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Abstract
TaSe2 nanosheets are prepared by means of ultrasound-assisted liquid phase exfoliation (LPE) and the nonlinear saturable absorption properties are experimentally investigated. The TaSe2-polyvinyl alcohol (PVA) film is then manufactured and a TaSe2-based saturable absorber (SA) is fabricated for demonstrating a Q-switched erbium-doped fiber laser (EDFL) for the first time. Taking advantage of the saturable absorption properties of the TaSe2-based SA, the obtained shortest Q-switched pulse duration is 1.53 µs with a pulse energy of 65.5 nJ. The results indicate that TaSe2 is a competent candidate for use as SA in promoting stable and flexible ultrafast lasers. The work may pave the way to researches and applications for photonics in the mid-infrared band.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Reliable and scalable ultrafast fiber lasers are one of the most sought-after applications due to the merits of, in contrast to continuous wave (CW), narrow pulse duration and high pulse energy. To date, the dominant technologies to modulate the laser operation from CW into pulsed regime are passive Q-switching and mode-locking based on saturable absorbers (SAs) [1–10]. Considering that SAs are indispensable to sharpen the pulse in temporal domain, researchers are constantly paying tremendous attention to exploring ideal SA materials with features of high nonlinearity, high power handling ability, fast response time, wide absorption band, ambient atmospheric-stability, low cost, low optical loss, et al [11–37]. The discovering of Graphene in 2004 inspires continuous exploration enthusiasm of a diverse range of two-dimensional (2D) materials with atomic layer thickness. In return, based on the desired nonlinear absorption characteristics of 2D materials with controllable thicknesses, significant development of ultrafast fiber lasers is motivated simultaneously [38–56].
Among them, transition metal dichalcogenides (TMDCs), a new kind of functional materials for optoelectronic applications, have aroused wide research interest recently [6–12]. TMDCs are layered materials with generic formula MX2, where M is a transition metal (Nb, Ta, Ti, Mo, W, …) and X a chalcogen (S, Se, Te). The layers, made of triangular lattices of transition metal atoms sandwiched by covalently bonded chalcogens, are held together by weak van der Waals forces, which is the reason why TMDCs can be readily exfoliated into thin flakes down to the single layer limit. Furthermore, TMDCs will transit from indirect bandgap to direct bandgap when the material changes from bulk to single layer, which indicates that it's possible to engineer the bandgap of TMDCs. The novel feature brings about some excellent optical properties, such as high carrier mobility, controllable optoelectronic properties and outstanding nonlinear optical absorption [6–8]. On aspect to the property of nonlinear optical absorption, however, the present TMDCs materials generally exhibit comparatively large band gaps in the visible or near-infrared wavelength region (∼1.8 eV for MoS2, ∼2.1 eV for WS2, ∼0.8 eV for WTe2) [8,9], hence the absorption is relatively weak at the mid-infrared wavelength. Such characteristics hamper the applications of TMDCs in mid-infrared fields. Fortunately, TaSe2, a representative of TMDCs, possesses an ultra-narrow band gap about 0.15 eV, which is similar to TiSe2 [38,50].
The crystal structure of TaSe2 consists of Se-Ta-Se layers containing Ta in trigonal prismatic coordination. The weak interlayer van der Waals bonding makes it easier to peel from the block to different layered films, as well as leads to several TaSe2 polytypes with different relative layer orientations and stacking arrangements. In this work, we specifically focus on the characteristics of 2H-TaSe2 and its application in Q-switched fiber lasers. As illustrated [41–50], the structure of the 2H-TaSe2 polytype has a two-layer repeat pattern (AcA BcB), within which the trigonal prismatic units are rotated 60 ° with respect to each other, and for monolayer 2H-TaSe2, the thickness and interlayer spacing are 0.7 nm and 0.64 nm, respectively. Furthermore, according to the researches reported, TaSe2-material has merits of high nonlinearity and ultrafast recovery time. For multi-layered-TaSe2 of 21 layers, the nonlinear refractive index and the third-order nonlinear susceptibility are n2 = 8.0×10−7 cm2/W, χ(3) = 1.37×10−7 (e.s.u.) when λ=532 nm, while for multi-layered-TaSe2 of 31 layers, n2 = 3.3×10−7 cm2/W, χ(3) = 1.58×10−7 (e.s.u.) when 671 nm. For TaSe2 monolayer, χ(3) = 3.10×10−10 (e.s.u.) when λ = 532 nm, and χ(3) = 1.64×10−10 (e.s.u.) when λ = 671 nm [49]. The mean values of extracted lifetimes are 6.8 ps for 45 µW and 2.5 ps for 140 µW of incident power at 405 nm excitation wavelength [60].
Although intensive efforts have been devoted to TaSe2 material study, it has not been researched systematically in optical, electrical, and other fields so far. It is especially worth emphasizing that, reports of ultra fast lasers based on TaSe2 SA is still scarce. Considering that TaSe2 has been predicted to have superb optical response [48,50], excellent performance for Q-switched fiber laser by TaSe2 SA is proposed in this article. In this contribution, a passively Q-switched EDFL operating at 1559 nm based on TaSe2 SA is demonstrated for the first time. The TaSe2 nanosheets are prepared by ultrasound-assisted LPE technique. The experimental investigation of the fabricated TaSe2-PVA film reveals that the modulation depth, saturation intensity and nonsaturable loss are 15.5%, 1.57 MW/cm2 and 14%, while the layer number is about 2-4. The TaSe2 SA is then assembled for the prospective Q-switched operation. In the passively Q-switched experiment, the pulse repetition rates vary from 12.61 kHz to 45.5 kHz with the increasing pump power. The obtained shortest pulse width is 1.53 µs with pulse energy of 65.5 nJ. On the one hand, the work presents a simple as well as low cost preparation process, compact laser system, high stability and competitive output results. What's more important, the work indicates that TaSe2 possesses excellent nonlinear absorption properties as well as outstanding thermal stability, which is feasible for extensive use as stable and flexible SAs in the near future. Additionally, it's worth emphasizing that our work may pave the way to a broadband and compact ultrafast photonics in the mid-infrared band.
2. Preparation and characterization of TaSe2 SA
Liquid phase exfoliation (LPE) method is one of the commonly adopted approaches to prepare 2D material nanosheets due to the characteristics of low-cost and easy-operation. The mechanism of LPE method results from the vibration of microbubbles in the solution. Shock waves arising from the collapse of countless microbubbles produce strong shear-forces, which is sufficient to exfoliate flakes from crystal by overcoming the weak van der Waals forces between adjacent layers. On aspect of the solvent, deionized (DI) water has merits such as environmental friendliness, matching well with the surface energy of 2D materials, and possessing low boiling point to simplify the preparation process [50]. So in this work, TaSe2 nanosheets is prepared by ultrasound-assisted LPE technique using DI water as solvent.
In the ultra-sounding process, 6 mg TaSe2 powder is dispersed into 12 ml DI water. The mixture is stirred for 30 minutes and then treated under bath sonication condition for 2.5 hours to obtain TaSe2 dispersion. The as-prepared TaSe2 solution subsequently undergoes high-speed centrifugation process at 3500 rpm for 30 minutes to obtain TaSe2 nanosheets. Then the supernatant liquor, as shown in Fig. 1(a), is collected for the characterization and saturable absorber (SA) fabrication. First, TaSe2-PVA film is manufactured. 280 mg polyvinyl alcohol (PVA) is added into 7 ml TaSe2 dispersion, and the mixture undergoes ultrasonic water bath process for another 2 hours to get uniformity. In the process, PVA is used as the polymer matrix both to form film to host the TaSe2 and to avoid its oxidation. Then, the polymer mixture is poured onto a quartz substrate and after slow evaporation under the ambient at room temperature, a filmy free-standing TaSe2-PVA composite is prepared. For the commonly used sandwich-structure Q-switcher, the TaSe2-PVA film is transferred onto the end faces of fiber connectors and the proposed TaSe2 SA finally gets ready for Q-switched operation.
Raman spectrum is an important means to test and verify 2D materials. A laser (LabRAM HR Evol) operating at wavelength of 532 nm is utilized in the Raman spectroscopy. As depicted in Fig. 1(b), the Raman spectrum measured from the prepared TaSe2 nanosheets at room temperature presents a pronounced spike at 240 cm-1, which agrees well with previously reported work [49]. On the one hand, it verifies that the sample is layered-TaSe2 with high purity. Considering that the weak interlayer van der Waals bonding leads to several TaSe2 polytypes, on the other hand, the Raman spectrum confirms the sample is 2H-TaSe2.
Energy-dispersive X-ray spectroscopy (EDS) of the 2H-TaSe2 nanosheets is tested and the results are presented in Fig. 2. The EDS with SEM as inset (scale bar is 50 µm) reveals that the atomic ratio of Ta and Se is 1:2, which illustrates the purity of the 2H-TaSe2 nanosheets.
Fig. 2. Energy dispersive X-ray spectroscopy data for 2H-TaSe2 nanosheets (SEM (inset), scale bar is 50 µm).
As shown in Fig. 3(a) and (b), morphological structure of the prepared 2H-TaSe2 is analyzed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), where the clear appearance and uniform lattice fringes of 2H-TaSe2 are presented. To determine the specific phase of the 2H-TaSe2 samples, the high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) image of the 2H-TaSe2 nanosheets are shown in Fig. 3(c) and (d). It’s seen that there is no obvious defect, and the lattice distance is about 0.22 nm corresponding to the (1,0,4) lattice plane. The well-resolved 2D lattice fringes and diffraction pattern reveal the crystalline nature of the exfoliated 2H-TaSe2 nanosheets.
Fig. 3. (a) SEM image of 2H-TaSe2 nanosheets; (b) TEM image of 2H-TaSe2 nanosheets; (c) HRTEM image of 2H-TaSe2 nanosheets; (d) The selected area electron diffraction of 2H-TaSe2 nanosheets.
Furthermore, as displayed in Fig. 4, the morphology of 2H-TaSe2 nanosheets are characterized by atomic force microscope (AFM). It exhibits that the sample has been uniformly dispersed. The cross-section heights of nanosheets along the marked lines are measured and shown in Fig. 4(b), from which the thicknesses of the selected section A and section B are averaged to be 4.5 nm. Therefore, the layer number is estimated to be 2-4 according to 0.64 nm of each layer spacing.
The intensity-dependent nonlinear saturable absorption property at 1550 nm of the TaSe2-PVA film is investigated by the commonly-used balanced twin detector measurement scheme. A fiber laser operating at central wavelength of 1550 nm with 6 ns pulse duration and 10 Hz repetition rate is utilized. The intensity-dependent nonlinear normalized saturable absorption curve of the TaSe2-PVA film is depicted as blue dots in Fig. 5. Note that as the light intensity gradually increases, the nonlinear optical transmission approximately increases up to 86% and remains saturated there, which exhibits typical saturable absorption characteristic. By a fitting analysis, presented as red line in Fig. 5, the data agree well with the curve of the saturable absorption formula:
where T(I) is transmission, ΔT is modulation depth, I is input laser intensity, Isat and Tns are saturation intensity and nonsaturable absorbance, respectively. According to the fitting analysis, the modulation depth, saturation intensity and nonsaturable loss of the prepared TiSe2-PVA film are provided to be 15.5%, 1.57 MW/cm2 and 14%, respectively. In our opinion, the low loss of the SA may contribute to the few layer number of the TaSe2 nanosheets.Figure 6 shows the linear transmittance spectrum of 2H-TaSe2-PVA film. Using a UV / VIS / NIR spectrophotometer (U-3500, Hitachi, Japan), the variation of transmittance vs. the wavelength range from 200 to 2000nm can be measured. The red line displays the transmittance of the TaSe2-PVA film on the quartz substrate, and the blue line displays the transmittance of the blank quartz substrate under the same conditions. Obviously, in the wavelength range from 400 to 2000nm, the transmittances of the TaSe2-PVA film and the blank quartz substrate are 90.9 ± 0.9% and 93 ± 0.2%, respectively. The net transmittance of the TaSe2-PVA film is about 97.9%, and the scattering loss is around 2.1%. The results further reveal that the prepared 2H-TaSe2-PVA film is an excellent absorber displaying a high absorbance from ultraviolet (UV) to near infrared (NIR). It is qualified for a wide application scope.
In general, a good SA should have a high modulation depth (∼ 10% for fiber lasers) and low saturation intensity [36]. As described above, the TaSe2 SA has relatively larger modulation depth of 15.5% and lower saturation intensity of 1.57 MW/cm2, which is especially suitable for ultrafast laser operation.
3. Passively Q-Switched EDFL experimental results and discussion
The TaSe2-SA based passively Q-switched EDFL with ring cavity structure is schematically depicted in Fig. 7. The input power source is a 980 nm laser diode (LD) with a maximum power of 515 mW, which is coupled into the laser cavity through a 980/1550 wavelength-division multiplexing (WDM). A 5.5 m long EDF (Nufern-EDFC-980-HP) is connected after the WDM as a gain medium. Considering that the erbium doping-concentration of this kind fiber is not as high as the generally utilized erbium-doped fiber of LIEKKI (Er110-4-125), so similar to [29,61], a relatively longer fiber is adopted. A polarization independent isolator (PI-ISO) is used to ensure unidirectional operation of signal inside the cavity. In order to optimize the laser output, a polarization controller (PC) is utilized to adjust the polarization state of circulating light and the cavity birefringence [18,20,52]. During the experiment, the Q-switched operation will be achieved when the laser mode is optimized by fine adjusting the three pieces of the PC between 0-180 °. Additionally, 20% of the laser power is extracted from the cavity by an optical coupler (OC). The total length of the cavity, including the EDF and the standard single mode fiber (SMF-28) is about 10 m. The Q-switched output performances are measured by an optical spectrum analyzer (OSA) (YOKOGAWA AQ6370B) for the wavelength spectrum, a digital oscilloscope (Tektronix DP04104) for the pulse train, and an optical power meter (Molectron PM3) for output power, respectively.
The continue-wave (CW) is observed initially with threshold pump power of 24 mW. Stable Q-switched pulse train occurs by adjusting the PC and properly tuning pump power up to 41 mW. The system operates in stable Q-switched state in the 41-515 mW pump range. In our opinion, the admirable low laser threshold may contribute to the large modulation depth of the SA device because it is beneficial to the occurrence of self-starting.
In the process of Q-switching, SAs with high nonlinearity, ultrafast recovery time, and large modulation depth will bring better optical performance. Except the TaSe2-material’s inherent features of high nonlinearity and ultrafast recovery time, as far as the current research situation is concerned, large modulation depth of the prepared TaSe2 SA is beneficial to speed up the process of pulse narrowing. As a result, during the pump power increasing up to 515 mW, the pulse duration is constantly narrowed down till to the shortest pulse width of 1.53 µs. A single pulse envelope with the corresponding pulse sequence is recorded and separately shown in Fig. 8(a) and the inset. Smooth and symmetric pulses are observed at the oscilloscope with no obvious jitter in pulse amplitude or pulse shape, indicating that the Q-switched operation is stable. Figure 8(b) shows the output spectrum of the system, which reveals a center wavelength of 1559 nm with 3 dB bandwidth of 1.6 nm.
Fig. 8. Output pulse when the pump power is 515 mW: (a) Single pulse, the inset is the output pulse sequence diagram; (b) Pulse spectrum.
The pulse properties in Q-switched lasers mainly depend on the nonlinear dynamics in gain medium and SA. A pulse is emitted once a certain stored energy in the cavity is reached. This leads to a dependence of repetition rate and pulse duration upon pump power. A greater pump power enables higher repetition rates and also results in shorter pulses. This is observed experimentally and recorded in Fig. 9. The pulse duration is reduced from 19.69 µs down to 1.53 µs, while the repetition rate can be tuned over a range of 12.61 kHz to 45.5 kHz along with the pump power increasing from 41 to 515 mW. Obviously, when the pump power is at low level, the pulse duration changes more intensely. While when the pump power increasing from 288 mW up to 515 mW, the narrowing trend of pulse duration slows down and finally the curve is almost flattened out. The phenomena predict the meaninglessness of further sharpening the pulses by means of increasing pump power only. It's suggested that shortening laser cavity and adopting SAs with larger modulation depth be involved to further compress laser pulses [50]. Aspect to our work, the pulse duration is successfully narrowed from 19.69 µs to 1.53 µs, which contributes to the prepared high quality 2H-TaSe2 SA with relatively higher modulation depth. And if the cavity length in our work is shortened continually, the pulse duration will become shorter.
Figure 10 depicts the relationships of output power and single pulse energy vs. pump power. When the pump power is 515 mW, the obtained maximum average output power is 2.98 mW with energy of 65.5 nJ for each pulse. In order to further improve output power and pulse energy, reliable techniques can be implemented such as optimizing TaSe2 SA parameters, cavity design, splitting ratio of the output coupler (OC), and increasing the pump power simultaneously.
The pulse trains at different pump power levels are shown in Fig. 11. During the whole experiment period for more than one month, the Q-switched pulse sequence remains highly stable. It reveals the fact that, on the one hand, the TaSe2-PVA film is free from damage at present pump power. On the other hand, the TaSe2-PVA film presents high stability and antioxidant capacity in ambient condition. All the merits are critical in the practical applications as optical devices.
RF spectrum analyzer is generally employed to monitor the output pulse trains in frequency domain and better to describe the noise of the Q-switched operation. However, there is no RF spectrum analyzer at present in our Lab. In order to test the long-term stability of the Q-switched system, an alternative method can be implemented, which is similar to [33–35] and means that the spectra of the Q-switched system is measured repeatedly for a time duration with a definite interval. In our work, the spectra of the Q-switched system is measured in a time duration of 2 hours with an interval of 30 min, which is shown in Fig. 12. It is seen that the Q-switched EDFL exhibits excellent stability at room temperature.
At high pump power, the Q-switched pulses tend to disappear due to the saturation of the SA. However, many successful Q-switched operation at high pump power has been reported. The maximum pump powers are even as high as 600 mW [50] and 680 mW [53], respectively. In our experiment, the TaSe2-based SA shows a high damage threshold and excellent thermal stability till the pump power increases up to 515 mW. Besides, during the experiment, when the TaSe2 SA is absent, only CW output is obtained. Once the SA is inserted into the cavity in the pump power range of 41-515 mW, the laser system operates in stable Q-switching regime. The results indicate that the TaSe2 SA is responsible for the passively Q-switched operation. In the experiment, there is no optical damage occurred to the TaSe2 SA, which reveals that the prepared TaSe2 SA can endure higher optical power density. However, the optical damage threshold of the TaSe2 SA can't be tested limited to the available pump power.
In order to figure out the pros and cons of our passively Q-switched operation incorporating with TaSe2 SA, the performance of current and previous EDFLs based on various 2D saturable materials are presented in Table 1. It's found that, compared with previous experimental materials, the modulation depth of TaSe2 SA prepared by LPE method in this work is of above average level. Contribute to the excellent nonlinear saturable absorption properties of TaSe2 and high quality TaSe2-PVA based SA, the repetition rate, pulse duration, pulse energy, and output power of our work are competitive with those lasers using other SAs.
Table 1. Comparison of passively Q-switched EDFL performance based on different 2D materials
There is no mode-locked operation observed during the experiment. If a mode-locked regime is scheduled, as having been discussed in [20,50], several significant factors should be considered. On the one hand, decreasing splice losses inside the laser cavity can increase optical intensity and the number of oscillating longitudinal modes, hence mode-locking operation will be stimulated easily. On the other hand, the formation of mode-locked pulse is the result of the dynamic balance between cavity dispersion, laser gain, cavity loss and various nonlinear effects, which are easily obtained within long-length ring laser cavity. So extension of cavity length is another key factor [37].
4. Conclusion
In our work, for the first time, TaSe2 nanosheets with large modulation depth, low saturation intensity, and high damage threshold are successfully prepared and employed as SA for demonstrating high performance passively Q-switched operation. The pulse repetition rates vary from 12.61 kHz up to 45.5 kHz with the increasing pump power. The obtained shortest pulse width is 1.53 µs with pulse energy of 65.5 nJ. The work presents a simple as well as low cost preparation process, compact laser system, high stability and competitive output results. What's more important, the results indicate that TaSe2 possesses outstanding thermal stability as well as comparative nonlinear absorption properties with other 2D materials. Thus, it's worth emphasizing that our work may pave the way to a broadband and compact photonics at the mid-infrared band in the near future.
Funding
Shandong University of Technology and Zibo City Integration Development Project (2018ZBXC052, 2019ZBXC120); Natural Science Foundation of Shandong Province (ZR2017MA051); National Natural Science Foundation of China (11304184, 11704226).
Disclosures
The authors declare no conflicts of interest.
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