1 Introduction

In the past few years, two-dimensional (2D) transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se, Te), attracted much attention as saturable absorbers for passive Q-switching of Yb-ion solid-state lasers. Up to now, passive Q-switching laser action induced by 2D MX2 has been realized in many different Yb-ion crystal lasers such as Yb:YAG [1], Yb:LGGG [2], Yb:LSO [3], Yb:KLu(WO4)2 [4], Yb:GdAl3(BO3)4 [5, 6], Yb:Ca3Y2(BO3)4 [7], Yb:YCa4O(BO3)3 (Yb:YCOB) [8], and Yb:LuPO4 [9,10,11,12]. Most of the work reported previously involved MoS2 [2, 4, 5, 7, 9], or WS2 [3, 6, 10, 11]; while the MoTe2, one member of the 2D ditellurides, was utilized only very recently in an Yb:YCOB laser [8].

As saturable absorber for passive Q-switching in Yb-ion lasers operating over the 1.0–1.1 µm spectral region, 2D MoTe2 seems to be more desirable than its other MX2 counterparts. This results from the fact that the bandgap of 2D MoTe2 is expected to fall into a range of 0.93–1.1 eV (the monolayer direct bandgap is 1.1 eV; whereas the bulk indirect one is 0.93 eV [13]), smaller than the photon energy of Yb-ion lasers that usually ranges from 1.24 to 1.13 eV (wavelengths of 1.0–1.1 µm), thus allowing valence- to conduction-band electronic transitions to occur leading to strong resonant absorption for the laser radiation. Passive Q-switching action induced by 2D MoTe2 has been demonstrated in an Yb:YCOB laser, producing an output power of 1.58 W at a repetition rate of 704 kHz, with a minimum pulse duration of 52 ns [8]. Given the impressive performance, it is also of interest to explore other Yb-ion lasers that can be passively Q-switched effectively by 2D MoTe2 saturable absorber.

Our recent work suggests the necessity for a sufficiently high output coupling of the laser resonator in realizing stable, high-power passively Q-switched operation with 2D saturable absorbers [10, 12]. Since only in this way to reduce the internal circulating laser power, the detrimental thermal effects occurring inside the 2D saturable absorber, which arises from the nonsaturable absorption and proves to be the major obstacle to achieving stable high-power Q-switched laser action, could be effectively mitigated. Consequently, only those Yb-ion lasers that are capable of operating under high enough output couplings are feasible to generate high-power pulsed radiation in passively Q-switched operation with such 2D saturable absorbers. According to this criterion, Yb-ion-doped orthovanadates appear to be unsuitable for this purpose, for their fairly low limit in output couplings applicable in high-power lasers [14].

The monoclinic Yb:KLu(WO4)2 (Yb:KLuW) crystal turns out to be a promising candidate for high-power passive Q-switching with these 2D MX2 saturable absorbers; it enables efficient laser operation under high output couplings. Besides this, the Yb:KLuW crystal has exhibited superior capability of generating short laser pulses through passive Q-switching with Cr4+:YAG [15,16,17], the most widely used traditional saturable absorber for Yb- and Nd-ion lasers in the 1-µm region.

In this work, we demonstrated a highly efficient Yb:KLuW/MoTe2 laser that was formed with a compact plane-parallel resonator, and that could be passively Q-switched under a wide range of output couplings from T = 10% to T = 90%, producing output power in 2-W level at repetition rates higher than 2 MHz, with a minimum pulse duration of 36 ns. The crucial role of the resonator output coupling in the determination of the passive Q-switching performance of the Yb:KLuW/MoTe2 laser is also revealed.

2 Description of experiment

The few-layer MoTe2 sample used in the experiment was a commercial product; it was made on a 0.35-mm-thick sapphire substrate by CVD method. The main parameters characterizing its absorption saturation behavior in the 1-µm region have been determined in our recent work [8]. We chose to utilize an Ng-cut Yb:KLuW crystal to take advantage of its strong absorption for the pumping radiation at 975 nm; it was 2 mm long, with a square aperture of 4 mm × 4 mm, and the Yb-ion concentration in the crystal was 3.5 × 1020 cm−3 (5.3 at. %). No antireflection coatings were made on its end faces. A plane-parallel resonator was employed to build the compact Yb:KLuW/MoTe2 laser. The plane reflector was coated for high reflectance at 1020–1200 nm, and for high transmittance at 975 nm. As the output coupler, a group of partially transmitting plane mirrors was used, whose transmittances (output couplings) ranged from T = 10% to T = 90% (at 1030 nm). The Yb:KLuW crystal, which was cooled by thermoelectric coolers maintaining a temperature of 10 °C, was placed close to the plane reflector; while the MoTe2/sapphire sample was positioned between the laser crystal and the output coupler, leaving a physical cavity length of 5.5 mm. To pump the Yb:KLuW/MoTe2 laser, a fiber-coupled (fiber core diameter of 105 µm and NA of 0.22) diode laser emitting at 975 nm (bandwidth less than 1 nm) was employed. Through a re-imaging unit, the pumping radiation from the diode was coupled onto the laser crystal, with a pump spot radius of about 70 µm.

3 Results and discussion

Stable passively Q-switched operation of the Yb:KLuW/MoTe2 laser could be realized, with the output coupling changed over the wide range from T = 10% to T = 90%. In all cases, linearly polarized laser oscillation was observed with E//Nm. In general, higher output coupling resulted in higher output power as well as shorter minimum pulse width, with the optimum determined to be T = 80%. Figure 1 shows the average output power versus the absorbed pump power (Pabs), measured for T = 70%, 80%, and 90% in stable passively Q-switched operation. The amount of Pabs is calculated from the incident pump power (Pin) by Pabs = ηaPin, with ηa denoting the unsaturated or small-signal absorption. For the 2-mm-long, Ng-cut Yb:KLuW crystal, the value for ηa was measured to be 0.50 under diode pumping at 975 nm. It needs to be pointed out that although the absorption peak is located at 981 nm for Yb:KLuW, pumping of the crystal at a shorter wavelength of 975 or 976 nm, which is more readily available from common InGaAs diodes, is also a practical choice [17]. In fact, effective pumping of this crystal could be achieved at different wavelengths ranging from 974 to 990 nm [18], owing to the strong absorption as well as the relatively large bandwidth of the absorption peak at 981 nm [19]. For T = 70%, the passive Q-switching threshold was measured to be Pabs = 1.59 W. With the pump power raised above the threshold, the output power could increase almost linearly with Pabs, reaching 1.89 W at Pabs = 5.04 W, with a slope efficiency that was as high as 63%. With the pump power being further increased, the Q-switched laser operation became unstable, owing to the increasingly strengthened thermal effects in the MoTe2 absorber. Under the optimal output coupling of T = 80%, the lasing threshold increased to 1.98 W; while the slope efficiency decreased to 56%. However, the maximum output power attainable in this case, prior to the onset of unstable pulsed laser action, was 2.06 W, which was measured at Pabs = 5.87 W, resulting in an optical-to-optical efficiency of 35%. Under a still higher output coupling of T = 90%, efficient, stable passively Q-switched operation could also be realized, generating a maximum output power of 1.76 W at Pabs = 6.26 W, with a slope efficiency of 51%.

Fig. 1
figure 1

Output power versus Pabs measured for the Yb:KLuW/MoTe2 laser under output couplings of T = 70%, 80%, and 90%

Full size image

The pulse repetition rate was found to depend on pumping level. Figure 2 depicts the variation of repetition rate with absorbed pump power for T = 70%, 80%, and 90%. For the cases of T = 70% and 80%, the dependence of repetition rate on Pabs proved to be similar: increasing first slowly and then rapidly as the pump power exceeded a certain level. The slow increasing of repetition rate at the initial stage stemmed from the progressive increase in the bleaching degree of the MoTe2 absorber, which is, like most other 2D MX2, characterized by a rather high saturation intensity (1.71 MW/cm2 [8]). The highest repetition rate, reached in the cases of T = 70% and 80%, was measured to be 2.15 and 2.18 MHz, respectively. For T = 90%, the repetition rate could increase roughly linearly over the entire operational region, reaching 1.44 MHz at the highest pump power of Pabs = 6.26 W.

Fig. 2
figure 2

Repetition rate versus Pabs measured for the Yb:KLuW/MoTe2 laser under output couplings of T = 70%, 80%, and 90%

Full size image

Figure 3 shows the pulse energy versus Pabs, which is calculated from the average output power and the corresponding pulse repetition rate (Figs. 2, 3). In each case, the pulse energy increased with pump power; after reaching its maximum, the energy would remain almost unchanged or would become reduced. The initial increase of pulse energy was also due to the progressively increasing saturation degree of the 2D MoTe2 absorber. One sees that the highest pulse energy, 1.30 µJ, was generated at Pabs = 5.04 W under the optimal output coupling of T = 80%.

Fig. 3
figure 3

Variation of pulse energy with Pabs, determined for the Yb:KLuW/MoTe2 laser operating under output couplings of T = 70%, 80%, and 90%

Full size image

Figure 4 illustrates the variation behavior of pulse duration with pump power for T = 70%, 80%, and 90%. Also because of the increasing degree of absorber saturation, the pulse duration dropped rapidly with the increase of pump power. The minimum pulse duration of 36 ns was obtained under the optimal output coupling (T = 80%); it was measured at Pabs = 5.04 W, at which the highest pulse energy (1.30 µJ) was also generated (Fig. 3), giving rise to a maximum peak power of 36.1 W. In the cases of T = 70% and 90%, the shortest pulse duration was measured to be 44 and 37 ns, respectively. One sees that at high pumping levels, the laser pulse became widened very rapidly, in particular for T = 70% and 80%. At the highest pumping level, the pulse duration increased to 152, 131, and 61 ns, for T = 70%, 80%, and 90%, respectively.

Fig. 4
figure 4

Pulse duration versus Pabs measured for the Yb:KLuW/MoTe2 laser under output couplings of T = 70%, 80%, and 90%

Full size image

Figure 5 shows two typical pulse trains measured for the case of T = 80%, of which Fig. 5a was recorded at Pabs = 5.04 W where the shortest laser pulse was generated; while Fig. 5b was recorded at Pabs = 5.87 W, the highest pump power applied. The pulse amplitude fluctuations existing in the pulse trains are calculated to be 3.0% (a) and 1.7% (b); whereas the timing jitters are 2.7% (a) and 1.1 (b). All the numbers refer to rms values (root mean square). The temporal profile of an individual laser pulse from the pulse train (a) is presented in Fig. 6, showing an FWHM width of 36 ns.

Fig. 5
figure 5

Laser pulse train for the Yb:KLuW/MoTe2 laser operating under output coupling of T = 80%, measured at Pabs = 5.04 W (a) and at Pabs = 5.87 W (b)

Full size image
Fig. 6
figure 6

Temporal profile of an individual laser pulse measured at Pabs = 5.04 W under output coupling of T = 80%

Full size image

In our experiment, we achieved stable passively Q-switched operation of the Yb:KLuW/MoTe2 laser, with the output coupling changed from T = 10% to T = 90%. The passive Q-switching performance demonstrated was found to depend strongly on the output coupling utilized. Figure 7 depicts the output coupling dependence of both the maximum output power and the minimum pulse width. With the output coupling increased from 10 to 60%, the output power attainable in stable Q-switched operation could be increased from 0.46 to 1.18 W. One can also see that increasing the output coupling from 60 to 70% would lead to a substantial increase in the achievable output power. As shown in Fig. 7, the minimum pulse width could be effectively reduced by increasing the output coupling of the laser resonator. Employing a very low output coupling of T = 10%, the minimum pulse width amounted to 114 ns; it was reduced to 36 ns with the output coupling increased to T = 80%.

Fig. 7
figure 7

Output coupling dependence of the maximum output power and minimum pulse width measured for the Yb:KLuW/MoTe2 laser

Full size image

Similar to the situation of passive Q-switching induced by other 2D saturable absorbers, there existed a pump region over which stable repetitively pulsed operation could be obtained. It was found in our experiment that this region was dependent on the output coupling utilized. Figure 8 depicts the threshold pump power for passive Q-switching, Pabs,th, and the maximum pump power applicable before the Q-switching became unstable, Pabs,max, against the output coupling of the laser resonator. Under some output couplings, instability was observed near the lasing threshold; but this unstable pump range proved very narrow (< 0.2 W). So the difference between Pabs,max and Pabs,th, ΔP = Pabs,max − Pabs,th, can be used to roughly represent the stable pump region, as is marked in Fig. 8 by a vertical line for each output coupling. With the output coupling increased, the stable pump region would become wider, which could be attributed to the weaker thermal effects occurring in the 2D absorber because of the less intense circulating laser radiation. For T = 90%, the stable pump region was measured to be ΔP ≈ 4 W, roughly four times wider than in the case of T = 10%.

Fig. 8
figure 8

Output coupling dependence of the threshold and the maximum pump power applicable for the Yb:KLuW/MoTe2 laser. The length of the vertical line roughly represents the stable pump region for each output coupling

Full size image

The oscillation wavelengths of the Yb:KLuW/MoTe2 laser also depended upon the resonator output coupling, but only changed slightly with the pumping level. Figure 9 shows a set of lasing spectra for T = 10%, 30%, 50%, 80%, and 90%, illustrating an evolution of laser emission spectrum with output coupling. As the output coupling was increased, the laser emission spectrum would shift toward short-wavelength side, because the increase in overall resonator losses would require higher gain for lasing, which was available only at shorter wavelengths. For the Yb:KLuW/MoTe2 laser operating under output couplings of T = 10–90%, the emission wavelengths covered a spectral region of 1025.2–1043.3 nm; while operating under the optimal T = 80%, the pulsed laser emitted at 1030.6 nm.

Fig. 9
figure 9

Laser emission spectra of the Yb:KLuW/MoTe2 laser, measured under output couplings of T = 10%, 30%, 50%, 80%, and 90%

Full size image

The short plane-parallel cavity employed in our experiment was very similar to those used in microchip lasers passively Q-switched with Cr4+:YAG saturable absorber. We chose to utilize such a compact resonator in the passively Q-switched Yb:KLuW/MoTe2 laser, to produce short laser pulses by taking advantage of its short cavity photon lifetime. One may notice some similarity in performance between the current laser and those that were passively Q-switched with Cr4+:YAG, e.g., the output power, the laser efficiency, and, to some extent, the pulse duration. On the other hand, however, one should also note those characteristics typical of 2D saturable absorber passive Q-switching, such as the increase of pulse energy and the decrease of pulse duration with increasing pump level. More importantly, the extremely high pulse repetition rates in excess of 2 MHz, which were reachable with the current Yb:KLuW/MoTe2 laser, turned out to differ greatly from those that were passively Q-switched by Cr4+:YAG saturable absorbers. For the latter, there exists an upper limit for the pulse repetition rates, which is around 300 kHz, and is imposed by the recovery time of the Cr4+:YAG crystal [20].

Table 1 lists the primary parameters for the passively Q-switched Yb:KLuW/MoTe2 laser operating under the optimum output coupling of T = 80%. For comparison, the results for the recently reported Yb:YCOB/MoTe2 laser [8], as well as for the previous Yb:KLuW/MoS2 laser [4], are also presented in the table. Compared to the Yb:YCOB laser, higher output power and shorter pulse width were achieved from the current Yb:KLuW laser. More importantly, the maximum pulse repetition rate reached here, 2.18 MHz, proved to be much higher than achieved in the Yb:YCOB laser, a result arising from the distinctive spectroscopic properties between the two monoclinic laser crystals. In fact, the maximum repetition rate achieved in the current experiment, turned out to be the highest ever reached in Yb-ion lasers that were passively Q-switched by 2D saturable absorbers. Up to now, the highest repetition rate reported is limited to 1.67 MHz, which was reached in an Yb:LuPO4 laser passively Q-switched by a 2D topological insulator [21]. While in comparison with the Yb:KLuW/MoS2 laser, which was the only work reported thus far on passive Q-switching laser action of the Yb:KLuW crystal induced by 2D MX2 saturable absorbers, the performance of the Yb:KLuW/MoTe2 laser demonstrated in our experiment represents a significant improvement in nearly all major aspects.

Table 1 Comparison of passive Q-switching performance between the current Yb:KLuW/MoTe2, and the previous Yb:YCOB/MoTe2 and Yb:KLuW/MoS2 lasers
Full size table

As saturable absorbers for passive Q-switching in diode-pumped solid-state lasers, 2D materials differ from the traditional saturable absorbers such as Cr4+:YAG or SESAM, in respect of modulation depth, saturation intensity/fluence, damage threshold, and material homogeneity. Consequently, distinct passive Q-switching performance is expected with these 2D materials serving as saturable absorbers. Compared to the widely used Cr4+:YAG, one evident and unique advantage possessed by 2D materials is their capability of producing high-repetition-rate passive Q-switching action, which is connected with their intrinsic extremely short recovery time (at most a few ps [22]). The results achieved in the present work suggest the possibility of producing multi-watt-level output power at repetition rates exceeding 2 MHz with pulse durations of several tens nanoseconds. The passive Q-switching performance will be further improved through optimization in the absorption saturation properties of 2D materials, and through improvements in their optical quality, damage threshold, and material homogeneity. Anyway, the development of 2D materials is just at its initial stage.

4 Conclusions

In summary, we have demonstrated an efficient Yb:KLuW laser that could be passively Q-switched by 2D MoTe2 saturable absorber under output couplings of 10–90%. A maximum average output power of 2.06 W at 1030.6 nm was produced at a repetition rate of 2.18 MHz, with optical-to-optical and slope efficiencies of 35% and 56%. The largest pulse energy, minimum pulse width, and highest peak power achieved were, respectively, 1.3 µJ, 36 ns, and 36.1 W. The results demonstrated in the current work reveal the potential of Yb:KLuW crystal in making high-repetition-rate, nanosecond, passively Q-switched solid-state lasers employing 2D saturable absorbers.