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Abstract
In this paper, a novel 2D ternary chalcogenide Ta2NiSe5 is fabricated into a saturable absorber by using the liquid phase exfoliation (LPE) method and successfully applied to the passive Q-switching experiment of Tm:YAP all-solid-state laser at 2 µm. The nonlinear optical absorption (NOA) properties of Ta2NiSe5 at 2 µm was measured by using an open-aperture Z-scan method, showing saturation fluence and modulation depth of 400 µJ/cm2 and 24%, respectively. By using Ta2NiSe5 as saturable absorber, the shortest pulse width of 740 ns and maximum repetition frequency of 71 kHz are obtained, respectively. This results prove that the Ta2NiSe5 is an excellent saturable absorber to achieve passively Q-switched laser at 2 µm, for the first time to the best of our knowledge.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
2 µm pulsed lasers played a vital role in the military, medical and civilian fields as powerful scientific research tools [1–4]. Pulsed lasers which are realized by Q-switching or mode-locking technology, especially passive Q-switching technology based on two-dimensional saturable absorbers, has become a research hotspot in recent years. The essential to produce high-quality pulses is the saturable absorber with high operation performance. Two-dimensional (2D) materials have the advantages of more in line with the requirements of integrated lasers, such as the unique planar characteristics, easy to manufacture, ultrafast carrier dynamic and superior saturable absorption properties, which determined that it is very suitable for the generation of ultrafast pulse. Currently, black phosphorus (BP) [5,6], single-wall carbon nanotubes (SWCNTs), [7] graphene, [8] transition metal disulfides (TMDs), [9–15] perovskite [16] due to its excellent optical properties have attracted research interest and reported in passive Q-switching bulk lasers at 2 µm. Emerging materials have also received a lot of attention from researchers. For example, bismuthene and antimonene have excellent electrical, thermal properties and nonlinear optical properties, which have also attracted the attention of many researchers [17,18]. However these materials have more or less defects that limit its application range. For example, BP is easily oxidized, the diameter of the single-wall carbon nanotubes determines its absorption wavelength, graphene possesses lower absorption rate, and the majority of TMD materials tend to have a wide energy band gap (1–2 eV) structure and high fabrication cost, which limit their application in electronics and photonics. Consequently, it is urgent to explore new saturable absorbers with exceptional properties to obtain high-efficient pulsed laser.
In recent years, 2D ternary chalcogenide have received extensive attention and research interest due to their particular structure, outstanding optical and electrical properties [19]. What is more, comparing with singe or binary system, the electronic or optoelectronic properties of ternary material system can be altered easily by regulating chemical element composition ratio, and this may results in more attractive applications than single and binary materials [20]. In 2018, H. Ahmad et al. used the principle of tunable ternary band gap and achieve 1 µm and 2 µm passive Q-switching based on WSSe saturable absorber [21]. In 2019, Zhang et al. demonstrated that the nonlinear optical properties of the ternary alloy ReS1.02Se0.98 at 2µm and apply it to passive mode-locked laser to produce ultrafast laser [22]. In 2020, a high-quality Ta2NiS5 saturable absorber was fabricated and applied in an ultrafast fiber laser by Junli Wang’s group. A 1.45 ps mode-locked fiber laser pulse was realized with a pulse repetition rate of 2.92 MHz, demonstrating eminent saturable absorption properties at 1.5 µm [23]. As a new member of the ternary family material, the Ta2NiSe5 material with S element replaced by Se has a crystal structure similar to that of Ta2NiS5. Ta2NiSe5 also has a layered crystal structure that is supported by vander Waals force interplay, and the electronic behavior along the interlayer direction system displays certain two-dimensional properties [24]. The structure is similar to zigzag chains constituted by [TaSe6]2 dimer chains and [NiSe4] tetrahedral single chains and are periodically arranged [25]. Additionally, there is a unique one-dimensional chain structure in each independent single layer, and the electronic behavior exhibits strong one-dimensional characteristics and in-plane anisotropy characteristics [24–27]. It is worth noting that, different from some TMDs with conversion of indirect band gaps into direct band gaps often occurs when switching to monolayer, bulk Ta2NiSe5 has been proved to be a direct band gap (Eg=0.36 eV) semiconductor [27,28], which keeps its direct band-gap feature even when thinning to monolayer [29,30], making it has a great application prospect in functional optoelectronic devices. These remarkable properties illustrate that Ta2NiSe5 will be a credible saturable absorber to generate pulsed laser. Although a lot of research has been done on Ta2NiSe5, the nonline saturable characteristics of ternary chalcogenide Ta2NiSe5 at 2 µm are rarely reported. Thus, the optoelectronic properties, especially in saturable absorption effect, are urgently needed to be explored.
In this paper, the Ta2NiSe5 nanosheets are fabricated for experiment by LPE method. We proposed and verified the saturable absorption characteristics of Ta2NiSe5 by experiment, and successfully realized the Q-switched Tm:YAP all-solid-state laser based on Ta2NiSe5 SA. Pulsed laser output with minimum pulse width of 740 ns, maximum repetition rate of 71 kHz, and maximum output power of 451 mW, corresponding to a pulse peak power and single pulse energy of 8.58 W and 6.35 µJ, respectively.
2. Preparation and characterization of Ta2NiSe5 SA
The Ta2NiSe5 SA was prepared by liquid phase exfoliation method (LPE), which is often used in preparing various two-dimensional materials because the method is simple and easy to operate. First, bulk Ta2NiSe5 single crystal with purity of 99.99% purchased from 6 Carbon Technology company was triturated into powder in an agate mortar. Then the powder was dissolved in ethanol and treated with ultrasonic for 3 hours to pulverize the remaining large particles. During the period, ice cubes are added intermittently, in order to prevent the physical structure of Ta2NiSe5 from damaging due to the thermal effect. Thereafter, the prepared liquor was centrifuged at 5000 rpm for 10 minutes to deposit the large-sized the bulk materials, with only the supernatant extracted for standby. After then, the supernatant was spin coated onto the output mirror with T = 5% transmission. Finally, after drying naturally for 5 hours at room temperature, the Ta2NiSe5 saturable absorber was successfully prepared.
In order to get the layer distribution of the prepared Ta2NiSe5 SA, the microscopic topography of the prepared Ta2NiSe5-SA was characterized by employing an atomic force microscopy (AFM). The AFM scans a large area image of the sample shown in Fig. 1(a), and it can be seen that the prepared Ta2NiSe5 nanosheet have good uniformity. The thickness of each place is approximately 21 nm observed from Fig. 1(b), corresponding to about 32 layers, with the interlayer spacing of Ta2NiSe5 0.66 nm [31]. A Raman spectrometer was stimulated by a 532 nm laser source to survey the Raman spectra. The Raman spectrum distribution results measured of the prepared Ta2NiSe5 SA are depicted in Fig. 1(c). We can obviously observe the two typical Raman peak of Ta2NiSe5 located at 93.34 cm−1 and 121.24 cm−1, respectively, and the two low-intensity peaks located at 173.05 cm−1 and 287.04 cm−1, respectively, which was identical with the description in the Ref [31]. The results reveal that the film Ta2NiSe5 crystal was successfully fabricated with good crystallinity.
Fig. 1. (a) AFM topography of Ta2NiSe5. (b) the corresponding thickness of Ta2NiSe5. (C) Raman spectrum of the prepared Ta2NiSe5 SA.
The nonlinear optical absorption of the sample was measured by open Z-Scan method. Figure 2 indicates that the Ta2NiSe5 SA absorption tends to saturate with the rise of input laser intensity. We use the following formula to fit the relationship between the incident light intensity and the sample transmittance:
where T(F) is the transmission rate, ΔT is the modulation depth, A is the normalization constant, F is the input fluence, Fsat is the saturation fluence and αns is the non-saturable loss. The modulation depth obtained by fitting and the saturation fluence were demonstrated to be 24% and 400 µJ/cm2, respectively. The sample transmission rate increases with the increasing pump power, indicating that Ta2NiSe5 has significant saturable absorption characteristics at 2 µm region.3. Experimental results and discussions
The experimental setup is schematically shown in Fig. 3. A simple plane-plane resonant cavity configuration was employed for the passively Q-switched laser. The gain medium is a c-cut 3 at.% Tm:YAP crystal with the volume 3×3×8mm3, and both sides of the crystal were coated with high-transmission (HT) film at 793 nm and 1980 nm. The pump source is a laser with a center wavelength of 793 nm, which was coupled by a fiber with a core diameter of 200 µm, and a numerical aperture of 0.22. The radius of the pump spot illuminates the center of the crystal 100 µm by a 1:1 optical focusing system. The side of the Tm:YAP crystal was wrapped in indium foil and placed in a copper radiator to eliminate the thermal effect. The crystal temperature was controlled by a water-cooled circulation system and maintained at 17℃. In the cavity, the input mirror is a plane mirror, coated for anti-relection (AR) at 780–810 nm and a high-reflection (HR) at 1.9–2.1 µm, and the output mirror (OM) is a plane mirror with transmission of 5%. at 1.9–2.1 µm.
The CW and passively Q-switched output power were firstly measured by a power meter. The relationships between the output power and the absorbed pump power are shown in Fig. 4(a). In the CW regime, the maximum output power of 1.75 W was obtained with the output coupler of T=5%, corresponding to a slope efficiency of 22.4%. Whereas, the maximum output power of 451 mW was achieved in the passively Q-switched regime, corresponding to a slope efficiency of 17.4%. In order to prevent the Ta2NiSe5 SA from damaging, the absorbed pump power was limited to 5.5 W.
Fig. 4. (a)Output power against absorbed pump power under CW and PQS operation. (b) Variation of Q-switching pulse width and repetition rate with the absorbed pump power. (c) The single pulse energy and pulse peak power versus absorbed pump power, and (d) Shortest pulse width and typical oscilloscope pulse trains with respect to different output powers.
Figure 4(c) shows the dependence of the pulse repetition rate and pulse width on the absorbed pump power. The performance of the Tm:YAP bulk laser were detected by a detector (Newport, Model 818-BB–51) and displayed on a digital oscilloscope (Tektronix DPO 7104C, 1 GHz bandwidth, 20 Gs s−1 sampling rates). As shown in Fig. 4(b), the pulse repetition rate increase with the augment of the absorbed pump power, while the pulse width decrease with it. Under the maximum pump power, the shortest pulse width and maximum pulse repetition rate (PRR) reaches 740 ns and 71 kHz, respectively. The relationships between the single pulse energy and peak power versus the absorbed pump power are depicted in Fig. 4(c). The highest single pulse energy of 6.35 µJ was obtained, corresponding to the maximum peak power of 8.58 W. The shortest pulse profile and the pulse train with the maximum repetition rate are shown in Fig. 4(d).
Table 1 summarizes the performance of the passive Q-switching bulk laser by employing different 2D materials as SA at 2 µm. It can be observed from the comparison in Table 1, the pulse laser generated by the multilayer Ta2NiSe5 as SA surpass most two-dimensional materials (graphene, MoS2, ReS2, ReSe2), except BP. However, BP hinders its applications in optoelectronic devices due to its shortcomings such as susceptible oxidation and unstability. This illustrates that Ta2NiSe5 is a potential and applicability material for high power and short pulse laser generation at 2 µm and indicates that it has significant application prospects in the field of optoelectronics. The laser performance still has potential to improve by some approaches, such as optimizing the cavity structure and improve the crystal quality by using chemical vapor deposition technology to prepare Ta2NiSe5 SA. And it is a good suggestion to use Ta2NiSe5 in 1.5 µm ultrafast laser and compare the performances with this paper and that of Ta2NiS5 [34] in detail.
Table 1. Summary of 2 µm PQS lasers with SAs based on different 2D materials
4. Conclusions
In summary, we successfully used the LPE method to prepare Ta2NiSe5 SA. To the best of our knowledge, it is the first time that a stable all-solid-state passively Q-switched Tm:YAP laser based on Ta2NiSe5 SA was demonstrated. Raman spectroscopy proves the structural integrity of the Ta2NiSe5 single crystal, and the optical non-saturable absorption properties of the Ta2NiSe5 was demonstrated by open-aperture Z-scan method. The achieved maximum output power, shortest pulse width and maximum repetition rate are 451 mW, 740 ns and 71 kHz, respectively, corresponding to the maximum single-pulse energy of 6.35 µJ and the maximum peak power of 8.58 W. In addition, the experimental results indicate that the multilayer Ta2NiSe5 has immeasurable potential in produces high-power and short laser pulses as SA.
Funding
National Natural Science Foundation of China (61775123, 61875106); Key Research and Development Program of Shandong Province (2019GGX104039, 2019GGX104053); National Key Research and Development Program of China (2017YFA0701000).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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