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Abstract
Recently, two-dimensional materials have shown excellent nonlinear absorption properties and have practically promoted the development of ultrafast lasers. As one of the typical III-VI semiconductors, InSe has already been widely applied in designing photo-sensors and other optical devices due to its outstanding electrical transport, quantum physics and dramatic photo-response properties. However, the nonlinear absorption characteristics of InSe have not been experimentally investigated within ultrafast lasers so far. In our work, the nonlinear absorption properties of InSe were investigated. InSe-PVA film was successfully prepared and employed for achieving a mode-locked Yb-doped fiber laser. A stable mode-locked operation with a maximum output power of 16.3 mW and a minimum pulse width of 1.37 ns at a pulse repetition rate of 1.76 MHz was obtained, and the corresponding pulse energy was as high as 9.26 nJ. Our findings suggest that InSe may have wide potential ultrafast photonic applications due to its suitable bandgap value and excellent nonlinear saturable absorption characteristics.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A decade of extensive investigations on two-dimensional (2D) materials have practically promoted the development of optoelectronics and laser devices [1–11]. Based on 2D materials as saturable absorbers (SAs), mode-locked fiber lasers have advanced towards low-cost, high-efficient, narrow pulse width and compact direction [1–16]. As is known, the discovery of graphene played a important role in exploring new two-dimensional SAs [1–6]. So far, inspired by graphene, various kinds of 2D materials including graphene oxide (GO), topological insulators (TIs) [7–11], transition metal dichalcogenides (TMDs) [12–15], black phosphorus (BP) [16-17], Mxenes [18] and so on have already shown excellent absorption performance in demonstrating pulsed fiber lasers operating within 1, 1.5 and 2 μm region. However, the exploration of new SA materials with high damage threshold, ultra-fast recovery time and wide absorption band has always been in a desired state.
Recently, III-VI compounds MX (M = In, Ga, X = S, Se, Te) have shown good performance in designing photo-sensors, memory devices, optical conversion materials and so on. Because, III-VI compounds exhibit suitable wide-range bandgaps, large electronic conductivity and photo-response, dramatic nonlinear effect, high damage threshold and so on [19–33]. Among the mentioned III-VI compounds, InSe has attracted special attention. InSe belongs to the family of layered metal-chalcogenide semiconductors. As is reported that each layers of the InSe consisted of 4 covalently bonded Se-In-In-Se atomic planes. Layers are held together by van der Waals interactions at an inter-layer distance of 0.83 nm [28–31]. Additionally, the cleaved facets of InSe layered crystals are atomically smooth and contain a low density of surface states [30–33]. Optical studies have proved that the band gap value of InSe dependent on the number of layers. Few layers InSe has an indirect band gap of 1.4 eV, meanwhile, the bandgap value of bulk InSe was 1.2 eV, the phenomenon is mainly due to the suppression of inter layer coupling [28–33]. Recently, Bandurin et al [30] employed exfoliation and subsequent encapsulation of few layer InSe in an inert atmosphere, which allowed them to fabricate monolayer InSe structures and field effect devices, they proved that InSe exhibited previously unattainable quality and stability under ambient conditions. Lei et al have studied the evolution of the electronic band structure and efficient photo-detection in atomic layers of InSe, a strong photoresponse of 34.7 mA/W and fast response time of 488 μs for a few layered InSe were recorded, their results suggested that InSe was a good material for thin film optoelectronic applications [31]. In 2017, based on the pulsed laser deposition method, Yang et al have designed a wafer-scale InSe nanosheets and demonstrated a InSe-based broadband phototransistors within the region from ultraviolet to near-infrared, their findings suggested that the pulsed laser deposition grown InSe would be a promising choice for future device applications [32]. In 2016, High-quality InSe layered crystals have also been successfully grown by chemical vapor transport method with ICl3 as the transport agent, the emission and absorption capability of multilayer InSe were studied by photo-luminescence and photo-conductivity [33].
However, to our knowledge, the nonlinear absorption properties of InSe have been rarely tested for demonstrating pulsed fiber lasers before. Due to its high nonlinear effect, high damage threshold and suitable bandgap value (1.2-1.4 eV), InSe is expected to have the same saturable absorption characteristics in comparison with the reported 2D materials (TIs, TMDs, BP, etc). In our experiment, InSe-PVA film was successfully prepared and employed as a saturable absorber. The saturation intensity and modulation depth were measured to be 15.6 MW⁄cm2 and 4.2%, respectively. A mode-locked Yb-doped fiber laser was demonstrated for testing the absorption properties of the film-type InSe-PVA SA. Stable mode-locked operations with a maximum output power of 16.3 mW and a minimum pulse width of 1.37 ns at a pulse repetition rate of 1.76 MHz was obtained. In the past, for achieving Yb-doped mode-locked lasers, various 2D materials including graphene oxide, TIs (Bi2Te3, Bi2Se3, Sb2Te3), TMDs (MoS2, WS2, SnS2) and BP have been used as SAs [34–51]. As is compared in Table 1 that InSe also has analogous saturable absorption characteristics and equal excellent performance in the field of ultrafast optics. Additionally, InSe have the advantage in obtaining large-energy pulse laser operations based on the fact that the 9.3 nJ pulse-energy obtained in our work was the largest of all so far. Besides, InSe exhibited previously unattainable quality and stability [30].the saturable absorption parameters of InSe have been experimental proved to be sensitive to the layers of the materials, which will be beneficial for flexible design of absorber parameters [52].
Table 1. Comparison of passively mode-locked Yb-doped lasers based on different 2D SAs.
2. Preparation and characterization of InSe-based SA
In our work, InSe powder was firstly prepared, the preparation progress was listed below. High-purity indium (99.999%) and selenium(99.999%) powders were mixed 1:1 according to the molar ratio and put in the quartz tube at a pressure of 10−3 Pa. Then the quartz tube was heated to 700 °C slowly in 24 h and the InSe was obtained after 72 h reaction. The samples were cooled to room temperature naturally and pulverized in the a glove box filled with inert nitrogen gas. After that, based on the prepared InSe powder, the InSe-PVA film was then prepared. Figure 1 shows the preparation progress of the film-type InSe-PVA SA. As is shown, commonly used liquid-phase exfoliation and spin coating methods were employed for preparing the InSe solution and the film-type InSe-PVA SA, respectively. Firstly, 0.1 g InSe powder was added into 40 ml 30% alcohol for the preparation of InSe solution. The InSe solution was placed in a high power ultrasonic cleaner for 6 hours and centrifuged for 20 min, thus, solution with few-layer InSe nanosheet will be obtained. Then, the InSe solution and 4 wt% PVA solution were mixed at the volume ratio of 3:4 and placed in the ultrasonic cleaner for 3 hours for preparing uniform InSe-PVA solution. After that, 100 μL InSe-PVA solution was spin coated on a sapphire substrate for the formation of InSe-PVA film. After the 100 μL InSe-PVA solution was dropped on the substrate, another substrate was employed for uniformly spinning the solution on the substrate, then, the coated substrate was placed into the drying oven at 25 °C for 12 hours. In this way, InSe-PVA film was successfully prepared. Finally, a 1*1 mm2 film was cut off from the substrate and placed at the end of the fiber ends for proposing as a sandwich-structured mode-locker.
The characterizations of the InSe nanosheets were investigated by using a Raman spectrometer (Horiba HR Evolution) and a X-ray Diffraction (XRD) (D8 advance Bruker). As is shown, Raman shifts locating at 115.1 cm−1(A1′), 175.8 cm−1 (E′-TO and E″), 202.9 cm−1 (A2″-LO) and 224.3 cm−1(A1′) were recorded and shown in Fig. 2(a), respectively. The recorded Raman peaks all correspond to the first-order scattering from optical phonons, which were in agreement with the previous reported works [28–33]. Besides, No other Raman shifts were observed in the spectrum, indicating that pure InSe nanosheet was successfully prepared in our work. The diffraction XRD spectrum from the InSe was showed in Fig. 2(b), as is shown, peaks corresponding to the (002), (004), (101), (006), and (008) planes in InSe, were recorded, respectively. Especially, the high diffraction peak at the (004) plane indicates that InSe nanosheets with well-layered structure and high crystallinity were successfully prepared.
Fig. 2 (a) The Raman spectrum of the InSe nanosheets. (b) The X-ray Diffraction of the InSe nanosheets
Also, the layered-structure and morphology characteristics of the used InSe powder were useful for analyzing the parameters of the SA. In the experiment, based a scanning electron microscopy (SEM) (Sigma 500), the SEM images under different resolution of the tested InSe are shown in Fig. 3(a) and (b), respectively. Obvious typical layered structures were depicted. In addition, as is shown that InSe powder exhibits large flat area, this structure suggests that single or few layers InSe nanosheets can be expected after the liquid-phase exfoliation. The TEM images of the InSe solution produced after centrifugation, which were recorded by a JEM-2100 under the resolutions of 20 and 200 nm, are provided in Fig. 3(c) and (d), it is obvious that few layers InSe nanosheets with large flat area were successfully obtained in the experiment. The SEM and TEM results proved that InSe powder exhibits obvious layered structure and is suitable for designing saturable absorbers.
Fig. 3 (a). The SEM image of the InSe nanosheets under the resolution of 1 μm. (b) The SEM image of the InSe nanosheets under the resolution of 2 μm. (c) The TEM image of the InSe nanosheets under the resolution of 20 nm. (d)The TEM image of the InSe nanosheets under the resolution of 200 nm.
As is known, the parameters of 2D material based SAs dependent on the thickness of the materials. In our experiment, the thickness characteristics of the prepared InSe nanosheets were also tested by an atomic force microscope (Bruker Multimode 8), as is shown in Fig. 4(a) and 3(b), the thicknesses of the marked samples arranged from about 2.6 to 5.1 nm, indicating that the layer numbers of the marked samples are about 3-6.
The characteristics of the prepared InSe-PVA film were also investigated experimentally. Firstly, based on the mentioned SEM (Sigma 500), the thickness of the InSe-PVA film was measured, the thickness of the film was measured to be about 24-30 μm. The film also exhibit relative uniform thickness. The linear transmission spectrums of InSe-PVA film on substrate, bare PVA film on substrate and the substrate were also provided to show the linear absorption from InSe nanosheet, as is shown in Fig. 5(b), the PVA film have little influence on the transmission of the substrate, however, the transmission of the InSe-PVA film decreased to about 86% from 92%, indicating that the saturable absorption was mainly due to the InSe nanosheets.
Based on a commonly used two-detector method (shown in the insert of Fig. 6), which were also reported in our previous works [52], the nonlinear optical saturable absorption response of the InSe-PVA film was investigated. Thereinto, a picosecond home-made Yb-doped mode-locked fiber laser with a maximum output power of 69 mW and a pulse width of 35 ps under a pulse repetition rate of 36.6 MHz was used as the pump source. A variable optical attenuator (VOA) was used for power regulation and A 50:50 output coupler was used for separating the incident pulses into two parts. The output powers were recorded by two power meters and shown in Fig. 6. Additionally, based on the following conventional formula [52]:
where T is transmission, Tns is non-saturable absorbance, ΔT is modulation depth, I is input intensity of laser, Isat is saturation intensity. The saturation intensity and modulation depth of the InSe-PVA film, which were obtained by fitting the experimental results, were 15.6 MW/cm2 and 4.2%, respectively.Fig. 6 The nonlinear absorption property of the InSe-PVA film. Insert. the setup of the power-dependent transmission technique.
3. Experimental details
The experimental setup of the InSe-PVA based Yb-doped mode-locked fiber laser is shown in Fig. 7. A 116.8 m-long ring laser cavity including a 980/1064 wavelength division multiplexer (WDM), a 32 cm long Yb-doped fiber (Liekki, Yb-1200, 4/125), two polarization controllers (PCs), a polarization-independent isolator (PI-ISO), a 100 m long single mode fiber (SMF), a InSe-PVA film-type SA, a 5 nm bandpass filter, and a 10:90 optical coupler (OC) was employed for investigating the performance of the mode-locked fiber laser. The pump source was a 980 nm laser diode with a maximum output power of 680 mW. The PCs and the PI-ISO were used for adjusting the polarization state and unidirectional operation in the cavity, respectively. The narrow bandwidth filter with a central wavelength of 1068 nm and 3 dB bandwidth of 5 nm was inserted into the cavity for suppressing the mode competition effect. The output performance of the fiber laser were record by a fast-speed photodetector (3G), a digital oscilloscope (DPO4054), a power meter (PM100D-S122C), a optical spectrum analyzer (AQ6317) and a spectrum analyzer (R&S FPC1000).
4. Results and discussions
In general, within mode-locked fiber lasers, the balance between the laser gain, the loss, the effect of various nonlinear optical progress and the net dispersion value of the cavity contributed to the formation of different soliton generations. Generally speaking, changing the length of the cavity to adjust the net dispersion value is the main method to achieve soliton generations. In our experiment, when the cavity length was about 116.8 m, mode-locked operation can be recorded by adjusting the state of the PCs and the pump power. In addition, large net dispersion value will lead to a wide pulse width, thus, in our work, the total length was set to be 116.8 m. As is known, self-mode-locked or Q-switched operations can always be obtained within a long-cavity fiber laser. In our experiment, the SA was firstly removed from the cavity for testing if self-mode-locked or Q-switched operations will be obtained, however, without the SA, no pulse operations were recorded by changing the pump power and the state of the PCs, which indicating that the pulse generations obtained in our work was due to the nonlinear absorption of the InSe-PVA SA. The characteristics of the continuous-wave operation were also investigated. In Fig. 8(b), the relationship between the output power and the pump power of the continuous-wave operation is depicted, the threshold power was 60 mW, meanwhile, the maximum output power was 38.2 mW under a pump power of 340 mW, corresponding to an optical to optical conversion efficiency of 11.2% and a slope efficiency of 13.6%. In the experiment, by inserting the SA into the cavity, when the pump power was higher than 185 mW, by adjusting the state of the PCs, stable mode-locked operations were obtained. Firstly, the emission spectrum of the mode-locked Yb-doped fiber laser was tested by a optical spectrum analyzer (AQ-6317) with a resolution of 0.02 nm and shown in Fig. 8(a), as is shown, the central wavelength and 3 dB spectrum bandwidth were 1068.36 and 1.72 nm, respectively. It is necessary to state that the bandgap value of InSe arranged from 1.2 to 1.4 eV (886-1033 nm), meanwhile, the operating photon energy of our work was about 1.16 eV, which indicating that the pulse operation was due to the sub-bandgap absorption within the InSe SA. Actually, passively Q-switched and mode-locked fiber lasers which were modulated by sub-bandgap absorption phenomena have also been widely reported before [40–52]. Because, in a finite system, the sub-bandgap absorption at low photon energies could be realized which attributed to energy levels within the bandgap arising from edge-state [12, 15, 52]. Thus, In our opinion, sub-bandgap absorption observed in our work was also attributed to the edge-state absorption of the InSe. The relationships between the average output powers and pump powers is also shown in Fig. 8(b), the maximum average output power was as high as 16.3 mW under a pump power of 335 mW, corresponding to an optical conversion efficiency of 4.87%. However, in comparison with the continuous-wave operation, the conversion efficiency of the mode-locked operation was lower. In addition, the threshold of our work was also higher than that of the previous works [34, 36, 41, 44–47], in our experiment, the insert loss of the SA was tested to be 1.8 dB, which is indicating that the higher threshold and lower optical conversion efficiency were mainly due to the large insert loss of the SA. As is described in Fig. 5(b) that the PVA film has little influence on the nonlinear absorption, which indicating that the insert loss was mainly due to the InSe nanosheets. Additionally, 1.8 dB was not high enough for the 185 mW threshold, in other words, the insert loss of the intra-cavity components also contributed to the high threshold. In our future work, we will focusing on reducing the total insert loss and the threshold power of the mode-locked operation. In the experiment, when the pump power was higher than 335 mW, the mode-locked operation became unstable, which was mainly due to the saturation of the SA under high pump power. So, for obtaining higher average output power, the decrease of the threshold is necessary.
Fig. 8 (a). The emission spectrum of the fiber laser. (b) The relationships between the average output power and pump power.
Figure 9(a) depicts a typical pulse train of the InSe-PVA based Yb-doped mode-locked laser, the pulse-to-pulse interval was 0.57 μs, corresponding to a pulse repetition rate of 1.76 MHz, which matches well with the total cavity length of 116.8 m. Thus, the corresponding maximum pulse energy was 9.3 nJ. The single pulse sharp of the mode-locked pulse train is shown in Fig. 9(b), the pulse width was 1.37 ns, as is mentioned that the 3dB spectrum width was 1.3 nm, corresponding to a time-bandwidth product (TBP) is about 61.9, which is a much higher than the theoretical limit value, indicating that the optical pulse is higher chirped, which is mainly attributed to the long length of the laser cavity. As is compared in Table 1 that narrow pulse generations are easier to be obtained within short-length laser cavity due to the smaller net total dispersion in the cavity. However, the length of the cavity is not as short as possible due to the reason that long-cavity length was beneficial to the formation of solitons. In general, for compressing the wide pulse width, external-cavity compression using grating pairs was an commonly used methods, thus, in our next work, we will try to compress the wide pulse width by using grating pairs. The radio frequency (rf) spectrum of the InSe-PVA based mode-locked laser was recorded by a spectrum analyzer (R&S FPC1000), Fig. 9(c) shows the radio frequency spectrum which was recorded with a bandwidth of 3 MHz and a resolution of 20 kHz, as is shown, the central frequency located at the fundamental repetition rate of 1.76 MHz, the signal-to-noise ratio is about 45 dB. In addition, the radio frequency spectrum within a wide bandwidth of 200 MHz is shown in Fig. 9(d). All the results exhibit that mode-locked pulses with high stability was obtained in our work. However, it is obvious that the RF spectrum shown in Fig. 9(c) contains two symmetrical shoulders and the base of the spectrum is not flat, this phenomenon indicates that the stability of the mode-locked operation can be future improved. Because, as is mentioned that the threshold of the mode-locked operation was 185 mW, under a high pump power, the InSe-PVA film will suffer from the effect of the thermal, which will have influence on the stability of the mode-locked operation. Thus, the RF spectrum exhibits unstable component. Accordingly, in our future works, we will make great effort to obtained a mode-locked operation with higher stability under a lower pump power.
Fig. 9 (a). Emission pulse train of the mode-locked laser. (b) The single pulse sharp of the mode-locked generation. (c) the RF spectrum located at 1.76 MHz with a bandwidth of 3 MHz. (d) the RF spectrum within a bandwidth of 200 MHz.
In Table 1, relatively complete parameters of two-dimensional material based passively mode-locked Yb-doped fiber lasers were depicted. As is shown, so far, various 2D materials including graphene, GD, TIs, BP and TMDs have been also employed as mode-lockers. maximum average output powers of 33.7 and 80 mW have been obtained within single-mode and multimode fiber lasers [41, 49]. In addition, based on Sb2Te3 as SA, the minimum pulse width of 6 ps was achieved in [42]. Through this contrast, we also found out that the 9.3 nJ pulse energy obtained in our work was the largest of all. On the one hand, the large pulse energy is related to the design of laser parameters such as the cavity length, on the other hand, the results shows that InSe has the potential for obtaining large pulse energy fiber lasers.
In conclusion, the nonlinear saturable absorption properties of InSe were investigated within a Yb-doped mode-locked laser, in our experiment. The saturation intensity and modulation depth of the InSe-PVA film were 15.6 MW/cm2 and 4.2%, respectively. Stable mode-locked Yb-doped laser operation with a maximum output power of 16.3 mW was obtained. The minimum pulse width was 1.37 ns at a pulse repetition rate of 1.76 MHz. Thus, the maximum pulse energy was as high as 9.3 nJ. Our experiment results fully proved that InSe had excellent nonlinear absorption characteristics and would have wide potential applications in the field of ultra-fast pulse lasers. In our opinion, passively mode-locked operation with large pulse energy will have wide applications in the field of industrial processing, surgical medicine and so on.
Funding
China Postdoctoral Science Foundation (2016M602177); Shandong Provincial Natural Science Foundation (ZR2016FP01); National Natural Science Foundation of China (nos 11747149).
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