. Opt. Mater. 2015, 3, 1447–1453. (13) Wang, Y.; Liu, S.; Yuan, J.; Wang, P.; Chen, J.; Li, J.; Xiao, S.; Bao, Q.; Gao, Y.; He, J., Ultra-broadband Nonlinear Saturable Absorption for Two-dimensional Bi2TexSe3-x Nanosheets. Sci. Rep. 2016, 6, 33070-33078. (14) Zhang, H.; Lu, S. B.; Zheng, J.; Du, J.; Wen, S. C.; Tang, D. Y.; Loh, K. P., Molybdenum disulfide (MoS‐) as a broadband saturable absorber for ultra-fast photonics. Opt. Express 2014, 22, 7249-7260. (15) Tani, S.; Blanchard, F.; Tanaka, K., Ultrafast carrier dynamics in graphene under a high electric field. Phys. Rev. Lett. 2012, 109, 166603-166608. (16) Chen, Y.; Mu, H.; Li, P.; Lin, S.; Shivananju, B. N.; Bao, Q., Optically driven black phosphorus as a


Page 19 of 28 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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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saturable absorber for mode-locked laser pulse generation. Opt. Eng. 2016, 55, 081317-081321. (17) Zhang, H.; Tang, D.; Knize, R. J.; Zhao, L.; Bao, Q.; Loh, K. P., Graphene mode locked, wavelength-tunable, dissipative soliton fiber laser. Appl. Phys. Lett. 2010, 96, 51-56. (18) Cizmeciyan, M. N.; Kim, J. W.; Bae, S.; Hong, B. H.; Rotermund, F.; Sennaroglu, A., Graphene mode-locked femtosecond Cr:ZnSe laser at 2500 nm. Opt. Lett. 2013, 38, 341-343. (19) Cho, W. B.; Kim, J. W.; Lee, H. W.; Bae, S.; Hong, B. H.; Choi, S. Y.; Baek, I. H.; Kim, K.; Yeom, D. I.; Rotermund, F., High-quality, large-area monolayer graphene for efficient bulk laser mode-locking near 1.25 µm. Opt. Lett. 2011, 36, 4089-4091. (20) Sun, Z.; Popa, D.; Hasan, T.; Torrisi, F.; Wang, F.; Kelleher, E. J. R.; Travers, J. C.; Nicolosi, V.; Ferrari, A. C., A stable, wideband tunable, near transform-limited, graphene-mode-locked, ultrafast laser. Nano Res. 2010, 3, 653-660. (21) Luo, Z.; Zhou, M.; Cai, Z.; Ye, C.; Weng, J.; Huang, G.; Xu, H., Graphene-Assisted Multiwavelength Erbium-Doped Fiber Ring Laser. IEEE Photonics Tech. Lett. 2011, 23, 501-503. (22) Maciejko, J.; Qi, X. L.; Karch, A.; Zhang, S. C., Fractional topological insulators in three dimensions. Phys. Rev. Lett. 2010, 105, 246809-246814. (23) Bernard, F.; Zhang, H.; Gorza, S. P.; Emplit, P. In Towards mode-locked fiber laser using topological insulators, Nonlinear Photonics, 2012. (24) Sotor, J.; Sobon, G.; Macherzynski, W.; Abramski, K. M., Harmonically mode-locked Er-doped fiber laser based on a Sb2Te3 topological insulator saturable absorber. Laser Phys. Lett. 2014, 11, 055102. (25) Luo, Z.; Huang, Y.; Weng, J.; Cheng, H.; Lin, Z.; Xu, B.; Cai, Z.; Xu, H., 1.06µm Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi2Se3 as a saturable absorber. Opt. Express 2013, 21, 29516-29522. (26) Zhao, C.; Zhang, H.; Li, H.; Luo, H.; Li, J.; Wang, L.; Liu, Y., 3-µm mid-infrared pulse generation using topological insulator as the saturable absorber. Opt. Lett. 2015, 40, 3659-3662. (27) Chen, C.; Xie, Z.; Feng, Y.; Yi, H.; Liang, A.; He, S.; Mou, D.; He, J.; Peng, Y.; Liu, X., Tunable Dirac Fermion Dynamics in Topological Insulators. Sci. Rep. 2013, 3, 2411-2417. (28) Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; Zant, H. S. J. V. D.; Castellanos-Gomez, A., Fast and broadband photoresponse of few-layer black phosphorus field-effect transistors. Nano Lett. 2014, 14, 3347-3352. (29) Han, C. Q.; Yao, M. Y.; Bai, X. X.; Miao, L.; Zhu, F.; Guan, D. D.; Wang, S.; Gao, C. L.; Liu, C.; Qian, D., Electronic Structures of Black Phosphorus Studied by Angle-resolved Photoemission Spectroscopy. Phys. Rev. B. 2014, 90, 678-680. (30) Hong, T.; Chamlagain, B.; Lin, W.; Chuang, H. J.; Pan, M.; Zhou, Z.; Xu, Y. Q., Polarized photocurrent response in black phosphorus field-effect transistors. Nanoscale 2014, 6, 8978-8983. (31) Huang, L.; Li, J., Tunable electronic structure of black phosphorus/blue phosphorus van der Waals p-n heterostructure. Appl. Phys. Lett. 2016, 108, 083101-083106. (32) Low, T.; Rodin, A. S.; Carvalho, A.; Jiang, Y.; Wang, H.; Xia, F.; Neto, A. H. C., Tunable optical


ACS Photonics 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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

properties of multilayers black phosphorus thin films. Phys. Rev. B. 2014, 90, 075434-075439. (33) Castellanosgomez, A.; Vicarelli, L.; Prada, E.; Island, J. O.; Narasimhaacharya, K. L.; Blanter, S. I.; Groenendijk, D. J.; Buscema, M.; Steele, G. A.; Alvarez, J. V., Isolation and characterization of few-layer black phosphorus. 2D Mater. 2014, 1, 025001-025012. (34) Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Min, H.; Hultman, L.; Gogotsi, Y.; Barsoum, M. W., Two-Dimensional Nanocrystals: Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4207-4207. (35) Anasori, B.; Lukatskaya, M. R.; Gogotsi, Y., 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2017, 2, 16098-16118. (36) Shein, I. R.; Ivanovskii, A. L., Graphene-like titanium carbides and nitrides Tin +1 Cn , Tin +1 Nn (n = 1, 2, and 3) from de-intercalated MAX phases: First-principles probing of their structural, electronic properties and relative stability. Compt. Mater. Sci. 2012, 65, 104–114. (37) Mashtalir, O., Chemistry of Two-Dimensional Transition Metal Carbides (MXenes). Degree Thesis, Drexel University, June, 2015. (38) Matthias, B. T.; Hulm, J. K., A Search for New Superconducting Compounds. Phys. Rev. 1952, 87, 799-806. (39) Xu, C.; Wang, L.; Liu, Z.; Chen, L.; Guo, J.; Kang, N.; Ma, X. L.; Cheng, H. M.; Ren, W., Large-area high-quality 2D ultrathin Mo2C superconducting crystals. Nat. Mater 2015, 14, 1135-1141. (40) Wang, L.; Xu, C.; Liu, Z.; Chen, L.; Ma, X.; Cheng, H. M.; Ren, W.; Kang, N., Magnetotransport Properties in High-Quality Ultrathin Two-Dimensional Superconducting Mo2C Crystals. ACS Nano 2016, 10, 4504-4510. (41) Xu, C.; Song, S.; Liu, Z.; Chen, L.; Wang, L.; Fan, D.; Kang, N.; Ma, X.; Cheng, H. M.; Ren, W., Strongly Coupled High-Quality Graphene/2D Superconducting Mo2C Vertical Heterostructures with Aligned Orientation. ACS Nano 2017, 11, 5906-5914. (42) Liu, Z.; Xu, C.; Kang, N.; Wang, L.; Jiang, Y.; Du, J.; Liu, Y.; Ma, X.; Cheng, H.; Ren, W., Unique Domain Structure of Two-Dimensional α-Mo2C Superconducting Crystals. Nano Lett. 2016, 16, 4243-4250. (43) Ghidiu, M.; Lukatskaya, M. R.; Zhao, M. Q.; Gogotsi, Y.; Barsoum, M. W., Conductive two-dimensional titanium carbide 'clay' with high volumetric capacitance. Nature 2014, 516, 78-81. (44) Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall'Agnese, Y.; Rozier, P.; Taberna, P. L.; Naguib, M.; Simon, P.; Barsoum, M. W.; Gogotsi, Y., Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide. Science 2013, 341, 1502-1505. (45) Shahzad, F.; Alhabeb, M.; Hatter, C. B.; Anasori, B.; Man, H. S.; Koo, C. M.; Gogotsi, Y., Electromagnetic interference shielding with 2D transition metal carbides (MXenes). Science 2016, 353, 1137-1140. (46) Khazaei, M.; Arai, M.; Sasaki, T.; Estili, M.; Sakka, Y., Two-dimensional molybdenum carbides: potential thermoelectric materials of the MXene family. Phys. Chem. Chem. Phys. 2014, 16, 7841-7849.


Page 21 of 28 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 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(47) Jhon, Y. I.; Koo, J.; Anasori, B.; Seo, M., Lee, J. H.; Gogotsi, Y.; Jhon Y. M., Metallic mxene saturable absorber for femtosecond mode-locked lasers. Adv. Mater. 2017, 29, 1702496. (48) Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y., 25th anniversary article: mxenes: a new family of two-dimensional materials. Adv. Mater. 2014, 26, 992-1005. (49) Li, P.; Chen, Y.; Yang, T.; Wang, Z.; Lin, H.; Xu, Y.; Li, L.; Mu, H.; Shivananju, B. N.; Zhang, Y., Two-Dimensional CH3NH3PbI3 Perovskite Nanosheets for Ultrafast Pulsed Fiber Lasers. ACS Appl. Mater. Interf. 2017, 9,12759–12765. (50) Bandyopadhyay,

D.;

Haldar,

B.;

Sharma,

R.

C.;

Chakraborti,

N.,

The

Ti-Mo-C

(Titanium-Molybdenum-Carbon) System. J. Phase. Equilib. 1999, 20, 332-336. (51) Sutherland, R., Handbook of Nonlinear Optics, Second Edition. Marcel Dekker Inc 2003. (52) Wang, K.; Szydłowska, B. M.; Wang, G.; Zhang, X.; Wang, J. J.; Magan, J. J.; Zhang, L.; Coleman, J. N.; Wang, J.; Blau, W. J., Ultrafast Nonlinear Excitation Dynamics of Black Phosphorus Nanosheets from Visible to Mid-Infrared. ACS Nano 2016, 10, 6923-6932. (53) Zhang, S.; Dong, N.; Mcevoy, N.; O'Brien, M.; Winters, S.; Berner, N. C.; Yim, C.; Li, Y.; Zhang, X.; Chen, Z., Direct Observation of Degenerate Two-Photon Absorption and Its Saturation of WS2 and MoS2 Monolayer and Few-Layer Films. ACS Nano 2015, 9, 7142-7150. (54) Wang, Y.; Mu, H.; Li, X.; Yuan, J.; Chen, J.; Xiao, S.; Bao, Q.; Gao, Y.; He, J., Observation of large nonlinear responses in a graphene-Bi2Te3 heterostructure at a telecommunication wavelength. Appl. Phys. Lett. 2016, 108, 183 (55) Wang, Y.; Liu, S.; Yuan, J.; Wang, P.; Chen, J.; Li, J.; Xiao, S.; Bao, Q.; Gao, Y.; He, J., Ultra-broadband Nonlinear Saturable Absorption for Two-dimensional Bi2TexSe3-x Nanosheets. Sci. Rep. 2016, 6, 33070-33079. (56) Keller, U., Keller, U. Recent developments in compact ultrafast lasers. Nature 2003, 424, 831-838. (57) Xia, H.; Li, H.; Lan, C.; Li, C.; Zhang, X.; Zhang, S.; Liu, Y., Ultrafast erbium-doped fiber laser mode-locked by a CVD-grown molybdenum disulfide (MoS2) saturable absorber. Opt. Express 2014, 22, 17341-17348. (58) Grelu, P.; Akhmediev, N., Dissipative solitons for mode-locked lasers. Nat. Photonics 2012, 6, 84-92. (59) Jeon, J.; Lee, J.; Ju, H. L., Numerical study on the minimum modulation depth of a saturable absorber for, stable fiber laser mode locking. J. Opt. Soc. Am. B. 2015, 32, 31-37. (60) Wise, F. W.; Ilday, F. Ö.; Lim, H., Generation of 2-nJ pulses from a femtosecond ytterbium fiber laser. Opt. Lett. 2003, 28, 660-662. (61) Wang, L. R.; Liu, X. M.; Gong, Y. K., Giant‐chirp oscillator for ultra‐large net‐normal dispersion fiber lasers. Laser Phys. Lett. 2010, 7, 63–67.


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Figure captions: Figure 1. Material characterizations of 2D α-Mo2C. (a) Typical optical images of 2D α-Mo2C crystals showing different regular shapes fabricated by CVD with a Cu/Mo foil as substrate and transferred onto the optical fiber end-facet (inset). Scale bar: 25 µm. (b) AFM image of the hexagonal ultrathin 2D α-Mo2C crystals with a thickness of 10.3 nm. (c) UV-visible-NIR absorption spectrum of 2D α-Mo2C on quartz substrate. Inset: optical image of α-Mo2C crystals on quartz sheet, scale bar: 50 µm. (d)-(e) TEM image of the 2D ultrathin 2D α-Mo2C crystal. Inset: SAED pattern along [100] direction. e) HRTEM image of the 2D ultrathin 2D α-Mo2C crystal. f) Transient absorption dynamics of 2D α-Mo2C crystals pumped and probed at the wavelength of 1550 nm. Figure 2. Open-aperture Z-scan results of 2D α-Mo2C crystals at 1550 nm. (a)-(c) Statistical distribution of the thickness of Sample-1 (S1), Sample-2 (S2) and Sample-3 (S3) with Gaussian fits overlaid. (d)-(f) Z-scan data of S1, S2, S3, respectively. The open dots refer to experimental data and the solid lines stand for analytical fit to the data. Figure 3. Typical mode-locking pulse output characteristics with central wavelength of 1602.6. (a) Typical mode-locking optical spectrum. (b) Mode-locking pulse train. c) Autocorrelation trace. (d) The radio frequency optical spectrum at the fundamental frequency and the wideband RF spectrum (inset). (e) The output mode-locked spectra measured every 4 hours showing long-term stability of the mode-locking state. (f) The central wavelength and the spectral bandwidth as a function of time. Figure 4. Typical mode-locking pulse output characteristics with central wavelengths of 1061.8 nm. (a) Typical mode-locking optical spectrum. (b) Mode-locking pulse train. (c) Autocorrelation trace. (d) The radio frequency optical spectrum at the fundamental frequency and the wideband RF spectrum (inset). Table 1. Nonlinear optical parameters of 2D α-Mo2C crystals concluded from the Z-scan measurement results. The relative errors are estimated as ±20%. Table 2. Comparison of the nonlinear absorption coefficient (β) and saturable absorption intensity (Is) of different 2D materials.


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Figure 1. Material characterizations of 2D α-Mo2C. (a) Typical optical images of 2D α-Mo2C crystals showing different regular shapes fabricated by CVD with a Cu/Mo foil as substrate and transferred onto the optical fiber end-facet (inset). Scale bar: 25 µm. (b) AFM image of the hexagonal ultrathin 2D α-Mo2C crystals with a thickness of 10.3 nm. (c) UV-visible-NIR absorption spectrum of 2D α-Mo2C on quartz substrate. Inset: optical image of α-Mo2C crystals on quartz sheet, scale bar: 50 µm. (d)-(e) TEM image of the 2D ultrathin 2D α-Mo2C crystal. Inset: SAED pattern along [100] direction. e) HRTEM image of the 2D ultrathin 2D α-Mo2C crystal. (f) Transient absorption dynamics of 2D α-Mo2C crystals pumped and probed at the wavelength of 1550 nm.


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Figure 2. Open-aperture Z-scan results of 2D α-Mo2C crystals at 1550 nm. (a)-(c) Statistical distribution of the thickness of Sample-1 (S1), Sample-2 (S2) and Sample-3 (S3) with Gaussian fits overlaid. (d)-(f) Z-scan data of S1, S2, S3, respectively. The open dots refer to experimental data and the solid lines stand for analytical fit to the data.


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Figure 3. Typical mode-locking pulse output characteristics with central wavelength of 1602.6. (a) Typical mode-locking optical spectrum. (b) Mode-locking pulse train. (c) Autocorrelation trace. (d) The radio frequency optical spectrum at the fundamental frequency and the wideband RF spectrum (inset). (e) The output mode-locked spectra measured every 4 hours showing long-term stability of the mode-locking state. (f) The central wavelength and the spectral bandwidth as a function of time.


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Figure 4. Typical mode-locking pulse output characteristics with central wavelengths of 1061.8 nm. (a) Typical mode-locking optical spectrum. (b) Mode-locking pulse train. (c) Autocorrelation trace. (d) The radio frequency optical spectrum at the fundamental frequency and the wideband RF spectrum (inset).


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Table 1. Nonlinear optical parameters of 2D α-Mo2C crystals concluded from the Z-scan measurement results. The relative errors are estimated as ±20%. Sample

Wavelength (nm)

S1

S2

S3

1550

1550

1550

(GW/cm2)

NLO absorption coefficient (β) (cm/GW)

Saturable absorption intensity (Is) (GW/cm2)

Modulation depth (αs) (％)

24.2

- (2.50 ± 0.50) ×103

0.74 ± 0.148

7.30 ± 1.46

6.8

- (9.20 ± 1.84) ×103

0.18 ± 0.036

8.60 ± 1.72

1.1

- (6.20 ± 1.24) ×104

0.01 ± 0.002

7.30 ±1.46

15.2

- (1.00 ± 0.20)

×104

45.00 ± 9.00

45.00 ± 9.00

4.0

- (2.30 ± 0.46)

×104

30.00 ± 6.00

45.00 ± 9.00

1.6

- (1.65 ± 0.33)

×105

3.50 ± 0.70

47.00 ± 9.40

18.3

- (0.80 ± 0.16) ×104

107.00 ± 21.40

59.70 ± 11.94

5.0

- (2.80 ± 0.56) ×104

31.00 ± 6.20

61.00 ± 12.20

1.4

- (6.80 ± 1.36) ×104

12.90 ± 2.58

55.00 ± 11.00

Laser density at the focus (I0)


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Table 2. Comparison of the nonlinear absorption coefficient (β) and saturable absorption intensity (Is) of different 2D materials. Material

Wavelength

Laser density at

NLO absorption

Saturable absorption

(nm)

the focus (I0)

coefficient (β)

intensity (IS)

(GW/cm2)

(cm/GW)

(GW/cm2)

Ref.

2D α-Mo2C

1550

24.2 ~ 1.1

- (10.0 ± 9.8) × 104

128.4 ~ 0.008

this work

BP

1550

--

- (1.8 ± 0.9) × 10-2

398 ± 163

52

Bi2Se3

1550

43.6 ~ 1.3

- (2.3 ± 0.5) × 104

132 ± 30

55

Graphene

1550

0.84

- 1.5 × 103

--

54

1030

43.0 ~ 6.2

- 250 ~ 50

--

53

-Bi2Te3 MoS2


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ACS Photonics

Graphical TOC Entry


Ultrathin 2D transition metal carbides for ultrafast pulsed fiber lasers

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