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PMMA Sandwiched [Bi.sub.2][Te.sub.3] Layer as a Saturable Absorber in Mode-Locked Fiber Laser.

1. Introduction

Topological insulator is a state of quantum matter with insulating bulk states like an ordinary insulator and highly conducting and massless spin-helical surface states on its surface [1-7]. These unique properties, which are derived from the combination of spin-orbit interactions and time-reversal symmetry, enable topological insulators as efficient saturable absorber materials for the generation of ultrafast pulse [8, 9]. Compared with traditional saturable absorbers, such as semiconductor saturable absorber mirrors (SESAMs) [10,11], single-walled carbon nanotubes (SWCNTs) [12], graphene [13], transitional metal dichalcogenides [14,15], black phosphorus [16,17], topological insulator saturable absorber possesses many good qualities, such as low saturable absorption threshold, large modulation depth [18, 19], short recovery time [20, 21], and wavelength independent saturable absorption [22]. To fabricate saturable absorbers with advanced performance, it is necessary to search for a facile and simple transfer technique of topological insulator nanosheets onto target substrates like optical fiber-ferrules or optical quartz substrate.

Several methods have been proposed for the transfer of saturable absorbers onto the optical fiber-ferrules, such as optical deposition [23], ink jet print [24], drop cast [25], and polymer composite [26]. Drop cast is favored for its simple and facility. However, its coffee ring effect is difficult to effectively restrain while drying the solute. Ink jet print method is tedious and prone to error and requires for precision instruments and skillful professional operators. Optical deposition was used to deposit carbon nanotubes and graphene onto cores of optical fiber ends. Though the method is simple, it requires precise control of the optical power and does not possess controllability of the deposition layer properties.

In this paper, we present a simple approach to improve the [Bi.sub.2][Te.sub.3] (one kind of topological insulator) transfer method. The [Bi.sub.2][Te.sub.3] nanosheets were deposited on the PMMA film, and then another PMMA film was placed onto the [Bi.sub.2][Te.sub.3] nanosheets to isolate air and water. By this process, the stability of [Bi.sub.2][Te.sub.3] based saturable absorber was greatly increased. The sandwiched structure of [Bi.sub.2][Te.sub.3] film was placed onto fiber end facet and introduced into fiber ring laser. Ultrafast pulse was obtained with sandwiched structure of [Bi.sub.2][Te.sub.3] film saturable absorber. It has a pulse width of about 505 fs and a repetition rate of 13.14 MHz. The stable mode-locking operation lasted at least 6 hours. All these results show that this PMMA sandwiched [Bi.sub.2][Te.sub.3] structure is an effective and stable saturable absorber, which could have potential applications in ultrafast pulse generation.

2. Saturable Absorber Preparation

2.1. Synthesis of [Bi.sub.2][Te.sub.3] Nanosheets. [Bi.sub.2][Te.sub.3] nanosheets were synthesized following a solvothermal method of Wang et al. [27]. Using a typical synthesis, a stoichiometric ratio of bismuth chloride ([BiCl.sub.3]) and sodium selenide ([Na.sub.2]Te[O.sub.3]) was dissolved in ethylene glycol with vigorous stirring. Then the mixture was transferred into the Teflon-lined stainless-steel autoclave and heated to 200[degrees]C. The autoclave was maintained at the reaction temperature for 36 h and then cooled to room temperature naturally. The black powders were collected by filtering, washed with distilled water and ethanol, and finally dried at 60[degrees]C in vacuum overnight. The as-grown and washed powders were dispersed in ethanol solution.

2.2. Characterization of [Bi.sub.2][Te.sub.3] Nanosheets. Scanning electronic microscope (SEM) measurements were carried out to characterize the morphology of [Bi.sub.2][Te.sub.3] nanosheets. The SEM image shows that the [Bi.sub.2][Te.sub.3] sample has a uniform size and shape in Figure 1. The obtained products are predominantly hexagonal-based plates, which matches well with the regular hexagonal lattice structure of [Bi.sub.2][Te.sub.3].

2.3. Fabrication of [Bi.sub.2][Te.sub.3] Based Saturable Absorber. Figure 2 shows the fabrication of [Bi.sub.2][Te.sub.3] based saturable absorber. The PMMA solution was spin coated onto quartz substrate and then the [Bi.sub.2][Te.sub.3] dispersion was also spin coated onto the PMMA film. Thus the [Bi.sub.2][Te.sub.3] based saturable absorber was fabricated. To enhance the mechanical strength and stability of the saturable absorber, another PMMA film was placed onto the [Bi.sub.2][Te.sub.3] nanosheets to isolate air and water. The sandwiched structure was peeled down from the quartz substrate. The [Bi.sub.2][Te.sub.3] based saturable absorber was completely made.

3. [Bi.sub.2][Te.sub.3] Saturable Absorber Based Fiber Laser

3.1. Design of Mode-Locked Fiber Laser. To construct a passively mode-locked ring fiber laser, the experimental configuration as in Figure 3 is employed, including the standard fiber-optic components such as wavelength division multiplexer (WDM), polarization controller (PC), coupler, optical isolator, and active fiber (AF). The fiber laser has a ring cavity configuration with a total cavity length of 15.22 m, which comprises a piece of 1 m erbium-doped fiber (EDF, LIEKKI Er 80-8/125) as AF with group velocity dispersion (GVD) of -20 [ps.sup.2]/km and the pigtails of fiber-optic components. All these pigtails are standard single mode fiber (SMF-28) with GVD of -23 [ps.sup.2]/km at 1550 nm. The pump, which was generated from a 975 nm laser diode (LD) source, is coupled into the cavity through a 980/1550 wavelength-division multiplexer (WDM), and a 10% fiber coupler is employed to output the pulsed laser. A polarization independent isolator (PI-ISO) is used to force the unidirectional operation of the ring cavity, and a polarization controller (PC) is used to fine adjust the polarization state of circulating light and the in-cavity birefringence. An optical spectrum analyzer (Ando AQ6317B) and an oscilloscope (Tektronix TDS3054B) combined with a 5 GHz photo-detector (Thorlabs SIR5) are employed to simultaneously monitor the optical spectra and temporal profile of the output pulse. The pulse duration is measured with a commercial second harmonic generation autocorrelator.

3.2. Mode-Locking Results and Discussions. The modelocking operation could be self-started with a mode-locking threshold of 63.7 mW. Figure 4 summarizes the characteristics of single soliton pulse of our fiber laser at pump power of 85.4 mW. Figure 4(a) shows that the single pulse train with a repetition rate of 13.14 MHz, which matches well with the cavity length, indicates that the laser operates in the mode-locking state. The corresponding optical spectrum, as shown in Figure 4(b), has obvious Kelly sidebands spectral indicating that the fiber laser operates in soliton regime. It has a central wavelength of 1570.45 nm and a 3 dB bandwidth of 5.35 nm. Correspondingly, as shown in Figure 4(d), the measured autocorrelation (AC) trace can be well fitted by hyperbolic secant function with a full width at half maximum (FWHM) of 756 fs, showing that the real pulse width is about 505 fs. The time-bandwidth product is 0.328, indicating that the obtained soliton pulse is almost transform limited. Figure 4(d) is corresponding measured RF spectrum with resolution bandwidth (RBW) of 10 Hz; the signal-to-noise ratio (SNR) of our fiber laser is up to 67.7 dB indicating that our fiber laser operates in a relatively stable regime. Moreover, as can be seen in long scale RF spectrum (the insert), there is not any excrescent frequency components except the fundamental and harmonic frequency components, further confirming the stability of our fiber laser and single soliton operation.

To investigate the long-term stability of the single soliton operation, we recorded the output spectra every hour over 6 h with fixed experimental setup, such as pump power of 90 mW, as shown in Figure 5. Neither the central wavelength drift nor new wavelength component was observed during our measurement and always exhibits the same profiles, showing excellent repeatability and superimposability. All these results confirm the fiber laser possesses a reasonably good stability that is suitable for practical applications.

During the entire measurement, neither the central wavelength drifting nor new wavelength components were observed, revealing that the mode-locked fiber laser shows long-term stability, which benefits from the PMMA sandwiched [Bi.sub.2][Te.sub.3] nanosheets saturable absorber. There are many other advantages to this novel PMMA-TI-PMMA structure. First, the spin coated layer spontaneously generated at the interface between the solution and air can remain very uniform. Second, the spin coated layer can be effectively protected by double PMMA layers, and the original morphology will be automatically kept while being transferred and installed. Third, the saturable absorber device can be effectively protected from oxidation, since it is isolated from air and water due to the double PMMA sandwich structure. Also, this mode-locked result shows no distinct different with previous reports [28-30], which means that this transfer process has no adverse effect to the [Bi.sub.2][Te.sub.3] based saturable absorber. All of these results lead to a conclusion: the PMMA sandwiched structure could protect well the [Bi.sub.2] [Te.sub.3] based saturable absorber and ensure no recede of the performance of saturable absorber.

4. Conclusions

Here, we fabricated a PMMA sandwiched [Bi.sub.2][Te.sub.3] nanosheets structure process. Based on this PMMA sandwiched [Bi.sub.2][Te.sub.3] nanosheets saturable absorber, we have demonstrated stable mode-locked fiber laser. Ultrafast pulse with a pulse width about 505 fs was obtained from the mode-locked fiber laser. These results suggest that the PMMA sandwiched [Bi.sub.2][Te.sub.3] nanosheets structure is a good saturable absorber candidate for ultrafast pulse generation. Also this technique could be developed to fabricate other stable two-dimensional materials based devices.

Data Availability

The experimental data used to support the findings of this study are all included within the article.

https://doi.org/10.1155/2018/7578050

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was partially supported by the Natural Science Foundation of Hunan Province, China (Grant no. 2018JJ2455).

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Guobao Jiang (iD), (1, 2) Yuan Zhou (iD), (1) Lulu Wang, (iD), (1) and Ying Chen (iD), (1, 2)

(1) School of Electronic Information and Electrical Engineering, Changsha University, Changsha 410003, China

(2) Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China

Correspondence should be addressed to Ying Chen; yingchen@ccsu.edu.cn

Received 21 June 2018; Accepted 6 December 2018; Published 18 December 2018

Guest Editor: Xinxing Zhou

Caption: Figure 1: SEM image of [Bi.sub.2][Te.sub.3] nanosheets.

Caption: Figure 2: Fabrication of PMMA sandwiched [Bi.sub.2][Te.sub.3] nanosheets saturable absorber.

Caption: Figure 3: The schematic of mode-locked fiber laser.

Caption: Figure 4: Single pulse mode-locking results of fiber laser.

Caption: Figure 5: Long-term stability of optical spectra.
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Title Annotation:Research Article; Polymethyl methacrylate
Author:Jiang, Guobao; Zhou, Yuan; Wang, Lulu; Chen, Ying
Publication:Advances in Condensed Matter Physics
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2018
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