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One-dimensional optical materials of microfibers by electrospinning
Materials Letters 66 (2012) 292–295
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
One-dimensional optical materials of microfibers by electrospinning Le Li a, Xinghua Yang a, b,⁎, Libo Yuan a a b
College of Science, Harbin Engineering University, Harbin 150001, China State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Information Road 17, Xi'an, Shannxi 710119, China
a r t i c l e
i n f o
Article history: Received 30 May 2011 Accepted 19 August 2011 Available online 25 August 2011 Keywords: Electrospinning Optical materials and properties Fiber technology Micro or nanofiber
a b s t r a c t Optical microfibers of PMMA were fabricated by electrospinning. The fibers with the diameter ranging from 300 nm to 1000 nm were obtained by electrospinning the solutions such as PMMA/DMF, PMMA/DMF/formic acid and PMMA/formic acid. The morphology and the diameter of the fibers were analyzed by scanning electron microscopy (SEM). Results showed that the sidewalls of the fibers were smooth and the diameters were uniform. The light with the wavelength of 488 nm, 532 nm and 650 nm could be launched into the fibers and guide along them. The simulation and experimental results showed that the fibers exhibited excellent optical properties. This method provided an effective and convenient way to fabricate highly uniform micro/nano scale optical waveguide neither using expensive equipments nor involving complex procedures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional micro/nano structures, such as nanowires, nanotubes and nanofibers, are playing important roles in physics, chemistry, environment and biology [1–4]. Among them, optical micro/nano fibers (MNFs) show great importance in a variety of optical applications due to their high index contrast and subwavelength diameter. They can guide light with a number of interesting optical characteristics such as high fraction of evanescent fields, tight optical confinement, manageable large waveguide dispersion, field enhancement and low optical loss through sharp bends [5,6]. Then, many efforts such as taper drawing technique [7] and bulk materials drawing method [8] have been employed to fabricate one-dimensional MNFs. On the other hand, electrospinning is one of the simple, versatile and cost-effective approaches for fabricating one-dimensional materials. It is a process that uses an electric field to control the formation and deposition of micro/nanofibers [9–12]. In this paper, we provide a novel method of electrospinning to fabricate microfibers which were used as waveguides. The results suggest that high quality poly(methyl methacrylate) (PMMA) fibers can be fabricated by electrospinning and used as waveguides with subwavelength diameter. These fibers have the potentials in various micro/nano-scale photonic devices.
2. Experimental PMMA is widely applied in optical devices because of its good optical properties such as high transparency (92% for visible light), relatively high refractive index (n= 1.49) and favorable mechanical properties. ⁎ Corresponding author. Tel./fax: + 86 451 82519850. E-mail address: [email protected] (X. Yang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.08.068
In this work, PMMA (MW = 150,000) were chosen as the optical materials for electrospinning. PMMA solutions were prepared by vigorous stirring in a water bath at 50 °C for about 120 min. In the process of electrospinning, the solutions were contained in a syringe with an 18-gage blunt-end needle (0.35 mm inner diameter) that was mounted in a syringe pump. The needle was connected with anode. The cathode was connected with an aluminum foil to collect the samples for morphologic characterization [Fig. 1(a)]. For the optical characterization, the aluminum foil was replaced by a parallel electrode to collect the samples. Then, single fiber was obtained by transferring the fiber from the parallel electrode to a piece of MgF2 [Fig. 1(b)]. A voltage of 10 kV was applied between the two electrodes. The distance between ground plate and the needle tip was 15 cm. The syringe pump delivered the polymer solutions at a controlled flow rate of 0.05 mL/min. All experiments were carried out at room temperature with relative humidity of about 40%. 3. Results and discussion In previous works, uniform PMMA fibers were fabricated by electrospinning the mixed solution of PMMA/Menthol/DMF/Cyclodextrin [13]. For optical waveguide, the addition of menthol and cyclodextrin would reduce the transparency of PMMA. Then, we realized the electrospinning of PMMA using dimethylformamide (DMF) or DMF/formic acid as the solvents. We found that the transparency of the fiber was improved. Table 1 exhibits the ideal compositions of the polymer solutions. The average diameters and the morphologies of the fibers are also given in this table. Fig. 1(c)–(f) shows the SEM images of the fibers with uniform morphologies. A series of PMMA solutions using DMF as solvent were electrospun. We found the concentration of the polymer solution should exceed a critical value in electrospinning because extensive chain entanglements were necessary to produce fibers [14]. When the concentration was below 10%,
L. Li et al. / Materials Letters 66 (2012) 292–295
293
Fig. 1. Schematic diagram of the setup for electrospinning and the SEM images of the PMMA fibers. (a) The setup for collecting the samples for SEM. (b) The setup for obtaining single PMMA fiber as one-dimensional waveguide. (c) 30 wt.% in DMF, (d) 25 wt.% in DMF/formic acid mixtures with solvent ratio of 1:1, (e) 25 wt.% in DMF/formic acid mixtures with solvent ratio of 3:1, (f) 25 wt.% in formic acid. The insets show higher magnification SEM images.
continuous fibers could not be fabricated due to the low viscosity of the solution. At the concentration of 25%, spindle-like beads were observed in the fiber. When the concentration increased to 30%, the bead disappeared and uniform fibers were fabricated with the average diameter of 915 nm [Table 1, Fig. 1(c)]. It could be seen that high concentrations reduced the formation of broken fibers and increased the diameter of the fibers [15]. However, when the
Table 1 Composition of the solutions and the diameter/morphology of the resulting electrospun fibers. Solutions (v/v)
PMMA% (w/w)
Average fiber diameter (nm)
Fiber morphology
DMF DMF DMF/Formic acid(3:1) DMF/Formic acid(1:1) DMF/Formic acid(1:3) Formic acid
25 30 25 25 25 25
– 915 – 625 410 297
Beads Bead-free Few beads Bead-free Bead-free Bead-free
concentration was higher than 33%, the viscosity of the polymer solution was high and we could not obtain uniform PMMA fibers. As is well known, electrospinning involves stretching of the solution in the fluid drop that is caused by repulsion of the charges at its surface. If the conductivity of the solution is increased, more charges can be carried by the jet and, in general, there are more chances to get fibers without beads [16]. Considering formic acid has high conductivity (6.4 × 10 − 5 S/cm, 25 °C), we investigated DMF/formic acid mixtures as the solvent to fabricate bead-free fibers with smaller diameter. Fig. 1 (d), (e) and (f) shows the morphologies of the fibers fabricated from the solutions with mixtures solvent (25% PMMA). When the ratio of formic acid to DMF was 0.3, the number of the beads greatly decreased comparing with the condition of pure DMF as solvent. When the ratio was 1, the average diameter of the fibers was 625 nm [Fig. 1(d)]. As that ratio increased to 3, the corresponding diameter decreased to 410 nm [Fig. 1(e)]. If the solvent was pure formic acid, the diameter of fibers decreased to 297 nm [Fig. 1(f)]. As a result, the addition of formic acid gradually decreased the diameters of the fibers. This was because the increase of the conductivity of the solution enhanced the split of the taylor cone in electric field. From the SEM images, we could observe
294
L. Li et al. / Materials Letters 66 (2012) 292–295
that the as-fabricated fibers have uniform diameters and very smooth surfaces. This morphology would minimize the light-scattering at the surfaces of the fibers when they are used as optical waveguide and sensing components. To investigate the optical characteristics of the fibers, we launched light into them by evanescent-field coupling method. Fig. 2(a) shows the schematic of this process. The coupling model between a tapered optical fiber (the diameter was 670 nm) and a PMMA fiber (the diameter was 600 nm) on the substrate of MgF2 was designed. Corresponding theoretical modeling was simulated by the beam propagation method (BPM). In Fig. 2(b), simulated results revealed that the microfiber with high fractional evanescent fields was very likely to suffer from the mode transition that leads to power leakage of the guided light on the substrate [17]. When the fiber was supported by SiO2 (n = 1.45), a serious power leakage occurred on the interface. However, when the substrate was replaced by MgF2 (n= 1.38), only a fraction of energy leaked into the substrate. If the tapered fiber and the PMMA fiber are parallelly placed together on this substrate, light can be efficiently launched into the fiber within a few micrometers' overlap. As shown in Fig. 2(c), power maps of the coupling region reveal that the light
from the taper (refraction index is 1.45) can be efficiently coupled into a PMMA fiber (refraction index is 1.49) through evanescent when they meet the condition of phase matching. Here, the wavelength of the light was 980 nm. The length of overlapping was 28.8 μm. The distance of lateral separation was 110 nm. The details of evanescent coupling under a microscope were shown in Fig. 3. We collected a PMMA fiber with the diameter of 600 nm on MgF2 substrate. A single-mode fiber was tapered down to a fiber with the diameter of 800 nm [18] and placed together with the PMMA fiber using three-dimensional micromanipulation platform under a microscope to form a coupler structure. The tapered fiber was used to evanescently couple the light into PMMA fiber by overlapping them in parallel on the MgF2 surface. Both the PMMA fiber and the tapered fiber were attracted on the MgF2 substrate by van der Waals force and electrostatic force. Then, we coupled light with different wavelengths of 488 nm, 532 nm and 650 nm into the core of the tapered fiber. As a result, we successfully observed that the light from the taper was efficiently coupled into the fiber in the part of overlapping (contacting length is 40 μm). The light in the PMMA fiber could be guided for at least 100 μm. The uniform and virtually unattenuated scattering along the fiber showed that, relative to the guided intensity, the scattering was small.
4. Conclusion In summary, optical PMMA microfiber was successfully fabricated by electrospinning technique. The microfibers have uniform diameters and very smooth surfaces. Simultaneously, the fibers performed excellent optical properties due to diameter uniformity and sidewall
Fig. 2. (a) Schematic diagram of evanescent launching between a PMMA fiber and a tapered fiber. (b) Power distribution simulated at the transverse cross plane of MgF2supported (left) and SiO2-supported (right) PMMA fiber. The wavelength of the light is 980 nm. (c) Power maps of evanescent coupling region.
Fig. 3. Microscope images of optical coupling with different wavelengths between the PMMA fiber and the tapered fiber. (a) 488 nm, (b) 532 nm, (c) 650 nm. The arrows show the direction of light propagation.
L. Li et al. / Materials Letters 66 (2012) 292–295
smoothness. The light with the wavelength of 488 nm, 532 nm and 650 nm could guide along the fiber. Moreover, this method provided an effective and convenient way to fabricate highly uniform nano or micro scale optical waveguide without expensive equipment and complex procedure. The fibers from electrospinning technique could find wide applications in various micro/nano-scale photonic and sensitive devices. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC) (project no. 61007053), Ph.D. Programs Foundation of Ministry of Education of China (20092304120022) and the Fundamental Research Funds for the Central Universities (HEUCF20111114). References [1] Osberg KD, Schmucker AL, Senesi AJ, Mirkin CA. Nano Lett 2011;11:820–4. [2] Dalvand P, Mohammadi MR, Fray DJ. Mater Lett 2011;65:1291–4. [3] Dong GP, Chi YZ, Xiao XD, Liu XF, Qian B, Ma ZJ, et al. Opt Express 2009;17: 22514–9.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
295
Yang XH, Wang LL, Yang S. Mater Lett 2007;61:2904–7. Tong LM, Gattass RR, Ashcom JB, He SL, Lou JY, Shen MY. Nature 2003;426:816–9. Bures J, Ghosh R. J Opt Soc Am A 1999;16:1992–6. Bayle F, Meunier JP. Appl Opt 2005;44:6402–11. Tong LM, Lou JY, Gattass RR, He SL, Chen XF, Liu L, et al. Nano Lett 2005;5:259–62. Yang PP, Zhan SM, Huang ZH, Zhai JL, Wang DJ, Xin Y, et al. Mater Lett 2011;65: 537–9. Bisht GS, Canton G, Mirsepassi A, Kulinsky L, Oh S, Dunn-Rankin D, et al. Nano Lett 2011;11:1831–7. Yang XH, Shao CL, Liu YC. J Mater Sci 2007;42:8470–2. Yang XH, Shao CL, Liu YC, Mu RX, Guan HY. Thin Solid Films 2005;478:228–31. Uyar T, Nur Y, Hacaloglu J, Besenbacher F. Nanotechnology 2009;20:125703. Shenoy SL, Bates WD, Frisch HL, Wnek GE. Polymer 2005;46:3372. Demir MM, Yilgor I, Yilgor E, Erman B. Polymer 2002;43:3303–9. Zheng J, He A, Li J, Xu J, Han CC. Polymer 2006;47:7095–102. Law M, Sirbuly DJ, Johnson JC, Goldberger J, Saykally RJ, Yang PD. Science 2004;305:1269–73. Zhu XL, Yuan LB, Yang J, Cao SX. Appl Opt 2009;48:5624–8.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
One-dimensional optical materials of microfibers by electrospinning Le Li a, Xinghua Yang a, b,⁎, Libo Yuan a a b
College of Science, Harbin Engineering University, Harbin 150001, China State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Information Road 17, Xi'an, Shannxi 710119, China
a r t i c l e
i n f o
Article history: Received 30 May 2011 Accepted 19 August 2011 Available online 25 August 2011 Keywords: Electrospinning Optical materials and properties Fiber technology Micro or nanofiber
a b s t r a c t Optical microfibers of PMMA were fabricated by electrospinning. The fibers with the diameter ranging from 300 nm to 1000 nm were obtained by electrospinning the solutions such as PMMA/DMF, PMMA/DMF/formic acid and PMMA/formic acid. The morphology and the diameter of the fibers were analyzed by scanning electron microscopy (SEM). Results showed that the sidewalls of the fibers were smooth and the diameters were uniform. The light with the wavelength of 488 nm, 532 nm and 650 nm could be launched into the fibers and guide along them. The simulation and experimental results showed that the fibers exhibited excellent optical properties. This method provided an effective and convenient way to fabricate highly uniform micro/nano scale optical waveguide neither using expensive equipments nor involving complex procedures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional micro/nano structures, such as nanowires, nanotubes and nanofibers, are playing important roles in physics, chemistry, environment and biology [1–4]. Among them, optical micro/nano fibers (MNFs) show great importance in a variety of optical applications due to their high index contrast and subwavelength diameter. They can guide light with a number of interesting optical characteristics such as high fraction of evanescent fields, tight optical confinement, manageable large waveguide dispersion, field enhancement and low optical loss through sharp bends [5,6]. Then, many efforts such as taper drawing technique [7] and bulk materials drawing method [8] have been employed to fabricate one-dimensional MNFs. On the other hand, electrospinning is one of the simple, versatile and cost-effective approaches for fabricating one-dimensional materials. It is a process that uses an electric field to control the formation and deposition of micro/nanofibers [9–12]. In this paper, we provide a novel method of electrospinning to fabricate microfibers which were used as waveguides. The results suggest that high quality poly(methyl methacrylate) (PMMA) fibers can be fabricated by electrospinning and used as waveguides with subwavelength diameter. These fibers have the potentials in various micro/nano-scale photonic devices.
2. Experimental PMMA is widely applied in optical devices because of its good optical properties such as high transparency (92% for visible light), relatively high refractive index (n= 1.49) and favorable mechanical properties. ⁎ Corresponding author. Tel./fax: + 86 451 82519850. E-mail address: [email protected] (X. Yang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.08.068
In this work, PMMA (MW = 150,000) were chosen as the optical materials for electrospinning. PMMA solutions were prepared by vigorous stirring in a water bath at 50 °C for about 120 min. In the process of electrospinning, the solutions were contained in a syringe with an 18-gage blunt-end needle (0.35 mm inner diameter) that was mounted in a syringe pump. The needle was connected with anode. The cathode was connected with an aluminum foil to collect the samples for morphologic characterization [Fig. 1(a)]. For the optical characterization, the aluminum foil was replaced by a parallel electrode to collect the samples. Then, single fiber was obtained by transferring the fiber from the parallel electrode to a piece of MgF2 [Fig. 1(b)]. A voltage of 10 kV was applied between the two electrodes. The distance between ground plate and the needle tip was 15 cm. The syringe pump delivered the polymer solutions at a controlled flow rate of 0.05 mL/min. All experiments were carried out at room temperature with relative humidity of about 40%. 3. Results and discussion In previous works, uniform PMMA fibers were fabricated by electrospinning the mixed solution of PMMA/Menthol/DMF/Cyclodextrin [13]. For optical waveguide, the addition of menthol and cyclodextrin would reduce the transparency of PMMA. Then, we realized the electrospinning of PMMA using dimethylformamide (DMF) or DMF/formic acid as the solvents. We found that the transparency of the fiber was improved. Table 1 exhibits the ideal compositions of the polymer solutions. The average diameters and the morphologies of the fibers are also given in this table. Fig. 1(c)–(f) shows the SEM images of the fibers with uniform morphologies. A series of PMMA solutions using DMF as solvent were electrospun. We found the concentration of the polymer solution should exceed a critical value in electrospinning because extensive chain entanglements were necessary to produce fibers [14]. When the concentration was below 10%,
L. Li et al. / Materials Letters 66 (2012) 292–295
293
Fig. 1. Schematic diagram of the setup for electrospinning and the SEM images of the PMMA fibers. (a) The setup for collecting the samples for SEM. (b) The setup for obtaining single PMMA fiber as one-dimensional waveguide. (c) 30 wt.% in DMF, (d) 25 wt.% in DMF/formic acid mixtures with solvent ratio of 1:1, (e) 25 wt.% in DMF/formic acid mixtures with solvent ratio of 3:1, (f) 25 wt.% in formic acid. The insets show higher magnification SEM images.
continuous fibers could not be fabricated due to the low viscosity of the solution. At the concentration of 25%, spindle-like beads were observed in the fiber. When the concentration increased to 30%, the bead disappeared and uniform fibers were fabricated with the average diameter of 915 nm [Table 1, Fig. 1(c)]. It could be seen that high concentrations reduced the formation of broken fibers and increased the diameter of the fibers [15]. However, when the
Table 1 Composition of the solutions and the diameter/morphology of the resulting electrospun fibers. Solutions (v/v)
PMMA% (w/w)
Average fiber diameter (nm)
Fiber morphology
DMF DMF DMF/Formic acid(3:1) DMF/Formic acid(1:1) DMF/Formic acid(1:3) Formic acid
25 30 25 25 25 25
– 915 – 625 410 297
Beads Bead-free Few beads Bead-free Bead-free Bead-free
concentration was higher than 33%, the viscosity of the polymer solution was high and we could not obtain uniform PMMA fibers. As is well known, electrospinning involves stretching of the solution in the fluid drop that is caused by repulsion of the charges at its surface. If the conductivity of the solution is increased, more charges can be carried by the jet and, in general, there are more chances to get fibers without beads [16]. Considering formic acid has high conductivity (6.4 × 10 − 5 S/cm, 25 °C), we investigated DMF/formic acid mixtures as the solvent to fabricate bead-free fibers with smaller diameter. Fig. 1 (d), (e) and (f) shows the morphologies of the fibers fabricated from the solutions with mixtures solvent (25% PMMA). When the ratio of formic acid to DMF was 0.3, the number of the beads greatly decreased comparing with the condition of pure DMF as solvent. When the ratio was 1, the average diameter of the fibers was 625 nm [Fig. 1(d)]. As that ratio increased to 3, the corresponding diameter decreased to 410 nm [Fig. 1(e)]. If the solvent was pure formic acid, the diameter of fibers decreased to 297 nm [Fig. 1(f)]. As a result, the addition of formic acid gradually decreased the diameters of the fibers. This was because the increase of the conductivity of the solution enhanced the split of the taylor cone in electric field. From the SEM images, we could observe
294
L. Li et al. / Materials Letters 66 (2012) 292–295
that the as-fabricated fibers have uniform diameters and very smooth surfaces. This morphology would minimize the light-scattering at the surfaces of the fibers when they are used as optical waveguide and sensing components. To investigate the optical characteristics of the fibers, we launched light into them by evanescent-field coupling method. Fig. 2(a) shows the schematic of this process. The coupling model between a tapered optical fiber (the diameter was 670 nm) and a PMMA fiber (the diameter was 600 nm) on the substrate of MgF2 was designed. Corresponding theoretical modeling was simulated by the beam propagation method (BPM). In Fig. 2(b), simulated results revealed that the microfiber with high fractional evanescent fields was very likely to suffer from the mode transition that leads to power leakage of the guided light on the substrate [17]. When the fiber was supported by SiO2 (n = 1.45), a serious power leakage occurred on the interface. However, when the substrate was replaced by MgF2 (n= 1.38), only a fraction of energy leaked into the substrate. If the tapered fiber and the PMMA fiber are parallelly placed together on this substrate, light can be efficiently launched into the fiber within a few micrometers' overlap. As shown in Fig. 2(c), power maps of the coupling region reveal that the light
from the taper (refraction index is 1.45) can be efficiently coupled into a PMMA fiber (refraction index is 1.49) through evanescent when they meet the condition of phase matching. Here, the wavelength of the light was 980 nm. The length of overlapping was 28.8 μm. The distance of lateral separation was 110 nm. The details of evanescent coupling under a microscope were shown in Fig. 3. We collected a PMMA fiber with the diameter of 600 nm on MgF2 substrate. A single-mode fiber was tapered down to a fiber with the diameter of 800 nm [18] and placed together with the PMMA fiber using three-dimensional micromanipulation platform under a microscope to form a coupler structure. The tapered fiber was used to evanescently couple the light into PMMA fiber by overlapping them in parallel on the MgF2 surface. Both the PMMA fiber and the tapered fiber were attracted on the MgF2 substrate by van der Waals force and electrostatic force. Then, we coupled light with different wavelengths of 488 nm, 532 nm and 650 nm into the core of the tapered fiber. As a result, we successfully observed that the light from the taper was efficiently coupled into the fiber in the part of overlapping (contacting length is 40 μm). The light in the PMMA fiber could be guided for at least 100 μm. The uniform and virtually unattenuated scattering along the fiber showed that, relative to the guided intensity, the scattering was small.
4. Conclusion In summary, optical PMMA microfiber was successfully fabricated by electrospinning technique. The microfibers have uniform diameters and very smooth surfaces. Simultaneously, the fibers performed excellent optical properties due to diameter uniformity and sidewall
Fig. 2. (a) Schematic diagram of evanescent launching between a PMMA fiber and a tapered fiber. (b) Power distribution simulated at the transverse cross plane of MgF2supported (left) and SiO2-supported (right) PMMA fiber. The wavelength of the light is 980 nm. (c) Power maps of evanescent coupling region.
Fig. 3. Microscope images of optical coupling with different wavelengths between the PMMA fiber and the tapered fiber. (a) 488 nm, (b) 532 nm, (c) 650 nm. The arrows show the direction of light propagation.
L. Li et al. / Materials Letters 66 (2012) 292–295
smoothness. The light with the wavelength of 488 nm, 532 nm and 650 nm could guide along the fiber. Moreover, this method provided an effective and convenient way to fabricate highly uniform nano or micro scale optical waveguide without expensive equipment and complex procedure. The fibers from electrospinning technique could find wide applications in various micro/nano-scale photonic and sensitive devices. Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC) (project no. 61007053), Ph.D. Programs Foundation of Ministry of Education of China (20092304120022) and the Fundamental Research Funds for the Central Universities (HEUCF20111114). References [1] Osberg KD, Schmucker AL, Senesi AJ, Mirkin CA. Nano Lett 2011;11:820–4. [2] Dalvand P, Mohammadi MR, Fray DJ. Mater Lett 2011;65:1291–4. [3] Dong GP, Chi YZ, Xiao XD, Liu XF, Qian B, Ma ZJ, et al. Opt Express 2009;17: 22514–9.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
295
Yang XH, Wang LL, Yang S. Mater Lett 2007;61:2904–7. Tong LM, Gattass RR, Ashcom JB, He SL, Lou JY, Shen MY. Nature 2003;426:816–9. Bures J, Ghosh R. J Opt Soc Am A 1999;16:1992–6. Bayle F, Meunier JP. Appl Opt 2005;44:6402–11. Tong LM, Lou JY, Gattass RR, He SL, Chen XF, Liu L, et al. Nano Lett 2005;5:259–62. Yang PP, Zhan SM, Huang ZH, Zhai JL, Wang DJ, Xin Y, et al. Mater Lett 2011;65: 537–9. Bisht GS, Canton G, Mirsepassi A, Kulinsky L, Oh S, Dunn-Rankin D, et al. Nano Lett 2011;11:1831–7. Yang XH, Shao CL, Liu YC. J Mater Sci 2007;42:8470–2. Yang XH, Shao CL, Liu YC, Mu RX, Guan HY. Thin Solid Films 2005;478:228–31. Uyar T, Nur Y, Hacaloglu J, Besenbacher F. Nanotechnology 2009;20:125703. Shenoy SL, Bates WD, Frisch HL, Wnek GE. Polymer 2005;46:3372. Demir MM, Yilgor I, Yilgor E, Erman B. Polymer 2002;43:3303–9. Zheng J, He A, Li J, Xu J, Han CC. Polymer 2006;47:7095–102. Law M, Sirbuly DJ, Johnson JC, Goldberger J, Saykally RJ, Yang PD. Science 2004;305:1269–73. Zhu XL, Yuan LB, Yang J, Cao SX. Appl Opt 2009;48:5624–8.