Deterministic Transfer of Large-Scale β-Phase Arsenic on Fiber End Cap for Near-Infrared Ultrafast Pulse Generation

Gray arsenic (β-phase) has aroused great attention in photonics and electronics applications, as a novel family member of two-dimensional (2D) elemental crystals of group-VA. Here, β-phase arsenic (β-As) bulk crystals were synthesized via the chemical vapor transport (CVT) method. Meanwhile, large-scale β-As nanoflake was transformed using the polydimethylsiloxane (PDMS)-assisted dry transfer method and was placed on the end cap of optical fiber with high coverage over the core area. Moreover, the β-As was used as a saturable absorber in ytterbium-doped fiber ring cavity resonance, and we demonstrated near-infrared ultrafast pulse fiber laser with the central wavelength, repetition rate, and signal-to-noise ratio (SNR) of 1,037.3 nm, 0.6 MHz, and 67.7 dB, respectively. This research demonstrates a 2D material small area deterministic transfer method and promotes the potential application of group-VA crystals in near-infrared ultrafast laser generation.

With the rapid development of novel 2D material preparation, at the same time, arsenic compounds (gallium arsenide, cadmium arsenide, and black arsenic phosphorus) are rising in industry and scientific research (Yoon et al., 2010;Zhang C. et al., 2019;Khalatpour et al., 2021). The raw material arsenic is widely used in many fields due to its excellent physical and chemical properties and has been thoroughly studied and recognized as building blocks for future photons and optoelectronic technologies. Along with the inspiration of the research on few-layer phosphorous allotrope, the few-layer arsenic allotrope is also concerned. They are both elemental layered materials derived from group VA (phosphorus, arsenic, antimony (Ji et al., 2016;Wu et al., 2017), and bismuth) (Zhang S. et al., 2016;Zhang et al., 2018;Niu et al., 2019;Wu and Hao, 2020). BP shows high mobility of up to about 1,000 cm 2 V −1 s −1 and is used in nano-electronic and photonic devices, while it is very unstable and degrades rapidly in ambient conditions (Wang et al., 2019b;Xu et al., 2020b). Purple phosphorus with a pyrolysis temperature above 512°C is the most stable phosphorus allotrope (Zhang L. et al., 2020), and for blue phosphorus, an indirect band gap semiconductor (Zhang J. L. et al., 2016;Zhang J. L. et al., 2020), the photoelectric response can reach the ultraviolet region. The different properties of materials corresponding to different structures have aroused people's interest in arsenic research. The research on few-layer α-As (black arsenic) started in 2018 (Zhong et al., 2018). The β-As (gray arsenic) thin film has also been developed in recent years. It has the same structure as blue phosphorus, which displays rhombohedral stacking of layers (Zhao et al., 2017). The wide bandgap (0-1.71ev) with adjustable layers are predicted in some theoretical articles (Kamal and Ezawa, 2015;Zhou et al., 2017). The higher carrier mobility in bulk gray arsenic has also been observed in few-layer arsenic (Hu et al., 2019). There are signs that β-As may become an excellent contender for a new generation of 2D nano-electronic, photonics devices. In addition, to obtain ultrashort pulse laser with a high signal-to-noise ratio and long-term stability, the quality of saturable absorber (SA) is a key component Hu et al., 2018;Wang et al., 2019c;Zhang M. et al., 2019). Researchers are looking for reliable ways to place the nanomaterials at the fiber core, such as a special platform with a small area. The scotch tape-assisted approach, light-induced deposition method, end-to-end self-assembly, and embedding the layered materials in transparent polymer were reported (Lee et al., 2016;Rusdi et al., 2016;Hu et al., 2017;Cuando-Espitia et al., 2019). However, nanoflake accurate transfer is still a huge challenge.
In this study, β-phase arsenic (β-As) bulk crystals were synthesized via the CVT method, and the morphology and structure were studied. With our PDMS-assisted accurate positioning dry transfer method, the gray arsenic nanoflake saturable absorber was prepared on the end cap of optical fiber with 100% yield. The g-As nanoflake-based ytterbiumdoped fiber laser can realize a stable mode-locked pulse with the central wavelength, repetition rate, and signal-to-noise ratio (SNR) of 1,037.3 nm, 0.6 MHz, and 67.7 dB, respectively.

SAMPLE PREPARATION AND CHARACTERIZATIONS
Growth Method β-As crystalline bulks were synthesized by using the CVT method (Xu et al., 2020c) in a furnace with two temperature zones, as schematically shown in Figure 1A, where the red area indicates the high temperature zone and the green area indicates the low temperature zone. Growth processes of β-As crystal are as follows: gray arsenic powders were used as precursor. First of all, 15 mg gray arsenic powder was put into a quartz tube (length of 120 mm, diameter of 20 mm) and subsequently sealed up under vacuum (<1 × 10 −2 Pa). Afterward, the as-sealed quartz tube was placed in the furnace (OTF-1200X), and the temperature program of high temperature zone was set as a program curve: heating up to 500°C within 5 h, keeping at 500°C for 1 h, and then cooling down to room temperature in 5 h. Finally, the product was obtained at the low temperature zone.

Characterization Apparatus
The morphology of the samples was investigated by optical microscopy (Nikon Eclipse LV100ND microscope) and atomic force microscopy (AFM, Bruker Dimension Icon 3,100). Scanning electron microscopy (SEM) and the corresponding energy-dispersive X-ray spectroscopy (EDX) characterizations were carried out on a SEM (Quanta FEG 250) instrument, with an operating voltage of 30 kV and a spot line of 5.0. Raman measurements were performed in a confocal Raman spectrometer (Renishaw inVia), with an excitation laser of 532 nm wavelength and a ×100 objective lens. X-ray diffraction (XRD) was performed with a powder X-ray diffractometer (Bruker AXS D8 Advance) system with Cu Kα irradiation (λ 1.5406 Å). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were performed on a TEM (Tecnai G2 F20 S-TWIN) instrument.

Characterization and Analysis
As shown in the optical image ( Figure 1B), the β-As single crystal is about 8 mm size, and obvious geometric corners are found on it. Its luminal appearance indicates that thickness can be reduced by mechanical stripping. SEM with EDX measurements was conducted to observe the morphology and analyze the chemical element of the β-As crystals. Figure 1C shows a flake with regular edges and corners. The EDX mapping verified the uniform distribution of unique arsenic element, as shown in Figure 1D. The powder XRD was applied to judge the crystal structure and the phase purity of the as-synthesized β-As crystals. In Figure 1E, the peaks match with the rhombohedral structure in space group R3m (PDF # 05-0,632) (Hu et al., 2019). The diffraction peak at about 25.28°and 52.04°can be well-indexed to the (003) and (006) plane, respectively. What is more, the XRD pattern demonstrates that the β-As crystals present a highly preferred orientation along the (00L) direction. Figure 1F shows a typical exfoliated β-As nanoflake after it was transferred onto the SiO2/Si substrate. It can be seen that the surface is uniform without impurities. The corresponding atomic force microscopy (AFM) image is shown in FIGURE 3 | Deterministic transfer of β-As nanoflake on fiber end cap. (A) The protocol of deterministic transfer: step 1, β-As bulk crystal mechanical exfoliation with blue tape; step 2, β-As nanoflake adhere; step 3, β-As nanoflake lifted with PDMS; step 4, the selected β-As nanoflake alignment; step 5, PDMS peeled off; step 6, deterministic transferred sample of β-As nanoflake. (B) Microscope digital image of the fiber end cap (diameter of red circle is 9 μm) before transfer. (C) Microscope digital image of fiber end cap after deterministic transfer of β-As nanoflake.
Frontiers in Materials | www.frontiersin.org September 2021 | Volume 8 | Article 721587  Figure 1G. It has a regular angle of 120°, and its height is about 335 nm. It is relatively difficult to exfoliate the β-As bulk to very thin flakes, which may result from a strong interlayer interaction. Raman spectrum was carried out as fingerprint authentication of materials. As shown in Figure 1H, two characteristic peaks with frequencies of 195.3 and 254.5 cm −1 are corresponding to E g (in-plane vibration) and A 1g (out-ofplane vibration) modes of β-As, respectively.
In order to further discuss the crystal structure, we investigated β-As with a rhombohedral A7 structure (Zhu et al., 2015). The crystal is a double-layered structure composed of many interlocking six-membered rings. The crystal structure model of the β-As was observed in different views in Figures 2A,B. It is a kind of buckled 2D hexagonal structure, with a lattice constant of a 3.76 Å, c 10.44 Å, and As-As bond length of 2.5 Å. Afterward, the β-As nanoflakes were transferred to the Cu grid for transmission electron microscopy characterizations. Figure 2B illustrates a hexagonal β-As nanoflake with an angle of 120°. As shown in Figure 2C, the electron diffraction pattern presents a refined hexagonal structure, which is consistent with the XRD data (PDF # 05-0,632). Figure 2D shows the high-resolution TEM (HRTEM) characterization of the same β-As nanoflake with distinct lattice fringes without obvious impurities. The lattice fringes marked in the figure shows a fixed spacing, which is measured as 1.88 Å. These crystal planes can be indexed to (110)  Deterministic Transfer on Fiber End Face β-As nanoflake was accurately transferred onto a fiber facet as an SA by the homemade 3D transfer platform (Figure 3). The protocol schematic of deterministic transfer was shown in Figure 3A. The specific dry transfer process is described as follows: First of all, the β-As bulk was thinned by mechanical exfoliation with a blue tape. Homemade polydimethylsiloxane (PDMS) stamp was used to adhere thinned β-As nanoflakes. As the PDMS is transparent, the thickness and size of β-As nanoflake can be roughly determined through it. When a suitable flake has been identified, the underlying fiber end face is fixed on the sample stage. The PDMS with target nanoflake is then fixed to the three-axis cantilever with the flakes facing the fiber core. When the PDMS and the fiber core are close to the focal plane of the microscope, it is possible to align the right flake toward the fiber core. Finally, the stamp is pressed against the fiber end face and peeled off very slowly. As shown in Figure 3B, the fiber core (diameter: 9 μm) was marked by a red circle in the dark field optical microscope image. After transfer, the fiber core was completely covered with a β-As nanoflake ( Figure 3C). In this way, the resulting nanoflakes have no bulges or wrinkles.

PULSE GENERATION AND DISCUSSION
The sandwiched structure β-As-SA was installed into the laser ring cavity. As illustrated in Figure 4A, the total length of the ring cavity is about 351 m. A laser diode (LD) operating at 976 nm wavelength was used to pump a 1.5-m-long ytterbium-doped fiber (YDF) (6/125) through a 980/1,060 nm wavelength division multiplexing (WDM). In order to realize unidirectional waveguide of the laser, a polarization-independent isolator (PI-ISO) was connected with the YDF. The SA device of sandwiched structure was placed between the optical coupler (OC) and 300-m single-mode fiber (SMF) (HI1060). We adopted two polarization controllers (PC) to adjust the phase of the laser oscillation mode. Besides, we used an OC of 20:80 ratio which has a 1 × 2 pigtail structure. The 20% laser output was separated from the laser cavity to measure the laser characteristics by the oscilloscope or optical spectrum analyzer (OSA). The remaining 80% laser was coupled into the laser cavity to form laser oscillation.
By increasing the pump power to 330 mW and adjusting the PC appropriately, the mode-locked pulse phenomenon took place. The state was not very stable at the beginning of the startup, and the pulse splits slightly. However, it kept an obvious pulse interval under 330-450 mW, as shown in Figure 4B. The corresponding output spectrum is illustrated in Figure 4C, with a slight change in the central wavelength of around 1,037 nm ( Figure 4D). The output pulse trace is shown in Figure 4E, with a pulse interval of 1.7 μs under 400 mW. The corresponding radio frequency (RF) spectrum is as shown in Figure 4F. The signal-to-noise ratio (SNR) is about 58.5 dB, and the repetition rate is 0.6 MHz. In the 3-day sampling period, the pulse sequence and spectral output are basically stable.

CONCLUSION
In conclusion, high-quality β-As were successfully synthesized, and an ytterbium-doped fiber mode-locked laser based on β-As-SA has been realized for the first time. A relative stable 1-μm pulsed laser was generated at a pump power ranging from 330 to 450 mW with an almost unchanged repetition rate of 0.6 MHz and a central wavelength around 1,037 nm. This work demonstrates a 2D material small area deterministic transfer method and promotes the research of near-infrared pulsed lasers based on 2D materials and also shows the great potential of group VA crystals for nonlinear photonic applications.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material; further inquiries can be directed to the corresponding authors.

AUTHOR CONTRIBUTIONS
JW and KZ conceived the experiments and supervised the project. QY and CC synthesized and characterized the samples. QY and KG performed the optical experiments and did the data analysis. HD and YZ contributed to the sample exfoliation and transfer.
Frontiers in Materials | www.frontiersin.org September 2021 | Volume 8 | Article 721587 6 TY and WS contributed to the device design and schematic drawing. All the authors contributed to the conception and manuscript writing.