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Abstract
Based on the saturable absorption feature of a two-dimensional (2D) nano-material, antimonene, the passively Q-switched operation for solid-state laser was realized for the first time. For the 946 and 1064 nm laser emissions of the Nd:YAG crystal, the Q-switched pulse widths were 209 and 129 ns, and the peak powers were 1.48, 1.77 W, respectively. For the 1342 nm laser emission of the Nd:YVO4 crystal, the Q-switched pulse width was 48 ns, giving a peak power of 28.17 W. Our research shows that antimonene can be used as a stable, broadband optical modulating device for a solid-state laser, which will be particularly effective for long wavelength operation.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In 2004, the appearance of graphene opened a new world of two-dimensional materials [1]. Thereafter, topological insulators [2] and transition-metal dichalcogenides [3] successively became the research hotspots as potential candidates for electronic and optoelectronic devices. In recent years, black phosphorus (BP) has attracted much attention as a material composed by group-V element [4–9]. Its electronic bandgap is adjustable by thickness, and such property fills up the lacuna left by graphene [8], topological insulators, and transition-metal dichalcogenides. In addition, the high carrier mobility [9] and strong intrinsic in-plane anisotropy [5] expand its applications in electronics and optoelectronics [8,10,11]. However, the thin BP samples are highly unstable in ambient atmosphere, and the monolayer or few-layer BP may degrade within hours. Such defect limits its application to a great extent. Besides, the lack of scalable, reliable synthesis methods is still a big impediment.
Therefore, people start to consider other group-V elements to prepare two-dimensional (2D) materials, such as the allotrope of phosphorus [12], arsenic (As), antimony (Sb), and bismuth (Bi) [13–18]. Among them, bulk Sb is semimetallic, which is predicted to become a topological insulator when the number of layers is below 22, and presents a quantum spin Hall phase below 8 layers [19]. Antimonene is very stable and possesses many advantages, such as good thermal conductivity [20], high mobility [16], excellent thermoelectric figure of merit [21], and good optical properties like high refractive index [22] as well as broad absorption band [23]. By time now, several methods have been used to make few-layer antimonene successfully, including molecular beam epitaxy [24], van der Waals epitaxy [25], liquid phase exfoliation [26], and mechanical isolation [27]. Some antimonene-based heterostructures and quantum dots were developed for electronic and optical applications [28–30]. Both the first-principle calculations and the spatial self-phase modulation (SSPM) experiments have manifested that antimonene can be used as excellent nonlinear photonics devices [23,30]. The electronic structure of the monolayer antimonene shows it is a semiconductor with an indirect band gap of 2.28 eV. While for few-layer antimonene, the band gap decreases to 0 eV and a metallic character is presented [13]. Correspondingly, the absorption band is greatly expanded. Under intense laser field, wideband SA effect can be realized because of the Pauli blocking of the conduction band and the depletion of the valence electrons. Recently, based on the saturable absorption effect of few-layer antimonene, the mode-locking of 552 fs and Q-switching of 1.3 ~2.2 μs were realized for erbium-doped fiber laser (~1558 nm) [31].
In present work, the few-layer antimonene sample was prepared from bulk Sb by liquid phase exfoliation method. By fitting the nonlinear absorption of prepared antimonene saturable absorber (SA) for nanoseconds laser pulses, the saturation intensities were determined to be 0.43 MW/cm2 at 532 nm and 0.53 MW/cm2 at 1064 nm, respectively. With antimonene as SA, the passively Q-switched laser operations were realized at 946, 1064 nm in Nd:YAG crystal, and 1342 nm in Nd:YVO4 crystal. Among them, the optimum experimental results came from the 1342 nm Nd:YVO4 laser, including the shortest pulse width of 48 ns, the largest single pulse energy of 1.36 μJ, and the highest peak power of 28.17 W.
2. Antimonene preparation and structure characterization
The few-layer antimonene was produced from bulk Sb by liquid phase exfoliation method. The experimental samples at different stages were shown in Fig. 1. Firstly, 200 mg of the bulk Sb with purity of 99.999% was milled in an agate mortar for 30 minutes. Then the produced Sb powder was collected and dispersed in absolute ethanol with ultrasonic pulverization for 2 hours. The dispersion was settled in atmosphere for 3 days. After removing the deposited particles, the remaining supernatant was used as the antimonene dispersion for various characterization. When it was coated on a quartz substrate, the dried component was use as the saturable absorber for solid-state laser experiments.
Fig. 1 Photographs of the original bulk Sb (a), the antimonene dispersion in absolute ethanol (b), and the quartz saturable absorber coated with antimonene (c).
A field-emission scanning electron microscope (SEM, Hitachi S-4800) was used to observe the sample morphology. The milled Sb powder displays representative solid agglomerates with typical dimensions of several micrometers, as shown in Fig. 2(a). The stacked, layered structure includes many cleavage steps and cleavage planes, which manifests that the binding force among layers is weak. This result coincides with the theoretical analyzing, i.e. the layer-to-layer interaction inside Sb is van der Waals force [13]. A typical image of antimonene sample obtained from the dispersion is presented in Fig. 2(b). The antimonene nanosheets exhibit a flaky morphology, which is significantly different with the appearance of bulk Sb. They are almost transparent due to the ultrathin nature.
The Atomic force microscopy (AFM, Nanoscope Multi Mode V, Digital Instruments/Bruker Systems) was used to measure sample thickness. The as-prepared antimonene dispersion was deposited on a SiO2 substrate and dried for 24 hours before AFM measurement. A typical AFM image of the Sb nanosheets is shown in Fig. 3. The average thickness is ~6 nm, corresponding to 15 layers. At some special positions, like the right end of the sampling line, the thickness increases to 12 nm, corresponding to 30 layers or so.
To further confirm that the experimental sample was few-layer antimonene, we measured the Raman spectra (excitation wavelength: 632 nm) of Sb powder and antimonene dispersion deposited on glass substrate, as shown in Fig. 4. For the Sb powder obtained from bulk material, there are two principal Raman peaks, one is Eg peak at 112.4 cm−1 which is an in-plane vibrational mode, the other is A1g peak at 148.8 cm−1 which origins from an out-of-plane vibrational mode. For the antimonene nanosheets, a representative test result is plotted with red curve, where the Eg peak is at 118.7 cm−1, and the A1g peak is at 155.1 cm−1. Referencing the research results of Ji et al [25], these data corresponds to a thickness of 12 nm or so, i.e. about 30 layers. In our measurements, compared with bulk Sb, antimonene nanosheets present the largest A1g peak blueshift to 156.2 cm−1, exhibited by the pink curve in Fig. 4. It corresponds to a thickness of 10 nm or so, i.e. about 25 layers. This curve, or to say, this thickness of antimonene nanosheets, is the measurement limit of our instrument. The thinner antimonene doesn’t show any measurable Raman signal. This situation is similar as what was reported in the reference [26], which the measurement ultimate was 17 layers. Generally speaking, compared with other familiar 2D materials like graphene, MoS2, or BP [32–34], the Raman response of antimonene is relatively weak. By time now, although several documents have experimentally revealed the relationship between Raman frequency shift and antimonene thickness [25–27,35], but as we have known no one has observed the Raman response of single layer antimonene, and the thinnest antimonene demonstrated by Raman experiment was 4 nm, i.e. 10 layers or so [35]. The very low intensity of the Raman signal for a monolayer (> 1000 times less Raman active than for the bulk case) precludes its detection [26]. The blue curve in Fig. 4 is the Raman spectrum of few-layer antimonene which was preserved in air for one month. It can be seen that it only presents a small change (red shift ~1 cm−1) compared to the fresh sample (red curve). This performance proves the good stability of few-layer antimonene in air, unlike black phosphorus.
In short, the AFM experiment indicates that our antimonene nanosheets are 15 ~30 layers (with thicknesses of 6 ~12 nm). It is further verified by the Raman spectrum measurement (≤ 30 layers, i.e. 12 nm), at the same time the good stability is demonstrated.
3. Linear transmission property
The linear transmission spectrum of the prepared antimonene SA was measured in the range of 200 - 2000 nm by an UV/VIS/NIR spectrophotometer (U-3500, Hitachi, Japan), as shown in Fig. 5. As a comparison, the transmission of the blank quartz substrate was also recorded under the same conditions. In the scope of 900 ~1500 nm, the transmittances of the antimonene SA and the quartz substrate are 81.5 ± 1.0% and 88.5 ± 1.0%, respectively. So the net transmittance of the antimonite nanosheets is about 92%, giving a scattering loss of 8%.
4. Saturable absorption performance
The saturable absorption property of prepared antimonene SA was directly characterized by measuring its transmittance variation with the incident power density. The fundamental wave (1064 nm) and its frequency doubling wave (532 nm) of a 10 Hz electro-optic Q-switched Nd:YAG laser were used as the light source, with pulse durations of 6 ns and 5 ns, respectively. As background experiment, the transmission of the blank quartz substrate was measured at different power intensities, and no nonlinear optical effect was observed. The saturable absorption experimental data are shown in Fig. 6, which can be fitted by the followed formulae [36]:
where T is the transmittance of the sample, A is a normalization constant, δT is the absolute modulation depth, I is the incident intensity, and Is is the saturation intensity. The absolute modulation depths of the antimonene SA were fitted to be 8.0%, 4.9% at 532, 1064 nm, respectively. Considering the 80%, 81% initial transmittance at these two wavelengths, in theory the ultimate transmittance can reach 86% beyond. The fitted saturation intensity Is are 0.43, 0.53 MW/cm2 at 532 and 1064 nm, respectively. As shown in Fig. 6, when the 1064 nm incident intensity is 1.6 MW/cm2, the actual modulation depth of the antimonene SA is 3.5% (81% → 84.5%), which implies a high remaining loss of 15.5%. During passively Q-switching operation of solid-state laser, it will led to a high pump threshold, small output power, and low conversion efficiency.Fig. 6 Variation of nonlinear transmittance with the incident power intensity for antimonene SA at 532 and 1064 nm.
5. Passively Q-switched operation
Using antimonene as SA, we realized passively Q-switched operations for different Nd3+ solid-state lasers. Figure 7 shows the schematic of the experimental set-up. The pump source was an 808 nm fiber-coupled diode laser (ϕ = 100 µm, N. A. = 0.22). Its output beam was delivered into the laser crystal by a focusing system whose transfer ratio was 1:1. For 946 nm and 1.06 μm laser operations, the gain medium was a 0.4 at.% doped Nd:YAG crystal with dimensions of 4 × 4 × 7 mm3. Its transmission surfaces were anti-reflection (AR) coated at 0.9 ~1.1 μm waveband. For 1.34 μm laser operation, the gain medium was a 0.5 at.% doped Nd:YVO4 crystal with dimensions of 3 × 3 × 5 mm3. Its transmission surfaces were simultaneously AR coated at 1.06 and 1.34 μm, to avoid the prior oscillation of 1.06 μm laser. The change of laser medium was because the Nd:YAG sample had a large loss at 1.3 μm, which was not appropriate for 1.3 μm laser experiments. The laser crystal was wrapped in indium foil and held in a copper block which was cooled by circulating water with fixed temperature of 15 °C. The antimonene coated quartz substrate acted as the passively Q-switcher. The laser resonator was composed of a flat mirror M1 and a concave mirror M2. M1 was AR coated at the pump wavelength of 808 nm, and high-reflection (HR) coated at working wavelength. M2 was partially transmitted at working wavelength. Under our existing conditions, the optimized M2 parameters for passively Q-switched laser operations were T = 5%, R = 50 mm @ 0.9 μm, T = 10%, R = 100 mm @ 1.06 μm, and T = 5%, R = 100 mm @ 1.34 μm, respectively. Here T was the transmittance at working wavelength and R was the curvature radius. For all of the laser experiments, the cavity length between M1 and M2 was kept nearly the same, i.e. 25 ± 2 mm. The average output power was measured by a power meter ((Powermax 500D, Molectron Inc.), and the temporal behaviors of laser pulses were recorded by a digital oscilloscope (DPO7104, Tektronix Inc.) with a photodiode detector.
When the antimonene SA was removed from the resonator, the continuous-wave (CW) laser output was measured. By inserting the antimonene SA into the laser cavity, the passively Q-switched operation was realized. The detailed results were presented in Figs. 8-10.
Fig. 8 946 nm passively Q-switched laser characteristics of antimonene. (a) Output power. The inset is the laser emission spectrum. (b) Pulse width and repetition frequency. (c) Single pulse energy and peak power. (d) Pulse train with a repetition rate of 268.3 kHz and the corresponding pulse profile with a width of 208.8 ns.
Fig. 9 1.06 μm passively Q-switched laser characteristics of antimonene. (a) Output power. The inset is the laser emission spectrum. (b) Pulse width and repetition frequency. (c) Single-pulse energy and peak power. (d) Pulse train with a repetition rate of 569.1 kHz and the corresponding pulse profile with a width of 129 ns.
Fig. 10 1.34 μm passively Q-switched laser characteristics of antimonene. (a) Output power. The inset is the laser emission spectrum. (b) Pulse width and repetition frequency. (c) Single-pulse energy and peak power. (d) Pulse train with a repetition rate of 28.65 kHz and the corresponding pulse profile with a width of 48.33 ns.
From Fig. 8(a), it can be seen that the maximum CW power is 1.32 W and the corresponding optical-to-optical efficiency is 32.8% with respect to the absorbed pump power of 4.02 W. The threshold pump power for the Q-switched operation of 946 nm is 2.4 W. The average output power is almost linear increasing with the absorbed pump power. In the pump region of 2.4 ~4 W, stable Q-switching operation is observed. The highest Q-switched average output power is 83 mW at a pump power of 4.02 W. When the pump power is elevated further, the pulse performance degrades apparently, although the antimonene SA has no obvious damage. The pulse sequence becomes disordered, and the satellite pulse starts to appear. The measured laser emission spectrum is shown in the inset of Fig. 8(a). The central wavelength locates at 946 nm. The pulse characteristics versus pump power at 946 nm are shown in Figs. 8(b)-8(d). With the increase of pump power, the repetition rate, single pulse energy, and peak power elevate, and the pulse width decrease. The achieved shortest pulse width is 208.8 ns, corresponding to a repetition rate of 268.3 kHz, as shown in Fig. 8(d). The largest pulse energy is 0.31 μJ and the highest peak power is 1.48 W.
Compared to 946 nm operation, the Q-switched threshold pump power reduces to 1.4 W at the wavelength of 1.06 μm, as shown in Fig. 9(a). The pump power region for stable Q-switching is 1.4 ~3.4 W. At an absorbed pump power of 3.35 W, the maximum Q-switched average output power is 130 mW. At the same pump power, a maximum CW output power of 1.21 W is obtained, corresponding to an optical-to-optical efficiency of 36.1%. The laser emission spectrum is shown in the inset of Fig. 9 (a), and the central wavelength locates at 1064 nm. The corresponding pulse characteristics are shown in Figs. 9(b)-9(d). The shortest pulse width is 129 ns, corresponding to a repetition rate of 569.1 kHz, as shown in Fig. 9(d). The largest pulse energy and the highest peak power are 0.23 μJ, 1.77 W, respectively.
For 1.34 μm passively Q-switched operation, the threshold pump power is 3.9 W, as shown in Fig. 10(a). At an incident pump power of 5.5 W, the maximum average output power is 39 mW. At the same pump power, a maximum CW laser power of 2.12 W is obtained, and the corresponding optical-to-optical efficiency is 38.5%. The laser emission spectrum is shown in the inset of Fig. 10(a), and the central wavelength is 1342 nm. As shown in Figs. 10(b) and 10(c), at the highest pump power, the repetition rate, pulse width, single pulse energy, and peak power are 28.65 kHz, 48.33 ns, 1.36 μJ, 28.17 W, respectively. The corresponding pulse train and single pulse profile are shown in Fig. 10(d).
At all of the three wavelengths, the passively Q-switched lasers demonstrate much higher laser threshold and much lower output power than the CW lasers. With 1.34 um laser as an example, the passively Q-switched threshold and the maximum output are 3.93 W, 39 mW, respectively, while the corresponding CW data are 0.05W, 2.12 W. The Q-switching conversion efficiency with respect to the CW mode is 1.8%. For 946 and 1064 nm lasers, the corresponding Q-switching conversion efficiencies are 6.3% and 10.7%, respectively. Such results indicate that the present SA component still has notable non-saturable loss. It is quite consistent with the linear transmission and saturable absorption measurement results. As indicated by Fig. 5 and Fig. 6, for the present antimonene SA component, the linear loss of few-layer antimonene is 7.5% (81% → 88.5%), including 5% saturable loss (81% → 86%) and 2.5% non-saturable loss (86% → 88.5%), while the loss of quartz substrate is 11.5% (88.5% → 100%). Since we use a common uncoated quartz glass as the SA substrate, and its large linear loss is absolutely non-saturable, the insertion loss has been drastically increased. So the main reason of the low Q-switching conversion efficiency with respect to the CW operation comes from the substrate, not the antimonene itself.
When the wavelengths and the laser mediums are similar, the passively Q-switching performance of solid-state laser is compared for antimonene and several famous 2D materials, as well as the commercial semiconductor saturable-absorber mirror (SESAM). As shown in Table 1, the wavelengths are fixed to be 946, 1064 and 1342 nm, corresponding to the 4F3/2 → 4I9/2, 11/2, 13/2 transitions of Nd3+ ions, respectively. For 946 and 1064 nm operations, the laser medium is fixed to be Nd:YAG crystal. For 1342 nm operation, the laser medium is fixed to be Nd:YVO4 crystal, or its alternate, Nd:GdVO4. Graphene, MoS2 or WS2, and BP are selected as the representative 2D nanomaterials. Under these limitations, two cases have not been reported, i.e. BP Q-switched 946 nm laser, and MoS2/WS2 Q-switched 1342 nm laser. Among the listed pulse data in Table 1, antimonene is a little better than MoS2 and inferior to graphene and SESAM at 946 nm, better than graphene, WS2, BP and only inferior to SESAM at 1064 nm, the best at 1342 nm. In summary, under the similar conditions like wavelength and laser medium, antimonene has exhibited comparable passively Q-switching properties with other famous 2D nanomaterials, as well as SESAM. At the same time, for nanomaterial passively Q-switched near infrared solid-state lasers, the performance of antimonene is more notable when the operating wavelength increases.
Table 1. Passively Q-switching performance for solid-state laser of antimonene and several typical 2D nanomaterials, as well as SESAM
6. Conclusions
By liquid phase exfoliation method the few-layer antimonene was prepared from bulk Sb material. The AFM measurement revealed that its thickness was below 12 ns, i.e. fewer than 30 layers, which was further confirmed by Raman spectra. By saturable absorption experiments of nanosecond laser pulses, the saturation intensities of antimonene were determined to be 0.43 MW/cm2 at 532 nm and 0.53 MW/cm2 at 1064 nm, respectively. Using antimonene as SA, the passively Q-switched operations were realized for 946, 1064 nm laser emissions of Nd:YAG crystal, and 1342 nm laser emission of Nd:YVO4 crystal. The best Q-switching results came from 1342 nm solid-state laser, giving the largest pulse energy of 1.36 μJ, the shortest pulse width of 48.33 ns, and the highest peak power of 28.17 W. The excellent stability and broadband saturable absorption characteristic of antimonene will help it to find wide applications in optics and optoelectronics fields.
Funding
Natural Science Foundation of Shandong Province (ZR2017MF031); National Natural Science Foundation of China (NSFC) (11704227); Scientific Research Foundation of Shandong University of Technology (4041/416033).
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