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Abstract
Transition metal dichalcogenides (TMDs), a family of two-dimensional layered materials exhibiting unusual electronic, optical, mechanical properties, have aroused much attention in recent years. Here, we focus on the nonlinear saturable absorption behavior of a new family member of TMDs, rhenium diselenide (ReSe2), and its application in pulsed laser generation. We have prepared the few-layer ReSe2 by the mechanical exfoliation method, and validated its ambient-stable nonlinear optical responses and all-optical tunable potential at 1.55 µm. In addition, the ambient-stable and parameter-tunable Q-switched fiber laser modulated by the ReSe2 has been realized.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The transition metal dichalcogenides (TMDs), one kind of layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into two-dimensional layers of single unit cell thickness, have attracted great attentions due to their excellent electronic and optical properties in recent years [1, 2]. The TMDs have unique optical characteristics, such as strong photoluminescence [3], ultrafast carrier dynamics [4] and a high nonlinear optical response [5]. These interesting properties pave the way for the development of novel application for photonics and optoelectronics [6, 7]. Beyond the linear optics regime, one typical successful application of TMDs is the nonlinear optical modulator, i.e. saturable absorber (SA) for pulsed laser and other nonlinear optical devices, which can verify the application potential of the nonlinear optical materials.
TMDs are a class of materials with the formula MX2, where M is a transition metal element from group IV (Ti, Zr, Hf and so on), group V (for instance V, Nb or Ta) or group VI (Mo, W and so on), and X is a chalcogen (S, Se or Te). These materials form layered structures of the form X–M–X, with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms. The present mostly studied monolayer TMDs have a 2H lattice structure, which means that their electronic and optical properties are in-plane isotropic. Recently, in-plane anisotropic 2D materials, such as black phosphorus (BP) [8–12], have been demonstrated for the applications in optoelectronics and photonics. BP shows anisotropic electronic and optical properties along the armchair and zigzag direction as a result of the orthorhombic crystal structure. In addition, the bandgap of BP can be widely tuned with the variation of layers [13–15]. However, due to the photo-oxidation and the absorption of water during several hours to days [16], BP exhibits weak environmental stability.
Rhenium diselenide (ReSe2), a new member of the TMDs family, has exhibited excellent optoelectronic performance [17–19]. Different from other TMDs, the ReSe2 structure is rather unique because of its in-plane 1D chains of Re atoms arranged in linked Re4 “diamond” shapes along the b axis [17]. The novel Re4 chains induce a distorted 1T phase of ReSe2 with triclinic symmetry, which can be considered as a distortion of Re atoms away from their ideal octahedral sites [18]. Interestingly, the presence of Re4 chains in ReSe2 makes it an optically biaxial material which shows exceptionally anisotropic electrical and optical behavior for linearly polarized light. Compared with the layered rhenium disulfide [20, 21], the ReSe2 shows stronger anisotropic optical behavior for the incident light. The relatively large birefringence of anisotropic 2D layered materials, such as BP, can facilitate accurate manipulation of light polarization with atomically controlled device thickness for various applications where integrated polarization-controllers at the nanoscale are required [22]. More interestingly, ReSe2 exhibits excellent environmental stability compared with BP.
In this contribution, we have demonstrated the fabrication of few layer ReSe2 by the mechanical exfoliation method, and found that the layered TMDs can exhibit stable and controllable nonlinear optical response. With the ReSe2 SA, the ambient-stable and parameter-tunable Q-switched fiber laser has been realized experimentally.
2. Material characterizations and experimental setup
The few-layer ReSe2 was prepared by the mechanical exfoliation method from the bulk counterpart, as shown in Fig. 1. By pressing the fiber ferrule onto the sample on the scotch tape, the few-layer ReSe2 can be transferred onto the fiber end facet, and thus the fiberized saturable absorber can be fabricated successfully. To illustrate the few-layer nature of the ReSe2 sample, the Raman spectrum of the mechanically exfoliated ReSe2 on the scotch tape was shown in Fig. 2(a). There are three distinct Raman peaks at 118 cm−1, 160 cm−1 and 174 cm−1 [19], which indicates it is a few layers structure. Meanwhile, Fig. 2(a) also shows the Raman spectra of the same ReSe2 after 3 weeks, which indicates that ReSe2 exhibits excellent environmental stability, consistent with the reported results [18, 23].
Fig. 1 The process of mechanical exfoliation method and optical image of the fiber end-facet (fiber cladding diameter of 125 μm, fiber core diameter of 9 μm).
Fig. 2 (a) The Raman spectra (red) of few-layer ReSe2 after 1 hour and the Raman spectra (blue) of ReSe2 after 3 weeks. (b) SEM image of few-layer ReSe2.
We have measured the nonlinear absorption of the ReSe2 using a 1560 nm mode-locked fiber laser whose repetition rate is ~20 MHz and pulse width is ~1.5 ps. Figure 3 shows the different transmittance the ReSe2 film by fitting the data with the following equation:
where α(In) is the absorption coefficient, αs is the saturable loss, αns is non-saturable absorbance, I and Is are input and the saturation intensities, respectively. To validate its modulation potential, we have also measured the intensity-dependent transmittance of the device modulated by a 980 nm laser as modulation light. In order to investigate the modulated absorption characteristics of ReSe2-SA, we added a modulation light with intensity of I2 in the Eq. (1) and the input signal light will experience attenuation as following form:where α(Im) is the saturable loss of ReSe2-SA with modulation light intensity of Im, and Is1 is the saturation intensity of input signal light with frequency of ω1. It is clear in Eq. (2) that αs(Im) can be described by αs0, which is the saturable loss of ReSe2-SA without I2, and Is2 (the saturation intensity of ReSe2-SA when signal light has the frequency of ω2) with the form:Fig. 3 (a) The schematic experimental setup to measure the nonlinear optical response of the ReSe2-SA. (b) The experimental data of the nonlinear absorption of ReSe2-SA with different modulation power. (c) The relationship between modulation depth and modulation power for the ReSe2-SA.
From Eq. (3), we can see clearly that the modulation depth of the ReSe2-SA Δα (corresponding to α(In, Im)- α(0, Im)) decreases continuously with the increasing modulation light power I2, which is in good agreement with the experimental results shown in Fig. 3(b). The inset of Fig. 3 shows the diagram of the fiber-compatible SA device.
The twin detector system was set up to test the nonlinear response of ReSe2 film as shown in Fig. 3(a). The fiber-compatible SA device was placed between two 980/1550 wavelength division multiplexers (WDMs). The reflection port (Port: 1550 nm) of WDM1 was connected to a home-made 1560 nm pulsed fiber laser (repetition rate: 20 MHz, pulse duration: 1.5 ps) and the pass port (Port: 980 nm) was connected to a 980 nm continuous laser diode. The reflection port of the other WDM2 was monitored with a power meter, with the pass port hanging in the air. The 1560 nm pulsed laser was used to pump the ReSe2 and the 980 nm continuous wave light was used to tune the absorption of ReSe2. The curves in Fig. 3(b) show the different transmission ratios of this ReSe2 by fitting the data. The maximum modulation depth is about 7%, which decreases slowly with the addition of modulation power. We have observed the nonlinear optical absorption around 1550 nm for the few-layer ReSe2, and we argue that the saturable absorption of the layered material can be attributed to the saturation of edge states, which arise from large edge to surface area ratios of few-layer ReSe2 flakes [24, 25]. Figure 3(c) shows the modulation depth decreases with the increasing modulation powers clearly. We introduced the modulation power at 0 mW, 8.6 mW, 17.1 mW, 25.2 mW, and we can get four different modulation depths. The modulation depth decreases with the increasing modulation powers clearly.
3. Experimental results and discussions
To test the tunable saturable absorption of the fiber-compatible ReSe2-SA, we constructed an erbium-doped fiber ring laser with a cavity configuration in Fig. 4. The gain medium of the laser cavity is a piece of 0.8 m, high concentration erbium doped fiber (LIEKKI Er80-8/125) with a group velocity dispersion (GVD) of 15.8 ps/nm/km, which is pumped by a 980 nm laser diode (LD). We used a 980/1550 WDM coupler to couple the pump laser into the fiber cavity. A polarization independent isolator and an intra-cavity polarization controller (PC) were used to force the unidirectional operation of the ring and adjust the cavity birefringence, respectively. The modulation light, a 980 nm continuous-wave laser, passes through a 980/1550 nm WDM and acts on the ReSe2-SA, then it will be filtered out by another 980/1550 nm WDM. The overall cavity length is about 8.6 m. Except the gain fiber, other fibers components are all single mode fiber (SMF-28), with a GVD of 18 ps/nm/km. The net cavity dispersion is calculated to be −0.091 ps2. An optical spectrum analyzer (Ando AQ-6317B) and a real-time oscilloscope with a bandwidth of 4 GHz (Agilent DSO9404A) together with a photodetector (MC PD-12D) are employed to simultaneously monitor the optical spectrum and the temporal evolution of the output pulse train. The output power of the Q-switched fiber laser was measured with a power meter (Ando, AQ2140).
In this experiment, self-started Q-switching state was obtained just by increasing the pump power up to 60 mW. For the fiber laser, the oscillation threshold was 45 mW. Figure 5 summarizes the typical characteristics of Q-switching state at a pump power of 140 mW. Figure 5(a) shows the typical Q-switching pulse train. It has a repetition rate of 16.64 kHz, corresponding to a pulse interval of 59.65 μs. No peak intensity modulation had been found on the pulse train, illustrating the high stability of Q-switching operation. There were no top amplitude fluctuations, as shown in Fig. 5(b). It has a symmetric Gaussian-like shape with a full width at half maximum (FWHM) of 4.98 μs. The corresponding optical spectrum is shown in Fig. 5(c), from which we can infer that the 3 dB spectral bandwidth of the Q-switching output is about 0.2 nm centered at 1566 nm. To investigate the stability of this Q-switched fiber laser, we also measured the corresponding Radio Frequency (RF) spectrum centered at 16.64 kHz, as shown in Fig. 5(d). The signal-to-noise ratio (SNR) is up to about 40.7 dB.
Fig. 5 The typical output characteristics of the Q-switched fiber laser under a pump power of 140 mW. (a) The pulse train of oscilloscope trace. (b) The corresponding single pulse trains. (c) The optical spectrum. (d) The corresponding RF spectrum.
Figure 6 shows the Q-switched pulse parameter variation with the pump power. Figure 6(a) demonstrates the evolution of the repetition rate and pulse duration with the pump power. It shows typical features of the Q-switching operation. With the increasing pump power, the repetition rate linearly increases from 6.64 kHz to 21.04 kHz, while the pulse duration varied in the range of 16.5 μs to 4.98 μs. Figure 6(b) is the relationship between the average output power and the pulse energy with the pump power. The average output power increased almost linearly with the pump power. Most importantly, at the maximum pump power of 140 mW, the maximum obtained pulse energy is up to 36 nJ, which is comparable to those of the Q-switch pulses obtained in Er-doped fiber lasers with other SAs, such as CNTs [26–30], Graphene [31–39], TI [40–47]. In addition, we have added a table to compare our results with the recent passively Q-switched fiber laser reports using some other TMD SAs, such as MoS2, MoSe2, WS2, WSe2, ReS2, and the ReSe2 has exhibited comparable pulsed laser performance [Table 1].
Fig. 6 (a) The evolution of pulse repetition rate and pulse duration and (b) the output average power and pulse energy with the increasing pump power.
Table 1. Performance summary of the Q-switched fiber lasers based on different TMDs-based saturable absorbers.
The Fig. 7(a) explains the relationship between output power and pump power, both pump power and modulation power (Pm means modulation light power in the frame at the top left corner) with the variable modulation light power. In order to confirm the modulation effects rather than pumping the EDF of modulation light, we measured the output power of the Q-switched fiber laser. Similarly, we measured the output power with the modulation power at 0 mW, 8.6 mW, 17.1 mW, 25.2 mW, respectively. As shown in Fig. 7(a), with the increasing of pump power, the monotonically increasing lines of output power are almost superposing together. Distinctly, the continuous 980 nm modulation light is extracted out with WDM2 and has no obvious impact on the intracavity light intensity and the output power. Figure. 7(b) shows the relationship between pulse duration and pump power. When we fix the modulation power at 0 mW, 8.6 mW, 17.1 mW, 25.2 mW, respectively, and we can get four lines which decrease successively by increasing the pump power from 60 mW to 140 mW. In order to fix the pump power at homologous abscissa values, the pulse width increases with the increasing modulation powers clearly. Figure 7(c) shows the relationship between repetition rate and pump powers from 60 mW to 140 mW. With the increasing pump power, we can get a noticeably regular curve between pump power and repetition rate. Then we introduced the modulation power at 0 mW, 8.6 mW, 17.1 mW, 25.2 mW, and we can get distinguishing lines with similar increasing tendency. By fixing the modulation power, the pulse repetition rate can be increased by increasing the pump power. To validate the stability of the Q-switched fiber laser, the optical spectrum of the fiber laser modulated by the same ReSe2 after 3 weeks has been shown in Fig. 7(d) for a pump power of 110 mW, which manifests the excellent environmental stability of ReSe2.
Fig. 7 (a) The relationship between output power and pump power with the variable modulation light power from 0 mW to 25.2 mW. (b)The relationship between width and pump power, with the variable modulation light power from 0 mW to 25.2 mW. (c)The relationship between repetition rate and pump power with the variable modulation light power from 0 mW to 25.2 mW. (d)The corresponding optical spectrum for a pump power of 110 mW after 3 weeks.
4. Conclusion
In summary, we have prepared the ambient-stable few-layer ReSe2 by the mechanical exfoliation method, and investigated its nonlinear saturable absorption experimentally. It has been found that the modulation depth of the few-layer ReSe2 could be gradually altered by an all-optical modulation method. In addition, we constructed a passively Q-switched fiber laser modulated by the ReSe2-SA, whose pulse width and repetition rate could be continuously tuned via varying the modulation light. Our experimental results can validate the stable and controllable nonlinear absorption behavior of the ReSe2, and can make inroad for the nonlinear optoelectronic devices’ applications with TMDs.
Funding
National Natural Science Fund Foundation of China (NSFC) (61475102, 11574079 and 61775056); and the Joint Equipment Pre-Research Foundation of the Ministry of Education of China (Grant 6141A02033404); Natural Science Foundation of Hunan Province under Grant 2017JJ1013.
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