Few-Layer Platinum Diselenide as a New Saturable Absorber for

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 21534−21540

Few-Layer Platinum Diselenide as a New Saturable Absorber for Ultrafast Fiber Lasers Jian Yuan,† Haoran Mu,†,‡ Lei Li,§ Yao Chen,† Wenzhi Yu,† Kai Zhang,∥ Baoquan Sun,† Shenghuang Lin,*,†,⊥ Shaojuan Li,*,† and Qiaoliang Bao*,†,‡

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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, People’s Republic of 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 § Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou, Jiangsu 221116, People’s Republic of China ∥ i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123 Jiangsu, P. R. China ⊥ Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong 999077, P. R. China ABSTRACT: A multilayer platinum diselenide (PtSe2) film was experimentally demonstrated as a new type of saturable absorber with the capability to deliver robust dissipative solitons in a passively mode-locked fiber laser. The PtSe2 film synthesized by chemical vapor deposition was placed onto the ferule of a single mode optical fiber through a typical dry transfer process. The nonlinear optical measurements reveal efficient saturable absorption characteristics in terms of a large modulation depth (26%) and low saturable intensity (0.346 GW cm−2) at the wavelength of 1064 nm. An all-fiber ring cavity was built, in which the PtSe2 film was sandwiched between two ferules as the saturable absorber and Ytterbiumdoped fiber was used as the optical gain medium. Robust dissipative soliton pulses with a 3 dB spectral bandwidth of 2.0 nm and a pulse duration of 470 ps centered at 1064.47 nm were successfully observed in the normal dispersion regime. Moreover, our mode-locked lasers also exhibit good long-term stability. Our finding suggests that multilayer PtSe2 may find potential applications in nonlinear optics and ultrafast photonics. KEYWORDS: 2D TMDs, platinum diselenide, nonlinear optical property, saturable absorbers, mode-locked fiber laser ultrafast carrier dynamics at the picosecond timescale10−12 and saturable absorption over a broad wavelength range from 1000 to 2500 nm.13 However, the absorption coefficient in graphene is relatively low (e.g., 2.3% per layer) because of zero band gap, which has impressed certain limitations for photonic applications in which a strong light−matter interaction is demanded. Fortunately, TMDCs with a sizable band gap and strong resonant optical absorption in specific wavelengths are making up the shortage of graphene.14 For example, Zhang et al. demonstrated mode-locked pulse generation centered at 1054.3 nm (bandwidth: 2.7 nm, pulse duration: 800 ps) using MoS2 as the SA.6 Khazaeizhad et al. revealed that a modelocked fiber laser with multilayer MoS2 film as the SA can achieve a relatively large bandwidth of 12.38 nm at the center laser wavelength of 1568 nm.15 Most recently, Luo et al.

1. INTRODUCTION The mode-locking technique based on saturable absorbers (SAs) has been proved to be one of the efficient techniques to realize ultrafast pulse generation because it has many merits such as good mechanical stability, high reliability, excellent beam quality, high power scalability, easy implementation, compactness, and low cost. SAs have been extensively studied in bulk ion-doped crystals1 and semiconductors,2 especially in semiconductor saturable absorption mirrors (SESAMs)3 for over 20 years, but the fabrication process of SESAM is complicated and the price is expensive. More recently, twodimensional (2D) materials, such as graphene,4,5 transition metal dichalcogenides (TMDs including MoS2, WS2, e.g.),6,7 and black phosphorus,8,9 have raised as a new family of SAs and attracted intensive research interest for developing new ultrafast photonic applications. Tremendous efforts have been implemented to fabricate the passively mode-locked pulse lasers employing 2D materials. In particular, graphene-based SAs have been extensively studied and developed because of their © 2018 American Chemical Society

Received: February 21, 2018 Accepted: May 30, 2018 Published: May 30, 2018 21534

DOI: 10.1021/acsami.8b03045 ACS Appl. Mater. Interfaces 2018, 10, 21534−21540

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Side view and top view of the crystal structure of a monolayer PtSe2 (b) schematic diagram of the PtSe2 film synthesis process using a vapor phase selenization method. (c) Optical micrograph of the PtSe2 film, scale bar is 20 μm, AFM image is shown inset, scale bar is 5 μm. (d) XRD of PtSe2 measured at room temperature. (e) Raman spectra of PtSe2 under 633 nm laser. (f) High-resolution XPS spectra of Pt 4f and Se 3d.

optical measurements, a large modulation depth and a relatively low saturable intensity were observed in a few nanometer thick PtSe2 film, which underpins the potential application for modelocked pulse lasers. By incorporating few-layer PtSe2 film into ytterbium-doped fiber (YDF) laser, a stable mode-locking state was realized. Also, picosecond ultrafast pulses were obtained with a large SNR of over 50 dB.

reported picosecond pulse generation (pulse duration: 1.45 ps, signal-to-noise ratio (SNR): 61.5 dB) at 1558 nm by using fewlayer MoSe2 as the SA.16 Nevertheless, from optical physics point of view, 2D materials with a small band gap (∼0.8 eV) are more suitable for infrared photonics and optical communication. However, previously reported TMD SAs such as MoS2, WS2, and so forth have considerably large band gaps (e.g., monolayer MoS2: 1.78 eV; monolayer WS2: 1.84 eV; monolayer MoSe2: 1.5 eV; monolayer WSe2: 1.52 eV).17,18 Consequently, the strong resonant absorption of these materials occurs at visible wavelengths.19,20 However, fiber lasers usually operate at infrared wavelength (1064−2000 nm, corresponding to photon energies of 0.5−1 eV), in which the participation of interband optical transition in the “perfect crystals” of these materials are impossible. Relying on the atomic defects and resulting intermediate states in the band gap of these materials, the mode-locking operation was demonstrated.6,15 However, the introduction of defects makes optical processes in the samples complex and nonlinear optical effects unreliable. Different from most of the members of 2D TMDs, platinum diselenide (PtSe2) has a semimetal-to-semiconductor transition when approaching the monolayer thickness.21 Previous research studies suggest that monolayer PtSe2 has an indirect band gap of 1.2 eV and bilayer PtSe2 has an indirect band gap of 0.20 eV.22 To this point, the small band gap of PtSe2 allows all photons in fiber lasers (with photon energies of 0.5−1 eV) jump directly from the valence band to conduction band, which results in robust saturable absorption and inherently stable mode-locking operation. Moreover, the results obtained from photoresponse experiments clearly indicate that it can be utilized as mid-infrared photoactive material with fast response and strong light absorption from visible to mid-infrared,23 which are highly desired for high frequency optoelectronics and ultrafast photonics at infrared wavelengths. As compared to extensively studied MoS2 and WS2 SAs, the saturable absorption of the 2D PtSe2 has not been investigated yet, and it is nontrivial to investigate the nonlinear absorption properties in 2D PtSe2 and develop ultrafast laser applications. In this article, the saturable absorption of few-layer PtSe2 was discovered and characterized. According to the nonlinear

2. RESULTS AND DISCUSSION As indicated by previous studies, the PtSe2 belongs to the D33d (P3m1) space group with the CdI2-type trigonal (1T) structure. The crystal structure of PtSe2 is shown in Figure 1a, in which one layer of Pt atoms is sandwiched between two layers of Se atoms.21 As shown in the schematic diagram of Figure 1b, the few-layer PtSe2 film was successfully synthesized via chemical vapor deposition (CVD) inside a double heating zone furnace. A platinum (Pt) film was predeposited onto a SiO2/Si substrate (with 300 nm oxide) via electron beam evaporation. The SiO2/ Si substrate with Pt film was located at the downstream of the quartz tube with high heating temperature. Also, 180 mg selenium (Se, 99.997%; Alfa Aesar) powder was filled in a crucible at the upstream of low-temperature heating zone in the quartz tube. After that, the carrier gas (argon) was purged into the furnace to remove the residual air before carrying out the experiment. When the temperature of the Pt film at the downstream reached 450 °C, the Se powder was immediately heated to 245 °C. Then, the temperature of the Pt/SiO2/Si substrate was kept at 450 °C for 2 h to form PtSe2 film. Figure 1c shows a representative optical image of the as-produced fewlayer PtSe2 film. The inset of Figure 1c depicts the morphology of PtSe2 film characterized by atomic force microscopy (AFM). The line profile of the flake reveals that the thickness is ∼10 nm, corresponding to ∼20 atomic layers. To investigate the crystalline property of the obtained samples, the X-ray diffraction (XRD) technique was employed and the corresponding XRD patterns can be found in Figure 1d. It can be seen that the PtSe2 film possesses one pronounced peak located at 2θ = 17.2° and two small diffraction peaks at around 2θ = 33.5° and 2θ = 52.5° corresponding to the (0 0 1), (0 0 2), and (0 0 3) crystal planes, respectively, which agrees very well with the previous reports on few-layer PtSe2,24−26 21535


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Figure 2. (a) TEM image of PtSe2 layer, the low magnification TEM image of PtSe2 layer on TEM grid is shown in the inset, the scale bar is 500 nm. (b) HRTEM image of PtSe2 layer. The scale bar is 2 nm. (c) SAED image. (d) Corresponding bright-field STEM image the rectangular region in (a). The scale bar is 20 nm. (e,f) are the corresponding Pt and Se elemental maps from the rectangular region in (d), respectively. The scale bar in (d) also applies to (e,f). (g) EDS spectrum and (h) table of elemental ratio for the PtSe2 layer.

Figure 3. Optical characterizations of 10 nm PtSe2 layer. (a) UV−visible−NIR absorption spectrum of PtSe2 layer on quartz sheet. (b) Saturable absorption data measured by an open-aperture Z-scan at 1064 nm, (c) nonlinear transmission curve of PtSe2 SA. (d) Power-dependent nonlinear optical transmission of PtSe2 SA at 1064 nm obtained from dual-detector measurements.

revealing the 2D nature of the PtSe2 film. Furthermore, a Raman spectroscope equipped with a 633 nm laser was also utilized to identify and characterize the PtSe2 layer (Figure 1e). The Raman spectrum of the PtSe2 film reveals two feature peaks located at ∼176 and ∼210 cm−1. The peak centered at ∼176 cm−1 is attributed to the Eg in-plane vibration mode of Se atoms moving away from each other within the layer, and the peak centered at ∼210 cm−1 corresponds to the A1g out-ofplane vibration mode of Se atoms in opposing directions, similar to other 1T structure materials such as HfS2, ZrS2, and CdI2.27,28 The distribution of the ratio of Pt and Se atoms for the PtSe2 sample has also been studied by X-ray photoelectron

spectroscopy (XPS). The corresponding spectra of the Pt 4f and Se 3d regions are displayed in Figure 1f, suggesting a high chemical purity. The binding energies for the Pt 4f7/2 and 4f5/2 are located at ∼72.95 and ∼76.32 eV and the binding energies for the Se 3d5/2 and Se 3d3/2 are located at 54.23 and 55.14 eV, respectively, in consistent with the reported values for PtSe2.26 The atomic ratio of Pt/Se can be calculated by using the ⎛ APt ⎞ ⎛ ASe ⎞ n(Pt) formula of n(Se) = ⎜ S 4f ⎟ /⎜ S 3d ⎟, where APt4f and ASe3d are the ⎝ Pt4f ⎠ ⎝ Se3d ⎠ areas of Pt 4f and Se 3d peaks and SPt4f and SSe3d are the corresponding sensitive factors.26,29 Accordingly, the calculated relative atomic ratio of Pt/Se is ∼0.48 (∼1:2), suggesting a nearly complete transformation from Pt to the selenide. 21536


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intensity is 53.4 MW/cm2. While the input power was decreased from high to low value (backward), the modulation depth is 26.4% and the saturation intensity is 55.2 MW/cm2. The results from both forward and backward scanning of the power agree very well with each other, attesting to good stability and high optical damage threshold. Notably, the modulation depth of PtSe2 film is much higher than that of multilayered graphene (∼8%) and MoS2 (∼13.6%) at the same wavelength,4,33 which indicates that the PtSe2 film can be used as a new nonlinear absorption material for pulse reshaping as that demonstrated in conventional SAs.6,34 Then, we investigated the mode-locking performance of PtSe2 SA by incorporating it into the ring cavity of a 1 μm fiber laser. The laser cavity we designed is schematically illustrated in Figure 4. The detailed setting up process is described in the

To further explore the atomic structure, crystallinity, and elemental composition of the PtSe2 films, the as-grown samples were transferred onto a copper grid using the poly(methyl methacrylate) assisted method and studied by transmission electron microscopy (TEM), a bright-field scanning TEM (STEM), and energy-dispersive X-ray spectroscopy (EDS). Figure 2a depicts a typical TEM image for the studied continuous PtSe2 film, and the inset exhibits its corresponding low-magnification TEM image. The high-resolution TEM (HRTEM) image (Figure 2b) of the studied PtSe2 nanosheet indicates that the crystal plane spacing of the sample is 0.261 nm, which corresponds to the (002) plane of PtSe2. Figure 2c shows the selected area electron diffraction (SAED) pattern of the PtSe2 layer, which indicates that the obtained sample is polycrystalline. The three distinguished dashed red circles are assigned to (001), (101), and (002) planes with lattice spacings of 5.23, 2.79, and 2.61 Å, respectively, which is also consistent with our XRD result. Figure 2d shows the studied area for the STEM EDS (energy-dispersive X-ray spectroscopy) mapping of Pt and Se elements. The corresponding mapping images can be found in Figure 2g,f, which confirms that the studied sample really consists of Pt and Se elements and the elemental uniformity of the as-grown PtSe2 film over the whole platelet. Notably, the STEM−EDS results as shown in Figure 2g,h also depict that the film having the weight percent of Se is calculated to be 45.25% and the relative atomic ratio of Pt/Se is about 0.49 (∼1:2), revealing a perfect match between the Pt and Se elements in the obtained sample. To study the optical properties (linear and nonlinear) of the PtSe2 film, the samples were transferred onto quartz substrates and measured by a UV-vis-IR absorption spectrometer and an open aperture Z-scan system. Figure 3a shows the corresponding optical absorption spectrum. It is found that the PtSe2 film owns a strong light absorption over a wide wavelength range from 250 to 2200 nm, which fully covers the visible to nearinfrared wavelengths. Figure 3b shows the open aperture Z-scan results measured at 1064 nm. The PtSe2 sample was placed on the Z-axis movement stage, and when the sample was gradually approaching to the focus point (Z = 0), which means that the incident light intensity was continually increased, the transmission rate of the PtSe2 sample was increasing. The dependence between the transmission rate of the sample and the incident light intensity was thereby recorded, as shown by the blue dots in Figure 3b, where the typically optical saturable absorption phenomenon was observed. The nonlinear transmission curve with respect to the incident light intensity can also be directly provided by extracting the Z-scan experimental data, which was shown in Figure 3c. Through fitting by the following formula:30,31 αS α( I ) = + αNS 1 + I /IS (1)

Figure 4. Experimental setup of the YDF laser ring cavity. WDM: wavelength division multiplex. YDF: Ytterbium-doped fiber. PC: polarization controller. Isolator: polarization-independent isolator.

Experimental Section. PtSe2 film was successfully transferred onto the cross section of a single-mode fiber ferule through a standard dry transfer process with the aid of polystyrene, as shown in the left inset of Figure 5a. Under the observation of the optical microscope (the right inset of Figure 5a), it can be seen that the surface topography of the PtSe2 SA is integrated and it fully covers the core area of the fiber end-facet. Before the measurement, the self-mode locking effect in the initial cavity, which could be possibly introduced by the polarization dependence of fiber components, should be excluded. The output characteristics of the laser cavity without the PtSe2 SA were checked. It is found that no mode-locking indication appeared under any polarization state while varying the pump power from low to very high value (∼600 mW). However, once the PtSe2 SA was incorporated into the laser cavity, by slightly adjusting the intracavity polarization state, the mode-locked pulse generation was successfully obtained at a threshold pump power of 115 mW. Subsequently, while increasing the pump power up to 286 mW, the mode-locked pulse generation was steadily obtained, as presented in Figure 5. The optical spectrum with steep edges in Figure 5a indicates that it is a typical dissipative soliton mode-locking state.35 The output spectrum reveals a central wavelength at 1064.47 nm with a 3 dB spectral bandwidth of 2 nm. As shown in Figure 5b, the repetition rate of the mode-locked pulse train is 4.08 MHz, in consistent with the cavity length (49.02 m). The pulse train in a large range is also displayed in the inset of Figure 5b, attesting to good stability. The single pulse envelope is depicted in Figure 5c. Through the fitting by Gaussian function,35 it is calculated that the full width at half maximum of the single

where αS is the saturable component (referred to as modulation depth), αNS is the nonsaturable component, and IS is the saturation absorption intensity. The saturable intensity is calculated to be 0.346 GW cm−2, and the normalized modulation depth is 26%. To further confirm the saturable absorption characteristics, the balanced dual-detector measurements were also implemented. The details of the configurations can be found in previous studies.32 Figure 3d shows the powerdependent nonlinear optical transmission data. While increasing the input power from low to high value (forward), the modulation depth is calculated to be 25.6% and the saturation 21537


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Figure 5. Laser mode-locking characteristics using the PtSe2 layer SA. (a) Optical spectra. Inset (right): Photograph of the fiber pigtail with a PtSe2 film coating on the fiber end facet. Inset (left): optical image of PtSe2 film covering on the core area of the fiber end facet. (b) Wide-band oscilloscope tracings. (c) Individual pulse profile. (d) rf spectral profile. Inset: the wideband rf spectrum. The pump power is 185 mW.

Figure 6. (a) Output power as a function of pump power at 1064 nm. (b) Long-term stability of the mode-locked dissipative soliton.

pulse, or the so-called pulse duration, is ∼470 ps. The peak power is 4.9 W and the pulse energy is 2.31 nJ. The pulse chirping is calculated to be as large as 249.6. It has been suggested that a large pulse chirping is one of the characteristics of dissipative solitons, because the laser cavity should be allnormal dispersion ,and the soliton pulses are formed by the balance among the nonlinearity, optical gain, cavity dispersion, and optical loss.35 Figure 5d shows the radio-frequency (rf) spectrum of the mode-locked pulses and a high SNR of 53 dB is observed. The corresponding wideband (0−300 MHz) radio frequency spectrum is depicted in the inset of Figure 5d, further confirming very stable operation regime of the laser output. The output power as a function of the input pump power is plotted in Figure 6a. As the pump power increased from 90 to 400 mW, the corresponding output power increases linearly from 1.04 to 12.19 mW, yielding a power conversion efficiency of around 3%. The relatively low output power under the mode-locking state is mainly due to the strong absorption of PtSe2. Nevertheless, it is noteworthy that the mode-locking state can be maintained while changing the pump power over a large range, suggesting robust mode-locking performance enabled by the PtSe2 SA. To investigate the long-term stability of our fiber lasers, the output spectra of the mode-locked dissipative soliton pulse

were continuously monitored for 12 h. The evolution of the central wavelength and the spectral bandwidth are shown in Figure 6b. It can be seen that the central wavelength slightly shifts from 1064.47 to 1064.49 nm and its 3 dB bandwidth has a relatively small variation from 1.99 to 2.06 nm. These results indicate that PtSe2 holds intriguing potential as a new type of nonlinear optical material for ultrafast and nonlinear optics.

3. CONCLUSIONS In summary, both linear and nonlinear optical properties of few-layer PtSe2 prepared by CVD were studied. The SA based on PtSe2 film has a large modulation depth of 26% and a relatively low saturation intensity of 0.346 GW cm−2 at the near-infrared band. We have successfully demonstrated that PtSe2 film can be used as an efficient nonlinear absorption media to generate mode-locking pulses in an YDF laser. The mode-locked pulse centered at 1064.47 nm has a pulse duration of 470 ps, a 3 dB spectral bandwidth of 2.0 nm, a repetition rate of 4.08 MHz, and a SNR of 53 dB. Considering many attractive merits, it is anticipated that few-layer PtSe2 may find applications in ultrafast and nonlinear optics at even longer wavelength where a nonlinear absorption material is required. 21538


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4. EXPERIMENTAL SECTION

Technology. Q.B. acknowledges support from the MCN Tech Fellowship. H.M. acknowledges the Monash Graduate Scholarship (MGS) and Monash International Postgraduate Research Scholarship (MIPRS) for his Ph.D. research.

4.1. Material Characterization. The morphology and thickness of the PtSe2 was examined using optical microscopy (Leica, DM2700 M) and AFM (Cypher S, Asylum Research). The crystal structure was investigated by XRD (PANalytical, Empyrean), and the crystallinity was verified by TEM (FEI Tecnai F30, acceleration voltage: 200 kV). The chemical atomic ratio was measured by using XPS (Japan, ULTRA DLD). The Raman spectra were measured at 633 nm on a micro-Raman spectrometer (Horiba Jobin-Yvon, LabRAM HR800). The estimated laser power reaching the sample is around 10 mW, and a high magnification objective lens (×100, NA = 0.9) was used. The grating of the spectrometer has 1800 lines/mm. The linear optical absorption were measured on a UV−vis−NIR spectrometer (Lambda 750, PerkinElmer). 4.2. Z-Scan Measurements. Z-scan measurements were performed to study the nonlinear index and the nonlinear absorption coefficient by using a femtosecond pulse laser. The few-layer PtSe2 sample under investigation was placed at Z-axis of the incident beam and moved along the light propagation direction. The transmittance of few-layer PtSe2 is defined as the ratio of the pulse energy passing the sample to the total incident pulse energy. The transmittance of the few-layer PtSe2 sample changes because the irradiance is changing, while the sample is moving to or from the focus point (z = 0). The details of the open-aperture Z-scan measurements were described in our previous report.30,35 4.3. Laser Performance Characterization. In the fiber laser cavity, an YDF (Thorlabs, Yb1200-4/125, 75 cm) was used as the gain medium, which was reversely pumped by a laser diode (975 LD, 975 nm) by a wavelength division multiplexer (WDM, 980/1060 nm). A polarization-independent isolator (ISO) was used to ensure the light propagation direction. A polarization controller (PC) is placed between the pump and SA to adjust the cavity polarization state as well as intracavity birefringence. A 10 dB port of a coupler was used to realize 10% output. The output time-domain profile and the frequency-domain spectra were simultaneously monitored by using an optical spectrum analyzer (Yokogawa, AQ6370C) and an oscilloscope (Tektronix, MDO3104, 500 MHz), and the output power was recorded by an optical power meter (EXFO, PM-1623).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Shenghuang Lin). *E-mail: [email protected]. Fax: (+86)-512-65882846 (Shaojuan Li). *E-mail: [email protected] (Q.B.). ORCID

Baoquan Sun: 0000-0002-4507-4578 Qiaoliang Bao: 0000-0002-6971-789X Author Contributions

J.Y., H.M., and L.L. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (nos. 61604102, 11404372, 51290273, and 91433107), National Key Research & Development Program (no. 2016YFA0201902), Australian Research Council (FT150100450, CE170100039, and IH150100006), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Collaborative Innovation Center of Suzhou Nano Science and 21539


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Few-Layer Platinum Diselenide as a New Saturable Absorber for

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