Selenium semiconductor core optical fibers

Selenium semiconductor core optical fibers G. W. Tang, Q. Qian, K. L. Peng, X. Wen, G. X. Zhou, M. Sun, X. D. Chen, and Z. M. Yang Citation: AIP Advances 5, 027113 (2015); doi: 10.1063/1.4908020 View online: http://dx.doi.org/10.1063/1.4908020 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mid-infrared Raman sources using spontaneous Raman scattering in germanium core optical fibers Appl. Phys. Lett. 102, 011111 (2013); 10.1063/1.4773884 Effect of bilayer geometry on the diffusion of Ni in amorphous Si and the consequent growth of silicides J. Vac. Sci. Technol. B 30, 061203 (2012); 10.1116/1.4757134 Observation of the Plateau-Rayleigh capillary instability in multi-material optical fibers Appl. Phys. Lett. 99, 161909 (2011); 10.1063/1.3653247 On the fabrication of all-glass optical fibers from crystals J. Appl. Phys. 105, 053110 (2009); 10.1063/1.3080135 Nd 3+ -doped fluoroaluminate glasses for a 1.3 μm amplifier J. Appl. Phys. 87, 2098 (2000); 10.1063/1.372145

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AIP ADVANCES 5, 027113 (2015)

Selenium semiconductor core optical fibers G. W. Tang,1,2,3 Q. Qian,1,2,3,a K. L. Peng,1,2,3 X. Wen,1,2,3 G. X. Zhou,1,2,3 M. Sun,1,2,3 X. D. Chen,1,2,3 and Z. M. Yang1,2,3 1

State Key Laboratory of Luminescent Materials and Devices and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510640, China 2 Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, South China University of Technology, Guangzhou 510640, China 3 Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510640, China

(Received 8 January 2015; accepted 2 February 2015; published online 9 February 2015) Phosphate glass-clad optical fibers containing selenium (Se) semiconductor core were fabricated using a molten core method. The cores were found to be amorphous as evidenced by X-ray diffraction and corroborated by Micro-Raman spectrum. Elemental analysis across the core/clad interface suggests that there is some diffusion of about 3 wt % oxygen in the core region. Phosphate glass-clad crystalline selenium core optical fibers were obtained by a postdrawing annealing process. A two-cmlong crystalline selenium semiconductor core optical fibers, electrically contacted to external circuitry through the fiber end facets, exhibit a three times change in conductivity between dark and illuminated states. Such crystalline selenium semiconductor core optical fibers have promising utility in optical switch and photoconductivity of optical fiber array. C 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4908020]

I. INTRODUCTION

Glass-clad optical fibers comprising semiconductor core have attracted great recent attention for their potential utility as novel waveguides for application in nonlinear optics, sensing, power delivery, optical switch, photodetecting devices and biomedicine.1–6 Over the last few years, impressive progress has been made in the nascent field of semiconductor optical fibers, from the fundamentals though to device demonstration.7–11 Glass-clad amorphous/crystalline unary (Si and Ge)12–16 and crystalline binary (InSb and ZnSe)17,18 semiconductor core optical fibers have been realized using a molten core method or a chemical vapor deposition (CVD). The CVD technique can be easily adapted to deposit various materials into a range of pore size, which can be used to fabricate semiconductor microstructured optical fibers (MOFs).12,13 However, there are some drawbacks of this technique with one of the most notable being the slow deposition rate, meaning that fabricating long lengths of fiber (several tens of meters to kilometers) is not currently possible.7,19 In contrast to the CVD method, the molten core approach is very useful for fabricating long lengths of low loss fiber.7,20 In general, a precursor core phase is set inside a glass tube, which serves as the cladding material.20 Then the glass cladding draws directly into fiber at a temperature where the core precursor phase is molten, which goes “along for the ride” and ultimately solidifies as the fiber cools.6,20 Employed here is a molten core approach to fabricate phosphate glass-clad selenium semiconductor core optical fibers. As an important elemental semiconductor, Se shows many promising properties, such as nonlinear optical response, a high photoconductivity, and a high infrared transparency, which results in their potential applications in electronic and optical electronic devices.21–25 Therefore, Se is a

a Author to whom correspondence should be addressed; electronic mail: [email protected]

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very intriguing core material for glass-clad semiconductor core optical fibers. It can be expected that the optical fibers containing selenium semiconductor core will have large potential utility in optical switch, photodetecting devices, nonlinear optics, sensing and infrared power delivery system.

II. EXPERIMENTS

In this work, Se powder of 99.9 % purity (Aladdin Industrial Corporation, Shanghai, China) was filled in a 8-cm-long phosphate glass tube, with outer diameter about 28 mm and inner diameter of 3 mm. A self-developed multi-component phosphate glasses (55 P2O5-18 K2O-13 BaO-14 Al2O3 wt %) were obtained by conventional melt-quenching method, and then were processed to preform with one end closed.26 And the cylindrical hole in the phosphate glass tube is about 7 cm long. The other end of the cylindrical hole in the preform was sealed after the Se powder filled in under vacuum condition. The continuous Se semiconductor core optical fibers (Se core fibers) were drawn using an optical fiber draw tower under an argon atmosphere at approximately 660◦C. Fibers were cleaved and their cross sections were observed by electron microscopy, which was performed using a Nova Nano SEM430 scanning electron microscope operated at 15 kV and a working distance of 9.2 mm under vacuum atmosphere. Elemental analysis was performed using energy dispersive x-ray spectroscopy (Shimadzu EPMA-1600), which were measured at several locations traversing the core in approximately 1 µm increments, in order to examine the distribution of elements spatially across the core/clad interface. The crystalline phase of the fiber was identified by X-ray powder diffractometer (X’Pert PROX, Cu Ka). The Micro-Raman (Renishaw RM2000) measurements were also as an evidence to identify the crystalline phase of the core. The current of the fibers between dark and illuminated (under illumination from 80 mW white LED) states were recorded using Keithley series 2400 source meter. All measurements were made at room temperature.

III. RESULTS AND DISCUSSION A. Morphological characterization

Fig. 1 provides the electron micrograph image of the phosphate glass-clad Se core optical fibers. As is observed, the fiber has an outer diameter of about 317 µm and inner diameter of 47 µm. There is a good circularity and uniformity as well as being no obvious discontinuities at the core/clad interface. And no obvious cracks or signs of bubble in the Se core fiber can be observed,

FIG. 1. Electron micrograph image of phosphate glass-clad, Se core optical fiber.


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FIG. 2. X-ray diffraction spectra for the Se powder, the core of the as-drawn fiber, and the core of the annealed fiber.

indicating the well-matched coefficients of thermal expansion between Se semiconductor core and the phosphate glass cladding. B. Structural characterization

The X-ray diffraction spectrum from the Se powder and the core of the fibers is shown in Fig. 2. The Se powder was identified to be hexagonal Se phase (JCPDS No. 06-0362) without other phase. It is noted that the core of the fibers was structural amorphous with only two broad diffuse humps detected in the curve. Upon exiting the drawing furnace the fibers rapidly cool and the core of the fibers are quenched into the amorphous state. Consequently, the amorphous Se core optical fibers (a-Se core fibers) can be obtained using a molten core method from the crystalline Se core precursor. As selenium 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 at 150◦C for 1 h.22,27 This temperature is substantially below the glass transition of the phosphate glass cladding (T g ∼ 480◦C),26 and the glass cladding is therefore unaffected by the annealing. The X-ray diffraction curve of the annealed fiber (Fig. 2) exhibits several characteristic diffraction peaks from the core of the annealed fibers, which are readily indexed to the hexagonal Se phase. Therefore, phosphate glass-clad crystalline Se semiconductor core optical fibers (c-Se core fibers) can be attained by a postdrawing annealing process. Fig. 3 presents the Micro-Raman spectra (532 nm excitation) from the core of the fibers and the Se powder. Clearly observed are characteristic peaks of crystalline Se at ∼233 cm−1 and ∼142 cm−1 in both the annealed fiber core and the Se powder. However, the predominant Raman band lies at 250 cm−1 in amorphous Se, which also can be found in the core of the as-drawn fibers.28–30 The X-ray diffraction and the Micro-Raman spectroscopy confirmed that phosphate glass-clad optical fibers containing amorphous Se core as well as crystalline Se core can be realized. C. Properties characterization

Fig. 4 shows the elemental (Se, O, P) profile across the core/clad interface of the a-Se core fibers. The zero relative distance represents the middle of the core. It can be seen that there is some diffusion of Se into the cladding region and, conversely, diffusion of oxygen and phosphorus into the core region. High quality semiconductor core fiber required lowest level of oxygen in the core, because the oxide precipitates could act as defects which will limit the performance of the fibers. The level of oxygen in this a-Se core fibers drawn at 660◦C is about 3 wt %, which is less than that found in Ge core fibers drawn at 1000◦C (4 wt % ),16 Si core fibers drawn at 1950◦C (9.7 wt %),14 InSb core fibers drawn at 700◦C (∼ 9 wt %).17 However, there is a question that the phosphate glasses chosen for the cladding might not seem a particularly appropriate choice of materials


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FIG. 3. Micro-Raman spectra of the Se powder, the core of the as-drawn fiber, and the core of the annealed fiber.

given its lack of transparency at the infrared wavelengths where the a-Se core could function as a waveguide. But, given the large refractive index difference between the a-Se core (refractive index, n ∼ 2.44 at 1310 nm) and the phosphate glasses cladding (n ∼ 1.53 at 1310 nm), very little of the optical power would propagate in the cladding and so the transparency of the cladding is less influential to the overall attenuation.17,24 The propagation loss of the a-Se core fiber at 1310 nm was measured to be 2.6 dB/cm by using the cutback method. With the present a-Se core fibers, losses are much higher, even though the appearance of the fibers and their other properties measurements are comparatively quite well. There are several strategies to mitigate attenuation that will follow in future work. Future fiber draws will employ the highest-purity Se crystal and a more appropriate glass cladding with high transparency at the infrared wavelengths. The two-cm-long a-Se core fiber and the c-Se core fiber can be easily connected to external circuitry by applying silver paint to both ends of a fiber segment, as shown in Fig. 5(a). Fig. 5(b) provides the voltage-current curve of the fibers between dark and illuminated states. As can be seen, there is no current in the a-Se core fibers neither in dark nor illuminated states due to its extremely low conductivity (