High-power passively Q-switched 2.0 μm all-solid-state laser based on a MoTe₂ saturable absorber

1. Introduction

Tm³⁺ (³F₄→³H₆) doped pulse laser sources operating around 2.0 μm waveband have attracted wide range of interests due to the emission wavelength located at the characteristic absorption spectrum of many gaseous pollutants and strong absorption by water molecules and biological tissue. This reason leads 2.0 μm source having extensive applications, such as medical treatment, atmospheric monitoring, molecular spectra, laser radar and pumping sources for optical parametric oscillators [1–4]. To generate pulse laser, passive Q-switching with saturable absorbers (SAs) is a convenient and effective method and has drawn great deal of attention. Due to the high mechanical stability and large absorption cross section, Cr:ZnSe and Cr:ZnS have been extensively employed as SA to generate PQS lasers in 2.0 μm spectral regime [5–7], but the expensive price and the low pulse repetition rate they generated limit their application of PQS lasers.

In recent years, 2D material based SAs have been widely used in generating pulse laser due to its advantage of compactness, convenience, and low-cost [8]. There are many 2D material based SAs have been investigated in 2.0 μm laser operation, for instance, graphene [9,10], black phosphorus (BP) [11,12], topological insulator (TI) [13], and transitional metal dichalcogenides (TMDs) [14–18]. Semiconducting TMDs, such as WS₂, MoS₂, WSe₂, and MoSe₂, stand out from those 2D materials, for its variable bandgap, strong photoluminescence, ultrafast carrier dynamics and high nonlinear optical response. MoTe₂, another novel member of the TMDs family, has 1T’ and 2H two different phase structures. 1T’ phase MoTe₂ has been confirmed a kind of weyl semi-metal material recently [19]. 2H phase MoTe₂ is kind of semiconductor material. Like other TMDs such as MoS₂ and WS₂, the band structure of MoTe₂ varies with the thickness of the nanosheets, from 0.88 eV indirect semiconductor for bulk to 1.02 eV direct semiconductor for monolayer [20]. Compared with MoS₂ and WS₂, MoTe₂ has smaller bandgap and higher electrical conductivity [21], which makes it a promising candidate for near or mid-infrared pulse laser generation. In recent years, the fabrication method, electronic application and semi-metallic properties of MoTe₂ materials are widely studied [22–24]. However, the saturable absorption properties are still rarely covered. Up to now, MoTe₂ have been used as SA to generate Er-doped-fiber mode-locked laser [25,26] and 3.0 μm all-solid-state passively Q-switched laser [27], but there are still no reports about its 2.0 μm solid-state pulse laser application.

In this paper, a high-power passively Q-switched 2.0 μm all-solid-state laser based on MoTe₂ SA was demonstrated. The shortest pulse width, maximum output power and highest repetition rate were 380 ns, 1.21 W, 144 kHz. The saturable absorption property of the MoTe₂ SA around 2.0 μm was investigated, through open-aperture Z-scan measurement By introducing Te vacancies in MoTe₂, we calculate the band structure via DFT method and find those vacancies could reduce the bandgap of MoTe₂. Moreover, as far as we know, it is the first time for MoTe₂ to be used as the saturable absorber in 2.0 μm laser operation, and the pulse width is shortest around 2.0 μm all solid-state laser with TMDs saturable absorber.

2. Preparation and characterization of MoTe₂ SA

By using liquid phase exfoliation method, high quality multi-layered MoTe₂ SA was fabricated. First, MoTe₂ bulk with purity of 99.99% was ground into powder, then dispersed those powder into alcohol and sonicate for 10 hours to separate large-size sheets. Second, the as-prepared solution was centrifuged at 2500 rmp for 15 mins, and the supernatant liquor was collected for standby. Finally, the processed MoTe₂ solution was dropped on the CaF₂ substrate and dried at room temperature for 24 hours. Finally, the MoTe₂ SA was prepared successfully.

To characterize the thickness of the MoTe₂ SA, atomic force microscopy (AFM) was taken. As shown in Fig. 1(a), the maximum thickness of the MoTe₂ SA was about 12 nm, corresponding to 18 layers MoTe₂ sheets respectively (the interlayer spacing in the MoTe₂ is 0.65 nm). Raman scattering spectrum of the MoTe₂ SA was characterized by Raman spectroscopy with a 632 nm laser source and the results was shown in Fig. 1(b). Two typical Raman peaks, A_1g at 179 cm⁻¹ and ${\text{E}}_{2g}^1$ at 240 cm⁻¹ were demonstrated. The peak ${\text{E}}_{2g}^1$ is much stronger than the peak A_1g attributing to the multilayer of the MoTe₂ SA, while the result accord with the previous reported results [28]. Figure 1(c) shows the transmission spectra of the MoTe₂ SA, indicated that the transmittance of the sample measured to be 83% at 2.0 μm.

 

Fig. 1 (a) AFM image and typical height profile of MoTe₂ SA. (b) Raman and (c) transmission spectrum of the prepared MoTe₂ SA at room temperature.

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Utilizing an open aperture Z-scan method based on a home-made SESAM mode-locked Tm: YAP laser at 1989 nm with pulse width of 3.0 ps and repetition rate of 92.3 MHz, the nonlinear optical properties of the MoTe₂ SA were measured. Focused by lens (f = 60 mm), the radius of the beam waist and the Rayleigh length was about 70 μm and 1250 μm, respectively. As shown in Fig. 2, the sharp and narrow peaks was shown by normalized Z-scan transmittance curve, demonstrate that the transmittance of the MoTe₂ SA increased with the rise of input laser intensity, indicating its stronger saturated absorption behavior. From the fitting curve, the nonsaturable loss, the saturation fluence, and the modulation depth were determined to be 14%, 2.26 μJ∕cm², and 6.0%, respectively.

 

Fig. 2 (a) Z-scan curve of MoTe₂ SA, and (b) nonlinear transmission versus energy intensity.

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The intrinsic bandgap of MoTe₂ is about 0.88-1.02 eV, corresponding to the optical absorption wavelength below 1400 nm. However, the 3.0 μm PQS laser operation of MoTe₂ has been reported [27]. In recent work, it has been proved that due to the defect-induced mid-gap states, the bandgap of MoTe₂ can be decreased by Te vacancies [29]. Energy-dispersive x-ray spectroscopy (EDS) imagine and the corresponding Te to Mo atomic ratio were shown in Fig. 3. In the MoTe₂ SA, the ratio of Te to Mo was about 1.64:1, implies that 18% Te vacancies existed.

 

Fig. 3 EDS image and the corresponding Mo to Te atomic ratio of the MoTe₂ sample.

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In order to gain deeper insight into the above phenomena, we used density functional theory (DFT) calculations of the electronic band structures. As shown in Fig. 4(a), the bandgap of the intrinsic MoTe₂ is about 1 eV, consistent well with the previous work, and the bandgap of MoTe₂ used in our experiment with 18% Te vacancies was calculated to be 0.33 eV, as shown in Fig. 4(b). The calculation results indicated those Te vacancies sufficiently reduce the band structure of MoTe₂, from 1.0 eV (1240 nm) to 0.33 eV (3758 nm). As a result, the optical absorption of the MoTe₂ SA can be extended to mid-infrared waveband.

 

Fig. 4 The Electronic band structures of (a) intrinsic MoTe₂, (b) MoTe₂ with 18% Te vacancies.

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3. Experimental results and discussions

A compact concave-plane resonator with a 25 mm length was used for investigating PQS Tm:YAP laser, the experimental setup is shown in Fig. 5. The pump source was a fiber-coupled laser diode with center wavelength at 792 nm, a numerical aperture of 0.22 and fiber core diameter of 400 μm. By using an optical focusing system, the pump light was focused into the Tm:YAP with a radius of 400 μm. The input mirror (IM) with the curvature radius of 200 mm was antireflection-coated at 0.78-0.81 μm (T > 95%) and high-reflection-coated at 1.8-2.1 μm (R > 99.5%). A flat output coupler (OCs) was employed in the cavity respectively, with partial transmission of 5% at 1.8-2.1 μm. A a-cut 3 at.% Tm:YAP crystal with the size of 3 × 3 × 8 mm³ long was employed as the gain medium for laser operation. For efficient thermal removal, the crystal was wrapped by indium foil and mounted in a copper block with water cooled at 15°C.

 

Fig. 5 Experimental setup of Tm:YAP Q-switched laser.

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The continuous-wave laser operation was investigated firstly. As shown in the Fig. 6(a), a maximum CW output power of 1.52 W was obtained, under the absorption pump power of 4.46 W, corresponding to an optical-to-optical conversion efficiency and slope efficiency of 34% and 43%. By using a laser spectrometer (APE WaveScan, APE Inc.) with a resolution bandwidth of 0.4 nm to measure the output spectrum, it can be observed that, the center wavelength was located at 1989 nm, as shown in Fig. 6(b).

 

Fig. 6 (a) Output power versus absorbed pump power under CW and PQS operation. (b) Spectrum of Tm:YAP CW and PQS laser. (c) The PQS laser beam profile.

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Then, the MoTe₂ SA was inserted in the cavity and stable passively Q-switched operation was realized. The pump threshold was measured to be ~1.0 W. Under the absorption pump power of 4.46 W, a maximum PQS laser output power of 1.21 W was obtained, corresponding to an optical-to-optical conversion efficiency of 27%, and a slope efficiency of 34%. The center wavelength of output PQS lasers was found blue-shifted, which might be the Q-switching process leading to depopulation of the ground-state level and as a result the reabsorption effect was reduced [30]. By using a laser beam profiling system (NanoScan by PHOTOH, Inc), the M² factor were fitted to be 1.42 in tangential direction and 1.26 in sagittal direction at the maximum average output power, and the PQS laser beam profile is shown in Fig. 6(c). The instabilities (output power, rms) of the PQS laser is measured to be 2.314% at 0.5 h.

The output pulse trains were recorded by an oscilloscope (Tektronix 1 GHz bandwidth) and detected by a fast InGaAs photodetector with a rise time of 35 ps, (EOT, ET-5000, USA). Without the MoTe₂ SA, disordered self-modulation pulses were observed. This phenomenon was also observed in other Tm³⁺ doped lasers and consistent with nonlinear dynamical chaos [31]. The self-modulation pulses turn into stable PQS pulses when the MoTe₂ SA was inserted in the cavity. The shortest pulse width of 380 ns was achieved with the repetition rates of 144 kHz. Figure 7 shows the shortest pulse profile and the stable pulse train at the highest repetition rate. To the best of our knowledge, it is the first time that 2.0 μm pulse laser operation based on the MoTe₂ SA, and the results represent the shortest pulses in 2.0 μm PQS all- solid-state lasers based on TMDs SAs.

 

Fig. 7 The shortest pulse profile and the stable pulse train at the highest repetition rate generated by using the MoTe₂ SA.

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The relationship of the pulse width and repetition rate versus the absorbed pump power is shown in Fig. 8(a). The pulse width decreased with the augment of absorbed pump power, while the pulse repetition rate increased. Figure 8(b) shows the calculated single pulse energy and peak power versus the absorbed pump power. A maximum single pulse energy was calculated as 8.4 μJ, corresponding to a maximum pulse peak power of 22.2 W.

 

Fig. 8 (a) The pulse width and repetition rate versus the absorbed pump power, (b) the single pulse energy and pulse peak power versus absorbed pump power.

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Table 1 summarized some results ever obtained with TMDs 2D materials in 2.0 µm all-solid-state lasers field. Compared with other TMDs SAs, the pulse width of 380 ns represent in this work sets a shortest record among those of PQS all-solid-states lasers around 2.0 μm waveband with TMDs SAs, and the output power, pulse energy, peak power are also stand out from the list. The results demonstrated the superiority of MoTe₂ acting as an optical modulator for achieving short pulses with high peak power and repetition rate.

Table 1. Summary of passively Q-switched solid-state 2 μm lasers based om TMDs SAs

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4. Conclusions

In conclusion, a diode-end-pumped 2.0 μm passively Q-switched all-solid-state lasers with a MoTe₂ SA was demonstrated for first time, to the best of our knowledge. The MoTe₂ SA was fabricated by liquid phase exfoliation method, the saturable absorption behavior among 2.0 μm was investigated by Z-scan method, and the electronic band structures indicated the saturated absorption waveband of the MoTe₂ SA can be extended to mid-infrared due to Te vacancies. Under the absorbed pump power of 4.46 W, a maximum output power of 1.21 W was obtained at the center wavelength of 1977 nm, with the shortest pulse width of 380 ns and a repetition rate of 144 kHz, corresponding to a maximum single-pulse energy of 8.4 μJ, and a maximum peak power of 22.2 W. Furthermore, the results sufficiently indicated that in the fields of generating high repetition rate and high power short pulse lasers, MoTe₂ was a kind of promising candidate for near or mid-infrared spectral range.