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Amorphous selenium (a-Se) core fibers with glass cladding have immediately been fabricated using the molten core method. The high aspect ratio of the a-Se core fibers and the presence of the glass cladding surrounding the Se core make it convenient to convert the a-Se core into a polycrystalline structure. It is found that the two-step thermal annealing process allows for the increase of crystal grain size and decrease of the structure defects in the Se core. Therefore, a low propagation loss of 1.5 dB/cm at 1310 nm has been realized for the polycrystalline selenium (c-Se) core optical fibers obtained by the wo-step annealing a-Se core fibers, first at a 80 °C low temperature annealing followed by a 207 °C high temperature annealing, which is much lower than that reported for c-Se core optical fibers (2.6 dB/cm).
© 2017 Optical Society of America
1. Introduction
Over the past several years, multimaterial fibers has been a research topic with an increasing emphasis in fiber optics and has the vision of integrating sophisticated materials inside fibers, which are not traditionally used in optical fibers [1–3]. The semiconductor waveguides in fiber form not only offer exceptional nonlinear optical and optoelectronic properties, but also serve as unifying structures between conventional fiber optics and planar semiconductor photonics [4–7]. It is well-known that silica fibers have high optical losses at mid-infrared wavelengths longer than 2.5 μm, while fluoride and chalcogenide glass fibers generally can guide only modest optical powers [8–10]. Crystalline semiconductor core optical fibers are preferred for the guidance of high optical powers at wavelengths well into the far-infrared for a wide range of applications such as nonlinear optics, biomedicine and sensing [8, 11]. However, one of the biggest challenges to the adoption of these fibers has been their relatively high optical losses.
High pressure chemical vapor deposition (HPCVD) has been an important technique to fabricate crystalline semiconductor core fibers, which can be easily adapted to deposit various semiconductor materials (e.g. Si, Ge and ZnSe) into silica capillaries or microstructured optical fibers [4, 9, 12]. However, it is not possible to achieve long lengths of fibers due to its slow deposition rate [13]. The molten core method have been developed effectively to integrate a wider class of materials into fibers, as long as a compatible glassy cladding can be found to encapsulate the core melts as it flows from preform to fibers at a high temperature at which the cladding glass can be in a viscous state and the core material simultaneously be melted [4, 14]. To date, very long length of continuous glass-clad crystalline unary (Si, Ge, and Te) [15–17] and crystalline binary (InSb and Sb2Se3) [18, 19] semiconductor core fibers have been successfully drawn by using the molten core method. However, these crystalline semiconductor core fibers made by the approach are usually polycrystalline, which have high optical losses due to the significant scattering from grain boundaries [20]. Therefore, post-fabrication treatments including laser and oven-annealing, which can increase the polycrystalline grain size and decrease in the defects in the semiconductor core, play a crucial role to realize low loss crystalline semiconductor core fibers for optical applications. Recently, polycrystalline Si core optical fibers with low optical loss (< 1 dB/cm at 2.2 μm) have been realized by modified thermal annealing of amorphous Si core optical fibers fabricated by HPCVD method [8]. Later, low loss (1 dB/cm at 2.0 μm) single crystal Si optical fibers have been demonstrated by using a mid-infrared CO2 laser (λ = 10.6 μm) to directionally re-crystallize crystalline Si core fibers fabricated by molten core method [21]. More recently, a low loss (~1.33 dB/cm at 2.0 μm) of long-length (~9 mm) single crystal Ge core fiber has been obtained by using a precisely controlled 488 nm laser to crystallize the amorphous Ge core fibers fabricated by HPCVD [20].
Besides unary semiconductor Si and Ge, Se semiconductor has a high photoconductivity and infrared transparency, nonlinear optical response and is extensively used in xerography plates, rectifying junctions as well as thin-film photovoltaics [4, 22–25]. Therefore, Se semiconductor is a very intriguing core material for semiconductor core fibers and fiber-integrated optoelectronic devices. One-dimensional crystalline Se semiconductor filament arrays have been fabricated by optical fiber drawing technique, which have potential utility in optical switch, photodetector, and unique date storage platform [26]. The phosphate glass has excellent optical, mechanical, and drawing properties. And it’s drawing temperature is about 660 °C, which is higher than the melting point of Se (217 °C), assuring the Se core is molten when the glass cladding is drawn into fibers. In our previous work [22], continuous phosphate glass-clad amorphous Se semiconductor core optical fibers with optical loss of 2.6 dB/cm at 1310 nm have been fabricated by the molten core method. And the Se core fibers have good circularity and uniformity as well as being no obvious discontinuities at the core/clad interface, which indicate that the phosphate glass is an appropriate cladding for Se core. However, the loss was too high to be measured when Se core was crystallized by a simple annealing treatment [22]. In this work, we demonstrate a specific post-processing technique in which a two-step thermal annealing is used to realize the low loss of crystalline Se core fibers. With precise control of the annealing technique, the process can increase crystal grain size and reduce defect density, thus resulting low optical losses of 1.5 dB/cm at 1310 nm.
2. Experimental
A self-developed multi-component phosphate glass (55P2O5-18K2O-13BaO-14Al2O3 wt %) was fabricated by molten core method, and then was processed to 8-cm-long cylindrical glass tube as the cladding of fiber preform [22, 27, 28]. Commercial Se powder of 99.99% purity was prepared to fill in the phosphate glass tube with one end closed, which has an outer diameter about 32 mm and inner diameter of 2 mm. The other end of the cladding glass tube was sealed after the Se powder was filled in the glove box. Then, the amorphous Se core fibers were drawn in an optical fiber draw tower under Ar atmosphere at approximately 660 °C. During the fiber drawing process, the heating temperature was kept gradual and slow proceeded. And a smaller core to glass cladding ration (1:8) was used to reduce the tendency of glass cracking. In addition, the fluid instability breakup of Se core will occur during the fiber drawing process when the fibers are drawn into a smaller diameter about 150 μm. Therefore, it is hard to directly obtain continuous phosphate glass-clad Se core fibers with core diameter less than 15 μm by using the molten core method. Se core fibers with smaller core diameter may be realized by using other methods such as HPCVD, fiber taper, and multiple casing drawing.
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. To determine the temperature at which a-Se core crystallizes, the micro-Raman spectra (532 nm excitation) were collected on the fiber core using a Renishaw RM2000 instrument. The glass transition temperature (Tg) and onset crystallization peak (Tx) and the top crystallization (Tp) of amorphous Se core 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. To further determine the structure and phase of crystals in core region, TEM sample was prepared in the FEI Helios 450S dual beam focused ion beam (FIB) system, and high-resolution transmission electron microscopy (HR-TEM) images and selected area electron diffraction (SAED) patterns were performed using transmission electron microscope (FEI Titan Themis 200, X-FEG TEM, America). Optical transmission losses were measured at 1310 nm by a standard cutback technique. The current of the annealed fiber between dark and illuminated states (under illumination from 532 nm DPSS laser) were recorded using Keithley series 2450 source meter. All measurements were made at room temperature.
3. Results and discussions
Figure 1 shows the SEM image of the cross section of the as-drawn Se core fiber. The fiber has a good circularity and uniformity with an outer diameter of about 175 μm and an inner diameter of about 20 μm. Obviously, there is a strong contrast between semiconductor core and glass cladding due to the different element distribution. There are no discontinuities at the core/clad interface and no obvious cracks or vacuum bubbles in the core, indicating good wettability between the Se semiconductor core and the phosphate glass cladding. The two-dimentional EDS mappings distribution of P, O, and Se were demonstrated in Figs. 1(b)-1(d). The distribution boundary of each element forms a circle, and the P and O is mainly in the glass-clad region, while the core is mainly composed of Se. These results mean that the core-clad structure is preserved completely and a little elemental diffusion occurred between core and cladding during the fiber drawing process.
Fig. 1 (a) The SEM image of the as-drawn Se core fiber. (b)-(d) The EDS mappings of the marked area in (a). Yellow, cyan and red denote element selenium (Se), phosphorus (P) and oxygen (O), respectively.
The DSC curve of a-Se core is shown in Fig. 2(a). The characteristic temperatures of Tg, Tx and Tp are 42 °C, 80 °C and 92 °C, respectively. As a-Se is an unstable glass and can easily go between amorphous and crystalline states, the amorphous Se core may be converted to the equilibrium crystalline state simply by annealing near the crystallization onset temperature (Tx) [29]. To determine the temperature at which selenium core crystallizes, a-Se core fibers were annealed at temperature ranging from 75 °C to 207 °C. The annealing temperature is substantially below the glass transition of the phosphate glass cladding (Tg ~480 °C) [22], so the glass cladding is unaffected by the annealing process. And there exist no cracks in Se core fibers after the thermal treatment. The Raman spectra collected under 532 nm excitation in Fig. 2(b) reveal that the Se core is amorphous until the annealing temperature is raised to 80 °C at which the spectra display the disappearance of the amorphous Se characteristic peak at 250 cm−1 and the appearance of two crystalline selenium characteristic peaks at 142 cm−1 and 233 cm−1. The DSC curve and the Raman spectra of the a-Se core confirmed that the crystallization temperature of a-Se core is 80 °C, which is also accordance with the crystallization temperature of amorphous selenium films [30]. Stress, surface physics and chemistry, and the cylindrical fiber geometry may all play a role in ascertaining the details of the origin of this lower crystallization temperature and further investigation is required.
Fig. 2 (a) The DSC curve of amorphous Se core. (b) The Raman spectra of annealed Se core fiber at different temperature.
As we know, one of the dominant optical loss mechanisms of polycrystalline state semiconductors in infrared range is scattering from grain boundaries. Thus, the strategy of increasing in crystal grain size arising from optimized annealing approaches has considerable potential to decrease optical loss [8]. On incipiently crystallizing amorphous selenium films, small crystallites emerge at the center of amorphous films whose number increases with further raising temperature [29]. These crystallites serve as nuclei for crystal growth and the amorphous material surrounding them gradually disappears in an annealing progress. This process continues until the growing crystals merge. For the polycrystalline selenium core fiber, recrystallization of grains at a higher temperature that is slighter under the melting point (217 °C) of selenium and much lower than softening point (660 °C) of glass-clad may further increase their size and eliminated stress-induced defects [8, 30]. It seems the formation of large grains and fewer defects in fiber core should be achieved in a better way with a two-step annealing process. That is to say, an appropriate low-temperature annealing should ideally lead to the nucleation of a-Se core and a subsequent high-temperature annealing should allow for further increase in grain size and few defects via recrystallization [30].
To investigate the hypothesis that a two-step annealing process will give the largest grain growth and fewer defects, two identical amorphous selenium fibers were annealed for 1 hour at temperature of 80 and 90 °C (slightly and somewhat above the Tx), respectively. These two fibers were then annealed at 207 °C for 1 hour to remove defects and allow for recrystallization. A control sample was annealed only at 207 °C for 1 hour. The Raman spectra of these three samples collected on an excitation wavelength of 532 nm are shown in Fig. 3. Using a Voigt function to fit the Raman spectra, the characteristic peaks of crystalline selenium appear at around 234 cm−1 has been assigned to vibration of Se-Se bonds [31]. It can be seen that the control sample annealed at only 207 °C has a Lorentzian FWHM of 9.3 cm−1. The sample annealed at 90 and 207 °C has a smaller FWHM of 9.2 cm−1, and the one annealed at 80 and 207 °C has the smallest FWHM, 9.0 cm−1. This peak width for the latter sample with the smallest FWHM indicates that it likely has the largest and most defect-free crystalline domains among the three different annealing process.
Fig. 3 The FWHM of the lorentzian Raman component of the Se core fibers annealed at different temperature.
To gain deeper insight into the grain structure and crystal growth of the selenium core during annealing, SEM images were collected on samples with different annealing processes. Figures 4(a)-4(c) show the grain sizes of Se crystals in the cores of the fibers annealed solely at 80 °C, solely at 207 °C, and at a 80 °C low temperature annealing followed by a 207 °C high temperature annealing, respectively. Clearly in Fig. 4(a), the Se crystals were homogeneously distributed in core region along with some defects which are thought to arise from insufficient stress relief when annealed first at 80 °C. The SEM image of the sample collected first annealed at 80 °C and after the second high temperature annealing at 207 °C in Fig. 4(c) shows much less grain boundary with fewer defects and the core is now comprised of larger grain of selenium than annealed solely at 80 °C. Larger crystals and fewer obvious grain boundary are observed in Fig. 4(b) on the sample annealed solely at 207 °C, indicating this annealing process exhibits grain growth directly with few nucleation growth process. Figure 4(d) presents the interface between the glass cladding and the semiconductor core after the fibers being annealed first at 80 °C and followed by 207°C, showing that the amorphous glass cladding and the crystalline core still have good interface even after thermal treatment. Thus, SEM analysis supports the hypothesis that a low temperature annealing of 80 °C followed by a 207 °C high-temperature annealing is advantageous in producing large-size grains with few defects.
Fig. 4 (a) and (b) The SEM images of polycrystalline Se core annealed at 80 °C and 207 °C, respectively. (c) The SEM images of polycrystalline Se core annealed first at 80 °C and followed by 207 °C. (d) The interface between the glass cladding and the core of the fibers after being annealed first at 80 °C and followed by 207 °C.
To further study the microstructure of this two-step annealing a-Se core fibers first annealed at 80 °C and followed by 207 °C, the TEM sample was prepared in a FIB system, and then the TEM, SAED and HR-TEM measurement were performed. Figure 5(a) is the characteristic TEM image of the interface between the phosphate glass cladding (left) and the Se core (right). Figure 5(b) displays the HR-TEM image of the selected area labeled one (yellow circle) about 400 nm in diameter in Fig. 5(a). It demonstrates that a crystal lattice fringe spacing is ~0.3 nm, agreeing well with the separation of the (101) crystal facets of the hexagonal phase Se. And its SAED pattern is shown in Fig. 5(c), which shows a single crystalline character. Figure 5(d) shows that the SAED pattern of the selected area labeled two (yellow circle) in Fig. 5(a), which has a typical polycrystalline character. Therefore, the Se core of two-step annealed fiber mainly consists of hexagonal Se particles and contains some larger single crystal grains of about 400 nm size in certain area.
Fig. 5 (a) TEM image of the c-Se fiber. (b) HR-TEM image of SAED 1 patterns in (a). (c) SAED image of (b). (d) SAED patterns of the selected area 2 of the c-Se fiber in (a).
To determine the optical losses of annealed optical fibers, the losses of fibers were measured at 1310 nm by a standard cutback technique. The optical losses observed at 1310 nm upon using different annealing strategies are summarized in Table 1. The fiber annealed solely at 80 °C exhibits a loss of 2.54 dB/cm and the fiber annealed solely at 90 °C has a loss of 2.46 dB/cm. These higher loss values has been expected for these fibers involving smaller grain sizes, more defects, and some amorphous regions. However, after the second annealing of the 80 °C fiber at 207 °C, the optical loss decreases to 1.50 dB/cm, the lowest value yet reported for selenium core fiber and much lower than that reported for large core size (~40 μm) amorphous selenium fibers (2.6 dB/cm) [22]. This lower loss is consistent with the large grain size and small defect density observed in SEM images. The second annealing at 207 °C for the 90 °C fiber improves the loss to 1.98 dB/cm. Finally, the loss for a fiber annealed solely at 207 °C with no prior low-temperature annealing is up to 2.18 dB/cm. We can conclude that the two-step annealing process is the most effective, in this investigation, for decreasing the optical loss of Se core fibers when the low-temperature annealing is performed close to the temperature of 80 °C. This observation above supports the hypothesis that appropriate low-temperature annealing should lead to crystal nucleus forming and a subsequent high-temperature annealing should allow for crystal nucleus growing in size and a quantity of structure defects decreasing. Further reduction in optical losses through realizing single crystal selenium core optical fibers can be anticipated in the future.
Table 1. Optical loss value of Selenium core fibers annealed at different temperatures
Photoconduction is a important characterization for crystalline semiconductor optical fibers and it has been reported that there exist some changes in conductivity between dark and illuminated states in crystalline semiconductor core fibers [22, 28]. To study the photoelectric properties of the annealed selenium core optical fiber at 80 °C and followed 207 °C, a c-Se core fiber with 2-cm-long was connected directly to external circuitry. Figure 6 shows that the voltage-current curve of the annealed crystalline selenium optical fiber under dark state and the state illuminated by 532 nm DPSS laser at the energy density of 200 mW/cm2. Some remarkable current changes can be observed between dark and illuminated states. The large difference with about three times change in conductivity between dark (1.6 × 10−7 Ω−1 cm−1) and illuminated states (4.7 × 10−7 Ω−1 cm−1) suggest c-Se optical fibers may have promising utility in photodetector and optical switch. The calculated conductivity of the polycrystalline Se core fiber in dark is much less than that of bulk single crystal Se (10−6 Ω−1 cm−1) [26, 32], which may be attributed to reflecting electrons at crystal grain boundaries or residual defects along the high aspect ratio core [33].
Fig. 6 Current-voltage characteristics of c-Se optical fiber annealed at 80 °C and followed at 207 °C in the dark and illuminated states.
4. Conclusion
In conclusion, amorphous selenium core fibers with multi-component phosphate glass cladding have immediately been obtained using molten core method. Then, the amorphous selenium core in fibers can convert to polycrystalline state by the modified thermal annealing process. Based on the systematically investigation, it is found that the two-step annealing process, first at 80 °C and followed at 207 °C, allows for obtaining larger Se crystal grain and fewer defects in the selenium core, by which a low optical loss of 1.50 dB/cm of the fibers at a wavelength of 1310 nm is thus realized. For the two-step annealed fiber, over three times change in conductivity between dark (1.6 × 10−7 Ω−1 cm−1) and illuminated states under 532 nm DPSS laser (4.7 × 10−7 Ω−1 cm−1) suggests the annealed Se core fiber may have promising utility in photodetector and optical switch.
Funding
China National Funds for Distinguished Young Scientists (61325024); the High-level Personnel Special Support Program of Guangdong Province (2014TX01C087); Fundamental Research Funds for the Central Universities (2015ZP019); the China State 863 Hi-tech Program (2014AA041902); National Natural Science Foundation of China (NSFC) (51472088, 61535014 and 51302086), the Science and Technology Project of Guangdong (2015B090926010).
References and links
1. M. A. Schmidt, A. Argyros, and F. Sorin, “Hybrid Optical Fibers: Hybrid Optical Fibers-An Innovative Platform for In-Fiber Photonic Devices,” Adv. Opt. Mater. 4(1), 13–36 (2016). [CrossRef]
2. 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]
3. G. Tao, A. F. Abouraddy, A. M. Stolyarov, and Y. Fink, “Multimaterial Fibers,” Int. J. Appl. Glass Sci. 3(4), 349–368 (2012). [CrossRef] [PubMed]
4. A. C. Peacock, J. R. Sparks, and N. Healy, “Semiconductor optical fibres: progress and opportunities,” Laser Photonics Rev. 8(1), 53–72 (2014). [CrossRef]
5. A. C. Peacock, U. J. Gibson, and J. Ballato, “Silicon optical fibres-past, present, and future,” Adv. Phys. X 1(1), 114–127 (2016).
6. A. C. Peacock and N. Healy, “Semiconductor optical fibres for infrared applications: A review,” Semicond. Sci. Technol. 31(10), 1–12 (2016). [CrossRef]
7. F. A. Martinsen, B. K. Smeltzer, J. Ballato, T. Hawkins, M. Jones, and U. J. Gibson, “Light trapping in horizontally aligned silicon microwire solar cells,” Opt. Express 23(24), A1463–A1471 (2015). [CrossRef] [PubMed]
8. S. Chaudhuri, J. R. Sparks, X. Ji, M. Krishnamurthi, L. Shen, N. Healy, A. C. Peacock, V. Gopalan, and J. V. Badding, “Crystalline Silicon Optical Fibers with Low Optical Loss,” ACS Photonics 3(3), 378–384 (2016). [CrossRef]
9. P. J. Sazio, A. Amezcua-Correa, C. E. Finlayson, J. R. Hayes, T. J. Scheidemantel, N. F. Baril, B. R. Jackson, D. J. Won, F. Zhang, E. R. Margine, V. Gopalan, V. H. Crespi, and J. V. Badding, “Microstructured optical fibers as high-pressure microfluidic reactors,” Science 311(5767), 1583–1586 (2006). [CrossRef] [PubMed]
10. C. E. Finlayson, A. Amezcua-Correa, P. J. A. Sazio, N. F. Baril, and J. V. Badding, “Electrical and Raman characterization of silicon and germanium-filled microstructured optical fibers,” Appl. Phys. Lett. 90(13), 132110 (2007). [CrossRef]
11. W. Yan, T. Nguyen-Dang, C. Cayron, T. D. Gupta, A. G. Page, Y. Qu, and F. Sorin, “Microstructure tailoring of selenium-core multimaterial optoelectronic fibers,” Opt. Mater. Express 7(4), 1388–1397 (2017). [CrossRef]
12. J. R. Sparks, R. He, N. Healy, M. Krishnamurthi, A. C. Peacock, P. J. Sazio, V. Gopalan, and J. V. Badding, “Zinc selenide optical fibers,” Adv. Mater. 23(14), 1647–1651 (2011). [CrossRef] [PubMed]
13. N. F. Baril, R. He, T. D. Day, J. R. Sparks, B. Keshavarzi, M. Krishnamurthi, A. Borhan, V. Gopalan, A. C. Peacock, N. Healy, P. J. Sazio, and J. V. Badding, “Confined high-pressure chemical deposition of hydrogenated amorphous silicon,” J. Am. Chem. Soc. 134(1), 19–22 (2012). [CrossRef] [PubMed]
14. S. Morris and J. Ballato, “Molten-core fabrication of novel optical fibers,” Am. Ceram. Soc. Bull. 92(2), 24–29 (2013).
15. 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]
16. J. Ballato, T. Hawkins, P. Foy, S. Morris, N. K. Hon, B. Jalali, and R. Rice, “Silica-clad crystalline germanium core optical fibers,” Opt. Lett. 36(5), 687–688 (2011). [CrossRef] [PubMed]
17. G. Tang, Q. Qian, X. Wen, G. Zhou, X. Chen, M. Sun, D. Chen, and Z. Yang, “Phosphate glass-clad tellurium semiconductor core optical fibers,” J. Alloys Compd. 633(6), 1–4 (2015). [CrossRef]
18. 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]
19. G. Tang, W. Liu, Q. Qian, G. Qian, M. Sun, L. Yang, K. Huang, D. Chen, and Z. Yang, “Antimony selenide core fibers,” J. Alloys Compd. 694(10), 497–501 (2016).
20. X. Ji, R. L. Page, S. Chaudhuri, W. Liu, S. Y. Yu, S. E. Mohney, J. V. Badding, and V. Gopalan, “Single-Crystal Germanium Core Optoelectronic Fibers,” Adv. Opt. Mater. 5(1), 1–6 (2017).
21. N. Healy, M. Fokine, Y. Franz, T. Hawkins, M. Jones, J. Ballato, A. C. Peacock, and U. J. Gibson, “CO2 Laser-Induced Directional Recrystallization to Produce Single Crystal Silicon-Core Optical Fibers with Low Loss,” Adv. Opt. Mater. 4(7), 1004–1008 (2016). [CrossRef]
22. G. Tang, Q. Qian, K. Peng, X. Wen, G. Zhou, M. Sun, X. Chen, and Z. Yang, “Selenium semiconductor core optical fibers,” AIP Adv. 5(2), 027113 (2015). [CrossRef]
23. B. Gates, B. Mayers, B. Cattle, and Y. Xia, “Synthesis and Characterization of Uniform Nanowires of Trigonal Selenium,” Adv. Funct. Mater. 12(3), 219–227 (2002). [CrossRef]
24. H. Yoo, R. A. Wibowo, A. Hölzing, R. Lechner, J. Palm, S. Jost, M. Gowtham, F. Sorin, B. Louis, and R. Hock, “Investigation of the solid state reactions by time-resolved X-ray diffraction while crystallizing kesterite Cu2ZnSnSe4 thin films,” Thin Solid Films 535(1), 73–77 (2013). [CrossRef]
25. R. Lechner, S. Jost, J. Palm, M. Gowtham, F. Sorin, B. Louis, H. Yoo, R. A. Wibowo, and R. Hock, “Cu2ZnSn(S,Se)4 solar cells processed by rapid thermal processing of stacked elemental layer precursors,” Thin Solid Films 535(20), 5–9 (2013). [CrossRef]
26. D. Deng, N. Orf, S. Danto, A. Abouraddy, J. Joannopoulos, and Y. Fink, “Processing and properties of centimeter-long, in-fiber, crystalline-selenium filaments,” Appl. Phys. Lett. 96(2), 023102 (2010). [CrossRef]
27. S. H. Xu, Z. M. Yang, T. Liu, W. N. Zhang, Z. M. Feng, Q. Y. Zhang, and Z. H. Jiang, “An efficient compact 300 mW narrow-linewidth single frequency fiber laser at 1.5 microm,” Opt. Express 18(2), 1249–1254 (2010). [CrossRef] [PubMed]
28. G. Tang, Q. Qian, X. Wen, X. Chen, W. Liu, M. Sun, and Z. Yang, “Reactive molten core fabrication of glass-clad Se0.8Te0.2 semiconductor core optical fibers,” Opt. Express 23(18), 23624–23633 (2015). [CrossRef] [PubMed]
29. M. Kawarada and Y. Nishina, “The Structure and Crystallization of Amorphous Se,” Jpn. J. Appl. Phys. 14(10), 1519–1528 (1975). [CrossRef]
30. A. Umehara, S. Nitta, H. Furukawa, and S. Nonomura, “Preparation and properties of nanocrystalline semiconductor selenium films,” Appl. Surf. Sci. 119(1), 176–180 (1997). [CrossRef]
31. K. Yang, Q. Cui, Y. Hou, B. Liu, Q. Zhou, J. Hu, H. K. Mao, and G. Zou, “Pressure-induced crystallization and phase transformation of amorphous selenium: Raman spectroscopy and x-ray diffraction studies,” J. Phys. Condens. Matter 19(42), 425220 (2007). [CrossRef]
32. J. L. Hartke, “Drift Mobilities of Electrons and Holes and Space-Charge-Limited Currentsin Amorphous Selenium Films,” Phys. Rev. 125(4), 1177–1192 (1962). [CrossRef]
33. 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]
