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Abstract
As a new member of two-dimensional (2D) phosphorene, 2D layered violet phosphorus (VP) has unique optoelectronic properties and good environmental stability, showing its huge advantages in optoelectronic applications. In this paper, the ultrafast nonlinear optical (NLO) properties of layered VP nanosheets at 1 µm band were explored, which exhibit an obvious saturable absorption response with a modulation depth of ∼1.97%. Meanwhile, the fast and slow carrier lifetimes of VP nanosheets at 1µm band were also determined as 295.9 fs and 2.36 ps, respectively, which are much shorter than that of most reported 2D materials. The excellent saturable absorption response combined with ultrashort carrier lifetimes indicate the prospect of layered VP nanosheets as a fast saturable absorber (SA) for ultrafast laser modulation. Then we demonstrated a Yb-doped fiber laser based on the VP-deposited taper-shaped fiber (TSF) SA, which delivers stable Q-switched mode-locked (QSML) pulses, dual-wavelength mode-locked pulses and 404-fs noise-like pulses. This work fully demonstrates the great potential of 2D VP materials for 1 µm ultrashort laser pulse generation.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Conventional soliton mode-locked fiber lasers operating in the anomalous dispersion regime are limited by soliton area theory, resulting in their low output single-pulse energy of less than ∼0.1 nJ [1]. In contrast, ultrashort soliton pulses operating in the passively mode-locked Yb-doped fiber laser with a normal dispersion regime usually possess high single-pulse energy due to the large chirp, which can satisfy the requirements of applications in industrial and scientific fields such as optical sensing, biomedical, nonlinear microscopy, and material processing [2–4]. Among them, noise-like pulse (NLP) generated in a normal dispersion cavity with a total intracavity dispersion value close to 0 has unique and excellent properties. Its temporal coherence is such low that it is difficult for the interfering signal to match the phase of the pulse, and thus it has a strong anti-interference capability [5,6]. Combined with its advantages of high single-pulse energy, narrow pulse width, and broad spectrum, it has a wide range of applications in the fields of optical coherence tomography, optical communications, optical metrology, and optical sensing [7–10]. Therefore, the generation and application of NLP is one of the hot topics of interest for researchers today.
Currently, NLP is mainly obtained by the nonlinear amplified loop mirror (NALM) or nonlinear polarization rotation (NPR) mode-locking techniques. There are few reports about the generation of NLP through two-dimensional (2D) saturable absorbers (SAs). Compared to NALM and NPR techniques, the method of mode-locked operation via real materials SAs has the advantages of facile cavity, ultrafast response, and easy self-starting [11]. The research on 2D nanomaterials SAs has become increasingly popular in recent years. Among them, the class of 2D phosphorene has been emphasized after graphene due to its direct bandgap and high mobility [12]. Recently a novel 2D anisotropic material of phosphorus, layered violet phosphorus (VP) nanosheet is successfully prepared and shows great potential in terms of optoelectronic properties and applications [13–17]. For example, 2D VP possesses good surface electronic properties [18], distinct p-type transport characteristics with a high Ion/Ioff ratio of 104 [14], and high carrier mobility (∼7000 cm2 V−1 s−1), indicating it a very promising optoelectronic material [19]. Meanwhile, the bandgap of VP can be effectively tuned by changing the thickness and applying external electric fields and stresses [18,20,21], providing a feasible way to design VP-based optoelectronic materials with precise bandgaps. In nonlinear optics, VP has a strong trionic effect with a high trionic binding energy of ∼109 meV at room temperature (higher than that of other common 2D materials), confirming that it is a superior system for investigating the interaction between light and matter [17]. Additionally, 2D VP features excellent photoelectric anisotropy, with an in-plane anisotropic photocurrent ratio of 11.5 (surpassing most reported 2D anisotropic materials), proving that it is an ideal platform for developing anisotropic multifunctional optoelectronic devices [22]. It is well known that the preparation and application of black phosphorus (BP) need to be carried out under protected conditions. In contrast, the environmental requirements for the VP are not harsh, and the thermal decomposition temperature is 52 degrees Celsius higher than that of BP [13]. Therefore, the VP is the most stable state of phosphorus and can maintain long-term stable work in air conditions. Additionally, as far as the field of laser applications, the VP has demonstrated good saturable absorption properties at 1.5 µm band and has been used as the SA in an all-fiber erbium-doped fiber laser (EDFL), which generated various mode-locked solitons such as conventional soliton, bright-dark soliton pairs and bound-state soliton molecule [23–25]. However, the nonlinear optical (NLO) characteristics of 2D VP materials and its ultrashort laser pulse modulation performance at 1 µm band are still unexplored, which has raised our concern.
In this paper, layered VP nanosheets were prepared by liquid-phase exfoliation (LPE) method, and their ultrafast NLO properties were explored at 1 µm band. The experimental results show good saturable absorption characteristics of VP nanosheets with a modulation depth of 1.97% and a saturation intensity of 11.7 MW/cm2. The ultrafast dynamics of photoexcited carriers in VP nanosheets were also investigated by the femtosecond transient absorption spectroscopy (fs-TAS) technique. The fitted fast and slow carrier relaxation lifetimes are 295.9 fs and 2.36 ps, which are much shorter than that of most reported 2D materials. These results demonstrate the excellent ultrafast NLO absorption response of the VP. As an application, we built an all-fiber ring-cavity ytterbium-doped laser using a VP-integrated TSF device as the SA. Stable QSML pulses, dual-wavelength mode-locked pulses, and NLP were obtained, in which the pulse width and the maximum pulse energy of the NLP are 404 fs and 7.905 nJ, respectively. This work demonstrates that 2D VP holds great promise as an ideal fast SA for the generation of 1µm ultrashort laser pulses with high single-pulse energy.
2. Preparation and characterization
Few-layered VP nanosheets were prepared by the LPE method. Firstly, 45 mg of VP crystals were taken and ground with a glass mortar to obtain the powder. Secondly, 20 ml of anhydrous alcohol was added to dissolve it initially as a suspension, and then it was sonicated for 12 h with a power of 100 W in a water-bath sonicator. Finally, the processed solution was centrifuged at 5000 rpm/min for 15 minutes to obtain the supernatant and a large amount of precipitate. The supernatant contained few-layered VP nanosheets.
The prepared 2D VP nanosheets were characterized to determine their morphology, structure, and optical properties. As displayed in Fig. 1(a), the transmission electron microscopy (TEM) image indicates that VP nanosheets have a 2D layered structure with regular edge shapes. The selected-area electron diffraction (SAED) image in Fig. 1(b) demonstrates a typical monocrystalline structure of VP nanosheets with two sets of crystal planes intersecting at an angle of 90° and a face spacing of 6.4 Å. The atomic force microscopy (AFM) was also carried out to determine the longitudinal thickness and the layer number of as-prepared VP nanosheets. The AFM image in Fig. 1(c) and the corresponding height distribution graph in Fig. 1(d) show that the thickness is ∼2.1 nm. According to the layer spacing of ∼1.1 nm derived from the previous reports [13], the layer number of VP nanosheets can be deduced to 1∼2. The Raman spectrum of VP nanosheets excited with a 532 nm laser is shown in Fig. 1(e). The weaker dielectric vibrational signals of P-P bonds were observed at 135.3, 173.6, 200.8, 242.4, and 267.6 cm-1, which originate from van der Waals forces, the integral rotation of atoms, and the bond angle distortion [14]. Among them, the more pronounced Raman peaks located at 200.8 and 267.6 cm-1 were due to the squeezing vibrational modes of phosphorus cages and the asymmetric radial breathing mode. Strong dielectric vibrational signals of P-P bonds were observed at 348.2, 366.0, 445.7, and 465.1 cm-1, which originate from the localized bond stretching and bending [14]. The monoclinic crystal nature of VP nanosheets can be deduced from the Raman spectrum. The linear optical absorption properties of few-layered VP nanosheets were characterized by an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectroscopy. The prepared VP nanosheets exhibit broadband linear optical absorption in a wide band from the UV to NIR as illustrated in Fig. 1(f). The step at 800 nm is caused by the device switching the excitation light source during the measurement.
Fig. 1. Characterization of the as-prepared VP nanosheets: (a) TEM image; (b) SAED pattern; (c) AFM image; (d) height profile corresponding to the line in panel (c); (e) Raman spectrum; (f) optical absorption spectrum from the UV to NIR band.
3. Results and discussion
3.1 Nonlinear saturable absorption properties
To measure the NLO absorption properties of layered VP nanosheets, we deposited them on the surface of a taper-shaped fiber (TSF) to construct a facile fiber device. The diameter at the tapered waist of the TSF was 22 µm, and the length of the tapered region was 3 cm. The clean TSF has a loss rate of 40% to ensure good coupling between material and laser under the evanescent field effect. The alcohol supernatant containing VP nanosheets was dropped onto the surface of the TSF (fixed on the quartz substrate) by a fine syringe. At the same time, a 976 nm continuous-wave (CW) laser with an output power of 150 mW was utilized to obtain an evanescent field effect, which causes the rapid evaporation of the alcohol solvent by the photothermal effect and allows the VP nanosheets to be rapidly deposited. After the drop-evaporation operation was repeated several times, the VP-TSF SA can be successfully fabricated with an insertion loss rate of 59%, demonstrating the successful deposition of VP nanosheets. The VP-integrated TSF (VP-TSF) device can be used for NLO measurements and laser experiments. Additionally, the thermal stability of the VP-TSF device was evaluated using a 1060 nm continuous wave laser with a maximum output power of 1.6 W. No damage occurred to the VP-TSF device, indicating that its optical damage threshold was higher than 1.6 W.
The NLO measurements of the VP-TSF device were performed by a homemade dual-balanced detection fiber system, as shown in Fig. 2(a). A nonlinear polarization rotating (NPR) mode-locked fiber laser amplified by an ytterbium-doped fiber (YDF) amplifier was used as the light source. The source had a center wavelength of 1030 nm, a pulse width of 243 fs, and a repetition frequency of 6.959 MHz. The incident laser intensity was modulated by a variable optical attenuator (VOA). A 1:4 output coupler was used to beam split the laser, with the port with a ratio of 20% serving as the reference value and the port with a ratio of 80% interacting with the VP-TSF. The NLO transmission curve of VP-TSF was recorded under laser beam excitation. As shown in Fig. 2(b), the VP nanosheets have a significant saturable absorption response. The recorded experimental data can be fitted by Eq. (1):
The modulation depth ΔT, saturation intensity Is, and non-saturable loss Tns of VP-TSF at 1 µm band were estimated to be ∼1.97%, ∼11.7 MW/cm2, and ∼57.9%, respectively. The good saturable absorption characteristics and low saturation intensity indicate the promise of the VP-TSF device as the SA for applications in pulsed laser modulation.
Fig. 2. (a) Schematic illustration of the homemade dual-balance detection fiber system; (b) NLO transmittance of VP-TSF SA with increasing laser intensity.
3.2 Ultrafast carrier dynamics
In addition to good saturable absorption properties, the generation of ultrashort laser pulses with femtosecond pulse widths puts demands on the response speed of the SA. Therefore, we further explored the ultrafast dynamics of photoexcited carriers in as-prepared VP nanosheets by the fs-TAS technique. A 400-nm pulsed laser was used as the pump light and the wavelength of the probe light is 1030 nm. ΔA is defined as the absorption difference of the probe light in the sample with and without pump light excitation. By adjusting the delay time between the probe light and the pump light, a time-dependent absorption difference ΔA can be obtained.
Figure 3 records the variation of ΔA signal with delay times in VP nanosheets after the pump light excitation. With the increasing delay time, the measured ΔA signal shows an exponential downward trend until leveling off, which indicates the interband recombination process of photogenerated carriers. The dynamic curve of ΔA signal was further fitted by Eq. (2):
where A1 and A2 are constants, and t is the delay time. The time constant τ1 represents the cooling time of hot electrons in the conduction band, also known as the fast relaxation lifetime. Meanwhile, the time constant τ2 is the slow relaxation lifetime during interband recombination of carriers. The fast and slow relaxation lifetimes of VP nanosheets are fitted as 295.9 fs and 2.36 ps, respectively, which are much shorter than those of other common 2D nanomaterials such as graphene/CHP (fast: 0.2-0.4 ps, slow: 1-11 ps), transition metal dichalcogenides (fast: 1-3 ps, slow: 70-400 ps), topological insulators (fast: 0.3-2 ps, slow: 3-23 ps), black phosphorene (fast: 0.36 ps, slow: 1.36-5.96 ps), and bismuthene (fast: 3 ps, slow: 420 ps) [11,12]. The ultrashort carrier lifetime reveals the ultrafast optical response of VP, demonstrating its huge potential to be used as a fast SA for generating 1 µm femtosecond laser pulses.3.3 Ultrafast mode-locked fiber lasers
The schematic diagram of the all-fiber ytterbium-doped fiber laser (YDFL) based on the VP-TSF SA is shown in Fig. 4. A 976 nm laser diode (LD) was used as the pump source and injected into the resonator through a 980/1030 wavelength division multiplexer (WDM). Using the stress birefringence effect, the laser was vibrated by a three-paddle polarization controller (PC) that regulates the intracavity polarization state. A polarization-independent isolator (PI-ISO) was utilized to ensure unidirectional operation within the optical resonator to avoid back reflection of the signal and burnt-hole effect. The output coupler (OC) inside the resonator splits the laser beam into two parts, 10% for output and 90% back into the cavity. A 1 m long ytterbium-doped fiber (SM-YSF-HI-HP, Corning Inc.) was used as the gain medium. All devices are connected by single-mode fibers (HI-1060, GVD = 22 ps2 /km, Corning Inc.). The total length of the resonator is 34.1 m. The net dispersion of the resonator is ∼0.69 ps2, ensuring that the resonator operates in the positive dispersion region. The laser pulse train was observed and recorded using a digital oscilloscope (DPO 7104C, Tektronix Inc., 1 GHz bandwidth, 2.5 GS/s sampling rate) with an InGaAs photodetector (ET-5000, EOT Inc., 3 GHz bandwidth). An optical spectrum analyzer (MS9740, Anritsu Inc., resolution of 0.07 nm) was used to measure the spectral data. A radio-frequency (RF) signal analyzer (FPC1000, ROHDE&SCHWARZ Inc.) and a second-harmonic-generation autocorrelator (Pulse check 150, APE GmbH) were used to detect the RF spectrum and pulse profile, respectively.
During the experiment, a TSF without deposited VP was first accessed into the resonator as SA, and the pulse sequence could not be obtained regardless of increasing the pump power and adjusting the PC. Subsequently, we accessed the VP-TSF into the resonator and obtained stable QSML pulses by adjusting the PC appropriately at the pump power of 272 mW. At this point, the output power of the QSML laser is 2.612 mW. The single pulse envelope and the typical pulse train with a repetition frequency of 46 kHz are displayed in Fig. 5(a). The optical spectrum of QSML pulses was also recorded as shown in Fig. 5(b), which features two spectral peaks of comparable intensity with central wavelengths of 1023.3 and 1036.5 nm, and 3 dB spectral widths of 1.1 and 1.4 nm, respectively. With such low pump power, the mode-locking operation could not be achieved by further regulating the PC.
Fig. 5. (a) QSML pulse train; (b) optical spectrum of QSML pulses; (c) transitional ML pulse train; (d) optical spectrum of transitional ML pulses; (e) NLP train; (f) optical spectrum of NLP.
Based on stable QSML operation, we were able to observe the evolution of QSML pulses into synchronized dual-wavelength mode-locked pulses by increasing the pump power to 330 mW and slightly adjusting the PC. The pulse train is shown in Fig. 5(c). The time interval between pulses is 150.9 ns, corresponding to a pulse repetition frequency of 6.58 MHz, which also corresponds to the optical resonator length of 34.1 m. Figure 5(d) measured the optical spectrum of mode-locked pulses, which still exhibit two spectral peaks with the central wavelengths of 1022.7 and 1036.5 nm, and the 3 dB pulse widths of 0.5 and 2.0 nm, respectively. This indicates that there are two longitudinal mode envelopes with different central frequencies oscillating synchronously in the mode-locking state in the resonator, which is in an equilibrium state of gain competition. This is therefore a typical synchronous dual-wavelength mode-locked operation.
Then we further increased the pump power to 360 mW and observed NLP output. The output average power is 4.147 mW. As illustrated in Fig. 5(e), the laser pulses are still transmitted at the fundamental frequency of 6.583 MHz, which is further proved by the measured RF spectrum in Fig. 6(a). From the inserted figure of the pulse sequence, it can be seen that the NLP mode-locking operation is very stable. From the optical spectrum of the NLP in Fig. 5(f), it can be found that the longitudinal mode envelope with lower center frequency wins the gain competition and the spectral peaks get broadened. The central wavelength is 1037.9 nm with a 3dB spectral width of 8.5 nm. On the other hand, the longitudinal mode envelope with a higher center frequency is unable to oscillate stably and gradually annihilates into a continuous noise signal. In the all-normal-dispersion regime, the formation of NLP is caused by the normal dispersion of the fiber together with the peak-power clamping effect caused by the saturable absorption properties of the VP-TSF SA [26]. In addition, the Raman effect may also have a non-negligible influence on promoting the formation of noise-like pulses in all-normal-dispersion fiber lasers [27]. Additionally, the VP-based SA also influences the inter-pulse interactions through pulse shaping and spectral filtering effects, which are crucial for the eventual formation and stable operation of noise-like pulses.
Fig. 6. (a) RF spectra; (b) autocorrelation trace of the laser pulse; (c) autocorrelation trace of the spike and its fitting curve (red); (d) optical spectra recorded for 8 h.
The detail output characteristics of the NLP were further measured as shown in Fig. 6. The RF spectrum was recorded with a resolution bandwidth (RBW) of 100 Hz, which shows the fundamental frequency of 6.583 MHz with a signal noise ratio (SNR) of 42.27 dB. The main reason for the low SNR is that NLP is essentially formed by clustering together many randomly evolving ultrashort pulses. There are time-domain intervals between ultrashort pulses, which results in an uneven distribution of energy in the signal, and finally causes the low SNR. Additionally, the high intracavity energy not only supports the stable operation of the NLP mode-locking but also supports the generation and transmission of some chaotic dispersive waves and continuous waves, which cause a strong background noise signal. The strong background noise may also result in a low SNR. The 1 GHz range RF spectrum measured with an RBW of 300 Hz demonstrates the good stability of the NLP mode-locking. For determining the pulse width of the NLP, we detected its intensity autocorrelation trace. As displayed in Fig. 6(b), a sharp pulse peak has a pedestal with steep edges, which is typical of NLP. The pedestal width is approximately 138.64 ps and the pedestal-to-peak intensity ratio is ∼0.7. A high pedestal also indicates the presence of a strong background signal, which is in keeping with the low SNR of NLP. The spike width fitted by Gauss fitting is estimated to be ∼404 fs as shown in Fig. 6(c), which represents the average pulse width of the small pulses within the NLP envelope. This femtosecond spike width is also much narrower than those of noise-like pulses based on other 2D nanomaterials saturable absorbers, such as chromium sulfide (Cr2S3) nanosheets, Bi2Te3 nanosheets, tellurene nanosheets, and WS2 nanosheets [4,28–30]. Figure 6(d) recorded the spectra over an 8h period. The spectral shape and amplitude do not show any change, demonstrating the excellent stability of the NLP output. In addition, the second validation data collection was conducted three months after the initial data collection, the results showed that VP-TSF SA could still perform the mode-locking experiments normally, demonstrating the long-term environmental stability and reliability of the VP material. By optimizing the preparation process, including the preparation of VP nanosheets and VP-TSF SA, it is possible to improve the laser modulation performance of VP-based SA and reduce the insertion loss, resulting in improved laser output performance such as lower mode-locking thresholds, higher signal-to-noise ratios, and average output power. For example, high-quality and uniform VP nanosheets can be obtained by adjusting parameters, such as sonication power, sonication time, and centrifugation speed, ensuring their excellent optical properties. On the other hand, the layered VP nanosheets can be distributed flatly and uniformly on the fiber surface by optimizing the parameters, such as solution concentration, pump power, and deposition time, to obtain high-efficiency coupling with the fiber laser. Additionally, the optimization of the laser cavity configuration also contributes to the improvement of the laser output performance.
The output average power of 1 µm YDFL based on the VP-TSF SA versus the pump power is summarized in Fig. 7. As the pump power increases, the laser output power increases almost linearly. At the same time, a transition from QSML operation to dual-wavelength mode-locking operation, and then to NLP mode-locking operation occurs. The thresholds of QSML, dual-wavelength mode-locking, and NLP mode-locking are ∼272, ∼330, and ∼360 mW, respectively. At the pump power of 416 mW, the NLP laser outputs an average power of 5.2 mW, which corresponds to a slope efficiency of 1.7% and a single-pulse energy of 7.905 nJ. Notably, at high pump powers, a free switch between QSML, dual-wavelength mode-locking, and NLP mode-locking operations can be achieved. This multi-state output of fiber lasers can expand many novel applications in various scenarios. However, when the power exceeds 500 mW, the NLP mode-locked operation becomes less stable. This is because too high pump power leads to an increase in the number of small pulses within the NLP envelope. Some waves that do not form small pulses are transmitted in the form of dispersive waves along with the NLP in the resonator. In this case, the dispersive wave causes drastic changes in the pulse width and peak power of the small pulses within the NLP envelope, as well as leading to a sharp decrease in the coherence between pulses and thus preventing the formation of a stable NLP mode-locking [31]. Additionally, to verify whether the laser can self-start, we kept the PC unchanged under the stable NLP mode-locked state and turned down the pump power to 0 mW. Subsequently, NLP can be reproduced again while the pump power increases to 358 mW, which demonstrates the good self-start property of VP-TSF-based YDFL.
4. Conclusion
In this paper, 1∼2-layered VP nanosheets were successfully prepared and their NLO saturable absorption properties at 1 µm band were experimentally investigated. An obvious saturable absorption response was observed with a fitted saturation intensity and a modulation depth of ∼11.7 MW/cm2 and ∼1.97%, respectively. The ultrafast dynamics of photoexcited carriers in VP nanosheets were also explored by the fs-TAS technique. The fast and slow carrier lifetimes were estimated to be 295.9 fs and 2.36 ps, respectively, which are much shorter than common 2D nanomaterials. These results indicate the capacity of VP nanosheets to act as a fast SA for the ultrashort laser pulse generation. Thus, a YDFL was demonstrated using the VP-TSF device as SA, which generated stable QSML pulses, dual-wavelength mode-locked pulses, and NLP. Among them, the NLP has a central wavelength of 1037.9 nm with a peak pulse width of 404 fs. This VP-based all-fiber YDFL with multiple types of output characteristics can meet the needs of multiple applications and will promote the development of ultrashort pulsed lasers. In conclusion, this work demonstrates the potential of the novel 2D VP nanomaterials for ultrafast photonics applications in the 1 µm band.
Funding
National Natural Science Foundation of China (12304466, 12174223, 12274263, 52072351); Natural Science Foundation of Shandong Province (ZR2022QF063); Postdoctoral Innovation Project of Shandong Province (SDCX-ZG-202201006); Qilu Young Scholar Program of Shandong University.
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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