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Abstract
A black arsenic-phosphorus saturable absorber (SA) was fabricated and experimentally applied for a passively Q-switched Er-doped fiber laser for the first time to the best of our knowledge. The high-quality black arsenic-phosphorus crystals were synthesized by the mineralizer-assisted chemical vapor transport (CVT) method, and the fewer-layer black arsenic-phosphorus thin film nanosheet SA was performed by a unique electrochemical delamination exfoliation procedure. Meanwhile, a stable passively Q-switched pulse was proposed based on the SA modified Er-doped ring fiber cavity, which had the repetition rate of 38.47 kHz, corresponding to a minimum pulse width of 5.26 µs, and a corresponding output power as high as 3.68 mW was obtained. The results suggest that black arsenic-phosphorus is a good choice to make practical two-dimensional saturable absorbers for potential ultrafast photonic applications due to its designable bandgap value and excellent optical characteristics.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The saturable absorber (SA) is a key component to modulate intracavity loss and generate ultrashort pulse in ultrafast laser systems. In the past decade, in order to satisfy the requirement of pulsed lasers generation, two-dimensional (2D) materials SAs, with the advantages of unique layered structures, easy manufacture, designable band gap and fast recovery time, have been rapidly developed [1]. In particular, typical 2D materials represented by graphene [2,3], hexagonal boron nitride (h-BN) [4], transition metal dichalcogenides (TMDCs) [5], topological insulators (TIs) [6], black phosphorus (b-P) [7] and MXenes [8] have practically promoted the development of producing better parameters of laser pulses due to their excellent properties. However, to further improve the properties of ultrashort laser, finding an efficient 2D material based SA with both designable bandgap and high stability in the mid-wave infrared range is still a great challenge [8,9].
Graphene has a zero band gap, leading to weak photon absorption. Analogously, TMDCs have a relatively large band gap (for example, 1.8 eV for MoS2 and 2.0 eV for WS2), so that the optical response mainly occurs in the visible range. So far, as a rising star of post-graphene 2D material, black phosphorus (b-P) based SAs have pushed the bandgap of 2D materials to the mid-wave infrared range (0.3 eV of bulk b-P) [10,11]. The b-P based SAs filled the gap between graphene and TMDCs [12] and induced pulse fiber laser in near-infrared range [13–19]. There is still a broad spectral range of the long-wave infrared over 4 µm that has not been covered by recently reported 2D materials. Therefore, 2D materials with more narrow band gaps should be suitable for the long-wave infrared range. Interestingly, a family of 2D material called black arsenic-phosphorus (b-AsP) have attracted more and more attention [20–26]. The b-AsP have the same orthorhombic crystal structure as b-P with P atoms replaced by As atoms and synthesized through a novel mineralizer-assisted chemical vapor transport (CVT) method similar to the growth of b-P crystals. The b-AsP materials can have a well designable chemical composition of As atoms (b-AsxP1-x, x from 0 to 0.83), which will cause a continuously adjustable band gap changing from 0.3 eV (pristine b-P) down to around 0.15 eV (b-As0.83P0.17) and push the infrared response range cover from 4 µm to 8 µm. Significantly, fewer-layer b-AsP possesses a band gap covering the main emission range of fiber pulse lasers and the resent researches focused on b-AsP were mainly for photodetection, which had not been applied to fiber lasers.
In this paper, we demonstrated a passively Q-switched Er-doped fiber laser with low pump power at 1.5 µm based on an innovative b-As0.83P0.17 SA. The high quality b-As0.83P0.17 crystals were synthesised by the mineralizer-assisted chemical vapor transport (CVT) method, and the few-layer nanoflakes of b-As0.83P0.17 were performed by a unique electrochemical delamination exfoliation procedure. To the best of our knowledge, it’s the first time to generate pulse laser based on the black arsenic-phosphorus SA. The output pulse had the maximum output power of 3.68 mW with a slope efficiency of ∼9.2%. The repetition rate could be varied from 25.99 kHz to 38.47 kHz, corresponding the pulse duration tuned from 9.18 µs to 5.26 µs. These experimental results revealed the superiority of b-AsP SA in ultrafast photonics due to its designable more narrow band gap and excellent optical characteristics, and should be suitable for the long-wave infrared range ultrafast photonical applications.
2. Sample preparation and characterization of the b-As0.83P0.17 saturable absorber
2.1 Crystal growth
The high-quality b-AsP crystals were fabricated by mineralizer-assisted chemical vapor transport (CVT) method, similar to b-P [27,28]. For the synthesis of b-AsP crystals, stoichiometric red P (chempur 99.999%), grey As (chempur 99.999%), Sn (20 mg, chempur 99.999%) and SnI4 (10 mg, chempur 99.999%) with total weight of 600 mg were sealed in evacuated silica glass tubes (length: 200 mm; inner diameter: 13 mm) under 10−3 Pa before the crystal growth. The tubes were placed horizontally in the heating zone of a quartz tube furnace. First, the furnace was heated to 750 °C at a rate of 5 °C/min. After holding at 750 °C for 2 hours, the furnace was cooled to 500 °C within 7.5 hours and held for another 3 hours at this temperature. After that, a further slow cooling process to 150 °C in 8 hours was carried out before finally cooling down to room temperature. The b-AsP crystals could be found at the cold end of the silica glass tubes. The arsenic composition obtained by this CVT method can be distributed from 0 to 0.83 and a typical sample with x ≈ 0.83 with a narrow band gap of around 0.15 eV was used in this paper. As shown in Fig. 1, this kind of layered b-As0.83P0.17 material, similar to b-P, has an orthorhombic crystal structure (A17 Type) with a pleated honeycomb lattice, resulting in strong in-plane covalent chemical bonds and weak interlayer van der Waals interactions.
2.2 Preparation of b-As0.83P0.17 SAs
The b-As0.83P0.17 flakes were performed by an electrochemical delamination exfoliation procedure [29]. Firstly, the as-grown b-As0.83P0.17 crystals were taken out from silica glass tubes and soaked in acetone for two days to remove SnI4. For the exfoliation process, b-As0.83P0.17 crystals were performed as cathode and a piece of platinum foil as anode in a two-electrode system, respectively. A piece of b-As0.83P0.17 crystal of weight around 200 mg was fixed at cathode with copper wires to avoid falling off during the exfoliation process. The electrolyte was made by dissolving tetra-n-butyl-ammonium bisulfate (TBA·HSO4) in anhydrous, deoxygenated propylene carbonate (PC) with a concentration of 0.1M. Then, the electrodes were set in parallel with a distance of 1-2 cm. An adjustable potential according to crystal size (-10 V ∼ -30 V) was applied to start the exfoliation. The whole process were kept in a glove box to avoid oxidation. After complete delamination, as-exfoliated b-As0.83P0.17 flakes with the electrolyte were transferred into a glass bottle sealed with Parafilm. In order to disperse b-As0.83P0.17 flakes evenly, slight bath sonication of 10 min should be necessary.
As shown in Fig. 2, the suspension of b-As0.83P0.17 flakes was directly filtered on an organic membrane (pore diameter 0.45 µm) and the optical photograph and SEM image of the film were depicted in Fig. 2(b) and (c), respectively. The obtained b-As0.83P0.17 thin film was then transferred onto the end face of a fiber adapter to form SA devices.
Fig. 2. (a,b) Photographs of b-As0.83P0.17 membrane fabricated by filtering. (c) The SEM image of the membrane.
2.3 Apparatus and characterization
For further characterization, some treatments were needed. The as-exfoliated b-As0.83P0.17 was centrifuged at 8000 rpm for 5 min and washed by anhydrous propylene carbonate and Deionization (DI) water twice. Afterwards, a moderate amount of as-washed BP flakes was dispersed into DI water by mild sonication for 15 min. At last, the supernatant was decanted for further fabrication and characterization. The morphology and elemental maps of b-As0.83P0.17 flakes were investigated by using scanning electron microscope (SEM, Quanta FEG 250). Transmission electron microscopy (TEM) image and SAED pattern were obtained from a Tecnai G2 F20 S-TWIN (field-emission) scanning transmission electron microscope operating at 200 kV. AFM characterization was carried out with Dimension 3100 system. To analysis element compositions of the samples, a SPECS NAP-XPS X-ray photoelectron spectroscopy (XPS) was used with a basic chamber pressure of UHV-25 mbar and an Al anode as the X-ray source. Both survey and high-resolution spectra were recorded using a beam diameter of 100 µm. Raman measurements were performed in a LABRAM HR Evolution and the excitation was provided by visible laser light (λ=532 nm) through a 100×objective. X-ray diffraction (XRD) was performed with Bruker AXS X-ray scattering systems with Ni-filtered Cu Kα radiation (1.5406 Å).
Since b-As0.83P0.17 has a same crystal structure but larger lattice constant than b-P (5.3 Å), it could also be exfoliated in liquid phase. As depicted in Fig. 3(a), during the liquid-phase electrochemical exfoliation procedure, TBA+ cations have smaller diameters (4.7 Å), leading to efficient intercalation in b-As0.83P0.17 layers happen with an external current flow. Then, the reduction of solvated protons (H+) which come from the dissociation of bisulfate anions (HSO4-) will produce hydrogen gas and make spacing between layers further increase. Finally, b-As0.83P0.17 flakes dispersed in PC solution could be obtained. Different potentials applied in exfoliation procedure will result in different reaction speeds. Particularly, when applying a potential of -20.0 V, the crystal volume expansion, together with small bubbles and solution color gradually turning brown, could be observed only 5 min in the solution of TBA·HSO4 dissolved in PC. Scanning electron microscopy (SEM, Fig. 3(b)) were used to explain the exfoliation process. Figure 3 (b) shows a typical layered crystal structure of b-As0.83P0.17 with well-defined layers before exfoliation. The structure deformed as cations intercalation and hydrogen generation, causing the increase of interlayer distance and finally dividing bulk into large amounts of flakes.
Fig. 3. (a) Schematic illustration of the liquid-phase electrochemical exfoliation procedure. (b) The typical structures of b-As0.83P0.17 crystals before exfoliation and after exfoliation. (c) The TEM micrograph of a b-As0.83P0.17 flake. (d) High-resolution TEM (HRTEM) image of the as-exfoliated b-As0.83P0.17 flake. (e) The corresponding selected area electron diffraction (SAED) pattern of the b-As0.83P0.17 flake.
The crystal structure of b-As0.83P0.17 flakes was expounded by transmission electron microscopy (TEM), as shown in Fig. 3(c-e). Figure 3(c) displays a typical TEM image of a thin b-As0.83P0.17 flake, showing pleated morphology and irregular shape with lateral size of several micrometers. Figure 3(d) shows a high resolution TEM (HRTEM) image selected from Fig. 3(c), which illustrates the orthorhombic crystal structure with intact lattice. Clear lattice fringes with the interplanar spacing of 3.3 Å (zigzag, [100] direction) and 4.7 Å (armchair, [001] direction) could be measured. The interplanar spacing of armchair-plane in b-As0.83P0.17 is larger than in b-P of 4.4 Å, indicating that the addition of As atoms could result in an increase in lattice constant. As shown in Fig. 3(e), the sharp and intense diffraction spots in the selected area electron diffraction pattern (SAED) also can be well indexed to an orthorhombic structure. The (100), (001) and (111) facets from Fig. 3(e) correspond to lattice constants of 3.3 Å, 4.7 Å and 5.8 Å, respectively, also showing high crystalline quality.
Figure 4 (a) shows a representative b-As0.83P0.17 flake dispersed on a silicon substrate with 285 nm SiO2 after the electrochemical exfoliation and the blue color indicates that its thickness is less than 20 nm. The atomic force microscopy (AFM) photograph of the flake in Fig. 4(a) and associated height variations are depicted in Fig. 4(b). The thickness was measured to be 14 nm. SEM image and element mappings shown in Fig. 5. demonstrate the elemental composition of b-As0.83P0.17, indicating that As atoms and P atoms are evenly distributed in the material. Figure 4 (c) illustrates the infrared absorption spectra of b-As0.83P0.17. The result shows that this material has an absorption edge of around 1284 cm−1, corresponding to a band gap of 0.159 eV. This value is consistent with the previous report and reaches the long-wave range. Raman spectra and XRD spectra of b-As0.83P0.17 are displayed in Fig. 4(d) and (e), compared with pristine b-P. Raman spectra of b-P (Fig. 4(d)) shows three characteristic peaks located at 362, 438 and 466 cm−1, corresponding to Ag1, B2g and Ag2 vibration modes, respectively. While, in b-As0.83P0.17, the existence of different bonds including As-As, As-P and P-P would show more Raman peaks than pure b-P or b-As (black arsenic). The P-P peaks b-As0.83P0.17 are located in the region larger than 380 cm−1 and P-As peaks are displayed in the range from 280 to 380 cm−1. Characteristic As-As peaks also can be found in the region below 280 cm−1. As shown in Fig. 4. (e), XRD spectra displays the main characteristic peaks of high crystalline b-P and b-As0.83P0.17. The peaks of (020), (021), (040) and (060) in b-As0.83P0.17 shifting to a small angle indicate an increase in lattice constant which is consistent with TEM. In the high-resolution spectrum of XPS, shown in Fig. 4. (f), the band associated with P 2p and As 3p shows the key features of crystalline b-As0.83P0.17. Finally, the suspension were directly filtered on an organic membrane.
Fig. 4. Characteristics of b-As0.83P0.17 flakes synthesized by electrochemical exfoliation method. (a, b) An optical photograph and atomic force microscopy (AFM) image of a b-As0.83P0.17 flake on the Si substrate with 285 nm SiO2. (c) Plots of infrared absorption of b-As0.83P0.17 flakes with an optical band gap of 0.159 eV. (d, e) X-ray diffraction (XRD) pattern and Raman spectra of b-As0.83P0.17 flakes compared with pure BP crystals. (f) Representative X-ray Photoelectron Spectroscopy (XPS) spectra of b-As0.83P0.17 flakes.
Fig. 5. (a) SEM image of a b-As0.83P0.17 flake and its EDX elemental mapping of As L (b) and P K (c).
The saturable absorption properties of b-As0.83P0.17 thin film were also characterized. The transmittance under different incident light power intensity was recorded, as shown in Fig. 6. The laser source is a home-made mode-locked Er-doped fiber laser, which has the central wavelength of 1559.5 nm, repetition rate of 26.2 MHz and pulse width of 1 ps. The fitted modulation depth and saturated intensity were 1.7% and 27.8 MW/cm2, respectively.
Fig. 6. Nonlinear transmission of the b-As0.83P0.17 thin film under different incident power intensities.
3. Generation of the ultrashort fiber laser
Based on the aforementioned b-As0.83P0.17 SA, an all-fiber Er-doped fiber (EDF) ring cavity was employed to generate ultrafast pulses, as shown in Fig. 7. The laser cavity comprised a piece of 2 m long EDF as the gain medium which was pumped by a 980 nm laser diode (LD) with a pump power range of 0∼780 mW, through a 980/1550 nm wavelength division multiplexer (WDM). The output of the EDF was then connected to a polarization-insensitive isolator that not only ensured the one-way propagation of the generated signal laser but also induced a filtering effect. Meanwhile, in order to adjust the polarization state of circulating light, a polarization controller was used. A 10/90 optical coupler (OC) was adopted to extract the energy. The b-As0.83P0.17 SA was linked between the OC and WDM to establish the ring cavity. The total length of the cavity is approximately 11 m. The 10% output port of the OC was connected with a 20/80 coupler. Thus, the output spectrum and temporal signal could be measured simultaneously by an optical spectrum analyzer and oscilloscope.
Fig. 7. Schematic illustration of the Er-doped all-fiber ring cavity pulse laser based on b-As0.83P0.17 SA. LD: laser diode; WDM: wavelength division multiplexer; EDF: Er-doped fiber; ISO: polarization-independent isolator; PC: polarization controller; SA: saturable absorber; OSA: optical spectrum analyzer; PD: photodetector.
Based on the above experimental setup, the Q-switched pulse could be observed at the threshold pump power of 15 mW. But the Q-switched operation was unstable. When the pump power was further increased to 20 mW, the stable pulse was realized. As shown in Fig. 8(a), the output power increased with increasing the pump power. The maximum output power was 3.68 mW at the pump power of 40 mW. The slope efficiency of the laser cavity is ∼9.2%. As mentioned above, the b-As0.83P0.17 SA was transferred onto the end face of a fiber adaptor, then it was inserted into the cavity. This may introduce additional loss into the laser cavity. The higher output power or conversion efficiency can be expected if tapered fiber coated with b-As0.83P0.17 SA is adopted or the optical devices in the cavity are optimized further. As shown in Fig. 8(b), the output pulse had two peak wavelengths 1559.9 nm and 1560.3 nm with the 3 dB bandwidth of 0.04 nm and 0.03 nm, respectively. The intensity of the wavelength component at 1559.9 nm is about 10 dB lower than the one at 1560.3 nm.
Fig. 8. Properties of the output pulse: (a) output power at different pump power. (b) optical spectrum of the laser. (c) repetition rate and pulse duration versus the pump power. (d) pulse energy and peak power versus the pump power.
Pulse characteristics under different pump power were recorded in detail. Figure 8(c) shows the repetition rates and pulse durations as a function of pump power. When the pump power started at 20 mW and continued until 40 mW, the frequency rate increased linearly from 25.99 kHz to 38.47 kHz. Under the same condition, the pulse duration decreased gradually from 9.18 µs to 5.26 µs. The pulse energy and peak power as a function of pump power are illustrated in Fig. 8(d). The pulse energy increased from 49.25 nJ to 96.40 nJ and the peak power varied from 5.36 mW to 18.19 mW. After repeated experiments, the instability of Q-switch laser pulse is proved to be caused by the over-saturation of the SA at high incident intensity instead of material damage by pump laser.
Figure 9. shows the Q-switched pulse characteristics at the pump power of 25 mW. The output pulse train is presented in Fig. 9(a), and the adjacent pulse interval was measured as 34.27 µs, which is corresponding to the repetition rate of 29.2 kHz. The single pulse profile is shown in Fig. 9(b). The pulse duration was 7.01 µs fitted by a Gaussian function. The fundamental radio frequency (RF) spectrum is depicted in Fig. 9(c). The measured fundamental frequency was 29.2 kHz with the resolution bandwidth (RBW) of 40 Hz. And the signal-to-noise ratio is ∼27.73 dB, which indicated that the Q-switched pulse operation was quite stable. The RF spectrum in a large range is also presented in Fig. 9(d).
Fig. 9. Characteristics of the Q-switched pulse: (a) pulse train. (b) pulse duration. (c) radio frequency (RF) spectrum. (d) RF spectrum in a large range of 230 kHz.
In this experiment, the extended stability of the b-As0.83P0.17 based SA was also evaluated through investigating the condition of the Q-switched laser pulse at the continuous pump power between 20 mW and 40 mW for at least 10 hours. During this period, the passively Q-switched performance could always be obtained and maintained a good stability without any obvious pulse distortion, indicating that our SA had an excellent working state.
From the above, some interesting results had been presented based on a novel 2D material b-As0.83P0.17 as the SA. To further verify the Q-switched pulse induced by the b-As0.83P0.17, the SA was removed from the cavity. Under this condition, no pulse generation could be observed only continuous wave output when we increased the pump power from 0 to the maximum value and adjusted the polarization controller, which indicated that the b-As0.83P0.17 was responsible for the passively Q-switched operation. The saturable absorption is attributed to the pulse generation.
4. Conclusions
In conclusion, we demonstrated a passively Q-switched pulse laser in the Er-doped fiber laser with a new 2D material b-As0.83P0.17 as the SA. Meanwhile, we also introduced the synthesis and characteristics of b-As0.83P0.17 in detail. To the best of our knowledge, it’s the first time to apply b-As0.83P0.17 SA in the pulse laser field. The maximum output power was 3.68 mW with a slope efficiency of ∼9.2%. The repetition rate varied from 25.99 kHz to 38.47 kHz and the pulse duration could be tuned from 9.18 µs to 5.26 µs. Our work proved that b-As0.83P0.17 is a promising candidate in the ultrafast fiber laser. We believe that b-As0.83P0.17 could serve as a broadband SA and should be used for the long-wave infrared range ultrafast photonic application.
Funding
National Natural Science Foundation of China (NSFC) (11574349, 61875223); Natural Science Foundation of Jiangsu Province (BK20170424); Key Research Program of Frontier Sciences of Chinese Academy of Sciences (QYZDB-SSW-SLH031); Natural Science Foundation of Hunan Province (2018JJ3610); Chinese Academy of Sciences (CAS) (Hundred Talent Program).
Acknowledgments
The authors would like to thank the support from the Vacuum Interconnected Nanotech Workstation of Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences.
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