Open Access
Add to BibSonomyAbstract
A random fiber laser is achieved based on the plasmonic feedback mechanism, which is constructed by first siphoning the polymer solution doped with silver nanoparticles into a 300-μm capillary tube and then evaporating the solvent. Strong amplification of the radiation can be obtained by employing the variable gain region, the fiber waveguide scheme and three-dimensional plasmonic feedback provided by the silver nanoparticles. Low-threshold directional random lasing is observed in the polymer fiber. This simple and straightforward approach facilitates the investigation of plasmonic random fiber lasers.
© 2016 Optical Society of America
1. Introduction
Random lasers, of particular interest, based on the localized surface plasmon resonance (LSPR) of metallic nanoparticles, have attracted significant attentions for their unique properties, such as without external cavity, simplicity in structures and plasmonic feedback [1–6]. The plasmonic feedback of metal nanoparticles or nanostructures enhances the electric field in the vicinity of the surface through localized surface plasmon resonance, producing an effect on the random lasing characteristics [7–10]. The feedback scheme consists of the plasmonic scattering and waveguide confinement mechanisms, and provides effective gain channels for the random lasers [11]. In our previous work, the strong plasmonic scattering by the gold nano-island structures on the silica substrate and the strong confinement by the active waveguide enable low pump threshold and high conversion efficiency of the random laser [12]. Recently, a concept of random fiber laser was brought birth for its unique merits, including directional output, simplicity and high lasing efficiency [13–20]. The characters of random lasers are reported in dye-doped liquid crystal [21,22] and the Ag NPs improves the performance of the random fiber laser [23,24]. Random fiber lasers represent a class of very interesting physical features, such as wavelength tunability [25], multiwavelength [26,27], Rayleigh scattering and narrow band [28–31]. So the random fiber lasers may lead to promising opportunities in networking, optical sensing and power delivery [32–36].
In this paper, a preparation method was provided to achieve the random fiber laser offering a simple and straightforward way of fabrication. The silver nanoparticles were directly embedded in the polymer fiber, avoiding the processes of spincoating and annealing. Strong random lasing emission can be observed when the pump intensity exceeds the threshold due to the fiber waveguide and the plasmonic feedback. The emission propagates in the polymer fiber, but the pump beam cannot travel long due to the high absorption. The emission spectra intensity increases by shortening the length of the polymer fiber, implies that miniaturization of such laser devices may be achieved by slicing the polymer fiber.
2. Fabrication
The AgNO3 powder of 1.7 g (10 mmol) was added to a toluene solution (40 mL), and the mixture was magnetic stirred vigorously for 30 minutes, resulting in an [Ag+] concentration of 0.25 mol/L. Then, the decanoic acid of 2.9 g (16.6 mmol) was dropped in the solution above, and the mixture solution was magnetic stirred at the temperature of 80 degree for 20 minutes. Subsequently, the n-butylamine of 1.67 mL was blended in at a speed of 2 drops per second. After that, the solution became milk white. When a 25 mL aqueous hydrazine as reducing reagent was added on a titration rate of 2 drops per second with vigorous stirring, a volume of gas was produced in this process. After the addition of reductant, the solution was stirred for a further 4 hours. Then the reaction products were suspending in 200 mL methanol, washed and precipitated in 50 mL acetone. Then, following centrifuging, resuspending in methanol, precipitating and washing with acetone, the desired dark-brown silver nanoparticles (Ag NPs) were fabricated. The Ag NPs were suspended in the xylene, forming a solution with 4 mg/mL concentration.
A typical conjugated polymer, poly [(9, 9- dioctylfluorenyl-2, 7-diyl)-alt-co-(1, 4-benzo-{2, 1′, 3}-thiadiazole)] (F8BT) was dissolved in xylene at a concentration of 25 mg/mL. The two xylene solutions of Ag NPs and F8BT were mixed as a volume ratio of 1:10 in an ultrasonic oscillating tank for 10 minutes for homogeneity. Thus the ink of Ag NPs and F8BT was produced. An end of a capillary tube with 300 μm inner diameter was dipped into the ink and the capillary tube siphoned the xylene solution of F8BT and Ag NPs to a height. After the solvent of xylene in the capillary tube evaporating, the solid mixture of F8BT and Ag NPs formed a cylindrical fiber with a diameter of 300 μm, forming the plasmonic random laser of polymer F8BT doped with Ag NPs. Then the polymer fiber was pushed out of the capillary tube by an optical fiber.
3. Random lasers and spectra analysis
Figure 1 shows the schematic and close-up view of the plasmonic random laser based on a polymer fiber. Figure 1(a) illustrates that the light randomly scattered by the Ag nanoparticles travels along complicated paths in the polymer fiber. The inset in the top-right corner of Fig. 1(a) is the scanning electron microscopy (SEM) image of an Ag NP with hexagon shape. The average diameter of the Ag NP is about 500 nm. The right part of the fiber in blue implies the effective penetration depth (z) of the pump beam, which corresponds to the gain region. Thus, in our experiment, the gain region will increase with increasing the pump intensity. Figure 1(b) demonstrates the side view of polymer fiber, with a length of about 1.6 mm after pushing out from the capillary tube. Figure 1(c) presents the optical photographs of front view, with the capillary tube. The cross section of capillary tube is a ring, and pointed out by a red arrow. The plasmonic random lasing was indicated by green arrows, and referred as the polymer fiber laser. The length of polymer fiber inside the capillary tube can be controlled by adjusting the liquid amount when siphoning. In the experiment, the F8BT fibers doped with the Ag NPs were with lengths from about 0.5 mm to 3 mm, corresponding to the lengths of liquid columns from about 5 mm to 3 cm. The polymer fibers are long enough to obtain directional random lasing due to the waveguide effect. The effect of the polymer fiber length on the laser emission will be discussed later.
Fig. 1 (a) Schematic of the plasmonic random lasing in polymer fiber. z denotes the effective penetration depth of the pump beam. The inset in the top-right corner is the SEM image of an Ag NP, and the scale bar is 150 nm. (b) Optical micrograph of the side view of the polymer fiber. (c) Optical micrograph of the front view of the polymer fiber. The scale bars in (b) and (c) represent 200 μm.
Figure 2(a) presents the extinction of Ag nanoparticles (in green), the photoluminescence spectra of F8BT (in blue) and F8BT doped Ag nanoparticles (in red), respectively. The extinction and photoluminescence spectra were measured using a spectrometer (Maya 2000 Pro, Ocean Optics). The broad plasmonic resonance spectrum (in green) of the Ag NPs is attributed to a broad distribution of diameter and overlaps the photoluminescence spectrum of the polymer F8BT, guaranteeing the enhancement of the radiation of polymer by the plasmonic resonance of Ag NPs. Figure 2(b) shows the photoluminescence lifetime of F8BT (in blue) and F8BT doped with Ag NPs (in red). The plasmon resonance of Ag NPs reduces the photoluminescence of F8BT molecules lifetime from about 1.2 to 1.0 ns.
Fig. 2 (a) The extinction of Ag NPs (in green), the photoluminescence spectra of the F8BT (in blue), and F8BT doped with Ag NPs (in red), respectively. (b) The photoluminescence decays of F8BT (in blue) and F8BT doped with Ag NPs (in red), respectively.
The plasmonic random lasers were excited by a femtosecond pump beam, which is with a wavelength of 400 nm, a repetition frequency of 1 kHz and a pulse length of 200 fs. The random laser emission from the polymer fiber at pump power densities is shown in Fig. 3(a). The spectra are centered at 567 nm for the plasmonic random fiber lasers, with the full width at half maximum (FWHM) less than 10 nm. The inset in Fig. 3(a) is the schematic diagram of the experimental setup. The angle θ is about 40° and expresses the receiving direction of spectrum detector deviating from the axis of the polymer fiber. Figure 3(b) shows the intensity and the FWHM of the output lasers as a function of pump power density. The pump density threshold is about 293 μJ/cm2 for F8BT fiber doped with Ag NPs. The inset of Fig. 3(b) is an optical micrograph of a polymer fiber when the pump laser lighted on the right end of the polymer fiber, with a laser power density much lower than the pump threshold of 293 μJ/cm2. The bright yellow zone on the right end of polymer fiber is owing to the amplified spontaneous emission from polymer fiber. The wrinkles on the polymer fiber are made in the mechanical extruding process and have almost no influence on the laser performance. The radiation from the polymer molecules is randomly and strongly scattered by the distributed Ag NPs doping in F8BT fiber. Thus, the Ag NPs provides a three-dimensional plasmonic feedback inside the polymer fiber due to the localized surface plasmonic resonance, enhancing the photoluminescence of polymer molecules significantly.
Fig. 3 (a) The spectra of the random laser emission at different pump power densities. The inset is the schematic setup of measurement, and the angle θ is about 40°. (b) The intensity and FWHM of the output lasers as a function of the pump power density, indicating the pump thresholds of about 293 μJ/cm2. The inset is an optical micrograph of a polymer fiber with the amplified spontaneous emission on the right end and the scale bars denotes 300 μm.
The pump laser and the random laser emission propagate in the polymer fiber, and are inevitably absorbed by the polymer molecules. The pure F8BT film and the F8BT film doped with Ag NPs were prepared by drying drops of solutions on the silica substrates. The thickness of both films were about 200 nm. The transmittance of polymer films were recorded by a Spectrophotometer (U-4100, Hitachi) and plotted in Fig. 4(a) for the pure F8BT film (black solid sphere) and the F8BT film doped with Ag NPs (red solid sphere). The absorption coefficient α was derived by the equation of α = -lnT/d from the Reference [37]. And the curves are shown in Fig. 4(a) for the pure F8BT film (black hollow circle) and the F8BT film doped with Ag NPs (red hollow circle). In this equation, T is the transmittance, and d = 200 nm denoting the thickness of polymer film. The difference between the spectra of two films is attributed to the localized surface plasmonic resonance of Ag NPs, as indicated by the red arrows in Fig. 4(a). In Fig. 3(a), the femtosecond pump beam of 400 nm was employed and the plasmonic random fiber laser centers at 567 nm. In Fig. 4(a), a blue line and a green line mark the locations of 400 nm and 567 nm, respectively. It can be seen that the pump beam suffers much stronger absorption than the laser emission beam. At pump beam wavelength of 400 nm, the absorption coefficient of polymer F8BT film doped with Ag NPs is 0.50 × 105/cm, which is similar to that of polymer F8BT film 0.75 × 105/cm, reported in the Reference [37].
Fig. 4 (a) The transmittance (in red) and the absorption coefficient α (in black) of polymer F8BT film doped with Ag NPs. The two lines at 400 nm (in blue) and 567 nm (in green) represent the wavelengths of pump and emission, respectively. (b) The pump intensity as a function of the penetration depth increment (Δz).
From absorption coefficient curve, the intensity of pump beam with 400 nm would reduce quickly in the polymer F8BT fiber doped with Ag NPs due to the higher absorption coefficient. Whereas, the excited random laser would travel much longer in the polymer fiber for the lower absorption coefficient at 567 nm. The more details would be discussed in the followings.
In order to investigate the effective penetration depth of pump beam in the polymer F8BT fiber doped with Ag NPs, the absorption formula, shown as I = I0 e-αL, was employed, where I, I0, α and L are transmitted light intensity, incident light intensity, absorption coefficient and penetration depth. Note that the penetration depth (L = 1/α) is longer than the effective penetration depth (z) in Fig. 1(a). The effective penetration depth equals zero when the pump intensity is below the threshold of the laser device. On the threshold of 293 μJ/cm2, the pump laser excites laser emission of 567 nm with a certain effective penetration depth in the polymer F8BT fiber doped with Ag NPs. With the increment of intensity, the pump beam will increase the effective penetration depth in polymer fiber. We assume the transmitted light intensity I, absorption coefficient α are set as 293 μJ/cm2, 0.5 × 105/cm, respectively. The penetration depth increment (Δz) is described as I0 = 293 × eαΔz, and is plotted in Fig. 4(b) under pump intensities. The penetration depth increment exceeds 150 nm when the pump intensity is up to 640 μJ/cm2, indicating that the gain region increases with the pump intensity rising. In other words, the gain region is related to the effective penetration depth, which can be adjusted by the pump intensity. It means that the performance of this laser device will be improved when the pump intensity increases.
In order to figure out the emission pattern of polymer fibers, the intensity of the output lasers as a function of the angle was plotted in Fig. 5. Figure 5(a) shows intensity measurement schematic diagram, and the pump beam illuminates the polymer fiber from the right end. The angle θ between the intensity detector and the axis of the polymer fiber has the vertex on the center of rotating platform. The detector probes the random laser intensities, rotating around the polymer fiber along the rotating track with a distance of about 20 mm. In our experiments, three fibers with lengths about of 0.5, 0.8 and 1.1 mm were chosen, and the pump power density is about 490 μJ/cm2. The pump power densities are higher than the pump threshold of 293 μJ/cm2 mentioned in Fig. 3, so the random lasers from polymer fibers strongly emit and are recorded by the spectrometer. The polymer F8BT fiber doped with Ag NPs has a low absorption coefficient at 567 nm for output of random laser. So the excited random laser can pass throughout of the polymer fiber. The intensities of output lasers at angles for the polymer fibers are shown in Fig. 5(b). The output of random laser with 567 nm passes through the polymer fiber and the intensity of output laser is partly directional along the axis of polymer fiber, implying that the fiber is a waveguide for the emission with 567 nm. The ratios of the energy between forward and backward emissions are 1.6, 1.8 and 1.4 for the polymer fibers with lengths of 0.5 mm, 0.8 mm and 1.1 mm, respectively. So the forward emission energy is greater than that of backward emission, meaning a directional emission.
Fig. 5 (a) Schematic diagram of the intensity measurement at different angles. The angle between the receiving direction of intensity detector and the axis of the polymer fiber is defined as θ, with the vertex on the center of the center of rotating platform. (b) The intensities of the output lasers as a function of the angle for three polymer fibers with lengths of 0.5 mm (in blue), 0.8 mm (in red) and 1.1 mm (in green), respectively. The left and right half planes are marked as Forward and Backward, respectively. The pump power density is about 490 μJ/cm2.
Generally, the pump beams penetrate the samples in the optical pump laser experiments, wasting a lot of pump intensity. However, the pump energy is almost completely utilized in our experiment, due to that the effective penetration depth of the polymer F8BT fiber doped with Ag NPs increases with the pump intensity rising. The pump beam with 400 nm cannot propagate long in the polymer F8BT fiber doped with Ag NPs due to the high absorption. However, the polymer F8BT fiber doped with Ag NPs waveguides the emission with 567 nm along the axis direction, guaranteeing the directional output of the random laser.
The siphonage method provides a fabrication of the long-polymer-fiber random laser. In the future, the long polymer fiber may be cut into short ones with different lengths, working as patch-typed random lasers for applications.
4. Conclusion
The plasmonic random laser in a polymer fiber based on plasmonic feedback was fabricated by a siphoning method. The fabrication is a simple and straightforward way to produce a directional random lasing in the polymer fiber. The performance of the random laser is significantly improved by the plasmon resonance of doped Ag NPs. The laser output is determined by the pump and fiber length. The effective penetration depth increases with the pump intensity rising. The emission propagates in the polymer fiber, but the pump beam cannot propagates long in the polymer fiber due to the high absorption. These results provide a utilized way for application of plasmonic random fiber lasers.
Acknowledgments
This work has been supported by the National Natural Science Foundation of China (NSFC) (11474014, and 11274031), and Fundamental Research Funds for the Central Universities (2014MS162).
References and links
1. D. Wiersma, “The physics and applications of random lasers,” Nat. Phys. 4(5), 359–367 (2008). [CrossRef]
2. C. Vanneste, P. Sebbah, and H. Cao, “Lasing with resonant feedback in weakly scattering random systems,” Phys. Rev. Lett. 98(14), 143902 (2007). [CrossRef] [PubMed]
3. T. Zhai, Y. Zhou, S. Chen, Z. Wang, J. Shi, D. Liu, and X. Zhang, “Pulse-duration-dependent and temperature-tunable random lasing in a weakly scattering structure formed by speckles,” Phys. Rev. A 82(2), 023824 (2010). [CrossRef]
4. X. Shi, Y. Wang, Z. Wang, S. Wei, Y. Sun, D. Liu, J. Zhou, Y. Zhang, and J. Shi, “Random lasing with a high quality factor over the whole visible range based on cascade energy transfer,” Adv. Opt. Mater. 2(1), 88–93 (2014). [CrossRef]
5. H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seeling, Q. H. Wang, and R. P. H. Chang, “Random laser action in semiconductor powder,” Phys. Rev. Lett. 82(11), 2278–2281 (1999). [CrossRef]
6. T. Zhai, Z. Xu, X. Wu, Y. Wang, F. Liu, and X. Zhang, “Ultra-thin plasmonic random lasers,” Opt. Express 24(1), 437–442 (2016). [CrossRef] [PubMed]
7. S. V. Frolov, W. Gellerman, M. Ozaki, K. Yoshino, and Z. V. Vardeny, “cooperative emission in pi-conjugated polymer thin films,” Phys. Rev. Lett. 78(4), 729–732 (1997). [CrossRef]
8. X. Meng, K. Fujita, Y. Zong, S. Murai, and K. Tanaka, “Random lasers with coherent feedback from highly transparent polymer films embedded with silver nanoparticles,” Appl. Phys. Lett. 92(20), 201112 (2008). [CrossRef]
9. T. Zhai, J. Chen, L. Chen, J. Wang, L. Wang, D. Liu, S. Li, H. Liu, and X. Zhang, “A plasmonic random laser tunable through stretching silver nanowires embedded in a flexible substrate,” Nanoscale 7(6), 2235–2240 (2015). [CrossRef] [PubMed]
10. X. Shi, Y. Wang, Z. Wang, Y. Sun, D. Liu, Y. Zhang, Q. Li, and J. Shi, “High performance plasmonic random laser based on nanogaps in bimetallic porous nanowires,” Appl. Phys. Lett. 103(2), 023504 (2013). [CrossRef]
11. Y. Wang, X. Shi, Y. Sun, R. Zheng, S. Wei, J. Shi, Z. Wang, and D. Liu, “Cascade-pumped random lasers with coherent emission formed by Ag-Au porous nanowires,” Opt. Lett. 39(1), 5–8 (2014). [CrossRef] [PubMed]
12. T. Zhai, X. Zhang, Z. Pang, X. Su, H. Liu, S. Feng, and L. Wang, “Random laser based on waveguided plasmonic gain channels,” Nano Lett. 11(10), 4295–4298 (2011). [CrossRef] [PubMed]
13. Z. N. Wang, Y. J. Rao, H. Wu, P. Y. Li, Y. Jiang, X. H. Jia, and W. L. Zhang, “Long-distance fiber-optic point-sensing systems based on random fiber lasers,” Opt. Express 20(16), 17695–17700 (2012). [CrossRef] [PubMed]
14. Z. Wang, H. Wu, M. Fan, Y. Rao, I. Vatnik, E. Podivilov, S. Babin, D. Churkin, H. Zhang, P. Zhou, H. Xiao, and X. Wang, “Random fiber laser: simpler and brighter,” Opt. Photonics News 25, 30 (2014).
15. S. Turitsyn, S. Babin, A. El-Taher, P. Harper, D. Churkin, S. Kablukov, J. Ania-Castanon, V. Karalekas, and E. Podivilov, “Random distributed feedback fibre laser,” Nat. Photonics 4(4), 231–235 (2010). [CrossRef]
16. R. Zhang, S. Knitter, S. Liew, F. Omenetto, B. Reinhard, H. Cao, and L. Dal Negro, “Plasmon-enhanced random lasing in bio-compatible networks of cellulose nanofibers,” Appl. Phys. Lett. 108(1), 011103 (2016). [CrossRef]
17. S. Babin, A. El-Taher, P. Harper, E. Podivilov, and S. Turitsyn, “Tunable random fiber laser,” Phys. Rev. A 84(2), 021805 (2011). [CrossRef]
18. M. Gagné and R. Kashyap, “Demonstration of a 3 mW threshold Er-doped random fiber laser based on a unique fiber Bragg grating,” Opt. Express 17(21), 19067–19074 (2009). [CrossRef] [PubMed]
19. W. L. Zhang, Y. J. Rao, J. M. Zhu, Z. X. Yang, Z. N. Wang, and X. H. Jia, “Low threshold 2nd-order random lasing of a fiber laser with a half-opened cavity,” Opt. Express 20(13), 14400–14405 (2012). [CrossRef] [PubMed]
20. I. D. Vatnik, D. V. Churkin, S. A. Babin, and S. K. Turitsyn, “Cascaded random distributed feedback Raman fiber laser operating at 1.2 μm,” Opt. Express 19(19), 18486–18494 (2011). [CrossRef] [PubMed]
21. L. Ye, J. Kang, H. Yang, B. Liu, Z. Hu, Y. Zu, Y. Liu, and Y. Cui, “Enhancement of random lasing assisted by Ag nanoparticle doped dye medium in solidified fiber,” Laser Phys. 26(4), 045001 (2016). [CrossRef]
22. S. Sebastian, C. Linslal, C. Vallabhan, V. Nampoori, P. Radhakrishnan, and M. Kailasnath, “Random lasing with enhanced photostability of silver nanoparticle doped polymer optical fiber laser,” Laser Phys. Lett. 11(5), 055108 (2014). [CrossRef]
23. S. Ferjani, V. Barna, A. De Luca, C. Versace, N. Scaramuzza, R. Bartolino, and G. Strangi, “Thermal behavior of random lasing in dye doped nematic liquid crystals,” Appl. Phys. Lett. 89(12), 121109 (2006). [CrossRef]
24. J. Lin and Y. Hsiao, “Manipulation of the resonance characteristics of random lasers from dye-doped polymer dispersed liquid crystals in capillary tubes,” Opt. Mater. Express 4(8), 1555–1563 (2014). [CrossRef]
25. A. Camposeo, F. Di Benedetto, R. Stabile, A. A. Neves, R. Cingolani, and D. Pisignano, “Laser emission from electrospun polymer nanofibers,” Small 5(5), 562–566 (2009). [CrossRef] [PubMed]
26. A. Pinto, O. Frazão, J. Santos, and M. Lopez-Amo, “Multiwavelength fiber laser based on a photonic crystal fiber loop mirror with cooperative Rayleigh scattering,” Appl. Phys. B 99(3), 391–395 (2010). [CrossRef]
27. Z. Hu, Y. Liang, K. Xie, P. Gao, D. Zhang, H. Jiang, F. Shi, L. Yin, J. Gao, H. Ming, and Q. Zhang, “Gold nanoparticle-based plasmonic random fiber laser,” J. Opt. 17(3), 035001 (2015). [CrossRef]
28. T. Zhu, X. Bao, and L. Chen, “A self-gain random distributed feedback fiber laser based on stimulated Rayleigh scattering,” Opt. Commun. 285(6), 1371–1374 (2012). [CrossRef]
29. S. Sugavanam, N. Tarasov, X. Shu, and D. V. Churkin, “Narrow-band generation in random distributed feedback fiber laser,” Opt. Express 21(14), 16466–16472 (2013). [CrossRef] [PubMed]
30. S. A. Babin, E. I. Dontsova, and S. I. Kablukov, “Random fiber laser directly pumped by a high-power laser diode,” Opt. Lett. 38(17), 3301–3303 (2013). [CrossRef] [PubMed]
31. H. Zhang, P. Zhou, H. Xiao, and X. Xu, “Efficient Raman fiber laser based on random Rayleigh distributed feedback with record high power,” Laser Phys. Lett. 11(7), 075104 (2014). [CrossRef]
32. Z. Hu, Q. Zhang, B. Miao, Q. Fu, G. Zou, Y. Chen, Y. Luo, D. Zhang, P. Wang, H. Ming, and Q. Zhang, “Coherent random fiber laser based on nanoparticles scattering in the extremely weakly scattering regime,” Phys. Rev. Lett. 109(25), 253901 (2012). [CrossRef] [PubMed]
33. C. J. de Matos, L. de S Menezes, A. M. Brito-Silva, M. A. Martinez Gámez, A. S. Gomes, and C. B. de Araújo, “Random fiber laser,” Phys. Rev. Lett. 99(15), 153903 (2007). [CrossRef] [PubMed]
34. P. Wang, Y. Wang, and L. Tong, “Functionalized polymer nanofibers: a versatile platformfor manipulating light at the nanoscale,” Light Sci. Appl. 2(10), e102 (2013). [CrossRef]
35. S. Turitsyn, S. Babin, D. Churkin, I. Vatnik, M. Nikulin, and E. Podivilov, “Random distributed feedback fibre lasers,” Phys. Rep. 542(2), 133–193 (2014). [CrossRef]
36. L. Cui, J. Shi, Y. Wang, R. Zheng, X. Chen, W. Gong, and D. Liu, “Retrieval of contaminated information using random lasers,” Appl. Phys. Lett. 106(20), 201101 (2015). [CrossRef]
37. M. Campoy-Quiles, G. Heliotis, R. Xia, M. Ariu, M. Pintani, P. Etchegoin, and D. Bradley, “Ellipsometric characterization of the optical constants of polyfluorene gain media,” Adv. Funct. Mater. 15(6), 925–933 (2005). [CrossRef]
