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Abstract
We report the generation of the robust nanosecond Nd:YVO4 solid-state laser and Er3+:ZBLAN fiber laser passively Q-switched by the tin selenide (SnSe) nanoflowers saturable absorbers. The SnSe nanoflowers prepared via chemical precipitation method exhibit broadband nonlinear optical absorption performance, and can modulate the separate lasers to deliver stable nanosecond pulse ∼1 µm and ∼2.8 µm successfully. The experimental results show that the low-dimensional transition-metal monochalcogenides hold great potential for the broadband robust saturable absorbers, and may pave an avenue toward developing high-performance broadband optoelectronic devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Robust nanosecond lasers have attracted significant attention due to their potential applications in industrial materials processing, optical communication, laser radar, laser ignition, nonlinear optics, etc [1–4]. Compared with the different techniques to generate pulsed laser, the passively Q-switched method with real saturable absorbers (SAs) has been developed rapidly with the evolution of the emerging nonlinear optical materials and cost-effective fabrication process. With the intensity-dependent nonlinear optical absorption, the SAs can modulate the laser system to deliver laser pulses with high peak power or energy via mode-locking or Q-switching technique. After the first generation of SAs, the dye, was used to passively mode lock the Nd:glass lasers in 1966, a large family of nonlinear optical materials have been identified to fulfill different laser requirements [5]. Along with the commercialization of semiconductor saturable absorber mirrors (SESAMs), the pulsed laser has developed quickly towards the goals of high-power, high-efficiency and high-integration [6–8]. However, the SESAMs suffer from the complicated fabrication procedures and limited operation bandwidth. With the evolving of the low-dimensional materials, different types of SAs have been proposed and adopted in lasers successfully to obtain the pulsed laser output [9–27]. However, the preparation and transfer processes inevitably lead to poor repeatability and reliability, which will be challenging to find the robust broadband SAs to modulate the pulsed lasers, especially towards the mid-infrared regime. Therefore, it is necessary to explore the broadband nonlinear optical material to improve the pulsed laser performance.
Inspired by the requirements for broadband, stable and cost-effective SAs, tin selenide (SnSe), a kind of transition-metal monochalcogenides (TMMCs), has aroused wide attention for its earth-abundance, less toxicity, excellent chemical stability and optoelectronic performance [28]. The SnSe has exhibited excellent application potential in thermoelectric, piezoelectric and photovoltaic fields [29,30]. With the increasing incident intensity, the SnSe low-dimensional materials, such as nanoflakes or nanosheets, show intensity-dependent nonlinear optical response around 1560 nm and ∼2 µm [31,32]. However, the broadband nonlinear optical response and the application of SnSe above 2 µm wavelength have not been explored. Moreover, the size, geometry and doping will influence SnSe’s absorption behavior, which can provide opportunities for versatile applications of SnSe in optoelectronic devices [33].
Here, we have prepared the SnSe nanoflowers via the chemical precipitation method, and demonstrated that the nanoflowers can act as a broadband Q-switcher operating ∼1.0 µm and ∼2.8 µm. Stable nanosecond pulses can be delivered by introducing SnSe-SA into a ∼1.0 µm Nd:YVO4 laser cavity and a ∼2.8 µm Er3+:ZBLAN fiber laser cavity. The experimental results show that the SnSe nanoflowers can be expected to be an excellent nonlinear optical material for broadband optoelectronic devices.
2. Preparation and characterization of SnSe-SA
SnSe, a representative layered TMMCs, has a layered orthorhombic structure (space group: Pnma) with alternating transition metal and chalcogenide atoms for the crumpled plane at room temperature [31]. The crystalline unit cell of SnSe is similar as black phosphorous but with much higher chemical stability against ambient conditions. Here, the low-dimensional SnSe sample has been prepared via the chemical precipitation method. Firstly, 3.2 g (0.04 mol) of Se powder was added to 200 mL (0.5 M) Na2SO3 aqueous solution, refluxed at 90° for 24 hours, and the supernatant clarified Na2SeSO3 solution was collected via centrifugation. Then, 10 mL(1 M) sodium citrate aqueous solution was dripped in 30 mL (1.5 M) NaOH aqueous solution which has dissolved 0.45 g SnCl2·2H2O with rapidly stirring. Finally, 6 mL prepared Na2SeSO3 solution was slowly added dropwise to form the black precipitate. The precipitate was centrifuged, washed with deionized water and ethanol several times, therewith dried at 70 °C in drying oven.
The morphology and height profile of SnSe sample has been characterized by the SEM and AFM, as shown in Fig. 1(a)-(d). The sample shows nanoflower morphology and its height are about 300-400 nm. Figure 2(a) shows the Raman spectrum of the SnSe sample, and the four Raman peaks locate at 70 cm−1, 106 cm−1, 129 cm−1, and 149 cm−1, corresponding to the A1g, B3g, A2g, and A3g mode, respectively. Figure 2(b) shows XRD patterns of the SnSe sample, and the characteristic peaks can be assigned to the crystal planes (201), (111), (311), (020), (511) and (420) (JCPDS No. 89-0232), respectively [34]. The XRD patterns reveal that the prepared SnSe is an orthonormal crystal (space group: Pnma (62)). In addition, the SnSe samples were further characterized by XPS, as shown in Fig. 2(c)-(d). The two peaks corresponding to Sn 3d3/2 (494.9 eV) and Sn 3d5/2 (486.4 eV) can be clearly seen in Fig. 2(c), and the peak corresponding to Se 3d can be seen in Fig. 2(d), which are consistent with the reported results in [35]. Figure 2(e) shows the linear transmission spectrum of SnSe nanoflowers, which shows that the sample has wavelength-dependent absorption, especially in the mid-infrared region. We have also carried out the transient absorption measurement for the sample with the pump-probe technique, as shown in Fig. 2(f). By fitting the experimental results, the SnSe nanoflowers exhibit the ultrafast processes with a lifetime of 80 ps and 1.2 ns, respectively. The first stage mainly includes the ultrafast process of carrier energy absorption and the process of carrier-carrier scattering, while the second stage mainly includes the cooling process of electron-hole recombination. The ultrafast relaxation process makes SnSe a promising nonlinear optical material for ultrafast applications.
Fig. 2. (a) Raman spectra of SnSe nanoflowers; (b) XRD patterns of SnSe nanoflowers; (c)-(d) High-magnification XPS patterns of Se 3d and Sn 3d of SnSe nanoflowers; (e) Linear transmission spectrum of SnSe nanoflowers; (f) Transient absorption kinetics of SnSe nanoflowers.
The open aperture (OA) Z-scan technique has been adopted to measure the nonlinear saturation absorption behavior of SnSe nanoflowers. To explore the broadband nonlinear saturable absorption of SnSe nanoflowers, the pulsed lasers around 1 µm (central wavelength 1064 nm, repetition rate 0.1 MHz, pulse duration 4 ns), 2 µm (central wavelength 1930 nm, repetition rate 32.3 MHz, pulse duration 2.8 ps) and 2.8 µm (central wavelength 2800 nm, repetition rate 1 kHz, pulse duration 35 fs) have been adopted. The OA Z-scan measurements result with SnSe sample was shown in Fig. 3, and we can obtain the saturation intensity and modulation depth of the SnSe by fitting the experimental data with the following formula:
where T(z), α0, I, IS, L and β represent the normalized transmittance, linear absorption coefficient, incident laser intensity, saturation intensity, sample thickness and TPA coefficient, respectively. By fitting the experimental results, the modulation depth and saturation intensity for SnSe nanoflowers are 3.3% and 3.51 kW/cm2 at 1064 nm, 5.5% and 0.06 MW/cm2 at 1930 nm, 1.4% and 17.5 GW/cm2 at 2800 nm, respectively. It can be seen that the SnSe nanoflowers show obvious broadband saturable absorption within the spectral range from ∼1.0 µm to ∼3.0 µm. In addition, the saturable intensity varies with the operating wavelength, which can result from the different excitation laser with different parameters, such as pulse energy, pulse duration and repetition rate, etc.Fig. 3. Nonlinear saturable absorption properties of the SnSe. (a) OA Z-scan trace and (b) the relationship between transmittance and input laser intensity at 1064 nm wavelength; (c) OA Z-scan trace and (d) relationship between transmittance and input laser intensity at 1930 nm wavelength; (e) OA Z-scan trace and (f) the relationship between transmittance and input laser intensity at 2800 nm wavelength.
3. Experimental results and discussions
3.1 Nd:YVO4 solid-state laser
To demonstrate the broadband nonlinear optical absorption performance of low-dimensional SnSe in the Q-switched Nd:YVO4 solid-state laser, the design of the laser resonator is shown in Fig. 4. The pump source was an 808 nm fiber coupled laser diode, and the numerical aperture (NA) and core diameter of the pigtail were 0.22 and 105 µm, respectively. The planar parallel polished Nd:YVO4 crystal has a 0.3 at.% doping concentration and a dimensions of 3×3×12 mm3. The Nd:YVO4 crystal was wrapped with indium foil and mounted inside a copper block. The copper block was connected to a water-cooled chiller with temperature set at 15 °C. The pump light was collimated by a lens L1 (LA1951-B, coating-B, f = 25.4 mm) and focused by a Lens L2 (LA1608-B, coating-B, f=75.0 mm). The design of the resonant cavity is a typical F-P cavity, which is composed of a pump input mirror M1 (high reflectivity (>99.5%) @1000-1100 nm & high transmittance (>97%) @808 nm) and an output coupling mirror M2 (∼95%@1000–1100 nm), and the cavity length is about 13 cm. We have used an optical power meter (1917-R), an optical spectrum analyzer (DSO9404A) and a photodiode (DSO9484A) for the measurements.
By slowly adjusting the position of SnSe-SA, stable Q-switched pulse trains can be delivered from the solid-state laser when the pump power was around 1 W. The output spectrum of the SnSe nanoflowers Q-switched solid-state laser has shown in Fig. 5(a), which locates at 1064.7 nm with FWHM 0.2 nm. The radio-frequency of the Q-switched Nd:YVO4 solid-state laser has been measured, as shown in Fig. 5(b). It can be obtained from the figure that the signal to noise ratio (SNR) was 32.3 dB, which confirms the relative stability of Q-switched pulse. The SNR of the pulse laser depends on many factors, such as cavity design of the laser, ambient temperature, stability of the pump source, the quality of SA, and so on. To improve the SNR of the pulsed laser, we can further optimize the cavity-type design of the laser and reduce the perturbations of the environment to improve the SNR of the pulsed laser. In particular, the preparation of high-quality SA is a key factor for the highly stable laser output. We have also obtained the output performance of the passively Q-switched solid-state laser with increasing pump power, as shown in Fig. 5(c). With the increase of pump power from 1 to 3 W, the pulse repetition rate increases monotonously from 299 to 438 kHz and the pulse width decreases monotonously from 821 to 406 ns. The dependence of the pulse energy and output power as the function of the incident pump power was shown in Fig. 5(d), and the maximum output power could reach up to 537 mW by increasing the incident pump power to 3 W.
Fig. 5. The output characteristics of SnSe nanoflower-based passively Q-switched all-solid-state laser. (a) Optical spectrum; (b) Radio-frequency output spectrum; (c) Pulse width and repetition rate as a function of the incident pump power; (d) Output power and pulse energy as a function of the incident pump power.
We have also confirmed the stable Q-switched operation from the oscilloscope traces at different scale duration, as shown in Fig. 6. We have not further increased the incident pump power above 3 W due to the thermal influences on the SnSe-SA under high power laser illumination. When the laser operates at the maximum incident pump power at 3 W, the delivered pulse width is 406 ns. By further reducing the length of cavity and increasing the modulation depth of the SA, the pulse duration could be further narrowed [36]. In the experiment, we haven’t obtained the stable mode-locking operation. Hönninger et al. have qualitatively analyzed the transition between Q-switched and mode-locked lasers, and realized that the transition between Q-switched and mode-locked status by adjusting the resonator parameters [37]. As for our Q-switched laser, we can optimize the laser cavity and the saturable absorption of SA to realize mode-locking, including changing the cavity configuration, the cavity loss, the cavity dispersion, optimizing the modulation depth of SA, etc. With the optimizations, the mode-locking operation can be anticipated in our future work.
Fig. 6. Typical Q-switched pulse trains at different time scale (5 µs /div and 1 µs/div) when the laser operating at maximum incident pump power.
3.2 Er3+:ZBLAN fiber laser
We have also explored the broadband nonlinear optical absorption performance of low-dimensional SnSe in the fiber laser towards the mid-infrared regime. Figure 7 depicts the linear cavity configuration of the Er3+-doped ZBLAN fiber laser with SnSe-SA as the Q-switcher. The pump source is a 975 nm fiber coupled laser diode, and the NA and core diameter of the pigtail were 0.2 and 105 µm, respectively. The 3.8 m Er3+-doped ZBLAN fiber acts as an amplifier, that is, a gain medium, has a doping concentration of 70000 ppm, and the diameter and NA of the core are 15 µm and 0.12, respectively. The inner cladding diameter is 240×260 µm (NA of 0.4). The fiber end near the pump source side is perpendicular-cleaved to act as output coupler, and the other end of the fiber is cleaved at an angle of 8° to suppress parasitic lasing. The resonator was composed of a perpendicular-cleaved optical fiber end surface and a gold mirror M2 integrated with SnSe nanoflowers. The 975 nm pump laser output from the pigtail was coupled into the gain fiber through a lens group consisting of lens L1 (coating-B, f=25.4 mm) and lens L2 (CaF2 lens, transmission 94% @ 975 nm, f=50 mm). The emerging laser beam from the fiber end cleaved at 8° angle was focused on SnSe-SA via the lens L3 (uncoated CaF2 lens, f=20 mm) and L4 (uncoated CaF2 lens, f=25.4 mm). The dichroic mirror M1 (HT@975 nm, HR@2.8 µm) located between the pump coupling systems was used to separate lasers from the pump laser. At the appropriate pump power, stable Q-switched pulse can be obtained by adjusting the position of SnSe-SA.
When the incident pump power was 0.26 W, stable Q-switched pulse was initially observed from the Er3+-doped ZBLAN fiber laser. We continue to increase the pump power up to 1.21 W, and the pulse operation was still stable. The stable pulse operation was maintained until the incident pump power was 1.69 W. Figure 8(a) has shown the output spectrum of the SnSe nanoflowers Q-switched Er3+-doped ZBLAN fiber laser, which locates at 2790 nm with FWHM 6 nm. At the same time, we also measured the RF spectrum, as shown in Fig. 8(b). The SNR was 34 dB, which confirms the stable Q-switching operation of the fiber laser. The relationship between pulse repetition rate and pulse width versus incident pump power was measured, as shown in Fig. 8(c). With the increase of pump power from 0.26 to 1.69 W, the pulse repetition rate increases monotonically from 29 to 105 kHz and the pulse width decreases monotonically from 5.2 to 0.58 µs. Figure 8(d) shows the pulse energy and output power as a function of the incident pump power, and we can see that the average output power increased nearly linearly with the incident pump power. We have also presented the oscilloscope traces of Q-switched pulse in different time scale at the maximum pump power in Fig. 9, from which we can see the excellent pulse stability of the mid-infrared laser pulse. It is necessary to mention that the oscillation laser intensity in the cavity for the Q-switched fiber laser cannot be directly evaluated with the measured nonlinear optical parameters under the ultrafast laser excitation. With the delivered output power and the cavity parameters, the oscillating laser intensity in the cavity can be evaluated on the order of ∼kW/cm2. In addition, the calculated laser damage threshold of SnSe nanoflowers is greater than 0.5 MW/cm2.
Fig. 8. The output characteristics of SnSe nanoflowers-based passively Q-switched Er3+:ZBLAN fiber laser. (a) Optical spectrum; (b) Radio-frequency output spectrum; (c) Pulse width and repetition rate as a function of the incident pump power; (d) Output power and pulse energy as a function of the incident pump power.
Fig. 9. Typical Q-switched pulse trains at different time scale (20 µs/div and 5 µs/div) when the laser operating at maximum incident pump power.
The IV–VI semiconductor SnSe with different dimension and shape shows different electronic band structures [30]. The bulk SnSe has the direct and indirect bandgaps 1.30 and 0.90 eV, respectively [38], while the reported SnSe nanoflower prepared by one-pot synthetic method shows the direct and indirect bandgaps 1.05 and 0.95 eV, respectively [39]. Here, it should be pointed out that the SnSe nanoflowers show saturable absorption ∼2.8 µm wavelength, where the photon energy is lower than the electronic bandgap of the reported SnSe materials. The saturable absorption in the mid-infrared regime can result from sub-bandgap saturable absorption behavior observed in some other low-dimensional materials, such as MoS2 and WS2 [40,41], which may be due to the Se vacancies and structure defects in the SnSe nanoflowers [32,42]. Furthermore, SnSe nanoflowers show broadband optical transmission ∼2.8 µm wavelength, as shown in Fig. 2(e), which indicates that the SnSe material in our experiment can absorb the light towards the mid-infrared regime. In addition, the SnSe nanoflowers SA adopted in the experiments shows lower saturation intensity, which is favorable to lower the threshold for pulsed laser.
In order to compare the performance of the passively Q-switched laser based on SnSe SA with other TMMCs materials, we summarized the reported results based on different TMMCs materials in Table 1. The results show that the SnSe can act as an excellent SA for the pulsed lasers.
Table 1. Comparison of pulsed lasers based on different TMMCs
4. Conclusions
We have explored the broadband nonlinear optical response and applications of TMMCs SnSe nanoflowers. By characterizing the SnSe nanoflowers prepared via chemical precipitation method, the low-dimensional TMMCs exhibits broadband nonlinear optical response both in near-infrared and mid-infrared spectral range. With the SnSe nanoflowers as the SA, we demonstrated the passively Q-switched Nd:YVO4 laser operation ∼1 µm with pulse width 406 ns, repetition rate 438 kHz, and a Q-switched Er3+:ZBLAN mid-infrared fiber laser around ∼2.8 µm with pulse width 583 ns, repetition rate 105 kHz. The experimental results show that the low-dimensional TMMCs hold great potential as broadband robust SAs, and may pave an avenue toward developing high-performance broadband optoelectronic devices.
Funding
National Natural Science Foundation of China (61775056, 61805076, 61975055); Natural Science Foundation of Hunan Province (2019JJ50080).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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