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Abstract
Group IVB (Hf) transition metal dichalcogenides (TMDs) have attracted significant interest in photoelectronics due to their predictable superior physical characteristics. In this work, the liquid phase exfoliation method is used to prepare the hafnium diselenide (HfSe2)/polyvinyl alcohol (PVA) saturable absorber (SA) device. The modulation depth (ΔT) is measured to be 6.65%. By using HfSe2/PVA as a Q-switcher, application in the fiber laser with the Q-switching state is demonstrated experimentally. The maximum single pulse energy is 167 nJ is and the slope efficiency is 7.7%. To our best knowledge, this is the first report of the use of HfSe2 as SA for large energy pulse generation. The experimental results prove that, because of its excellent nonlinear optical absorption properties, HfSe2 could promote the development of Hf-based TMDs in the field of ultrafast photonics.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Two-dimensional (2D) materials have received extensive attention for materials, optoelectronic and medical applications, including transistors, optical switching, solar cells, high-speed wavelength conversion, and saturable absorbers (SAs) [1–5]. Compared with zero-dimensional or one-dimensional (1D) nano-structure materials, 2D materials are linked by Van der Walls bond to form atomically thin 2D layered structure, possessing compatibility with current nano-material preparation technology [6–9]. In addition, the large surface-to-volume ratio of 2D layered materials can create stronger interaction between light and materials, which makes these 2D layered materials widely used in optoelectronic devices [10–16]. Especially, owing to the outstanding characteristics of large third-order nonlinear susceptibility, wideband absorption, and ultrafast carrier dynamics, these 2D materials have potential applications in ultrafast optics [17–19]. Graphene, with zero-gap structure and extremely high mobility of 200000 cm2/V/s, has demonstrated as a kind of ultra-wide-band SAs for pulsed lasers applications [20–22]. However, graphene, as a representative group-IVA mono-elemental material, has weak optical modulation depth and lacks electronic bandgap, which limits its applications. Topological insulators (TIs), whose narrow bandgap enhances their ability to absorb the broadband spectrum, expanding their potential applications to diverse fields. But the thermal damage threshold should be improved [23–25]. MXene [26], lead monoxide (PbO) [27], metal–organic frameworks [28] and perovskite [29–31] have unique physical and chemical properties, are becoming a hotspot. Nevertheless, the nonlinear absorption properties remain to be enhanced. Group-VA mono-elemental materials, represented by black phosphorus, antimonene and bismuthene, with a broad range of band-gaps, covering the wavelength ranging from near-infrared to visible light. They have successfully realized mode-locking operation, injecting new vitality into the study of ultrafast lasers [32–39]. Transition metal dichalcogenides (TMDs) possess the remarkable qualities of non-zero bandgap and third-order optical nonlinearity depending on layers number, which can act as SAs and have been widely investigated in nonlinear optics [40–42].
TMDs are with the formula MX2. X represents a chalcogen atom, such as S, Se, or Te. M acts as a substitute for a transition metal atom from group IVB (Ti, Zr, Hf) or group VIB (Mo, W). The trilayer sheets assembled by strong covalent bonding interactions. In the last few years, VIB (Mo and W) or VIIB (Re) group TMDs have been proved to have broadband nonlinear optical response, ultrafast electron relaxation ability, which makes them apply to passive Q-switchers, optical limiter, and mode-lockers [43–46]. In comparison with the mentioned VIB or VIIB group TMDs, IVB group TMDs also have attracted attention because of higher carrier mobility and tunneling current density [47–49]. HfSe2 is a representative IVB group TMD, whose band gap is predicted to be in a small value (0.9-2 eV). The calculated carrier mobility of HfSe2 can reach to 2500 cm2/V/s, which is much higher than that of MoS2 (400 cm2/V/s) [50]. HfSe2-based ultrafast and ultrasensitive phototransistors have been demonstrated [51]. However, the nonlinear optical properties and ultrafast photonic devices based on HfSe2 remain unexplored so far.
In this paper, we have demonstrated large energy passively Q-switched EDF laser with HfSe2 as Q-switcher. The HfSe2/PVA SA is prepared by liquid phase exfoliation technology and the nonlinear optical absorption characters are also examined experimentally. The ΔT of HfSe2/PVA is measured to be 6.65%. Based on HfSe2/PVA, Q-switched fiber laser is established with pump power varying from 87 to 160 mW. The slope efficiency is calculated to be 7.7%. The central wavelength of Q-switched fiber laser locates at 1561 nm. The maximum single pulse energy is 167 nJ. As far as we know, this is the first demonstration of using HfSe2 as SA for pulse generation in fiber laser. The experimental results show that Hf-based TMDs could be developed as a kind of effective candidate for pulsed laser applications and other nonlinear optoelectronic devices.
2. Experiments
2.1 HfSe2/PVA preparation
Weak bonding interaction force between the HfSe2 layers makes their bulk form exfoliate into layer structure easily. In this experiment, liquid phase exfoliation method is used to prepare HfSe2 nanosheets dispersion, which is a widely explored technique [52]. Deionized water, as an inorganic solvent, showing good solubility to carry out the exfoliation of 2D materials with excellent stability. So we use the deionized water as solvent. The 5 mg HfSe2 powder is dispersed into 10 ml deionized water. For dissolving the powder completely, the dispersion is sonicated for 5 hours. In further step, the HfSe2 dispersion is centrifuged at 5000 r/minute for 30 min. The photograph of HfSe2 dispersion is shown in Fig. 1(a). It can be seen that the HfSe2 dispersion shows medium yellow color. The Raman spectrum of HfSe2 is excited by 532 nm laser. It is observed that there are two Raman modes (Eg and A1g) appeared at 146 cm−1 and 203 cm−1 in Fig. 1(b), which consistents with previously published results [53]. The atomic force microscopy (AFM) is used to observe the morphology of HfSe2 nanosheets. The cross section height profile along three dotted lines are shown in Fig. 2, indicating the thickness of HfSe2 nanosheets distributes among 8-15 nm. In order to increase the processability and compatibility, the HfSe2 nanosheets are embedded into PVA. Via dissolving the PVA powder in deionized water and putting it into ultrasonic stirrer for 1 hour, we can obtain the liquid PVA solution. Then 5 mL HfSe2 dispersion and 10 mL PVA solution are blended uniformly by a magnetic stirrer. In next step, the mixture are poured into polystyrene cells. For the further evaporating process, we put these cells filled with mixture into an oven. After two days, the HfSe2/PVA thin film is formed, coating on the wall and bottom of cells. The HfSe2/PVA film on the wall of cell can be used as SAs.
2.2 Q-switched fiber laser setup
The experimental setup of EDF laser with HfSe2 as Q-switcher is shown in Fig. 3. As many other reports [54,55], we use the ring-shaped structure laser cavity to generate pulse. The total length of laser cavity is 15.12 m. The laser is amplified by a piece of EDF (38 cm), whose absorption coefficient is 110 dB/m at 976 nm. Through a wavelength division multiplexer (WDM), a 976 nm laser diode (LD) with the maximum power of 450 mW is used as the pump source. To keep the laser pulse sequence operates unidirectionally, we use a polarization independent isolator (PI-ISO). To obtain large energy pulse output, a 30/70 optical coupler (OC) is used. A polarization controller (PC) is within cavity to make the laser polarization sates adjustable. 5 m single mode fibers (SMFs) are added into the cavity for obtaining enough nonlinearity. The HfSe2/PVA is connected into laser cavity via a flange to form a sandwich-like structure. The optical spectrum are measured by an optical spectrum analyser (YOKOGAWA AQ6370D) with the minimum resolution of 0.02 nm. The optical pulse performance are monitored by a 1 GHz digital oscilloscope (Rohde & Schwarz RTO1014) combined with a 5 GHz InGaAs photodetector (Thorlabs DET08CFC). The average output power is measured by a digital power meter (JDSU OLP-85).
3. Results and discussion
3.1 Nonlinear optical characteristics of HfSe2/PVA
With an all-fiber balance twin-detector measurement setup, we study the nonlinear optical saturable absorption characteristics of HfSe2/PVA type SA. A self-made nonlinear polarization rotation mode-locked EDF laser (pulse duration is 510 fs and repetition rate is 21.6 MHz) is used as laser source. The fiber laser operates at 1560 nm. The measured experimental data are fitted by the following formula:
Where ${\alpha _s}$, ${\alpha _{ns}}$ and ${I_{sat}}$ represent the saturable absorption, nonsaturable absorption and saturation intensity, respectively. The function of 1-$\alpha (I )$ is used to fit the transmission intensity. The nonlinear transmission curve of HfSe2/PVA SA is exhibited in Fig. 4. The ΔT, ${I_{sat}}$ and ${\alpha _{ns}}$ are estimated to be around 6.65%, 18.8 MW/cm2 and 37.35%.3.2 HfSe2 Q-switched fiber laser
The Q-switching threshold of HfSe2/PVA EDF laser is 70 mW. In the pump power range of 70-160 mW, the HfSe2/PVA EDF laser can always keep Q-switching operation. Figure 5(a) demonstrates pulse trains evolution with variation of pump power. The pulse trains are measured at different pump powers. The pulse trains generated from HfSe2 Q-switched fiber laser have no intensity modulation as pump power increased, indicating the Q-switching operation is very stable. As is shown in Fig. 5(b), the optical spectrum is record at the pump power of 160 mW, which has a central wavelength of 1561 nm.
Fig. 5. (a) Typical oscilloscope traces the Q-switched pulse trains under different pump power; (b) the optical spectrum of Q-switched EDF with HSe2/PVA.
The typical features of HfSe2 Q-switched fiber laser are illustrated in Fig. 6. As shown in Fig. 6(a), with pump power increasing from 70 to 120 mW, the pulse width decreases from 25.55 µs to 5.1 µs. While when pump power increases from 120 to 160 mW, the pulse width remains almost unchanged. Under the pump power of 160 mW, we measure the shortest pulse duration to be 4.5 µs. The relationship between repetition rate and pump power is also demonstrated in Fig. 6(a). The repetition rate varies from 12.97 kHz to 45 kHz as pump power increases from 70 mW to 160 mW. The linear relationship between average output power and pump power is observed in Fig. 6(b). The slope efficiency is calculated to be 7.7%. As pump power increases to 160 mW, the average output power boosts to 7.5 mW, corresponding to the maximum single pulse energy of 167 nJ. In order to evaluate the long term stability of Q-switching state, the experiments have been performed over 48 hours at pump power of 160 mW. The pulse trains remain reasonably stable and no damage of the HfSe2/PVA SA is observed. To verify whether the Q-switching operation is purely contributed by the saturable absorption of the HfSe2, we have performed the comparison experiments. When we remove the HfSe2/PVA out of the laser oscillator, we could not observe Q-switching operation despite of rotating the PC and adjusting the pump power. The comparative results show that Q-switching operation is indeed contributed by the saturable absorption of the HfSe2. The passive mode-locking operation has not been observed in this work. It is widely accepted that the mode-locking operation usually needs high intra-cavity power. The HfSe2/PVA Q-switcher is prepared by mixing the HfSe2 nanosheets into PVA film. The PVA has the low thermal damage. So the Q-switching fiber laser works in the 70-160 mW pump range. Besides, the 30/70 optical coupler is used. Therefore, the intra-cavity power is relative low. Thus, the passive mode-locking phenomenon does not appear due to the relative low intra-cavity power.
Fig. 6. (a) The repetition rate and pulse width as functions of pump power; (b) Output power and pulse energy as function of pump power.
4. Conclusion
In conclusion, HfSe2/PVA SA has been prepared using liquid phase exfoliation method and inserted into EDF laser cavity as Q-switcher to generate large energy pulse. It is the first demonstration that the HfSe2 exhibits excellent nonlinear optical absorption property. For HfSe2 Q-switched fiber laser, the maximum single pulse energy is 167 nJ and the slope efficiency is 7.7%. This work shows great potential in laser technology of Hf-based TMDs. For the development of group IVB TMDs material frontiers, more optical properties should be studied.
Funding
National Natural Science Foundation of China (61705183); Natural Science Foundation of Shaanxi Province, China (2019JQ-446); Young Talent Fund of University Association for Science and Technology in Shaanxi, China (20190113); Science Research Foundation of the Education Department of Shaanxi Province, China (19JK0811).
Disclosures
The authors declare no conflicts of interest.
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