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Abstract
MXenes are a class of two-dimensional layered structure ternary metal carbide or/and nitride materials. Recently, the MXene V2CTx has demonstrated excellent long-term stability, strong saturable absorption, and fast optical-switching capability, used to generate Q-switched and ultrashort pulsed lasers. However, bound-state fiber lasers based on V2CTx have not been reported yet. In this study, V2CTx is combined with a D-shaped fiber to form a saturable absorber device, whose modulation depth is measured to be 1.6%. By inserting the saturable absorber into an Er-doped fiber laser, bound states with different soliton separation and munbers are successfully obtained. Additionally, bound states with a compound soliton structure, such as the (2 + 2)- and (2 + 1)-type, are also realized. Our findings show that V2CTx can be developed as an efficient ultrafast photonics candidate to further understand the complex nonlinear dynamics of bound-state pulses in fiber lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Passive mode-locked fiber lasers have been researched extensively due to their potential applications in fundamental science, biomedicine, and industry [1–7]. Based on the comprehensive interaction between dispersion, nonlinearity, gain and loss, fiber lasers can transmit not only conventional single solitons but also multi solitons. Therefore they can serve as an ideal platform for studying complex pulsed lasers. Depending on the soliton interaction, multiple solitons exhibit various interesting dynamical behaviors in fiber lasers, such as soliton clusters [8,9], soliton rain [10,11], harmonic solitons [12,13], and bound-state solitons [14–19]. Among these, bound-state solitons, a distinctive existing form in mode-locked fiber lasers, consist of two or more closely spaced pulses with relatively fixed temporal separation and phase difference. Compared with single soliton operation, bound-state counterparts have received much more attention not only because it is beneficial to reveal nonlinear interaction mechanism between solitons but also helps to develop larger telecommunication capacity in optical fiber transmission lines [20–23]. The mode-locked fiber laser is a good platform to study bound-state solitons, wherein the saturable absorber (SA) acts as a mode-locker, playing a key role in the generation of multiple solitons.
Recently, 2D layered material-based SAs have been widely studied because of their excellent wideband optical response, large third order nonlinearity, tunable bandgaps, ultrafast carrier mobility, and strong interplay between the laser and the materials. Materials such as graphene [24–26], topological insulators [27–29], black phosphorus [30], and transition metal dichalcogenides [31–35], have been extensively employed in mode-locked fiber lasers. MXene materials, which are a kind of metal carbide and metal nitride materials with a 2D layered structure, were first assembled by Gogotsi et al. [36]. The general formula of an MXene material is Mn + 1XnTx, where M is a transition metal element, X is a carbon or/and nitrogen element, and T is the surface termination (such as hydroxyl, oxygen or fluorine). The value of n is 1, 2, or 3. To date, properties of MXenes, from morphological to optical, electric, and magnetic, have been studied [37]. In addition, they have been used as SAs for mode-locked laser systems successfully. Jiang et al. demonstrated the MXene Ti3C2Tx can be applied as a SA for mode-locked operations at 1.06 and 1.55 µm. Consequently, a highly stable ultrafast laser with a pulse width of 159 fs in the telecommunication window is readily achieved [38]. To advance the utilize of the broadband nonlinear optical responses of Ti3C2Tx, the printed side-polished fiber and gold mirror have been integrated to laser resonators, and ultrafast laser operations covering the optical spectrum of 1-3 µm have been realized [39]. Wu et al. realized an all-optical device based on a microfiber knot resonator deposited with MXene Ti2CTx to generate high-performance phase and intensity modulation with a fast response time, good modulation efficiency, and high stability [40]. Although the common MXenes (Ti3C2Tx or Ti2CTx) exhibit excellent performance as a SA in mode-locked fiber laser, they suffer from oxidation under environmental conditions easily, significantly hindering their widespread application [41,42].
V2CTx, a member of the Mxene family, has been widely researched for its outstanding electron and ionic conductivity, and high energy density [43]. More significantly, recent studies show that V2CTx possesses good long-term stability under ambient conditions [44,45]. Therefore, high-performance SAs are expected to be achieved using MXene V2CTx. Wu et al. proposed a high-performance all-optical modulator for actively Q-switched pulse generation based on a microfiber knot resonator deposited with MXene V2CTx [46]. Huang et al. employed V2CTx as an excellent SA in passively mode-locked fiber lasers, and a 206th harmonic order was successfully obtained with a corresponding maximum repetition rate of 1.01 GHz [47]. However, no bound-state solitons have been reported to be observed in V2CTx based mode-locked fiber lasers.
In this study, the nonlinear optical characteristics as well as application of V2CTx in bound-state fiber lasers are systematically studied. A V2CTx based SA is fabricated by depositing MXene V2CTx nanosheets on a D-shaped fiber with a modulation depth of approximately 1.6%. By inserting the nanosheets into a fiber laser, not only conventional solitons, but also various bound-state solitons are observed, such as bound states with the same soliton number and different soliton separation, bound states with a different soliton number and same soliton separation, and (2 + 2)-type and (2 + 1)-type bound states. Our experimental results can provide a useful supplement to efficiently improve the bound state family.
2. Experimental section
2.1 MXene V2CTx characterization
Based on the unit cell of pristine V2C, functionalized V2CTx MXene structures with T terminations (such as hydroxyl, oxygen or fluorine) are constructed [48]. In general, V2CTx MXene has an ABC stacking order in a hexagonal crystal lattices, where carbon atoms fill the octahedral site between two layers of vanadium atoms, as shown in Fig. 1(a). And the inter-layer distance is about 1.08. Presently, the reported synthetic methods for MXene fabrication primarily include bottom-up synthesis, solution (HF and HCl + LiF), and high-temperature fluoride melting. Among these, HF acid etching has many advantages compared with the other methods, such as high practicability, low cost, and mature preparation process. Hence, it was employed to fabricate the V2CTx solution in this experiment. The preparation process is similar to a previous report [47]. The prepared V2CTx dispersion liquid is displayed in Fig. 1(b). As shown in Fig. 1(c), the elemental constituent of the prepared V2CTx is analyzed via energy dispersive spectroscopy (EDS). Accordingly, the content of silicon was found to be 43.2 wt%, resulting from the substrate where the V2CTx nanosheets were located. In addition, 0.3 wt% aluminum was found, due to the table of the measuring instrument. Scanning electron microscope (SEM) was used to study the structure of V2CTx, whose image is shown in Fig. 1(d), wherein its 2D layered structure can be observed. Atomic force microscopy (AFM) was used to determine the thickness of the nanosheets, as shown in Fig. 1(e), and a red line is marked along the cross-section. As shown in Fig. 1(f), the line demonstrates the thickness of the nanosheets to be approximately 35.5 nm.
Fig. 1. (a) Atomic structure of V2CTx. (b) V2CTx dispersion liquid. (c) EDS spectrum. (d) SEM image. (e) AFM image of the exfoliated V2CTx nanosheets and the corresponding (f) height profile.
2.2 Band structure and density of states
V2CTx MXene structures are decorated with T terminations, primarily including -O, -F, and -OH. These groups are randomly distributed on the surface, so calculation the band structure of V2CTx is unmanageable. In order to understand the band characteristics of V2CTx, band structure and density of states of pristine V2C were calculated. Figure 2(a) illustrates the electronic band structure of bulk V2C. Both lowest point of the lowest unoccupied molecular orbital and the highest point of the highest occupied molecular orbital are situated at point Γ. In addition, it reveals that bulk V2C is a zero-band gap metallic material. From Fig. 2(b), it is evident that the contribution of V-d orbitals near the Fermi level can be clearly discerned. Figure 2(b) demonstrates that V-d orbitals exhibit a higher density of states near the Fermi level, implying that electrons in V-d orbitals primarily contribute to the formation of electronic states around the Fermi level. This observation strongly suggests the pivotal role of the V element in the electronic structure of the V2C system, especially in governing the electronic properties around the Fermi level. This result is crucial for understanding the electronic properties of V2C and its potential applications. it can be seen that the energy band in the range of -4 to -3 ev below the Fermi level can be regarded as bonding states between the C-p and the V-d orbitals. Band structure and density of states of monolayer V2C are given in Figs. 2(c) and 2(d). It could be found that the density of states of monolayer V2C is very similar to that of bulk V2C.
Fig. 2. Band structure of (a) monolayer and (c) bulk V2C. Density of states of of (b) monolayer and (d) bulk V2C.
2.3 Fabrication of the V2CTx-based SA and experimental setup
An evanescent field interaction V2CTx SA is fabricated in this experiment. The D-shaped fiber used here is obtained by polishing a standard single-mode fiber on one side and with a length of approximately 20 mm. This D-shaped fiber is fastened onto a glass slide with the D-region facing upwards. The V2CTx-based SA is prepared by depositing V2CTx nanosheets onto the surface of D-shaped fiber. To illustrate the nonlinear optical properties of the fabricated V2CTx-based SA, a balanced double detector measurement system is developed, as shown in Fig. 3(a). An ultrafast fiber laser is employed as the optical source, and an attenuator is used to adjust the output power of this fiber laser. Then, the laser is split into two paths using a 10/90 optical coupler, and the V2CTx-based SA is located in one of the branches. A dual channel power meter records the power of two branches.
Fig. 3. Experimental setup of (a) double detector measurement system and (b) mode-locked fiber laser.
An Er-doped fiber laser is constructed to investigate the performance of as-prepared V2CTx-SA, as shown in Fig. 3(b). The laser system is pumped by a laser diode (LD) with a central wavelength of 976 nm. The light is coupled into the cavity via a polarization-independent tap-isolator-wavelength-division multiplexer (PI-TIWDM). In addition, the PI-TIWDM ensures unidirectional propagation in the cavity, allowing for 30% intracavity power as the output. Further, a polarization controller (PC) is inserted into the laser cavity to tune the polarization states of the propagating light. Thereby, a D-shaped fiber covered with V2CTx nanosheets serves as the SA to accomplish mode-locked operation. The total cavity length is approximately 7.2 m, comprising of 0.8 m Er-doped fiber (EDF) and 6.4 m single-mode fiber.
3. Results and discussion
3.1 Nonlinear optical properties and fundamental mode-locked solitons
By using experimental setup displayed in Fig. 3(a), nonlinear optical properties of the V2CTx-based SA were measured, and the experimental data are illustrated in Fig. 4(a). The SA exhibits typical saturable absorption characteristics of a corresponding increase in transmission with an increase in pulse intensity. As shown in Fig. 4(a), the modulation depth of the SA is extrapolated as 1.6%. Then the SA was inserted in the fiber laser cavity, as shown in Fig. 3(b). When the pump power of the fiber laser is increased to 65 mW, continuous waves can be obtained. The mode-locking operation is achieved with appropriate PC states, as the pump power is up to 95 mW. Accordingly, multiple pulses circulating in the laser cavity can be detected. When the pump power is decreased to 81 mW, single-pulse mode-locking operation is observed, as shown in Fig. 4. The mode-locking spectrum is presented in Fig. 4(a), with a central wavelength and 3 dB spectral bandwidth of 1570.3 and 6.2 nm, respectively. The Kelly sidebands are located on both sides of the spectrum, demonstrating that the laser is operating in the conventional soliton state. As shown in Fig. 4(b), the pulse shape of mode-locking operation is measured by an autocorrelator, giving a pulse duration of approximately 1.04 ps by assuming a Sech2 pulse shape. Figure 4(c) is the output pulse-train with an adjacent pulse separation of ∼34.8 ns. The uniform amplitudes indicate that the fiber laser is operating in a relatively stable mode-locking state. No modulation or Q-switching was observed in the experiment, no matter how the PC and pump power were adjusted. Although D-shaped fiber with nonlinear polarization rotation effect can will affect the pulse performance, no mode-locking operation can be achieved by removing V2CTx nanosheets from D-shaped fiber.
Fig. 4. (a) Nonlinear optical properties of the V2CTx-based SA. (b) Optical spectrum, (c) AC trace and (d) Pulse train of passive mode-locking laser performance with pump power of 81 mW.
3.2 Various bound-state operations
By slightly adjusting the PC states, two-solition bound state operation can be obtained at a pump power of 91 mW. Figure 5(a) shows the typical bound-state spectrum, wherein spectral modulation is evident, which is an important feature of bound-state pulses. The enlarged spectrum is shown in Fig. 5(b), and the modulation period is measured as 3.16 nm. The characteristics in the temporal domain were recorded by an intensity autocorrelator, as shown in Fig. 5(c). Three peaks with an intensity ratio of 1:2:1 are generated in the autocorrelation trace, indicating that the bound-state soliton consists of two equal intensity single solitons [49]. The pulse separation is measured as 2.6 ps, which agrees well with the 3.16 nm spectral modulation period. The oscilloscope showing a pulse-train with a fundamental repetition rate is notable. Accordingly, we can conclude that the fine details inside the bound-state soliton cannot be resolved by the temporal resolution of the detection device. Further, when maintaining pump power and slightly rotating the polarization states of the PC, the soliton separation inside the bound states increases without affecting the soliton number. The spectra presented in Figs. 5(d), 5(g), and 5(j) show more pronounced modulation. The enlarged spectra, shown in Figs. 5(e), 5(h), and 5(k), indicate the modulation periods as 1.13, 0.56, and 0.44 nm, respectively. The corresponding bound-state pulses with soliton separations of 7.3, 14.8, and 18.8 ps are illustrated in Figs. 5(f), 5(i), and 5(l).
Fig. 5. Bound states with the same soliton number and different soliton separation. (a) The measured spectrum, (b) enlarged spectrum, and (c) autocorrelation trace with a soliton separation of 2.6 ps. (d) The measured spectrum, (e) enlarged spectrum, and (f) autocorrelation trace with a soliton separation of 7.3 ps. (g) The measured spectrum, (h) enlarged spectrum, and (i) autocorrelation trace with a soliton separation of 14.8 ps. (j) The measured spectrum, (k) enlarged spectrum, and (l) autocorrelation trace with a soliton separation of 18.8 ps.
When the pump power is marginally increased, keeping the polarization states of the PC constant, multi-soliton bound states are obtained, as shown in Fig. 6. Its spectrum is shown in Fig. 6(a). The modulation period is 3.16 nm, which is the same as that shown in Fig. 5(a). As shown by the blue line in Fig. 6(b), we can infer that 6-soliton bound states are formed in the fiber laser at a pump power of 110 mW. The soliton separation is 2.6 ps, corresponding to a modulation period of 3.16 nm. When the pump power is further increased, the spectral profile and the modulation period show negligible change. In addition, as presented by the red and purple lines in Fig. 6(b), the soliton number inside the bound states increases while the soliton separation remains constant.
Fig. 6. Bound states with different soliton numbers and same soliton separation. (a) Measured spectrum. (b) Autocorrelation traces for 6-soliton, 7-solition, and 8-soliton bound states.
Experimentally, more novel types of compound bound states were achieved. Unlike the bound states discussed above, they demonstrate relatively complicated AC traces. Figures 7(a) and 7(b) illustrate the (2 + 2)-type bound state formed by four solitons. First, a two-soliton bound state behaves as a unit, and then two sets of such units are bound together. A spectrum of this bound state is also different from those discussed above. As shown in Fig. 7(a), twofold modulation can be observed with spectral modulation periods of 0.35 and 3.42 nm, corresponding to a soliton separation of 23.3 and 2.4 ps, respectively. The AC trace is shown in Fig. 7(b), and the pulse-to-pulse separation in the two-soliton bound state and two set units is 2.4 and 23.3 ps, respectively, which agrees well with the spectral modulation period.
Fig. 7. (a) The measured spectrum (inset: enlarged spectrum) and (b) autocorrelation trace of (2 + 2)-type bound state. (c) The measured spectrum (inset: enlarged spectrum) and (d) autocorrelation trace of (2 + 1)-type bound state.
Additionally, another unusual bound state, called the (2 + 1)-type bound state, is obtained in our experiment. This bound state consists of a soliton pair and a single soliton, as illustrated in Figs. 7(c) and 7(d). Its double modulated optical spectrum is shown in Fig. 7(c). There spectral periods are 0.86 and 3.42 nm, respectively. The corresponding autocorrelation trace is displayed in Fig. 7(d), which is different from that of the (2 + 2)-type bound state. Figure 7(d) shows that the structure of side components lacks main peaks. The separation of the soliton pair is 2.4 ps, corresponding to a spectral modulation period of 3.42 nm. Further, the separation between the soliton pair and single soliton is 9.5 ps, corresponding to a modulation period of 0.86 nm.
In this experiment, various types of bound states were observed. The principle of their formation is as follows: in the laser cavity, a single soliton would split into multiple solitons when nonlinearity was sufficiently high. Once multiple solitons were generated, they evolved into various modes depending on the interaction between the solitons and dispersive waves in the cavity. Without any external control, multiple solitons are usually randomly distributed in the laser cavity. However, they can be automatically arranged and positioned in cavities with different time intervals if the pump power and PC rotation are adjusted. Once the phase difference of these solitons is constant, they would merge and generate various bound states. Essentially, all solitons inside a bound state are tied together by the interaction amongst them, and they have high temporal phase coherence.
4. Conclusions
We investigated various bound-state pulses in a fiber laser using a V2CTx SA. A fundamental mode-locking operation was realized at 1558.8 nm, with a central wavelength and 3 dB spectral bandwidth of 1570.3 and 6.2 nm, respectively. Taking advantage of the outstanding nonlinear effect and remarkable saturable absorption of the V2CTx SA, both bound states with the same/different soliton number and different/same soliton separation were observed. By adjusting the intracavity polarization and pump power, (2 + 2)- and (2 + 1)-type bound states were also realized successfully. The experimental results demonstrated that the V2CTx SA can serve as an excellent nonlinear photonic device, which can be employed in mode-locked fiber lasers for the research of various soliton interactions.
Funding
National Natural Science Foundation of China (12075190, 62005212, 62305299); New Star Project of Science and Technology of Shaanxi Province (2022KJXX-69); Innovation Capability Support Program of Shaanxi (2021TD-09); Fund for Outstanding Young Talents of China Academy of Space Technology (Xi’an) (Y21-RCFYJQ1-03); Young Elite Scientists Sponsorship Program by CAST (2022QNRC001); Young Talent fund of University Association for Science and Technology in Shaanxi, China (20210112); Open Foundation of State Key Laboratory of Transient Optics and Photonics (SKLST202207); The Youth Innovation Team of Shaanxi Universities; Natural Science Foundation of Zhejiang Province (LQ23F050004).
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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