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Abstract
By using a few-layered tungsten disulfide (WS2) nanosheet based saturable absorber (SA) prepared with the ultrasonic assisted vertical evaporation method, a diode-pumped singly Q-switched laser is demonstrated. At an incident pump power of 6.54 W, a maximum output power of 1.18 W with a minimum pulse duration of 53 ns is obtained from the singly WS2-SA Q-switched laser. To the best of our knowledge, this is the shortest pulse duration ever achieved from a diode-pumped solid-state passively Q-switched laser with a WS2-SA at 1.06 μm. Based on this, a sub-nanosecond KTiOPO4 (KTP) based intracavity optical parametric oscillation (IOPO) pumped by a hybrid Q-switched laser with a WS2-SA and an acousto-optic modulator (AOM) is realized. At an incident pump power of 10.2 W and an AOM repetition rate of 10 kHz, a maximum output power of 233 mW with a minimum pulse duration of 810 ps for signal wave is obtained. The experimental results indicate that the IOPO pumped by the hybrid Q-switched laser with an AOM and a WS2-SA can generate a signal wave with a shorter pulse duration and a higher peak power than that pumped by the Q-switched laser with an AOM and a graphene-SA or a molybdenum disulfide SA (MoS2-SA).
© 2017 Optical Society of America
1. Introduction
In recent years, two-dimensional (2D) layered materials have been widely investigated from fundamental lasers to practical applications due to their large nonlinearity and unique electronic band structure [1–3]. Since graphene was successfully employed as SA [4, 5], other kinds of 2D materials, such as topological insulators (TIs) and transition-metal dichalcogenides (TMDs), have been paid much attention owing to their unique optoelectronic properties [6–10]. Although conventional SAs such as semiconductor saturable absorber mirrors (SESAMs) have merits of flexible design of parameters, large ratio of saturable to non-saturable losses, and high damage threshold, they are still required to accommodate specific wavelength and bandwidths [11, 12]. Due to the low saturation fluence, broadband absorption, and low cost of fabrication, carbon nanotubes (CNTs) have been widely investigated in the past several years [13, 14]. However, the problems of cluster-induced losses and heavily depending on diameter and clarity severely restrict their applications [15]. TMDs have the advantages of broadband absorption, high third-order nonlinear susceptibility and ultrafast carrier dynamics, which make them become promising potential materials for SAs. As one kind of TMDs, ungsten disulfide (WS2) has received extensive attentions since it was reported [16–26]. Similar to MoS2, WS2 also possesses many exotic electronic and optical properties such as high carrier mobility and strong spin-orbit coupling [17, 18], high photoluminescence efficiency [19], indirect-to-direct bandgap transition, and strain dependent band structure tunability [20]. In [21], it was shown that WS2 has a better thermal conductivity (2.2 Wm−1K−1) than that of MoS2 (0.85 Wm−1K−1), which indicates that WS2-SA would have a better SA performance than that of MoS2 for its higher thermal stability. Up to now, WS2-SAs have been widely applied for passively Q-switched or mode-locked lasers at wavelengths of 0.64μm, 1.06 μm, 1.55 μm and 2 μm [22–25]. Recently, a 1.06 μm passively Q-switched laser with a few-layered WS2 as SA has been reported and the shortest pulse duration is as short as 56 ns [26], which is the shortest pulse duration ever obtained from the Q-switched solid-state lasers with 2D materials. Given its outstanding performance in a Q-switched laser system, it is expected to perform similarly well in a hybrid Q-switched laser system.
Coherent sources in the spectral region of 1.5-1.6 µm have a lot of applications in the fields of remote sensing, surgery, communications and laser radar. In order to obtain these kinds of laser sources, traditional methods such as employing erbium (Er)-doped crystal as laser gain medium or utilizing the stimulated Raman scattering of media [27–31] are adopted. However, the Q-switched Er-doped laser usually has a relatively large pulse duration [27, 28], while the stimulated Raman scattering has a high threshold and low efficiency due to the small nonlinearities χ(3) [29–31]. Another efficient method to obtain such laser sources is the optical parametric oscillation (OPO). Up to now, OPO has been proved to be an efficient way to obtain coherent sources from near ultraviolet (UV) to far-infrared (FIR) wave band owing to its advantages of compactness, low threshold and high efficiency [32–45]. With respect to coherent sources in the spectral region of 1.5-1.6 µm, one can acquire them by employing a 1.06 μm laser to pump a KTiOPO4 (KTP) or KTiOAsO4 (KTA) OPO [32–36]. However, the reported intracavity OPOs (IOPOs) usually generate signal waves with relatively long pulse widths (more than 3 ns) [37–40]. In order to obtain even higher peak power and shorter pulse width, it is essential to choose a high-quality pump laser with a shorter pulse duration. The recently developed hybrid-loss modulated technique, i.e. simultaneously employing an active Q-switch and a passive Q-switch in the resonator can efficiently generate such pump light [46–48]. In a hybrid-loss modulated laser system, the active Q-switch allows the laser medium to store energy and control the pulse repetition rate, while the saturable absorber usually acts as a passive Q-switch and further shapes the pulses. Thus, the rising and the falling edges of the pulses experience double modulation losses, resulting in short pulse widths and high peak powers. Using such hybrid-loss modulated Q-switched lasers to pump OPOs, signal wave pulses with short pulse durations and stable pulse repetition rates as well as high peak powers can be achieved [49–51]. Considering the outstanding saturation absorption performance of WS2-SA in solid-state Q-switched laser, by using a hybrid Q-switched laser with an AOM and a WS2-SA as pump laser, IOPO with a high peak power and a short pulse duration is expected.
In this letter, a diode-pumped passively Q-switched laser with a self-made WS2-SA is firstly presented. At an incident pump power of 6.54 W, a maximum output power of 1.18 W with a minimum pulse duration of 53 ns is obtained. Subsequently, a sub-nanosecond KTP-based IOPO pumped by a hybrid Q-switched laser (HIOPO) with a WS2-SA and an AOM is realized. At an incident pump power of 10.2 W and an AOM repetition rate of 10 kHz, a maximum output power of 233 mW with a minimum pulse duration of 810 ps is achieved. In comparison with the IOPOs pumped by an hybrid Q-switched laser with an AOM and other kinds of 2D materials such as graphene-SA or MoS2-SA, the IOPO pumped by the hybrid Q-switched laser with an AOM and a WS2-SA can generate signal wave with a shorter pulse width and a higher peak power.
2. Preparation and characterization of WS2-SA
The few-layered WS2 nanosheet was obtained by employing the ultrasonic assisted vertical evaporation method. Firstly, 20 mg bulk WS2 powder was put into 10 ml 0.1% SDS (solid dodecyl sulfate) aqueous solution. Secondly, to obtain WS2 aqueous dispersion with high absorption, the WS2 aqueous solution was ultrasonically agitated for 10 hours. The upper portion of the centrifuged solution was decanted to a bottle and diluted, then poured into a polystyrene cell. At last, a hydrophilic quartz substrate was inserted vertically into the cell and the polystyrene cell was placed at the atmosphere for gradual evaporation. The quartz surface with WS2 nanosheet adhered on was successfully prepared for characterization and laser operation.
As shown in Fig. 1(a) and 1(b), the distribution of lateral dimensions and flake thickness of WS2 are measured via atomic force microscopy (AFM). From Fig. 1(b), we can see that the thickness of WS2 is about 7 nm. As we know, the height of a single layer of WS2 is about 0.6 nm [52], hence, the layer numbers of this WS2 sample are approximately 11-12. Figure 1(c) gives the scanning electron microscopy (SEM) image of the WS2 piece, from which a layered structure could be recognized at the edge of the sample. The Raman spectrum (excited by 514.5 nm laser) of the WS2-SA is shown in Fig. 1(d). As shown in Fig. 1(d), two peaks related to longitudinal acoustic mode 2LA(M) and out-of-plane A1g vibrational modes are located at 351.5 cm−1 and 419.8 cm−1, respectively. As reported in [53], with the increasing of WS2 layer numbers, the intensity of A1g mode increases, while the intensity of 2LA(M) mode has a decrease tendency. That is to say the ratio of I2LA(M)/IA1g decreases with the increase of WS2 layer numbers. For the bulk material, the ratio of I2LA(M)/IA1g is smaller than 1, while for the monolayer WS2-SA, the ratio is larger than 2. It can be seen from Fig. 1(d) that the ratio of I2LA(M)/IA1g is about 0.7, which indicates the layer numbers of WS2 coincide with the results shown in Fig. 1(a) and 1(b).
Fig. 1 Characterization of the prepared WS2-SA: (a) AFM image; (b) height variation; (c) SEM image; (d) Raman spectra; (e) Linear transmittance spectrum; (f) Nonlinear transmittance curve versus input pulse influence.
Figure. 1(e) gives the linear transmittance of the as-prepared WS2-SA. From Fig. 1(e), we can see the WS2-SA has broadband absorption and the measured transmission of WS2-SA at 1064 nm is 86%. Besides, in order to confirm the saturable absorption capability of the WS2-SA sample near 1 μm, the nonlinear optical characteristics was measured by the balanced twin-detector measurement technique. The laser source was an AOM Q-switched solid-state laser at 1.06 μm. The corresponding optical transmittances with respect to different input pulse fluence are shown in Fig. 1(f). From Fig. 1(f) the modulation depth, saturation intensity, and initial transmittance are estimated to be 4.9%, 143.4 mJ/cm2 and 87.6%, respectively. Chhowalla et al. [2] pointed out that the direct bandgap of monolayer WS2 was ~2.0 eV (~630 nm) and the indirect bandgap was ~1.4 eV (~886 nm), hence the saturable absorption that occurs at 1064 nm may be attributed to the sub-bandgap absorption, which was considered to be induced by the edge-induced sub-bandgap states and imperfection of TMDs nanosheets [54–56].
3. Laser design and characterization
3.1 Q-switching operation
At first, we investigated the characteristics of a Q-switching laser with a WS2-SA. Figure 2 depicts the schematic diagram of the hybrid Q-switched laser with an AOM and a WS2-SA, while it was employed as the singly WS2-SA Q-switched laser cavity when the AOM was removed. The pump source emitting at wavelength of 808 nm was a 30 W commercial fiber-coupled diode laser (FAP-I system, Coherent Inc.) with a core diameter of 400 µm. A coupling lens system with a focal length of 45 mm and a coupling efficiency of 90% was used to re-image the pump beam into the laser medium. The laser cavity consisted of an input mirror M1, a laser gain medium, an AOM, a WS2-SA, and an output coupler mirror M2. M1 was a plane mirror with high-reflectivity (HR) coated at 1064 nm (R>99.8%) and high-transmission (HT) coated at 808 nm (T = 85%). The laser gain medium was a 4 × 4 × 10 mm3 (10 mm in thickness) a-cut Nd:YVO4 crystal with 0.5 at.% Nd3+ concentration. One surface of Nd:YVO4 crystal was antireflection (AR) coated at 808 nm and 1064 nm, and the opposite face was AR coated at 1064 nm. In order to alleviate the heat deposition, the Nd:YVO4 crystal was wrapped with indium foil and mounted in copper holders maintained at 17°C. The 47-mm-long AO Q-switch (GSQ27-3, the 26th institute, CETC, China) was AR coated at 1064 nm (R<0.2%) on both surfaces and the modulation rate could be tuned from 1 kHz to 50 kHz. M2 was a plane mirror with T = 10% at 1064 nm. The physical cavity length from M1 to M2 was 62 mm for the hybrid Q-switched laser with the AOM and the WS2-SA. A DPO 7104C digital oscilloscope (1 GHz bandwidth and 20 GS/s sampling rate, Tektronix Inc., USA) and a fast InGaAs photo detector with a rising time of 0.4 ns (New Focus, 1611) were used to measure the pulse characteristics of the laser. A wavescan laser spectrometer (Resolution: 0.4 nm, APE GmbH, Germany) and a MAX 500AD laser power meter (Coherent Inc., USA) were employed to measure the laser spectrum and the average output power, respectively.
The laser characteristics of a singly Q-switched laser with a WS2-SA and a hybrid Q-switched laser with an AOM and a WS2-SA are shown in Fig. 3. Figure 3(a) shows the average output powers of these two kinds of lasers versus the incident pump powers. Figures 3(b) and 3(c) give the pulse durations of these two kinds of lasers and the pulse repetition rates of WS2-SA Q-switched laser with respect to the incident pump powers. For the hybrid Q-switched laser with the AOM and the WS2-SA, at the maximum incident pump power of 6.54 W, the maximum output powers at different AOM repetition rates of 10 kHz, 20 kHz and 30 kHz are 0.88 W, 0.92 W, 0.98 W, corresponding to the minimum pulse durations of 4.8 ns, 5.2 ns, and 5.8 ns, respectively. As shown in Fig. 3(a) and 3(c), for the singly Q-switched laser with the WS2-SA, at the maximum incident pump power of 6.54 W, the maximum output power of 1.18 W and the minimum pulse duration of 53 ns are obtained. This is the shortest pulse duration ever obtained from diode-pumped WS2-SA Q-switched laser at 1.06 μm, as far as we know. Figure 3(d) gives a temporal pulse train from the passively Q-switched laser with the WS2-SA at the incident pump power of 6.54 W, indicating good Q-switching stability. Figure 3(e)-3(f) show the typical temporal pulse profiles from the passively Q-switched laser with the WS2-SA and the hybrid Q-switched laser with the AOM and the WS2-SA at an incident pump power of 6.54 W. It can be seen from Fig. 3(e)-3(f) that at the same pump power, the hybrid Q-switched laser has a significant pulse duration compression in comparison with passively Q-switched laser with the WS2-SA. For the singly Q-switched laser with the WS2-SA, the quartz substrate for WS2-SA might be damaged by the strong fundamental laser intensity and unabsorbed pump power intensity when the pump power exceeded 7 W, so the pump power was limited below 6.54W.
Fig. 3 Laser characteristics of the singly Q-switched laser with a WS2-SA and hybrid Q-switched laser with a WS2-SA and an AOM.
3.2 OPO operation
In this part, we study the laser characteristics of KTP-based IOPO pumped by the hybrid Q-switched laser with the AOM and the WS2-SA. For comparison, we also tried the IOPO pumped by the singly Q-switched laser with the WS2-SA. However, due to the long pulse duration and high pulse repetition rate of singly Q-switched laser with the WS2-SA, when the KTP crystal was inserted into the Q-switched laser cavity, the power intensity in the KTP crystal couldn’t reach the OPO oscillation threshold. Hence, the IOPO pumped by the singly Q-switched laser with the WS2-SA was not realized. Figure 4 gives the experimental setup of the IOPO pumped by a hybrid Q-switched Nd:YVO4 laser with the AOM and the WS2-SA. In comparison with Fig. 2, a KTP crystal was inserted into the cavity and the mirror M2 was replaced by a mirror M3. A 20 mm-long KTP crystal was cut for type-IInon-critical phase-matching (θ = 90°, φ = 0°) configuration to eliminate the walk-off effect and hold a wide temperature low-sensitivity range. With this configuration, the signal laser wavelength was calculated to be 1572 nm when the fundamental laser wavelength was 1064 nm [57]. The input face of the KTP crystal was HR coated at 1572 nm (R>99.7%) and AR coated at 1064 nm (R<0.2%), while the other face was AR coated both at 1064 and 1572 nm (R<0.5%). M3 was a plane mirror with partial-reflectivity (PR) coated at 1572 nm (T = 15%). The OPO and fundamental wave cavity lengths were 23 and 93 mm, respectively.
Fig. 4 Experimental setup of KTP based IOPO pumped by a hybrid Q-switched Nd:YVO4 laser with an AOM and a WS2-SA.
Figure 5 gives the average output powers and pulse durations of signal waves for the HIOPO configuration with respect to pump powers at three different AOM repetition rates of 10 kHz, 20 kHz and 30 kHz, respectively. From Fig. 5(a), we can see that the average output powers of HIOPO increases with the increase of the repetition rates of the AOM. The power threshold of HIOPO at the different AOM repetition rates of 10 kHz, 20 kHz and 30 kHz was around 5.5 W. According to the results shown in Fig. 3, the oscillation threshold of intracavity power intensity in the KTP was estimated to be 100 MW/cm2. Under a maximum incident pump power of 10.2 W, the average output powers of HIOPO signal waves at the AOM repetition rates of 10 kHz, 20 kHz and 30 kHz were measured to be 233 mW, 254 mW and 278 mW, respectively. Figure 5(b) shows the dependences of the pulse durations of HIOPO on the incident pump powers at three different AOM repetition rates. Under the same pump power, one can see that the higher the AOM repetition rates are, the broader the pulse durations of HIOPO signal waves would be. At the maximum pump power of 10.2 W, the shortest pulse durations for the signal waves at AOM repetition rates of 10 kHz, 20 kHz and 30 kHz were 0.81 ns, 0.83 ns and 0.86 ns, respectively. The emission wavelengths of fundamental and signal waves from HIOPO are found to locate at 1064 nm and 1572 nm, respectively, which are shown in the Fig. 5(c).
Fig. 5 (a)-(b): Average output powers and pulse durations of the HIOPO versus incident pump power at the AOM repetition rates of 10, 20 and 30 kHz. (c): A typical spectrum of the HIOPO at an incident pump power of 10.2 W and an AOM repetition rate of 10 kHz.
Figure 6 depicts the typical temporal pulse profiles of depleted fundamental wave and signal wave for the HIOPO at the incident pump power of 10.2 W and an AOM repetition rate of 10 kHz. To obtain the temporal pulse shape of signal wave for the HIOPO, we placed a HR mirror coated at 1064 nm before the probe to eliminate the pump light, while the temporal pulse shape of the depleted fundamental wave was measured from the reflected light by the same HR mirror at 1064 nm. It can be seen from Fig. 6 that the pulse durations of depleted fundamental and signal wave are 1.8 ns and 0.81 ns, respectively.
Fig. 6 Temporal pulse shapes of signal and fundamental lights at an incident pump power of 10.2 W and an AOM repetition rate of 10 kHz.
By employing the equations: and, the single pulse energies and peak powers of signal wave can be calculated, where E, Ps, Pp and τ are the single pulse energy, the average output power, the peak power and the pulse duration of signal wave, respectively, and f is the repetition rate of the AOM. Figure 7 shows the single pulse energies and peak powers of signal wave for HIOPO versus incident pump powers. As shown in Fig. 7, the highest single pulse energy of 23.3 μJ and the highest peak power of 28.7 kW are obtained at an incident pump power of 10.2 W and an AOM repetition rate of 10 kHz.
Fig. 7 Single pulse energies and peak powers of the HIOPO versus incident pump power at the AOM repetition rates of 10 kHz, 20 kHz and 30 kHz.
Furthermore, we compare the laser performances of the HIOPO pumped by the hybrid Q-switched laser with the AOM and the WS2-SA with those ever obtained in our previous works [50, 51], which are shown in Table 1, in which other kinds of 2D materials including a monolayer graphene-SA and a MoS2-SA were employed in the IOPOs pumped by the hybrid Q-switched lasers. As shown in Table 1, the IOPO pumped by the hybrid Q-switched laser with the WS2-SA can generate higher output power, shorter pulse duration and higher peak power in comparison with the IOPOs pumped by the hybrid Q-switched laser with the graphene-SA or the MoS2-SA. As for the carrier dynamics of graphene, the intraband carrier relaxation time and the interband relaxation time are about 10-150 fs and 0.4-1.7 ps, respectively [58, 59]. The longer interband relaxation can act as the modulation switching by changing the intrinsic electron and hole carrier densities of graphene. Therefore, graphene is a suitable saturable absorber for the generation of mode-locking or Q-switching pulses. However, the modulation depth of graphene is dependent on layer number, which mainly affects the Q-switched pulse duration. The monolayer graphene usually has very small modulation depth, resulting in the relatively long pulse duration [60, 61]. As for TMDs such as WS2 and MoS2, the saturable absorption is attributed to the existence of the edge-induced sub-bandgap states and the imperfection of TMDs nanosheets [54–56]. When the TMDs seminconductor is excited by a light with photon energy higher than gap energy, electrons are transferred from valence band to conduction band. Under strong excitation, electrons at valence band are excited into conduction band and the states in valence band become depleted, while the finial states in the conduction band are partially occupied. Further exciation from valence band is prevented and no absorption is induced, leading to a saturable absorption condition that the low-intensity light experiences large loss. In fact, the TMDs materials’ thermal stability has important influence on the performances of their Q-switched laser. In [21], Chen et al. demonstrated that the WS2-SA Q-switching fiber laser had more stable pulses trains, higher output power and shorter pulse duration in comparison with the MoS2-SA Q-switching fiber laser due to WS2-SA’s high thermal conductivity. This is consistent with the results obtained from HIOPOs in this work. It should be noted that the time resolution of our experimental facilities was relatively low, hence, the measured pulse duration of our work may be wider than the real pulse duration [62]. However, due to the reason that the measurement conditions are similar in comparison with our previous results, we believe that the experimental results can stand out the advantage of WS2-SA. Therefore, we think the WS2-SA can be considered as a promising candidate in generating high-performance solid-state Q-switched lasers and OPOs.
Table 1. Comparisons of the IOPOs pumped by the hybrid Q-switched lasers with an AOM and different 2D materials.
4. Conclusion
In summary, by using a WS2 nanosheet as a SA, a diode-pumped singly Q-switched laser with a WS2-SA is presented, from which a maximum output power of 1.18 W and a minimum pulse duration of 53 ns are obtained at an incident pump power of 6.54 W. To the best of our knowledge, this is the shortest pulse duration ever obtained from the diode pumped passively Q-switched solid-state laser with the WS2-SA at 1.06 μm. A sub-nanosecond KTP-based IOPO pumped by a hybrid Q-switched laser with a WS2-SA and an AOM is realized, from which a maximum output power of 233 mW and a minimum pulse duration of 810 ps are obtained at an incident pump power of 10.2 W and an AOM repetition rate of 10 kHz. The experimental results show that in comparison with the IOPO pumped by the hybrid Q-switched laser with an AOM and other kinds of 2D materials such as graphene-SA and MoS2-SA, the IOPO pumped by the hybrid Q-switched laser with the AOM and the WS2-SA can generate signal wave with shorter pulse width and higher peak power. The results also indicate that WS2-SA is a promising candidate for SAs in generating high-performance Q-switched solid-state lasers and OPO sources.
Funding
National Natural Science Foundation of China (61775119, 61378022, 61475088); the Young Scholars Program of Shandong University (2015WLJH38).
References and links
1. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, and A. K. Geim, “Fine structure constant defines visual transparency of graphene,” Science 320(5881), 1308 (2008). [PubMed]
2. M. Chhowalla, H. S. Shin, G. Eda, L. J. Li, K. P. Loh, and H. Zhang, “The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets,” Nat. Chem. 5(4), 263–275 (2013). [PubMed]
3. S. Wang, H. Yu, H. Zhang, A. Wang, M. Zhao, Y. Chen, L. Mei, and J. Wang, “Broadband few-layer MoS2 saturable absorbers,” Adv. Mater. 26(21), 3538–3544 (2014). [PubMed]
4. Q. L. Bao, H. Zhang, Y. Wang, Z. H. Ni, Y. L. Yan, Z. X. Shen, K. P. Loh, and D. Y. Tang, “Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater. 19, 3077–3083 (2009).
5. W. Cai, S. Jiang, S. Xu, Y. Li, J. Liu, C. Li, L. Zheng, L. Su, and J. Xu, “Graphene saturable absorber for diode pumped Yb:Sc2SiO5 mode-locked laser,” Opt. Laser Technol. 65, 1–4 (2015).
6. G. Sobon, “Mode-locking of fiber lasers using novel two-dimensional nanomaterials: graphene and topological insulators,” Photon. Res. 3(2), A56–A63 (2015).
7. F. J. Jia, H. Chen, P. Liu, Y. Z. Huang, and Z. Q. Luo, “Nanosecond-pulse, dual-wavelength passively Q-switched c-cut Nd:YVO4 laser using a few-layer Bi2Se3 saturable absorber,” IEEE J. Sel. Top. Quantum Electron. 21(1), 1601806 (2015).
8. R. Wang, B. Ruzicka, N. Kumar, M. Bellus, H. Chiu, and H. Zhao, “Ultrafast and spatially resolved studies of charge carriers in atomically-thin molybdenum disulfide,” Phys. Rev. B 87, 161403 (2013).
9. A. P. Luo, M. Liu, X. D. Wang, Q. Y. Ning, W. C. Xu, and Z. C. Luo, “Few-layer MoS2-deposited microfiber as highly nonlinear photonic device for pulse shaping in a fiber laser,” Photon. Res. 3(2), A69–A78 (2015).
10. C. Cheng, H. Liu, Y. Tan, J. R. Vázquez de Aldana, and F. Chen, “Passively Q-switched waveguide lasers based on two-dimensional transition metal diselenide,” Opt. Express 24(10), 10385–10390 (2016). [PubMed]
11. A. Rutz, R. Grange, V. Liverini, M. Haiml, S. Schon, and U. Keller, “1.5 μm GaInNAs semiconductor saturable absorber for passively modelocked solid-state lasers,” Electron. Lett. 41, 321–323 (2005).
12. M. Hoffmann, S. Schilt, and T. Südmeyer, “CEO stabilization of a femtosecond laser using a SESAM as fast opto-optical modulator,” Opt. Express 21(24), 30054–30064 (2013). [PubMed]
13. H. Kataura, Y. Kumazawaa, Y. Maniwaa, I. Umezub, S. Suzukic, Y. Ohtsukac, and Y. Achibac, “Optical properties of single-wall carbon nanotubes,” Synth. Met. 103, 2555–2558 (1999).
14. J. Liu, Y. Wang, Z. Qu, and X. Fan, “2 μm passive Q-switched mode-locked Tm3+:YAP laser with single-walled carbon nanotube absorber,” Opt. Laser Technol. 44(4), 960–962 (2012).
15. T. Schibli, K. Minoshima, H. Kataura, E. Itoga, N. Minami, S. Kazaoui, K. Miyashita, M. Tokumoto, and Y. Sakakibara, “Ultrashort pulse-generation by saturable absorber mirrors based on polymer-embedded carbon nanotubes,” Opt. Express 13(20), 8025–8031 (2005). [PubMed]
16. A. Klein, S. Tiefenbacher, V. Eyert, C. Pettenkofer, and W. Jaegermann, “Electronic band structure of single-crystal and single-layer WS2 influence of interlayer van der Waals interactions,” Phys. Rev. B 64(20), 205416 (2001).
17. H. Zeng, G.-B. Liu, J. Dai, Y. Yan, B. Zhu, R. He, L. Xie, S. Xu, X. Chen, W. Yao, and X. Cui, “Optical signature of symmetry variations and spin-valley coupling in atomically thin tungsten dichalcogenides,” Sci. Rep. 3, 1608 (2013). [PubMed]
18. W. Zhao, Z. Ghorannevis, L. Chu, M. Toh, C. Kloc, P.-H. Tan, and G. Eda, “Evolution of electronic structure in atomically thin sheets of WS2 and WSe2.,” ACS Nano 7(1), 791–797 (2013). [PubMed]
19. Y. Chen, J. Xi, D. O. Dumcenco, Z. Liu, K. Suenaga, D. Wang, Z. Shuai, Y. S. Huang, and L. Xie, “Tunable band gap photoluminescence from atomically thin transition-metal dichalcogenide alloys,” ACS Nano 7(5), 4610–4616 (2013). [PubMed]
20. L. Britnell, R. M. Ribeiro, A. Eckmann, R. Jalil, B. D. Belle, A. Mishchenko, Y. J. Kim, R. V. Gorbachev, T. Georgiou, S. V. Morozov, A. N. Grigorenko, A. K. Geim, C. Casiraghi, A. H. Castro Neto, and K. S. Novoselov, “Strong light-matter interactions in Heterostructures of atomically thin films,” Science 340(6138), 1311–1314 (2013). [PubMed]
21. B. Chen, X. Zhang, K. Wu, H. Wang, J. Wang, and J. Chen, “Q-switched fiber laser based on transition metal dichalcogenides MoS2, MoSe2, WS2, and WSe2.,” Opt. Express 23(20), 26723–26737 (2015). [PubMed]
22. Y. J. Cheng, J. Peng, B. Xu, H. Yang, Z. Q. Luo, H. Y. Xu, Z. P. Cai, and J. Weng, “Passive Q-switching of a diode-pumped Pr:LiYF4 visible laser using WS2 as saturable absorber,” IEEE Photonics J. 8(3), 1501606 (2016).
23. D. Mao, S. Zhang, Y. Wang, X. Gan, W. Zhang, T. Mei, Y. Wang, Y. Wang, H. Zeng, and J. Zhao, “WS2 saturable absorber for dissipative soliton mode locking at 1.06 and 1.55 µm,” Opt. Express 23(21), 27509–27519 (2015). [PubMed]
24. S. H. Kassani, R. Khazaeinezhad, H. Jeong, T. Nazari, D. Yeom, and K. Oh, “All-fiber Er-doped Q-switched laser based on tungsten disulfide saturable absorber,” Opt. Express 5(2), 373–379 (2015).
25. C. Luan, K. Yang, J. Zhao, S. Zhao, L. Song, T. Li, H. Chu, J. Qiao, C. Wang, Z. Li, S. Jiang, B. Man, and L. Zheng, “WS2 as a saturable absorber for Q-switched 2 micron lasers,” Opt. Lett. 41(16), 3783–3786 (2016). [PubMed]
26. W. J. Tang, Y. G. Wang, K. J. Yang, J. Zhao, S. Z. Zhao, G. Q. Li, D. C. Li, T. Li, and W. C. Qiao, “1.36 W passively Q-switched YVO4/Nd:YVO4 laser with a WS2 saturable absorber,” IEEE Photonics Technol. Lett. 29(5), 470–473 (2017).
27. A. Levoshkin, A. Peirov, and J. E. Montage, “High efficiency diode pumped Q-switched Yb:Er:glass laser,” Opt. Commun. 185(4–6), 399–405 (2000).
28. N. W. H. Chang, N. Simakov, D. J. Hosken, J. Munch, D. J. Ottaway, and P. J. Veitch, “Resonantly diode-pumped continuous-wave and Q-switched Er:YAG laser at 1645 nm,” Opt. Express 18(13), 13673–13678 (2010). [PubMed]
29. J. T. Murray, R. C. Powell, N. Peyghambarian, D. Smith, W. Austin, and R. A. Stolzenberger, “Generation of 1.5 µm radiation through intracavity solid-state Raman shifting in Ba(NO3)2,” Opt. Lett. 20(9), 1017–1019 (1995). [PubMed]
30. N. Takei, S. Suzuki, and F. Kannari, “Compensation of thermal lensing in an eye-safe cascade Raman laser with Ba(NO3)2 crystal,” in Conference on Lasers and Electro-Optics, Pacific Rim, 99(3), 744–755 (1999).
31. Y. T. Chang, Y. P. Huang, K. W. Su, and Y. F. Chen, “Diode-pumped multi-frequency Q-switched laser with intracavity cascade Raman emission,” Opt. Express 16(11), 8286–8291 (2008). [PubMed]
32. Y. F. Chen, S. W. Chen, S. W. Tsai, and Y. P. Lan, “High-repetition-rate eye-safe optical parametric oscillator intracavity pumped by a diode-pumped Q-switched Nd:YVO4 laser,” Appl. Phys. B 76(3), 263–266 (2003).
33. Y. F. Chen, S. W. Chen, L. Y. Tsai, Y. C. Chen, and C. H. Chien, “Efficient sub-nanosecond intracavity optical parametric oscillator pumped with a passively Q-switched Nd:GdVO4 laser,” Appl. Phys. B 79(7), 823–825 (2004).
34. W. Y. Cheng, S. Z. Zhao, Z. Zhuo, X. M. Zhang, and Y. Wang, “Laser-diode side-pumped actively Q-switched eye-safe intracavity optical parametric oscillator,” Opt. Lasers Eng. 46(1), 12–17 (2008).
35. R. F. Wu, K. S. Lai, H. Wong, W. J. Xie, Y. Lim, and E. Lau, “Multiwatt mid-IR output from a Nd:YALO laser pumped intracavity KTA OPO,” Opt. Express 8(13), 694–698 (2001). [PubMed]
36. W. J. Sun, Q. P. Wang, Z. J. Liu, X. Y. Zhang, F. Bai, X. B. Wan, G. F. Jin, X. T. Tao, and Y. X. Sun, “High efficiency KTiOAsO4 optical parametric oscillator within a diode-side-pumped two-rod Nd:YAG laser,” Appl. Phys. B 104(1), 87–91 (2011).
37. M. Wang, K. Zhong, J. Mei, S. Guo, D. Xu, and J. Yao, “Simultaneous dual-wavelength eye-safe KTP OPO intracavity pumped by a Nd:GYSGG laser,” J. Phys. D Appl. Phys. 49, 065101 (2016).
38. Y. Huang, H. Cho, K. Su, and Y. Chen, “Dual-wavelength intracavity OPO with a diffusion-bonded Nd:YVO4/Nd:GdVO4 crystal,” IEEE Photonics Technol. Lett. 28, 1123 (2016).
39. J. Guo, G. He, Z. Jiao, and B. Wang, “Efficient high-peak-power and high-repetition-rate eye-safe laser using an intracavity KTP OPO,” Laser Phys. 25, 035403 (2015).
40. H. Huang, D. Shen, J. He, H. Chen, and Y. Wang, “Nanosecond nonlinear Čerenkov conical beams generation by intracavity sum frequency mixing in KTiOAsO4 crystal,” Opt. Lett. 38(4), 576–578 (2013). [PubMed]
41. G. Marchev, P. Dallocchio, F. Pirzio, A. Agnesi, G. Reali, V. Petrov, A. Tyazhev, V. Pasiskevicius, N. Thilmann, and F. Laurell, “Sub-nanosecond, 1-10 kHz, low-threshold, non-critical OPOs based on periodically poled KTP crystal pumped at 1064 nm,” Appl. Phys. B 109(2), 211–214 (2012).
42. C. Kieleck, A. Berrou, B. Donelan, B. Cadier, T. Robin, and M. Eichhorn, “6.5 W ZnGeP2 OPO directly pumped by a Q-switched Tm3+-doped single-oscillator fiber laser,” Opt. Lett. 40(6), 1101–1104 (2015). [PubMed]
43. A. Dergachev, D. Armstrong, A. Smith, T. Drake, and M. Dubois, “3.4-mum ZGP RISTRA nanosecond optical parametric oscillator pumped by a 2.05-µm Ho:YLF MOPA system,” Opt. Express 15(22), 14404–14413 (2007). [PubMed]
44. N. Hendaoui, A. Peremans, P. G. Schunemann, K. T. Zawilski, and V. Petrov, “Synchronously pumped OPO for picoseond pulse generation in the mid-infrared near 6.45 µm using AgGaS2 and CdSiP2: a comparative study,” Laser Phys. 23, 085401 (2013).
45. V. Petrov, J. J. Zondy, O. Bidault, L. Isaenko, V. Vedenyapin, A. Yelisseyev, W. D. Chen, A. Tyazhev, S. Lobanov, G. Marchev, and D. Kolker, “Optical, thermal, electrical, damage, and phase-matching properties of lithium selenoindate,” J. Opt. Soc. Am. B 27(9), 1902–1927 (2010).
46. P. Datta, S. Mukhopadhyay, S. Das, L. Tartara, A. Agnesi, and V. Degiorgio, “Enhancement of stability and efficiency of a nonlinear mirror mode-locked Nd:YVO4 oscillator by an active Q-switch,” Opt. Express 12(17), 4041–4046 (2004). [PubMed]
47. Z. Li, Z. Xiong, N. Moore, G. Lim, W. Huang, and D. Huang, “Pulse width reduction in AO Q-switched diode-pumped Nd:YVO4 laser with GaAs coupler,” Opt. Commun. 237, 411 (2004).
48. F. Hajiesmaeilbaigi, H. Razzaghi, M. Mahdizadeh, and M. R. A. Moghaddam, “High-average-power diode-side-pumped double Q-switched Nd:YAG laser,” Laser Phys. Lett. 4(4), 261–264 (2007).
49. J. Wang, S. Z. Zhao, G. Q. Li, K. J. Yang, D. C. Li, J. An, and M. Li, “Pulse compression in laser-diode-pumped doubly Q-switched intracavity optical parametric oscillator considering Gaussian distribution of intracavity photon densities,” Jpn. J. Appl. Phys. 46(4), 1505–1510 (2007).
50. J. P. Qiao, J. Zhao, K. J. Yang, S. Z. Zhao, G. Q. Li, D. C. Li, T. Li, W. C. Qiao, and H. W. Chu, “Intracavity KTP OPO pumped by a doubly Q-switched laser with AOM and a monolayer graphene saturable absorber,” Opt. Mater. 50, 234–237 (2015).
51. J. Qiao, S. Zhao, K. Yang, J. Zhao, G. Li, D. Li, T. Li, and W. Qiao, “Hybrid Q-switched laser with MoS2 saturable absorber and AOM driven sub-nanosecond KTP-OPO,” Opt. Express 25(4), 4227–4238 (2017). [PubMed]
52. H. R. Gutiérrez, N. Perea-López, A. L. Elías, A. Berkdemir, B. Wang, R. Lv, F. López-Urías, V. H. Crespi, H. Terrones, and M. Terrones, “Extraordinary room-temperature photoluminescence in triangular WS2 monolayers,” Nano Lett. 13(8), 3447–3454 (2013). [PubMed]
53. A. Berkdemir, H. R. Gutierrez, A. R. Botello-Mendez, N. P. López, A. L. Elías, C. I. Chia, B. Wang, V. H. Crespi, F. L. Urias, J. C. Charlier, H. Terrones, and M. Terrones, “Identification of individual and few layers of WS2 using Raman Spectroscopy,” Sci. Rep. 3, 1755 (2013).
54. R. C. T. Howe, R. I. Woodward, G. Hu, Z. Yang, E. J. R. Kelleher, and T. Hasan, “Surfactant-aided exfoliation of molybdenum disulfide for ultrafast pulse generation through edge-state saturable absorption,” xs Solidi. B 253(5), 911–917 (2016).
55. M. Zhang, G. Hu, G. Hu, R. C. T. Howe, L. Chen, Z. Zheng, and T. Hasan, “Yb- and Er-doped fiber laser Q-switched with an optically uniform, broadband WS2 saturable absorber,” Sci. Rep. 5, 17482 (2015). [PubMed]
56. M. Trushin, E. J. R. Kelleher, and T. Hasan, “Theory of edge-state optical absorption in two-dimensional transition metal dichalcogenide flakes,” Phys. Rev. B 94, 155301 (2016).
57. H. Chu, J. Zhao, T. Li, S. Zhao, K. Yang, D. Li, G. Li, W. Qiao, Y. Sang, and H. Liu, “KTP OPO with signal wave at 1630 nm intracavity pumped by an efficient σ-polarized Nd,MgO:LiNbO3 laser,” Opt. Mater. Express 5(3), 684–689 (2015).
58. J. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, and M. Spencer, “Measurement of ultrafast carrier dynamics in epitaxial graphene,” Appl. Phys. Lett. 92, 042116 (2008).
59. M. Breusing, C. Ropers, and T. Elsaesser, “Ultrafast carrier dynamics in graphite,” Phys. Rev. Lett. 102(8), 086809 (2009). [PubMed]
60. H. Lin, X. Huang, X. Liu, D. Sun, W. Zhu, and Y. Xu, “Passively Q-switched c-cut Nd:YVO4 laser using graphene-oxide as a saturable absorber,” Optik (Stuttg.) 127(10), 4545–4546 (2016).
61. G. Xie, J. Ma, P. Lv, W. Gao, P. Yuan, L. Qian, H. Yu, H. Zhang, J. Wang, and D. Tang, “Graphene saturable absorber for Q-switching and mode locking at 2μm wavelength,” Opt. Mater. Express 2(6), 878–883 (2012).
62. T. Li, S. Zhao, Z. Zhuo, K. Yang, G. Li, and D. Li, “Dual-loss-modulated Q-switched and mode-locked YVO4/Nd:YVO4/KTP green laser with EO and Cr4+:YAG saturable absorber,” Opt. Express 18(10), 10315–10322 (2010). [PubMed]
