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Passively Q-switched erbium-doped fiber laser based on gold nanorods
Optik 125 (2014) 5789–5793
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Passively Q-switched erbium-doped fiber laser based on gold nanorods Tao Jiang a , Guanshi Qin b,∗ , Weiping Qin b,∗ , Jun Zhou a a b
Institute of Photonics, Faculty of Science, Ningbo University, Ningbo 315211, China College of Electronic Science & Engineering, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 17 October 2013 Accepted 29 May 2014 Keywords: Fiber lasers Q-switching Gold nanorods Saturable absorber
a b s t r a c t We have demonstrated that the gold nanorods (GNRs) can be used as effective saturable absorbers for passively Q-switched erbium-doped fiber laser (EDFL). Two types of GNRs were synthesized by a seedmediated growth method. The longitudinal surface plasmon resonance wavelengths of the GNRs with different aspect ratios were around 1068 and 1442 nm, respectively. The GNRs were mixed with sodium carboxymethylcellulose (NaCMC) to form GNRs-NaCMC films with different absorption wavelengths. Using these GNRs-NaCMC films, 9.6-s pulses with a repetition rate of ∼22.9 kHz and 10.4-s pulses with a repetition rate of ∼22.4 kHz were obtained from the passively Q-switched EDFLs at a pump power of ∼120 mW, respectively. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Due to their large third-order nonlinearity and short response time as picoseconds, noble metals have been applied in many fields such as ultrafast communication, optical information processing, all-optical switching, optical data storage, and optical limiting [1–4]. In recent years, many works have been focused on the theoretical study of nonlinear property and the detection of nonlinear absorption coefficient of both gold composite materials and gold films [5–8]. For instance, the optical nonlinearities enhancement of a gold film with random roughness has been obtained [9]. Gold nanocrystals (GNCs)-doped photonic crystal fibers have been produced and their optical limiting behavior has been observed [7]. Using the four-wave mixing deduced by nonlinearities of gold nanostructure films, nonlinear negative refraction at optical frequencies has been demonstrated experimentally [10]. The surface plasmon excitation of GNCs can induce resonant enhancement of the local electric field resulting in amplifying of the third-order optical nonlinearities, second harmonic signal, Raman scattering, etc. [11]. Particularly, GNCs have shown saturable absorption properties for their surface plasmon resonance [11,12]. On the other hand, owing to their flexibility and low cost, highly stable and compact passively Q-switched pulsed fiber lasers have been widely used in optical imaging, medicine, material processing,
∗ Corresponding authors. Tel.: +86 574 87600744; fax: +86 574 87600744. E-mail addresses: [email protected] (G. Qin), [email protected] (W. Qin). http://dx.doi.org/10.1016/j.ijleo.2014.07.011 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
military, fiber communications, and fiber optical sensing [13–15]. Although transition metal ions doped bulk crystals, bleachable dyes, and semiconductor saturable absorber mirrors (SESAMs) have been commonly applied as absorbers in passively Q-switched lasers, it requires extra elements (e.g., lens and mirrors) to focus the output light from fibers into bulk saturable absorber (SA), which makes lasers complex [16,17]. Recently, much attention has been paid to passively Q-switched fiber lasers with singlewall carbon nanotubes (SWNTs) and graphene as SAs due to their numerous advantages, including good compatibility with optical fibers, minimized dimension, extremely high nonlinearities, low saturation intensity, subpicosecond recovery time, wide operating range, and simple manufacturing process [18–21]. In addition, SWNTs and graphene can be homogeneous dispersed in polymers such as polyvinylalcohol, polyamide, sodium carboxymethylcellulose (NaCMC), and polymethylmethacrylate to decrease scattering losses [22]. Furthermore, much research has been carried out on the synthesis of SWNTs with tunable sizes and graphene with fewer layers to get better Q-switching performance such as tunable laser operating wavelength, low insertion loss, low pump thresholds, and high optical-to-optical efficiency [19,22]. By contrast, little attention has been paid to the research on noble metals such as gold, silver, and copper with promising optical properties using as the Q-switching element [22–24]. Especially, GNCs have been proven to possess excellent nonlinear optical properties in previous works [25,7,26] and the passively Q-switching induced by gold nanoparticles (GNPs) has been demonstrated in our experiment [27]. However, the main absorption wavelength of
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GNPs was at about 523 nm, far from the laser operating wavelength we applied. Although a broad absorption band from 500 to 2000 nm was caused by the aggregation of GNPs, the concentration of GNPs in the film should be exactly tuned and the red shift of absorption band to 1561 nm is difficult to control. If gold nanorods (GNRs) with longitudinal surface plasmon resonance (LSPR) bands near 1561 nm were used, more stable passively Q-switched erbiumdoped fiber laser (EDFL) may be obtained. The ability to manipulate the structure and composition of nanoscale materials gives opportunities to create novel machines with superior performance. It is important that the properties of materials can be manufactured in more flexible ways. The LSPR bands of GNRs can be tuned by simply changing their aspect ratios, and therefore, the passively Q-switching at different wavelengths may be realized easily. As a result, GNRs with perfect saturable absorption properties are thus other suitable absorbers for application in passively Q-switched lasers besides SWNTs and graphene. In this work, we reported on the application of GNRs as SAs in passively Q-switched EDFLs. Using the SAs with different absorption wavelengths, 9.6-s pulses with repetition rate of ∼22.9 kHz and 10.4-s pulses with repetition rate of ∼22.4 kHz were obtained from passively Q-switched EDFLs at a pump power of ∼120 mW, respectively. By varying the pump power from 25 to 120 mW, the average output power can be adjusted from 0.08 to 4.77 mW and 0.06 to 4.70 mW, respectively. Our experiment results indicated that SAs based on GNRs are also promising passively Q-switcher with low fabrication cost, polarization insensitivity, and environmental robustness.
Fig. 1. TEM images of the S-GNRs (a) and the L-GNRs (b), AFM images of the S-GNRsNaCMC film (c) and the L-GNRs-NaCMC film (d).
2. Experimental details 2.1. Sample preparation The colloidal GNRs were synthesized by a seed-mediated growth method [28]. Firstly, the seed solution for GNRs was prepared by using fresh NaBH4 as reductant and aged at 25 ◦ C for 2 h. Then, the growth solution was prepared. 10 mL of hexadecyltrimethyl ammonium bromide (CTAB) (0.2 M) together with 10 mL of 5-bromosalicylic acid (5-BMSA) (0.2 M) were mixed with 4 mM AgNO3 solution. 1 mL of HAuCl4 (10 mM) and, if necessary, a small amount of HCl (37 wt% in water, 12.1 M) was added. After 15 min of slow stirring, 1 mL of ascorbic acid (0.1 mM) was added, and the solution was vigorously stirred for 30 s until colorless. Finally, 0.8 mL of seed solution was injected into the growth solution. The mixture was stirred for 30 s and left undisturbed at 30 ◦ C for 12 h for GNRs growth. The resulting precipitates were separated by centrifugation and washed with deionized water. Stable suspensions of GNRs in 1 wt% aqueous solution of NaCMC (medium viscosity, Sigma) were prepared by ultrasonication. This suspension was kept for 48 h and no precipitation was observed. The GNRs-NaCMC films were formed by casting the solution onto a flat substrate, followed by a slow drying at room temperature. 2.2. Characterization The sizes and morphologies of the obtained GNRs were observed by a JEM-2000 EX transmission electron microscope (TEM). As shown in Fig. 1(a), the obtained GNRs have an average diameter of 14.0 ± 4.0 nm and an average length of 95.0 ± 6.5 nm (corresponding to an average aspect ratio of about 6.8). Small fraction of spherical NPs also exists in the as-synthesized sample. Fig. 1(b) shows that the GNRs obtained from precursor solution with lower pH value (HCl was added) have an average diameter of 12.0 ± 4.0 nm and an average length of 100.0 ± 20.5 nm (corresponding to larger aspect ratios from 8.4 to 11.4). However, more
Fig. 2. Optical absorption spectra of the GNPs-NaCMC film, the S-GNRs-NaCMC film and the L-GNRs-NaCMC film (The inset shows the photographs of the corresponding aqueous solutions).
acidic environment causes a greater percentage of quasi-spherical NPs and wider size distribution of the synthesized GNRs. The atomic force microscopy (AFM) images of the prepared films were shown in Fig. 1(c) and (d). The surfaces of the two thin films were both smooth and crack-free. The absorption spectra of GNRs-NaCMC films with different SPR bands were compared to that of GNPs-NaCMC film we prepared previously [27]. All the three GNC-NaCMC films have broadband absorption from 500 to 2000 nm as shown in Fig. 2. However, the main absorption peak of GNPs-NaCMC film was at 530 nm far from the laser operating wavelength (1561 nm). The film of GNRs with small aspect ratio (S-GNRs) has a main absorption peak centered at around 1068 nm. The film of GNRs with large aspect ratio (L-GNRs) maintains broad absorption band between 1246 and 1442 nm. However, the intensity of the infrared band is relatively lower. The inset of Fig. 2 shows the photographs of the corresponding aqueous solutions of GNPs, S-GNRs, and L-GNRs. 3. Results and discussion To investigate the saturable absorption property of the two GNRs-NaCMC films, we measured the dependence of the transmission on the pump peak power density for the two films by using a 1560 nm pulsed fiber laser with a pulse width of 200 fs, as shown
T. Jiang et al. / Optik 125 (2014) 5789–5793
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Fig. 3. Dependence of the transmission at 1560 nm on the pump peak power density for the S-GNRs-NaCMC film (a) and the L-GNRs-NaCMC film (b).
Fig. 5. (a) Emission spectrum, (b) typical pulse train, (c) single pulse profile of the Q-switched EDFL with S-GNRs-NaCMC film under the pump power of ∼120 mW, and (d) dependence of the pulse width (solid circles) and the repetition rate (hollow squares) on the pump power.
Fig. 4. Schematic illustration of the proposed GNCs based Q-switched EDFL.
in Fig. 3. The measured data exhibited evidently the feature of saturable absorption. Non-bleachable losses (˛0 ), modulation depths (T), and saturation power densities (Psat ) were determined to be 24.6%, 6.9%, and 5.16 MW/cm2 for S-GNRs and 25.4%, 6.7%, and 5.09 MW/cm2 for L-GNRs respectively. Although their modulation depths were smaller than that of the GNPs-NaCMC film, the GNRsNaCMC film could also be used to induce Q-switching. The GNRs-NaCMC film was placed between two fiber connectors to form a fiber-compatible SA, and then integrated into a laser cavity as shown in Fig. 4. The insertion losses of GNRs-NaCMC films were measured both to be around 1.1 dB. The fiber laser was pumped by a 980 nm laser diode (LD) through a 980/1550 nm wavelengthdivision multiplexer (WDM). A 20-cm-long Liekki Er80-8/125 EDF was used as the gain medium. A polarization controller was used in the cavity to optimize the laser performance. Unidirectional light propagation was ensured by an optical isolator (ISO). A 90/10 1 × 2 fiber coupler functioned as an output mirror to couple 10% output from the EDFL cavity. The rest of the cavity consisted of a SMF-28 single mode fiber and the total cavity length was 5 m. The output lasers were analyzed by using an optical spectrum analyzer and a digital oscilloscope. For the S-GNRs-NaCMC film, the detection results are as follow. Continuous wave operation started at 20 mW pump power. The pulsed laser oscillation was achieved as soon as the pump power exceeded the threshold of ∼25 mW with a careful adjustment to the cavity elements. Fig. 5(a) shows the typical output optical spectrum of the EDFL centered at 1561 nm depends on a pump power of ∼120 mW. The full width at half maximum (FWHM) of the spectrum is 1–2 nm, which is much shorter than that achieved by SWNTs or graphene mode-locked lasers [29]. Fig. 5(b) shows the pulse train for a typical laser output at 120 mW pump power. The time interval between adjacent pulses is about 43.5 s corresponding to a repetition rate of ∼23.0 kHz. Fig. 5(c) plots a typical single pulse shape with a FWHM of ∼9.6 s, which is comparable to that of the fiber lasers Q-switched with other SAs, but much longer than that of the mode-locked fiber lasers [15,29]. Fig. 5(d) presents the dependence of the pulse width (solid circles) and repetition rate (hollow squares) on the pump power. It is seen that the pulse width becomes smaller and the repetition rate becomes larger with increasing the pump power, which presents a
Fig. 6. (a) Emission spectrum, (b) typical pulse train, (c) single pulse profile of the Q-switched EDFL with L-GNRs-NaCMC film under the pump power of ∼120 mW, and (d) dependence of the pulse width (solid circles) and the repetition rate (hollow squares) on the pump power.
typical feature of passively Q-switching unlike mode-locking that is operating at high repetition rate of MHz in a fundamental mode determined by the cavity length. The reason is that the pump rate for the upper laser level increases with the rise of pump power and SA gains much more saturation energies. The resultant pulse duration is ranging from 36.4 to 9.6 s and the repetition rate varies from 8.26 to 22.94 kHz with a 95 mW pump power variation. The L-GNRs-NaCMC film with a LSPR band between 1246 and 1442 nm was also capable of initiating passively Q-switching in the same EDFL, although the absorption intensity of infrared band is lower than that of visible band. The threshold pump power was the same as low as 25 mW. When the pump power is increased from 25 to 120 mW, the pulse width decreases from 39.9 to 10.4 s and the repetition rate increases from 9.8 to 22.39 kHz as shown in Fig. 6. For 95 mW variation of the pump power, the output power of the two passively Q-switched lasers increases linearly from 0.08 to 4.77 mW and 0.06 to 4.70 mW, respectively (Fig. 7). The optical-to-optical efficiencies of the two lasers are both about 4%. The EDFLs based on GNRs-NaCMC films could be Q-switched easily with high reproducibility. The pulse train was stable during several hours of operation. The damage of GNRs-NaCMC film only occurred at much higher pump power (>120 mW). As it is shown,
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4. Conclusions
Fig. 7. Output power of the Q-switched EDFL with S-GNRs-NaCMC film (a) and LGNRs-NaCMC film (b) as a function of the pump power.
both the S-GNRs and the L-GNRs can induce Q-switched pulse output at 1561 nm, therefore the absorption intensities and wavelengths are all significant for the saturable absorption properties. As a consequence, materials with proper structures could be designed as SAs to realize pulse lasers. Compared with the EDFL based on SGNRs, the Q-switched performance of the laser based on L-GNRs is not so better, even though the LSPR band of L-GNRs has been tuned to 1442 nm near the laser operation wavelength of ∼1561 nm. This phenomenon may be caused by that the infrared absorption intensity of L-GNRs is relatively lower than that of S-GNRs. In fact, the synthesis of L-GNRs is more complicated (HCl is needed and the pH value should be adjusted precisely). A practical SA should possess both good Q-switched ability and fairly simple manufacturing process. After an overall consideration of these aspects, we regard the S-GNRs as more suitable for inducing Q-switched pulse at operation wavelength, i.e. 1.55 m for telecommunication applications in our experiment. Attention should be paid to that the Q-switched performance of the EDFL based on GNRs is essentially the same with that based on GNPs; actually the pulse width of the laser based on the second one was narrower. As we have known, the saturable absorption properties of SAs are related to various factors, such as scattering losses of nanorods, non-saturable absorbance of impurities, polymer matrixes, and surfactants [30]. In the synthesis process of GNRs, CTAB together with 5-BMSA was used. These surfactants inevitably existed in the SA films although the resulting GNRs were washed with deionized water. At the same time, the sizes of GNRs (with average length of about 100 nm) are larger than those of GNPs (with average diameter of about 20 nm). Therefore, more scattering losses can be inferred in the GNRs-NaCMC film, so the insertion loss was a little larger and the pulse width was narrower than those obtained with GNPs-NaCMC film. On the other hand, the wide band absorption of GNPs-NaCMC film caused by the aggregation of GNPs can only be maintained for about a month. The reason for the disappearing of wide absorption band we have not known exactly, perhaps the aggregation of the GNPs is not so stable. As a consequence, the GNPs may have more potential use in the visible wavelength pulse lasers. By contrast, the absorption bands of GNRs centered at longer than 1000 nm are determined by their natural aspect ratios and can be retained easily for a long time. The stable of GNRs is significant in the practical application, although they are more difficult to synthesize than GNPs and the Q-switched performances of EDFL based on them are not better than that based on GNPs. In order to actualize better passively Q-switched operation by using GNRs with sharper LSPR peak near laser operation wavelength, the synthesis method will be improved to get uniform GNRs with larger aspect ratio and fewer shape impurities in the future. We will also study the effect of the film thickness and the GNRs concentration on the Q-switched quality of pulse lasers. Moreover, if broadly tunable LSPR peaks of GNRs were realized, passively Q-switching at different operating wavelengths can also be anticipated.
In conclusion, we have demonstrated GNRs-NaCMC films with LSPR bands greater than 1000 nm could be used as SAs experimentally. When these SAs were inserted into EDFLs, self-start and stable passively Q-switched pulses were realized for a threshold pump power of ∼25 mW. 9.6-s pulses with repetition rate of ∼22.9 kHz and 10.4-s pulses with repetition rate of ∼22.4 kHz were obtained at a pump power of ∼120 mW, respectively. By varying the pump power from 25 to 120 mW, the average output power can be adjusted from 0.08 to 4.77 mW and 0.06 to 4.70 mW, respectively. As is well known, the LSPR wavelength of GNRs can be tuned by changing their aspect ratios. Therefore the use of GNRs for Q-switching at other wavelengths can also be expected based on our work. The GNRs-NaCMC film may find practical applications in fiber communications, optical data storage, optical limiters, and sensors.
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant nos. 60908001, 61077033, and 61275153), the Foundation of Zhejiang Educational Commission (grant nos. Y201430403 and Y201430419), K.C. Wong Magna Foundation in Ningbo University, China.
References [1] E.L.F. Filho, R.B. Silva, R.G.S. Filho, A.M.B. Silva, A. Galembeck, C.B. de Araújo, High-order nonlinearity of silica-gold nanoshells in chloroform at 1560 nm, Opt. Express 18 (2010) 21636–21644. [2] P. Lu, K. Wang, H. Long, M. Fu, G. Yang, Size-related third-order optical nonlinearities of Au nanoparticle arrays, Opt. Express 18 (2010) 13874–13879. [3] J.T. Seo, Q. Yang, W.J. Kim, J. Heo, S.M. Ma, J. Austin, W.S. Yun, S.S. Jung, S.W. Han, B. Tabibi, D. Temple, Optical nonlinearities of Au nanoparticles and Au/Ag coreshells, Opt. Lett. 34 (2009) 307–309. [4] L. Polavarapu, V. Mamidala, Z.P. Guan, W. Ji, Q.H. Xu, Huge enhancement of optical nonlinearities in coupled Au and Ag nanoparticles induced by conjugated polymers, Appl. Phys. Lett. 100 (2012) 023106. [5] F.Z. Henari, A.A. Dakhel, Investigation of nonlinear optical properties of gold nanograins embedded in indium oxide films by reflection Z-scan using continuous laser, J. Appl. Phys. 108 (2010) 123109. [6] Y. Tsutsui, T. Hayakawa, G. Kawamura, M. Nogami, Tuned longitudinal surface plasmon resonance and third-order nonlinear optical properties of gold nanorods, Nanotechnology 22 (2011) 275203. [7] L. Bigot, H.E. Hamzaoui, A.L. Rouge, G. Bouwmans, F. Chassagneux, B. Capoen, M. Bouazaoui, Linear and nonlinear optical properties of gold nanoparticle-doped photonic crystal fiber, Opt. Express 19 (2011) 19061–19066. [8] H. Baida, D. Mongin, D. Christofilos, G. Bachelier, A. Crut, P. Maioli, N.D. Fatti, F. Vallee, Ultrafast nonlinear optical response of a single gold nanorod near its surface plasmon resonance, Phys. Rev. Lett. 107 (2011) 057402. [9] P. Wang, Y. Lu, L. Tang, J. Zhang, H. Ming, J. Xie, F.H. Ho, H.H. Chang, H.Y. Lin, D.P. Tsai, Surface-enhanced optical nonlinearity of a gold film, Opt. Commun. 229 (2004) 425–429. [10] S. Palomba, S. Zhang, Y. Park, G. Bartal, X. Yin, X. Zhang, Optical negative refraction by four-wave mixing in thin metallic nanostructures, Nat. Mater. 11 (2011) 34–38. [11] F. Chen, S. Dai, T. Xu, X. Shen, C. Lin, Q. Nie, C. Liu, J. Heo, Surfaceplasmon enhanced ultrafast third-order optical nonlinearities in ellipsoidal gold nanoparticles embedded bismuthate glasses, Chem. Phys. Lett. 514 (2011) 79–82. [12] K.H. Kim, A. Husakou, J. Herrmann, Saturable absorption in composites doped with metal nanoparticles, Opt. Express 18 (2010) 21918–21925. [13] R. Paschotta, R. Häring, E. Gini, H. Melchior, U. Keller, H.L. Offerhaus, D.J. Richardson, Passively Q-switched 0.1-mJ fiber laser system at 1.53 m, Opt. Lett. 24 (1999) 388–390. [14] M. Laroche, A.M. Chardon, J. Nilsson, D.P. Shepherd, W.A. Clarkson, S. Girard, R. Moncorg, Compact diode-pumped passively Q-switched tunable Er–Yb doubleclad fiber laser, Opt. Lett. 27 (2002) 1980–1982. [15] Z.D. Peng, W. Li, D. Bo, L.W. Ki, Tunable passively Q-switched erbium-doped fiber laser with carbon nanotubes as a saturable absorber, IEEE Photon. Technol. Lett. 22 (2010) 9–11. [16] G.J. Spuhler, S. Reffert, M. Haiml, M. Moser, U. Keller, Output-coupling semiconductor saturable absorber mirror, Appl. Phys. Lett. 78 (2001) 2733– 2735.
T. Jiang et al. / Optik 125 (2014) 5789–5793 [17] W.J. Tang, D.C. Li, S.Z. Zhao, G.Q. Li, K.J. Yang, Optimization of the passively Qswitched laser with a novel central semiconductor saturable absorption mirror (C-SESAM), Optik 122 (2011) 1558–1564. [18] H.H. Liu, K.K. Chow, S. Yamashita, S.Y. Set, Carbon-nanotube-based passively Q-switched fiber laser for high energy pulse generation, Opt. Laser Technol. 45 (2013) 713–716. [19] B. Dong, J. Hu, C.Y. Liaw, J. Hao, C. Yu, Wideband-tunable nanotube Qswitched low threshold erbium doped fiber laser, Appl. Opt. 50 (2011) 1442–1445. [20] Z. Wang, S. Zhu, Y. Chen, M. Wu, C. Zhao, H. Zhang, Multilayer graphene for Q-switched mode-locking operation in an erbium-doped fiber laser, Opt. Commun. 300 (2013) 17–21. [21] J. Liu, J. Xu, P. Wang, Graphene-based passively Q-switched 2 m thuliumdoped fiber laser, Opt. Commun. 285 (2012) 5319–5322. [22] T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P.H. Tan, A.G. Rozhin, A.C. Ferrari, Nanotube–polymer composites for ultrafast photonics, Adv. Mater. 21 (2009) 3874–3899. [23] A. Lin, X. Liu, P.R. Watekar, Y. Chung, W.T. Han, Ag nanocrystal-incorporated germano-silicate optical fiber with high resonant nonlinearity, Appl. Phys. Lett. 93 (2008) 021901–021903.
5793
[24] J. Jayabalan, A. Singh, S. Khan, R. Chari, Third-order nonlinearity of metal nanoparticles: isolation of instantaneous and delayed contributions, J. Appl. Phys. 112 (2012) 103524. [25] C. Corbari, M. Beresna, P.G. Kazansky, Saturation of absorption in noble metal doped nanocomposite glass film excited by evanescent light field, Appl. Phys. Lett. 97 (2010) 261101. [26] K.H. Kim, U. Griebner, J. Herrmann, Theory of passive mode locking of solidstate lasers using metal nanocomposites as slow saturable absorbers, Opt. Lett. 37 (2012) 1490–1492. [27] T. Jiang, Y. Xu, Q.J. Tian, L. Liu, Z. Kang, R.Y. Yang, G.S. Qin, W.P. Qin, Passively Q-switching induced by gold nanocrystals, Appl. Phys. Lett. 101 (2012) 151122. [28] X.C. Ye, L.H. Jin, H. Caglayan, J. Chen, G.Z. Xing, C. Zheng, V.D. Nguyen, Y.J. Kang, N. Engheta, C.R. Kagan, C.B. Murray, Improved size-tunable synthesis of monodisperse gold nanorods through the use of aromatic additives, ACS Nano 6 (2012) 2804–2817. [29] H.Y. Chen, C.X. Huang, Time-domain ABCD matrix analysis of passively modelocked Tm-doped fiber laser, Optik 124 (2013) 1490–1492. [30] A.G. Rozhin, V. Scardaci, F. Wang, F. Hennrich, I.H. White, W.I. Milne, A.C. Ferrari, Generation of ultra-fast laser pulses using nanotube mode-lockers, Phys. Stat. Sol. (B) 243 (2006) 3551–3555.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Passively Q-switched erbium-doped fiber laser based on gold nanorods Tao Jiang a , Guanshi Qin b,∗ , Weiping Qin b,∗ , Jun Zhou a a b
Institute of Photonics, Faculty of Science, Ningbo University, Ningbo 315211, China College of Electronic Science & Engineering, Jilin University, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 17 October 2013 Accepted 29 May 2014 Keywords: Fiber lasers Q-switching Gold nanorods Saturable absorber
a b s t r a c t We have demonstrated that the gold nanorods (GNRs) can be used as effective saturable absorbers for passively Q-switched erbium-doped fiber laser (EDFL). Two types of GNRs were synthesized by a seedmediated growth method. The longitudinal surface plasmon resonance wavelengths of the GNRs with different aspect ratios were around 1068 and 1442 nm, respectively. The GNRs were mixed with sodium carboxymethylcellulose (NaCMC) to form GNRs-NaCMC films with different absorption wavelengths. Using these GNRs-NaCMC films, 9.6-s pulses with a repetition rate of ∼22.9 kHz and 10.4-s pulses with a repetition rate of ∼22.4 kHz were obtained from the passively Q-switched EDFLs at a pump power of ∼120 mW, respectively. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Due to their large third-order nonlinearity and short response time as picoseconds, noble metals have been applied in many fields such as ultrafast communication, optical information processing, all-optical switching, optical data storage, and optical limiting [1–4]. In recent years, many works have been focused on the theoretical study of nonlinear property and the detection of nonlinear absorption coefficient of both gold composite materials and gold films [5–8]. For instance, the optical nonlinearities enhancement of a gold film with random roughness has been obtained [9]. Gold nanocrystals (GNCs)-doped photonic crystal fibers have been produced and their optical limiting behavior has been observed [7]. Using the four-wave mixing deduced by nonlinearities of gold nanostructure films, nonlinear negative refraction at optical frequencies has been demonstrated experimentally [10]. The surface plasmon excitation of GNCs can induce resonant enhancement of the local electric field resulting in amplifying of the third-order optical nonlinearities, second harmonic signal, Raman scattering, etc. [11]. Particularly, GNCs have shown saturable absorption properties for their surface plasmon resonance [11,12]. On the other hand, owing to their flexibility and low cost, highly stable and compact passively Q-switched pulsed fiber lasers have been widely used in optical imaging, medicine, material processing,
∗ Corresponding authors. Tel.: +86 574 87600744; fax: +86 574 87600744. E-mail addresses: [email protected] (G. Qin), [email protected] (W. Qin). http://dx.doi.org/10.1016/j.ijleo.2014.07.011 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
military, fiber communications, and fiber optical sensing [13–15]. Although transition metal ions doped bulk crystals, bleachable dyes, and semiconductor saturable absorber mirrors (SESAMs) have been commonly applied as absorbers in passively Q-switched lasers, it requires extra elements (e.g., lens and mirrors) to focus the output light from fibers into bulk saturable absorber (SA), which makes lasers complex [16,17]. Recently, much attention has been paid to passively Q-switched fiber lasers with singlewall carbon nanotubes (SWNTs) and graphene as SAs due to their numerous advantages, including good compatibility with optical fibers, minimized dimension, extremely high nonlinearities, low saturation intensity, subpicosecond recovery time, wide operating range, and simple manufacturing process [18–21]. In addition, SWNTs and graphene can be homogeneous dispersed in polymers such as polyvinylalcohol, polyamide, sodium carboxymethylcellulose (NaCMC), and polymethylmethacrylate to decrease scattering losses [22]. Furthermore, much research has been carried out on the synthesis of SWNTs with tunable sizes and graphene with fewer layers to get better Q-switching performance such as tunable laser operating wavelength, low insertion loss, low pump thresholds, and high optical-to-optical efficiency [19,22]. By contrast, little attention has been paid to the research on noble metals such as gold, silver, and copper with promising optical properties using as the Q-switching element [22–24]. Especially, GNCs have been proven to possess excellent nonlinear optical properties in previous works [25,7,26] and the passively Q-switching induced by gold nanoparticles (GNPs) has been demonstrated in our experiment [27]. However, the main absorption wavelength of
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GNPs was at about 523 nm, far from the laser operating wavelength we applied. Although a broad absorption band from 500 to 2000 nm was caused by the aggregation of GNPs, the concentration of GNPs in the film should be exactly tuned and the red shift of absorption band to 1561 nm is difficult to control. If gold nanorods (GNRs) with longitudinal surface plasmon resonance (LSPR) bands near 1561 nm were used, more stable passively Q-switched erbiumdoped fiber laser (EDFL) may be obtained. The ability to manipulate the structure and composition of nanoscale materials gives opportunities to create novel machines with superior performance. It is important that the properties of materials can be manufactured in more flexible ways. The LSPR bands of GNRs can be tuned by simply changing their aspect ratios, and therefore, the passively Q-switching at different wavelengths may be realized easily. As a result, GNRs with perfect saturable absorption properties are thus other suitable absorbers for application in passively Q-switched lasers besides SWNTs and graphene. In this work, we reported on the application of GNRs as SAs in passively Q-switched EDFLs. Using the SAs with different absorption wavelengths, 9.6-s pulses with repetition rate of ∼22.9 kHz and 10.4-s pulses with repetition rate of ∼22.4 kHz were obtained from passively Q-switched EDFLs at a pump power of ∼120 mW, respectively. By varying the pump power from 25 to 120 mW, the average output power can be adjusted from 0.08 to 4.77 mW and 0.06 to 4.70 mW, respectively. Our experiment results indicated that SAs based on GNRs are also promising passively Q-switcher with low fabrication cost, polarization insensitivity, and environmental robustness.
Fig. 1. TEM images of the S-GNRs (a) and the L-GNRs (b), AFM images of the S-GNRsNaCMC film (c) and the L-GNRs-NaCMC film (d).
2. Experimental details 2.1. Sample preparation The colloidal GNRs were synthesized by a seed-mediated growth method [28]. Firstly, the seed solution for GNRs was prepared by using fresh NaBH4 as reductant and aged at 25 ◦ C for 2 h. Then, the growth solution was prepared. 10 mL of hexadecyltrimethyl ammonium bromide (CTAB) (0.2 M) together with 10 mL of 5-bromosalicylic acid (5-BMSA) (0.2 M) were mixed with 4 mM AgNO3 solution. 1 mL of HAuCl4 (10 mM) and, if necessary, a small amount of HCl (37 wt% in water, 12.1 M) was added. After 15 min of slow stirring, 1 mL of ascorbic acid (0.1 mM) was added, and the solution was vigorously stirred for 30 s until colorless. Finally, 0.8 mL of seed solution was injected into the growth solution. The mixture was stirred for 30 s and left undisturbed at 30 ◦ C for 12 h for GNRs growth. The resulting precipitates were separated by centrifugation and washed with deionized water. Stable suspensions of GNRs in 1 wt% aqueous solution of NaCMC (medium viscosity, Sigma) were prepared by ultrasonication. This suspension was kept for 48 h and no precipitation was observed. The GNRs-NaCMC films were formed by casting the solution onto a flat substrate, followed by a slow drying at room temperature. 2.2. Characterization The sizes and morphologies of the obtained GNRs were observed by a JEM-2000 EX transmission electron microscope (TEM). As shown in Fig. 1(a), the obtained GNRs have an average diameter of 14.0 ± 4.0 nm and an average length of 95.0 ± 6.5 nm (corresponding to an average aspect ratio of about 6.8). Small fraction of spherical NPs also exists in the as-synthesized sample. Fig. 1(b) shows that the GNRs obtained from precursor solution with lower pH value (HCl was added) have an average diameter of 12.0 ± 4.0 nm and an average length of 100.0 ± 20.5 nm (corresponding to larger aspect ratios from 8.4 to 11.4). However, more
Fig. 2. Optical absorption spectra of the GNPs-NaCMC film, the S-GNRs-NaCMC film and the L-GNRs-NaCMC film (The inset shows the photographs of the corresponding aqueous solutions).
acidic environment causes a greater percentage of quasi-spherical NPs and wider size distribution of the synthesized GNRs. The atomic force microscopy (AFM) images of the prepared films were shown in Fig. 1(c) and (d). The surfaces of the two thin films were both smooth and crack-free. The absorption spectra of GNRs-NaCMC films with different SPR bands were compared to that of GNPs-NaCMC film we prepared previously [27]. All the three GNC-NaCMC films have broadband absorption from 500 to 2000 nm as shown in Fig. 2. However, the main absorption peak of GNPs-NaCMC film was at 530 nm far from the laser operating wavelength (1561 nm). The film of GNRs with small aspect ratio (S-GNRs) has a main absorption peak centered at around 1068 nm. The film of GNRs with large aspect ratio (L-GNRs) maintains broad absorption band between 1246 and 1442 nm. However, the intensity of the infrared band is relatively lower. The inset of Fig. 2 shows the photographs of the corresponding aqueous solutions of GNPs, S-GNRs, and L-GNRs. 3. Results and discussion To investigate the saturable absorption property of the two GNRs-NaCMC films, we measured the dependence of the transmission on the pump peak power density for the two films by using a 1560 nm pulsed fiber laser with a pulse width of 200 fs, as shown
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Fig. 3. Dependence of the transmission at 1560 nm on the pump peak power density for the S-GNRs-NaCMC film (a) and the L-GNRs-NaCMC film (b).
Fig. 5. (a) Emission spectrum, (b) typical pulse train, (c) single pulse profile of the Q-switched EDFL with S-GNRs-NaCMC film under the pump power of ∼120 mW, and (d) dependence of the pulse width (solid circles) and the repetition rate (hollow squares) on the pump power.
Fig. 4. Schematic illustration of the proposed GNCs based Q-switched EDFL.
in Fig. 3. The measured data exhibited evidently the feature of saturable absorption. Non-bleachable losses (˛0 ), modulation depths (T), and saturation power densities (Psat ) were determined to be 24.6%, 6.9%, and 5.16 MW/cm2 for S-GNRs and 25.4%, 6.7%, and 5.09 MW/cm2 for L-GNRs respectively. Although their modulation depths were smaller than that of the GNPs-NaCMC film, the GNRsNaCMC film could also be used to induce Q-switching. The GNRs-NaCMC film was placed between two fiber connectors to form a fiber-compatible SA, and then integrated into a laser cavity as shown in Fig. 4. The insertion losses of GNRs-NaCMC films were measured both to be around 1.1 dB. The fiber laser was pumped by a 980 nm laser diode (LD) through a 980/1550 nm wavelengthdivision multiplexer (WDM). A 20-cm-long Liekki Er80-8/125 EDF was used as the gain medium. A polarization controller was used in the cavity to optimize the laser performance. Unidirectional light propagation was ensured by an optical isolator (ISO). A 90/10 1 × 2 fiber coupler functioned as an output mirror to couple 10% output from the EDFL cavity. The rest of the cavity consisted of a SMF-28 single mode fiber and the total cavity length was 5 m. The output lasers were analyzed by using an optical spectrum analyzer and a digital oscilloscope. For the S-GNRs-NaCMC film, the detection results are as follow. Continuous wave operation started at 20 mW pump power. The pulsed laser oscillation was achieved as soon as the pump power exceeded the threshold of ∼25 mW with a careful adjustment to the cavity elements. Fig. 5(a) shows the typical output optical spectrum of the EDFL centered at 1561 nm depends on a pump power of ∼120 mW. The full width at half maximum (FWHM) of the spectrum is 1–2 nm, which is much shorter than that achieved by SWNTs or graphene mode-locked lasers [29]. Fig. 5(b) shows the pulse train for a typical laser output at 120 mW pump power. The time interval between adjacent pulses is about 43.5 s corresponding to a repetition rate of ∼23.0 kHz. Fig. 5(c) plots a typical single pulse shape with a FWHM of ∼9.6 s, which is comparable to that of the fiber lasers Q-switched with other SAs, but much longer than that of the mode-locked fiber lasers [15,29]. Fig. 5(d) presents the dependence of the pulse width (solid circles) and repetition rate (hollow squares) on the pump power. It is seen that the pulse width becomes smaller and the repetition rate becomes larger with increasing the pump power, which presents a
Fig. 6. (a) Emission spectrum, (b) typical pulse train, (c) single pulse profile of the Q-switched EDFL with L-GNRs-NaCMC film under the pump power of ∼120 mW, and (d) dependence of the pulse width (solid circles) and the repetition rate (hollow squares) on the pump power.
typical feature of passively Q-switching unlike mode-locking that is operating at high repetition rate of MHz in a fundamental mode determined by the cavity length. The reason is that the pump rate for the upper laser level increases with the rise of pump power and SA gains much more saturation energies. The resultant pulse duration is ranging from 36.4 to 9.6 s and the repetition rate varies from 8.26 to 22.94 kHz with a 95 mW pump power variation. The L-GNRs-NaCMC film with a LSPR band between 1246 and 1442 nm was also capable of initiating passively Q-switching in the same EDFL, although the absorption intensity of infrared band is lower than that of visible band. The threshold pump power was the same as low as 25 mW. When the pump power is increased from 25 to 120 mW, the pulse width decreases from 39.9 to 10.4 s and the repetition rate increases from 9.8 to 22.39 kHz as shown in Fig. 6. For 95 mW variation of the pump power, the output power of the two passively Q-switched lasers increases linearly from 0.08 to 4.77 mW and 0.06 to 4.70 mW, respectively (Fig. 7). The optical-to-optical efficiencies of the two lasers are both about 4%. The EDFLs based on GNRs-NaCMC films could be Q-switched easily with high reproducibility. The pulse train was stable during several hours of operation. The damage of GNRs-NaCMC film only occurred at much higher pump power (>120 mW). As it is shown,
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4. Conclusions
Fig. 7. Output power of the Q-switched EDFL with S-GNRs-NaCMC film (a) and LGNRs-NaCMC film (b) as a function of the pump power.
both the S-GNRs and the L-GNRs can induce Q-switched pulse output at 1561 nm, therefore the absorption intensities and wavelengths are all significant for the saturable absorption properties. As a consequence, materials with proper structures could be designed as SAs to realize pulse lasers. Compared with the EDFL based on SGNRs, the Q-switched performance of the laser based on L-GNRs is not so better, even though the LSPR band of L-GNRs has been tuned to 1442 nm near the laser operation wavelength of ∼1561 nm. This phenomenon may be caused by that the infrared absorption intensity of L-GNRs is relatively lower than that of S-GNRs. In fact, the synthesis of L-GNRs is more complicated (HCl is needed and the pH value should be adjusted precisely). A practical SA should possess both good Q-switched ability and fairly simple manufacturing process. After an overall consideration of these aspects, we regard the S-GNRs as more suitable for inducing Q-switched pulse at operation wavelength, i.e. 1.55 m for telecommunication applications in our experiment. Attention should be paid to that the Q-switched performance of the EDFL based on GNRs is essentially the same with that based on GNPs; actually the pulse width of the laser based on the second one was narrower. As we have known, the saturable absorption properties of SAs are related to various factors, such as scattering losses of nanorods, non-saturable absorbance of impurities, polymer matrixes, and surfactants [30]. In the synthesis process of GNRs, CTAB together with 5-BMSA was used. These surfactants inevitably existed in the SA films although the resulting GNRs were washed with deionized water. At the same time, the sizes of GNRs (with average length of about 100 nm) are larger than those of GNPs (with average diameter of about 20 nm). Therefore, more scattering losses can be inferred in the GNRs-NaCMC film, so the insertion loss was a little larger and the pulse width was narrower than those obtained with GNPs-NaCMC film. On the other hand, the wide band absorption of GNPs-NaCMC film caused by the aggregation of GNPs can only be maintained for about a month. The reason for the disappearing of wide absorption band we have not known exactly, perhaps the aggregation of the GNPs is not so stable. As a consequence, the GNPs may have more potential use in the visible wavelength pulse lasers. By contrast, the absorption bands of GNRs centered at longer than 1000 nm are determined by their natural aspect ratios and can be retained easily for a long time. The stable of GNRs is significant in the practical application, although they are more difficult to synthesize than GNPs and the Q-switched performances of EDFL based on them are not better than that based on GNPs. In order to actualize better passively Q-switched operation by using GNRs with sharper LSPR peak near laser operation wavelength, the synthesis method will be improved to get uniform GNRs with larger aspect ratio and fewer shape impurities in the future. We will also study the effect of the film thickness and the GNRs concentration on the Q-switched quality of pulse lasers. Moreover, if broadly tunable LSPR peaks of GNRs were realized, passively Q-switching at different operating wavelengths can also be anticipated.
In conclusion, we have demonstrated GNRs-NaCMC films with LSPR bands greater than 1000 nm could be used as SAs experimentally. When these SAs were inserted into EDFLs, self-start and stable passively Q-switched pulses were realized for a threshold pump power of ∼25 mW. 9.6-s pulses with repetition rate of ∼22.9 kHz and 10.4-s pulses with repetition rate of ∼22.4 kHz were obtained at a pump power of ∼120 mW, respectively. By varying the pump power from 25 to 120 mW, the average output power can be adjusted from 0.08 to 4.77 mW and 0.06 to 4.70 mW, respectively. As is well known, the LSPR wavelength of GNRs can be tuned by changing their aspect ratios. Therefore the use of GNRs for Q-switching at other wavelengths can also be expected based on our work. The GNRs-NaCMC film may find practical applications in fiber communications, optical data storage, optical limiters, and sensors.
Acknowledgments This work was supported by the National Natural Science Foundation of China (grant nos. 60908001, 61077033, and 61275153), the Foundation of Zhejiang Educational Commission (grant nos. Y201430403 and Y201430419), K.C. Wong Magna Foundation in Ningbo University, China.
References [1] E.L.F. Filho, R.B. Silva, R.G.S. Filho, A.M.B. Silva, A. Galembeck, C.B. de Araújo, High-order nonlinearity of silica-gold nanoshells in chloroform at 1560 nm, Opt. Express 18 (2010) 21636–21644. [2] P. Lu, K. Wang, H. Long, M. Fu, G. Yang, Size-related third-order optical nonlinearities of Au nanoparticle arrays, Opt. Express 18 (2010) 13874–13879. [3] J.T. Seo, Q. Yang, W.J. Kim, J. Heo, S.M. Ma, J. Austin, W.S. Yun, S.S. Jung, S.W. Han, B. Tabibi, D. Temple, Optical nonlinearities of Au nanoparticles and Au/Ag coreshells, Opt. Lett. 34 (2009) 307–309. [4] L. Polavarapu, V. Mamidala, Z.P. Guan, W. Ji, Q.H. Xu, Huge enhancement of optical nonlinearities in coupled Au and Ag nanoparticles induced by conjugated polymers, Appl. Phys. Lett. 100 (2012) 023106. [5] F.Z. Henari, A.A. Dakhel, Investigation of nonlinear optical properties of gold nanograins embedded in indium oxide films by reflection Z-scan using continuous laser, J. Appl. Phys. 108 (2010) 123109. [6] Y. Tsutsui, T. Hayakawa, G. Kawamura, M. Nogami, Tuned longitudinal surface plasmon resonance and third-order nonlinear optical properties of gold nanorods, Nanotechnology 22 (2011) 275203. [7] L. Bigot, H.E. Hamzaoui, A.L. Rouge, G. Bouwmans, F. Chassagneux, B. Capoen, M. Bouazaoui, Linear and nonlinear optical properties of gold nanoparticle-doped photonic crystal fiber, Opt. Express 19 (2011) 19061–19066. [8] H. Baida, D. Mongin, D. Christofilos, G. Bachelier, A. Crut, P. Maioli, N.D. Fatti, F. Vallee, Ultrafast nonlinear optical response of a single gold nanorod near its surface plasmon resonance, Phys. Rev. Lett. 107 (2011) 057402. [9] P. Wang, Y. Lu, L. Tang, J. Zhang, H. Ming, J. Xie, F.H. Ho, H.H. Chang, H.Y. Lin, D.P. Tsai, Surface-enhanced optical nonlinearity of a gold film, Opt. Commun. 229 (2004) 425–429. [10] S. Palomba, S. Zhang, Y. Park, G. Bartal, X. Yin, X. Zhang, Optical negative refraction by four-wave mixing in thin metallic nanostructures, Nat. Mater. 11 (2011) 34–38. [11] F. Chen, S. Dai, T. Xu, X. Shen, C. Lin, Q. Nie, C. Liu, J. Heo, Surfaceplasmon enhanced ultrafast third-order optical nonlinearities in ellipsoidal gold nanoparticles embedded bismuthate glasses, Chem. Phys. Lett. 514 (2011) 79–82. [12] K.H. Kim, A. Husakou, J. Herrmann, Saturable absorption in composites doped with metal nanoparticles, Opt. Express 18 (2010) 21918–21925. [13] R. Paschotta, R. Häring, E. Gini, H. Melchior, U. Keller, H.L. Offerhaus, D.J. Richardson, Passively Q-switched 0.1-mJ fiber laser system at 1.53 m, Opt. Lett. 24 (1999) 388–390. [14] M. Laroche, A.M. Chardon, J. Nilsson, D.P. Shepherd, W.A. Clarkson, S. Girard, R. Moncorg, Compact diode-pumped passively Q-switched tunable Er–Yb doubleclad fiber laser, Opt. Lett. 27 (2002) 1980–1982. [15] Z.D. Peng, W. Li, D. Bo, L.W. Ki, Tunable passively Q-switched erbium-doped fiber laser with carbon nanotubes as a saturable absorber, IEEE Photon. Technol. Lett. 22 (2010) 9–11. [16] G.J. Spuhler, S. Reffert, M. Haiml, M. Moser, U. Keller, Output-coupling semiconductor saturable absorber mirror, Appl. Phys. Lett. 78 (2001) 2733– 2735.
T. Jiang et al. / Optik 125 (2014) 5789–5793 [17] W.J. Tang, D.C. Li, S.Z. Zhao, G.Q. Li, K.J. Yang, Optimization of the passively Qswitched laser with a novel central semiconductor saturable absorption mirror (C-SESAM), Optik 122 (2011) 1558–1564. [18] H.H. Liu, K.K. Chow, S. Yamashita, S.Y. Set, Carbon-nanotube-based passively Q-switched fiber laser for high energy pulse generation, Opt. Laser Technol. 45 (2013) 713–716. [19] B. Dong, J. Hu, C.Y. Liaw, J. Hao, C. Yu, Wideband-tunable nanotube Qswitched low threshold erbium doped fiber laser, Appl. Opt. 50 (2011) 1442–1445. [20] Z. Wang, S. Zhu, Y. Chen, M. Wu, C. Zhao, H. Zhang, Multilayer graphene for Q-switched mode-locking operation in an erbium-doped fiber laser, Opt. Commun. 300 (2013) 17–21. [21] J. Liu, J. Xu, P. Wang, Graphene-based passively Q-switched 2 m thuliumdoped fiber laser, Opt. Commun. 285 (2012) 5319–5322. [22] T. Hasan, Z. Sun, F. Wang, F. Bonaccorso, P.H. Tan, A.G. Rozhin, A.C. Ferrari, Nanotube–polymer composites for ultrafast photonics, Adv. Mater. 21 (2009) 3874–3899. [23] A. Lin, X. Liu, P.R. Watekar, Y. Chung, W.T. Han, Ag nanocrystal-incorporated germano-silicate optical fiber with high resonant nonlinearity, Appl. Phys. Lett. 93 (2008) 021901–021903.
5793
[24] J. Jayabalan, A. Singh, S. Khan, R. Chari, Third-order nonlinearity of metal nanoparticles: isolation of instantaneous and delayed contributions, J. Appl. Phys. 112 (2012) 103524. [25] C. Corbari, M. Beresna, P.G. Kazansky, Saturation of absorption in noble metal doped nanocomposite glass film excited by evanescent light field, Appl. Phys. Lett. 97 (2010) 261101. [26] K.H. Kim, U. Griebner, J. Herrmann, Theory of passive mode locking of solidstate lasers using metal nanocomposites as slow saturable absorbers, Opt. Lett. 37 (2012) 1490–1492. [27] T. Jiang, Y. Xu, Q.J. Tian, L. Liu, Z. Kang, R.Y. Yang, G.S. Qin, W.P. Qin, Passively Q-switching induced by gold nanocrystals, Appl. Phys. Lett. 101 (2012) 151122. [28] X.C. Ye, L.H. Jin, H. Caglayan, J. Chen, G.Z. Xing, C. Zheng, V.D. Nguyen, Y.J. Kang, N. Engheta, C.R. Kagan, C.B. Murray, Improved size-tunable synthesis of monodisperse gold nanorods through the use of aromatic additives, ACS Nano 6 (2012) 2804–2817. [29] H.Y. Chen, C.X. Huang, Time-domain ABCD matrix analysis of passively modelocked Tm-doped fiber laser, Optik 124 (2013) 1490–1492. [30] A.G. Rozhin, V. Scardaci, F. Wang, F. Hennrich, I.H. White, W.I. Milne, A.C. Ferrari, Generation of ultra-fast laser pulses using nanotube mode-lockers, Phys. Stat. Sol. (B) 243 (2006) 3551–3555.