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Abstract
The chemical vapor deposition (CVD) method was employed to fabricate a two-dimensional (2D) MoS2 nanosheet and the related characteristics were measured. By using a new crystal Tm:Ca(Gd,Lu)AlO4 as laser medium, a laser-diode (LD) pumped dual-loss-modulated Q-switched laser with AOM and a MoS2 nanosheet was first presented as far as we know. The minimum pulse duration of 82 ns and the maximum peak power of 589 W were obtained under the modulation rate of 3 kHz. In comparison with the singly Q-switched laser with AOM or MoS2, the dual-loss-modulated Q-switched laser could generate shorter pulse width and higher peak power. The maximum compression ratio of pulse duration was 9.85 and the highest enhancement factor of peak power was 123. The experimental results hit a conclusion that 2D MoS2 nanosheet is potential in pulse laser at ∼2 µm and dual-loss-modulated Q-switching operation can compress the pulse duration and improve the peak power.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
All-solid-state lasers are of great research interest for its high efficiency, stable performance and good reliability [1]. Especially, Q-switched lasers at 2 µm waveband are ideally suitable for medical applications, plastic material processing, interferometric sensing, pumping source of optical parametric oscillators and so on [2,3]. Recently, a novel disordered “mixed” calcium aluminate crystal, Tm:Ca(Gd,Lu)AlO4 (space group I4/mmm, lattice constants: a = 3.6446 Å, c = 12.2157 Å, optically uniaxial), attracts the attention of researchers [4]. The tetragonal host, CaGdAlO4, possess high thermal conductivity of 6.9 (6.3) Wm−1K−1 along a-axis (c-axis) and weak dependence of concentration [5]. Negative thermo-optic coefficients of CaGdAlO4 crystal result in positive but weak thermal lensing [6]. Different from the charge and ionic radii of Ca2+ cations (1.180 Å) and Gd3+ cations (1.107 Å), the Tm3+ (1.052 Å) and Lu3+ (1.032 Å) cations enter the sites of CaGdAlO4, which makes the crystal local disordered and affects the crystal field, leading to inhomogeneous broadening of Tm3+ emission spectra (∼2 µm, 3F4 to 3H6, FWHM of bandwidth is ∼200 nm). Additionally, the Tm3+ cross-relaxation process may bring a high quantum efficiency. The luminescence decay time of the upper laser level is measured to be 3.2 ms. Lately, Pan et al. first realized a CW and wavelength-tunable Tm:Ca(Gd,Lu)AlO4 laser which could generate 1.82 W at 1945 nm [4]. To our knowledge, there was no report on Q-switched laser by using Tm:Ca(Gd,Lu)AlO4 crystal.
Since the unique viable birth of graphene in 2004 [7], much attention has been paid to 2D materials [8]. Black phosphorus [9–11], topological insulators [12,13], transition-metal dichalcogenides (TMDs) [14–16] were applied as saturable absorbers (SAs) in the pulse laser system. Atomic-layered TMDs, molybdenum disulfide (MoS2) particularly, has been seen as a potential 2D material for its distinctive thickness dependent optical and electronic properties, such as large modulation depth, high damage threshold and excellent stability [17]. Distinct from the indirect optical bandgap about 1.3 eV of bulk MoS2, monolayer MoS2 possess a 1.8 eV direct optical bandgap [17–19]. The weak van der Waals interaction force connects the layers which can be cleavage easily. Saturable absorption of MoS2 nanosheet in the infrared region may be ascribed to the nonlinear two-photon absorption or the edge-induced states and stoichiometric defects which resulted in the sub-bandgap absorption [20,21]. Up to now, 2D MoS2 SAs have been widely applied for pulse lasers at wavelengths of ∼2 µm [22–25]. Wang et al. realized a Q-switched Tm,Ho:YGG laser using MoS2 with the narrowest pulse duration of 410 ns [22]. Niu et al. achieved a doubly passively Q-switched Tm:YAP laser whose shortest pulse duration is 249.4 ns [23]. Kong et al. achieved a passively Q-switching and Q-switched mode-locking Tm:CLNGG laser with MoS2 saturable absorber mirror [24]. Luan et al. accomplished a Tm,Ho:YAP laser generating pulses with a duration as short as 435 ns [25].
Plenty of methods have been put forward to fabricate 2D materials and nanosheets have been proved to be an efficient way for pulse lasers. However, some shortcomings also exist [26]. For example, the molecular beam epitaxy and vapor-liquid-solid methods are long-running and costly process [27,28]. The liquid phase exfoliation and hydrothermal intercalation exfoliation methods are so large randomness that hard to control the thickness and uniformity of exfoliated nanosheets [29,30]. In the polyol and hydrothermal synthesis methods [31,32], the aggregation of nanosheets limit the application. For CVD technique, it holds the merits of uniform coating, precise control of layer numbers and domain size. Now, 2D materials grown by CVD have been widely used in many fields. Here, the CVD method is introduced to fabricate 2D MoS2 nanosheet [33].
In this paper, a dual-loss-modulated Q-switched Tm:Ca(Gd,Lu)AlO4 laser with AOM and MoS2 nanosheet (CVD) at ∼2 µm was realized. Under the absorbed pump power of 5.37 W at ∼792 nm, the minimum pulse duration of 82 ns and the maximum peak power of 589 W were obtained. In contrast to the singly actively and passively Q-switched lasers, the dual-loss-modulated Q-switched lasers can generate shorter pulse duration and higher peak power.
2. MoS2 SA fabrication and characterization
The 2D MoS2 SA was synthesized by the CVD method. Several pieces of sapphire wafer were used as substrates where MoS2 nanosheet grown. As the precursors, 40 mg of MoO3 and 80 mg sulfur powders were placed in CVD reactor and heated to ∼650°C. After maintained for 30 min, the CVD reactor was naturally cooled. The size, thickness and quality of MoS2 nanosheet were connected with the growth temperature, quantity of precursors, chamber purity and pressure [34].
To verify the MoS2 nanosheet on the sapphire substrate, a powerful nondestructive characterization means, Raman spectroscopy, was employed to test the crystalline structures. Two Raman active modes, E12g and A1g, could be detected as shown in Fig. 1. The E12g mode, represents the opposite vibration of two S atoms in regard to the Mo atom, travelled from 383 cm−1 (bulk MoS2) to 388 cm−1. The A1g mode at 408 cm−1 represents the out-of-plane vibration of S atoms in opposite directions [35]. Layers MoS2 was identified to grow successfully by this reliable Raman spectroscopy.
Scanning electron microscope (SEM) was employed to observe the MoS2 nanosheet based on sapphire substrate for analyzing surface morphologies. Layered MoS2 with lateral size of hundreds nanometers were dispersed on the substrate, as shown in Fig. 2, proving the growth success.
As can be seen from the determined by atomic force microscopy (AFM) measurements in Fig. 3, relatively uniform MoS2 nanosheet could be observed, which show a thickness of dozens of nanometers (thickness of monolayer MoS2 is ∼0.65 nm [36]). One could hold that the 2D MoS2 SA was successfully prepared.
Figure 4 displays the absorption spectrum near 2 µm of the MoS2 nanosheet measured by UV-VIS-NIR spectrophotometer (UV-3600, SHIMADZU). Four absorption peaks (A, B, C, and D) correspond to different electronic transitions. In contrast to the baseline, it can be seen that a broad absorption peak exists at ∼2 µm, which may have effect on passively Q-switching operation.
A balanced twin-detector measurement system whose laser source is an actively Q-switched laser was employed to measure the nonlinear transmission of the MoS2 nanosheet. The pulse duration is about 200 ns and the PRR is 5 kHz in the spectral range of ∼2 µm. From the fitted curve in Fig. 5, the modulation depth of MoS2 nanosheet was 3.6%, corresponding to the non-saturable absorbance coefficients of 13.7% (including the sapphire substrate). Saturated input pulse fluence of MoS2 nanosheet was measured to 725 µJ/cm2. The modulation depth and saturation input pulse fluence are both dependent on layer number [20]. In our measurements, several sets of experiments were carried out and different positions of the SA showed similar results.
3. Experimental setup
The experimental setup for Q-switching operation was shown in Fig. 6. The pump laser was a commercial LD (FOCUSLIGHT, wavelength: 792 nm, core diameter: 200 µm) and focused to the middle of laser crystal through a coupling lens system (1:1). The laser crystal is a-cut Tm:Ca(Gd,Lu)AlO4 (3.5 at.% Tm3+ and 5.2 at.% Lu3+, the stoichiometric crystal formula is CaGd0.913Lu0.052Tm0.035AlO4) which has a aperture of 3 × 3 mm2 and thickness of 4 mm. The crystal is anti-reflection coated at ∼2 µm and ∼790 nm, and the absorption rate is measured to 80% at 792 nm. The Tm:Ca(Gd,Lu)AlO4 crystal was wrapped with indium foil for its four lateral sides and then installed to a water-cooled heat abstractor (287 K). For convenience and efficiency, V-type cavity (physical length: 160 mm) was employed to insert the AOM (QSG27-2000-3QE, CETC, length: 50 mm) and MoS2 nanosheet. The laser cavity consisted of a flat input mirror M1 coated for high transmittance (> 99.5%) from 750 to 850 nm and for high reflectivity (> 99.5%) from 1800 to 2200 nm, the concave mirror M2 (R = −75 mm) coated the same as M1, and a flat output coupler (OC) M3 coated for transmission of 3% (1%, 2%) at the laser wavelength ∼2 µm. A premium long-pass filter (FELH1100, Thorlabs) was used to move out the leaking pump laser. To obtain the output powers, a laser energy meter (EPM 1000, Molectron) was utilized. A photodetectors (ET-5000, EOT) and a digital oscilloscope (DPO4102B-L, Tektronix) were applied to record laser pulse temporal characteristics.
4. Experimental results and discussion
The dual-loss-modulated Q-switching operation can be realized when both AOM and MoS2 nanosheet were inserted into the cavity simultaneously. If the AOM or the MoS2 nanosheet is removed away, the singly passively or actively Q-switching operation is realized. The CW operation can be realized without any Q-switched modulated subject.
The CW laser with 1%, 2%, and 3% output transmission were realized in V-type cavity and the output powers were recorded, shown in Fig. 7. One can see that the CW laser with larger output transmission had larger slope efficiency and higher threshold. Using 3% output transmission, the slope efficiency was 8.9% and the threshold was 1.23 W (absorbed pump power), while they became 6.0%, 1.00 W, and 4%, 0.88 W with 2% and 1% output transmission. Considering thermal damage of the Tm:Ca(Gd,Lu)AlO4 crystal, the absorbed pump power was not higher than 5.37 W. Note that the maximum output powers were 340 mW for 3% output transmission under the absorbed pump power of 5.37 W, the 3% transmission was employed to follow up the experiment.
Figure 8 gives the average output power versus pump power for the Q-switching operation. As can be seen in Fig. 8, when the absorbed pump power increase from 1.57 W to 5.37 W, the tendency of seven average output power curves of Q-switched lasers were all linear rise. Maximum output powers were 182 mW, 187 mW, 192 mW, 176 mW, 145 mW, 151 mW, and 163 mW for actively Q-switched lasers at PRR of 3 kHz, 5 kHz, and 7 kHz, the passively Q-switched laser, and the dual-loss-modulated Q-switched lasers at PRR of 3 kHz, 5 kHz, and 7 kHz. With higher frequency, the average output powers of actively and dual-loss-modulated Q-switched lasers were higher, which resulted from the efficiency utilization of population inversion.
Fig. 8. Average output powers of actively, passively, and dual-loss-modulated (D) Q-switched lasers versus pump power
Figure 9 shows the pulse durations versus the absorbed pump power under Q-switching performance. The pulse durations all shortened with increasing absorbed pump power. The accumulation of population inversion linked up with the rate of pulse formation. Compared to the others, the Q-switched lasers with PRR of 3 kHz possessed minimum pulse durations, which were 116 ns and 82 ns of actively and dual-loss-modulated Q-switched lasers at the absorbed pump power of 5.37 W. Pulse durations were 159 ns and 123 ns of actively and dual-loss-modulated Q-switched lasers when the frequency were set as 5 kHz, and 193 ns and 154 ns for 7 kHz. It can be seen that the dual-loss-modulated Q-switched can generate shorter pulse width. A compression ratio tc of the pulse durations was defined as:
where ts and td were the pulse durations of the actively (passively) and dual-loss-modulated Q-switched lasers, respectively. When it comes to the actively Q-switched lasers, the compression ratios tc were 1.41, 1.29 and 1.25 at 3 kHz, 5 kHz and 7 kHz under the absorbed pump power of 5.37 W. And the tc were 9.85, 6.57 and 5.25 in comparison to the passively Q-switched lasers.Fig. 9. Pulse durations of actively, passively, and dual-loss-modulated (D) Q-switched lasers versus pump power.
The repetition rate of the pulse laser is also an important parameter. For the singly passively Q-switched laser with MoS2, the PRR increased progressively from 21 kHz to 45 kHz with increasing absorbed pump power, shown in Fig. 10. For the actively and dual-loss-modulated Q-switched lasers, the PRRs equal the modulation rate of AOM.
According to the pulse durations, PRRs and average output powers, the pulse peak powers can be calculated, shown in Fig. 11. From Fig. 11, one can see that the dual-loss-modulated Q-switched lasers generated higher pulse peak power than actively and passively Q-switched lasers, especially in higher absorbed pump power. At the absorbed pump power of 5.37 W, the pulse peak powers of dual-loss-modulated Q-switched lasers were 589 mW, 246 mW and 151 mW respectively. They became 523 mW, 235 mW and 142 mW for actively Q-switched lasers and 4.8 mW for passively Q-switched laser. An enhancement factor Pi of the peak powers was defined as:
where Ps and Pd were the peak powers of the actively (passively) and dual-loss-modulated Q-switched lasers. At the absorbed pump power of 5.37 W, Pi were 1.13 (123), 1.04 (51), and 1.06 (31) with PRR of 3 kHz, 5 kHz and 7 kHz in comparison to the actively (passively) Q-switched lasers.Fig. 11. Pulse peak powers of actively, passively, and dual-loss-modulated (D) Q-switched lasers versus pump power
Figure 12 showed the temporal Q-switched pulse profiles under the highest absorbed pump power of 5.37 W. The large pulse-to-pulse fluctuation existed in the passively Q-switched laser, and the interval of pulses was unstable. These phenomena were improved in actively Q-switched lasers and get further improvement in the dual-loss-modulated Q-switched lasers where PRRs were controlled by AOM and pulse-to-pulse fluctuations were relatively small.
Fig. 12. Temporal traces of Q-switched pulse train. (a) Passively (b) Actively, 3 kHz. (c) Dual-loss-modulated (D), 3 kHz. (d) Actively, 5 kHz. (e) Dual-loss-modulated (D), 5 kHz. (f) Actively, 7 kHz. (g) Dual-loss-modulated (D), 7 kHz.
The scanning knife-edge method was employed for analyzing the beam quality factor (M2). After a focusing lens, the beam radii of output pulse lasers along the axis were recorded and simulated, shown in Fig. 13. The fitted value of M2 factors in horizontal and vertical direction were 1.27 and 1.34, proving the existence of multiple modes in the V-type cavity. Within the margin of error, the beam quality factors of dual-loss-modulated Q-switched lasers at PRR of 3 kHz, 5 kHz and 7 kHz were measured to be approximately similar.
Table 1 summarizes the results about performance of different SAs at ∼2 µm all-solid-state lasers. In comparison with other reports, our results demonstrate the comparable output power, shorter pulse width and relatively higher pulse peak power. From Table 1, one can draw a conclusion confidently that MoS2 is a promising nanomaterial for obtaining ∼2 µm pulse lasers.
Table 1. A performance comparison of Q-switched 2 µm all-solid-state lasers
5. Conclusion
In conclusion, 2D MoS2 nanosheet was fabricated by CVD method and the related characteristics were measured. By using a new crystal Tm:Ca(Gd,Lu)AlO4 as laser medium, dual-loss-modulated Q-switched Tm:Ca(Gd,Lu)AlO4 laser with AOM and MoS2 nanosheet at ∼2 µm was realized. The experimental results demonstrate that MoS2 nanosheet and Tm:Ca(Gd,Lu)AlO4 crystal showed well-performance in pulse laser at ∼2 µm. Especially, the dual-loss-modulated Q-switched laser can generate shorter pulse duration and higher peak power.
Funding
National Natural Science Foundation of China (61775119); Natural Science Foundation of Shandong Province (ZR2018MF033); Taishan Young Scholar Program of Shandong Province; Qilu Young Scholar Program of Shandong University.
Disclosures
The authors declare no conflicts of interest.
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