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Nano-seconds pulsed Er:Lu2O3 laser using molybdenum ditelluride saturable absorber
Optics and Laser Technology 121 (2020) 105791
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Nano-seconds pulsed Er:Lu2O3 laser using molybdenum ditelluride saturable absorber
T
⁎
Yangyang Lianga, Tao Lia, , Jia Zhaoa, Wenchao Qiaoa, Guiqiu Lia, Tianli Fenga, Shuaiyi Zhangb, Shenzhi Zhaoa a School of Information Science and Engineering, and Shandong Provincial Key Laboratory of Laser Technology and Application, Shandong University, Jinan 250100, China b Advanced Optoelectronic Materials and Technologies Engineering Laboratory of Shandong, Qingdao University of Science & Technology, Qingdao 266061, China
H I GH L IG H T S
nanoflakes were successfully exfoliated by liquid phase exfoliation. • MoTe saturable absorption property of the film at ~3 μm was characterized. • The laser at ~3 μm based on the MoTe film was realized. • Q-switched • 2 dimensional MoTe film is an outstanding passive modulator at ~3 μm. 2
2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: MoTe2-SA Q-switched lasers Solid state lasers Mid-infrared lasers
The stable nano-seconds pulsed Er:Lu2O3 laser were realized by employing a molybdenum ditelluride (MoTe2) saturable absorber (SA), which was fabricated with the liquid phase exfoliation (LPEx) method. Under the absorbed power of 7.24 W, the maximum average output power of 0.52 W was achieved in a repetition rate of 105 kHz and a pulse duration of 317 ns at 2.85 μm, corresponding to a pulse energy of 4.94 μJ and a peak power of 15.6 W.
1. Introduction Mid-infrared laser is among the most intensively developed laser sources in recent years. Laser radiation with wavelength of ~3 μm has various of applications in gas sensing [1] infrared countermeasure [2] and is promising in space communication. This radiation is also a proper laser source for nonlinear laser frequency process such as optical parameter oscillators (OPO) and high order harmonic generation [3]. In medical application, it attracts even more interests, the reason is that the wavelength of ~2.94 μm corresponds a strong absorption peak of water (~10000 cm−1), which is significantly contained in organs and biology tissues. The radiation energy can be mainly absorbed by the target tissue, thus, the thermal damage to the surrounding tissue could be dramatically decreased [4]. Thus, the drastic demands in applications promote an even explosive researching tendency of the laser radiation at ~3 μm. 3 μm lasers could be achieved from the stimulated emission of the 4 I11/2 → 4I13/2 transition of Er3+ ion, and the radiation at 2.69 μm was
⁎
firstly observed in 1967 [5]. But owing to the thermal effect of the host materials, which was induced by the low stokes efficiency (~35%) and the strong multiphonon relaxing, optimizing a high performance laser at ~3 μm was really hard [6]. However, the novel high quality sesquioxide crystals combining excellent thermomechanical property, comparable low phonon energies and even strong crystal field make themselves ideal host materials for the high performance mid-IR laser operations [7]. As a favorable sesquioxide crystal, the cubic Lu2O3 has already been applied in high performance 1 μm [8], 2 μm [9] and 3 μm [10,11] laser operations. The experimental performances of graphene [12], CN [13] and other two dimension (2D) materials [14,15] in passively Q-switched or mode-locked lasers show us the untapped potential to achieve high performance mid-IR lasers as SAs. Among all types of stuffs, transition metal dichalcogenide (TMD) is one of the most attractive and capable candidates, for not only typical layer structure, but also the variation band-gap, which depending on their composition, structure and dimensionality [16–19]. Similar to other TMD materials, in same layer of
Corresponding author. E-mail address: [email protected] (T. Li).
https://doi.org/10.1016/j.optlastec.2019.105791 Received 19 August 2018; Received in revised form 4 May 2019; Accepted 25 August 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 121 (2020) 105791
Y. Liang, et al.
Fig. 1. Characterization of MoTe2 exfoliation: (a) TEM imagine, (b) AFM imagine and (c) height variation of MoTe2 film, (d) linear transmission and nonlinear transmittance (insert) of YAG-based MoTe2 film at 2.85 μm.
Fig. 2. Configuration of the laser system.
pulsed laser was 0.522 W, corresponding to a repetition rate of 105 kHz and a pulse duration of 317 ns, resulting in a pulse energy of 4.94 μJ and a peak power of 15.6 W.
MoTe2 samples, the stable Te-Mo-Te units are periodically arranged and combined with the strong covalent bond, while between layers, it is the effect of weak van der Waals force [16,18,19]. Attributed to the two different combinations, MoTe2 shows a typical layer structure and also has a great potential of being exfoliated into multilayer or even monolayer nano-flakes from bulk samples. In many TMDs, the band-gap varies in both value and type with the different layer structure. For MoTe2, an in-direct band-gap of 0.9 eV in bulk samples and a direct band-gap of 1.1 eV in monolayer structure was reported [20–22]. In both types above, the band gap is bigger than the energy of mid-IR photon, thus single–photon absorption at these energies cannot occur. However, the experimental broad band saturable absorption of the low dimension TMD materials in mid-IR could be attributed to the sub-band absorption induced from crystallographic defect state [23] or the edge states[24]. Based on the outstanding properties of Er:Lu2O3 and MoTe2 material, in this letter, we demonstrate a nano-seconds passively Q-switched laser operation at ~2.85 μm modulated by a homemade multilayer MoTe2-SA. The layered MoTe2 specimen was fabricated by the LPEx method. Stable CW laser operations were built up in the first step, and then was modulated to be stable Q-switched laser systems by introducing our homemade MoTe2-SA into the laser cavity. Under the maximum absorbed pump of 7.4 W, the optimum output power of stable
2. Preparation and characterization of multilayer MoTe2 The effectiveness and maneuverability of LPEx has been proved in massive semiconductor fabrication, multilayer or even monolayer films can be successfully exploited from the bulk sample after hours of process [25,26]. The selection of proper solvent is one of the key factors for the high quality films exfoliation. In our fabrication, the layered MoTe2 specimen was exfoliated from powder sample in the pure ethanol. Through 6 h ultrasonic processing, the sample in multilayer could be successfully prepared. Then suspension liquid was centrifuged for 10 min with rotate speed of 1500 rps, and the top dispersion was collected for further characterization and laser experiment. TEM is an efficient method to measure the ultrathin samples. The TEM measurement of MoTe2 exfoliations is shown in Fig. 1(a), it reveals our homemade exfoliations was in the sub-micrometer dimension. In order to get a height cognition of exfoliation in details, height imagine formed by AFM and the corresponding chart are shown in Fig. 1(b) and (c), from figure mentioned, one could hold that our nano-flakes with lateral size of few hundred nanometers, and with variable height 2
Optics and Laser Technology 121 (2020) 105791
Y. Liang, et al.
Fig. 3. Dependence of average output power on absorbed pump power in (a) CW and (b) Q-switched laser operations; spectrum of (c) CW (peak: 2.85 μm) and (d) Qswitched (peak: 2.85 μm) laser operations.
Fig. 4. Top: variation of pulse duration and repetition rate with absorbed pump power; bottom: typical pulse train and the temporal profile of the stable pulse.
exfoliation with layers varing from 20 to 40 was successfully prepared. The MoTe2-SA was fabricated by dropping the dispersion on a YAG basement and then naturally dried for 10 h. The linear optical transmittance of our homemade specimen at wavelength ranging from 2600 to 3100 was measured by an UV–VIS-NIR spectrophotometer (U-4100, Hitachi, Japan) and the data was visualized in Fig. 1(d), the result states that in this demarcated range, the linear transmittance rate over 92.6% is notable, and also in the figure, the highlight also points out that at lasing wavelength of 2.85 μm, the transmittance is 98.2%, or a linear absorption of 1.8%. And the nonlinear optical transmission was characterized by an active q-switched laser with a repetition rate of 4 kHz and lasing at ~ 2.85 μm, the variation of the response with the increasing single pulse energy is shown in the insert drawing of Fig. 1(d), and the typical saturable absorption property at emission wavelength of our homemade MoTe2-SA can be clearly observed.
Fig. 5. Variation of pulse energy and peak power with absorbed pump power.
varying from 15 to 25 nm. The typical height of a monolayer material is about 0.65 nm[22], thus, it is convincing to state that our MoTe2 3
Optics and Laser Technology 121 (2020) 105791
Y. Liang, et al.
3. Laser experiment
absorbed pumping source with power of 7.24 W at ~976 nm, the best performance of Q-switched laser was achieved with 4.94 μJ/317 ns pulses in repetition rate of 105 kHz at ~2.85 μm, corresponding to a maximum average output power of 0.52 W and a peak power of 15.6 W.
Stable CW and Q-switched lasers were both built up in a simple configured laser system. As shown in Fig. 2, a fiber coupled ~976 nm LD laser was collimated and incident to the center of the front end of the 3*3*10 mm cubic Er:Lu2O3, and the crystal was placed in the linear cavity which was composed of flat input mirror (M1) and curve output mirror (M2). Three different output couplers (OCs) with optical transmittances of 1%, 3% and 5% at 2.85 μm were employed here to identify the laser performances under different losses. The output power of all operations was measured by a PM100D power meter together with an S314C power head (Thorlabs Inc., USA). The measurement of the output power and its dependence with the absorbed pump power were shown in Fig. 3(a) and (c). For CW operations, the lasing thresholds of the three systems was measured to be 1.27 W, 1.84 W and 2.37 W, corresponding to OCs with transmittances of 1%, 3% and 5%. Under the maximum absorbed pump power of 7.24 W, the output powers were 0.78 W, 0.98 W and 0.59 W, separately, and the slop efficiencies were calculated to be 11.9%, 18.5% and 12.6%. Insert the prepared multilayer MoTe2-SA into the laser cavities, the stable Q-switched lasers were also achieved. In this condition, the lasing thresholds altered to be 2.05 W, 2.38 W and 2.55 W. Keeping the maximum absorbed pump power the same, the measurements of the average output power were 0.35 W, 0.52 W and 0.33 W, and the slop efficiencies altered to be 6.3%, 10.8% and 6.8% respectively. According to the linear fitting of the experiment data in Fig. 3(a) and (c), the linear increase tendency of output powers with the absorbed pump power in CW laser operations, and nearly linear in pulsed laser operations can also be clearly observed. In CW and Q-switched lasers, the measurements of the emitting wavelengths both stabilized at~2.85 μm and were shown in Fig. 3 (c) and (d). The laser pulse trains were detected by a HgCdTe IR detector with a response time of 1 ns (PVI-4TE-4, Vigo System S.A.) and visualized by a DPO 7104C digital oscilloscope with a rise time of 350 ps, bandwidth of 1 GHz and sampling rate of 5 GS/s (Tektronix DPO 7102, USA). A simple conclusion that pulse duration decreases while lasing repetition rate increases with the increasing absorbed pump power could be indicated from the previous studies of passively Q-switched lasers [6,14,15,19,27], and same tendencies of pulsed duration and repetition rate could be observed in our pulsed lasers in top of Fig. 4. However, we could also observe some fluctuations of both targets when compared with other pulsed lasers which were also modulated by multilayer TMD [19,28,29], here we attribute it to the Inhomogeneity of our homemade MoTe2-SA. In our experiment, when coupling transmittance is 3%, the stable Q-switched laser with the shortest stable pulse duration of 317 ns was achieved under the maximum absorbed pump power of 7.24 W and the corresponding repetition rate is 105 kHz. The temporal profile of shortest pulse together with the laser trains are both shown in the bottom of Fig. 4. The maximum repetition rate of 108 kHz was obtained under the same maximum pump power with the coupling transmittance of 1%. The dependency of the emission peak power and pulse energy on the absorbed pump power under different coupling transmittances are shown in Fig. 5. Obviously, both pulse energy and peak power increase with the increasing absorbed pump power in all laser systems with different losses. As is painted, when absorbed power exceeds 5.5 W, the laser operations with both biggest peak power and hugest pulse energy could be obtained by employing the OC with transmittance of 3%. In our experiment, the maximum pulse energy of 4.94 μJ and peak power of 15.6 W were obtained under the absorbed pump power of 7.24 W.
Acknowledgment This work was supported by the National Key Research and Development Program of China (2016YB1102201); the National Natural Science Foundation of China (NSFC) (61605100); the Open Project of State Key Laboratory of Crystal Material (No. KF1702); and Dr. Tianli Feng thanks Qilu Young Scholars Program of Shandong University. References [1] P. Werle, A. Popov, Application of antimonide lasers for gas sensing in the 3–4-µm range, Appl. Opt. 38 (9) (1999) 1494–1501, https://doi.org/10.1364/AO.38. 001494. [2] D.H. Titterton, et al., Development of a mid-infrared laser for study of infrared countermeasures techniques, in Technologies for Optical Countermeasures. 2004. Doi: 10.1117/12.578214. [3] K.A. Temelkov, et al., Experimental study on the spectral and spatial characteristics of a high-power He–SrBr 2 laser, J. Phys. D Appl. Phys. 42 (11) (2009) 115105, , https://doi.org/10.1088/0022-3727/42/11/115105. [4] A. Zajac, et al., Electrooptically Q-switched mid-infrared Er:YAG laser for medical applications, Opt. Express 12 (21) (2004) 5125–5130, https://doi.org/10.1364/ OPEX.12.005125. [5] M. Robinson, D.P. Devor, THERMAL SWITCHING OF LASER EMISSION OF Er3+ AT 2.69 μ AND Tm3+ AT 1.86 μ IN MIXED CRYSTALS OF CaF2:ErF3:TmF3. Appl. Phys. Lett., 1967. 10(5), p. 167–170. Doi: 10.1063/1.1754895. [6] M. Fan, et al., Multilayer black phosphorus as saturable absorber for an Er:Lu2O3 laser at ~3 μm, Photonics Res. 4 (5) (2016), https://doi.org/10.1364/PRJ.4. 000181. [7] C. Krankel, Rare-earth-doped sesquioxides for diode-pumped high-power lasers in the 1-, 2-, and 3-μm spectral range, IEEE J. Sel. Top. Quantum Electron. 21 (1) (2015) 250–262, https://doi.org/10.1109/JSTQE.2014.2346618. [8] L. Hao, et al., Spectroscopy and laser performance of Nd:Lu2O3 crystal, Opt. Express 19 (18) (2011) 17774–17779, https://doi.org/10.1364/OE.19.017774. [9] P. Koopmann, et al., Efficient diode-pumped laser operation of Tm:Lu2O3 around 2 μm, Opt. Lett. 36 (6) (2011) 948–950, https://doi.org/10.1364/OL.36.000948. [10] T. Li, et al., Efficient high-power continuous wave Er:Lu2O3 laser at 2.85 μm, Opt. Lett. 37 (13) (2012) 2568–2570, https://doi.org/10.1364/OL.37.002568. [11] L. Wang, et al., Room temperature continuous-wave laser performance of LD pumped Er:Lu(2)O(3) and Er:Y(2)O(3) ceramic at 2.7 mum, Opt Express 22 (16) (2014) 19495–19503, https://doi.org/10.1364/OE.22.019495. [12] Q. Bao, et al., Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers, Adv. Funct. Mater. 19 (19) (2009) 3077–3083, https://doi.org/10.1002/ adfm.200901007. [13] M. Fan, et al., Graphitic C3N4 as a new saturable absorber for the mid-infrared spectral range, Opt. Lett. 42 (2) (2017) 286–289, https://doi.org/10.1364/OL.42. 000286. [14] B. Chen, et al., Q-switched fiber laser based on transition metal dichalcogenides MoS2, MoSe2, WS2, and WSe2, Opt. Express 23 (20) (2015) 26723–26737, https:// doi.org/10.1364/OE.23.026723. [15] Y. Cheng, et al., Passive Q-switching of a diode-pumped Pr:LiYF4 visible laser using WS2 as saturable absorber, IEEE Photonics J. 8 (3) (2016) 1–6, https://doi.org/10. 1109/JPHOT.2016.2550804. [16] M. Chhowalla, et al., The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets, Nat. Chem. 5 (4) (2013) 263–275, https://doi.org/10. 1038/NCHEM.1589. [17] D. Shi, et al., Preparation and thermoelectric properties of MoTe2 thin films by magnetron co-sputtering, Vacuum 138 (2017) 101–104, https://doi.org/10.1016/j. vacuum.2017.01.030. [18] M. Xu, et al., Graphene-like two-dimensional materials, Chem. Rev. 113 (5) (2013) 3766–3798, https://doi.org/10.1021/cr300263a. [19] Y. Liang, et al., Passively Q-switched Er:YAG laser at 1645 nm utilizing a multilayer molybdenum ditelluride (MoTe2) saturable absorber, Laser Phys. Lett. 15 (9) (2018), https://doi.org/10.1088/1612-202X/aacfae. [20] T. Böker, et al., Band structure ofMoS2, MoSe2, andα−MoTe2:angle-resolved photoelectron spectroscopy andab initiocalculations, Phys. Rev. B 64 (23) (2001), https://doi.org/10.1103/PhysRevB.64.235305. [21] H. Guo, et al., Double resonance Raman modes in monolayer and few-layer MoTe2, Phys. Rev. B 91 (20) (2015), https://doi.org/10.1103/PhysRevB.91.205415. [22] C. Ruppert, O.B. Aslan, T.F. Heinz, Optical properties and band gap of single- and few-layer MoTe2 crystals, Nano Lett 14 (11) (2014) 6231–6236, https://doi.org/10. 1021/nl502557g. [23] S. Wang, et al., Broadband few-layer MoS2 saturable absorbers, Adv. Mater. 26 (21) (2014) 3538–3544, https://doi.org/10.1002/adma.201306322. [24] M. Trushin, E.J.R. Kelleher, T. Hasan, Theory of edge-state optical absorption in two-dimensional transition metal dichalcogenide flakes, Phys. Rev. B 94 (15)
4. Conclusion and discussion In conclusion, multilayer MoTe2 films was fabricated in the first step and then introduced into an Er:Lu2O3 laser operation lasing at ~2.85 μm as an SA. The stable Q-switch laser was achieved. Under 4
Optics and Laser Technology 121 (2020) 105791
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nitride nanosheet film, Opt. Express 25 (11) (2017) 12796–12803, https://doi.org/ 10.1364/OE.25.012796. [28] M. Fan, et al., Watt-level passively Q-switched Er:Lu(2)O(3) laser at 2.84 mum using MoS(2), Opt. Lett. 41 (3) (2016) 540–543, https://doi.org/10.1364/OL.41. 000540. [29] M. Fan, et al., Continuous wave and ReS2 passively Q-switched Er: SrF2 laser at approximately 3 mum, Opt. Lett. 43 (8) (2018) 1726–1729, https://doi.org/10. 1364/OL.43.001726.
(2016), https://doi.org/10.1103/PhysRevB.94.155301. [25] G.S. Bang, et al., Effective liquid-phase exfoliation and sodium ion battery application of MoS2 nanosheets, ACS Appl. Mater Interfaces 6 (10) (2014) 7084–7089, https://doi.org/10.1021/am4060222. [26] Y. Hernandez, et al., High-yield production of graphene by liquid-phase exfoliation of graphite, Nat. Nanotechnol. 3 (9) (2008) 563–568, https://doi.org/10.1038/ nnano.2008.215. [27] M. Fan, et al., Passively Q-switched Ho, Pr:LiLuF4 laser with graphitic carbon
5
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Nano-seconds pulsed Er:Lu2O3 laser using molybdenum ditelluride saturable absorber
T
⁎
Yangyang Lianga, Tao Lia, , Jia Zhaoa, Wenchao Qiaoa, Guiqiu Lia, Tianli Fenga, Shuaiyi Zhangb, Shenzhi Zhaoa a School of Information Science and Engineering, and Shandong Provincial Key Laboratory of Laser Technology and Application, Shandong University, Jinan 250100, China b Advanced Optoelectronic Materials and Technologies Engineering Laboratory of Shandong, Qingdao University of Science & Technology, Qingdao 266061, China
H I GH L IG H T S
nanoflakes were successfully exfoliated by liquid phase exfoliation. • MoTe saturable absorption property of the film at ~3 μm was characterized. • The laser at ~3 μm based on the MoTe film was realized. • Q-switched • 2 dimensional MoTe film is an outstanding passive modulator at ~3 μm. 2
2
2
A R T I C LE I N FO
A B S T R A C T
Keywords: MoTe2-SA Q-switched lasers Solid state lasers Mid-infrared lasers
The stable nano-seconds pulsed Er:Lu2O3 laser were realized by employing a molybdenum ditelluride (MoTe2) saturable absorber (SA), which was fabricated with the liquid phase exfoliation (LPEx) method. Under the absorbed power of 7.24 W, the maximum average output power of 0.52 W was achieved in a repetition rate of 105 kHz and a pulse duration of 317 ns at 2.85 μm, corresponding to a pulse energy of 4.94 μJ and a peak power of 15.6 W.
1. Introduction Mid-infrared laser is among the most intensively developed laser sources in recent years. Laser radiation with wavelength of ~3 μm has various of applications in gas sensing [1] infrared countermeasure [2] and is promising in space communication. This radiation is also a proper laser source for nonlinear laser frequency process such as optical parameter oscillators (OPO) and high order harmonic generation [3]. In medical application, it attracts even more interests, the reason is that the wavelength of ~2.94 μm corresponds a strong absorption peak of water (~10000 cm−1), which is significantly contained in organs and biology tissues. The radiation energy can be mainly absorbed by the target tissue, thus, the thermal damage to the surrounding tissue could be dramatically decreased [4]. Thus, the drastic demands in applications promote an even explosive researching tendency of the laser radiation at ~3 μm. 3 μm lasers could be achieved from the stimulated emission of the 4 I11/2 → 4I13/2 transition of Er3+ ion, and the radiation at 2.69 μm was
⁎
firstly observed in 1967 [5]. But owing to the thermal effect of the host materials, which was induced by the low stokes efficiency (~35%) and the strong multiphonon relaxing, optimizing a high performance laser at ~3 μm was really hard [6]. However, the novel high quality sesquioxide crystals combining excellent thermomechanical property, comparable low phonon energies and even strong crystal field make themselves ideal host materials for the high performance mid-IR laser operations [7]. As a favorable sesquioxide crystal, the cubic Lu2O3 has already been applied in high performance 1 μm [8], 2 μm [9] and 3 μm [10,11] laser operations. The experimental performances of graphene [12], CN [13] and other two dimension (2D) materials [14,15] in passively Q-switched or mode-locked lasers show us the untapped potential to achieve high performance mid-IR lasers as SAs. Among all types of stuffs, transition metal dichalcogenide (TMD) is one of the most attractive and capable candidates, for not only typical layer structure, but also the variation band-gap, which depending on their composition, structure and dimensionality [16–19]. Similar to other TMD materials, in same layer of
Corresponding author. E-mail address: [email protected] (T. Li).
https://doi.org/10.1016/j.optlastec.2019.105791 Received 19 August 2018; Received in revised form 4 May 2019; Accepted 25 August 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 121 (2020) 105791
Y. Liang, et al.
Fig. 1. Characterization of MoTe2 exfoliation: (a) TEM imagine, (b) AFM imagine and (c) height variation of MoTe2 film, (d) linear transmission and nonlinear transmittance (insert) of YAG-based MoTe2 film at 2.85 μm.
Fig. 2. Configuration of the laser system.
pulsed laser was 0.522 W, corresponding to a repetition rate of 105 kHz and a pulse duration of 317 ns, resulting in a pulse energy of 4.94 μJ and a peak power of 15.6 W.
MoTe2 samples, the stable Te-Mo-Te units are periodically arranged and combined with the strong covalent bond, while between layers, it is the effect of weak van der Waals force [16,18,19]. Attributed to the two different combinations, MoTe2 shows a typical layer structure and also has a great potential of being exfoliated into multilayer or even monolayer nano-flakes from bulk samples. In many TMDs, the band-gap varies in both value and type with the different layer structure. For MoTe2, an in-direct band-gap of 0.9 eV in bulk samples and a direct band-gap of 1.1 eV in monolayer structure was reported [20–22]. In both types above, the band gap is bigger than the energy of mid-IR photon, thus single–photon absorption at these energies cannot occur. However, the experimental broad band saturable absorption of the low dimension TMD materials in mid-IR could be attributed to the sub-band absorption induced from crystallographic defect state [23] or the edge states[24]. Based on the outstanding properties of Er:Lu2O3 and MoTe2 material, in this letter, we demonstrate a nano-seconds passively Q-switched laser operation at ~2.85 μm modulated by a homemade multilayer MoTe2-SA. The layered MoTe2 specimen was fabricated by the LPEx method. Stable CW laser operations were built up in the first step, and then was modulated to be stable Q-switched laser systems by introducing our homemade MoTe2-SA into the laser cavity. Under the maximum absorbed pump of 7.4 W, the optimum output power of stable
2. Preparation and characterization of multilayer MoTe2 The effectiveness and maneuverability of LPEx has been proved in massive semiconductor fabrication, multilayer or even monolayer films can be successfully exploited from the bulk sample after hours of process [25,26]. The selection of proper solvent is one of the key factors for the high quality films exfoliation. In our fabrication, the layered MoTe2 specimen was exfoliated from powder sample in the pure ethanol. Through 6 h ultrasonic processing, the sample in multilayer could be successfully prepared. Then suspension liquid was centrifuged for 10 min with rotate speed of 1500 rps, and the top dispersion was collected for further characterization and laser experiment. TEM is an efficient method to measure the ultrathin samples. The TEM measurement of MoTe2 exfoliations is shown in Fig. 1(a), it reveals our homemade exfoliations was in the sub-micrometer dimension. In order to get a height cognition of exfoliation in details, height imagine formed by AFM and the corresponding chart are shown in Fig. 1(b) and (c), from figure mentioned, one could hold that our nano-flakes with lateral size of few hundred nanometers, and with variable height 2
Optics and Laser Technology 121 (2020) 105791
Y. Liang, et al.
Fig. 3. Dependence of average output power on absorbed pump power in (a) CW and (b) Q-switched laser operations; spectrum of (c) CW (peak: 2.85 μm) and (d) Qswitched (peak: 2.85 μm) laser operations.
Fig. 4. Top: variation of pulse duration and repetition rate with absorbed pump power; bottom: typical pulse train and the temporal profile of the stable pulse.
exfoliation with layers varing from 20 to 40 was successfully prepared. The MoTe2-SA was fabricated by dropping the dispersion on a YAG basement and then naturally dried for 10 h. The linear optical transmittance of our homemade specimen at wavelength ranging from 2600 to 3100 was measured by an UV–VIS-NIR spectrophotometer (U-4100, Hitachi, Japan) and the data was visualized in Fig. 1(d), the result states that in this demarcated range, the linear transmittance rate over 92.6% is notable, and also in the figure, the highlight also points out that at lasing wavelength of 2.85 μm, the transmittance is 98.2%, or a linear absorption of 1.8%. And the nonlinear optical transmission was characterized by an active q-switched laser with a repetition rate of 4 kHz and lasing at ~ 2.85 μm, the variation of the response with the increasing single pulse energy is shown in the insert drawing of Fig. 1(d), and the typical saturable absorption property at emission wavelength of our homemade MoTe2-SA can be clearly observed.
Fig. 5. Variation of pulse energy and peak power with absorbed pump power.
varying from 15 to 25 nm. The typical height of a monolayer material is about 0.65 nm[22], thus, it is convincing to state that our MoTe2 3
Optics and Laser Technology 121 (2020) 105791
Y. Liang, et al.
3. Laser experiment
absorbed pumping source with power of 7.24 W at ~976 nm, the best performance of Q-switched laser was achieved with 4.94 μJ/317 ns pulses in repetition rate of 105 kHz at ~2.85 μm, corresponding to a maximum average output power of 0.52 W and a peak power of 15.6 W.
Stable CW and Q-switched lasers were both built up in a simple configured laser system. As shown in Fig. 2, a fiber coupled ~976 nm LD laser was collimated and incident to the center of the front end of the 3*3*10 mm cubic Er:Lu2O3, and the crystal was placed in the linear cavity which was composed of flat input mirror (M1) and curve output mirror (M2). Three different output couplers (OCs) with optical transmittances of 1%, 3% and 5% at 2.85 μm were employed here to identify the laser performances under different losses. The output power of all operations was measured by a PM100D power meter together with an S314C power head (Thorlabs Inc., USA). The measurement of the output power and its dependence with the absorbed pump power were shown in Fig. 3(a) and (c). For CW operations, the lasing thresholds of the three systems was measured to be 1.27 W, 1.84 W and 2.37 W, corresponding to OCs with transmittances of 1%, 3% and 5%. Under the maximum absorbed pump power of 7.24 W, the output powers were 0.78 W, 0.98 W and 0.59 W, separately, and the slop efficiencies were calculated to be 11.9%, 18.5% and 12.6%. Insert the prepared multilayer MoTe2-SA into the laser cavities, the stable Q-switched lasers were also achieved. In this condition, the lasing thresholds altered to be 2.05 W, 2.38 W and 2.55 W. Keeping the maximum absorbed pump power the same, the measurements of the average output power were 0.35 W, 0.52 W and 0.33 W, and the slop efficiencies altered to be 6.3%, 10.8% and 6.8% respectively. According to the linear fitting of the experiment data in Fig. 3(a) and (c), the linear increase tendency of output powers with the absorbed pump power in CW laser operations, and nearly linear in pulsed laser operations can also be clearly observed. In CW and Q-switched lasers, the measurements of the emitting wavelengths both stabilized at~2.85 μm and were shown in Fig. 3 (c) and (d). The laser pulse trains were detected by a HgCdTe IR detector with a response time of 1 ns (PVI-4TE-4, Vigo System S.A.) and visualized by a DPO 7104C digital oscilloscope with a rise time of 350 ps, bandwidth of 1 GHz and sampling rate of 5 GS/s (Tektronix DPO 7102, USA). A simple conclusion that pulse duration decreases while lasing repetition rate increases with the increasing absorbed pump power could be indicated from the previous studies of passively Q-switched lasers [6,14,15,19,27], and same tendencies of pulsed duration and repetition rate could be observed in our pulsed lasers in top of Fig. 4. However, we could also observe some fluctuations of both targets when compared with other pulsed lasers which were also modulated by multilayer TMD [19,28,29], here we attribute it to the Inhomogeneity of our homemade MoTe2-SA. In our experiment, when coupling transmittance is 3%, the stable Q-switched laser with the shortest stable pulse duration of 317 ns was achieved under the maximum absorbed pump power of 7.24 W and the corresponding repetition rate is 105 kHz. The temporal profile of shortest pulse together with the laser trains are both shown in the bottom of Fig. 4. The maximum repetition rate of 108 kHz was obtained under the same maximum pump power with the coupling transmittance of 1%. The dependency of the emission peak power and pulse energy on the absorbed pump power under different coupling transmittances are shown in Fig. 5. Obviously, both pulse energy and peak power increase with the increasing absorbed pump power in all laser systems with different losses. As is painted, when absorbed power exceeds 5.5 W, the laser operations with both biggest peak power and hugest pulse energy could be obtained by employing the OC with transmittance of 3%. In our experiment, the maximum pulse energy of 4.94 μJ and peak power of 15.6 W were obtained under the absorbed pump power of 7.24 W.
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4. Conclusion and discussion In conclusion, multilayer MoTe2 films was fabricated in the first step and then introduced into an Er:Lu2O3 laser operation lasing at ~2.85 μm as an SA. The stable Q-switch laser was achieved. Under 4
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