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Abstract
In this work, γ-graphyne (γ-GY), a novel two-dimensional (2D) carbon allotrope, is demonstrated to have excellent ultrafast saturable absorption properties superior to that of graphene in the near infrared region. For practical application, the γ-GY nanosheets were directly dispersed in toluene in the existence of polymer, and were easily fabricated into flexible thin films. As proof-of-concept, the performances of γ-GY as a saturable absorber in passively Q-switched lasers and in all-optical switches were investigated. A minimum pulse width of 241 ns with the maximum pulse energy of 0.76 µJ and the corresponding peak power of 3.15 W was achieved for a γ-GY Q-switched solid-state laser at 1.06 µm. The γ-GY all-optical switching shows a rising/falling time of 1.46 ms/ 2.13 ms.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As a new family of carbon nanomaterials, graphynes (GYs), first predicted by Baughman et al. [1] and composed of ethynyl units (sp-hybridized) and aromatic moiety rings (sp2-hybridized carbon) with large conjugate structure [2–5], show promising prospects in the fields of energy, catalysis and optoelectronics [6–11], etc. Like graphene, GYs have two-dimensional (2D) structure and Dirac cones as well as superior electrical properties, such as high carrier mobility and small carrier effective masses [12]. Most importantly, GYs were predicted to have natural semiconductor band gap, which originates from the overlap of carbon 2pz orbitals and inhomogeneous π-bonding between the carbon atoms with sp2 and sp hybridization [13–16].
GYs family contain graphyne (GY), graphiyne (GDY), graphtriyne (GTY), and so on, on the basis of the number of ethyne units between two neighboring aromatic rings. Currently, γ-GY and γ-GDY with the adjacent benzene rings linked by one and two acetylenic linkages (−C≡C−), respectively, show the most stable structure in the GYs family [17,18] and are synthesized experimentally by Y. L. Li et al. and X. L. Cui et al. [4,10,19], respectively. K. Srinivasu et al. predicted that the band gap of γ-GY and γ-GDY is 2.23 and 1.18 eV, respectively [6], which have been confirmed experimentally.
Owing to the presence of acetylenic groups, GYs are expected to have potential applications in optoelectronic devices [20–22]. Most recently, Y. Zhao et al. reported the mode-locking performances of γ-GDY saturable absorber (SA) in near infrared region. A mode-locked laser pulse with a repetition rate of 12.05 MHz and a pulse width of 734 fs at a center wavelength of 1564.70 nm was obtained using the γ-GDY as SA from an erbium-doped fiber laser at the 1.5 µm [23]. However, the nonlinear optical (NLO) performances of GYs such as saturable absorption behavior and all-optical switching performances, especially when compared to its famous analogue graphene, are still not investigated limited by the access of the materials. Compared to semi-metallic graphene and other 2D materials such as unstable black phosphorus (BP) and semiconducting transition metal dichalcogenides (TMDs), γ-GY may show higher potentials in practical applications for that it is semiconducting with high stability, and has a large delocalized π-conjugated structure which is expected to endow it outstanding optical and electronic properties.
In this work, we successfully dispersed the synthesized γ-GY powders by poly(methyl methacrylate (PMMA) in toluene using liquid-phase exfoliation method and investigated the ultrafast NLO performances of γ-GY in PMMA and its applications in ultrafast laser pulse generation and all-optical switches in comparison with graphene in near infrared range. The all-optical switch is based on polarization-dependent thermo-optic effect. When the γ-GY absorbed the energy of the control beam (980 nm), a change in the refractive index of γ-GY and tapered fiber is introduced owing to the thermo-optical effect. The anisotropism of γ-GY in thickness leads to the phase shift caused by the change of the refractive index polarization-dependent. We named the two-orthogonal axes as x and y, respectively, which corresponded to the polarizations for which the incident light had the maximum and minimum phase shift, respectively. As a result, the two components of the signal beam (1550 nm) along the x and y polarization directions experienced different phase changes after interacting with the sample, and their phase difference was determined by the control beam. Finally, the two components interfered with each other when they reached the polarizer and the signal output after the polarizer was controlled by the availability of the pump pulsed light.
2. Results and discussion
2.1. Materials characterization
The γ-GY powders were synthesized using benzene and CaC2 as precursors by Q.D Li et al. [11,19]. Figure 1(a) shows the schematic atomic structure of the γ-GY, which is composed of benzene rings and ethynyl linkages with porous structure. We found that γ-GY powders can be well dispersed in organic solvent 1-Methyl-2-pyrrolidinone (NMP) rather than Ethanol-H2O by liquid-phase exfoliation method (Fig. 1(b)). However, the low concentration of the dispersions was unfavorable for the linear and NLO studies as well as practical applications. We therefore employed PMMA to disperse γ-GY. Figure 1(c) shows the effective stable dispersion of γ-GY by PMMA in toluene with magnetically stirring for 48 h after centrifugation for 30 min at a centrifugation rate of 2000rpm, which can be easily fabricated into flexible thin films by solvent evaporation process at a temperature of 50 °C as shown in Fig. 1(e). This method is also effective for the dispersion of other 2D materials, such as graphene, WS2, antimonene (Sb), tellurium (Te) and so on. The 2D layered structure can be observed from the transmission electron microscope (TEM) in Fig. 1(d) with a lattice spacing of ∼0.370 nm (inset in Fig. 1(d)), corresponding to (220) crystal plane [11]. Raman spectrum in Fig. 1(f) shows two relatively strong characteristic bands of γ-GY with the D band around 1358 cm−1 to the defects and disorder structures of the carbonaceous solid and the G band at 1587 cm−1 to the sp2-hybridized carbon of benzene rings [24,25]. In addition, another two weak bands around 1946 and 2181 cm−1 are observed, which are attributed to the stretching vibration of sp-hybridized carbon according to the reported theoretical and experimental results [19,24]. The full width half maximums (FWHMs) for the D and G bands are calculated to be around ∼76 and 81 cm−1, respectively, comparable to those reported in previous work [25]. Combined with the TEM results, it can be concluded that the obtained γ-GY has good crystallinity. According to the absorption spectra in Fig. 1(g), the γ-GY has an optical band gap of ∼2.6 eV, which is in accordance with the theoretical prediction [6]. In addition, the γ-GY exhibits a weak absorption peak at ∼1167 nm compared to the pure PMMA film. It can be ascribed to the defect-related absorption according to the Raman results where a relatively strong D band associated with defects is observed. The γ-GY used in the following part is in PMMA thin film state without specific illumination.
Fig. 1. Characterizations for γ-GY. (a) Schematic atomic structure of γ-GY. (b, c) γ-GY powders dispersed by NMP, Ethanol-H2O and PMMA-toluene, respectively, by magnetic stirring for 48 h after centrifugation at 2000rpm for 30 min. (d) TEM image for γ-GY nanosheets dispersed by NMP. Inset is enlarged image in the square frame. (e) γ-GY-PMMA thin films obtained from γ-GY in PMMA-toluene dispersions by solvent evaporation method at 50 °C for 2 days after centrifugation at 2000rpm for 30 min. (f) Raman spectrum of the GY powders. (g) Absorption spectra of γ-GY-PMMA thin film and pure PMMA. Inset is the Tauc plot curve of the γ-GY-PMMA thin film.
2.2. The NLO properties of γ-GY
To reveal the NLO performances of γ-GY, we carried out an open-aperture z-scan technology using a mode-locked fiber laser operating at 1030 nm with 340 fs pulses at a 100 Hz repetition rate. The total transmittance through the sample is measured as a function of incident laser intensity, while the sample is sequentially moved through the focus of a lens (along the z-axis). Figure 2 shows that the γ-GY exhibited obvious saturable absorption with the optical transmission increasing with the incident beam intensity when the sample position z approaches the focal point of focusing lens (z = 0 mm). The host PMMA showed neglectable NLO responses, demonstrating that the saturable absorption comes from the γ-GY.
Fig. 2. Open-aperture z-scan experimental results for GY-PMMA and graphene-PMMA with 340 fs laser at 1030 nm.
The above z-scan curves were fitted using the following equation according to the NLO theory [26]:
where α0 and αNL are the linear and NLO coefficient, respectively.Table 1 shows the fitting parameters, where the film thickness (L) is measured using digital outside micrometer with a resolution of 0.001 mm. The NLO coefficient (αNL) is fitted to be −0.461 and −0.411 cm GW−1 for γ-GY- and graphene-PMMA, respectively, where the value for graphene is in the same order with that reported in our previous work [27]. Thus, γ-GY shows a stronger nonlinear response than graphene, which can be attributed to the existence of sp hybridized carbon with large conjugate structure. Accordingly, the saturated intensity (Isat) of γ-GY-PMMA was calculated to be ∼47 GW cm−2 using the equation αNL≅ -α0/Isat, lower than that of graphene-PMMA (∼53 GW cm−2). The saturable absorption of γ-GY can be attributed to the existence of defect energy levels in γ-GY like 2D MoS2 in the case of that the excitation photon energy (1.2 eV) is much smaller than the γ-GY band gap (∼ 2.6 eV) [28], predicting a broadband saturable absorption properties of γ-GY. Generally, high modulation depth as well as large αNL with low Isat are expected for a good SA [29]. The results signify that the γ-GY holds great potential as a passive Q-switcher for generating ultrafast laser pulses.
Table 1. Linear and NLO parameters of the γ-GY-PMMA thin films excited at 1030 nm in the femtosecond region.
2.3. The Q-switching performance of γ-GY
As a proof, we investigated the Q-switching performance of γ-GY as SA in near infrared laser at 1.06 µm. The laser experimental setup is shown in Fig. 3(a). A compact linear cavity was employed for the passively Q-switched laser. The laser gain medium is a c-cut Nd:YVO4 with dimension of 3×3×6 mm and 1 at% Nd-doped. Concave mirror M1 with a radius of 200 mm was employed as the input mirror, which was high transmission coated at 808 nm, and high reflection coated at 1064 nm. The flat mirror M2 with transmission of 10% at 1064 nm was employed as the output coupler. The length of whole laser cavity was about 20 mm. The as-prepared γ-GY sample was inserted into the cavities to modulate the cavity loss. Before inserting the SA inside the cavity, a continuous wave (CW) laser was obtained with a maximum output power of 890 mW as shown in Fig. 3(b). After inserting the γ-GY SA into the cavity, a stable Q-switched pulse laser was achieved when the pump power reached 0.56 W. As shown in Fig. 3(b), the maximum Q-switched output power was 233 mW under the pump power of 2.2 W. The pulse laser has a center wavelength of 1063.9 nm, which shows the similar bandwidth to the CW laser (Fig. 3(c)). Figure 3(d) shows the typical Q-switched single pulse at the maximum output power, and its corresponding pulse train which looks uniform and stable (inset in Fig. 3(d)). Figure 3e shows the variation trends of repetition rate and pulse width with the increasing of the pump power. The pulse width decreased from 933 ns to 241 ns, while the pulse repetition rate increased from 115 kHz to 307 kHz. According to the above data, we can estimate the maximum pulse energy to be 0.76 µJ, and its corresponding peak power to be 3.15 W. Compared to the reported performances of passively Q-Switched laser based on graphene SA [30,31], which has a minimum 450 ns pulse duration, our passively Q-Switched laser based on γ-GY SA exhibits a narrower pulse width. This is probably related with the higher carrier mobility of γ-GY than that of graphene, enabling γ-GY with faster carrier relaxation time which results in narrower pulse width for the Q-switched fiber laser based on γ-GY SA [30,32]. It should be noted that no optical damage was found for the sample during the Q-switched laser operation under our experimental conditions with the intracavity energy intensity as 0.11 MW/cm2. The results demonstrate that γ-GY could be a promising photonic material for near-infrared lasers.
Fig. 3. (a) Scheme of the Nd:YVO4 solid-state laser with a γ-GY saturable absorber (SA). (b) CW output power and pulse output power versus absorbed pump power. (c) The spectrum of CW and Q-switched laser pulse with γ-GY SA. (d) Autocorrelation trace with measurement data. Inset is the oscilloscope trace. (e) Repetition rate and pulse duration versus absorbed pump power.
2.4. The all-optical switching performance of γ-GY
All-optical switching, as the key component in photonic devices, plays an important role for signal processing in many applications including optical communication and computation [33,34]. Figure 4(a) shows the scheme of the γ-GY optical switch using polarization interference with 1.55-µm light as the signal beam and 980-nm light as the control beam [35]. Two fiber polarization controllers (PCs) were used to adjust the polarization of the signal and control beams independently. The two beams were combined using a 0.98/1.55 µm wavelength de-multiplexer (WDM) and channeled into the γ-GY-PMMA coated fiber connecters. When the control light passed, γ-GY absorbed the energy of control light and a change in its refractive index was introduced due to thermo-optical effect, which will result in a modulation for the following signal beam [35]. Figure 4(b) represents the output of the 1.55 µm pulse train as modulated by the control beam with a 20% duty cycle, from which a good on/off stability mode can be observed.
Fig. 4. Demonstration of all-optical switch based on γ-GY. (a) Schematic setup for testing the performance of a γ-GY optical-switch based on polarization interference. (b) The long-term output waveform of the γ-GY optical-switch, showing its output stability. (c) Comparison of the signal input to the outcome from the γ-GY optical switch. The rise time and fall time were measured to be 1.46 and 2.13 ms, respectively. (d-e) Effect of duty cycles of the control pulse on the output of the γ-GY optical switch with the control pulse’s peak intensity (d) or pulse energy (e) being constant. (f,h) Enlarged images from the marked area in (g) showing the performance in signal decay of γ-GY and graphene when the control pulse was turn on (f) or off (h). (g) Comparison of the output waveforms based on γ-GY and graphene optical-switch, showing the different recovery lifetimes. The rising/falling times are determined to be 1.51 ms/3.94 ms for γ-GY, while 1.18 ms/1.74 ms for graphene. Solid lines are from the fit with a two-exponential model. Inset table: the exponents, γ1 and γ2, are the lifetimes of the two processes.
Figure 4(c) shows the response of γ-GY optical switch where the rising/falling times were determined to be 1.46 ms/2.13 ms, respectively, following the 10-90% rule. These response times are better than the values reported in similar devices based on graphene (rise/fall time 9.1 ms/3.2 ms) and 2D WS2 optical switches (rise time 7.3 ms) [36,37]. The role of the control pulse on the output of the γ-GY optical switch was investigated as a function of the energy/power of the control pulses. As shown in Figs. 4(d-e), the switching time of the signal light depended on both the peak power and the pulse energy of the control light. It can be observed that the γ-GY optical switch reveals faster rising time (0.69 ms) and slower falling time (4.9 ms) at higher pulse energy (10% duty cycle) compared to the rising/falling time of 1.74 ms/2.71 ms at lower pulse energy (50% duty cycle) as shown in Fig. 4(d). Similarly, the same trend is observed when increasing the power of the control pulse, which results in the rising/falling time varying from 0.95 ms/3.34 ms (10% duty cycle), to 1.55 ms/2.27 ms (20% and 30% duty cycles), and 1.85 ms/2.08 ms (40% and 50% duty cycles) as shown in Fig. 4(e). According to the mechanisms of the γ-GY all-optical switch, i.e., thermo-optic effect, the falling time of the switch mainly depend on the heat dissipation of γ-GY. This means higher heat dissipation will lead to faster falling time with correspondingly slower rising time [38]. Moreover, the heat dissipation difference caused by the sample preparation can be neglected in the case that the γ-GY-PMMA thin film and the graphene-PMMA thin film were fabricated using the same method under the same conditions, i.e., by combination of liquid-phase exfoliation method and solvent evaporation method. The results suggest that the γ-GY has smaller thermo-optic coefficient than graphene.
3. Conclusions
In summary, we realized the dispersion of γ-GY nanosheets by liquid-phase exfoliation method with the assistance of polymer in toluene, which is beneficial for practical applications. Based on the fabricated flexible γ-GY-PMMA thin film, we revealed its ultrafast saturable absorption and Q-switching performances in near infrared region. The results demonstrate that γ-GY-PMMA has superior saturable absorption response and better Q-switching performances in compared to its analogue graphene. γ-GY optical switch is also fabricated based on thermo-optic effect, showing a rising/falling time of 1.46 ms/ 2.13 ms. Our work show that γ-GY has great potentials for a variety of photonic applications such as ultrafast Q-switchers and all-optical switches.
Funding
National Natural Science Foundation of China (51302285, 61675217, 61975221); Natural Science Foundation of Shanghai (19ZR1479300); the Key Research Program of Frontier Science of CAS (QYZDB-SSW-JSC041); Program of Shanghai Academic Research Leader (17XD1403900); the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB16030700).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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