Synthesis of high quality silver nanowires and their applications in ultrafast photonics

Silver nanowires are widely used in catalysts, surface enhanced Raman scattering, microelectronic equipment, thin film solar cells, microelectrodes and biosensors for their excellent conductivity, heat transfer, low surface resistance, high transparency and good biocompatibility. However, the optical nonlinearity of silver nanowires has not been further explored yet. In this paper, three silver nanowire samples with different concentrations are produced via a typical hydrothermal method. Their applications to fiber lasers are implemented to prove the optical nonlinearity of silver nanowires for the first time. Based on three kinds of silver nanowires, the mode-locked operation of fiber lasers is successfully realized. Moreover, the fiber laser based on the silver nanowire with a concentration of 2 mg/L demonstrates the shortest pulse duration of 149.3 fs. The experiment not only proves the optical nonlinearity of silver nanowires, but also has some enlightenment on the selection of the optimum concentration of silver nanowires in the consideration of ultrashort pulse output. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

In view of the outstanding performance of AgNW in the fields of electricity, heat, magnetism and catalysis, we are trying to find out whether it performs optical nonlinearity in optics field. It is a simple and effective way to prove the optical nonlinearity of AgNW with the aid of fiber lasers. Because of the optical nonlinearity of AgNW, it can be used as an ultrafast optical switch to achieve optical pulse shaping [40][41][42][43]. Generally speaking, there are two parameters to determine the saturable absorber performance of a material: relaxation time and third-order nonlinear polarizability [44]. During the relaxation process, the fast relaxation time is the main factor affecting the shaping ability of pulses [45,46]. It has reported that the relaxation time of AgNW is shorter than that of graphene [47]. This result indicates that the potential of AgNW to produce ultrashort pulses is comparable to that of graphene. Similarly, the third-order nonlinear polarizability has been proved with the Z-scan technique [48]. By comparison, the nonlinear refractive index of AgNW is comparable to or even better than that of TMDs. In summary, AgNW has a satisfactory performance in both relaxation time and nonlinearity.
In this paper, the optical nonlinearity of AgNW is proved experimentally. The mode-locked Er-doped fiber (EDF) laser based on AgNW is implemented. The AgNW is synthesized via a typical hydrothermal method. To explore whether the solution concentration has an effect on the performance of the EDF laser, the comparative experiments are implemented with three solutions of different concentration under the same conditions. By comparison, the EDF laser based on the AgNW with the concentration of 2 mg/L performs best in terms of ultrashort pulse output. The shortest pulse duration is 149.3 fs.

Preparation and characterization
In this work, AgNW was synthesized via a typical hydrothermal method [49]. As shown in Fig.  1(a), silver nitrate (AgNO3), polyvinylpyrrolidone (PVP), and copper chloride dihydrate (CuCl2·2H2O) were mixed together and stirred for 10 min to form precursor solution. After 3h of 130 °C heating, AgNW was obtained. Surface metallization modification of the fiber connector was then performed via 1500 rpm spin coating using AgNW/EtOH solution in Fig.  1(a). The surface color of fiber connector remained white after naturally drying, owing to the slender diameter and low fill-factor of AgNW in Fig. 1(b). Figure 1(c) indicates the diameter distribution of AgNW. In order to collect the crystallographic information of as-prepared AgNW, X-ray diffraction (XRD) was employed. Results show three characteristic peaks from 2θ = 30 to 70 degrees, corresponding to (111), (200), and (220) crystal faces of silver in Fig.  1(d). There is no other diffraction peak attended, indicating the highly purity of as-prepared AgNW. X-ray photoelectron spectroscopy (XPS) peak-differentiation-imitating analysis around Ag3d region showed well separated spin-orbit components with a gap of 6.0 eV in Fig.  1(e), which also prove that the exist of metallic silver rather than silver nitrate residue or silver oxide [50,51]. The left one of Fig  It can be seen that the AgNW is attached to the effective area of the optical fiber. Micromorphologies of AgNW were characterized via a transmission electron microscope (TEM) and a scanning electron microscope (SEM). High-resolution TEM image also shows uniform nanowire structure of AgNW with a diameter of ~50 nm in Fig. 1(g). Selected area electron diffraction (SAED) reveals a polycrystalline ring, backing well the XRD diffraction result provided before. Surface metal fill-factor is a critical factor in the laser response. By controlling the concentration of AgNW in AgNW/EtOH solution, different fill-factor AgNW coating were obtained. To further explore and contrast the mode locking performances of fiber connectors, AgNW coating layer with different concentration of AgNW from 2 mg/L to 0.5 mg/L were pre-prepared on three fiber connectors in Fig. 1(h).
To characterize the nonlinear saturable absorption properties of AgNW, the double-balanced detection method was adopted. The beam from the laser source was divided into two parts, one passing through the AgNW SA and the other passing through the single-mode fiber as a reference. By changing the power of the light, the relationship between the power intensity and transmittance of the AgNW SA is revealed. As shown in Fig. 2, three AgNW SAs with different concentrations have great disparity in the nonlinear absorption properties. The curve in Fig. 2 is fitted by the following formula, where T is the transmittance, s  is the saturable loss, ns  is the non-saturable loss, I is the input intensity, and sat I is the saturation intensity. The AgNW SA with concentration of 2 mg/L exhibits the maximum modulation depth of 59.5% among three SAs as shown in Fig. 2(a). The modulation depth of the AgNW SA with concentration of 1 mg/L and 0.5 mg/L decrease in turn, which are given as 34.19% and 21.1%, respectively.

Experiment
In order to confirm the optical nonlinearity of AgNW, we have adopted the traditional EDF ring laser. The construction of the proposed nonlinear optical verification device based on AgNW is shown in Fig. 3. The laser diode (LD) which pump EDF (0.6 m) through a wavelength division multiplexer (WDM, 980/1550 nm) operated at 980 nm with the maximum power of 630 mW. The isolator (ISO) is employed to guarantee unidirectional light propagation. By carefully adjust the polarization controller (PC), the polarization state and operation state will be optimized to a certain extent. By means of a 20% fiber coupler, the real-time status of the cavity can be monitored by the test equipment. The corresponding pulse profile, spectrum, frequency spectrum is identified by oscilloscope (Tektronix DPO3054), optical spectrum analyzer (Yokogawa AQ6370C) and RF spectrum analyzer (Agilent E4402B), respectively.

Result and discussion
To explore whether solution concentration has an effect on the performance of EDF lasers, three different AgNW concentrations are adopted to conduct experiments. In the first experiment, the AgNW SA with concentration of 2 mg/L is used, and the corresponding laser performances are summed up in Fig. 4. The mode-locking operation is realized when the pump power run up to 160 mW. As shown in Fig. 4(a), the pulse interval is 12.68 ns observed from the oscilloscope trace. The long-time output spectrum is demonstrated in Fig. 4(b). Obviously, there are some sidebands on both sides of the spectrum, which is the characteristic spectrum of standard soliton mode-locking. The EDF laser based on the AgNW SA with the concentration of 0.5 mg/L operates at 1559.8 nm with the bandwidth of 28 nm. As shown in Fig. 4(c), the autocorrelation trace is revealed consistent well with the track of sech 2 function, which corresponds the pulse duration of 149.3 fs. The corresponding time bandwidth product of realized EDF laser is calculated as 0.5151, which indicates that the whole mode-locking system is slightly chirped. Meanwhile, the corresponding radio frequency (RF) spectrum with the span of 100 kHz and resolution of 3 Hz is shown in Fig. 4(d). The fundamental repetition rate is 78.8363 MHz. The signal-to-noise ratio (SNR) is measured as 62.8 dB, which illustrates the exceptional stability of the mode-locking operation. The inset in Fig. 4(d) reveals the frequency multiplication spectrum measured in the wide span of 1000 MHz. The calculated maximum pulse energy and optical damage threshold of the SAs is 328.52 μJ and 0.515 mJ/cm 2 , respectively.  Subsequently, the AgNW with solution concentration of 1 mg/L and 0.5 mg/L are also successfully applied to the same laser. When the concentration of AgNW is 1 mg/L, the corresponding repetition rate and the pulse duration are 78.69 MHz and 156.9 fs as shown in Figs. 5(a) and 5(b). Analogously, the repetition rate and pulse duration of the EDF laser based on AgNW with the concentration of 0.5 mg/L are given as 78.7368 MHz and 168.7 fs in Figs. 5(c) and 5(d). More detailed data on three different mode-locked lasers are listed in Table 1.
The contrast results show that the trend of concentration and pulse duration are not consistent. That result indicates that there exists an optimum concentration to achieve ultrafast pulse output.
In addition, the laser performance comparison of AgNW with the traditional SAs has also been implemented. The pulse duration of 149.3 fs in our experiment is shorter compared with most EDF lasers in Table 2. This not only proves the optical nonlinearity of AgNW, but also indicates it has great advantages in realizing ultrashort pulses.

Conclusion
In this paper, three AgNW samples with different concentrations have been prepared via a typical hydrothermal method. The characterization of three different SAs based on AgNW have indicated the high modulation depth and small insertion loss of them. In the experiment, three kinds of samples have been successfully applied in the EDF lasers. In addition, through the laser performance comparison of AgNW with different concentrations, it is found that there exists an optimum concentration in achieving short pulse output for AgNW. The pulse duration of 149.3 fs is also superior to most EDF lasers based on some classical materials. Our experiment not only demonstrates the optical nonlinearity of AgNW, but also enlightens that the AgNW with optimal concentration is competitive for the development and promotion of ultrafast mode-locked pulses.