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Abstract
Few-layer bismuthene is an emerging two-dimensional material in the fields of physics, chemistry, and material science. However, its nonlinear optical property and the related photonics device have been seldom studied so far. Here, we demonstrate a sub-200 fs soliton mode-locked erbium-doped fiber laser (EDFL) using a microfiber-based bismuthene saturable absorber for the first time, to the best of our knowledge. The bismuthene nanosheets are synthesized by the sonochemical exfoliation method and transferred onto the taper region of a microfiber by the optical deposition method. Stable soliton pulses centered at 1561 nm with the shortest pulse duration of about 193 fs were obtained. Our findings unambiguously imply that apart from its fantastic electric and thermal properties, few-layer bismuthene may also possess attractive optoelectronic properties for nonlinear photonics, such as mode-lockers, Q-switchers, optical modulators and so on.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Soliton lasers, which can generate soliton pulses in the picoseconds or femtosecond range, have attracted tremendous interests, owing to their versatile applications in optical communication, optical sensing, biomedical research, and radar system [1–3]. So far, several actively/passively mode-locked techniques have been exploited for generating soliton pulses in the lasers. Compared with active schemes, passive schemes share more benefits, including compactness, simplicity and flexibility in design [4]. The key element in the passively mode-locked laser is a nonlinear optical element, called a saturable absorber (SA), which turns the continuous-wave output into a train of pulses. In 1966, De Maria and associates firstly demonstrated the ultrashort pulses (ps level) in a passively mode-locked Nd:glass laser with a SA [5]. Since then, researchers among the ultrafast photonics community never stopped to seek for new SAs. Driven by the strong demand, two-dimensional materials [6–18], including graphene, topological insulators (TIs), and transition metal dichalcogenides (TMDCs), have been developed to be efficient SAs for mode-locked/Q-switched lasers at the wavelength range of 0.6-3.5 μm due to their ultrafast carrier dynamics and broadband saturable absorption. Notably, the aforementioned 2D materials also have some weak points. For example, graphene has no band-gap and a weak modulation depth (~2.3%/layer), which restraint its applications in situations where strong light-matter interaction is required [19–21]. The fabrication process of the mostly explored TIs [22–25] and TMDCs [26–31] are relatively complicated. Thus, the exploration of novel SAs is still a long-standing goal.
Recently, Group VA monolayer materials include black/blue phosphorene, arsenene, antimonene, and bismuthene, have attracted huge interests due to their interesting properties such as topological nontrivial states, tunable band-gaps ranging from 0.36 to 2.6 eV and high carrier motilities as high as several thousands of cm2 V−1s−1 [32]. Since 2014, Black phosphorus (BP) emerges as a hot-spot material with layer-dependent band-gap, strong light-matter interaction, high carrier mobility, and anisotropic electronic transportation [33]. BP has been recently developed as SAs in pulsed lasers and ultrafast modulators in all-optical signal processing systems [34–43]. However, limited by its poor stability and ease oxidation under ambient condition, researchers started to explore other Group VA materials with similar properties but high reliability beyond phosphorene [44]. Antimonene, another group VA material, has been theoretically predicted with enhanced stability and remarkable nonlinear optical properties [45–48]. Accordingly, it presents high carrier mobility, excellent ultraviolet response, and high nonlinear refractive index [49–52]. Study found that, free-standing antimonene is indirect, its band-gap can be varied from indirect to direct under biaxial strain. Although antimonene is semiconducting and ultrastable in air, its indirect band-gap might give rise to low photoelectric response, which may delimit its application in ultrafast photonics. Therefore, it becomes urgent to develop a suitable and stable SA device for soliton lasers.
Bismuthene is a single-layer form of bulk bismuth (Bi) in group VA and has attracted huge interests owing to its unique electronic-transport, semi-metallic bonding, spin-orbit interaction, and enhanced long-term stability [53–56]. With a lattice structure similar as graphene, bismuthene can become a topological semimetal or semiconductor [57–61]. Study found that, bismuthene possesses a stable low buckled hexagonal structure and layer-dependent energy gap from almost zero to 0.55 eV [62], suggesting that it may be a promising broadband optical material ranging from terahertz, mid-infrared toward near-infrared regime. Lu et al. also revealed the saturable absorption behavior of few-layer bismuthene at the visible band [63]. However, exploration of bismuthene-based femtosecond-level soliton laser is still lack.
Here, we demonstrated a sub-200 fs soliton fiber laser with a bismuthene-deposited fiber taper as a mode-locker. The obtained results indicate that the bismuthene-based photonic device could be indeed a good candidate of nonlinear device for potential application fields, such as ultrafast nonlinear optics, biomedical research and radar system.
2. Synthesis, characterization, and nonlinear optical properties of few-layer bismuthene
Currently, the large-sized few-layer bismuthene used in our experiment were synthesized by the sonochemical exfoliation method, as illustrated in Fig. 1. Initially, bulk bismuth (99.999%, Aladdin) was grinded into bismuth powder and added into a bottle with ethanol solution. Subsequently, 0.3275 mL of bismuth ethanol solution and 9.6725 mL of ethanol were placed in a glass bottle and kept under ice-bath sonication and probe sonication for 15 h, respectively. Then, the resulting suspension was centrifuged for 20 min at 7000 rpm. Finally, we obtained the bismuthene dispersion, as depicted in Fig. 2(a).
Fig. 2 Sample and characterization of few-layer bismuthene: (a) Photograph of bismuthene dispersion, (b) Raman spectrum (Inset: bismuthene crystal structure), (c) AFM image, and (d) TEM image.
In this experiment, structure characterization of few-layer bismuthene is very important. Therefor, we firstly provide its Raman spectrum, as shown in Fig. 2(b). The disappearance of the Eg band at 70 cm−1 and A1g band at 97 cm−1 of bismuthene in this spectrum is attributed to the Rayleigh rejection filter cutoff at ~100 cm−1. The top side view structure of synthesized bismuthene is also illustrated in the inset of Fig. 2(b). The atomic force microscopy (AFM) image of the bismuthene presents that the nanoflake has a height of ~9 nm with an irregular profile, as shown in Fig. 2(c). It can be seen from Fig. 2(d) that, the lateral size of bismuthene is about 0.6 μm, as depicted in the transmission electron microscopy (TEM) image.
The procedure of the fabrication of microfiber-based bismuthene device as follows. Firstly, we fabricated a microfiber with a waist diameter of ~16 μm and taper length of ~1 mm by using the fused biconical taper method [29]. Then, the bismuthene nanosheets were perfectly transferred into the waist of microfiber by using optical deposition method. After preparing the microfiber-based SA, we could observe the existence of the evanescent field through injecting the visible light, as shown inset of Fig. 3.
Fig. 3 The nonlinear saturable absorption curve of the microfiber-based bismuthene device (Inset: its corresponding red light image).
Furthermore, we investigate the nonlinear optical property of the bismuthene-deposited fiber device by using the power-dependent transmission technique. Its experimental setup is similar to the previous report [6]. The nonlinear transmission measurement was carried out using a femtosecond laser source (central wavelength: 1550 nm, repetition rate: 50 MHz, pulse duration: 500 fs). By continuously adjusting the input power, we could record the optical transmittance under different input intensities, as shown in Fig. 3. The data for transmission are fitted according to a simple two-level model as follows:
whereandrepresents the transmission rate and the modulation depth, respectively. andis the input pulse energy and the saturation energy, respectively. is the nonsaturable loss.According to the Eq. (1), we can provide the nonlinear parameters of the bismuthene device, such as the modulation depth of 5.6%, saturation intensity of 48.2 MW/cm2 and nonsaturable loss of 62.3% at 1550 nm, respectively. Correspondingly, the insertion loss of the microfiber-based bismuthene SA is about 5.5 dB. Notably, the saturation intensity of this bismuthene device is comparable to that of Bi2Te3 [22–24], but its nonsaturable loss is relatively larger, which may originated from the absorption and scattering of few-layer bismuthene and taper structure itself. Presumably, by further engineering the fabrication quality of microfiber and thickness and uniformity of bismuthene nanosheets, the smaller cavity loss could be obtained.
3. Experimental setup
To check the laser performance by using the prepared bismuthene device, it was inserted into a fiber ring laser cavity. The experimental setup of the proposed fiber laser is sketched in Fig. 4. The laser cavity consists of a piece of 4.3 m highly doped Erbium-doped fiber (Core active L900, EDF) with dispersion parameter of −16.3 ps/(km∙ nm) and peak absorption of 14.5 dB/m at 1530 nm and 19 m single mode fiber (SMF) with dispersion parameter of 18 ps/(km∙ nm). The total net cavity dispersion is ~-0.35 ps2. A fiber-pigtailed 976 nm laser diode (980-500-B-FA, LD) with maximum power of 350 mW via a fused 980/1550 wavelength-division multiplexer (WDM) is used to pump source and a 10:90 optical coupler (OC) is employed to extract the output of the laser beam, respectively. A polarization independent isolator (ISO) and a polarization controller (PC) were used to force the unidirectional operation of the ring cavity and adjust the polarization state of the propagation light, respectively. The pulse performance of the laser was monitored by a power meter, an optical spectrum analyzer (ANDO AQ-6317B) with spectral resolution of 0.01 nm, a photo-detector (Thorlabs PDA 12.5 GHz) combined with a 1 GHz mixed oscilloscope (Tektronix MDO4054-6, 5 GHz/s) and a commercial autocorrelator (APE, PulseCheck), respectively.
Fig. 4 Experimental setup of the proposed fiber laser. (Inset: the bismuthene-deposited microfiber device).
4. Results and discussions
Before carrying out this experiment, we measured the operation characteristics of the laser when no bismuthene device and the microfiber only inserted in the cavity, respectively. Whatever adjusting the pump power and polarization states of PCs, there is only continue wave (cw) lasing, which excludes the possibility of the nonlinear polarization rotation and microfiber mode-locking. Then, we inserted the bismuthene-assisted microfiber into a ring laser cavity, as sketched in Fig. 4. Initially, cw emitting started at an incident pump power of about 30 mW and self-started mode-locking operation occurred at about 50 mW. Clearly, the stable soliton pulses with Kelly sidebands can be achieved when the pump power increases from 100 mW to 350 mW, as shown in Fig. 5. The 3-dB bandwidth of soliton pulses varies from 3.35 to 14.6 nm, respectively.
Fig. 5 Typical optical spectra of soliton pulse at different pump power: (a) 100 mW, (b) 200 mW, (c) 300 mW, and (d) 350 mW.
Next, we provide the typical characteristics of soliton pulse under the pump power of 250 mW in our proposed laser. Figure 6(a) shows the optical spectrum of soliton pulse. Its center wavelength locates at 1561 nm with a 3-dB bandwidth of 14.4 nm. Clearly, there is a pair of Kelly sidebands distributed symmetrically on the spectrum, indicating that the state is in the soliton regime, according to the soliton theory [64]. Figure 6(b) shows the oscilloscopes trace of the soliton pulse with a time span of 2 μs. It can be seen that, there is a soliton pulse propagating in the cavity with a period of 113.4 ns, which corresponding to a fundamental frequency rate of 8.85 MHz. To confirm that the soliton pulse exists in our cavity, it needs to measure its pulse profile.Thereafter, the autocorrelation trace of the soliton pulse in the picosecond time scale was measured by using a commercial autocorrelator, as shown in the inset of Fig. 6(b). If a sech2 pulse profile is assumed for fitting, its duration is ~193 fs. The time-bandwidth product (TBP) of the soliton pulse can be calculated by the equation
where c, λc, and δλ represent the light speed, center wavelength, and 3-dB bandwidth of the mode-locked optical spectrum.Fig. 6 Typical characteristics of soliton pulse at 250 mW: (a) output optical spectrum, (b) its corresponding oscilloscope trace (inset: the autocorrelation trace) and (c) RF spectrum, and (d) The output power versus the pump power of the soliton pulse.
In this experiment, these parameters are τpulse = 193 fs, λc = 1561 nm, and δλ = 14.4 nm, respectively. Thus, the TBP of the pulses is about 0.342, which is slightly larger than the theoretical transform limit value (0.315) and indicates that the soliton pulse is nearly perfect.
To investigate the operation stability of the soliton pulse, we also measured its radio frequency (RF) spectrum. Its fundamental peak located at the repetition rate of 8.85 MHz with a signal-to-noise ratio of ~55 dB, as shown in Fig. 6(c), indicating that the stable soliton pulse operation was achieved. Figure 6(d) shows the average output power of the fiber laser as a function of pump power. When the pump power is 350 mW, the laser maximum average power of 5.6 mW is obtained, which limited by the available pump power up to 350 mW. The laser exhibits a slope efficiency of 1.7%. To evaluate the long-term stability of the soliton laser, we continuously monitor the evolution of optical spectra every 1-hour over 8 hours, as shown in Fig. 7. Notably, their central spectral peak locations, spectral bandwidth, spectral intensity remained reasonably stable over the time period, indicating the good stability of the soliton laser. Different from the previous reports [6, 26], we did not observe the multiple-pulse state in the cavity with the pump power of up to 350 mW, which may be caused by the larger insert loss of bismuthene-deposited fiber device.
Table 1 summarized the soliton mode-locked EDFL performance of this work with those reported based on the BP, antimonene, and bismuthene SAs. It was found that the soliton mode-locked EDFLs based on BP SA had the highest repetition rate (60.5 MHz), the minimum pulsewidth (242 fs), and the average output power was 80 mW. The antimonene-based SA achieved the soliton mode-locking with a repetition rate of 10.27 MHz and pulse width of 552 fs, respectively. Clearly, in the reported soliton mode-locked EDFLs with group VA 2D materials-based SAs, we achieved minimum pulse duration of 193 fs. Compared with the previous work [63], the pulse duration of our laser was decreased by 3 times, and the output power was increased by 5 times, respectively. We believed that, the performance of soliton laser can be improved by further optimizing the SA parameters (such as fiber taper waist, bismuthene thickness) and the cavity parameters (such as cavity length, polarization states).
Table 1. Summary of Soliton Mode-locked EDFLs with Group VA 2D Materials-Based SAs
Similar to other 2D materials such as graphene [6–9], TIs [22], TMDCs [26–31], and BP [34–41], the saturable absorption process of photons in the bismuthene can be explained by Pauli blocking, as illustrated in Fig. 8. As a new kind of 2D materials, bismuthene exhibits a perfect energy-band structure of Dirac-cone type [62]. Physically, any electron can be excited into the conduction band when the intensity of incident light is larger than the bandgap of bismuthene. Then, the distribution rapidly thermalizes and cools down to form a hot Fermi-Dirac distribution. Immediately, the newly created electron-hole pairs could block some of the originally possible interband optical transitions around the Fermi energy and decrease the absorption of photons. The hot carriers are then cooled further due to the intraband phonon scattering. Finally, electrons and holes recombine until the equilibrium distribution is restored. This describes the linear optical transition under low excitation intensity. However, as the light intensity increase to a higher level, the photocarriers increase instantaneously and fill the energy states near the edge of the conduction and valence band, the absorption is blocked, due to the Pauli blocking principle. Eventually, the photons at specific wavelength can transparently transmit the bismuthene without absorption. Notably, the direct bandgap range of few-layer bismuthene is from 0.36 to 0.99 eV [44]. Thus, it is very suitable for developing middle- and near-infrared photonics devices.
Fig. 8 Saturable absorption mechanism of bismuthene: (Left) Interband transition of the electron due to light excitation; (Middle) Hot carriers lend to thermal balance; (Right) Blocking of absorption for light.
5. Conclusions
In conclusion, by fabricating a bismuthene-deposited microfiber as a mode-locker, we have demonstrated a sub-200 fs soliton mode-locked fiber laser with a 3-dB bandwidth of 14.4 nm. The soliton laser has a pump threshold of 50 mW at 976 nm, pulse width of about 193fs, repetition rate of 8.85 MHz and average output power of 5.6 mW, respectively. These are the shortest soliton pulses so far generated from a mode-locked laser based on the group VA 2D materials SAs. This work clearly evidences that few-layer bismuthene possesses the desired optical properties for laser photonics and can be considered as another 2D material, paving the way for bismuthene-based nonlinear photonics.
Funding
Equipment Pre-Research Field Foundation (6140414040116CB01012); National Natural Science Foundation (NSFC) (61575051); 111 Project of Harbin Engineering University (B13015).
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