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A few-layer WS2 nanosheet was prepared by the ultrasonic assisted vertical evaporation method and its corresponding characteristics were measured. By simultaneously employing the WS2 nanosheet and electro-optic modulator (EOM) as modulator, a dual-loss-modulation Q-switched and mode-locked (QML) Nd:Lu0.15Y0.85VO4 sub-nanosecond laser with controllable repetition rate and high peak power was presented. The output performances versus the pump power were measured. At the pump power of 8.92 W, the shortest pulse duration was measured to be about 467 ps, corresponding to a peak power of 731 kW. To our knowledge, this is the highest output peak power with WS2 saturable absorbers (SAs) reported.
© 2017 Optical Society of America
1. Introduction
Short-pulse lasers with high peak power are becoming ubiquitous tools in a wide variety of applications, including biological medicine, military and industrial materials processing [1–3]. In addition to gain media [4], new saturable absorbers (SAs) play an important role for the development of pulsed lasers. In recent years, two-dimensional (2D) materials as SAs have received much attention, such as graphene [5–7], topological insulators (TIs) [8, 9], black phosphorus (BP) [10] and transition mental dichalcogenides (TMDs) [11,12]. Especially, as layered materials, TMDs have strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into monolayer (2D) or few-layer (quasi-2D) flakes. What’s more, by manipulating the size or atomic defects, the bandgaps of TMD flakes can be engineered. These remarkable properties have opened up new opportunities for thin TMD flakes in optoelectronics and nonlinear photonics [13, 14].
In the last few years, TMD materials have received significant researches and are considered to be a kind of promising SAs for pulsed lasers due to their broadband saturable absorption, high third-order nonlinear susceptibility and ultrafast carrier dynamics [11–14]. The general formula of TMDs can be described as MX2, where M is a transition metal (Mo, W and so on) and X is a chalcogen element (S, Se or Te). Because of the introduction of the stoichiometric defects (non-ideal atomic ratio) and the edge states effect, layered TMD materials possess broadband linear and non-linear absorption features [15, 16]. The thickness dependent electronic band structure and the enhanced, broadband and ultra-fast nonlinear optical property endow them with many new optical properties. Among them, few-layer MoS2 nanosheet is a relatively early and widely used material in passively mode locking and Q-switch operation as SA for its exotic electronic and optical properties [11, 17]. Until now, the few-layer MoS2 has been successfully applied in lasers at wavelengths of 1 μm, 1.42 μm, 1.55 μm, 1.9 μm, 2.1 μm and 3 μm [18–20]. Similar to MoS2, WS2 is another kind of TMD material with outstanding SA performance, such as larger nonlinear optical response and stronger light-matter interaction [21]. In order to compare the nonlinear optical characteristics of TMDs, Bohua Chen et al. reported a ring-cavity Q-switched fiber laser based on four different metal dichalcogenides (MoS2, MoSe2, WS2, WSe2) SAs with identical cavity configuration [22]. In comparison with MoS2, WS2 is a better choice to obtain high-power and stable pulses for its better optical properties and thermal stability. However, unlike graphene that has extremely high thermal conductivity, flexibility and mechanical stability, TMDs may show a lower optical damage threshold because of their relatively poorer thermal and mechanical property. To overcome this difficulty, Lu Li et al. put forward that the laser damage threshold of TMD-SA based on inorganic materials substrate can be increased dramatically, and the damage threshold of WS2/FM was measured as high as 406 MW/cm2 [23]. In recent years, WS2 has been successfully applied as SAs for passively mode-locking or Q-switched lasers at 640 nm, 1.06 μm, 1.55 μm and 2μm [12, 15, 24–26]. However, the aforementioned studies were mainly concentrated on the single passive modulation regime. Up to date, the dual-loss-modulation QML operation based on WS2-SA has not been reported.
In this paper, by simultaneously employing electro-optic modulator (EOM) and as-prepared WS2-SA, we firstly investigated the output performance of the diode-pumped dual-loss-modulated QML Nd:Lu0.15Y0.85VO4 laser. To our knowledge, the mixed laser crystal is more useful to generate shorter pulse width with higher peak power in QML laser in comparison with the single crystals owning to the reduction of the stimulated emission cross sections and the extension of the fluorescence lifetime [27]. Here the mixed crystal Nd:Lu0.15Y0.85VO4 is employed as the gain medium with the expectation of higher pulse energy and shorter pulse duration. The experimental results show that the Q-switched envelope duration of the doubly QML laser was greatly compressed and the amplitude stability was significantly improved. Especially, when the pump power exceeded 6.28, 7.18 and 8.92 W, only one mode-locking pulse underneath a Q-switching envelope was generated with sub-nanosecond pulse durations at the modulation frequencies of 1, 2 and 3 kHz, respectively. This is the first demonstration, to the best of our knowledge, sub-nanosecond single mode-locking laser pulse with low repetition rate and high peak power was achieved using WS2 materials. In particular, the peak power we obtained is the largest in comparison with that of mode-locked or Q-switched lasers based on WS2-SA reported previously.
2. Preparation and characterization of few-layered WS2 nanosheets
The few-layer WS2 nanosheet we used was fabricated by the ultrasonic assisted vertical evaporation technique. At the first step, 20 mg WS2 powder was poured into 10 ml 0.1% SDS (sodium dodecyl sulfate) aqueous solution. Secondly, the WS2 aqueous solution was ultrasonically agitated for 10 hours to obtain WS2 aqueous dispersion with high absorption. With the ultrasonic process, the dispersed solution of WS2 can be centrifuged to remove sedimentation of large WS2 clusters. Thirdly, the upper portion of the centrifuged solution was decanted to a bottle and diluted and poured into a polystyrene cell. Then a hydrophilic quartz substrate was inserted vertically into the cell. The polystyrene cell was placed at the atmosphere for gradual evaporation. Finally, the WS2 sheets adhered to the quartz surface and prepared for the characterization and laser operation. Compared with the horizontal evaporation method, the vertical evaporation method can get a better quality for the WS2 sample. Therefore, most WS2 are dispersed very well on the quartz substrate.
Characterization of the as-prepared WS2 was performed by using the atomic force microscopy (AFM), Raman spectra, linear transmittance spectrum and nonlinear transmittance curve. Figure 1(a) and 1(b) show the AFM results of WS2 film. The height difference between the quartz substrate and the sample surface reveals that the film has the average thickness of ~7 nm, indicating that the layer number of WS2 flake is approximately 8~9. The lateral dimension of the flakes is about ~300-600 nm. A Raman spectroscopy system (excited by 514.5 nm laser) was utilized to further investigate the atomic structural arrangement of WS2, as shown in Fig. 1(c), two peaks located at 349.2 cm−1 and 419.7 cm−1 are related to the longitudinal acoustic mode 2LA(M) (i.e., E2g mode) and out-of-plane A1g vibrational modes. For WS2, the absolute intensity of the 2LA(M) mode increases with the decreasing number of layers, while the intensity of the A1g displays the opposite behavior. The intensity ratio could be linked to the number of layers [28]. The larger the number of layers, the smaller the ratio obtained. For the bulk WS2 and monolayer WS2, the ratios are always smaller than 1 and larger than 2, respectively. Here, the ratio of our WS2 sample is about 1.2, which indicates a few-layer structure of WS2 film. Obviously, the Raman result agrees well with the AFM measurements.
Fig. 1 (a) AFM image of WS2 film; (b) the corresponding height profiles of WS2 film; (c) Raman spectra of WS2-SA; (d) Linear-transmittance spectrum of WS2-SA; (e) Nonlinear transmittance curve of the WS2-SA versus the input pulse fluence; (f) the experimental setup for saturable absorption measurement.
Figure 1(d) shows the typical UV-Vis spectra of few-layered WS2, it can be found that the WS2-SA possesses broadband saturable absorption response from the visible to the near-infrared band. Thus, besides the 1.06 µm wavelength, it also can be used at the other wavelengths. The measuring results are analogous to the results of other few-layer TMD materials [11, 17], which mean a good broadband absorption of the WS2 film. The measured transmission of the WS2 film is about 86% at 1064 nm. In order to further confirm the saturable absorption capability of the WS2-SA sample at 1 μm, the nonlinear optical characteristics of the WS2 nanosheet was measured by the balanced twin-detector measurement technique. Figure 1(e) and 1(f) show the obtained nonlinear transmission curve of the sample versus the input pulse fluence and the experimental setup for saturable absorption measurement. The laser source is a homemade picosecond solid-state mode-locking laser at 1.06 μm. From the nonlinear absorption curve, the modulation depth and the saturation intensity were estimated to be about 5.1% and 179.4 μJ/cm2.
3. Experimental setup and results
3.1 Experimental setup
The schematic setup for the low repetition rate subnanosecond Nd:Lu0.15Y0.85VO4 laser with EOM and WS2 is shown in Fig. 2. A Z-type folded cavity is employed with the lengths of the three cavity arms L1, L2 and L3 of 55, 76 and 9 cm, respectively. The pump source emitting at 808 nm was a commercial fiber coupled laser diode (FAP-I system, Coherent Inc., USA). The pump beam was collimated and focused into the laser gain medium with a spot radius of 200 μm. A 3 × 3 × 10 mm3 mixed crystal Nd:Lu0.15Y0.85VO4 with the Nd-doping concentration of 0.38 at.% was employed as the gain medium. Both surfaces of the Nd:Lu0.15Y0.85VO4 crystal were anti-reflection (AR) coated at 808 nm and 1064 nm. In order to efficiently dissipate the heat deposition, the laser crystal was mounted in a copper block water-cooled to 12 °C. A flat mirror M1, AR coated at 808 nm on two surfaces and high-reflection (HR) coated at 1064 nm on the pump surface, was adopted as the input mirror. The spherical concave mirrors M2 and M3 were both HR coated at 1064 nm with radii of curvature of 500 mm and 150mm, respectively. Flat mirror M4 with 10% transmission at 1064 nm was employed as output mirror. With a polarizer and a λ/4 plate, an EOM (BBO crystal) was employed as the active modulator for its fast switching and excellent hold-off ability which is beneficial to compress the pulse duration of Q-switched envelope, while a WS2 nanosheet was used as the passive saturable absorbers. In the experiments, the WS2-SA was placed at a tight focusing position (i.e., beam waist near M4) to minimize the mode-locking buildup time. According to the ABCD matrix, the average beam waist radius at the position of the absorber is about 115 µm. The pulse characteristics were recorded by a 16G digital oscilloscope (Agilent DSO-X91604A, 80 G samples/s sampling rate, USA) and a fast pin photodiode detector (New Focus 1414) with a rise time of 14 ps. The output power is measured by a PM100D Energy/Power Meter (Thorlabs Inc., USA).
3.2 Experimental results and discussion
In the doubly QML Nd:Lu0.15Y0.85VO4 laser with EOM and WS2, there are two operating stages, i.e., the QML stage and the sub-nanosecond single mode-locking stage. Firstly, stable QML pulses can be generated under a small pump power. During the QML stage, the repetition rate of the Q-switched envelope is controlled by the EOM while the repetition frequency of the mode-locking pulses underneath the Q-switched envelope is determined by the cavity roundtrip transmit time. With the increase of the pump power, more inversion population density can be generated in the gain medium, then the pulse width of the Q-switched envelope can be gradually compressed. Especially, when the Q-switched envelope duration is shorter than the cavity roundtrip transmit time, only one mode-locked pulse with sub-nanosecond pulse duration exists in a Q-switched envelope, resulting in a single mode-locked pulse train with a repetition rate equal to the modulation frequency of EOM. Then the laser enters the sub-nanosecond single mode-locking stage.
Figure 3 shows the change process of the number of mode-locking pulses under a Q-switched envelope with the variation of the pump power at 1 kHz modulation frequency. Obviously, the number of mode-locked pulses decreases with the increase of the pump power. As shown in Fig. 3(a)-3(c), when the pump power is 2.67, 4.42 and 6.28 W, there are seven, four and one mode-locked pulses underneath a Q-switching envelope, respectively. When the pump power is equal to or higher than 6.28 W, the laser operates at a stable sub-nanosecond single mode-locking stage. In order to prove the high stability of the sub-nanosecond laser, the pulse train obtained at 1 kHz under the pump power of 8.92 W is described in Fig. 3(d) and the corresponding data is recorded for pulse stability calculation. By calculating, the pulse-to-pulse amplitude fluctuation factor (the ratio between the largest deviation and the mean pulse amplitude) of the 1 kHz sub-nanosecond laser is less than 3%. In addition, to check the long term stability of our laser system, this dual-loss modulated QML Nd:Lu0.15Y0.85VO4 laser was kept operating for at least 5 hours every day during one week. After about a 10-minute warming up on each day, the laser ran at a stable QML sub-nanosecond pulse status and no damage of the WS2-SA was observed during the whole experimental progress.
Fig. 3 Pulse shapes of the dual-loss-modulated QML laser at different pump powers with 1 kHz repetition rate: (a) 2.67 W; (b) 4.42 W; (c) 6.28 W; (d) 8.92 W.
In order to further investigate the work mechanism of the dual-loss-modulation configuration, the pulse characteristics of single modulated lasers are recorded as a comparison in our experiment. When the EO modulator was removed, the lasers still operated on QML state with WS2-SA and the pure mode locking operation cannot be obtained, which may result from not enough high power density at WS2-SA. However, the pulse widths and the pulse repetition rates of the Q-switched envelopes are much larger than those of the dual-loss-modulated QML laser. Under the pump power of 8.92 W, the envelope pulse width and the envelope repetition rate of the singly passively QML laser with WS2-SA can reach 146 ns and 252 kHz, respectively, corresponding to a large pulse-to-pulse amplitude fluctuation, which are shown in Fig. 4(a) and 4(b). Obviously, in addition to controlling the pulse repetition frequency, the EO modulator also plays a significant role in compressing the pulse width of the Q-switched envelope and enhancing the stability of the QML laser. Similarly, when the SA was removed, at the pump power of 8.92 W and the modulated frequency of 1 kHz, the shortest pulse durations of 108 ns can be obtained from the actively QML laser with EO, which is shown in Fig. 4(c). Thus, it can be concluded that the dual-loss-modulation mechanism has a very distinct advantage in compressing the pulse width of the Q-switched envelope and enhancing the stability of the QML laser.
Fig. 4 Temporal shape of (a) the Q-switched envelope and (b) the pulse train in the singly passively QML laser with WS2-SA; (c) temporal shape of the Q-switched envelope in the singly QML laser with EOM at the pump power of 8.92 W.
Figure 5(a) shows the average output powers of the dual-loss modulated QML laser versus the pump power at different repetition rates. From the figure, one can see that the output powers increase almost linearly with increasing pump power. Besides, the higher the modulation frequency, the higher the output power is generated. At the pump power of 8.92 W, the maximum average output powers under 1, 2, and 3 kHz are 341, 376, and 431 mW, respectively. At the modulated frequency of 1 kHz, the threshold pump power and the slope efficiency of the laser are about 0.75 W and 4.2%, respectively. According to the threshold pump power and the slope efficiency, the total insertion loss of the laser cavity is about 0.09.
Fig. 5 Dependence of (a) the average output power and (b) the pulse durations on the pump power at different modulation frequencies. Solid symbols: the QML stage; Open symbols: the single mode-locking stage.
Figure 5(b) gives the pulse durations versus the pump power for different modulation frequencies. It can be found that different modulation frequencies have different single mode-locking threshold pump powers. Here, the single mode-locking threshold pump power is defined as the value of the pump power for which the stable sub-nanosecond mode-locked pulse output is just reached. For the modulation frequencies of 1, 2 and 3 kHz, the threshold pump powers are about 6.28, 7.18 and 8.92 W, respectively. At the pump power of 8.92 W, the shortest mode-locking pulse durations are about 467, 543 and 829 ps for the modulation frequencies of 1, 2 and 3 kHz, respectively. When the pump power increases farther, the pulse duration could not be further compressed observably. We think this phenomenon is mainly caused by the dispersion and chirp in the cavity. Besides, the measurement accuracy of devices and the non-ideal experimental environment also affect the pulse compression of the dual-loss-modulated QML laser.
According to the average output powers and the pulse repetition rates, the pulse energies of Q-switched envelopes can be calculated, which is exhibited in Fig. 6(a). A low repetition rate is beneficial to improve the energy of the single mode-locked pulse. The maximum pulse energies are 341.5, 188, and 143.7 μJ under 1, 2, and 3 kHz modulation frequencies, respectively. At the single mode-locking pulse stage, almost all the pulse energy of a Q-switched envelope is concentrated into the single mode-locking pulse. The pulse energy we measured can be regarded as the pulse energy of the single mode-locking pulse. As expected, this kind of the QML laser can generate sub-nanosecond pulses with very high pulse energy.
Fig. 6 (a) The pulse energies and (b) the peak powers versus the pump power. Solid symbols: the QML stage; Open symbols: the single mode-locking stage.
According to the pulse energies and the pulse durations, the pulse peak powers are calculated, as exhibited in Fig. 6(b). One can see that the peak powers grow exponentially as the rise of the pump power. At the single mode-locking stage, a low modulation frequency also has obvious advantage in generating high peak power. At the pump power of 8.92 W, the maximum peak powers are 731, 692 and 519 kW for 1, 2 and 3 kHz, respectively. To our knowledge, the peak power we obtained is much higher than those generated by other pulsed lasers based on WS2-SA reported previously [12, 13, 24–26, 29–31].
In order to further prove the robustness of the laser system, the beam quality for the high peak power sub-nanosecond laser with WS2-SA was measured by employing the 90.0/10.0 scanning-knife-edge method. For the modulation frequency of 1 kHz, the beam quality factors (M2) in the horizontal and longitudinal planes are 1.82/1.53, which are hardly affected by the modulation frequency. The experimental results also indicated that the WS2-SA is a promising candidate and have huge potentials for high-performance solid-state ultra-short pulsed laser. Besides the current wavelength, the dual-loss-modulated single mode-locking technique is also applicable to other wavelengths, such as 0.53 nm and 1.3 nm. Further investigations in this respect are on the way.
4. Conclusion
In conclusion, by simultaneously employing EOM and as-prepared WS2 film, a low repetition rate sub-nanosecond Nd:Lu0.15Y0.85VO4 laser with high stability and high peak power is demonstrated for the first time. At a pump power of 8.92 W, the shortest pulse duration of 467 ps and the highest pulse energy of 341.5 μJ can be obtained, corresponding to a peak power of 731 kW. The experimental results show that the dual-loss modulation technology with EOM and WS2-SA is an efficient method for generating high peak power sub-nanosecond pulsed laser.
Funding
National Natural Science Foundation of China (61378022, 61475088); the Youth Future Project of Shandong University (2016WLJH25).
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