Ultrafast fiber lasers play a significant role in our society with many aspects ranging from fundamental physics to industrial purposes. Searching for high-performance saturable absorbers (SAs) is vital to the developments of ultrafast fiber lasers. Gold nanorods (GNRs) have been discovered to possess saturable absorption effect. However, a major obstacle to make the GNRs as high-performance and practical SA is the low optical damage threshold. To overcome this drawback, herein we proposed the nanocomposites with gold nanorods/silica core-shell structure (GNRs@SiO2) as a high-performance SA for ultrashort pulse generation in a fiber laser. The GNRs@SiO2 SA presents a modulation depth of 4.2% and nonsaturable loss of 45.6%. With the proposed GNRs@SiO2 SA, 379 fs pulse was directly obtained from the fiber laser. The achieved results demonstrated that the GNR@SiO2 could be indeed a good candidate of high-performance SA towards practical applications in the field of ultrafast photonics.
© 2015 Optical Society of America
Nano-sized noble metal particles, whose diameters are within 100 nm, have attracted considerable interests in the industrial and scientific fields due to great variety of their functionalities such as electrical, magnetic properties, catalytic activity, and so on [1,2 ]. In recent years, interests in investigating various applications of metal nanoparticles are strongly stimulated by their unique characteristics. Since the characteristics of metal nanoparticles are demonstrated to be related to the distribution of their physical dimensions and shapes , how to fabricate the metal nanoparticles with desirable size and morphology would be the key to investigate their properties as well as applications. So far, metal nanoparticles with various shapes, i.e., nanoplates and nanoprisms, have been prepared by employing the advanced approaches [4,5 ]. As a representative type of metal nanoparticles, gold nanorods (GNRs), with their varied size, low reactivity, and anisotropy shape, possess special properties not seen in spherical gold nanoparticles [6,7 ]. As for the optical properties, GNRs could exhibit unique nonlinear optical response such as large optical nonlinearities , surface-enhanced Raman scattering (SERS) , and much efficient fluorescence originating from the local field enhancement near the surface plasmon resonance . In addition, the peak position of the surface plasmon resonance (SPR) could be flexibly tuned across a broad spectral range, covering the visible and near-infrared region by manipulating aspect ratios of the GNRs , indicating that GNRs could be used in different wavebands where the nonlinear effects are needed. Therefore, GNRs would be a promising nonlinear optical material which could find important applications in the field of nonlinear optics.
On the other hand, ultrafast fiber lasers, operating through the principle of mode-locking, have attracted considerable attention due to the versatile applications such as material processing, medicine, sensing and optical communications. A key step to achieve ultrashort pulse in fiber lasers is to introduce the saturable absorption effect into the cavity [12–15 ]. Therefore, there is always a strong motivation to develop high-performance saturable absorbers (SAs) [16–22 ]. Recently, by measuring the nonlinearly optical response with Z-scan technique, GNRs were found to possess saturable absorption effect at longitudinal SPR . Based on the achieved results, the GNRs can be expected to be functioned as the promising SA for ultrafast fiber lasers. Indeed, taking advantages of saturable absorption effect, the GNRs were introduced into the laser cavity to achieve passive mode-locking operation [24–26 ]. However, an intrinsic drawback for fabrication of GNRs SA is that the photo-thermal effect would cause the optical damage of GNRs . As we know, the pulse generated from fiber lasers would possess higher peak power when it operates in shorter duration regime. And the higher peak power would induce larger photo-thermal effect. Thus, the GNRs could not endure high peak pulse power in the laser cavity. In this case, only the mode-locked pulses with durations of tens or hundreds of picoseconds could be achieved in the fiber lasers based on the filmy type of GNRs-SAs [24,25 ], which leads to inferior laser performance and limits their uses in practical applications of ultrafast laser community. In fact, it is well known that silica has a high melting temperature and high optical transparency. With a modified Stöber approach, the silica could be successfully coated around the GNRs to form a core/shell structure (GNRs@SiO2) . By fabricating GNRs with core/shell structure, GNRs could not only preserve their intrinsically optical properties, but also improve their thermal stability [29,30 ]. Therefore, from the viewpoint of expanding the application fields of GNRs, a question naturally arises as to whether the GNRs@SiO2 could be employed as a high-performance SA towards practical applications for ultrafast fiber lasers.
In this work, we reported on the generation of 379 fs pulse directly from an ultrafast fiber laser by employing GNRs@SiO2 as a high-performance SA. The proper GNRs and GNRs@SiO2 are obtained by careful control during the chemical synthesis. The polyvinyl alcohol (PVA)-based GNRs@SiO2 SA shows a modulation depth of 4.2% and a nonsaturable loss of 45.6%. By virtue of saturable absorption and high photo-thermal stability of GNRs@SiO2, the fiber laser delivers a stable ultrafast pulse-train with a repetition rate of 8.93 MHz. These results suggest that the GNR@SiO2 SA could be indeed a good candidate of ultrafast saturable absorption device for ultrafast lasers, and also expand the applications of GNRs in the fields of nonlinear optics towards practical applications.
2. Preparation and characterization of GNRs@SiO2 film
GNRs were prepared by modified three-step seed-mediated method . Briefly, gold seed solution was formed by adding 0.6 mL of 0.01 M sodium borohydride solution into 10 mL of solution consisting of 0.1 M cetyltrimethylammonium bromide (CTAB) and 2.5 × 10−4 M chloroauric acid (HAuCl4). Next, the aqueous growth solution was prepared by gently introducing 380.7 μL of 0.0788 M ascorbic acid into 54 mL of mixed solution containing 0.1 M CTAB, 2.5 × 10−4 M HAuCl4, and 2.5 × 10−4 M nitric acid. The mixture was shaken until the solution turned colorless. Then, 4.5 mL of the growth solution was added to flasks A and B and 45 mL to flask C. Since the aspect ratios of the GNRs corresponding to the optical response at 1550 nm waveband is about 11.8, which is much longer than that at 800 nm. Thus, for synthesizing ultra-long GNRs, 400 μL of the gold seed solution was added to flask A and then 400 μL of the solution in flask A was transferred to flask B. Finally, 4mL of the new growth solution in flask B was transferred to flask C. After 24 hours of undisturbed incubation, the ultra-long GNRs could be formed.
Coating GNRs with silica was carried out by means of the improved Stöber method [28,32 ]. 20 mL of centrifuged GNRs solution, which was re-dispersed in ultrapure water, was mixed with 28wt% ammonia solution and vigorous stirred for 15 seconds. The pH of the GNRs solution was adjusted to 10 through the addition of ammonia. Then, 6 mL of 10 mM tetraethyl orthosilicate (TEOS, in ethanol) was quickly introduced into aforementioned GNRs solution at every 30 minutes interval for 2 hours. The mixture was allowed to react for 12 hours till the GNRs@SiO2 were produced. Finally, the GNRs@SiO2 solution were centrifuged and re-dispersed in ethanol.
To characterize the prepared GNRs, a transmission electron microscopy (JEM-2100) was used to observe the morphology of GNRs. Figure 1(a) shows TEM image of GNRs obtained by the present synthetic method. Large quantities of GNRs have been formed with an average length and width of 254 nm and 18 nm, respectively. The corresponding aqueous GNRs solution is yellowish-brown, as shown in Fig. 1(b). Figure 1(c) illustrates the histogram of aspect ratio distribution of the GNRs. The nanorod aspect ratios vary from 9.5 to 18.5. According to the discrete dipole approximation method , the longitudinal SPR band is approximately from 1330 nm to 2200 nm, which obviously covers the 1550 nm operation waveband of EDF laser used in this work. Figure 1(d) presents the prepared GNRs@SiO2 by hydrolysis and condensation of TEOS in alkaline water. As can be seen here, the GNRs@SiO2 nanocomposites are observed as elliptical shape. The TEM image clearly shows that each of GNRs has been encapsulated separately in a silica shell with the homogeneous thickness of ~32 nm. The thickness of the shell can be precisely controlled by varying the added amount of TEOS.
In order to obtain fiber-compatible SA, we prepared thin film of GNRs@SiO2 dispersed in polyvinyl alcohol (PVA) by simply drying a colloid in the presence of dissolved PVA . Briefly, 1 mL of GNRs@SiO2 solution was mixed with 1 mL of PVA solution, and then the mixture was vigorous stirred by a vortex mixer for 1 hour for uniform dispersion. Finally, a PVA based GNRs@SiO2 film is obtained by dumping mixture on a slide glass, followed by drying in air at room temperature. The fabricated PVA based GNRs@SiO2 film was presented in Fig. 2(a) . Then the fiber-compatible SA is formed by attaching GNRs@SiO2 film into the fiber end facet, as shown in Fig. 2(b).
By using ultraviolet-visible-near infrared (UV-vis-NIR) spectrophotometer (SHIMADZU, UV-3600), the absorption spectra of GNRs@SiO2 film can be characterized. In Fig. 3(a) , we compare the UV-vis-NIR absorption spectra of the GNRs and GNRs@SiO2 film. After the silica coating, the transverse SPR band of GNRs@SiO2 shows no noticeable peak shift, while the longitudinal SPR band redshifts slightly attributed to the rise of the local refractive index around the GNRs. However, the absorption spectra of GNRs@SiO2 and GNRs film show a relatively consistent trend, that is, their longitudinal SPR absorption band have a rising trend toward the near-infrared region, suggesting GNRs@SiO2 still have the same optical properties as GNRs and could be used as SA in erbium-doped fiber (EDF) laser operating at telecommunication waveband.
For further investigating the characteristics of the fabricated GNRs@SiO2 film, the nonlinear absorption was measured by using balanced twin detector measurement technology as we previously used in . In this experiment, the pump source was a home-made femtosecond pulse laser (center wavelength: 1554.4 nm, repetition rate: 26 MHz, pulse duration: ∼500 fs). Figure 3(b) provides the saturable absorption data of the GNRs@SiO2 SA film and the corresponding fitting curve as a function of average optical power. As can be seen here, the modulation depth and nonsaturable loss are ~4.2% and ~45.6%, respectively. Based on these experimental results, we can deduce that the GNRs@SiO2 film could be applied in fiber laser for implementing mode-locking. Moreover, to verify the nonlinear polarization characteristics of the fabricated GNRs@SiO2 film, the polarization-dependent loss was measured in our experiment. As can be seen in Fig. 3(c), no evident polarization-dependent loss could be observed, suggesting that the GNRs@SiO2 film used for this work possesses too small polarization-dependent loss to be considered.
3. Laser performance and discussions
To check the laser performance of PVA based GNRs@SiO2 SA, it was incorporated into a laser cavity to see whether ultrashort pulse could be generated or not. The schematic of the passively mode-locked EDF ring laser was shown in Fig. 4 . A 5m-long EDF was pumped by 980 nm laser diode through a 980/1550 nm wavelength division multiplexer (WDM). A polarization-independent isolator (PI-ISO) was utilized to guarantee the unidirectional operation. Two polarization controllers (PCs) were employed to adjust the polarization state of light and optimize the laser performance. The as-prepared PVA based GNRs@SiO2 SA, which was placed between two fiber connectors, played a key role in passive mode-locking operation. A 10/90 fiber coupler was used to output the laser. The total cavity is around ~23 m in length, corresponding to a fundamental repetition rate of 8.93 MHz. The spectrum was monitored by an optical spectrum analyzer (Anritsu MS9710C), and the mode-locked pulse-train was detected by a digital oscilloscope (Agilent DSO-X 3052A) with a high-speed photodetector (New Focus P818-BB-35F, 12.5 GHz). The radio frequency (RF) spectrum of the mode-locking operation was recorded by employing a radio frequency spectrum analyzer (RFSA, Advantest R3131A). Moreover, the pulse duration was measured using a commercial autocorrelator (Femtochrome FR-103XL).
In the experiment, continuous wave (cw) operation of the proposed fiber laser started at a pump power of ~6 mW. Then by simply increasing the pump power to 25 mW, the self-started mode-locking occurred by virtue of saturable absorption effect of GNRs@SiO2. For optimizing the mode-locking operation, the pump power was further increased to 40 mW. Figure 5(a) shows the typical mode-locked spectrum obtained at the pump power of ~40 mW. Here, the central wavelength of the mode-locked spectrum is 1559.8 nm and the 3-dB spectral bandwidth is 6.93 nm. The obvious Kelly sidebands appear symmetrically on the mode-locked spectrum, indicating that the fiber laser operates in the soliton regime with anomalous dispersion . Figure 5(b) presents the corresponding mode-locked pulse-train. The fundamental repetition rate is 8.93 MHz, which is determined by the cavity length of ~23 m. For better clarity, the pulse-train measured in a larger range is also presented in the insert of Fig. 5(b), showing that no evident power fluctuations could be observed. Then the duration of the mode-locked pulse was identified by using a commercial autocorrelator. As can be seen in Fig. 5(c), the mode-locked laser delivers the pulse-train with 379 fs duration if the fit of the sech2 pulse shape is assumed. Therefore, the time-bandwidth product is 0.324, suggesting that the output pulse is slightly chirped. Note that no pedestal was observed on the autocorrelation trace, as shown in the inset of Fig. 5(c), which reveals the excellent quality of mode locking operation. Based on the achieved results, the performance of the proposed fiber laser based on GNRs@SiO2 SA is comparable to those using two-dimensional materials based SAs [13–21,37–40 ].
To verify the laser stability, we have measured the radio-frequency (RF) spectrum of the mode locking operation with a resolution bandwidth of 300 Hz, as presented in Fig. 6(a) . The fundamental frequency peak locates at 8.93 MHz, corresponding to the fundamental cavity repetition rate. The signal-to-noise ratio (SNR) is ~60 dB, indicating the high stability of mode-locking operation. Moreover, the inset of Fig. 6(a) shows RF spectrum measured under the 800 MHz span. Here, it can be clearly seen that the RF spectrum is free of modulation, further suggesting that the fiber laser operates well in cw mode-locking state. In addition to the RF measurements, the mode-locked spectrum is also recorded every 30-minutes for 2.5 hours at a fixed pump power of ~40 mW, as depicted in Fig. 6(b). No drift of central wavelength and variation of spectral bandwidth were directly observed, which definitely confirms that the mode-locked operation possesses a reasonably good long-term stability. After the mode-locking operation, the optical damage of PVA-based GNRs@SiO2 SA was tested. To this end, we increased the 980 nm pump power gradually. However, the fiber laser could sustain mode-locking operation well at the maximum pump power of 350 mW in our experiment, indicating that the optical damage of GNRs SA was effectively suppressed by coating GNRs with silica. In the experiment, we have also checked the role of GNRs in passive mode-locking operation. To this end, GNRs@SiO2 SA was removed from the laser cavity. However, in this case no passively mode-locked pulse could be obtained even if the PCs were rotated in a large range. The comparative results demonstrated that the saturable absorption of GNRs@SiO2 was responsible for the ultrashort pulse generation of the fiber laser.
In ultrafast fiber lasers, it is well known that the quality of mode-locked pulse is greatly affected by the performance of SA. Regarding the GNRs SA, it was found that the obvious photo-thermal effect could be easily observed in the GNRs, which would cause a rod-to-sphere shape transition at higher optical power. Although the photo-thermal effect would be favorable for some applications such as drug delivery, photothermal therapy and optical storage [41–43 ], it indeed impairs the performance of GNRs-SA. That is to say, if the GNRs mode-locked pulse was in the pico-/femto-second regime, the high peak power of the mode-locked pulse would make the GNRs experience a rod-to-sphere shape transition, which could lead to the loss of mode-locking operation of a fiber laser. Therefore, without other improved methods, the fiber laser is difficult to achieve femtosecond pulse regime which is favorable for the practical applications. Here we can conclude that the major obstacle to make the GNRs as high-performance SA is the low optical damage threshold due to photo-thermal effect. In order to guarantee the safe and robust operation of GNRs avoiding the thermal damage, a preferred approach is to employ GNRs@SiO2 as SA by coating the surface of GNRs with silica. In this case, the shapes of the GNRs are effectively stabilized by the silica shell at high optical power [29,44 ]. Thus, the nonlinear optical response of GNRs could be kept even if the optical power imposed on the GNRs was high, which is particularly favorable for the applications in the field of nonlinear optics such as ultrafast lasers. With the GNRs@SiO2 SA, it was found that femtosecond pulse could be easily obtained by simply inserting a GNRs@SiO2 SA into laser cavity due to the excellent photo-thermal stability and saturable absorption ability of GNRs@SiO2, suggesting that GNRs@SiO2 has significant advantages over GNRs when applying in ultrafast lasers as SA. In the experiment, the fiber laser deliver 379 fs pulse by employing the prepared GNRs@SiO2 SA. It should be noted that, in addition to the quality of the SA, the pulse duration could be also influenced by the cavity dispersion. Therefore, it is believed that the pulse duration could be further shortened by optimizing the cavity dispersion. In fact, in order to compare the laser performances by the conventional GNRs and core-shell structural GNRs, we have also constructed a fiber laser mode-locked by the conventional GNRs in our experiments. In this case, the fiber laser could only achieve mode-locking operation with the pulse duration of 3.5 ps at the pump power of 35 mW. And when the pump power was increased to 45 mW, the fiber laser would lose mode locking state, indicating that the GNRs have been damaged by the photo-thermal effect. The comparative results indeed demonstrated that the GNRs@SiO2 SA could endure much higher optical damage threshold than that of conventional GNRs, which provide powerful evidence that the GNRs@SiO2 could be an excellent nonlinear optical material for practical applications where the strong light-matter interaction is needed.
In summary, we proposed the nanocomposites GNRs@SiO2 as a high-performance SA for ultrashort pulse generation in a fiber laser. The evident saturable absorption effect was identified by measuring the nonlinear optical response of GNRs@SiO2, which shows a modulation depth of 4.2% and nonsaturable loss of 45.6%, respectively. With the PVA based GNRs@SiO2 SA, the 379 fs ultrashort mode-locked pulse could be easily generated from the fiber laser. The obtained results indicate that the GNR@SiO2 SA could be indeed excellent nonlinear optical material for fabrication of saturable absorption device for ultrafast lasers, and also might shed some light on the applications of GNRs in the fields of nonlinear optics towards practical applications.
This work was supported in part by the National Natural Science Foundation of China (Grant Nos. 61378036, 61307058, 11304101, 11074078, 11474108), the Key Program of Natural Science Foundation of Guangdong Province, China (Grant No. 2014A030311037), the Scientific and Technological Innovation Project of Higher Education Institute, Guangdong, China (Grant No. 2013KJCX0051), the Scientific Research Foundation of Graduate School of South China Normal University, China (Grant No. 2014bsxm06), and the Open Fund of the Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques (South China University of Technology). Z.-C. Luo acknowledges the financial support from the Guangdong Natural Science Funds for Distinguished Young Scholar (Grant No. 2014A030306019), Program for the Outstanding Innovative Young Talents of Guangdong Province (Grant No. 2014TQ01X220), and the Pearl River S&T Nova Program of Guangzhou (Grant No. 2014J2200008).
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