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Abstract
A noble transition metal dichalcogenide, bilayers platinum diselenide (PtSe2), has a narrow bandgap (0.21 eV) and high charge carrier mobility. This metal was manufactured for use as a saturable absorber via the chemical vapor deposition method. The saturable absorption properties of samples, at a wavelength of 2.0 μm, were characterized by the open aperture Z-scan method. An all-solid-state 2.0 μm passively Q-switched laser was achieved experimentally based on the as-prepared bilayers PtSe2 saturable absorber. The maximum average output power, shortest pulse width, highest single-pulse energy, and highest pulse peak power of this laser were 1.41 W, 244 ns, 24.3 μJ, and 99.6 W, respectively.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, two-dimensional (2D) saturable absorber (SA) materials have been a development hotspot in the laser modulation field because of advantages that include compactness, low cost and broadband properties [1–4]. Transition metal dichalcogenides (TMDs), as a representative example of these 2D materials, are composed of a transition metal (M) atomic layer located between two chalcogen (X) atomic layers. Like graphene and other van der Waals solids, TMDs are characterized by weak noncovalent bonding between their layers and strong covalent in-plane bonding [5–7]. Layered TMDs present a variety of exciting properties that are not seen in their bulk counterparts because of quantum confinement and surface effects [6,7]. When compared with graphene, black phosphorus (BP) and other layered 2D materials that have been used as SAs [8–11], layered TMDs offer the advantages of strong absorption and spin–orbit coupling, oxidation resistance, bandgap tunability and high oscillator strength [3,4,12], which makes them more suitable for saturable absorption applications, and these materials have been widely used as SAs [13–16]. However, most layered TMDs have a large bandgap that lies in the visible and near-infrared range that hinders their use in mid-infrared range applications. One solution that has been used to reduce the bandgap is the introduction of defects [12,17,18], but this process is harmful to the lattice structure, is difficult to control quantitatively and reduces the charge carrier mobility (low charge carrier mobility is another disadvantage of layered TMDs that has limited the applications of these materials) [5]. It is therefore essential to find a new and superior layered TMD material that can overcome these shortcomings.
Platinum diselenide (PtSe2) is a TMD that crystallizes in a centrosymmetric CdI2-type structure with space group Pm1 (no. 164) and point group D3d (−3m). The structure can be regarded as being composed of hexagonal close-packed Se atoms with Pt atoms occupying the octahedral sites in alternate Se layers. The adjacent unoccupied Se layers are held together by weak van der Waals interactions that give rise to the quasi-2D nature of the material [19]. Bulk PtSe2 was confirmed as being a type-II Dirac semimetal with strong electron–phonon interactions [19–21], while monolayer PtSe2 has proved to be semiconducting, with a bandgap of 1.2 eV, but its bandgap was significantly reduced to 0.21 eV (corresponding to a response wavelength of 6000 nm) in a bilayers. If the thickness of the material continues to increase, it will ultimately become a metallic conductor without a bandgap [22–24]. In addition, the no-bandgap materials are unfit to act as saturable absorbers because it is not conducive to the accumulation of the upper level particles [12]. Meanwhile, the charge carrier mobility of PtSe2 has been predicted to be among the highest reported in TMDs and is comparable to that of BP [25,26]. These merits of PtSe2 (especially bilayers PtSe2) compensate ideally for the defects presented by other TMDs and mean that it has considerable potential for use on saturable absorption applications. Recently, passively mode-locked ytterbium-doped and Er-doped fiber laser and a Nd:LuVO4 laser based on multilayered PtSe2 films have been reported [27–29]. However, there have been no reports to date on saturable absorption properties of bilayers PtSe2 and applications beyond 2 μm wavelengths.
In this work, a bilayers PtSe2 SA was fabricated by chemical vapor deposition (CVD). The saturable absorption behavior of the material at 2.0 μm was investigated using open-aperture Z-scan measurements. When the sample was used for a diode-pumped solid-state Tm:YAP laser, a maximum average output power of 1.14 W was achieved at a pulse width of 244 ns and with a repetition rate of 58 kHz. The corresponding maximum single pulse energy and peak power were calculated to be 24.3 µJ and 99.6 W, respectively.
2. Synthesis and characterization of bilayer PtSe2 SA
Many fabrication techniques can be used to produce reliable layered 2D materials, and these techniques can be separated broadly into two approaches: top-down exfoliation approaches (e.g., mechanical exfoliation, liquid phase exfoliation) and bottom-up growth approaches (e.g., CVD, pulsed laser deposition) [2,7,30–32]. When compared with the top-down exfoliation methods with the disadvantage of poor scalability and low yield render; bottom-up growth techniques, and especially CVD methods, offer merits that include excellent uniformity, strong reliability and high repeatability [2,3]. Those properties are essential for translation of the electronic and optical properties of these materials into new applications [7]. What’s more, the CVD offers a scalable method for the fabrication of controllable layer 2D materials, makes it possible to study the properties of materials with the change of the number of layers. In this article, using a bottom-up CVD method, the uniform bilayers PtSe2 was successfully grew on a sapphire substrate.
The Raman spectra (excited using a 532 nm laser source) of the bilayers PtSe2 SA are shown in Fig. 1(a). Three typical Raman peaks were clearly identified and are labeled as Eg, A1g and LO. The A1g mode indicates the out-of-plane vibrations of Se atoms that are moving away from each other, while the Eg mode indicates intra-layer in-plane vibration of Se atoms that are moving in opposite directions, and the longitudinal optical (LO) peak is a combination of the out-of-plane (A2u) and in-plane (Eu) vibrations of the Pt and Se atoms, respectively [24,25]. Figure 1(b) shows the absorption spectra of the substrate and three sections of the bilayers PtSe2 SA over the wavelength range from 1000 to 2500 nm. These spectra clearly indicate that the sample show high optical uniformity and smooth absorption behavior in the infrared wavelength bands. Atomic force microscopy (AFM) images and typical height profiles of the prepared bilayers PtSe2 SAs are shown in Fig. 1(c) respectively, to illustrate the surface topographies and thicknesses of the material. The material is grown uniformly on the sapphire substrate by CVD. In addition, by a process of comparison with substrates from which the grown materials had been removed, the height of the bilayers PtSe2 SA was found to be approximately 1.3 nm, which perfectly corresponds to a thickness of two layers (the thickness of monolayer PtSe2 is approximately 0.63 nm).
Fig. 1 (a) Raman spectra and (b) absorption spectra of the bilayer PtSe2 SAs. (c) Atomic force microscopy (AFM) images and typical height profiles of the bilayer PtSe2 SAs.
The nonlinear saturable absorption properties of the PtSe2 SA at the operating wavelength of 2.0 μm were characterized using the open aperture Z-scan technique. The laser source was a self-made Tm:YAP mode-locked laser with a pulse width of 3 ps and a repetition rate of 65 MHz at a center wavelength of 1988 nm. The measured data was fitted by using the formulas in [15]. When focused using a calcium fluoride lens (focal length f = 100 mm), the radius of the beam waist and the Rayleigh length were approximately 100 μm and 1500 μm, respectively. As shown in Fig. 2(a), when the bilayer PtSe2 SA moves towards the focal point (i.e., Z = 0), the fitted Z-scan curve turns into a peak shape, thus illustrate that the transmittance of the sample tended towards saturation in the focal region. This demonstrated the strong saturable absorption capability of the sample. Three sections on the bilayer PtSe2 SA were measured. The results were similar to each other, which also demonstrate the uniform optical properties of the SA. In the nonlinear transmittance curves that are shown in Fig. 2(b), the increasing transmittance of the sample gradually slowed with increasing input laser intensity. Through a process of fitting of these curves, the unsaturated losses, the saturation fluence, and the modulation depth were determined to be 10.5%, 3.2 μJ∕cm2, and 6.6%, respectively.
Fig. 2 (a) Open aperture Z-scan curves of different sections of the SA and (b) nonlinear transmittance curves for the bilayer PtSe2 SA at 2.0 μm.
3. Experimental results and discussion
To investigate the passively Q-switched (PQS) performances of the prepared PtSe2 SA samples, a Tm:YAP laser with a compact 20-mm-long concave-plane resonator was used in the setup shown in Fig. 3. The input mirror (IM) had a radius of curvature of 200 mm and was antireflection-coated for the 0.78–0.81 μm range (transmittance T > 95%) and high-reflection-coated for the 1.8–2.1 μm range (reflectivity R > 99.5%), while the output coupler (OC) was flat and gave partial transmission of 5% in the 1.8–2.1 μm range. The gain medium was an a-cut uncoated 3 at.% Tm:YAP crystal with dimensions of 3 × 3 × 8 mm3. The crystal was wrapped in an indium foil and placed in a copper heat sink with water cooling at 15°C to remove heating effects. The pump source was a 792 nm fiber-coupled laser diode with a numerical aperture of 0.22 and a fiber core diameter of 400 μm. Using an optical focusing system, the pump light beam was focused into the gain medium with a radius of 400 μm.
The output power versus absorbed pump power characteristics of the PQS laser during operation with the bilayers PtSe2 SA are shown in Fig. 4(a). The maximum output powers of 1.88 W for continuous wave (CW) operation and 1.41 W for PQS operation were attained at an absorption pump power of 4.95 W, which corresponded to slope efficiencies of 44% for the CW laser and 35% for the PQS laser. As shown in Fig. 4(b), the central wavelengths of the CW and PQS lasers were located at 1994 and 1987 nm, respectively. The center wavelength of the PQS operation was slightly blue-shifted, which may have been caused by an insertion loss introduced by the SA.
Fig. 4 (a) Output power versus absorbed pump power during continuous wave (CW) and PQS operation. (b) Output spectra of the CW and PQS lasers.
Using an oscilloscope (Tektronix; bandwidth: 1 GHz) and a fast InGaAs photodetector with a rise time of 35 ps, (ET-5000, EOT, USA), the shortest pulse width of 244 ns and the highest repetition rate of 58 kHz were detected at an absorption pump power of 4.95 W. Figure 5 shows the stable pulse train at the highest repetition rate and the shortest pulse profile that was generated using the bilayers PtSe2 SA. This stable and smooth PQS pulse train demonstrated the excellent modulation capabilities of the bilayers PtSe2 SA.
Fig. 5 (a) Stable pulse train at the highest repetition rate and (b) the shortest pulse profile generated using the bilayer PtSe2 SA.
As Fig. 6(a) shows, the pulse width decreased with increasing absorbed pump power, while the pulse repetition rate increased correspondingly. In addition, Fig. 6(b) shows that the calculated single pulse energy and the peak power both increased with increasing absorbed pump power. The maximum single pulse energy was calculated to 24.3 μJ, which corresponded to a maximum pulse peak power of 99.6 W. During PQS laser operation, the instability (root mean square (rms) output power) was measured to be 2.206% @ 1 h, and no palpable damage was observed on the bilayers PtSe2 SA.
Fig. 6 (a) Pulse width and repetition rate versus absorbed pump power, and (b) single pulse energy and pulse peak power versus absorbed pump power.
The properties of a selection of 2.0 μm PQS lasers based on 2D materials are summarized in Table 1. As shown in the Table 1, our bilayers PtSe2 SA provides decided advantages over the other materials in terms of its narrow pulse width, high pulse energy and high peak power, besides BP in [34]. But our bilayers PtSe2 SA has advantages over BP in homogeneity and oxidation resistance, the sample can still be used after one mouth at room temperature. These merits indicate that PtSe2 is a promising material for high-power short laser pulse generation.
Table 1. Comparison of 2.0 μm PQS lasers with SAs based on different 2D materials.
4. Conclusions
In conclusion, a bilayers PtSe2 SA was manufactured using the CVD method. The saturable absorption properties of the sample at 2.0 μm was investigated by a open-aperture Z-scan device. Then, a diode-pumped all-solid-state PQS Tm:YAP laser based on use of the bilayers PtSe2 SA was demonstrated. From an absorbed pump power of 4.95 W, a maximum output power of 1.41 W was generated at a center wavelength of 1987 nm, with the shortest pulse width of 244 ns and the repetition rate of 58 kHz corresponding to a maximum single-pulse energy of 24.3 μJ and maximum peak power of 99.6 W. To the best of our knowledge, it is the first time that bilayers PtSe2 was employed as SA for 2.0 μm solid-state lasers. In addition, the experimental results indicated that bilayers PtSe2 was a promising candidate for generation of high-power short laser pulses as a broadband saturable absorber.
Funding
National Natural Science Foundation of China (NSFC) (61675116); Key R&D Program of Shandong Province (2017CXCC0808); Major Research and Development Program for Public Welfare in Shandong (2017GGX20134); and Young Scholars Program of Shandong University (2017WLJH48).
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