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Abstract
As a novel material with narrow band gap and natural van der Waals heterostructures (vdWH), francketie has potential applications in the field of optoelectronic fields. However, few studies have applied its nonlinear optical absorption properties to ultrafast fiber lasers. Here, we synthesized francketie nanosheets via the liquid-phase exfoliation (LPE) method. By incorporating the polyvinyl alcohol (PVA), a franckeite-PVA saturable absorber (SA) was fabricated to achieve a mode-locked Yb-doped fiber laser (YDFL) for the first time to the best of our knowledge. The saturation intensity and modulation depth of the SA were measured about 75 MW∕cm2 and 7%, respectively. The proposed franckeite-based YDFL demonstrates stable mode-locked operation with the maximum single energy of 5.35 nJ and the pulse duration of 1.57 ns. Our experimental results fully prove that franckeite may have wide potential for designing ultrafast photonics devices with low cost, high stability and excellent performance.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrashort pulse fiber lasers have attracted extensive attention of researchers because of their great value in basic research and industrial applications [1–3]. To date, various techniques have been proposed to generate ultrashort pulses, such as active methods through electro-optic or acousto-optic modulators and passive methods using saturable absorbers (SAs) [4–6]. Among them, passive SAs are excellent candidates for modulation devices due to their remarkable advantages of compactness, stability, and simplicity. For example, semiconductor saturable absorber mirror (SESAM) is currently the main form of commercial SA, which can be designed by molecular beam epitaxy growth techniques and defect engineering with good stability [7]. Ultra-long period grating (ULPG) was also employed as an efficient pulse-shaping device to obtain multi-wavelength mode-locked pulse output, recently [8]. Nonetheless, the disadvantage of SESAM and ULPG is that their fabrication procedure is complicated and it is tough for them to operate over a wide wavelength range. As a promising alternative for pulse modulation, two-dimensional (2D) materials possess unique and attractive strongpoints, such as fast response time, a wide range of nonlinear optical response and inexpensive fabrication cost. To date, many 2D materials have been developed as effective SA devices for ultrafast pulse generation, including carbon materials [9,10], topological insulators (TIs) [11,12], transition metal dihalides (TMDs) [13,14], black phosphorous (BP) [15,16] and other novel nanomaterials such as layered metal dichalcogenides(LMCs) [17,18], Mxenes [19,20], metal organic framework (MOF) [21], perovskite [22] and hydrazone organics [23]. However, limited by the synthesis technology and the energy band structure of the materials, there are still some challenges in the practical application of the above materials. For example, the lower absorption efficiency of graphene often leads to the lower modulation depth (2.3% per layer), which is detrimental to the interaction with intense light. TMDs and LMCs are not suitable for generating mid-infrared pulse laser due to the optical bandgap. As a direct bandgap semiconductor with tunable forbidden bandwidth (0.3∼2.0 eV), although BP can be employed as a good candidate for broadband SA, whose nonlinear optical absorption can be covered from 400 nm to 1930nm [15], its thermal stability and antioxidant capacity should be further improved, and the temperature and humidity of the surrounding environment need to be strictly controlled. And for some materials that are still in the early stage of exploration, such as Mxenes, MOF and perovskite, they still suffer from the disadvantages of complicated fabrication process and low damage threshold. Therefore, the exploration of novel SA materials with high performance will continue unabated.
Recently, van der Waals heterostructures (vdWH) composed of vertically stacked different 2D layered semiconductors have opened up great opportunities for designing next-generation integrated electronic and optoelectronic systems [24–26]. It can break through the limitation of the thickness of a single atomic layer and obtain unique physical properties, thus providing new possibilities for tuning the optoelectronic properties of 2D materials at the atomic level. As one of the sulfosalt minerals with naturally occurring vertical vdWH, franckeite (Pb5Sn2FeSb2S14) is stacked by alternating pseudo-tetragonal(Q) PbS-like layer and pseudo-hexagonal(H) SnS2 layers [27]. Furthermore, franckeite is also a p-type material with narrow bandgap (∼0.7 eV) and its photocurrent responsivity is larger than that of most few-layer BP. Therefore, it has great potential applications in advanced optoelectronic fields [28,29]. Owing to its attractive features, the use of franckeite has been demonstrated in many fields, such as field-effect devices, photodetectors and electrocatalysts [30,31]. Unfortunately, there are few reports on franckeite in the field of nonlinear optics. Li et al. reported the nonlinear optical (NLO) behavior of franckeite, demonstrating that it possesses the broadband NLO response and large third-order susceptibility. The in-band and inter-band carrier recovery times of franckeite-nanosheets were measured in the order of picoseconds by pump-probe technology, indicating that franckeite has the prospect of being used as a slow SA in ultrafast passive mode-locked lasers [32]. Recently, anisotropic NLO effects, such as Raman scattering and third-harmonic generation have also been investigated based on the incommensurate heterostructures of franckeite [33]. The strong optical nonlinearities and broadband NLO response indicate that franckeite has the capability to serve as ultrafast photonics devices.
However, as far as we know, the NLO properties of franckeite have rarely been used for the design of pulse modulator in fiber lasers. In view of its naturally occurring vdWH, high nonlinear and high damage threshold, franckeite is expected to be a more reliable SA candidate compared with the 2D materials reported before. In this paper, franckeite-based SA was used in a Yb-doped fiber lasers (YDFL) for passively mode-locked operation, with saturation density and modulation depth of 75 MW/cm2 and 7%, respectively. The YDFL operating at 1063.94 nm has a repetition rate of 1.76 MHz and pulse width of 1.57 ns. In addition, when we increase the pump power to 488 mW, the average output power of the laser is 9.42 mW, corresponding to the maximum single pulse energy of 5.35 nJ. Meanwhile, the laser pulses proved to be highly stable with the signal-to-noise ratio (SNR) of 50 dB and the spectral stability measurement further demonstrates the long-term stability of the laser output. In comparison with the pulse performance of other 2D material-based passive mode-locked YDFL, franckeite-based mode-locked fiber laser offers significant advantages in terms of starting threshold and output single pulse energy. The performance of our YDFL suggests that franckeite can be a reliable pulse modulator in ultrafast fiber laser and may have potential to achieve high pulse energy output.
2. Preparation and characterization of a franckeite-based SA
The preparation process of franckeite-based SA is shown in Fig. 1.The franckeite alcohol dispersion solution with a mass concentration of 10 mg/mL was first prepared. Under ultrasonic cleavage and centrifugation treatment, the dispersion containing few layers of franckeite nanosheets was obtained. Then the above solution was mixed with 4 wt% polyvinyl alcohol (PVA) matrix solution in equal volume, and the two were thoroughly blended by sonication again to form the homogeneous franckeite-PVA solution. After that, the mixture was carefully spread on a clean glass slide with a pipette, and franckeite-PVA film could be obtained after 24 h of evaporation treatment in the drying oven at 30°C. The final fiber-type SA was designed by sandwiching the thin film between two connectors.
The scanning electron microscope (SEM, JSM-7610F) and transmission electron microscope (TEM, JEM-2100) were used to characterize the morphology feature of franckeite nanosheets. As shown in Fig. 2(a), the SEM topographic image of franckeite at 20,000x magnification exhibits the layered structure of nanosheets after LPE. In Fig. 2(b), TEM images of the prepared franckeite dispersion solution under the optical resolution of 100 nm was provided, demonstrating that we successfully fabricated franckeite nanosheets with large surface area that exhibit good flatness. The energy dispersive X-ray spectroscopy (EDS) was employed to analyze the element component and stoichiometry of the franckeite nanosheets. Figure 2(c) clearly displays that the corresponding peaks affiliated with S/Pb/Sn/Pb elements and the atomic ratio of them is 53.21: 25.32: 11.82: 9.65, which is shown in the inset of Fig. 2(c). Furthermore, we also investigated the crystal structure of the franckeite nanosheets by employing the X-ray Diffraction (XRD). It can be seen from the Fig. 2(d) that peaks corresponding to the (004), (005), (100) and (008) planes of the franckeite were recorded, matching well with previous report [28]. What’s more, the significant appearance of the (100) diffraction peak in the XRD pattern reconfirms the successful preparation of layered franckeite nanosheets with high purity and crystallinity.
Fig. 2. Characterization of the layered franckeite. (a) The SEM image of the franckeite nanosheets. (b) The TEM images of the franckeite nanosheets under the optical resolutions of 100 nm. (c) EDS of the franckeite nanosheets and the corresponding atomic ratio. (d) XRD results of the franckeite nanosheets.
In order to better evaluate the performance of the fabricated franckeite-based SA, we also further characterized the franckeite-PVA films. Figure 3(a) is the SEM image of the cross section of the franckeite-PVA film, which exhibits a relative uniform thickness distribution and the measured thickness was about 81.1 µm. The linear absorption spectrum of the composite film is given by Fig. 3(b), exhibiting that franckeite-based SA has absorption over a wide spectral range. In addition, the absorption of pure PVA film with the equivalent thickness was also tested for comparison. It can be clearly observed that the absorption of the franckeite-PVA film at 1064 nm is about 9.2% whereas the pure PVA sample has almost no absorption of light, indicating that franckeite nanosheets in our sample are responsible for the saturable absorption.
Fig. 3. (a) The cross-section SEM image of the franckeite-PVA. (b) The absorption spectrum of franckeite-PVA.
The variation curve of nonlinear transmittance of franckeite-based SA with incident light intensity was recorded by a dual detector transmission measurement system. Based on a home-made mode-locked YDFL (central wavelength: 1064 nm, pulse width: 35 ps, repetition rate: 68 MHz), a variable attenuator is utilized to regulate the optical power incident on the sample. The pulsed laser is divided equally into two paths by an OC with a 50:50 splitting ratio, and the power of each branch is measured separately by a calibrated optical power meter. Figure 4 gives the saturable absorption curve of franckeite-PVA, which was fitted by the following equation [34]
where T is the transmission rate, Tns is the non-saturable loss, ΔT is the modulation depth, I is the input intensity of laser, Isat is the saturation intensity. By fitting and analyzing the results obtained in the measurement, the saturation intensity, modulation depth and non-saturable loss of franckeite-PVA were 75MW/cm2, 7%, and 47%, respectively. It can be predicted that such a low saturation intensity can effectively reduce the mode-locking threshold of the laser.3. Experimental setup
The experimental configuration of the proposed passively mode-locked YDFL applying the franckeite SA is shown in Fig. 5. In this 117 m long ring cavity, the pump light was generated by a 980 nm laser diode (LD) with a maximum output power of 500 mW and injected into the ring laser cavity via a 980/1064 nm wavelength division multiplexer (WDM). A piece of 0.28 m Yb-doped fiber (YDF, LIEKKI Yb1200) was employed as the gain medium, with an absorption of 1200 dB/m at 976 nm and group velocity dispersion (GVD) of 24.22 ps2/km. Two polarization controllers (PCs) were placed at different locations in the YDFL to adjust the polarization-dependent loss of the circulating laser. The laser unidirectional operation was guaranteed by polarization independent (PI-ISO). Besides, the dispersion and nonlinearity of the resonant cavity are regulated by a segment of 100 m-long single-mode fiber (SMF-28) with GVD of 17.7 ps2/km, which is also conducive to enhance the stability of the output pulse. And a fiber filter with bandwidth of 2 nm and center wavelength at 1064 nm was inserted to the cavity to provide additional amplitude modulation to restrain the mode competition. The laser output beam was extracted through the 20% output port of a 20:80 optical coupler (OC). Finally, the output performances of the fiber laser were measured by the following instruments: a fast-speed InGaAs photodetector (3GHz), a 1 GHz digital oscilloscope (Tektronix MDO4104C) with a bandwidth of 1GHz and sampling rate of 5 GS/s, a power meter (OPHIR Nova II), an optical spectrum analyzer (Anritsu, MS9740B) and a spectrum analyzer (R&S, FPC1000). The setting of 117 m long cavity with 0.28 m YDF was chosen after optimization. The pump light of 976 nm can be fully absorbed with the gain fiber of 0.28 m. And the selected 100 m SMF can effectively balance the large nonlinearity introduced by franckeite in the cavity. Besides, for an all-normal-dispersion laser, the pulse formation is the result of the combined effect of intracavity gain, loss, dispersion, nonlinear effects, and filtering effects. Under the selected cavity parameter settings, the mode-locked pulses can remain self-consistent with stable output performance and pure spectrum. Therefore, the choice of total cavity length and YDF length is reasonable.
4. Results and discussion
Firstly, we investigated the output performance of the laser without inserting the franckeite-PVA mode-locker into the cavity. No matter how we change the pumping power and the polarization state in the cavity, there is no emergence of mode-locking pulse, which excludes the possibility of self-mode locking. After inserting the franckeite-PVA film, the mode locked operation was achieved when the pump power steadily increased to 56 mW, and the low starting threshold of YDFL mainly due to the low insertion loss of SA, which was measured as ∼0.71 dB. When the pump power reached 105 mW, the emission spectrum with the center wavelength of 1063.91 nm was obtained, as shown in Fig. 6(a), and the corresponding 3 dB bandwidth was 0.37 nm. And there was no extra peak corresponding to the CW component in the spectrum, indicating the high spectral purity of the mode-locking operation. Figure 6(b) demonstrates the variation of the pulse spectrum with increasing pump power, the spectrum is just slightly broadened due to SPM effect. It should be pointed out that under strong pumping conditions, the spectral shape remains unchanged without pulse splitting.
Fig. 6. (a) The optical spectrum of the fiber laser at 105 mW. (b) spectra at different pump powers. (c) the pulse train of the mode-locked operation. (d) the single pulse shape of mode-locked fiber laser.
The single pulse shape at 488 mW is shown in Fig. 6(c), the pulse width obtained in the experiment is 1.57 ns, which is much larger than the theoretical calculation due to the large cavity dispersion value caused by the 100 m long SMF. It should be noted that the pulse front showed a slight slope which may be caused by two reasons from our opinion. On the one hand, it could be related to the frequency chirp. As is known, when the pulse width is longer than the transform limit, chirp is bound to occur. And the GVD effect will cause different frequency components to propagate at different velocities in the time domain. As a result, the pulse edge is skewed in the time domain. On the other hand, the measurement error caused by the limitation of response time of the detector and oscilloscope may also make the slight incline of pulse leading edge. The pulse train of the mode-locked laser is displayed in Fig. 6(d), the time interval between adjacent pulses is 0.57µs, which matches well with the cavity length of the laser. From our point of view, the pulse duration is not only determined by the cavity parameters but also the preparation of SA. Typically, the possibility of narrowing the pulse width can be achieved by shortening the length of cavity. However, the dispersion provided by the long cavity length can balance the high nonlinearity of the material, which is more favorable for the generation of mode-locked pulses. Besides, considering the inverse relationship between pulse width and modulation depth [35], a narrower pulse width can be obtained by further improving the modulation depth of SA. Therefore, in our next work, we will further optimize the structure of the resonant cavity and the preparation of SA materials to achieve ultrashort pulse output with higher output power. In the experiment, we found that when the input power exceeded 488 mW, the mode-locking pulse no longer operated in a stable state, but if the pump power reduced below 488 mW, a stable mode-locking phenomenon was observed again, indicating that the SA was not damaged and unstable mode-locked operation was caused by its oversaturation effect.
Figure 7 depicts the average output power and single pulse energy as a function of pump power. It is obvious to see that as the input power gradually grows from 56 mW to 488 mW, the average output power changes from 0.58 to 9.42 mW and the corresponding slope efficiency is calculated as 2.15%. Besides, it is worth to mention that the maximum single pulse energy in the experiment is as high as 5.35 nJ. We believe the low slope efficiency and output power can be determined by the loss distribution in the cavity, such as the unsaturated loss of SA (∼47%) and splicing loss of the fiber. In the further work, we will focus on improving the average output power and conversion efficiency of the mode-locked operation by enhancing the growth quality of the material and optimize the thickness of the film. Furthermore, we also investigated the damage threshold of franckeite-based SA. Based on the obtained single pulse energy of 5.35 nJ, the maximum energy intensity of the proposed YDFL is estimated to be ∼22.1 mJ/cm2. Because the material was not damaged in the experiment, the actual optical damage threshold of the franckeite polymer film modulator should be higher than 22.1 mJ/cm2. Regrettably, we are currently unable to give the specific optical damage threshold above 22.1 mJ/cm2 due to the limit of the maximum pump power in our lab. But we believe the proposed franckeite-PVA film can tolerate higher laser intensity due to the unique structure of franckeite and the large thickness of the fabricated film.
Additionally, we measured the radio frequency (RF) spectrum of different ranges of the mode locked laser at the resolution bandwidth (RBW) of 1 kHz. As is depicted in Fig. 8(a), the central frequency is 1.76 MHz, which matches the pulse repetition rate very well. And the SNR is about 50 dB, indicating that the laser works in a stable mode-locked state. The RF spectrum with a span of 100 MHz in Fig. 8(b) shows that there is almost no fluctuation in the height of the peak frequency, reconfirming that the laser pulse has lasting stability. Furthermore, we recorded the spectral evolution of the mode-locked pulse over a continuous period of 90 min at the pump power of 488 mW. As shown in Fig. 8(c), there is no shift in center wavelength and little change in spectral intensity. All the results can prove that the laser output in our experiment is of high stability.
Fig. 8. (a) The RF spectrum located at 1.76 MHz. (b) RF spectrum with bandwidth of 100 MHz. (c) Spectra stability measurement of mode-locked operation.
Table 1 provides a relatively comprehensive comparison of our work with the output pulse performance of other 2D materials-based passive mode-locked YDFL.As is shown, a variety of 2D materials have been used as mode-lockers, including GO, TIs, TMDs and BP etc. Although the average output power of 27 mW based on BP-SA is the maximum value in the list, the SNR of output pulses is only 39 dB [42]. Though the contrast, we find that the mode-locked pulse obtained based on franckeite has the lowest mode-locked threshold and relatively high SNR. In addition, the single pulse energy of 5.35 nJ achieved using franckeite-SA is second only to that of Ref. [45]. The output of high pulse energy is not only related to the cavity parameters of the YDFL, such as the dispersion map of the resonator, but also depends strongly on the performance of the SA, indicating that franckeite holds great promise for obtaining high-energy pulses.
Table 1. Comparative performance of passively mode-locked YDFL based on different 2D SAs.
5. Conclusion
In summary, passive mode-locked YDFL was proposed using franckeite-based SA as modulator, whose saturable intensity and modulation depth were 75MW/cm2 and 7%, respectively. Stable mode locked pulses were recorded with repetition rate of 1.76 MHz and SNR of 50 dB. The maximum single pulse energy can up to 5.35 nJ with the output power of 9.42 mW. We have not measured the nonlinear optical response of franckeite-based SA at other wavelengths in this work, but the broadband nonlinear optical properties of layered franckeite nanosheets have demonstrated in Ref. [32]. Therefore, we deem that the designed SA can have potential to be used for other wavelengths. In a word, a franckeite-polymer film can be acted as a promising pulse modulator with stable and excellent performance in ultrafast fiber lasers
Funding
Natural Science Foundation of Shandong Province (ZR2019MF047, ZR2020MF126, ZR2019MF043).
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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