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Abstract
In this paper, two-dimensional Graphdiyne and Hexakis-[(trimethylsilyl)ethynyl]benzene nanosheets were prepared using the liquid-phase exfoliation method and were then successfully applied to 1.06 µm passively Q-Switched all-solid-state lasers. The Hexakis-[(trimethylsilyl)ethynyl]benzene was applied for the first time in passively Q-Switched all-solid-state lasers, as we know. For Graphdiyne, the Q-Switched pulse achieved a narrowest pulse width of 415 ns, a maximum repetition frequency of 244.2 kHz, a maximum pulse energy of 133.53 nJ, and peak power of 321.77 mW was obtained. While, the narrowest pulse width, maximum repetition frequency, maximum pulse energy, and peak power for Hexakis-[(trimethylsilyl)ethynyl]benzene are approximately 398.4 ns, 297.1 kHz, 89.61 nJ, and 220.39 mW respectively. The findings demonstrate the promising potential of both candidates as saturable absorbers for signal modulation in solid-state lasers.
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1. Introduction
In recent years, a variety of saturable absorbers have been utilized in passively Q-switched all-solid-state lasers, showcasing significant potential for applications such as laser marking, distance measurement, microsurgery, and materials processing [1]. The diode-pumped, passively Q-switched Nd:YAG lasers exhibit superior compactness, cost-effectiveness, and efficiency compared to other laser types, rendering them highly suitable for a diverse range of applications [2,3]. The utilization of a saturated absorber enables the attainment of shorter pulse durations, increased repetition rates, and enhanced peak power in pulse laser modulation [4]. And more and more two-dimensional (2D) saturable absorbers with excellent optical properties are being discovered, such as graphene [5,6], topological insulator, MoS2 [7–9]and black phosphorus [10–12]. This provides scholars with greater motivation and inspiration to explore materials that are more economical, easier to prepare, and possess good absorption properties. In 2018, Feng et al. utilized MXene Ti3C2Tx as a saturable absorber in passively Q-switched all-solid-state lasers [13]. In 2018, X Su et al. adopted two-dimensional ReS2 as saturable absorber in the solid-state lasers experiment, which demonstrated the potential of ReS2 for generating Q-switched and mode-locked pulsed lasers [14]. In 2019, Q. Yang et al. demonstrated the use of γ-graphyne as a saturable absorber to achieve passive Q-switched solid-state lasers emitting wavelengths of 639 nm, 1342 nm, and 1932nm [15].
Graphdiyne (GDY) [16], follows fullerene(0D) [17], carbon nanotubes (1D) [18], graphene (2D) [19], both are all-carbon nanostructured material. While, GDY by virtue of the combination of sp- and sp2- hybridization that distinguishes it from other carbon-group materials, giving it a number of unique structures and properties, which has attracted widespread attention in the field of two-dimensional materials [20]. GDY exhibits excellent physical and chemical properties, making it highly promising for applications in optoelectronics, energy storage, catalysis, sensing, etc [21–25], based on its structural characteristics. As we have known, few studies have been reported on the characterization and application of HEB-TMS (a precursor for the synthesis of GDY), and most of these studies only focus on synthesizing high-quality GDY from it. The structure of HEB-TMS also includes two types of carbon atom structures with sp- and sp2-hybridization modes, which bear some resemblance to the structure of GDY [20]. In 2015, E Sheka performed a further study on the triple carbon bonds and structure of GDY [26]. In 2015, S Jalili et al. found that the intrinsic hole and electron mobility of GDY NTs could reach the orders of 102 and 104 cm2 V−1 s−1, respectively [27]. Since there are fewer studies on the application of 2D GDY nanosheets and 2D HEB-TMS nanosheets in solid-state lasers, this has inspired and motivated our research.
In this study, 2D GDY and 2D HEB-TMS nanosheets were prepared and characterized. And utilizing GDY and HEB-TMS as saturated absorbers for achieving signal modulation, a 1.06 µm passively Q-switched solid-state laser was constructed. The narrowest pulse widths are 398.4 ns and 415 ns, the maximum repetition frequencies are 297.1 kHz and 244.2 kHz, the peak powers are 220.39 mW and 321.77 mW, and the single pulse energies are 89.61 nJ and 133.53 nJ for GDY and HEB-TMS, respectively, thereby indicating a better prospect for the application of our chosen saturable absorber. The experimental results demonstrate the significant untapped potential of these two materials in the realm of solid-state lasers.
2. Sample preparation
In this experiment, 2D GDY nanosheets and 2D HEB-TMS nanosheets were successfully prepared using the liquid phase exfoliation (LPE) method, as illustrated in Fig. 1. The primary experimental equipment employed included sample powder, a clean mortar, and anhydrous ethanol.
Fig. 1. Powders used and nanosheets dispersions prepared for experiments (a) GDY powder (a’)2D GDY nanosheets (b) HEB-TMS powder (b’)2D HEB-TMS nanosheets
The sample preparation process is as follows. Weigh 3 mg of GDY powder (Fig. 1(a)) and transfer it into a clean and sterile mortar for grinding for approximately 5 hours using the assisted exfoliation method with anhydrous ethanol. The preparation of HEB-TMS nanosheets are similar to that of GDY. After undergoing ultrasonication and centrifugation, a dispersion of GDY and HEB-TMS nanosheets are obtained, as depicted in Fig. 1(a’) and Fig. 1(b’), respectively. Furthermore, after a 10-hour resting period, no discernible macroscopic precipitates are observed in the GDY and HEB-TMS dispersion, indicating their excellent dispersion properties. It is demonstrated that the effective preparation of 2D GDY nanosheets and 2D HEB-TMS nanosheets was successfully achieved by LPE method.
3. Characterization
The scanning electron microscopy (SEM) images of GDY powder and GDY nanosheets are shown in Fig. 2(a) and Fig. 2(b), respectively. The SEM images of the GDY powder revealed a significant presence of pore-like structures, which are formed by an extensive aggregation of nanoparticles. The statement also suggests that GDY can be prepared on a two-dimensional scale through techniques such as LPE or mechanical stripping. The SEM of the GDY nanosheets is shown in Fig. 2(b), with a size distribution ranging within a few micrometers. The atomic force microscopy (AFM) results in Fig. 2(c) and Fig. 2(d) illustrate that the as-prepared GDY nanosheets have a thickness of approximately 4 nm, with no discernible fluctuations in their thickness.
Fig. 2. SEM images of (a) GDY powder and (b) GDY nanosheets, (c) AFM image of a GDY nanosheet, (d) Height relative to that corresponding to the trajectory in figure c
The Raman spectra also responds to the structural features of the GDY powder as shown in Fig. 3. The peak of the shear vibration of the sp2- hybridized carbon atom in the aromatic ring structure is shown at 1350 cm-1 (D band). The peak at 1572 cm-1 (G band) is due to the stretching vibration of the carbon atom in the same phase in the aromatic ring. The ratio of the peak intensities in the D band to that in the G band is 0.8, indicating a higher degree of crystalline ordering within the crystal structure. In addition, the vibrations of the conjugated bis-alkynyl bond -C≡C-C≡C- at 1926cm-1 and 2194 cm-1 can be observed in the Raman results of the inset in Fig. 3 [28].
The SEM and AFM images of HEB-TMS and HEB-TMS nanosheets are shown in Fig. 4. The morphological characteristics of HEB-TMS powder and nanosheets are depicted in Fig. 4(a) and Fig. 4(b), respectively, revealing a distinct bulk structure for the HEB-TMS powder with evident delamination occurring at the edges. This indicates that it is capable of being stripped from layer to layer by the LPE method. Figure 4(b) shows that the size range of the HEB-TMS nanosheets is around a few micrometers. The delineated regions of lines 1, 2, and 3 in Fig. 4(c) correspond to the three height profiles from bottom to top in Fig. 4(d), respectively. The AFM characterization revealed that the thickness of the HEB-TMS nanosheets measured approximately 3.49-3.94 nm. Due to the potential occurrence of folding or aggregation of the nanosheets during the drying process in AFM preparation, it is possible that the actual thickness of the prepared nanosheets may fall below this range.
Fig. 4. (a) SEM image of HEB-TMS crystal, (b) SEM image of HEB-TMS nanosheets, (c) AFM image of HEB-TMS nanosheets, (d) corresponding heights of AFM image
The Raman results of HEB-TMS nanosheets are shown in Fig. 5. The Raman activity peaks of HEB-TMS nanosheets are located at 1312, 1514, 2148, and 2899 cm-1, respectively. The Raman peaks at 1312 and 1514 cm-1 are caused by two different vibrational modes of carbon atoms on the benzene ring. The Raman peak at 2148 cm-1 is caused by the stretching vibration of the carbon-carbon triple bond, and the Raman peak at 2899 cm-1 is caused by the stretching vibration of the C-H bond.
4. Experimental section
The saturable absorption properties of 2D GDY nanosheets and 2D HEB-TMS nanosheets were investigated in this experiment using a dual-arm probing method. Figure 6 illustrates the optical path configuration of the dual-arm probing system.
The laser emits light, which then enters the Granger mirror and subsequently passes through the lens. Finally, the beam is split into two by the beam splitter. One beam enters the energy meter A, and the other beam passes through the sample and enters the energy meter B. In this experiment, the power density of the emitted light is controlled by rotating the Granger mirror. The transmittance trend of the sample is observed to vary with the increase in power density of the light, ultimately yielding saturable absorption response data for 2D nanosheets under 1064 nm laser illumination.
In this experiment, a picosecond pulsed laser with an output wavelength of 1064 nm, a pulse width of 350 ps, a spot diameter of 2 mm, and a frequency of 100 Hz was used as the light source. And the data obtained from the experiment are fitted using Eq. (1).
In Eq. (1), where T(I) is the normalized transmittance, ΔT is the modulation depth, I is the incident light intensity, Isat is the saturation intensity, and Tns is the unsaturated loss.
Figure 7 and Fig. 8 shows the experimental results of GDY nanosheets and HEB-TMS nanosheets. The sample transmittance increases as the incident light power density gradually intensifies, until it reaches a certain threshold (saturation threshold). At this point, the sample transmittance stabilizes and remains constant for a period of time. Fitted with Eq. (1), the modulation depth, saturation intensity and non-saturation loss of the GDY nanosheets are 17.14%, 0.84 WM/cm2 and 11.75%, respectively, as evidenced by the data in Fig. 7. And the modulation depth, saturation intensity and non-saturation loss of the HEB-TMS nanosheets are 7.74%, 1.25 WM/cm2 and 14.66%, respectively, as shown in Fig. 8. The experimental results provide evidence for the saturable absorption properties of 2D nanosheets in both materials.
For solid laser experiments, the schematic diagram is shown in Fig. 9. The pump light is a diode laser with an output wavelength of 808 nm and parameters of φ=100 µm, N.A. = 0.22. The pump output beam is directed through a focusing system onto a 0.4 at.% doped Nd:YAG crystal with dimensions of 4 × 4 × 7 mm3 and coated with a highly reflective film at 1064 nm. The crystal is continuously cooled down by water cooling in order to prevent damage caused by high temperatures during the experiment. The laser resonant cavity consists of plane mirror M1 and concave mirror M2. The plane mirror M1 is coated with antireflection film at 808 nm and highly reflective film at 1064 nm. Mirror M2 has a transmittance T of 10% at 1.06 µm with a radius of curvature R of 100 mm. The output power and pulse train of the solid-state laser are recorded and saved by a power meter (Powermax 500D, Molectron Inc.) and an oscilloscope (DPO7104, Tektronix Inc.).
The results of the passively Q-Switched experiments of GDY and HEB-TMS at 1.06 µm are shown in Fig. 10 and Fig. 11. When there is no saturable absorber present in the laser resonant cavity, the output signal of the laser system becomes continuous wave (CW), and the passively Q-switched signal can be detected upon introduction of the saturable absorber.
Fig. 10. Passively Q-switched results of GDY at 1064 nm. (a) CW and Q-switched output power (insert: output spectrum of GDY Q-switched); (b) Pulse width and repetition rate; (c) Pulse energy and peak power; (d) Q-switched pulse
Fig. 11. Passively Q-switched results of HEB-TMS at 1064 nm. (a) CW and Q-switched output power (insert: output spectrum of HEB-TMS Q-switched); (b) Pulse width and repetition rate; (c) Pulse energy and peak power; (d) Q-switched pulse
The emergence threshold of Q-Switched pulse, as depicted in Fig. 10(a), is approximately 0.92 W. Subsequently, the output power of Q-Switched gradually amplifies with an increase in absorbed pump power. The maximum output power is about 32.61 mW when the absorbed pump power is 1.16 W, according to the optical-to-optical conversion efficiency is about 34.7%. And the Q-Switched conversion efficiency is about 8%. In the experiment, a stable Q-Switched output can be obtained with an absorption pump power in the range of 0.92-1.16W. When the pump power continues to increase, there is a significant change in the stability of the Q-Switched pulse, resulting in concurrent pulses. However, during this time, the GDY saturated absorber is not damaged and the experimental results are basically consistent when repeating the test. The inset part of Fig. 10(a) displays the emission spectrum, with the center wavelength located at 1064 nm. The trend of the pulse width and repetition frequency of the GDY with the absorbed pump power is shown in Fig. 10(b). As the absorbed pump power increases, the pulse width gradually decreases while the repetition frequency shows an opposite trend. The narrowest pulse width and the maximum repetition frequency is 415 ns and 244.2 kHz, respectively. The highest pulse energy and peak power is 133.53 nJ and 321.77 mW, respectively, as shown in Fig. 10(c). Figure 10(d) shows the corresponding Q-Switched pulse sequence and single pulse images at 244.2 kHz and 415 ns.
The experimental results of HEB-TMS are shown in Fig. 11(a). In the range of absorbed pump power of 1.12-1.80 W, a passively Q-switched signal can be observed. At the absorbed pump power of 1.80 W, the maximum CW output power is 788 mW, and the maximum Q-Switched output power is 26 mW. The optical-to-optical conversion efficiency is calculated to be approximately 43.78%, and the Q-Switched conversion efficiency is about 3%. The inset portion of Fig. 11(a) shows the emission spectrum and the center wavelength is found to be located at 1064 nm. Figure 11(b) shows the pulse width and repetition frequency. As the absorbed pump power increases, the pulse width gradually decreases while the repetition frequency gradually increases. The minimum pulse width and maximum repetition frequency are about 398.4 ns and 297.1 kHz, respectively. Figure 11(c) shows the variation of pulse energy and peak power with the absorbed pump power. The maximum pulse energy and peak power are 89.61 nJ and 220.39 mW, respectively. The pulse train and single pulse at 398.4 ns and 297.1 kHz are shown in Fig. 11(d).
The experimental data of passively Q-switched all-solid-state laser of GDY and HEB-TMS indicate that they have low Q-Switching conversion efficiency during the experiment, and non-saturated loss is a prevalent phenomenon. This is partly attributed to the choice of substrate for preparing the saturable absorber. During the experimental process, the uncoated quartz substrate exhibits significant light loss to the laser, resulting in a decrease in Q-Switched conversion efficiency.
In Table 1, the performance of several materials in passively Q-switched solid-state lasers is compared. The pulse width of GDY is better than that of Bi2Se3 in comparison. Although the pulse width of GDY is not as good as BP, it has obvious advantages in terms of pulse energy. For HEB-TMS, it achieves lower pulse width and higher peak power and pulse energy than that of Bi2Se3. The experimental results show that both have great potential for applications, especially HEB-TMS has a broad prospect that remains unexplored.
Table 1. Comparison of passively Q-switched solid-state lasers for two-dimensional materials
5. Conclusions
In this experiment, HEB-TMS was used as the saturable absorber to generate pulsed lasers in the near-infrared range for the first time. GDY was also applied in the solid-state lasers for comparison. Characterisation and nonlinear experiments of GDY and HEB-TMS show that both materials have significant saturable absorption properties. Both materials had been applied as saturable absorbers in the 1.06 µm passively Q-Switched all-solid-state laser. Although the peak power and pulse width energy for the all-solid-state laser with the HEB-TMS saturable absorber were smaller than that with the GDY saturable absorber, but it could get a narrower pulse width comparing to that with GDY saturable absorber. This work demonstrated the feasibility of GDY and HEB-TMS as saturable absorbers and this discovery had greatmarket promise and potential applications in ultrafast laser. Our work not only filled the gap in the field of lasers, but also lays a solid foundation for further research on HEB-TMS in future.
Funding
National Natural Science Foundation of China.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant No. 11704227).
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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