Enhanced thermal stability of oleic-acid-capped PbS quantum dot optical fiber amplifier Xiaolan Sun,1,* Rong Dai,1 Juanjuan Chen,1 Wei Zhou,2 Tingyun Wang,1 Alan R. Kost,3 Chia-Kuang (Frank) Tsung,4 and Zesheng An2 1
The Key Lab of Specialty Fiber Optics and Optical Access Networks, Shanghai University, Shanghai 200072, China 2 Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 200444, China 3 College of Optical Sciences, The University of Arizona, Tucson, AZ 85721-0094, USA 4 Department of Chemistry, Boston College, MA 02467, USA *[email protected]
Abstract: Poor thermal stability has remained a severe obstacle for practical applications of optical fiber amplifiers based on quantum dots (QDs). We demonstrate that thermal stability at elevated temperatures can be achieved by using oleic-acid-capped QDs. Optical fiber amplifiers using oleic-acid-capped QDs for the gain medium exhibited stable gain of more than 5 dB at 1550 nm between 25 °C and 50 °C that did not degrade upon cooling. In contrast, fiber amplifiers employing oleylamine-capped QDs exhibited reduced gain when heated and subsequently cooled. ©2014 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.4510) Optical communications; (160.4236) Nanomaterials.
References and links V. Sukhovatkin, S. Hinds, L. Brzozowski, and E. H. Sargent, “Colloidal quantum-dot photodetectors exploiting multiexciton generation,” Science 324(5934), 1542–1544 (2009). 2. V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. J. Eisler, and M. G. Bawendi, “Optical gain and stimulated emission in nanocrystal quantum dots,” Science 290(5490), 314–317 (2000). 3. F. Lelarge, B. Dagens, J. Renaudier, R. Brenot, A. Accard, F. van Dijk, D. Make, O. Le Gouezigou, J.-G. Provost, F. Poingt, J. Landreau, O. Drisse, E. Derouin, B. Rousseau, F. Pommereau, and G.-H. Duan, “Recent advances on InAs/InP quantum dash based, semiconductor lasers and optical amplifiers operating at 1.55 mu m,” IEEE J. Sel. Top. Quantum Electron. 13(1), 111–124 (2007). 4. C. Meuer, J. Kim, M. Laemmlin, S. Liebich, D. Bimberg, A. Capua, G. Eisenstein, R. Bonk, T. Vallaitis, J. Leuthold, A. R. Kovsh, and I. L. Krestnikov, “40 GHz small-signal cross-gain modulation in 1.3 μm quantum dot semiconductor optical amplifiers,” Appl. Phys. Lett. 93(5), 051110 (2008). 5. S. A. McDonald, G. Konstantatos, S. G. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina, and E. H. Sargent, “Solution-processed PbS quantum dot infrared photodetectors and photovoltaics,” Nat. Mater. 4(2), 138–142 (2005). 6. T. Rauch, M. Boeberl, S. F. Tedde, J. Fuerst, M. V. Kovalenko, G. Hesser, U. Lemmer, W. Heiss, and O. Hayden, “Near-infrared imaging with quantum-dot-sensitized organic photodiodes,” Nat. Photonics 3(6), 332– 336 (2009). 7. J. Tang, K. W. Kemp, S. Hoogland, K. S. Jeong, H. Liu, L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, K. W. Chou, A. Fischer, A. Amassian, J. B. Asbury, and E. H. Sargent, “Colloidal-quantum-dot photovoltaics using atomic-ligand passivation,” Nat. Mater. 10(10), 765–771 (2011). 8. E. H. Sargent, “Infrared quantum dots,” Adv. Mater. 17, 515–522 (2005). 9. Y. Ding, R. Aviles-Espinosa, M. A. Cataluna, D. Nikitichev, M. Ruiz, M. Tran, Y. Robert, A. Kapsalis, H. Simos, C. Mesaritakis, T. Xu, P. Bardella, M. Rossetti, I. Krestnikov, D. Livshits, I. Montrosset, D. Syvridis, M. Krakowski, P. Loza-Alvarez, and E. Rafailov, “High peak-power picosecond pulse generation at 1.26 µm using a quantum-dot-based external-cavity mode-locked laser and tapered optical amplifier,” Opt. Express 20(13), 14308–14320 (2012). 10. M. Matsuura, N. Calabretta, O. Raz, and H. J. S. Dorren, “Multichannel wavelength conversion of 50-Gbit/s NRZ-DQPSK signals using a quantum-dot semiconductor optical amplifier,” Opt. Express 19(26), B560–B566 (2011). 11. C. Cheng, H. Jiang, D. Ma, and X. Cheng, “An optical fiber glass containing PbSe quantum dots,” Opt. Commun. 284(19), 4491–4495 (2011). 1.
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 519
12. P. R. Watekar, L. Aoxiang, J. Seongmin, and H. Won-Taek, “1537 nm emission upon 980 nm pumping in PbSe quantum dots doped optical fiber,” in OFC/NFOEC 2008. 2008 Optical Fiber Communication Conference/National Fiber Optic Engineers Conference (2008), pp. 3030–3032. 13. S. Kawanishi, T. Komukai, M. Ohmori, and H. Sakaki, “Photoluminescence of semiconductor nanocrystal quantum dots at 1550 nm wavelength in the core of photonic bandgap fiber,” in CLEO '07. 2007 Conference on Lasers and Electro-Optics (2007), pp. 1343–1344. 14. C. Cheng, “A multiquantum-dot-doped fiber amplifier with characteristics of broadband, flat gain, and low noise,” J. Lightwave Technol. 26(11), 1404–1410 (2008). 15. F. Pang, X. Sun, H. Guo, J. Yan, J. Wang, X. Zeng, Z. Chen, and T. Wang, “A PbS quantum dots fiber amplifier excited by evanescent wave,” Opt. Express 18(13), 14024–14030 (2010). 16. X. Sun, Y. Dong, C. Li, X. Liu, G. Liu, and L. Xie, “PbSe quantum dots fiber amplifier based on sol-gel selfassembly method,” in Passive Components and Fiber-Based Devices VII, P. P. Shum, ed. (SPIE, 2011). 17. X. Sun, L. Xie, W. Zhou, F. Pang, T. Wang, A. R. Kost, and Z. An, “Optical fiber amplifiers based on PbS/CdS QDs modified by polymers,” Opt. Express 21(7), 8214–8219 (2013). 18. H. Guo, F. Pang, X. Zeng, and T. Wang, “PbS quantum dot fiber amplifier based on a tapered SMF fiber,” Opt. Commun. 285(13-14), 3222–3227 (2012). 19. H. Guo, F. Pang, X. Zeng, and T. Wang, “Gain characteristics of quantum dot fiber amplifier based on asymmetric tapered fiber coupler,” Opt. Fiber Technol. 19(2), 143–147 (2013). 20. X. Ji, D. Copenhaver, C. Sichmeller, and X. Peng, “Ligand bonding and dynamics on colloidal nanocrystals at room temperature: The case of alkylamines on CdSe nanocrystals,” J. Am. Chem. Soc. 130(17), 5726–5735 (2008). 21. B. Fritzinger, I. Moreels, P. Lommens, R. Koole, Z. Hens, and J. C. Martins, “In Situ observation of rapid ligand exchange in colloidal nanocrystal suspensions using transfer NOE nuclear magnetic resonance spectroscopy,” J. Am. Chem. Soc. 131(8), 3024–3032 (2009). 22. I. Moreels, Y. Justo, B. De Geyter, K. Haustraete, J. C. Martins, and Z. Hens, “Size-tunable, bright, and stable pbs quantum dots: a surface chemistry study,” ACS Nano 5(3), 2004–2012 (2011). 23. L. Cademartiri, J. Bertolotti, R. Sapienza, D. S. Wiersma, G. von Freymann, and G. A. Ozin, “Multigram scale, solventless, and diffusion-controlled route to highly monodisperse PbS nanocrystals,” J. Phys. Chem. B 110(2), 671–673 (2006). 24. S. F. Wuister, C. de Mello Donegá, and A. Meijerink, “Luminescence Temperature Antiquenching of WaterSoluble CdTe Quantum Dots: Role of the Solvent,” J. Am. Chem. Soc. 126(33), 10397–10402 (2004). 25. S. F. Wuister, A. van Houselt, C. de Mello Donegá, D. Vanmaekelbergh, and A. Meijerink, “Temperature antiquenching of the luminescence from capped CdSe quantum dots,” Angew. Chem. Int. Ed. Engl. 43(23), 3029–3033 (2004).
1. Introduction Quantum dots (QDs) have been under intensive investigation in optical devices due to their high quantum yield, tunable spectral profile and facile preparation [1–7]. Optical amplifiers are important components in optical communication technologies, and commercial optical amplifiers are based on rare-earth doped fibers. Though they are efficient and have high gain, rare-earth doped fibers cannot be applied to ultra-broad band optical communication due to their relatively narrow spectral bandwidth and lack of tunability. The spectral profile of QDs, e.g. PbS QDs, can be easily tuned to cover all optical communication bands [8]. Therefore, several studies have investigated QDs doped into amplifiers of various device architectures, including planar and fiber geometries [4, 9–11]. For QDs-based optical fiber amplifiers, different strategies have been explored to incorporate QDs into optical fiber amplifiers. For example, PbSe QDs were doped into silica optical fibers by using modified chemical vapor deposition (MCVD) technology, and an amplified spontaneous emission at 1537 nm has been achieved [12]. By filling the PbSe QD solution into a photonic bandgap fiber, PL at 1554 nm has been observed [13]. In addition, Cheng et al. realized a multiQD-doped fiber amplifier that used nanocrystals of different sizes [14]. Although these previous studies show the great potential of QD-based fibers, most of the approaches suffer from various drawbacks such as high temperature fabrication, low amplification, or coupling problems with single mode fibers (SMFs). There have been few investigations of the performance of QD-based fiber amplifiers at elevated temperatures. This is unfortunate because high-temperature stability is critical for application in wide-ranging environments. To solve the coupling problem of fiber amplifiers with SMFs, our laboratory has developed a tapered twin SMF coupler structure onto which QDs-doped silica can be coated #199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 520
using a sol-gel process [15–17]. The QDs in the silica coating around the tapered region are excited by an evanescent wave, through which amplification of signals is achieved. The characteristics of this tapered twin SMF coupler have been theoretically evaluated [18, 19]. While this structure is promising, poor thermal stability associated with QDs and hence vanishing amplification signals at higher temperature severely hinder applications for this structure. In order to resolve this thermal instability issue, we have exploited the usage of PbS/CdS core/shell QDs that have shown higher quantum yield and thermal stability [17]. In comparison with a PbS-based amplifier, the PbS/CdS-based amplifier did indeed exhibit better thermal stability. Nevertheless, the gain of the PbS/CdS-based amplifier also decreased with increasing temperature and the gain was essentially absent at 55 °C. Given the limited success using PbS/CdS QDs, other methods are needed to ensure that QDs-based optical fiber amplifiers can reliably maintain their performance in a reasonable temperature range. Ligands play an important role in determining both the optical properties and stability of QDs [20, 21]. The QDs we investigated previously were capped with oleylamine ligands. Acid-containing ligands have stronger binding affinity towards the PbS surface than amine-based ligands [22], and PbS QDs capped with oleic acid have higher quantum yield and higher stability. As a result, optical amplifiers with oleic-acid-capped QDs may be expected to have better thermal stability. Herein, we report a tapered twin SMF coupler optical fiber amplifier based on oleic-acidcapped PbS QDs (OLA-QDs) that maintained its amplification from 25 °C to 50 °C. OLAQDs were prepared by ligand exchange starting from oleylamine-capped QDs (OLAm-QDs) [22]. A second key to the success of this stable fiber amplifier was to use a tailor-made welldefined amphiphilic block copolymer to disperse the hydrophobic OLA-QDs into silica sol. The amphiphilic block copolymer enabled the formation of a stable QDs dispersion in silica sol due to a higher degree of phase segregation, compared with the random amphiphilic copolymer used for our previous report [17]. 2. Synthesis of PbS QDs OLAm-QDs were synthesized according to the procedure of Cademartiri et al [23]. The QDs were then ligand-exchanged with oleic acid to produce OLA-QDs [22]. Their photoluminescence (PL) spectra and temperature-dependent PL intensity are shown in Fig. 1. After ligand exchange, the OLA-QDs exhibit higher PL intensity than the OLAm-QDs at the same QD concentration. More importantly, the PL intensity of OLA-QDs increases with temperature, showing a PL temperature antiquenching effect [24, 25], and the PL intensity is essentially reversible during a heating-cooling cycle. While the exact nature of the antiquenching effect in this particular case is unknown at the current stage, it may be related to the QD surface state rearrangement as previously suggested for other QDs [24, 25]. On the other hand, the PL intensity of OLAm-QDs is reduced upon heating, as expected, and the PL intensity does not recover during the cooling process. These results are consistent with oleic acid binding more strongly to the PbS QD surface than oleylamine, leading to an improved electronic passivation of the surface [22].
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 521
Fig. 1. PL spectra of PbS QDs in chloroform (A) and PL intensity temperature profiles in 1octadecene (B).
3. Fabrication and characteristics of QDs-based optical fiber amplifiers Both OLAm- and OLA-capped QDs are hydrophobic. Thus they were first modified with a designer amphiphilic block copolymer, poly(polyethylene glycol methyl ether methacrylate)28-block-poly(dodecyl methacrylate)50, to disperse the QDs into silica sol, which was then coated on the tapered region of the optical fiber coupler [15, 17]. By using this twin fiber structure, a signal and a pump can be injected into the active region simultaneously. The pump excites the doped QDs through an evanescent wave and the signal interacts with excited QDs through an evanescent wave and then can be amplified. The test system is illustrated in Fig. 2. A 980 nm laser diode is used as the pump, a 1550 nm semiconductor light emitting diode is used as the signal source, and the amplified signal is analyzed on an optical spectrum analyzer.
Fig. 2. The test set-up for quantum dots fiber amplifiers.
Figure 3 shows the output spectra of the fiber amplifiers based on OLAm-QDs and OLAQDs when the input signal or the pump is used individually and the amplified spectrum when both the signal and pump are simultaneously applied. At ambient temperature, both fiber amplifiers show signal enhancement at 1550 nm with a 100 mW 980 nm pump - 4.8 dB for OLAm-QD and 6.47 dB for OLA-QD. The gain for both amplifiers increases with increasing pump power and reaches saturation values of 5.06 dB at 140 mW for OLAm-QD and 7.56 dB at 140 mW for OLA-QD. These results are consistent with the more stable nature and the higher solution PL intensity of OLA-QD in comparison with OLAm-QD.
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 522
Fig. 3. Output spectra with input signal only, pump only, and signal + pump for fiber amplifiers based on OLAm-QDs (A) and OLA-QDs (B). Dependence of gain at 1550 nm on pump power for fiber amplifiers based on OLAm-QDs and OLA-QDs (C).
4. Thermal stability of QDs-based optical fiber amplifiers Encouraged by the enhanced solution PL properties and higher amplification signals of OLAQDs in comparison with those of OLAm-QDs, we next compared the thermal stability of fiber amplifiers based on both types of QDs (Fig. 4). For the fiber amplifier based on OLAmQDs, the gain undergoes an obvious reduction from 4.8 dB to 2.96 dB upon heating from 25 °C to 50 °C, similar to the reduction in PL intensity for OLAm-QDs in solution. When cooled the gain does not recover, but continues to degrade to even lower values. For the fiber amplifier based on OLA-QDs, the gain experiences only a slight drop from 6.02 dB to 5.44 dB on heating from 25 °C to 50 °C, representing only 9.6% gain loss. More importantly, the gain recovers and essentially returns to its original value upon cooling, suggesting that heating the fiber amplifier does not lead to permanent damage to the OLA-QDs due to the superior thermal stability of OLA-QDs. In contrast to the thermal profile of the PL of OLAQDs in solution, which shows an antiquenching effect, the gain of the OLA-QD-doped fiber exhibits a slight decrease on heating. These results reflect the effects of the microenvironment in which the QDs reside on their surface and optical properties. It is likely that in solution the ligands are more flexible and are able to reorganize to adopt a more optimized passivation of the QD surface on heating, while in the solid state this type of ligand reorganization is to a large extent suppressed and the usual thermal quenching effect is exhibited.
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 523
8 OLAm-QD, heating OLAm-QD, cooling
7
OLA-QD, heating OLA-QD, cooling
Gain (dB)
6 5 4 3 2 1
25
30
35
40
45
50
o
Temperature ( C)
Fig. 4. Temperature-dependent gain of QDs-based optical fiber amplifiers at a pump power of 100 mW.
5. Conclusion The addition of capping ligands has profound, beneficial effects on the strength and thermal stability of luminescence for PbS QDs. For this work, we examined the performance of an optical amplifier with a gain region comprised of OLA-QDs (ligand-capped) coated on the tapered region of twin SMF couplers. The OLA-QDs-based optical amplifier exhibited gain that decreased only slightly at elevated temperatures and that returned to its original values when cooled. The results are consistent with use of the OLA-QDs-based optical amplifiers for applications such as broadband optical communications in environments for which the ambient temperature may vary over a wide range and for which thermal control may not be economical. Acknowledgments The work was funded by National Natural Science Foundation of China (61006083, 61377040, 61205172, 61275090, 60937003).
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 524
The Key Lab of Specialty Fiber Optics and Optical Access Networks, Shanghai University, Shanghai 200072, China 2 Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 200444, China 3 College of Optical Sciences, The University of Arizona, Tucson, AZ 85721-0094, USA 4 Department of Chemistry, Boston College, MA 02467, USA *[email protected]
Abstract: Poor thermal stability has remained a severe obstacle for practical applications of optical fiber amplifiers based on quantum dots (QDs). We demonstrate that thermal stability at elevated temperatures can be achieved by using oleic-acid-capped QDs. Optical fiber amplifiers using oleic-acid-capped QDs for the gain medium exhibited stable gain of more than 5 dB at 1550 nm between 25 °C and 50 °C that did not degrade upon cooling. In contrast, fiber amplifiers employing oleylamine-capped QDs exhibited reduced gain when heated and subsequently cooled. ©2014 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (060.4510) Optical communications; (160.4236) Nanomaterials.
References and links V. Sukhovatkin, S. Hinds, L. Brzozowski, and E. H. Sargent, “Colloidal quantum-dot photodetectors exploiting multiexciton generation,” Science 324(5934), 1542–1544 (2009). 2. V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. J. Eisler, and M. G. Bawendi, “Optical gain and stimulated emission in nanocrystal quantum dots,” Science 290(5490), 314–317 (2000). 3. F. Lelarge, B. Dagens, J. Renaudier, R. Brenot, A. Accard, F. van Dijk, D. Make, O. Le Gouezigou, J.-G. Provost, F. Poingt, J. Landreau, O. Drisse, E. Derouin, B. Rousseau, F. Pommereau, and G.-H. Duan, “Recent advances on InAs/InP quantum dash based, semiconductor lasers and optical amplifiers operating at 1.55 mu m,” IEEE J. Sel. Top. Quantum Electron. 13(1), 111–124 (2007). 4. C. Meuer, J. Kim, M. Laemmlin, S. Liebich, D. Bimberg, A. Capua, G. Eisenstein, R. Bonk, T. Vallaitis, J. Leuthold, A. R. Kovsh, and I. L. Krestnikov, “40 GHz small-signal cross-gain modulation in 1.3 μm quantum dot semiconductor optical amplifiers,” Appl. Phys. Lett. 93(5), 051110 (2008). 5. S. A. McDonald, G. Konstantatos, S. G. Zhang, P. W. Cyr, E. J. D. Klem, L. Levina, and E. H. Sargent, “Solution-processed PbS quantum dot infrared photodetectors and photovoltaics,” Nat. Mater. 4(2), 138–142 (2005). 6. T. Rauch, M. Boeberl, S. F. Tedde, J. Fuerst, M. V. Kovalenko, G. Hesser, U. Lemmer, W. Heiss, and O. Hayden, “Near-infrared imaging with quantum-dot-sensitized organic photodiodes,” Nat. Photonics 3(6), 332– 336 (2009). 7. J. Tang, K. W. Kemp, S. Hoogland, K. S. Jeong, H. Liu, L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, K. W. Chou, A. Fischer, A. Amassian, J. B. Asbury, and E. H. Sargent, “Colloidal-quantum-dot photovoltaics using atomic-ligand passivation,” Nat. Mater. 10(10), 765–771 (2011). 8. E. H. Sargent, “Infrared quantum dots,” Adv. Mater. 17, 515–522 (2005). 9. Y. Ding, R. Aviles-Espinosa, M. A. Cataluna, D. Nikitichev, M. Ruiz, M. Tran, Y. Robert, A. Kapsalis, H. Simos, C. Mesaritakis, T. Xu, P. Bardella, M. Rossetti, I. Krestnikov, D. Livshits, I. Montrosset, D. Syvridis, M. Krakowski, P. Loza-Alvarez, and E. Rafailov, “High peak-power picosecond pulse generation at 1.26 µm using a quantum-dot-based external-cavity mode-locked laser and tapered optical amplifier,” Opt. Express 20(13), 14308–14320 (2012). 10. M. Matsuura, N. Calabretta, O. Raz, and H. J. S. Dorren, “Multichannel wavelength conversion of 50-Gbit/s NRZ-DQPSK signals using a quantum-dot semiconductor optical amplifier,” Opt. Express 19(26), B560–B566 (2011). 11. C. Cheng, H. Jiang, D. Ma, and X. Cheng, “An optical fiber glass containing PbSe quantum dots,” Opt. Commun. 284(19), 4491–4495 (2011). 1.
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 519
12. P. R. Watekar, L. Aoxiang, J. Seongmin, and H. Won-Taek, “1537 nm emission upon 980 nm pumping in PbSe quantum dots doped optical fiber,” in OFC/NFOEC 2008. 2008 Optical Fiber Communication Conference/National Fiber Optic Engineers Conference (2008), pp. 3030–3032. 13. S. Kawanishi, T. Komukai, M. Ohmori, and H. Sakaki, “Photoluminescence of semiconductor nanocrystal quantum dots at 1550 nm wavelength in the core of photonic bandgap fiber,” in CLEO '07. 2007 Conference on Lasers and Electro-Optics (2007), pp. 1343–1344. 14. C. Cheng, “A multiquantum-dot-doped fiber amplifier with characteristics of broadband, flat gain, and low noise,” J. Lightwave Technol. 26(11), 1404–1410 (2008). 15. F. Pang, X. Sun, H. Guo, J. Yan, J. Wang, X. Zeng, Z. Chen, and T. Wang, “A PbS quantum dots fiber amplifier excited by evanescent wave,” Opt. Express 18(13), 14024–14030 (2010). 16. X. Sun, Y. Dong, C. Li, X. Liu, G. Liu, and L. Xie, “PbSe quantum dots fiber amplifier based on sol-gel selfassembly method,” in Passive Components and Fiber-Based Devices VII, P. P. Shum, ed. (SPIE, 2011). 17. X. Sun, L. Xie, W. Zhou, F. Pang, T. Wang, A. R. Kost, and Z. An, “Optical fiber amplifiers based on PbS/CdS QDs modified by polymers,” Opt. Express 21(7), 8214–8219 (2013). 18. H. Guo, F. Pang, X. Zeng, and T. Wang, “PbS quantum dot fiber amplifier based on a tapered SMF fiber,” Opt. Commun. 285(13-14), 3222–3227 (2012). 19. H. Guo, F. Pang, X. Zeng, and T. Wang, “Gain characteristics of quantum dot fiber amplifier based on asymmetric tapered fiber coupler,” Opt. Fiber Technol. 19(2), 143–147 (2013). 20. X. Ji, D. Copenhaver, C. Sichmeller, and X. Peng, “Ligand bonding and dynamics on colloidal nanocrystals at room temperature: The case of alkylamines on CdSe nanocrystals,” J. Am. Chem. Soc. 130(17), 5726–5735 (2008). 21. B. Fritzinger, I. Moreels, P. Lommens, R. Koole, Z. Hens, and J. C. Martins, “In Situ observation of rapid ligand exchange in colloidal nanocrystal suspensions using transfer NOE nuclear magnetic resonance spectroscopy,” J. Am. Chem. Soc. 131(8), 3024–3032 (2009). 22. I. Moreels, Y. Justo, B. De Geyter, K. Haustraete, J. C. Martins, and Z. Hens, “Size-tunable, bright, and stable pbs quantum dots: a surface chemistry study,” ACS Nano 5(3), 2004–2012 (2011). 23. L. Cademartiri, J. Bertolotti, R. Sapienza, D. S. Wiersma, G. von Freymann, and G. A. Ozin, “Multigram scale, solventless, and diffusion-controlled route to highly monodisperse PbS nanocrystals,” J. Phys. Chem. B 110(2), 671–673 (2006). 24. S. F. Wuister, C. de Mello Donegá, and A. Meijerink, “Luminescence Temperature Antiquenching of WaterSoluble CdTe Quantum Dots: Role of the Solvent,” J. Am. Chem. Soc. 126(33), 10397–10402 (2004). 25. S. F. Wuister, A. van Houselt, C. de Mello Donegá, D. Vanmaekelbergh, and A. Meijerink, “Temperature antiquenching of the luminescence from capped CdSe quantum dots,” Angew. Chem. Int. Ed. Engl. 43(23), 3029–3033 (2004).
1. Introduction Quantum dots (QDs) have been under intensive investigation in optical devices due to their high quantum yield, tunable spectral profile and facile preparation [1–7]. Optical amplifiers are important components in optical communication technologies, and commercial optical amplifiers are based on rare-earth doped fibers. Though they are efficient and have high gain, rare-earth doped fibers cannot be applied to ultra-broad band optical communication due to their relatively narrow spectral bandwidth and lack of tunability. The spectral profile of QDs, e.g. PbS QDs, can be easily tuned to cover all optical communication bands [8]. Therefore, several studies have investigated QDs doped into amplifiers of various device architectures, including planar and fiber geometries [4, 9–11]. For QDs-based optical fiber amplifiers, different strategies have been explored to incorporate QDs into optical fiber amplifiers. For example, PbSe QDs were doped into silica optical fibers by using modified chemical vapor deposition (MCVD) technology, and an amplified spontaneous emission at 1537 nm has been achieved [12]. By filling the PbSe QD solution into a photonic bandgap fiber, PL at 1554 nm has been observed [13]. In addition, Cheng et al. realized a multiQD-doped fiber amplifier that used nanocrystals of different sizes [14]. Although these previous studies show the great potential of QD-based fibers, most of the approaches suffer from various drawbacks such as high temperature fabrication, low amplification, or coupling problems with single mode fibers (SMFs). There have been few investigations of the performance of QD-based fiber amplifiers at elevated temperatures. This is unfortunate because high-temperature stability is critical for application in wide-ranging environments. To solve the coupling problem of fiber amplifiers with SMFs, our laboratory has developed a tapered twin SMF coupler structure onto which QDs-doped silica can be coated #199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 520
using a sol-gel process [15–17]. The QDs in the silica coating around the tapered region are excited by an evanescent wave, through which amplification of signals is achieved. The characteristics of this tapered twin SMF coupler have been theoretically evaluated [18, 19]. While this structure is promising, poor thermal stability associated with QDs and hence vanishing amplification signals at higher temperature severely hinder applications for this structure. In order to resolve this thermal instability issue, we have exploited the usage of PbS/CdS core/shell QDs that have shown higher quantum yield and thermal stability [17]. In comparison with a PbS-based amplifier, the PbS/CdS-based amplifier did indeed exhibit better thermal stability. Nevertheless, the gain of the PbS/CdS-based amplifier also decreased with increasing temperature and the gain was essentially absent at 55 °C. Given the limited success using PbS/CdS QDs, other methods are needed to ensure that QDs-based optical fiber amplifiers can reliably maintain their performance in a reasonable temperature range. Ligands play an important role in determining both the optical properties and stability of QDs [20, 21]. The QDs we investigated previously were capped with oleylamine ligands. Acid-containing ligands have stronger binding affinity towards the PbS surface than amine-based ligands [22], and PbS QDs capped with oleic acid have higher quantum yield and higher stability. As a result, optical amplifiers with oleic-acid-capped QDs may be expected to have better thermal stability. Herein, we report a tapered twin SMF coupler optical fiber amplifier based on oleic-acidcapped PbS QDs (OLA-QDs) that maintained its amplification from 25 °C to 50 °C. OLAQDs were prepared by ligand exchange starting from oleylamine-capped QDs (OLAm-QDs) [22]. A second key to the success of this stable fiber amplifier was to use a tailor-made welldefined amphiphilic block copolymer to disperse the hydrophobic OLA-QDs into silica sol. The amphiphilic block copolymer enabled the formation of a stable QDs dispersion in silica sol due to a higher degree of phase segregation, compared with the random amphiphilic copolymer used for our previous report [17]. 2. Synthesis of PbS QDs OLAm-QDs were synthesized according to the procedure of Cademartiri et al [23]. The QDs were then ligand-exchanged with oleic acid to produce OLA-QDs [22]. Their photoluminescence (PL) spectra and temperature-dependent PL intensity are shown in Fig. 1. After ligand exchange, the OLA-QDs exhibit higher PL intensity than the OLAm-QDs at the same QD concentration. More importantly, the PL intensity of OLA-QDs increases with temperature, showing a PL temperature antiquenching effect [24, 25], and the PL intensity is essentially reversible during a heating-cooling cycle. While the exact nature of the antiquenching effect in this particular case is unknown at the current stage, it may be related to the QD surface state rearrangement as previously suggested for other QDs [24, 25]. On the other hand, the PL intensity of OLAm-QDs is reduced upon heating, as expected, and the PL intensity does not recover during the cooling process. These results are consistent with oleic acid binding more strongly to the PbS QD surface than oleylamine, leading to an improved electronic passivation of the surface [22].
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 521
Fig. 1. PL spectra of PbS QDs in chloroform (A) and PL intensity temperature profiles in 1octadecene (B).
3. Fabrication and characteristics of QDs-based optical fiber amplifiers Both OLAm- and OLA-capped QDs are hydrophobic. Thus they were first modified with a designer amphiphilic block copolymer, poly(polyethylene glycol methyl ether methacrylate)28-block-poly(dodecyl methacrylate)50, to disperse the QDs into silica sol, which was then coated on the tapered region of the optical fiber coupler [15, 17]. By using this twin fiber structure, a signal and a pump can be injected into the active region simultaneously. The pump excites the doped QDs through an evanescent wave and the signal interacts with excited QDs through an evanescent wave and then can be amplified. The test system is illustrated in Fig. 2. A 980 nm laser diode is used as the pump, a 1550 nm semiconductor light emitting diode is used as the signal source, and the amplified signal is analyzed on an optical spectrum analyzer.
Fig. 2. The test set-up for quantum dots fiber amplifiers.
Figure 3 shows the output spectra of the fiber amplifiers based on OLAm-QDs and OLAQDs when the input signal or the pump is used individually and the amplified spectrum when both the signal and pump are simultaneously applied. At ambient temperature, both fiber amplifiers show signal enhancement at 1550 nm with a 100 mW 980 nm pump - 4.8 dB for OLAm-QD and 6.47 dB for OLA-QD. The gain for both amplifiers increases with increasing pump power and reaches saturation values of 5.06 dB at 140 mW for OLAm-QD and 7.56 dB at 140 mW for OLA-QD. These results are consistent with the more stable nature and the higher solution PL intensity of OLA-QD in comparison with OLAm-QD.
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 522
Fig. 3. Output spectra with input signal only, pump only, and signal + pump for fiber amplifiers based on OLAm-QDs (A) and OLA-QDs (B). Dependence of gain at 1550 nm on pump power for fiber amplifiers based on OLAm-QDs and OLA-QDs (C).
4. Thermal stability of QDs-based optical fiber amplifiers Encouraged by the enhanced solution PL properties and higher amplification signals of OLAQDs in comparison with those of OLAm-QDs, we next compared the thermal stability of fiber amplifiers based on both types of QDs (Fig. 4). For the fiber amplifier based on OLAmQDs, the gain undergoes an obvious reduction from 4.8 dB to 2.96 dB upon heating from 25 °C to 50 °C, similar to the reduction in PL intensity for OLAm-QDs in solution. When cooled the gain does not recover, but continues to degrade to even lower values. For the fiber amplifier based on OLA-QDs, the gain experiences only a slight drop from 6.02 dB to 5.44 dB on heating from 25 °C to 50 °C, representing only 9.6% gain loss. More importantly, the gain recovers and essentially returns to its original value upon cooling, suggesting that heating the fiber amplifier does not lead to permanent damage to the OLA-QDs due to the superior thermal stability of OLA-QDs. In contrast to the thermal profile of the PL of OLAQDs in solution, which shows an antiquenching effect, the gain of the OLA-QD-doped fiber exhibits a slight decrease on heating. These results reflect the effects of the microenvironment in which the QDs reside on their surface and optical properties. It is likely that in solution the ligands are more flexible and are able to reorganize to adopt a more optimized passivation of the QD surface on heating, while in the solid state this type of ligand reorganization is to a large extent suppressed and the usual thermal quenching effect is exhibited.
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 523
8 OLAm-QD, heating OLAm-QD, cooling
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Fig. 4. Temperature-dependent gain of QDs-based optical fiber amplifiers at a pump power of 100 mW.
5. Conclusion The addition of capping ligands has profound, beneficial effects on the strength and thermal stability of luminescence for PbS QDs. For this work, we examined the performance of an optical amplifier with a gain region comprised of OLA-QDs (ligand-capped) coated on the tapered region of twin SMF couplers. The OLA-QDs-based optical amplifier exhibited gain that decreased only slightly at elevated temperatures and that returned to its original values when cooled. The results are consistent with use of the OLA-QDs-based optical amplifiers for applications such as broadband optical communications in environments for which the ambient temperature may vary over a wide range and for which thermal control may not be economical. Acknowledgments The work was funded by National Natural Science Foundation of China (61006083, 61377040, 61205172, 61275090, 60937003).
#199002 - $15.00 USD (C) 2014 OSA
Received 7 Oct 2013; revised 9 Dec 2013; accepted 10 Dec 2013; published 3 Jan 2014 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.000519 | OPTICS EXPRESS 524
