Open Access
Add to BibSonomyAbstract
Phosphate glass-clad optical fibers comprising amorphous Se0.8Te0.2 semiconductor core were fabricated by a reactive molten core approach. The Se0.8Te0.2 crystals were precipitated in core region by a postdrawing annealing process, which were confirmed by X-ray diffraction, micro-Raman spectra, electron probe X-ray micro-analyzer, and transmission electron microscope measurement results. A two-cm-long crystalline Se0.8Te0.2 semiconductor core optical fiber, electrically contacted to external circuitry through the fiber end facets, exhibits a two-orders-of-magnitude change in conductivity between dark and illuminated states. The great discrepancy in light and dark conductivity suggests that such crystalline Se0.8Te0.2 semiconductor core optical fibers have promising applications in optical switch and photoconductivity of optical fiber array.
© 2015 Optical Society of America
1. Introduction
Recently, glass-clad optical fibers compromising amorphous/crystalline semiconductor core have attracted great attention for their potential utility as novel waveguides for applications in nonlinear optics, sensing, power delivery, biomedicine, optical switch, and photodetecting devices [1–3 ]. Although the field of semiconductor core optical fibers is still in its relative infancy, impressive progress has been made in a fairly short amount of time, from the fundamentals through to device demonstration [4–6 ].
Of the approaches employed to fabricate glass-clad semiconductor core optical fibers, the molten core method seems most practical to achieve long lengths of low loss fibers [1, 4 ]. This method requires that the core material melt at a temperature where the cladding glass is sufficiently soft so as to draw into optical fibers [7,8 ]. More recently, the molten core drawing process has been used as a new nanocomposite synthesis tool whereby the preform reduction zone serves as the role of a chemical reaction crucible, in which materials can physically mix, chemically react, and produce new compounds that precipitate directly into the fiber [7,9,10 ]. It opens the door to a wide variety of other potential core materials that cannot easily be directly melted or drawn into fiber such as incongruent melting or high vapor pressure compounds [7].
Selenium (Se) and tellurium (Te) are elemental semiconductors, and they exhibit unique combinations of many useful and interesting properties, such as photoconductivity, nonlinear optical response, and high infrared transparency, which results in their potential applications in electronic and optical electronic devices [11–13 ]. Photoconductivity is a useful mechanism whereby light transmitted through a novel fiber with a semiconducting crystalline core can be modulated or switched. The increase in carrier density produced by photo-excitation of the fiber core will induce phase shifts in a transmitted signal, or can amplitude modulate or switch the fiber through free-carrier absorption [14]. Trigonal tellurium (t-Te), as well as trigonal selenium (t-Se) and Se-Te alloys, have a highly anisotropic crystal structure consisting of helical chains of covalently bound atoms, which in turn bound together through van der Waals interactions into a hexagonal lattice [15,16 ]. Some references have reported that the t-Se and t-Te can form solid solution in the alloys and it is possible to control the properties of alloys such as photoconductivity by fine-tuning the elemental composition of Se and Te [12,16 ]. Photoconductivity measurements were performed on crystalline germanium-core optical fiber [14]. However, relative to germanium, the Se-Te alloy has larger photoconductivity [17]. Therefore, Se-Te alloy is a very intriguing core material for photoconductive optical fibers. However, the glass-clad amorphous/crystalline Se-Te alloy semiconductor core optical fibers have not been reported.
In this study, a reactive molten core method was used to fabricate semiconductor core optical fibers. Specifically, the fibers utilized a self-developed phosphate glass cladding with core compositions in the initial preform ranging from crystalline Se and Te powders to stoichiometric Se0.8Te0.2. The core of the as-drawn fibers was found to be amorphous. What is more, the Se0.8Te0.2 crystals were precipitated in core region by a postdrawing annealing process. The crystalline Se0.8Te0.2 alloy semiconductor core optical fiber shows great discrepancy in conductivity between illuminated and dark states, suggesting it has promising utility in optical switch and photoconductivity of optical fiber array.
2. Experimental
A self-developed multi-component phosphate glass (55P2O5-18K2O-13BaO-14Al2O3 wt %) was obtained by conventional melt-quenching method, and then was processed to cylindrical preform with one end closed [18–20 ]. Se powder and Te powder of 99.99% purity (Aladdin Industrial Corporation, Shanghai, China) in a molar proportion of 4:1 were mixed and then filled in the 8-cm-long phosphate glass preform, with outer diameter 16 mm and inner diameter of 3 mm. The other end of the preform was also closed after the mixed powder was filled in under vacuum condition. The continuous Se0.8Te0.2 semiconductor core fibers were drawn in an optical fiber draw tower under an argon atmosphere at approximately 660 °C.
The as-drawn fibers were cleaved and their cross sections were observed by field electron-scanning electron microscopy (FE-SEM, ZEISS Merlin, Germany). FE-SEM equipped with an energy-dispersive X-ray spectrometer (EDS) was used to study the distribution of elements of the fiber. Electron probe X-ray micro-analyzer (EPMA-1600, Shimadzu, Japan) was used to examine the distribution of elements spatially across the core/clad interface. The crystalline phase of the core was identified by X-ray powder diffractometer (XRD) (X’Pert PROX, Cu Kα). The glass transition temperature (Tg) and crystallization peak temperature (Tp) were determined by a Netzsch STA 449C Jupiter different scanning calorimeter (DSC) at a heating rate of 10 °C /min from 25 to 300 °C under N2 atmosphere. The micro-Raman spectra (532 nm excitation) were collected on the fiber core using a Renishaw RM2000 instrument. To further determine the structure and phase of crystals in core region, high-resolution transmission electron microscopy (HR-TEM) images and selected area electron diffraction (SAED) patterns were performed using transmission electron microscope (TEM, JEM-2100F).
Diffuse reflectance measurements, used to measure band gap width, were performed on a Perkin-Elmer Lambda 950/UV/Vis/NIR spectrophotometer, equipped with an integrating sphere. The core was grounded, placed in the sample cup and compared to a similarly prepared 100% reflectance standard, BaSO4. Vis/NIR spectra were recorded in the range of 400-1800 nm in diffuse reflectance mode and converted to the absorbance coefficient F(R) by the Kubelka-Munk equation. The current of the Se0.8Te0.2 core fibers between dark and illuminated (under illumination from a 200 mW/cm2 808 nm HeNe laser) states were recorded using Keithley series 2400 source meter. It is noted that in some cases both the core and the phosphate cladding were analyzed and in selected cases the phosphate cladding was removed by etching in HF acid. All measurements were made at room temperature.
3. Results and discussions
3.1 Morphological characterization
Figure 1 shows the SEM image of the as-drawn Se0.8Te0.2 core fiber. The fiber has a good circularity and uniformity with an outer diameter of about 280 μm and inner diameter of about 55 μm. As can be seen, there are no obvious discontinuities at the core/clad interface and no obvious cracks or signs of bubbles in the core, indicating the well-matched coefficients of thermal expansion and good wettability between the Se0.8Te0.2 semiconductor core and phosphate glass cladding. The two-dimentional EDX mappings distribution of P, O, Se, and Te were illustrated in Figs. 1(b)-1(e). The distribution of P and O is mainly in the glass-clad region. Meanwhile, the core is mainly composed of Se and Te.
Fig. 1 (a) SEM image of the as-drawn Se0.8Te0.2 core fiber. (b)-(e) The EDX mappings of the marked area in (a). Yellow, red, green, and light blue denote element phosphorus (P), oxygen (O), selenium (Se), and tellurium (Te), respectively.
3.2 Phase characterization
Figure 2(a) presents the XRD spectra from the core of as-drawn fiber and the core of the fiber after being annealed at 150 °C for 1 h. Obviously, the as-drawn Se0.8Te0.2 core fiber is structurally amorphous as only two broad diffuse humps were detected in the curve. The DSC curves of amorphous Se0.8Te0.2 core is shown in Fig. 2(b). The characteristic temperatures Tg and Tp are 70 °C, 130 °C, respectively. As Se0.8Te0.2 is an unstable glass and can easily go between amorphous and crystalline states, the amorphous Se0.8Te0.2 (a-Se0.8Te0.2) core may be converted to the equilibrium crystalline state simply by annealing near the crystallization peak temperature (Tp) [21]. The annealing temperature is substantially below the glass transition of the phosphate glass cladding (Tg ~480 °C) [20]. Therefore, the glass cladding is unaffected by the annealing process. Several characteristic diffraction peaks [Fig. 2(a)] emerge clearly after thermal treatment at 150 °C for 1 h, which appear between the peaks of pure Te (JCPDS Card No. 36-1452) and pure Se (JCPDS Card No. 06-0362), revealing that Se0.8Te0.2 was crystallized from the amorphous core. The lattice parameters of the crystalline Se0.8Te0.2 (c-Se0.8Te0.2) were calculated as a = 4.366 Å and c = 4.954 Å. These parameters fall directly between those pure Se and pure Te, which means that homogeneous solid solution was formed. Since Se and Te are completely isomorphous, and can form a completely miscible solid solution in which the Se and Te atoms are randomly distributed along the helical chain in the form of a random copolymer [the inset of Fig. 2(a)] [22]. By fine-tuning the molar proportion of Se and Te powder, it is possible to obtain phosphate glass-clad optical fibers containing different Se-Te alloys by using the reactive molten core method.
Fig. 2 (a) XRD spectra for the core of as-drawn fiber and the core of annealed fiber. (b) DSC curves of amorphous Se0.8Te0.2 core. The inset of Fig. 2(a) shows the hexagonal lattice occupied by trigonal-phase alloy of Se0.8Te0.2.
Figure 3 shows the micro-Raman spectra of the a-Se0.8Te0.2 core of as-drawn fiber and the c-Se0.8Te0.2 core of fiber after being annealed at 150 °C for 1 h. The vibrational Raman bands corresponding to Te-Te, Se-Te and Se-Se bond are indicated in Fig. 3. It can be seen that the dominating amorphous Se line, around 250 cm−1, while the crystalline Se line, around 235 cm−1 [23]. The line which appears at about 200 cm−1 has been assigned to vibration of neighboring amorphous Se-Te bonds [24]. According to the Ref. 25, the crystalline Se-Te line appears at about 170 cm−1. The line related with the crystalline Te-Te vibrations is at about 150 cm−1. However, the line related with amorphous Te-Te vibrations cannot be found in the a-Se0.8Te0.2 core due to the low concentration of tellurium. The XRD spectra and micro-Raman spectra indicate that the core of as-drawn fibers was amorphous and a pure phase of trigonal Se0.8Te0.2 phase was identified in annealed fibers, which demonstrates that Se0.8Te0.2 crystals were precipitated in core region by a postdrawing annealing process.
Fig. 3 Micro-Raman spectra of the a-Se0.8Te0.2 core of as-drawn fiber and the c-Se0.8Te0.2 core of annealed fiber.
3.3 Microstrcture characterization
The elemental (Se, Te, O, P) profile across the core/clad interface of the fibers after being annealed at 150 °C for 1 h is provided in Fig. 4 . The zero relative distance represents the middle of the core. The core possesses a composition of nearly 69Se-28Te in weight percent, which is 80Se-20Te in molar percent and can be consistent with the initial core composition. The results also confirm that phase-pure Se0.8Te0.2 crystals were precipitated in core region by simply thermal treatment. There are some diffusion of Se and Te into the cladding region and, conversely, diffusion of oxygen and phosphorus into the core region. Since diffusion is a thermally-activated process, a lowering of the processing temperature favors a reduced degree of diffusion [26]. It can be noted that the level of oxygen in c-Se0.8Te0.2 core fibers drawn at 660 °C is about 3 wt %, which is less than that found in InSb core fibers drawn at 700 °C (~9 wt %), Si core fibers drawn at 1950 °C (~9.7 wt %) [27, 28 ]. High quality semiconductor core fiber requires lowest level of oxygen in the core, because the oxide precipitates can act as defects which will limit the performance of the fiber. The propagation loss of the a-Se0.8Te0.2 core fibers at 1550 nm was measured to be 2.0 dB/cm by using the cutback method. The loss is much high, however, the other properties measurements are comparatively quite well.
Fig. 4 Elemental profiles (relative elemental composition as a function of position across the fiber) for the annealed fiber.
The TEM, HR-TEM and SAED images of the c-Se0.8Te0.2 core of fibers annealed at 150 °C for 1 h are illustrated in Fig. 5 . The SAED pattern in Fig. 5(b) indicates that the Se0.8Te0.2 crystal in selected area exhibits a single crystalline character. The HR-TEM image in the inset of Fig. 5(a) demonstrates a crystal lattice fringe with the spacing d value of ~0.38 nm, which corresponds to the (100) crystal facet of the trigonal phase Se0.8Te0.2. The SAED pattern in Fig. 5(d) indicates that the Se0.8Te0.2 crystal in selected area exhibits a polycrystalline character. It is well known, the forming conditions of single crystal are very harsh. C. McMillen et al. have reported that annealing of germanium fibers can greatly enhance single crystallinity, and the single crystal regions were observed routinely in lengths greater than 8 mm with the longest being about 15 mm [29]. Therefore, according to the TEM results, XRD patterns and relevant studies, it can be reasonable to inferred that c-Se0.8Te0.2 core in fibers is polycrystalline overall though with some single crystalline Se0.8Te0.2 nanocrystals. Future effort could focus on fabricating low-loss glass-clad single crystalline semiconductor core optical fibers.
Fig. 5 (a) and (c) TEM images of the c-Se0.8Te0.2 core of annealed fiber. (b) and (d) SAED patterns of the c-Se0.8Te0.2 in (a) and (c), respectively. The inset of Fig. 5(a) shows the HR-TEM image of the c-Se0.8Te0.2.
3.4 Properties characterization
The diffuse reflectance spectra, which were usually used to calculate the optical energy gap [30], and can be converted to the absorption coefficient by the Kubelka-Munk Eq. (1) [31]:
where R is the measured reflectivity, S is the scattering coefficient. The absorption coefficient α near the absorption edge, has dependence on the energy of incident light hν, of the form (2),where is the photon energy, is the optical energy gap. From a plot of vs., and fitting a straight line, the value ofcan be calculated.Figure 6(a) shows the measured absorbance spectra as a function of wavelength in the range of 400-1800 nm, for the a-Se0.8Te0.2 core of as-drawn fiber and the c-Se0.8Te0.2 core of fiber after being annealed at 150 °C for 1 h. As can be seen, both of the amorphous and crystalline Se0.8Te0.2 alloy are highly absorbing in the visible optical region [30]. Therefore, the photoconductivity measurements on Se0.8Te0.2 core optical fibers can be performed under a visible light irradiation. The absorption edge of the c-Se0.8Te0.2 core shifts to longer wavelength, when compared with the a-Se0.8Te0.2 core. Plots of versus the energy of absorbed light for a-Se0.8Te0.2 core and c-Se0.8Te0.2 core are shown in Fig. 6(b). The of a-Se0.8Te0.2 core and c-Se0.8Te0.2 core were found to be 1.26 eV and 1.12 eV, respectively.
Fig. 6 (a) The experimental date of absorbance spectra for a-Se0.8Te0.2 core and c-Se0.8Te0.2 core. (b) Plots of (αhν)1/2 versus the energy of absorbed light.
Figures 7(a)-7(c) shows the grain size of Se0.8Te0.2 crystals in core of the fibers annealed at 130 °C/1 h, 150 °C/1 h, 170 °C/1 h, respectively. It can be clearly seen that the Se0.8Te0.2 crystals were distributed homogenously in core region with a uniform size. Meanwhile, the grain size of Se0.8Te0.2 crystals increased with the enhancement of thermal treatment temperature. Figure 7(d) presents the interface between the glass cladding and the semiconductor core after the fibers being annealed at 150 °C for 1 h, showing that the amorphous glass cladding and the crystalline core still have good interface even after thermal treatment. Sketch and photograph of electrical contact to an external circuit by connecting the fiber end facets is shown in Fig. 7(e). A two-cm-long fiber, which was cut out from the original continuous Se0.8Te0.2 core fibers, can be easily connected to external circuitry by applying silver paint to both ends of a fiber segment. Figure 7(f) compares the voltage-current curve of Se0.8Te0.2 core fibers between dark and illuminated states (under illumination from a 200 mW/cm2 808 nm HeNe laser). As can be seen, there is no current in the as-drawn Se0.8Te0.2 core fibers neither in dark nor illuminated states due to its extremely low conductivity (﹤10−12 Ω−1 cm−1) [30]. However, the photocurrents of the annealed fibers, which were thermal treatment at different temperature, have great difference. It is worth noting that there is a two-orders-of-magnitude change in conductivity between dark and illuminated states in the fiber after being annealed at 150 °C for 1 h, suggesting it has promising applications in optical switch and photoconductivity of optical fiber array. The different sensitivity to illumination of the fibers annealed at different temperature can be associated with the microstructure of Se0.8Te0.2 crystals in core. The smaller grain size of Se0.8Te0.2 crystals in core of the fiber after being annealed at 130 °C for 1 h, results in more grain boundary, which will hinder the migration of carrier and increase the reflection of electrons at grain boundaries. When the fibers annealed at 170 °C for 1 h, the grain size of Se0.8Te0.2 crystals in core is significantly increased. However, they were not evenly distributed in core region, which will also hinder the migration of carrier. The Se0.8Te0.2 crystals were closely and homogenously distributed in core region with a uniform size after the fibers being annealed at 150 °C for 1 h. Therefore, the fiber which annealed at 150 °C for 1 h is more sensitive to 808 nm laser illumination.
Fig. 7 (a)-(c) SEM images illustrating grain size of Se0.8Te0.2 crystals in core as a function of the fibers after being annealed at different temperatures. (d) Shows the interface between the clad and the core of the fibers after being annealed at 150 °C for 1 h. (e) Sketch and photograph of electrical contact to an external circuit by connecting the fiber end facets. (f) Current-voltage characteristics of Se0.8Te0.2 core fibers in the dark and under illumination.
It has been reported that there is only three times change in conductivity between dark and illuminated states in c-Se core fibers [18]. And there is no photocurrent in c-Te core fibers [19]. This work gives a practical example that by fine-tuning the elemental composition of Se and Te of core material and using the reactive molten core method, it is possible to tune the photoconductivity of the crystalline Se-Te alloy semiconductor core in optical fibers. Moreover, the photoconductive optical fibers are created with the ability of detecting illumination along their entire length and are unaffected by the changes in the local environment due to contamination or humidity as they have glass cladding [32]. The electrical conductivity of the c-Se0.8Te0.2 core fibers, which annealed at 150 °C for 1 h, can be estimated from the applied voltage, measured current, and fiber dimensions. The calculated conductivity of the c-Se0.8Te0.2 core fibers in dark is 2.0 × 10−7 Ω−1 cm−1, which is larger than that of c-Se filaments (10−8 Ω−1 cm−1), but less than that of bulk single crystal Se (10−6 Ω−1 cm−1) [33, 34 ]. The increased surface-to-volume ratios, defects, impurities of the c-Se0.8Te0.2 core and reflection of electrons at grain boundaries may be the reasons for the less conductivity [35].
4. Conclusion
In conclusion, phosphate glass-clad amorphous Se0.8Te0.2 alloy semiconductor core optical fibers have been fabricated by using a reactive molten core method. The Se0.8Te0.2 crystals were precipitated in core region by a postdrawing annealing process as evidenced by XRD spectra, micro-Raman spectra, EPMA, and TEM measurement results. The c-Se0.8Te0.2 core fiber has a two-orders-of-magnitude change in conductivity between dark and illuminated states, suggesting it has promising applications in optical switch and photoconductivity of optical fiber array. This work also verifies that by fine-tuning the initial core composition of Se and Te powder and using the reactive molten core method, it is possible to tune the photoconductivity of the crystalline Se-Te alloy semiconductor core in optical fibers.
Acknowledgments
This research was supported by the China State 863 Hi-tech Program (2013AA031502 and 2014AA041902), NSFC (11174085, 51132004, and 51302086), Guangdong Natural Science Foundation (S2011030001349), and China National Funds for Distinguished Young Scientists (61325024).
References and links
1. A. C. Peacock, J. R. Sparks, and N. Healy, “Semiconductor optical fibres: progress and opportunities,” Laser Photonics Rev. 8(1), 53–72 (2014). [CrossRef]
2. G. M. Tao, A. M. Stolyarov, and A. F. Abouraddy, “Multimaterial fibers,” Int. J. Appl. Glass Sci. 3(4), 349–368 (2012). [CrossRef] [PubMed]
3. J. Ballato and P. Dragic, “Rethinking optical fiber: new demands, old glasses,” J. Am. Ceram. Soc. 96(9), 2675–2692 (2013). [CrossRef]
4. S. Morris and J. Ballato, “Molten-core fabrication of novel optical fibers,” Bull. Am. Ceram. Soc. 92(4), 24–29 (2013).
5. A. F. Abouraddy, M. Bayindir, G. Benoit, S. D. Hart, K. Kuriki, N. Orf, O. Shapira, F. Sorin, B. Temelkuran, and Y. Fink, “Towards multimaterial multifunctional fibres that see, hear, sense and communicate,” Nat. Mater. 6(5), 336–347 (2007). [CrossRef] [PubMed]
6. M. Bayindir, F. Sorin, A. F. Abouraddy, J. Viens, S. D. Hart, J. D. Joannopoulos, and Y. Fink, “Metal-Insulator-Semiconductor optoelectronic fibres,” Nature 431(7010), 826–829 (2004). [CrossRef] [PubMed]
7. J. Ballato, C. McMillen, T. Hawkins, P. Foy, R. Stolen, R. Rice, L. Zhu, and O. Stafsudd, “Reactive molten core fabrication of glass-clad amorphous and crystalline oxide optical fibers,” Opt. Mater. Express 2(2), 153–160 (2012). [CrossRef]
8. S. Morris, T. Hawkins, P. Foy, J. Ballato, S. W. Martin, and R. Rice, “Gladding glass development for semiconductor core optical fibers,” Int. J. Appl. Glass Sci. 3(2), 144–153 (2012). [CrossRef]
9. C. Hou, X. Jia, L. Wei, A. M. Stolyarov, O. Shapira, J. D. Joannopoulos, and Y. Fink, “Direct atomic-level observation and chemical analysis of ZnSe synthesized by in situ high-throughput reactive fiber drawing,” Nano Lett. 13(3), 975–979 (2013). [CrossRef] [PubMed]
10. S. Morris, T. Hawkins, P. Foy, C. McMillen, J. Fan, L. Zhu, R. Stolen, R. Rice, and J. Ballato, “Reactive molten core fabrication of silicon optical fiber,” Opt. Mater. Express 1(6), 1141–1149 (2011). [CrossRef]
11. B. Zhou and J. J. Zhu, “A general route for the rapid synthesis of one-dimensional nanostructured single-crystal Te, Se and Se-Te alloys directly from Te or/and Se powders,” Nanotechnology 17(6), 1763–1769 (2006). [CrossRef]
12. Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, and H. Yan, “One-dimensional nanostructres: synthesis, characterization, and applications,” Adv. Mater. 15(5), 353–389 (2003). [CrossRef]
13. T. I. Lee, S. Lee, E. Lee, S. Sohn, Y. Lee, S. Lee, G. Moon, D. Kim, Y. S. Kim, J. M. Myoung, and Z. L. Wang, “High-power density piezoelectric energy harvesting using radially strained ultrathin trigonal tellurium nanowire assembly,” Adv. Mater. 25(21), 2920–2925 (2013). [CrossRef] [PubMed]
14. R. Davis, R. Rice, A. Ballato, T. Hawkins, P. Foy, and J. Ballato, “Toward a photoconducting semiconductor RF optical fiber antenna array,” Appl. Opt. 49(27), 5163–5168 (2010). [CrossRef] [PubMed]
15. L. I. Berger, Semiconductor Materials (CRC Press, 1997).
16. B. Mayers and Y. Xia, “One-dimensional nanostructures of trigonal tellurium with various morphologies can be synthesized using a solution-phase approach,” J. Mater. Chem. 12(6), 1875–1881 (2002). [CrossRef]
17. A. Zakery and S. R. Elliott, “Optical properties and applications of chalcogenide glass: a review,” J. Non-Cryst. Solids 330(1–3), 1–12 (2003). [CrossRef]
18. G. W. Tang, Q. Qian, K. L. Peng, X. Wen, G. X. Zhou, M. Sun, X. D. Chen, and Z. M. Yang, “Selenium semiconductor core optical fibers,” AIP Adv. 5(2), 027113 (2015). [CrossRef]
19. G. W. Tang, Q. Qian, X. Wen, G. X. Zhou, X. D. Chen, M. Sun, D. D. Chen, and Z. M. Yang, “Phosphate glass-clad tellurium semiconductor core optical fibers,” J. Alloys Compd. 633, 1–4 (2015). [CrossRef]
20. S. H. Xu, Z. M. Yang, Z. M. Feng, Q. Y. Zhang, Z. H. Jiang, and W. C. Xu, “Efficient fibre amplifiers based on a highly Er3+/Yb3+ codoped phosphate glass fibre,” Chin. Phys. Lett. 26(4), 047806 (2009). [CrossRef]
21. M. F. Kotkata and M. K. El-Mously, “A survey of amorphous Se-Te semiconductors and their characteristic aspects of crystallization,” Acta Phys. Hung. 54(3), 303–312 (1983).
22. B. Mayers, B. Gates, Y. Yin, and Y. Xia, “Large-scale synthesis of monodisperse nanorods of Se/Te Alloys through a homogeneous nucleation and solution growth process,” Adv. Mater. 13(18), 1380–1384 (2001). [CrossRef]
23. G. Lucovsky, A. Mooradian, W. Taylor, G. B. Wright, and R. C. Keezer, “Identification of the fundamental vibrational modes of trigonal, a-monoclinic and amorphous selenium,” Solid State Commun. 5(2), 113–117 (1967). [CrossRef]
24. A. Mendoza-Galván, E. García-García, Y. V. Vorobiev, and J. González-Hernández, “Structural, optical and electrical characterization of amorphous SexTe1-x thin film alloys,” Microelectron. Eng. 51–52, 677–687 (2000). [CrossRef]
25. R. Geick, E. F. Steigmeier, and H. Auderset, “Raman effect in selenium-tellurium mixed crystals,” Phys. Stat. Solidi B 54(2), 623–630 (1972). [CrossRef]
26. J. Ballato, T. Hawkins, P. Foy, B. Yazgan-Kokuoz, R. Stolen, C. McMillen, N. K. Hon, B. Jalali, and R. Rice, “Glass-clad single-crystal germanium optical fiber,” Opt. Express 17(10), 8029–8035 (2009). [CrossRef] [PubMed]
27. J. Ballato, T. Hawkins, P. Foy, C. McMillen, L. Burka, J. Reppert, R. Podila, A. M. Rao, and R. R. Rice, “Binary III-V semiconductor core optical fiber,” Opt. Express 18(5), 4972–4979 (2010). [CrossRef] [PubMed]
28. J. Ballato, T. Hawkins, P. Foy, R. Stolen, B. Kokuoz, M. Ellison, C. McMillen, J. Reppert, A. M. Rao, M. Daw, S. R. Sharma, R. Shori, O. Stafsudd, R. R. Rice, and D. R. Powers, “Silicon optical fiber,” Opt. Express 16(23), 18675–18683 (2008). [CrossRef] [PubMed]
29. C. McMillen, T. Hawkins, P. Foy, D. Mulwee, J. Kolis, R. Stolen, R. Rice, and J. Ballato, “On crystallographic orientation in crystal core optical fibers,” Opt. Mater. 32(9), 862–867 (2010). [CrossRef]
30. K. V. Reddy and A. K. Bhatnagar, “Electrical and optical studies on amorphous Se-Te alloys,” J. Phys. D Appl. Phys. 25(12), 1810–1816 (1992). [CrossRef]
31. H. El-Zahed, A. El-Korashy, and M. Dongol, “Optical parameter studies of as-deposited and annealed Se1-xTex films,” Thin Solid Films 259(2), 203–211 (1995). [CrossRef]
32. F. Sorin, A. F. Abouraddy, N. Orf, O. Shapira, J. Viens, J. Arnold, J. D. Joannopoulos, and Y. Fink, “Multimaterial photodetecting fibers: a geometric and structural study,” Adv. Mater. 19(22), 3872–3877 (2007). [CrossRef]
33. D. S. Deng, N. D. Orf, S. Danto, A. F. Abouraddy, J. D. Joannopoulos, and Y. Fink, “Processing and properties of centimeter-long, in-fiber, crystalline-selenium filaments,” Appl. Phys. Lett. 96(2), 023102 (2010). [CrossRef]
34. J. L. Hartke, “Drift mobilities of electrons and holes and space-charge-limited currents in amorphous selenium films,” Phys. Rev. 125(4), 1177–1192 (1962). [CrossRef]
35. E. J. Menke, M. A. Thompson, C. Xiang, L. C. Yang, and R. M. Penner, “Lithographically patterned nanowire electrodeposition,” Nat. Mater. 5(11), 914–919 (2006). [CrossRef] [PubMed]
