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Abstract
An experimental investigation was carried out to evaluate the potential of few-layer Bi2Te3 topological insulator in use as a saturable absorber for passive Q-switching of compact solid-state lasers in the 1-μm spectral region. By incorporating a sapphire-based few-layer Bi2Te3 sample into a Yb:LuPO4 laser that was formed with a 4-mm plane-parallel resonator, we realized efficient, high-power, high-repetition-rate pulsed laser operation. Depending on the output coupling utilized, single- or dual-wavelength laser action could be achieved. A maximum output power of 5.02 W at 1014.5 nm was produced at a pulse repetition rate of 1.67 MHz, with an optical-to-optical efficiency of 41% and a slope efficiency of 54%; while operating at 1004.9/1012.7 nm, the pulsed laser could produce an output power of 3.94 W at 1.38 MHz, with a pulse duration being as short as 34 ns. The largest pulse energy and highest peak power achieved were 3.0 μJ and 85.3 W. The results demonstrated in our experiment reveal the great potential of the few-layer Bi2Te3 topological insulator in the development of pulsed compact solid-state lasers in the 1-μm region.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Topological insulators possess many extraordinary properties, of which the most peculiar is that they are insulators in interior; but at their surface there exist metallic states originating from the topological invariant [1]. In general, topological insulators are heavy-element, small-bandgap semiconductors [1]. So far the strong topological insulators that have been known include Bi2Te3, Bi2Se3, and Sb2Te3, which are semiconductor crystals of a rhombohedral structure belonging to the point group D3d [2, 3]. Another member in this class of compounds, Sb2Se3, is a topologically trivial insulator, because the spin-orbit coupling in this crystal proves to be not strong enough to significantly modify the electronic structure [1, 3].
For a topological insulator, its surface electronic bands can cross the bulk bandgap and enclose a single Dirac point at the Brillouin-zone center, forming a single Dirac cone (a linear energy-momentum relationship) [1, 2]. Such a band structure is similar to that of graphene, suggesting that topological insulators, like graphene, are also likely to be promising broadband saturable absorbers.
The saturable absorption property of topological insulators was first confirmed in 2012, in Bi2Se3 at a wavelength of 1550 nm [4]. Shortly thereafter, the absorption saturation behavior was also observed at 800 nm, as well as in microwave region [5, 6]. In 2013 passively Q-switched laser action induced by a Bi2Se3 saturable absorber was realized in a Nd:GdVO4 laser operating at 1063 nm [5], which was the first time a topological insulator was utilized as saturable absorber for passive Q-switching of solid-state lasers. Since then passive Q-switching, with Bi2Se3 or Bi2Te3 acting as saturable absorber, has been demonstrated in various solid-state lasers operating in a wide spectral region from visible to mid-infrared, including Nd:Lu2O3 at 1.08 μm [7]; Nd:YAG at 1.06 or 1.3 μm [8]; Nd:LiYF4 at 1.3 μm [9]; Yb:KGW or Yb:GdAl3(BO3)4 at 1.04 μm [10–12]; Er:YAG at 1.6 μm [13]; Tm:LuAG, Tm:YAG, or Tm:YAP at 2 μm [14–17]; Er:YSGG at 3 μm [17]; and Pr:LiYF4 at 0.6 μm [18].
Among all the solid-state lasers that were passively Q-switched with Bi2Se3 or Bi2Te3 as saturable absorber, pulsed output power in excess of 2 W was generated only at 2 μm, from a Tm:LuAG or a Tm:YAP laser; the shortest pulse duration achieved was about 230 ns [14, 15, 17]. For Yb- or Nd-ion lasers in the 1-μm spectral region, the pulsed output power still remains at 1-W level [11], and the shortest pulse duration obtained was 370 ns [12].
Given the fact that the pulsed output power, achievable from passively Q-switched Yb-ion lasers with GaAs saturable absorber, has been in the 5-W level with pulse duration in a range of 70−160 ns [19, 20]; whereas using Cr4+:YAG as saturable absorber, the pulsed output power could readily be scaled to 10−15 W level, with the shortest pulse duration being less than 30 ns [21, 22], one sees the passive Q-switching performance in the 1-μm region, demonstrated so far with solid-state lasers employing topological insulator as saturable absorber, is still much inferior to that achieved with traditional saturable absorbers. Therefore, much work is needed to evaluate the potential of these topological insulators in generating high-power, short-duration pulsed laser radiation. To this end, it is crucial to choose an appropriate laser medium suitable for passive Q-switching.
In this work we investigated the passive Q-switching performance of an Yb:LuPO4/Bi2Te3 laser. Recent work has shown that Bi2Te3 could act as an effective saturable absorber in the 2−3 μm mid-infrared region [14–17]. As a relatively new laser crystal among the wide variety of Yb-ion laser materials, Yb:LuPO4 has exhibited promising pulsed properties in passive Q-switching laser action induced by GaAs, Cr4+:YAG, or MoS2 saturable absorber [20, 23–26]. This Yb-ion crystal turns out to be rather unique; it enables very high output couplings to be utilized in passive Q-switching laser action, which will greatly reduce the possibility of optical damage to the internal elements of the laser resonator. This might be of particular importance for passive Q-switching with two-dimensional (2D) saturable absorbers like Bi2Te3, which may suffer from relatively low optical damage threshold.
In this paper we report the results obtained from our experiment on the Yb:LuPO4/Bi2Te3 laser, which was built by use of a very compact 4 mm long plane-parallel resonator. Stable passive Q-switching laser action was realized; average output power of 4−5 W could be produced at repetition rates higher than 1.3 MHz, with slope efficiencies around 50%; the shortest pulse duration measured was 34 ns. The passive Q-switching laser performance demonstrated in our experiment, to some extent, has already been comparable to that achieved with GaAs, revealing the great potential of Bi2Te3 for passive Q-switching of solid-state lasers in the more common 1-μm spectral region.
2. Description of experiment
The few-layer Bi2Te3 sample studied in this experiment was a commercial product (Sixcarbon Tech, Shenzhen, China). It was made on a 0.35 mm thick sapphire substrate by the CVD method. As the laser medium, an uncoated, 1.0 mm thick Yb:LuPO4 crystal plate (thickness along a crystallographic axis) was utilized. The Yb-ion concentration was Nt = 1.85 × 1021 cm−3 (15 at. %). A simple plane-parallel resonator was employed to fabricate this Yb:LuPO4/Bi2Te3 laser. The plane reflector was coated for high reflectance at 1010−1200 nm (≥ 99.8%) and for high transmittance at 975 nm (> 95%); a group of plane output couplers were used, whose transmissions (output couplings) ranged from T = 10% to T = 90%. The Yb:LuPO4 crystal plate was fixed on a thin copper heat-sink, which was cooled by using thermoelectric coolers, maintaining a temperature of 5 °C. Inside the resonator the laser crystal was positioned very close to the reflector mirror; while the Bi2Te3/Sapphire sample was placed between the laser crystal and the output coupler, leaving a physical cavity length of 4 mm. A 25-W 975-nm fiber-coupled diode laser (fiber core diameter of 105 μm and NA of 0.22; emission bandwidth less than 0.5 nm) was utilized to pump the Yb:LuPO4/Bi2Te3 laser. The pumping beam was focused by a re-imaging unit and then was coupled onto the laser crystal with a pump spot radius of about 70 μm.
3. Results and discussion
The property of absorption saturation of the few-layer Bi2Te3 in the 1-μm region was first investigated. Figure 1 shows the transmission (T) versus incident intensity (I), which was measured by using the standard z-scan technique. For this measurement, a mode-locked picosecond Nd fiber laser at 1064 nm was employed. Fitting the measured data in accordance with the relationship T(I) = 1 − ΔTexp(−I/Isat) − Ans [27], one can obtain a modulation depth of ΔT = 1.1%; a saturation intensity of Isat = 2.42 MW/cm2; and a non-saturable absorption loss of Ans = 54.0%. The Raman spectrum measured for the Bi2Te3/Sapphire sample is presented as an inset to Fig. 1, showing the vibration modes of E2g at 100.8 cm−1 and A21g at 134.2 cm−1, which are characteristic of few-layer Bi2Te3 [14].
Fig. 1 Transmission versus incident intensity, measured for the Bi2Te3/Sapphire sample by z-scan method. Open circles: measured data; solid line: fitting curve. Inset: Raman spectrum for the Bi2Te3/Sapphire sample.
The absorption properties measured for the Bi2Te3 sample differ largely from those reported in Ref. 8, where the modulation depth, saturation intensity, and non-saturable loss are respectively ΔT = 14.7%; Isat = 4.6 kW/cm2; and Ans = 12.25%. The non-saturable loss, a less intrinsic parameter, depends critically on the film homogeneity, defects, impurities, and the interface between the few-layer sample and its substrate. The value for this parameter can thus vary from one sample to another. The amount of modulation depth is mainly determined by the film thickness or layer numbers, and can be increased by simply increasing the layer number. The saturation intensity is the most intrinsic parameter characterizing the saturable absorption caused by both the surface or edge electrons and bulk electrons of a topological insulator, and can be affected by the ratio of surface to bulk electrons. As a consequence, the magnitude of Isat will strongly depend on the specific microscopic structure of a topological insulator. In fact, the values of Isat reported for Bi2Te3 or Bi2Se3 turn out to be very different, ranging from 0.7 kW/cm2 (at 2 μm) to 10.1 GW/cm2 (at 800 nm) [15, 28].
Stable passive Q-switching operation of the Yb:LuPO4/Bi2Te3 laser could be realized, with output coupling changed over a wide range from T = 10% to T = 90%. It was found, however, that with an output coupling lower than T = 30%, the Q-switched operation would become unstable at a relatively low output level. For stable high-power Q-switched operation, the output coupling utilized had to be sufficiently high. Figure 2 shows the average output power as a function of absorbed pump power (Pabs), measured under output couplings of T = 10%, 30%, 50%, 80%, and 90%. The fraction of pump power absorbed was ηa0 = 0.91, measured at a very low pumping level (small-signal or unsaturated value); and the absorbed pump power is calculated from incident pump power by Pabs = ηa0Pin. In all cases the laser radiation was linearly polarized with E//c (π-polarized). Employing the coupler of T = 10%, the maximum output power attainable in stable operation amounted only to 1.24 W, generated at Pabs = 5.13 W; under a higher output coupling of T = 30%, the output power achievable increased to 2.80 W. Under conditions of T = 50%, which proved to be the optimal for high-power operation, the Q-switching laser threshold was reached at Pabs = 2.30 W; above which the output power scaled with Pabs, with a slope efficiency amounting to 54%. A maximum output power of 5.02 W was generated at Pabs = 12.2 W, resulting in an optical-to-optical efficiency of 41%. In the cases of T = 80% and T = 90%, the highest output power produced was 3.94 and 2.69 W, respectively.
Fig. 2 Average output power versus Pabs, measured for T = 10%, 30%, 50%, 80%, and 90%. The slope efficiency, ηs, is given for each output coupling.
Given the large non-saturable absorption loss of the Bi2Te3 sample, it seems difficult to understand the high slope efficiencies reached in the cases of T ≤ 50%. One reasonable explanation is as follows: due to the inhomogeneity of the Bi2Te3 film, the non-saturable absorption loss is location dependent; thus very probably, the actual loss experienced by the internal laser field might not be so large. To confirm this, a modified Findlay-Clay analysis can be carried out with the help of the relation, Pth = Pth0 + (Pth0/δ)ln(1/R), here Pth is the measured lasing threshold under output coupling of T = 1 − R; δ denotes the round-trip internal losses; and Pth0 represents the threshold pump power for the case of T = 0 (R = 1) [29]. Making use of the measured threshold values (Fig. 2), Pth = 1.19; 1.55; 2.30; 2.95; 3.30 W for T = 10%; 30%; 50%; 80%; 90%, one can determine the single-pass total internal losses (δ/2) as 0.345. Taking into account the reabsorption loss of the 1 mm thick Yb:LuPO4 crystal, σabsNtl = 0.185 (σabs = 0.10 × 10−20 cm2 at 1015 nm [30]), one obtains the amount of the non-saturable absorption loss of the Bi2Te3 film: 0.16, which is much less than measured by the z-scan method. It is also worth pointing out that due to the presence of cavity coupling, caused by the uncoated rear surface of the sapphire substrate opposite to the plane coupler [31], an effective reflectivity, Reff, should be used to replace R in the above relation. The values for the effective reflectivity can be calculated to be Reff = 0.94; 0.82; 0.67; 0.41; 0.30 for T = 10%; 30%; 50%; 80%; 90%, in accordance with a simple coupled-cavity theory [32].
Figure 3(a) illustrates the dependence of pulse repetition rate upon pump power. As the pumping level was raised, the repetition rate would become increased, a typical behavior for passive Q-switching laser action. Besides this, an unusual feature is noticeable: the very high repetition rates reached in each case. One sees from Fig. 3(a) that even in the vicinity of Q-switching threshold, the repetition rates measured could still reach roughly 400−500 kHz. These results turned out to differ greatly from those obtained previously from other solid-state lasers that were passively Q-switched by Bi2Se3 or Bi2Te3, where the repetition rates were limited to < 200 kHz [7–18]. Given the high output couplings employed here, this point seems to be even more extraordinary. The physical reason could be attributed in part to the extremely high gain of the Yb:LuPO4 crystal. The most important reason, however, was the incomplete bleaching of the Bi2Te3 saturable absorber during the passive Q-switching process. Due to its extremely high saturation intensity, the Bi2Te3 could only be partially saturated, in particular at low pumping levels under high output couplings. One can see that in the case of T = 50%, a repetition rate as high as 1.67 MHz was reached at the highest pump power of Pabs = 12.2 W; and even under high output coupling of T = 90%, the repetition rate could still reach 1.31 MHz. In the cases of T = 10% and T = 30%, the highest repetition rates measured in stable Q-switched operation were 0.48 MHz and 0.63 MHz, respectively.
Fig. 3 Pulse repetition rate (a) and pulse energy (b) versus Pabs, measured (calculated) for T = 50%, 80%, and 90%.
Figure 3(b) shows the variation of pulse energy with absorbed pump power. The pulse energy is calculated from the average output power and the corresponding repetition rate. One sees that at low pumping levels, the pulse energy could increase roughly linearly with Pabs; while at high pumping levels, it tended to reach a certain magnitude that would remain almost unchanged. Such a pulse energy varying behavior proved to be common for all cases, stemming from the increasing degree of saturation or bleaching of the Bi2Te3 absorber. Once the complete bleaching state could eventually be reached, the resulting pulse energy would remain fixed, independent of pump power. Due to its fairly large non-saturable absorption, the Bi2Te3 sample would be heated up during the laser operation, resulting in additional losses caused by thermal, mechanical, and optical effects occurring at the Bi2Te3/Sapphire interface. The addition of internal losses would slow down the increase of repetition rate with pump power, allowing more pulse energy to be generated. This might be another reason responsible for the increase of pulse energy with increasing pump power. One may also notice the unusual pulse energy increasing occurred at high pump power (Pabs > 11 W) in the case of T = 80%, which was obviously resulted from the irregular variation of repetition rate (Fig. 3(a)). This behavior was repeatable, provided the same location on the Bi2Te3 film was utilized. The reduction in repetition rate implied the increase of internal losses, resulted probably from thermal effects that occurred at the specific location of the absorber. The largest pulse energy, 3.0 μJ, was generated at Pabs = 12.2 W in the case of T = 50%.
The variation of pulse duration with pump power was examined in each case. The measured results are depicted in Fig. 4(a) for T = 50%, 80%, and 90%. One sees that for each case, the pulse duration decreased with Pabs, from an initial value around 200 ns to several tens of ns. Qualitatively, such variation behavior is typical of passive Q-switching by fast saturable absorbers; however, it proves to be contrary to the general theory of passive Q-switching. In fact, the reduction in pulse duration upon raising the pumping level, just as the increase in pulse energy, originated also from the progressively increasing degree of absorption saturation in the Bi2Te3 absorber. As the saturable absorber could be completely bleached at sufficiently high pump power, the pulse duration would remain more or less unchanged. This feature can be clearly seen from the high pump power region in Fig. 4(a). Compared to the previous Nd:YAG/Bi2Te3 laser generating shortest pulses of 576 ns [8], much shorter pulse durations were obtained with the present Yb:LuPO4 laser. One obvious reason for this was the much shorter cavity length utilized here (18 times shorter than used in Ref. 8), for theoretically, the pulse duration is inversely proportional to the cavity length. Another important reason might be attributed to the extremely high output couplings used, which could greatly reduce the decay time of the internal laser intensity after reaching its peak value.
Fig. 4 Pulse duration (a) and peak power (b) versus Pabs, measured (calculated) for T = 50%, 80%, and 90%.
Figure 4(b) shows the peak power as a function of Pabs, which is calculated from the pulse energy and duration. The highest peak power, 85.3 W, was generated at Pabs = 12.2 W in the case of T = 80%; for T = 50% and 90%, the highest peak power, 54.5 and 55.4 W, proved to be very close. Inside the laser resonator, the highest internal peak power was reached in the case of T = 50%, giving rise to a maximum on-axis intensity of 4.3 MW/cm2 (assuming a mode spot radius of 70 μm). This might be regarded as the upper limit of intensity in the Bi2Te3 sample for generating stable Q-switched laser operation. If we assume the damage threshold to be 2−5 times greater than this limit value, we can estimate the damage threshold for the Bi2Te3 sample to be roughly 8−22 MW/cm2, for high-repetition-rate (MHz) several-tens-ns laser pulses at 1 μm.
Illustrated in Fig. 5(a) is a pulse train, which was measured at the highest pump power of Pabs = 12.2 W in the case of T = 50%. The pulse amplitude fluctuations were estimated to be 0.7% (rms value); whereas the timing jitters were 2.8% (rms value). The temporal profile of an individual pulse is depicted as an inset, showing FWHM duration of 55 ns. Figure 5(b) shows the temporal profile of the shortest laser pulse for the cases of T = 80% and T = 90%. In both cases, the shortest pulse was generated at Pabs = 12.2 W, with pulse duration measured to be 34 ns (T = 80%) and 37 ns (T = 90%). The shortest pulse durations obtained under lower output couplings of T = 10% and 30% were, respectively, 131 and 70 ns.
Fig. 5 (a) Pulse train measured at Pabs = 12.2 W in the case of T = 50%. The temporal profile of an individual pulse is presented as an inset. (b) Temporal profile of the shortest laser pulse generated in the cases of T = 80% and T = 90%..
The Q-switched lasing spectrum was monitored. Depending on the magnitude of output coupling employed, the laser emission lines occurred in a wavelength range of 1003−1015 nm for T = 50%−90%. Under a certain output coupling, the lasing wavelengths varied only slightly with pumping level. Figure 6 shows a typical lasing spectrum for each case, measured at an intermediate pump power of Pabs = 7.8 W. One can see that the pulsed laser emitted at a single wavelength of 1014.5 nm in the case of T = 50%; whereas dual-wavelength oscillation could be realized under higher output coupling conditions, occurring at 1004.9, 1012.7 nm for T = 80%, and at 1003.1, 1006.9 nm for T = 90%. A calculation of π-polarized gain cross-section versus wavelength for the Yb:LuPO4 crystal shows that under certain high excitation levels (the fraction of Yb ions excited ≥ 0.36), laser action will occur in an emission band of 1001−1011 nm. This agrees with the lasing spectrum for T = 80% and 90% (a high output coupling necessarily requires a high excitation level). The dual-wavelength laser oscillation occurred because the overall (net) resonator gain reached maximum at the two specific wavelengths, resulting from the combined effects of the gain element and the plane reflector mirror of the resonator, whose reflectance would become reduced for wavelengths shorter than 1010 nm.
The quality of output laser beam was also examined. Figure 7 depicts the measured beam spot radius as a function of propagation distance. The measurement was conducted at Pabs = 5.9 W (output power of 1.0 W) in the case of T = 80%. By fitting the measured data according to the propagation law for a Gaussian beam, one determines the beam quality factor as M2 = 1.10 in the horizontal direction (x); and M2 = 1.12 in the vertical direction (y). A laser beam pattern is presented as an inset of Fig. 7. The good beam quality at high output levels obtained with the plane-parallel resonator, was due to the existence of thermal lensing in the laser crystal, which could not only help to stabilize the cavity, but could also effectively reduce the fundamental mode size and propagation divergence. A simple calculation indicates that as the thermal focal length decreases from fT = ∞ to fT = 50 mm, the TEM00 mode radius of the current 4 mm long cavity will be reduced from 197 to 70 μm, realizing perfect mode matching with the pump beam of 70 μm in spot radius.
Fig. 7 Spot radius versus propagation distance, measured at Pabs = 5.9 W in the case of T = 80%. Inset: the beam pattern.
The passive Q-switching performance demonstrated with the Yb:LuPO4/Bi2Te3 laser is summarized in Table 1, which lists the primary parameters characterizing a passively Q-switched laser. 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. To give a direct comparison, we also list, in Table 1, the results previously reported for other Yb- or Nd-ion lasers in the 1-μm region that were passively Q-switched by Bi2Te3 or Bi2Se3 topological insulator saturable absorber. In the last part of Table 1, the results for a Nd:GdVO4 and an Yb:LuVO4 lasers, which were passively Q-switched by Cr4+:YAG saturable absorber, are also presented. One can see that compared with the best record reported thus far, the output power as well as lasing efficiency obtained from the current Yb:LuPO4/Bi2Te3 laser could be increased by a factor of 4−5; whereas the pulse duration could be reduced by one order of magnitude. Furthermore, the pulse repetition rate could be as high as 1.67 MHz, one order of magnitude higher than ever reached in other Yb- or Nd-ion solid-state lasers employing Bi2Te3 or Bi2Se3 saturable absorber. In fact, the capability of inducing high-repetition-rate passive Q-switching laser action proves to be the major advantage for such topological insulator saturable absorbers, which originates from their inherent, extremely short relaxation or recovery time (less than 1 ps [5]). For the traditional Cr4+:YAG, the recovery time is longer than 3 μs, imposing an upper limit of ~300 kHz for the pulse repetition rates [33, 34]. The highest repetition rate achieved so far in passive Q-switching of Nd-ion lasers with Cr4+:YAG was 252 kHz, obtained from a diode-side-pumped Nd:GdVO4 bounce laser [33]; while for Yb-ion lasers, the maximum repetition rate, 286 kHz, was reached with an Yb:LuVO4 laser [34]. Other performance parameters for these two lasers are also given in Table 1.
Table 1. Primary Parameters Characterizing a Passively Q-switched Laser, Giving a Comparison of the Current Yb:LuPO4/Bi2Te3 Laser with Previously Reported Yb- or Nd-ion Lasers Passively Q-switched by Bi2Te3, Bi2Se3, or Cr4+:YAG Saturable Absorber
The Yb:LuPO4 crystal employed in our experiment, which was of an Yb-ion doping level of 15 at. %, could enable high gain to be reached. Indeed, Nd-ion crystals such as Nd:YAG, is also capable of providing high gain. To generate short pulses in passively Q-switched laser action, however, a sufficiently large population inversion density proves to be essential. In this regard, the previous Nd:YAG/Bi2Te3 laser appeared to be much inferior to the current laser; the average population inversion density reached there could be much lower, given its 16 times lower brightness of pump source, 4 times larger pump spot area, and 7 times longer crystal leading to much larger pump beam divergence [8]. Among Yb-ion crystals, Yb:YAG may be more suitable for making a high-gain laser, due to its larger emission cross-section. Experimentally, however, neither the Yb:YAG nor other Yb-doped gallium garnets has exhibited promising performance comparable to Yb:LuPO4, in passively Q-switched laser operation induced by GaAs, a traditional fast semiconductor saturable absorber [20, 23, 35]. In fact, according to our work, the Yb-ion crystals that are able to compete with Yb:LuPO4 in GaAs passive Q-switching, are at present limited to the monoclinic rare-earth calcium oxyborates, Yb:YCa4O(BO3)3 and Yb:GdCa4O(BO3)3 [19, 36]. Indeed, similar passive Q-switching laser performance has been demonstrated in our latest experiment on the two oxyborate crystals employing Bi2Te3 saturable absorber, producing multi-watt output power with pulse durations in the sub-100 ns level. To conclude, the crucial factor in choosing an Yb-ion crystal for a passively Q-switched laser using topological insulator saturable absorber, is the ability of operating under very high output couplings. The superior passive Q-switching performance of Yb:LuPO4 might be attributed to its comprehensive spectroscopic, thermal, and optical properties.
4. Summary
In conclusion, we have carried out an investigation to evaluate the potential of few-layer Bi2Te3 topological insulator, in use as saturable absorber for passive Q-switching of solid-state lasers in the 1-μm spectral region. By incorporating a sapphire-based few-layer Bi2Te3 into a compact Yb:LuPO4 laser that was formed with a 4 mm long plane-parallel cavity, we realized efficient stable Q-switched operation under high output coupling conditions. A maximum output power of 5.02 W at 1014.5 nm was produced at a pulse repetition rate of 1.67 MHz under an output coupling of 50%, the optical-to-optical and slope efficiencies being 41% and 54%; whereas under a greater output coupling of 80%, a dual-wavelength pulsed operation was achieved, producing, at 1004.9/1012.7 nm, an output power of 3.94 W at 1.38 MHz, with the pulse duration as short as 34 ns. The largest pulse energy and highest peak power generated were 3.0 μJ and 85.3 W. The experimental results demonstrated here not only reveal the great potential of Bi2Te3 topological insulator as saturable absorber for passive Q-switching, but also imply a significant progress toward the development of high-power, high-repetition-rate compact pulsed solid-state lasers based on such topological insulator saturable absorbers.
Funding
National Natural Science Foundation of China (NSFC) (11574170).
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