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Abstract
Manganese oxide nanosheets (MnO2 NSs) with two-dimensional formation and typically ultrathin thickness have gained a great deal of attention due to their excellent physical and chemical properties. However, the potential capability of MnO2 NSs in laser application has been rarely explored. Here, we first report the MnO2 NSs as the saturable absorber (SA) for generating Q-switched pulsed laser. The MnO2 NSs–based SA not only shows a broadband absorption, but also possesses nonlinear saturable absorption feature. By integrating the MnO2 NSs-SA into an erbium-doped fiber laser cavity, a stable passively Q-switched operation at central wavelength of ∼1558 nm was realized with a threshold pump power of 220 mW, and 1.26 μs pulse width with a repetition rate of 92.35 kHz was obtained. Our results indicate that the MnO2 NSs can serve as promising candidates for constructing optical pulsed lasers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, growing interest has been focused on the saturable absorption properties of two-dimensional (2D) materials due to their broadband absorption, ultrafast carrier dynamics, and planar characteristic [1–3]. Hence, many 2D nanomaterials can be utilized as saturable absorbers (SAs) for mode-locking or Q-switching to produce pulsed laser. As a well-known 2D nanomaterial, graphene, first reported by Bao et al. in 2009, has been used as an excellent SA for realizing pulsed laser [4]. With the extensive application of graphene–based SAs, other graphene–like 2D materials also have been considered to be SA candidates for photonic applications. For instance, a 2D topological insulator as SA for ultra-short pulse generation was first reported by Zhao et al. in 2012 [5]. Also, in 2014, Wang et al. first demonstrated the pulsed laser operation by using layered transition metal dichalcogenide as SA [6]. Since then, many other 2D nanomaterials like black phosphorus have been developed as SAs for pulsed laser generation [7,8]. However, these 2D nanomaterials suffer from some intrinsic shortcomings. For example, graphene has low modulation depth [9], while topological insulators require complicated fabrication process [10]. Most layered transition metal dichalcogenides have a large bandgap and need complex tuning of defects, and black phosphorus is easy to be oxidized [11,12]. Therefore, it is necessary to explore a novel SA based on 2D nanomaterial for generating pulsed laser.
Ultrathin 2D layered MnO2 have attracted wide attention owing to its distinctive physical/chemical properties, including large specific surface areas [13], abundant active sites [14], ultrahigh mechanical flexibility [15,16], excellent electron transfer capacity and prominent light absorption [17]. Over the past few decades, considerable efforts have been made to construct 2D layered MnO2–based nanomaterials for a variety of applications, such as supercapacitors [18–20], batteries [21,22], catalysis [23–25], biosensing [26] as well as bioimaging and therapy [27–29]. Despite the widespread applications, to the best of our knowledge, there is still a lack of research on MnO2 nanosheets (MnO2 NSs) for optoelectronic devices, particularly nonlinear–based SA for generating pulsed laser.
In this paper, we first demonstrated the application of MnO2 NSs as a novel SA for constructing pulsed laser (Fig. 1). 2D ultrathin MnO2 NSs were synthesized by a bottom-up method. The MnO2 NSs possess nonlinear saturable absorption property in near-infrared (NIR) wavelength region. By sandwiching MnO2 NSs − based SA between two fiber connectors in an erbium-doped fiber laser (EDFL), stable Q-switched pulses at ∼1558 nm were successfully obtained. This work may open up new perspectives for the exploration of MnO2 NSs in nonlinear optical devices and photonic applications.
Fig. 1. Schematic illustration of passively Q-switched operation for pulsed laser generation using MnO2 NSs–based SA.
2. Preparation and characterization of MnO2 NSs and MnO2 NSs-based SA
In our experiment, the preparation process required two steps to form MnO2-based SA films. Firstly, the ultrathin MnO2 NSs were synthesized according to a previously reported approach [30]. Briefly, aqueous solution of sodium dodecyl sulfate (32 mL, 0.1 M) and H2SO4 (1.6 mL, 0.1 M) were dissolved in deionized water (283.2 mL) under vigorous stirring at 95°C for 15 min. Then KMnO4 solution (3.2 mL, 0.05 M) was added quickly into the mixture solution to start the redox reaction. After reaction for 60 min, a brown suspension of MnO2 NSs was obtained. The purified MnO2 NSs were obtained by centrifugation and wash with ultrapure water and alcohol and finally redispersed in water. In the second step, the as-prepared MnO2 NSs suspension and 1 wt% aqueous solution of carboxymethylcellulose sodium (NaCMC) were mixed by ultrasonication. Then, the stable mixed solution was kept for 48 h and without any precipitates. The MnO2–based SA films were prepared by resuspending the solution onto a flat substrate, followed by drying overnight at room temperature.
The morphology and nanostructure of resulting MnO2 NSs were investigated by transmission electron microscope (TEM), scanning electron microscopy (SEM), and high-resolution TEM (HRTEM). TEM image shows that the as-synthesized MnO2 NSs have a typically ultra-thin sheet-like morphology, with an average lateral dimension of about 200 nm [ Fig. 2(a)]. The HRTEM image along with the corresponding fast Fourier transform pattern [Fig. 2(b)] clearly confirm the high crystallinity of the MnO2 NSs. The lattice fringes with two characteristic lattice distances of 0.24 nm and 0.36 nm are in good agreement with the (100) and (002) crystal planes, respectively. The SEM images of the MnO2 NSs at different scales (1 μm and 100 nm) exhibit uniform morphology of flaky structure [Figs. 2(c) and 2(d)]. Abundant flat sheets of lamellae with lateral size of several hundred nanometers can be observed. The thickness of the MnO2 NSs discerned by atomic force microscopy (AFM) is shown in Fig. 2(e). The corresponding height profiles [Fig. 2(f)] show that the average thickness of the NSs is about 1.3 nm, confirming ultrathin 2D nanosheets with 1∼2 MnO2 monolayers. The chemical composition and elemental distribution of the resulting MnO2 NSs were analyzed by the energy dispersive X-ray spectroscopy (EDX) measurement. The EDX spectrum shows the existence of the elements (Mn and O) in the NSs, except for most of C and Cu signals from the carbon-supported Cu grid [ Fig. 3(a)]. Furthermore, by scanning randomly selected the NSs [Fig. 3(b)], the corresponding elemental mappings [Figs. 3(c) and 3(d)] of Mn and O indicate the homogeneous distribution of the constituents in the whole area of the wrinkle–shape NSs.
Fig. 2. (a) TEM image of MnO2 NSs. (b) HRTEM image and corresponding fast Fourier transform pattern of MnO2 NSs. (c) SEM image of MnO2 NSs on a silicon substrate. (d) SEM image of (c) at higher magnification. (e) AFM image of MnO2 NSs on a silicon substrate. (f) The corresponding height profiles along the lines of AFM image in (e).
Fig. 3. EDX spectrum (a) and a scanning TEM image (b) of the MnO2 NSs, and its corresponding element mapping images of (c) Mn and (d) O.
The crystal structure of the as-prepared MnO2 NSs was determined by X-ray diffraction (XRD). As shown in Fig. 4(a), major diffraction peaks at 2θ = 12.1°, 24.2°, 36.7° and 65.9° can be indexed to the (001), (002), (100), and (110) facets of the δ-MnO2 phase (JCPDS No. 18–0802), respectively, suggesting the formation of pure birnessite-type MnO2. Furthermore, the Raman spectrum of the MnO2 NSs in Fig. 4(b) exhibits a stronger characteristic peak at 653 cm−1 for the Mn-O lattice vibrations, in accordance with the previous report about δ-phase MnO2 [31,32]. To gain insight into the surface chemical composition and oxidation state, the MnO2 NSs sample was characterized by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 4(c), the XPS spectrum of the Mn shows two typical Mn2p3/2 and Mn2p1/2 bands centered at around 642.0 and 653.8 eV, respectively, indicating that the oxidation state is Mn (IV) [33]. In addition, the band of O1s exhibits two kinds of oxygen located at 529.6 and 532.3 eV [Fig. 4(d)], which are attributed to O in octahedral [MnO6] and in interlayer H2O, respectively [30]. This result is consistent with the EDX and Raman analysis.
Fig. 4. (a) XRD profile of MnO2 NSs. (b) Raman spectrum of MnO2 NSs. (c) XPS spectrum of MnO2 NSs for Mn2p. (d) XPS spectrum of MnO2 NSs for O1s. (e) FT-IR spectrum of MnO2 NSs. (f) TGA and DSC curves of MnO2 NSs.
Fourier transform infrared spectroscopy (FTIR) was used to characterize the presence of specific functional groups in the MnO2 NSs. The FTIR spectrum shows the octahedral [MnO6] vibration characteristic absorption peak at 493 cm−1, and the interlayer water absorption bands at 3425 and 1626 cm−1 [Fig. 4(e)] [34]. The existence of interlayer water corresponds to the 2D layered structure of MnO2. The small absorption bands at 2918 and 1132 cm−1 are due to the small quantity of residual sodium dodecyl sulfate in the preparation of MnO2 NSs [35]. The thermal stability of the resulting MnO2 NSs was performed by thermogravimetric analysis coupled with differential scanning calorimetry (DSC-TGA). The TG curve indicates that the weight loss is about 21% from 0°C to 1200°C [Fig. 4(f)]. The weight losses lower than 220°C and from 220°C to 535°C are attributed to the evaporation of surface water and interlayer water, and the further weight losses from 535°C to 578°C and 925°C to 970°C are associated to the phase transformations of MnO2 to Mn2O3 and Mn2O3 to Mn3O4, respectively [30,36]. The weight losses are in accordance with the endothermic peaks in the DSC curve. Taken together, these results demonstrated that the 2D ultrathin MnO2 NSs were successfully prepared.
Subsequently, the linear optical absorption of the MnO2 NSs was investigated. As presented in Fig. 5(a), MnO2 NSs solution shows a strong bandgap absorption in 250–800 nm, and a broad optical absorption in the NIR spectral region (800–2200 nm). The apparent NIR absorption can be attributed to the localized surface plasmon resonance (LSPR) absorption arising from the strong oscillation of incident light with free carriers in the MnO2 NSs, which can be evidenced by the influence of solvent medium refractive on the absorption peak [Figs. 5(b) and 5(c)] [37–40]. As shown in Fig. 5(c), a ∼65 nm red shift in the strongest peak position is observed with an increase in solvent refractive index from 1.36 to 1.48, indicating that LSPR effect is the main mechanism responsible for the absorption of MnO2 NSs in NIR region.
Fig. 5. (a) The absorption spectrum and photograph of the MnO2 NSs solution. (b) The absorption spectra of MnO2 NSs in three different refractive index solvents: ethanol (∼1.36), dimethylformamide (DMF, ∼1.43) and dimethyl sulfoxide (DMSO, ∼1.48). (c) Relationship between the absorption peak and the solvent refractive index. (d) The absorption spectra of MnO2–NaCMC and NaCMC films.
In order to fabricate the MnO2 NSs–based SA film, MnO2 NSs were mixed in an aqueous solution of NaCMC. Also, the optical absorption ability of the obtained the film was investigated. As depicted in Fig. 5(d), MnO2–NaCMC film exhibits a strong absorption ranging from 800 to 2000 nm. By contrast, the only NaCMC film shows a very weak absorption in the entire wavelength range, which is associated to the negligible host background band. This unique optical absorption feature of MnO2 NSs make them a promising candidate for broadband applications.
To demonstrate the saturable absorption of the MnO2 NSs-SA film, the nonlinear transmission property was investigated using a homemade pulsed fiber laser at 1558 nm with a pulse width of ∼800 fs. The measured data, fitted by the equation α (I) = αs /(1 + I/Is) + αns (where α (I) is the absorption coefficient, αs and αns are the saturable and nonsaturable absorption components, and I and Is are the injected and saturable intensities, respectively), are shown in Fig. 6(a) [41,42]. The modulation depth, saturation intensity and non-saturable loss were determined to be 6.00%, 24.37 MW/cm2 and 32.00%, respectively, suggesting that MnO2 NSs film could be a good candidate for Q-switching. To further examine the nonlinear optical property of the MnO2 NSs, we employed a well–developed open aperture Z-scan technique with a pulsed fiber laser (1558 nm, 1.5 ps) as the excitation source. The measured Z-scan curve is presented in Fig. 6(b). The nonlinear saturable absorption characteristic can be clearly observed. The Z-scan data were fitted by using the following formula:
Fig. 6. (a) The measured nonlinear saturable absorption results of MnO2 NSs at 1558 nm with a modulation depth of 6%. (b) The corresponding Z-scan curve.
3. Results and discussion
To verify the feasibility of MnO2 NSs as a SA for generating pulsed lasers, an EDFL with ring cavity configuration was constructed, as shown in Fig. 7. The pump source was a 980 nm laser diode (LD). A 980/1550 nm wavelength division multiplexer (WDM) was used to couple the 980 nm pump light into a ring cavity. A 25 cm long EDF was used as the gain medium. Such EDF has a core/cladding of 8/125 μm, a numerical aperture (NA) of 0.13 and absorption of 80 dB/m at 1530 nm. In addition, a polarization independent isolator (ISO) was employed to enforce the unidirectional operation of fiber laser. In order to obtain a good performance of Q-switching, a polarization controller (PC) was employed in the cavity for the optimum polarization state. The MnO2 NSs SA film was integrated into the EDFL cavity to induce Q-switched operation. The obtained pulse signals were delivered through a 10% optical coupler (OC). The characteristics of Q-switched laser were monitored by using an optical spectrum analyzer (Yokogawa, AQ6375D), an oscilloscope (Tektronix, DPO4104B), together with a 12.5 GHz photodetector (Newport, 818-BB-51F), a radio-frequency (RF) analyzer (Agilent, E4411B), and a power meter (Thorlabs, PM100D).
Fig. 7. Schematic of the erbium-doped fiber laser (EDFL). Laser diode (LD), wavelength division multiplexer (WDM), erbium-doped fiber (EDF), isolator (ISO), polarization controller (PC) and optical coupler (OC).
For pump powers below 220 mW, the output laser operated in a continuous wave regime. Interestingly, when the pump power exceeded 220 mW, the laser began to operate in a stable Q-switched regime. The emission spectrum of Q-switched EDFL at a pump power of ∼800 mW is shown in Fig. 8(a). The central operation wavelength locates at 1558 nm. The pulse train and corresponding single pulse profile of the Q-switched EDFL are presented in Figs. 8(b) and 8(c), respectively. The interval of two adjacent pulses is about 1.08 μs, corresponding to a repetition rate of 92.35 kHz. The duration of a single pulse is 1.26 μs. Figure 8(d) exhibits the dependence of pulse duration and repetition rate on the pump power. By increasing the pump power from 220 to 800 mW, the pulse duration decreased from 2.26 to 1.26 μs and repetition rate varied from 59.03 to 92.35 kHz, exhibiting a typical feature of passively Q-switched lasers.
Fig. 8. Q-switched pulse output characterization in the EDFL cavity based on MnO2 NSs SA. (a) Q-switched optical spectrum. (b) Q-switched output pulse train. (c) Single Q-switched pulse profile. (d) Pulse duration and repetition rate as a function of pump power. (e) Output power versus the pump power of the Q-switched fiber laser. (f) Radio-frequency output spectrum of Q-switched fiber laser.
Furthermore, the output power of the Q-switched EDFL as a function of pump power was further investigated. As shown in Fig. 8(e), the output power was enhanced linearly from 5.46 to 17.18 mW as pump power increased over the range from 220 to 800 mW. The corresponding slope efficiency was about 2.2%. When the maximum output power was 17.18 mW, the maximum pulse energy reached up to 190 nJ. In addition, we measured the RF spectrum of the Q-switching operation in order to verify the stability of Q-switched EDFL [Fig. 8(f)]. The fundamental frequency peak located at 92.35 kHz, which corresponds to the above obtained repetition rate for the pump power. The intensity of RF signal was measured as 45 dB, indicating the good stability of Q-switching operation. To confirm the long-term stability of MnO2 NSs SA, the emission spectra of the Q-switched laser were measured every 15 min for 2 hours. As shown in Fig. 9, the emission spectra of the Q-switched laser were not obviously changed in wavelength and intensity. These results demonstrated that MnO2 NSs–SA film was stable and promising for constructing Q-switched lasers.
4. Conclusions
In summary, we have prepared MnO2 NSs through a bottom-up approach, and studied their linear and nonlinear optical responses. The strong saturable absorption behavior exhibited in these MnO2 NSs shows a great promise in pulsed laser generation. As a proof-of-concept, by applying MnO2 NSs–SA as a pulse modulator, a stable passively Q-switched laser at central wavelength of ∼1558 nm, the maximum pulsed repetition rate of 92.35 kHz, the maximum output power of 17.18 mW, and the shortest pulse duration of 1.26 μs, was achieved for the first time. This work not only offers a novel 2D nanomaterial for the generation of pulse laser, but also broadens the scope of application of MnO2 NSs. We believe that MnO2 NSs–based SA will open up more opportunities for nonlinear optic and ultrafast photonic devices.
Funding
National Natural Science Foundation of China (11774132, 61527823, 61827821, 61875071); Opened Fund of the State Key Laboratory on Integrated Optoelectronics; Tsinghua National Laboratory for Information Science and Technology Cross-discipline Foundation; Key Technology Research and Development Project of Jilin Province (20180201120GX); Major Science and Technology Tendering Project of Jilin Province (20170203012GX).
Disclosures
The authors declare no conflict of interest.
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