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Abstract
We demonstrate a monolayer MoS2-based saturable absorber (SA) synthesized by chemical vapor deposition (CVD). The as-grown SA has high spatial homogeneity and shows excellent saturable absorption properties in the 2 µm wavelength region. A higher modulation depth of 21.3% and lower saturation intensity of 0.47 MW/cm2 are achieved compared with those of other documented MoS2 SAs to date. When applying the MoS2 SA to a Tm:YAG ceramic laser, we observe a Q-switched pulse train with a shortest pulse duration of 388.0 ns under a pulse repetition rate of 82 kHz, which corresponds to a single pulse energy of 5.4 uJ and a peak power of 14 W. We believe that CVD constitutes an effective method towards high-quality MoS2 saturable absorbers that possess remarkable application potential in the 2 µm wavelength region.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulsed laser sources operating near 2 µm wavelength region have attracted a lot of attentions due to the numerous applications in different fields. For example, the pulsed single-frequency 2 µm laser sources have been widely used in the field of laser remote sensing for detecting the concentration of important atmospheric gases like H2O and CO2 in the atmospheric air. Since there exists an atmospheric window near 2 µm wavelength region, the pulsed 2 µm laser source is very suitable for wind speed detection in a Doppler LIDAR [1,2]. With a pulsed 2 µm laser sources, the emission wavelength of laser can be efficiently extended to mid-infrared wavelength region via the nonlinear frequency conversion process. The mid-infrared laser is the key to realize laser countermeasure in a combat [3]. 2 µm pulsed lasers also can be used for laser surgery due to its special blood coagulation effect which can suppress bleeding when cutting the human tissue. Meanwhile, 2 µm lasers also can be found a lot of applications in industrial material processing, biomedical imaging, et. Al. [4,5]. Therefore, a robust 2 µm pulsed laser is the basis to realize the above mentioned applications. Due to the compactness and low-cost, the passive Q-switching method based on saturable absorbers (SAs) is considered to be the most efficient way to reach such kind of laser pulses. In a passive Q-switching laser, a SA is the key to realize excellent laser performance. At present, carbon-based low dimensional materials, such as carbon nanotubes and graphene, have been employed for passive Q-switching 2 µm lasers [6–8]. However, precise control of the diameter and purity of carbon nanotubes remains a challenge, and thus a SA based on uniform carbon nanotube layers is quite difficult to realize [6]. Moreover, the carbon nanotube SA is vulnerable to the surrounding environment, making it easily deteriorated at a high temperature aerobic condition. Graphene can be grown uniformly, yet its application is limited by the inevitable oxidation in the presence of water [7]. Recently emergent two-dimensional topological insulators (TIs) hold similar advantages as the above-mentioned carbon-based materials, such as ultrafast saturation recovery time and a wide absorption band. Although TIs have shown good saturable absorption proprieties in the spectral range from 1 µm to 3 µm [9–12], their long-term properties are still influenced by the oxidation to some extent [13]. Molybdenum disulfide (MoS2), a typical transition-metal dichalcogenide (TMD), with a hexagonal crystalline lattice, is rather stable in air and thus could provide a longer lifetime, making it suitable for solid-state lasers [14]. On the other hand, the modulation depth is an important parameter that evaluates the performance of a saturable absorber. A large modulation depth is favorable for achieving short laser pulses. MoS2 SAs with different modulation depth have been reported in the spectral range between 1 µm and 3 µm [15–22]. A multi-layer MoS2 SA showed a state-of-the-art modulation depth of up to 21% in the 2 µm spectral region recently [23], which is larger than the most reported values in TMDs (WS2 or WTe2) based SAs [24]. The large modulation depth is favorable for a short Q-switching pulse generation. For example, a previous study shows that the MoS2 SA is superior to WS2 in a short Q-switching pulse generation [25]. Besides, the MoS2 SA was more efficient in the 2 µm wavelength region compared to that at 1 µm, due to a lower saturation intensity [26].
There are several methods for the preparation of MoS2 SAs, including liquid-phase exfoliation (LPE), thermal decomposing (TD), pulsed laser deposition (PLD), vapor phase growth (VPG), and physical vapor deposition (PVD) [18,23,27]. Despite the existing techniques, fabricating a MoS2 SA with large area, high crystallinity, as well as structural homogeneity is still challenging. With LPE, the uniformity and thickness of the MoS2 SA cannot be precisely controlled, which often yields flakes randomly located on the substrate [23,28–30]. The other three methods, i.e., PLD, VPG, and PVD, often yield small crystalline flakes. Compared to the above-mentioned techniques, CVD possesses the advantages of low cost, high production yield, excellent scalability, as well as controllable sample quality and homogeneity [31]. In particular, the thickness of MoS2 layers can be tuned with the growth parameters [32]. Despite the well-recognized feasibility of MoS2 as SA for solid-state 2 µm lasers, the unique advantages of CVD-grown MoS2, in particular, with the precisely controlled thickness and a high spatial homogeneity, have not been adequately investigated in Q-switched bulk lasers so far.
In the present contribution, we demonstrate a high-quality monolayer MoS2 SA with good homogeneity and uniform thickness synthesized by the CVD method. The SA has a saturation intensity of 0.47 MW/cm2, a non-saturable absorbance of 11.9%, and a modulation depth of 21.3%. The large modulation depth endows the SA with a short pulse generation ability, which eventually enables a 388 ns pulse in a 2 µm Tm:YAG ceramic laser. Compared to previous reports, the CVD grown MoS2 SA shows unique advantages in realizing 2 µm solid-state lasers.
2. Preparation and characterization of MoS2 SA
The employed CVD furnace for MoS2 growth is shown in Fig. 1, in which the horizontal tube is comprised by a low temperature zone and a high temperature zone. In order to realize the sufficient mixing of Mo and S in the gas phase, the high purity Ar acts as the carrier gas, with its flow direction indicated by the arrow in Fig. 1. Before growth, flushing with the high pure Ar was performed to eliminate the residual oxygen and other contaminates inside the tube. Ultra-pure MoO3 (∼99.998%) and S (∼99.98%) powder serve as the sources with their location and distance marked in Fig. 1. The sapphire substrate that hangs above the MoO3 powders is placed in the high temperature zone. The sulphur powder kept at the low temperature zone is placed about 20 cm away from the substrate. It is known that the introduction of interfacial defects and tensile strains or stacking of several monolayers in sheets can strongly affect the band structure of MoS2 [26]. Considering the saturable absorption of MoS2 relies on its bandgap, so with this method we can indirectly adjust the saturable absorption properties of MoS2. During the growing period, the thickness and defects of MoS2 are strongly dependent on the ambient temperature inside the furnace. In the chemical reaction stage, a higher ambient temperature is beneficial to form the monolayer MoS2 and a strong tensile strain, while the low ambient temperature is good for the formation of multi-layer MoS2. Thus by simply controlling the ambient temperature one could adjust the thickness of MoS2. But when the temperature is higher than 1050 K, the formation of MoS2 becomes very difficult. So in the experiment, the temperature of the two zones is ramped gradually to 483 K and 1050 K. In the annealing stage, the tensile strains can be further strengthen by fast decreasing of the ambient temperature. Simultaneously, the interfacial defects also can be introduced during this fast-cooling process. So after a complete reaction of 10 mins, we use 15 sccm Ar flow for fast-cooling down. A picture of the fabricated MoS2 on the sapphire wafer is given as the inset at the bottom right of Fig. 2(a).
Fig. 2. (a) Optical microscopy image of the as-grown MoS2 on sapphire. Scale bar: 400 µm. Inset: as-grown MoS2 covering the whole sapphire wafer (the bottom right) and the scanning electron microscope (SEM) image (the top left). (b) AFM topographic image of the same sample in (a) with a dimension of 4 µm×4 µm×10 nm. (c) The height profile of MoS2 over a length of 2 µm. (d) The thickness distribution of the MoS2 film. (e) Raman spectrum of the CVD synthesized MoS2. (f) The nonlinear transmission of the fabricated MoS2 SA. Inset: the experimental setup for nonlinear transmission measurement.
In order to evaluate the homogeneity of the CVD fabricated MoS2, we take an optical microscope (OM) image, a scanning electron microscope (SEM) image, and an atomic force microscope (AFM) image respectively, which are shown as Fig. 2(a) and Fig. 2(b). The bright spot in Fig. 2(a) indicates the original nucleation sites, and the gray area is mainly the synthetic MoS2. The monolayer MoS2 covers about 80% space of the sapphire substrate, while the rest part is dominated by multi-layered MoS2, distributing randomly on the substrate. Figure 2(b) is the AFM image with a scanning size of area of 4 µm×4 µm, which indicates the height of the MoS2 film is around 0.94 nm [see Fig. 2(c)] with a slight fluctuation of about 0.15 nm [see Fig. 2(d)]. Considering the single layer MoS2 has a thickness of 0.65 nm [31], the synthesized film has a monolayer structure. The monolayer structure is further verified by the measured Raman spectrum in Fig. 2(e), which has two characteristic peaks (in-plane mode $\mathrm{E}_{2 \mathrm{g}}^{1}$ and out-of-plane mode A1g [32]) with a separation of 20 cm−1. The measured small-signal transmission of the monolayer MoS2 around 2 µm approaches to 91%, which is close to the reported values [20,26,29]. To obtain the saturable absorption characteristics of the monolayer MoS2 at 2µm, its nonlinear transmission is measured by a pair of balanced twin-detectors [33] [inset figure of Fig. 2(f)]. The pump source is a Tm:YAG laser with a center wavelength of 1985nm and pulse duration of 200 ns. The obtained data is theoretically fitted with the following equation [Fig. 2(f)]:
Where $\Delta T$ and Tns are the modulation depth and non-saturable absorption of SA, respectively. I and Isat are the pulse intensity and saturation intensity of the input pulse, respectively. Based on the fitted result, the modulation depth of 21.3%, saturation intensity of 0.47 MW/cm2 and non-saturable absorption of 11.9% are determined.
For a parallel comparison, we summarize the saturable absorption characteristics of MoS2 SA fabricated by different methods at 2 µm in Fig. 3. The x-axis in Fig. 3 is modulation depth, while the y-axis are saturation intensity and non-saturable absorption respectively. From Fig. 3(a), it can be found that the CVD synthesized MoS2 SA has the lowest saturation intensity of 0.47 MW/cm2, which is slight lower than that of graphene (about 0.6 MW/cm2) [26]. Meanwhile, the CVD synthesized MoS2 SA has the state-of-art modulation depth and a moderate non-saturable absorption loss [see Fig. 3(b)]. Despite a very high purity and homogeneity, the CVD synthesized MoS2 still has a high non-saturable absorption losses of 11.9%, which is not superior to the previous reported results. We attribute this to the scattering induced by the nucleation and the sapphire substrate. The saturable absorption characters of the CVD synthesized MoS2 SA are quite similar with that reported in Ref. [23], in which the SA is synthesized by LPE method. But the SA reported in that work has a bad uniformity, which shows a large thickness variation about 2 nm. Furthermore, due to the roughness surface and non-uniform thickness of the MoS2 SA, the LPE method fabricated MoS2 SAs exhibit a huge difference between each other. In comparison, these disadvantages rarely occur in the CVD fabricated MoS2 SAs, which with a good characterization result as shown above.
Fig. 3. The saturable absorption characteristics of MoS2 SA fabricated with different methods in the 2 µm wavelength region. Different color indicates different fabrication method. X-axis indicates the modulation depth and Y-axis gives the saturation intensity in Fig. 3(a) and the non-saturable absorption in Fig. 3(b).
3. Experiment setup and discussion
We further investigate the saturable absorption properties of the CVD synthesized MoS2 SA at 2 µm wavelength region by applying it to a passive Q-switching Tm:YAG ceramic laser system (Fig. 4). A 3 mm×3 mm×6 mm Tm: YAG (5 at. %) ceramic with the advantages of stable chemical properties, ease of fabrication, and capable of high concentration doping is selected as the gain medium. The pump light is coupled out through an optical fiber from a 793 nm laser diode (LD), then focused into the Tm: YAG ceramic with a 1:1 telescope. In order to effectively remove the generated heat, the gain medium is water-cooled at 13 °C by a cooling system. The laser cavity is built with two plan mirrors (M1 and M2) with a total length of 3.5 cm. The input plan mirror M1 is highly reflective (HR) coated from 1820 nm to 2150 nm (reflectivity > 99.9%) and anti-reflection (AR) coated around 790 nm. The plan mirror M2 with a transmittance of 3% from 1820 nm to 2150 nm acts as an output coupler. After M2, a filter is employed to block the pump laser. The Q-switching pulse profile is detected by an InGaAs PIN detector (EOT, ET-5000) and observed with an oscilloscope (Tektronix, DPO 4102B-L). The average output power is measured with a laser power meter (CNI, TS35).
The power performances of the continuous-wave (cw) and passive Q-switching (PQS) laser are shown in Fig. 5(a). The maximum absorbed pump power is limited within 8.3 W for getting a stable Q-switching operation. The thresholds for cw and PQS are both close to the absorbed pump power of 2 W. The slight threshold difference arises from the small saturated intensity of the monolayer MoS2 SA and the low intracavity loss induced by the flat surface of MoS2 SA. The slope efficiencies of 10.4% and 7.0% are achieved for cw and PQS regimes respectively. The maximum output powers of 660 mW is obtained in cw regime, corresponding to a power instability of 1.8% measured within 60 min. The introduced MoS2 SA deteriorates the laser output, resulting in a maximum output power of 445 mW with a power instability of 6.2% under the same absorbed pump power. The emission wavelength of the Tm:YAG laser is measured by employing a NIR spectrometer (Avaspec, NIR256-2.5TEC) with a resolution of 7 nm [Fig. 5(b)]. In the cw regime, single wavelength radiation at 2015 nm with a full width at half maximum (FWHM) of 16.8 nm is achieved [inset figure of Fig. 5(b)]. Despite the low resolution of the spectrometer, we still observe a stable dual-wavelength operation at 1996nm and 2027nm in the PQS regime. We believe the dual-wavelength emission is induced by the etalon effect from the substrate of the MoS2 SA.
Fig. 5. (a) The power performance in cw and PQS regimes (inset: the statistical distributions of maximum output power for 60 mins). (b) The emission spectra in cw (inset figure) and PQS regimes.
The beam quality factors (M2 factors) shown in Fig. 6(a) in PQS regime are measured by using the 90.0/10.0 scanning-knife edge method at the maximum average output power. The determined M2 factors from the fitted curve are 1.27 and 1.22 in tangential and sagittal direction respectively. These values are slight larger than the ideal values of 1 in each direction, which arise from the thermal lens effect of gain medium and the slight mode mismatching between pumping beam (105 µm) and intracavity mode (120 µm).
Fig. 6. (a) M2 factors in tangential and sagittal direction. (b) The pulse profile and the pulse train at the maximum absorbed pump power in Q-switching regime with MoS2 SA.
Figure 6(b) shows the pulse profile at the maximum average output power of 445 mW. A shortest pulse duration of 388 ns is obtained at a pulse repetition rate of 82 kHz. Considering the Q-switching pulse still exists stably at the maximum average output power of 445 mW, we estimate the damage threshold of MoS2 is higher than 1.5 MW/cm2. Figure 7(a) gives the pulse durations and repetition rates at different absorbed pump power. Due to the increasing intracavity photon intensity, the bleaching time of the SA is shortened, resulting in a decreasing of the pulse duration from 1325 ns to 388 ns and an increasing of the pulse repetition rate from 39 to 82 kHz. For a passive Q-switching laser pulse, further shorten the Q-switching pulse duration could be achieved by decreasing the cavity length and improving the modulation depth of SA according to the Ref. [35]. For example, a much shorter pulse duration of 175 ns is achieved in a 2 µm microchip laser which only has a cavity length of 4 mm [26]. Figure 7(b) shows the single pulse energy and pulse peak power versus the absorbed pump power. The maximum single pulse energy of 5.4 µJ is achieved at the absorbed pump power of 8.3 W, corresponding to a peak power of 14 W.
Fig. 7. (a) The pulse duration and repetition rate versus the absorbed pump power. (b) The single pulse energy and pulse peak power versus absorbed pump power.
The reported MoS2 passive Q-switching properties in the 2 µm bulk laser is summarized in Table 1. The pulse duration of 388 ns is only slightly larger than the Q-switching microchip laser reported in Ref. [26]. However, we find that the modulation depth (0.32%) of the MoS2 fabricated by PLD in Ref. [26] is much smaller than our case. So we believe the pulse duration in the microchip laser could be further shortened with a CVD prepared MoS2 SA. Meanwhile, the pulse duration of 388 ns is shorter than that obtained from other low dimensional materials (WS2, Black phosphorus, WSe2, BN, Graphene oxide, Gold nanorods, SnSe2, Single-walled carbon nanotubes, and Graphene) summarized in Ref. [33]. The maximum output power of 445 mW, single pulse energy of 5.4 µJ and peak power of 14 W are also larger than most of the reported values in Table 1. Above analyses could demonstrate the monolayer MoS2 SA is more suitable for short Q-switching laser pulse generation with high pulse energy in 2 µm wavelength region. Moreover, the CVD method is reliable to obtain such high quality monolayer MoS2.
Table 1. Comparison of passive Q-switching 2 µm solid-state lasers based on MoS2 SA.
4. Conclusion
In this work, a monolayer MoS2 SA is fabricated by CVD and its saturable absorption properties are characterized in the 2 µm spectral region. The CVD prepared MoS2 has a good homogeneity and uniform thickness. A large modulation depth of 21.3% and a low saturation intensity of 0.47 MW/cm−2 are demonstrated, which has not been reported before for MoS2 SAs. Using the MoS2 SA, a passively Q-switched Tm:YAG ceramic laser with multi-wavelength emission around 2 µm is realized. The obtained output characteristics, i.e., 388 ns pulse duration, 5.4 µJ single pulse energy and 14 W peak power, are better than most of the previously reported results. We believe the CVD fabricated MoS2 SA with the advantage of large area, excellent crystalline quality and homogeneity is highly suitable for 2 µm pulse laser generation.
Funding
Qilu Young Scholar Program of Shandong University; Taishan Scholar Foundation of Shandong Province (tsqn201812010); Deutsche Forschungsgemeinschaft (182087777 - SFB 951); Natural Science Foundation of Shandong Province (ZR2018MF033); National Natural Science Foundation of China (61775119).
Disclosures
The authors declare no conflicts of interest.
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