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Abstract
We report on passive Q-switching action induced by a few-layer MoTe2 saturable absorber in an Yb:YCa4O(BO3)3 (Yb:YCOB) microchip laser. With a sapphire-based few-layer MoTe2 incorporated into the 4 mm long plane-parallel resonator of the Yb:YCOB microchip laser, efficient stable passively Q-switched operation was achieved under output couplings of 40%−70%, producing, at an incident pump power of 5.0 W, an average output power of 1.58 W at a repetition rate of 704 kHz with a slope efficiency of 36%; the pulse energy and peak power were respectively 2.25 μJ and 40.8 W, while the shortest pulse duration obtained was 52 ns.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Few-layer semiconducting transition metal dichalcogenides (TMDs), MX2 (M = Mo, W; X = S, Se, Te), have been recognized as a promising class of two-dimensional (2D) broadband saturable absorbers for passive mode-locking or Q-switching in fiber, waveguide, and solid-state lasers operating at wavelengths ranging from the visible to mid-infrared [1–9]. Apart from their superiority of being applicable to a wide variety of lasers, these 2D TMDs also prove to be advantageous over traditional saturable absorbers in the 1-μm spectral region for passive Q-switching of Yb- or Nd-ion lasers, such as Cr4+:YAG and GaAs, in generating high-repetition-rate pulsed laser radiation, owing to their intrinsic, extremely short relaxation or recovery time.
In the field of passive Q-switching of Yb- and Nd-ion lasers in the 1-μm region, most previous studies of 2D TMDs were concentrated on MoS2 [8,10–13] or on WS2 [9,14–16]; while the diselenide members, MoSe2 and WSe2, have not yet received much attention, the only known relevant work was limited to an Yb:YAG/MoSe2 and an Yb:LuPO4/WSe2 lasers [17,18]. Compared to the situation of MoSe2 and WSe2, research work on the ditelluride members appears to be even less. Only very recently has 2D MoTe2 been utilized for passive mode-locking of a Tm fiber laser at 1.9 μm [19], and for passive Q-switching of an Er:YAG laser at 1.6 μm [20].
In comparison with those of disulfides or of diselenides, transition metal ditellurides possess smaller bandgaps in both monolayer and bulk states. For instance, monolayer MoTe2 has a direct bandgap of 1.1 eV, compared to 1.8 eV for MoS2 and 1.57 eV for MoSe2; whereas the indirect bandgap of bulk MoTe2 amounts only to 0.93 eV, against 1.29 eV for MoS2 and 1.1 eV for MoSe2 [2,19]. So the bandgap of a few-layer MoTe2 will be smaller than the photon energy in the 1-μm region (1.24−1.13 eV, for photons at 1.0−1.1 μm), allowing valence- to conduction-band electronic transitions through photon absorption, which is different from the cases of MoS2 and MoSe2. As a consequence, few-layer MoTe2 might be more suitable for use as saturable absorber in the 1-μm region, due to its strong interaction with radiation field.
In this paper we report on efficient passively Q-switched operation of an Yb:YCOB microchip laser at 1.03 μm, realized with a sapphire based few-layer MoTe2 acting as saturable absorber. Our work demonstrates the possibility of producing, from Yb-ion lasers employing few-layer MoTe2 saturable absorbers, watt-level or even multi-watt output power at repetition rates of several hundreds of kHz, with pulse durations of several tens of nanoseconds.
2. Description of experiment
The few-layer MoTe2 sample was prepared by CVD method on a 0.35 mm thick sapphire substrate (a commercial product provided by Sixcarbon Tech, Shenzhen, China). The number of layers of the sample ranged from 12 to 15. The details for the production process of few-layer MoTe2 by the CVD technique have been reported recently [21,22]. As the laser medium for the microchip laser, an uncoated, 1.5 mm thick, X-cut Yb:YCOB crystal plate was utilized, whose Yb-ion concentration was 25 at. %. To build the microchip laser, a plane reflector that was coated for high reflectance at 1020−1200 nm (≥ 99.8%) and for high transmittance at 976 nm (> 95%), and a plane output coupler were employed to form a 4 mm long plane-parallel resonator. Inside the resonator, the laser crystal plate was placed close to the plane reflector; whereas the MoTe2/Sapphire sample was positioned between the laser crystal and the output coupler. The Yb:YCOB crystal plate was cooled with thermoelectric coolers, maintaining a temperature of 5 °C; no active cooling was made for the MoTe2/Sapphire sample. The laser was pumped by a 976-nm fiber-coupled diode laser, whose fiber core diameter and NA were respectively 100 μm and 0.22. By use of a re-imaging unit, the pump beam was focused and delivered into the laser crystal with a pump spot radius of about 70 μm.
3. Results and discussion
Figure 1a shows the transmission spectrum over a wavelength range of 1000−1100 nm, measured for the MoTe2/Sapphire sample (red line) as well as for a 0.35 mm thick sapphire plate (blue line). The transmittance of the MoTe2/Sapphire increases slightly with wavelength, from 50.0% at 1000 nm to 53.7% at 1100 nm. Also presented in Fig. 1a are the optical images for the MoTe2/Sapphire sample (lower right) and for the sapphire plate (upper right).
Fig. 1 (a) Transmission spectra and optical images for the MoTe2/Sapphire sample as well as for a 0.35-mm sapphire plate. (b) Transmission versus incident intensity, measured for the MoTe2/Sapphire by z-scan method. Inset: Raman spectrum of the MoTe2/Sapphire.
The absorption saturation properties of the MoTe2/Sapphire sample in the 1-μm region were also investigated, with the help of the standard z-scan technique. The probe beam was provided by a 1.06-μm mode-locked picosecond Nd-ion fiber laser. Figure 1b depicts the transmission (T) as a function of incident probe intensity (I). By fitting the measured data according to the relation, T(I) = 1 − ΔTexp(−I/Isat) − Ans [2], one obtains the modulation depth ΔT = 0.9%; the saturation intensity Isat = 1.71 MW/cm2; and the non-saturable loss Ans = 47.1%. The recovery time of 2D MoTe2 saturable absorber was measured in a very recent work as 2.3 ps [19]. The Raman spectrum of the MoTe2/Sapphire sample was also measured, which is presented as the inset of Fig. 1b. The features appearing in the Raman spectrum seem to suggest 1T′ phase of the MoTe2 film, rather than 2H phase [21].
With the MoTe2/Sapphire sample incorporated into the Yb:YCOB microchip laser, we obtained stable passively Q-switched operation under output couplings of T = 40%−70%. The average output power versus incident pump power (Pin) measured for T = 40%, 60%, and 70%, is depicted in Fig. 2, showing the Q-switched output characteristics. The fraction of incident pump power absorbed by the 1.5 mm thick Yb:YCOB crystal, was measured to be 0.88 (small-signal or unsaturated value). The magnitude of this absorption fraction was found to depend largely on the incident pump power, decreasing from the small-signal value of 0.88 to 0.35 at Pin = 5.0 W. Such strong absorption saturation prevents reliable estimation of the absorbed pump power at a certain pumping level. So the performance of the present laser is characterized with respect to incident pump power. In all cases, the pulsed laser radiation was linearly polarized with E//Z. For T = 40%, the Q-switching laser threshold was reached at Pin = 0.41 W; above the threshold, the output power produced could increase roughly linearly with pump power, reaching 1.58 W at Pin = 5.0 W. The slope efficiency determined for the Q-switched laser operation was 36%. With the pumping level being raised further, the output power could continue to increase; however, the stability of the Q-switched operation would deteriorate rapidly, due to the increasingly enhanced thermal effects occurring in the few-layer MoTe2 and/or at the interface between the film and the sapphire substrate. To evaluate the passive Q-switching efficiency, we measured the output power when the laser was operated in continuous-wave (cw) mode, which was realized with a 0.35 mm thick uncoated sapphire plate to replace the MoTe2/Sapphire sample, while keeping the resonator unchanged. A cw output power of 2.11 W was measured at Pin = 5.0 W under output coupling of T = 40%. Thus one can estimate the efficiency of the passive Q-switching action induced by the few-layer MoTe2 in the Yb:YCOB microchip laser to be 75%.
Under a higher output coupling of T = 60% or T = 70%, the Q-switched laser operation became less efficient, with slope efficiency reduced to 34% (T = 60%) and 29% (T = 70%). The reduction in slope efficiency can be attributed to the fact that the output couplings utilized here were far apart from the optimal for achieving the highest lasing efficiency. Despite this, however, a higher output coupling might prove to be advantageous, because the stable Q-switched operational region could be extended to a higher pumping level, as illustrated in Fig. 2. In the case of T = 60%, the maximum output power of 1.55 W was produced at Pin = 5.39 W; while for T = 70%, the highest output power of 1.36 W was generated at Pin = 5.77 W. The physical reason for the wider operational region was the lower internal circulating laser intensity under a higher output coupling, which could reduce the detrimental nonsaturable absorption in the few-layer MoTe2 absorber, mitigating the thermal effects which could lead to instability of passive Q-switching. It was found in our experiment that when the output coupling employed was lower than T = 40%, the Q-switched laser action would become unstable at relatively low pumping levels, limiting the output power attainable to less than 1 W. Furthermore, under a lower output coupling than T = 40%, optical damage could occur to the MoTe2 film at relatively high pumping levels.
To estimate the insertion loss of the MoTe2 saturable absorber, we measured the threshold pump power under different output couplings in the range of T = 30%−70%, when the laser was operated in Q-switched as well as in cw mode. For cw operation, a 0.35-mm sapphire plate was inserted to keep the resonator unchanged. From a modified Findlay-Clay analysis [23], we obtain a value of 9.3% for the single-pass internal losses of the resonator containing the 0.35-mm sapphire plate; while for the resonator containing the MoTe2/sapphire, the single-pass internal losses are determined to be 13.2%. Thus we can estimate the net insertion loss of the MoTe2 saturable absorber to be 3.9%.
The emission spectrum of the Yb:YCOB/MoTe2 microchip laser varied with the output coupling utilized. However, its dependence on the pumping level proved to be very limited. Figure 3 shows the lasing spectra for T = 40%, 60%, and 70%, which were measured at Pin = 4.6 W. One sees that for each case, only a single laser emission line was observed, suggesting the presence of strong longitudinal mode discrimination during the pulse formation process. For the output couplings of T = 40%−70%, the lasing wavelength of the laser covered a range of 1032.5−1035.5 nm. The shift of laser emission line toward short-wavelength side upon increasing the output coupling, resulted from the requirement for higher gain under a higher output coupling; with the increase in excitation level, the gain maximum would move to shorter wavelengths.
Fig. 3 Q-switched lasing spectra of the Yb:YCOB/MoTe2 microchip laser, measured at Pin = 4.6 W for T = 40%, 60%, and 70%.
The pulse repetition rate of the Q-switched laser action was measured at different pumping levels. The results are depicted in Fig. 4(a) for T = 40%, 60%, and 70%. In the case of T = 40%, the repetition rate increased with Pin, from 195 kHz in the vicinity of the lasing threshold to 704 kHz at Pin = 5.0 W, the highest pumping level applied. For higher output couplings, the range of repetition rates was measured to be 313−741 kHz (T = 60%) and 400−690 kHz (T = 70%).
Fig. 4 Pulse repetition rate (a) and pulse energy (b) versus Pin, measured (calculated) for T = 40%, 60%, and 70%.
One may notice from Fig. 4(a) an unusual behavior exhibited in the low pumping region (Pin < ~1.5 W), where a larger output coupling led to higher repetition rates, contrary to the expectation on the basis of the current passive Q-switching theory. The few-layer MoTe2, like any other 2D TMD, is a fast saturable absorber characteristic of very high saturation intensity. During the passive Q-switching process, it could not be completely saturated when the internal laser intensity was not high enough. So at a certain pump power in the low pumping region, the bleaching degree of the few-layer MoTe2 reachable in the case of T = 70% would be the smallest, thus resulting in the highest repetition rate. With the pump power increased to such a level that the bleaching degrees, achieved under different output couplings, became comparable, this unusual behavior would disappear, as confirmed in the high pumping region of Pin > ~3.5 W.
Figure 4(b) shows the variation of pulse energy with Pin for T = 40%, 60%, and 70%, which is calculated from the measured data for average output power and pulse repetition rate. For each case, the pulse energy increased with pump power, due to the increasingly strengthened bleaching degree of the few-layer MoTe2 saturable absorber. As the absorber could eventually become almost completely saturated, the pulse energy would remain unchanged, as encountered in usual passive Q-switching induced by a conventional “slow” saturable absorber such as Cr4+:YAG. The highest pulse energy, 2.25 μJ, was generated under the lowest output coupling of T = 40%; for higher output couplings, the maximum energy produced was 2.09 and 1.97 μJ, for T = 60% and 70%, respectively.
Figure 5(a) illustrates the dependence of pulse duration on pump power for the passively Q-switched Yb:YCOB/MoTe2 microchip laser. Under each output coupling, the laser pulse was found to become shortened with increasing pump power, which also proved to be typical of passive Q-switching action induced by fast saturable absorbers of high saturation intensity, stemming from the progressively increasing degree of absorption saturation in the few-layer MoTe2 absorber. The shortest pulse duration was measured to be 52 ns at Pin = 4.58 W under output coupling of T = 40%; in the cases of higher output couplings, the minimum pulse width was measured to be 62 ns (T = 60%) and 67 ns (T = 70%).
Fig. 5 Pulse duration (a) and peak power (b) versus Pin, measured (calculated) for T = 40%, 60%, and 70%.
From the pulse energy and duration, one can calculate the peak power achieved at different pumping levels under a certain output coupling. The results are depicted in Fig. 5(b) for T = 40%, 60%, and 70%. In each case, the peak power increased with pump power. For T = 40%, the peak power reached 40.8 W at Pin = 5.0 W; while under higher output couplings, the peak power achievable amounted to 32.2 W (T = 60%) and 28.5 W (T = 70%).
Figure 6(a) presents a pulse train for T = 60%, recorded at Pin = 3.76 W at which the shortest laser pulse was generated under this output coupling. The pulse amplitude fluctuations and the timing jitters, which are exhibited in the pulse train, are calculated to be respectively 2.8% and 6.3%, both of which refer to rms value. The temporal profile of an individual pulse is presented as the inset, showing pulse duration (FWHM) of 62 ns. Illustrated in Fig. 6(b) is the temporal profile of the shortest laser pulse for T = 40%, which was measured at Pin = 4.58 W; and for T = 70%, which was measured at Pin = 5.39 W. One notices that for the shortest pulse generated under output coupling of T = 40%, the temporal profile proved to be fairly asymmetric, with the trailing edge dropping more rapidly. Such an asymmetric pulse shape implies more internal losses added at the second-half stage after reaching the peak of internal laser intensity. These additional losses might result from two-photon absorption (TPA), which could occur in few-layer TMD saturable absorbers at sufficiently high incident intensities, as observed in the case of WSe2 [2]. Besides the TPA, free-carrier absorption occurring in the few-layer MoTe2 might also contribute to the addition of internal losses.
Fig. 6 (a) Pulse train measured at Pin = 3.76 W under output coupling of T = 60%. The inset shows the temporal profile of an individual pulse. (b) Temporal profile of the shortest laser pulse generated in the cases of T = 40% and T = 70%.
The output beam quality of the Yb:YCOB/MoTe2 microchip laser was also studied. Figure 7 depicts the measured beam spot radius (w) versus the propagation distance (z); the inset shows a laser beam pattern. The measurement was conducted at Pin = 3.35 W (output power of 1.0 W) under output coupling of T = 40%. By fitting the measured data in accordance with the Gaussian beam propagation law, the beam quality factor was determined to be M2 = 1.18 in the horizontal direction (x); and M2 = 1.21 in the vertical direction (y).
Fig. 7 Spot radius (w) versus propagation distance (z), measured at an output power of 1.0 W under output coupling of T = 40%. Inset: the beam pattern.
In our experiment an X-cut Yb:YCOB crystal sample was utilized, because this crystal orientation led to the best passive Q-switching performance. Y- and Z-cut crystal samples, having the same thickness and Yb-ion concentration, were also tested in a preliminary experiment. The maximum output power achievable proved to be lower, and the pulse duration longer than obtained in the case of X-cut crystal.
It seems to be instructive to make a comparison of passive Q-switching performance between the current Yb:YCOB/MoTe2 microchip laser and the recently reported Yb:LuPO4/MoS2 laser which was formed with a 5 mm long plane-parallel resonator, and was operated under similar output couplings [12]. We list in Table 1, for the two lasers, the primary parameters that are used to characterize the passive Q-switching laser action. These parameters include Pmax, the maximum average output power; PRR, the highest pulse repetition rate; Ep, the largest pulse energy; tp, the shortest pulse duration; Pp, the highest peak power; ηs, slope efficiency; and λl, lasing wavelength. Also listed in Table 1 are the parameters for an Er:YAG laser operating at 1645 nm, which was passively Q-switched with a multilayer MoTe2/YAG serving as saturable absorber [20]. This was the only work reported so far on passive Q-switching induced by 2D MoTe2 saturable absorber. One notes the very similar performance of the current laser with the Yb:LuPO4/MoS2 laser under output coupling of T = 50%; but shorter pulse duration was achieved in the present experiment. It is also worth mentioning that the slope efficiency for the Yb:YCOB laser is determined with respect to incident pump power, rather than absorbed pump power, as for other lasers [12,20]. Due to its roughly two times smaller emission cross-section compared to the Yb:LuPO4 crystal, the Yb:YCOB proves to be inferior to the latter, in generating short pulses by passive Q-switching. Therefore, the results demonstrated in our experiment may suggest a greater ability of 2D MoTe2 saturable absorber to generate short laser pulses in passive Q-switching. Through a comparison, one also sees the great difference between the current laser and the Er:YAG/MoTe2 in repetition rate, and, in particular, in pulse duration.
Table 1. Comparison of Passive Q-switching Performance of the Current Yb:YCOB/MoTe2 Microchip Laser with the Recently Reported Yb:LuPO4/MoS2 and Er:YAG/MoTe2 Lasers
4. Summary
In conclusion, we have realized efficient stable passively Q-switched operation of an Yb:YCOB microchip laser with a few-layer MoTe2 acting as saturable absorber. With 5.0 W of pump power incident onto the 1.5 mm thick laser crystal, an average output power of 1.58 W at 1035.5 nm was produced at repetition rate of 704 kHz with a slope efficiency of 36%; the resulting pulse energy and peak power were respectively 2.25 μJ and 40.8 W, while the shortest pulse duration achieved was 52 ns. Our work represents the first demonstration of high-repetition-rate passive Q-switching laser action in the 1-μm region, induced by 2D MoTe2 saturable absorber, with pulse duration as short as several tens of nanoseconds.
Funding
National Natural Science Foundation of China (NSFC) (11574170).
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