Chalcogenide glasses are a promising group of materials for remote sensing applications. Two compositions from the Ge-Sb-Se/S system are investigated as core and cladding glasses in a step index fiber (SIF). Following thermomechanical and refractive index measurements, mid-infrared (MIR) light guiding is demonstrated through an 8 m length of SIF with a Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding. Using a single distillation procedure, Ge20Sb10Se70 at. % glass fibers are shown to have low optical loss across the 2 to 10 µm wavelength range with the lowest baseline loss shown as 0.44 dB/m at 6.4 µm.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
The mid-infrared (MIR) region of light, found between 3-25 µm wavelength , covers an important diagnostic tool known as the molecular fingerprint . Due to the strong vibrational resonances of almost all organic species, MIR light offers a unique opportunity to study the key building blocks of biological tissues via a non-destructive spectroscopy technique . This is particularly promising for several remote sensing applications, not least towards the in-vivo diagnosis of early-stage cancer. Since MIR absorption bands can be five orders of magnitude stronger than the overtones and combination vibrational absorption bands of the NIR region , it is easier to distinguish between the relevant and competing MIR features. Consequently, a raft of new MIR photonic components is now being developed, including passive optical fibers which will form the basis of an imaging probe.
As one of only a few materials transparent to MIR light, chalcogenide glasses have the additional benefit of being vitreous in nature i.e. making it possible for them to be fiber-drawn. Based on one or more of the chalcogen elements (S, Se or Te) , the ternary Ge-Sb-Se chalcogenide system is of particular interest for medical applications, as it is thought to be less toxic than its arsenic-containing counterpart . Defined by 5-35 atomic % (at. %) Ge, 5-40 at. % Sb and the remainder Se , Ge-Sb-Se compositions exhibit robustness, relatively good thermal, mechanical and chemical properties and transparency across the 2-16 µm wavelength region [7–10]. It has already been shown that Ge-Sb-Se glasses, including those from the GexSb10Se90-x at. % system, are best described by their chemically ordered network model (CONM) , which often exhibits an extremum for a number of properties, such as glass transition temperature (Tg), density and refractive index (n) . Due to this chemical threshold, it is often difficult to identify two compositions which have appropriate thermal and optical properties, particularly if a low numerical aperture (NA) is desired. Ou et al.  and Zhang et al.  recently demonstrated successful coupling of Ge-Sb-Se/ Ge-Sb-Se and Ge-Sb-Se/Ge-Se (core/cladding) glasses, respectively, for supercontinuum generation. However, both SIFs had a high NA. For applications that require lower NA, Wang et al.  and Guery et al.  have shown that the substitution of Se for S, has little effect on the atomic glass structure and that any observed changes in physical properties are caused primarily by a difference in -S- and -Se- bond strengths. Consequently, with increasing S content in the Ge-Sb-Se/S glass system, both the linear and nonlinear refractive indices decrease, whilst the Tg and optical band gap increase .
For the chalcogenide optical fibers to perform at their optimum, they must be prepared with high purity. However, the MIR region of light also embraces a number of absorption bands relating to atmospheric impurities such as oxygen, hydrogen and carbon [16–18]. Therefore, to achieve low optical losses in Ge-Sb-Se/S SIFs, it is necessary to synthesize the glasses from high purity starting elements with subsequent distillation, all under high vacuum. In addition to the constituent elements, gettering chemicals such as Al and TeCl4 are often introduced during the fabrication of high-purity chalcogenide glasses [19–21], so that [O] and [H] impurities are minimized, respectively.
This paper reports an initial investigation of two compositions from the Ge20Sb10Se70-xSx at. % system and assesses their suitability to act as core and cladding glasses in a multimode, large core SIF. A distillation procedure was investigated, specifically for the core glass composition. In section 2 the experimental methods of glass melting and annealing are set out, as well as the distillation technique and methods of characterization of the glasses in terms of XRD (X-ray diffractometry), imaging and analytical SEM (scanning electron microscopy), and thermal analysis to measure: glass transition temperatures and the temperature coefficients of viscosity. The approach to measuring refractive index dispersion of the glasses is reported, together with the fiber drawing process and method of fiber loss characterization. In section 3 the results are discussed in terms of numerical aperture and zero dispersion wavelength of the SIF and the effect of glass distillation on fiber optical loss.
2.1 Glass melting and annealing
All of the Ge-Sb-Se/S glasses investigated in this paper, were prepared via the traditional melt-quench technique . Each chalcogenide glass was made within a vitreous silica ampoule (Multilab, UK), which was initially HFaq. (aq. is aqueous) etched and annealed to remove residual stress. Carbonaceous deposits and/or physi-sorbed or chemi-sorbed water on the silica ampoule’s internal surface were also removed, by air and then vacuum baking at 10−5 Pa at 1000 °C for 6 hours each. Prior to batching, all Ge-Sb-Se/S glass melting was begun by purifying elemental Se (99.999%; Materion, ABSCO Materials), Sb (99.9999%, Cerac, ABSCO Materials) and S (99.999% Materion, ABSCO Materials) via a bake-out procedure to remove surface impurities. Although it may be possible to purify the surface of elemental Ge through techniques such as acid washing (although this may leave hydrogen impurities), it is technically difficult to purify Ge through the bake-out procedure since elemental Ge and Ge-oxides have low, and similar vapor pressures [22,23]. Therefore, as-bought Ge (99.999%, Materion, ABSCO Materials) was batched, with purified Se, Sb and S, straight into the prepared silica glass ampoule. All weighing and batching was carried out inside a dry nitrogen glovebox (MBraun 150B-G, ≤0.1 ppm O2 and ≤0.1 ppm H2O). For the Ge-Sb-Se glass used to investigate high-purity unstructured optical fibers, 1000 wt. ppm TeCl4 (Alfa Aesar, UK) was also batched at this stage. Using an oxygen-propane (BOC gas) torch (Jencons’ Junior Jet 7, UK), the silica glass ampoule and its contents were sealed under a vacuum of 10−3 Pa. Chalcogenide glass melting was carried out in a modified rocking furnace (Instron), during which the raw materials were slowly heated to 900 °C and homogenized for 24 hours. After cooling to 700 °C, the ampoule was removed from the furnace and the Ge-Sb-Se/S melt was quenched in situ, inside the ampoule, using an external nitrogen (BOC) gas flow jetted at the exterior of the ampoule surface to cool its contents. Immediately after quenching, the ampoule, containing the Ge–Sb–Se/S product, was annealed at its onset-Tg from differential scanning calorimetry (DSC)  for 0.5 hours and then allowed to cool to room temperature.
2.2 Ge-Sb-Se distillation
Following the initial melting and annealing of homogenized Ge-Sb-Se + 1000 wt. ppm TeCl4, this chalcogenide glass boule was broken into approximately 2-5 mm diameter pieces and batched into the charged end of a silica glass distillation rig (Multilab, UK), which had been prepared via the same procedure outlined in section 2.1. Once again, batching was carried out within the dry nitrogen glovebox. At the same time, 700 wt. ppm Al (Alfa Aesar, UK) was also batched with the Ge-Sb-Se + 1000 wt. ppm TeCl4 pieces so to act as a [O] getter during subsequent heating. Through careful temperature control, the charged end of the silica glass distillation rig was heated to approximately 700 °C. Ge-Sb-Se glass distillation occurred in an open system, under a drawing vacuum.
2.3 Glass characterization
X-ray diffractometry (XRD) was used to analyze the amorphous nature of the chalcogenide glasses using a Siemens D500 diffractometer, with CuKα radiation in the 2θ range 10°- 70°, and with a step size of 0.05°; 40 seconds was spent at each step, resulting in a run time of 13.2 hours. The onset-Tg was measured using differential scanning calorimetry (DSC) on 20 mg of as-annealed Ge-Sb-Se/S chunks, with dimensions ≤2 mm, in sealed aluminum pans. Each DSC sample was run 3 times in a Perkin Elmer Pyris 1 DSC analyzer, under flowing Ar gas and with a heating and cooling rate of 10 °C/min. The mean onset-Tg value (to ± 2 °C) was calculated from both the second and third runs. Viscosity-temperature measurements were carried out using a modified parallel plate technique, in a Perkin Elmer TMA7 thermomechanical equipment with flowing He (BOC) at 20 ml/min. A constant load of 50 mN was applied to samples with dimensions of 1.6 mm height and 4.7 mm diameter, whilst 400 mN was applied to samples with dimensions of 4.1 mm height and 4.7 mm diameter. Scanning electron microscopy (SEM), conducted on a FEI Philips XL30 SEM, was used to image Ge-Sb-Se/S samples under the backscattered electron (BSE) mode. When combined with energy dispersive X-ray spectroscopy (EDX), using Oxford Instruments INCA Energy software, the compositions of carbon-coated Ge-Sb-Se/S samples were measured with an accuracy of approx. ±0.5 at. % for elements heavier than oxygen.
2.4 Fiber-drawing and characterization of Ge-Sb-Se/S glass fibers
All of the Ge-Sb-Se/S optical fibers presented in this paper, were drawn from bulk glass preforms, which had been prepared via various techniques. Unstructured Ge-Sb-Se fibers were directly drawn from as-annealed glass rods (∼70 mm long and 10 mm diameter). Unstructured Ge-Sb-Se-S fiber was drawn from a 4.7 mm diameter, ∼90 mm long glass rod which had been extruded from an as-annealed 15 mm diameter boule . Ge-Sb-Se/S SIF was drawn from a co-extruded 10 mm diameter preform  comprising a Ge-Sb-Se core and Ge-Sb-Se-S cladding. Both the Ge-Sb-Se core and Ge-Sb-Se-S cladding glasses used for this co-extrusion, were as-annealed 29 mm diameter glass boules which had been polished to a 1 µm finish on the mating surfaces. Extrusions were carried out using an in-house built, vertical extruder, under flowing N gas. Fiber-drawing took place on a customized Heathway draw-tower (housed in a class 10,000 cleanroom), during which the draw-zone was purged using nitrogen gas (BOC) which had been freeze-dried as it traveled through a copper-coil immersed in liquid nitrogen at 77 K. The chalcogenide preforms were softened and drawn to fiber using a short graphite ring-receptor, which was heated by radio frequency from a copper coil.
Optical fiber loss measurements were conducted on fiber samples ∼7-18 m in length and 200 ± 10 µm diameter, via the cut-back method . Using two set-ups, the optical fiber attenuation was first measured using an InSb cooled detector (JUDSON, UK) in the 2-5.5 µm wavelength range and then an MCT (HgCdTe) cooled detector (Kolmar Technologies, UK) in the 5-10 µm wavelength range. Both measurements were made with a Globar source and a KBr beam splitter. So that the peaks of impurity bands could be estimated, optical fiber loss measurements were also conducted on a shorter length (approx. 0.5 m) of the same fiber under investigation.
Thin film refractive index samples were prepared from 20 mm lengths of 200 ± 10 µm diameter unstructured Ge20Sb10Se70 at. % and Ge20Sb10Se67S3 at. % optical fibers, which had been hot-pressed between two tungsten carbide plates, under vacuum (10−4 Pa) at Tg +40 °C (viscosity 108 Pa.s) with a maximum pressure of 700 N. Thin films were annealed at the onset-Tg and allowed to cool in-situ. Using an improved Swanepoel method, with a two-term Sellmeier model, refractive index measurements were found across the 2-25 µm MIR region and had an accuracy of <0.4% and standard deviation of precision < ±0.002. This accuracy was determined using a 3.1 µm interband cascade laser (ICL) [27,28] fabricated at NRL (which emitted >200 mW cw at 25 °C) by comparison with a benchmark refractive index value obtained from prism measurements on Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding glasses. Both the thin film and prism refractive index samples were taken from the same batch. A full description of the improved Swanepoel method can be found elsewhere .
3. Results and discussion
3.1 Ge-Sb-Se/S step-index fibers
Following the work by Parnell et al. , prospective core and cladding compositions were chosen from the Ge20Sb10Se70-xSx atomic % (at. %) chalcogenide glass system. With an overall aim of producing low NA SIFs, for greater selectivity in a future MIR imaging probe application, Ge20Sb10Se70 at. % was selected as the core glass and Ge20Sb10Se67S3 at. % as the cladding. For the Ge20Sb10Se70-xSx at. % glasses specifically relevant to this work, it was predicted that a 3 at. % substitution of Se for S would result in a 3 ± 1 °C increase in Tg and a 0.02 ± 0.01 decrease in refractive index . Well matched Tgs are required for the co-processing of core and cladding glasses to fabricate the SIF. Using DSC analysis, the onset-Tg of each composition was measured and is presented in Table 1. From this Table, the Ge20Sb10Se70 at. % core glass exhibits an onset-Tg of 214 °C, while the Ge20Sb10Se67S3 at. % cladding glass has an onset-Tg of 224 °C. This ΔT of 10 °C was larger than the predicted 3 °C, yet is particularly promising when considering the very large change in onset-Tg values when 5 at. % Se is substituted for Ge in the Ge-Sb-Se ternary glasses, viz.: Ge15Sb10Se75 at. % (160.1 °C), Ge20Sb10Se70 at. % (214 °C) and Ge25Sb10Se65 at. % (316 °C) .
As well as studying the Tg of the core and cladding glasses, thermomechanical analysis was conducted to investigate the viscoelastic behavior associated with the extrusion and fiber-drawing process. Figures 1(a) and (b) present the full viscosity-temperature curves for the Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding glasses, respectively; the average viscosity-temperature results for the extrusion and fiber-drawing processes are presented in Table 1.
Since the quality of the test piece can significantly impact the accuracy of the TMA viscometry results, the Ge-Sb-Se/S samples were carefully prepared from a 4.7 mm diameter, extruded rod, along which several cuts were made perpendicular to the rod axis. For both compositions, three cylindrical samples were cut with heights of 1.6 mm and another three with heights of 4.1 mm. The two sample geometries allowed the full range of the viscosity-temperature curves to be measured, considering sample spreading. Figure 1, presents the viscosity-temperature curves for all six samples. At relevant extrusion (107.5 Pa.s) and fiber-drawing (104.5 Pa.s) viscosities, the temperature was recorded for each sample curve. Therefore, the values presented in Table 1 are an average result calculated from three samples. Corresponding errors indicate the associated standard deviation. Since the errors were found to be small, they are presented only in Table 1 and not in Fig. 1.
Results showed that the temperature required to reach the estimated extrusion viscosity of 107.5 Pa.s was similar for both the Ge20Sb10Se70 at. % core (274.7 ± 0.4 °C) and the Ge20Sb10Se67S3 at. % cladding (277.7 ± 0.4 °C) glasses. Likewise, the temperature required to achieve a fiber-drawing viscosity of approx. 104.5 Pa.s was also close, with a ΔT of 4 °C between Ge20Sb10Se70 at. % core (357.8 ± 0.7 °C) and the Ge20Sb10Se67S3 at. % cladding (361.9 ± 0.3 °C). These results suggest that the two prospective compositions have similar thermal coefficients of viscosity above Tg and should, therefore, successfully co-extrude.
A final observation is that both viscosity-temperature curves show a slight deviation from linearity, at 339 °C for the Ge20Sb10Se70 at. % core and at 329 °C for the Ge20Sb10Se67S3 at. % cladding. This change in slope indicates a deviation from Arrhenian behavior, and possibly the breaking of different bonds on the atomic level.
In Fig. 2, the refractive index measurements of the Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding glasses are shown from 2 to 25 µm wavelength. Overall, the dispersion curves presented in Fig. 2 show that the substitution of 3 at. % Se for S in the Ge20Sb10Se70-xSx at. % system reduces the refractive index of the glass. At 3.1 µm wavelength, the Ge20Sb10Se70 at. % core thin film had a refractive index of 2.553 whilst the cladding Ge20Sb10Se67S3 at. % thin film had a refractive index of 2.540. With a refractive index difference of approx. 0.01, also observed by Wang et al. , a SIF composed of a Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding would give an NA of 0.25 at 3.1 µm wavelength. Ideally, SIFs in a hyperspectral imaging probe might have a slightly higher NA for superior light gathering. However, tailoring the Ge-Sb-Se/S compositions for higher NA, would in turn tend to increase detrimentally the difference in thermal properties.
Based on the refractive index measurements presented in Fig. 2, the calculated NA for the Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding glasses is presented as a function of wavelength in Fig. 3 (a), along with the material dispersion in Fig. 3(b). Since the MIR long-wavelength fundamental vibrational absorption bands of the Ge20Sb10Se67S3 at. % cladding is at a shorter wavelength than that of the Ge20Sb10Se70 at. % core, the refractive index dispersion of Ge20Sb10Se67S3 at. % decreases faster than the Ge20Sb10Se70 at. %. For this reason, the predicted NA increases with wavelength and from Fig. 3 (a), the lowest calculated NA (0.24) is at 4.7 µm whilst the highest (0.36) is at 23 µm wavelength. From Fig. 3 (b), the Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding thin films have zero dispersion at 6.7 µm and 6.1 µm wavelengths, respectively.
Following these individual investigations of the Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding glasses, both glass melts were combined for co-processing, firstly via a co-extrusion and then fiber-drawing to a 200 ± 5 µm diameter SIF. The optical fiber-loss was measured in an ∼8 m length of this SIF (see Fig. 4), along with the individual fiber-losses of unclad Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding glasses for comparison. Since none of the chalcogenide glasses had been purified beyond an initial bake-out procedure, relatively large impurity bands were expected and found e.g., the Se-H feature at 4.5 µm (10.51 dB/m) and Ge-O at 7.9 µm (19.05 dB/m). However, the absorption bands proved effective in demonstrating successful waveguiding in the SIF, as follows.
When light travels in a SIF, some portion of the optical mode inevitably occupies the cladding due to the evanescent wave phenomenon. Therefore, it was anticipated that traces of the Ge20Sb10Se67S3 at. % cladding impurity bands e.g. S-H at 4.04 µm (42.0 dB/m) and S-O at approx. 8.85 µm (14.8 dB/m), would show in the SIF fiber-loss results. However, as demonstrated in Fig. 4, there is very little difference between the SIF fiber-loss and that of the unstructured Ge20Sb10Se70 at. % fiber. The absence of impurity vibrational absorptions associated with the Ge20Sb10Se67S3 at. % glass suggests that there was a successful mismatch between the refractive indices of the Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding glasses, which promoted efficient propagation through the SIF core, accompanied by relatively weak evanescent field penetration into the cladding glass. The lowest loss for the Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding SIF was measured as 0.72 dB/m at 6.06 µm wavelength.
To confirm the presence of a core-cladding structure, SEM-EDX analysis was conducted on SIF cleaves used for the fiber-loss measurements in Fig. 4. From a backscattered electron SEM image, presented in Fig. 5 (a), two distinct regions can be identified, compromising a large circular core and a 10 µm thick cladding. With an overall SIF diameter of 200 µm, the core to cladding ratio was found to be 95%. Using SEM-EDX, each region was confirmed as a Ge20.4Sb10Se69.6 ±0.5 at. % core and a Ge20.4Sb9.9Se66.5S3.2 ±0.5 at. % cladding i.e. close to that nominally batched. Furthermore, the elemental mapping of S, as presented in Fig. 5 (b), shows a homogenous cladding in the SIF.
3.2 Low optical loss Ge-Sb-Se fibers
To minimize the optical absorption bands associated with oxygen, hydrogen and/or carbon, a distillation procedure was developed. Focusing on the Ge20Sb10Se70 at. % core glass, 1000 ppm wt. TeCl4 and 700 ppm wt. Al were added, to act as [H] and [O] getters, respectively. With adequate temperature control, the Ge20Sb10Se70 at. % glass was shown to distil successfully at temperatures close to 693 °C in an open system, under a drawing vacuum. Following a re-melt at 900 °C for 10 hours the, nominally batched, distilled Ge20Sb10Se70 at. % glass was quenched and annealed, before being drawn into 200 ± 10 µm diameter, uncladded fiber.
The optical fiber loss measurement presented in Fig. 6, for an 18 m section of the distilled Ge20Sb10Se70 at. % glass (solid line), shows remarkably low loss across the 3-8 µm region: 0.62 ± 0.19 dB/m. The lowest baseline loss was measured as 0.44 dB/m at 6.42 µm wavelength. Although a small Se-H absorption can be seen at 4.6 µm, it is considered extremely difficult to remove all hydrogen impurities  and even the purest reported chalcogenide glasses exhibit a small [H] spectral absorption band centered at 4.6 µm [26,32,33]. To the best of the Authors’ knowledge, this result is currently the lowest loss reported to date for Ge-Sb-Se optical fibers. To highlight the improved MIR transparency, the optical loss spectrum for the undistilled Ge20Sb10Se70 at. % optical fiber (presented in Fig. 4), is shown for comparison as the dashed line in Fig. 6. These results show that distilling the Ge20Sb10Se70 at. % glass with [O] and [H] getters significantly reduces the Se-H and Ge-O absorption bands.
Using SEM-EDX, the chemical composition of the distilled Ge20Sb10Se70 at. % glass was confirmed as Ge21.1Sb10.6Se68.3 ±0.5 at. % (cf. as batched: Ge20Sb10Se70). This measurement and standard deviation were based on several EDX points taken at regular intervals along the full 18 m length of the optical fiber. Although the final distilled glass composition did not exactly match that of the nominally-batched Ge20Sb10Se70 at. % glass, the standard deviation of the EDX measurements indicate good homogeneity of the glass.
Two compositions from the Ge20Sb10Se70-xSx at. % glass system have been combined successfully to produce SIFs with low NA. Using DSC and TMA, it was found that the prospective Ge20Sb10Se70 at. % core glass and the Ge20Sb10Se67S3 at. % cladding glass, had similar thermal properties. To achieve an extrusion viscosity of 107.5 Pa.s there was a 3 °C temperature difference between the Ge20Sb10Se70 at. % core glass (274.7 ± 0.4 °C) and the Ge20Sb10Se67S3 at. % cladding glass (277.7 ± 0.4 °C). Likewise, to achieve a fiber-drawing viscosity of 104.5 Pa.s there was a 4 °C temperature difference between the Ge20Sb10Se70 at. % core glass (357.8 ± 0.7 °C) and the Ge20Sb10Se67S3 at. % cladding glass (361.9 ± 0.3 °C). Furthermore, the measured refractive indices of 2.553 (Ge20Sb10Se70 at. %) and 2.540 (Ge20Sb10Se67S3 at. %) at 3.1 µm wavelength gave a predicted SIF NA of 0.25. The zero dispersion wavelengths were also calculated to be 6.7 µm and 6.1 µm for the Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding glasses, respectively. The two compositions were successfully drawn into a large core SIF, with the lowest optical baseline loss measured as 0.72 dB/m at 6.06 µm wavelength. To improve further the optical loss, a distillation procedure was developed specifically for high-purity Ge20Sb10Se70 at. % optical fibers. Using Al as an [O] getter and TeCl4 as a [H] getter, pre-homogenized Ge20Sb10Se70 at. % glass was successfully distilled at temperatures close to 693 °C. Optical loss measurements made on an 18 m length of unstructured, nominally-batched Ge20Sb10Se70 at. % fiber, drawn from the remelted distillate, revealed a remarkably low loss of 0.62 ± 0.19 dB/m across the 3-8 µm region. The lowest baseline loss of 0.44 dB/m, at 6.42 µm wavelength, appears to indicate that the Ge-Sb-Se fiber has the highest purity reported to date.
Engineering and Physical Sciences Research Council (EPSRC) (EP/P013708/1).
This work was supported by the Engineering and Physical Sciences Research Council [grant number EP/P013708/1] through project COOL (COld-cOntainer processing for Long-wavelength mid-infrared fibreoptics). European Cooperation in Science and Technology (EU COST) (MP1401) supported mobility. The Author: H. Parnell acknowledges, with thanks, the financial support of a PhD scholarship from the Faculty of Engineering, University of Nottingham, UK.
1. A. B. Seddon, “A Prospective for New Mid-Infrared Medical Endoscopy Using Chalcogenide Glasses,” Int. J. Appl. Glass Sci. 2(3), 177–191 (2011). [CrossRef]
2. M. J. Baker, E. Gazi, M. D. Brown, J. H. Shanks, P. Gardner, and N. W. Clarke, “FTIR-based spectroscopic analysis in the identification of clinically aggressive prostate cancer,” Br. J. Cancer 99(11), 1859–1866 (2008). [CrossRef]
3. A.B. Seddon, “Mid-infrared photonics for early cancer diagnosis,” in 16th International Conference on Transparent Optical Networks (ICTON), Graz, 2014, pp.1–4.
4. P. W. France, M. G. Drexhage, J. M. Parker, M. W. Moore, S. F. Carter, and J. V. Wright, Fluoride Glass Optical Fibres (CRC Press, Inc, 1990).
5. A. B. Seddon, “Chalcogenide glasses: A review of their preparation, properties and applications,” J. Non-Cryst. Solids 184, 44–50 (1995). [CrossRef]
6. W. H. Wei, L. Fang, X. Shen, and R. P. Wang, “Transition threshold in GexSb10Se90−x glasses,” J. Appl. Phys. 115(11), 113510 (2014). [CrossRef]
7. J. A. Savage, P. J. Webber, and A. M. Pitt, “An assessment of Ge-Sb-Se glasses as 8 to 12µm infra-red optical materials,” J. Mater. Sci. 13(4), 859–864 (1978). [CrossRef]
8. A. R. Hilton and D. J. Hayes, “The interdependence of physical parameters for infrared transmitting glasses,” J. Non-Cryst. Solids 17(3), 339–348 (1975). [CrossRef]
9. M. Frumar, H. Tichá, J. Klikorka, and P. Tomíška, “Optical absorption in vitreous GeSb2Se4,” J. Non-Cryst. Solids 13(1), 173–178 (1973). [CrossRef]
10. M. D. Rechtin, A. R. Hilton, and D. J. Hayes, “Infrared transmission in Ge-Sb-Se glasses,” J. Electron. Mater. 4(2), 347–362 (1975). [CrossRef]
11. H. Parnell, D. Furniss, Z. Tang, N. C. Neate, T. M. Benson, and A. B. Seddon, “Compositional dependence of crystallization in Ge–Sb–Se glasses relevant to optical fiber making,” J. Am. Ceram. Soc. 101(1), 208–219 (2018). [CrossRef]
12. H. Ou, S. Dai, P. Zhang, Z. Liu, X. Wang, F. Chen, H. Xu, B. Luo, Y. Huang, and R. Wang, “Ultrabroad supercontinuum generated from a highly nonlinear Ge-Sb-Se fiber,” Opt. Lett. 41(14), 3201–3204 (2016). [CrossRef]
13. B. Zhang, Y. Yu, C. Zhai, S. Qi, Y. Wang, A. Yang, X. Gai, R. Wang, Z. Yang, and B. Luther-Davies, “High Brightness 2.2-12 µm Mid-Infrared Supercontinuum Generation in a Nontoxic Chalcogenide Step-Index Fiber,” J. Am. Ceram. Soc. 99(8), 2565–2568 (2016). [CrossRef]
14. R. Wang, Q. Xu, H. Liu, Y. Sheng, and X. Yang, “Structure and physical properties of Ge15Sb20Se65-xSx glasses,” J. Am. Ceram. Soc. 101(1), 201–207 (2018). [CrossRef]
15. G. Guery, J. D. Musgraves, C. Labrugere, E. Fargin, T. Cardinal, and K. Richardson, “Evolution of glass properties during a substitution of S by Se in Ge28Sb12S60−xSex glass network,” J. Non-Cryst. Solids 358(15), 1740–1745 (2012). [CrossRef]
16. D. Lezal, “Chalcogenide glasses- survey and progress,” J. Optoelectron. Adv. Mater. 5(1), 23–34 (2003).
17. V. G. Borisevich, V. V. Voitsekhovsky, I. V. Scripachev, V. G. Plotnichenko, and M. F. Churbanov, “Investigation of the influence of extrinsic hydrogen on the optical properties of chalcogenide glasses in the system As-Se,” Vysokochist. Vesch. 1, 65 (1991).
18. M. F. Churbanov, G. E. Snopatin, V. S. Shiryaev, V. G. Plotnichenko, and E. M. Dianov, “Recent advances in preparation of high-purity glasses based on arsenic chalcogenides for fiber optics,” J. Non-Cryst. Solids 357(11-13), 2352–2357 (2011). [CrossRef]
19. E. V. Karaksina, V. S. Shiryaev, T. V. Kotereva, and M. F. Churbanov, “Preparation of high-purity Pr(3+) doped Ge–Ga–Sb–Se glasses with intensive middle infrared luminescence,” J. Lumin. 170(1), 37–41 (2016). [CrossRef]
20. V. Shiryaev and M.F. Churbanov, “Preparation of high-purity chalcogenide glasses,” Chapter 1 in Chalcogenide Glasses, J.L Adam and X. Zhang, eds. (Woodhead Publishing Limited, 2014).
21. G. E. Snopatin, V. S. Shiryaev, V. G. Plotnichenko, E. M. Dianov, and M. F. Churbanov, “High-purity chalcogenide glasses for fiber optics,” Inorg. Mater. 45(13), 1439–1460 (2009). [CrossRef]
22. W. L. Jolly and W. M. Latimer, “The Equilibrium Ge(s) + GeO2(s) = 2GeO(g). The Heat of Formation of Germanic Oxide,” J. Am. Chem. Soc. 74(22), 5757–5758 (1952). [CrossRef]
23. S. Pizzini, Physical Chemistry of Semiconductor Materials Processing (John Wiley & Sons, Ltd.2015), Chap. 5.
24. D. Furniss and A.B. Seddon, “Thermal Analysis of Inorganic Compound Glasses and Glass-Ceramics,” Chap. 10 in Principles and Applications of Thermal Analysis, P. Gabbott, ed. (Blackwell Publishing Ltd, 2008).
25. S. D. Savage, C. A. Miller, D. Furniss, and A. B. Seddon, “Extrusion of chalcogenide glass preforms and drawing to multimode optical fibers,” J. Non-Cryst. Solids 354(29), 3418–3427 (2008). [CrossRef]
26. Z. Tang, V. S. Shiryaev, D. Furniss, L. Sojka, S. Sujecki, T. M. Benson, A. B. Seddon, and M. F. Churbanov, “Low loss Ge-As-Se chalcogenide glass fiber, fabricated using extruded preform, for mid-infrared photonics,” Opt. Mater. Express 5(8), 1722–1737 (2015). [CrossRef]
27. I. Vurgaftman, R. Weih, M. Kamp, J. R. Meyer, C. L. Canedy, C. S. Kim, M. Kim, W. W. Bewley, C. D. Merritt, J. Abell, and S. Höfling, “Interband Cascade Lasers,” J. Phys. D: Appl. Phys. 48(12), 123001 (2015). [CrossRef]
28. M. Kim, C. S. Kim, C. L. Canedy, W. W. Bewley, C. D. Merritt, I. Vurgaftman, and J. R. Meyer, “Recent advances of interband cascade lasers and LEDs,” Proc. SPIE10939 (2019).
29. Y. Fang, D. Jayasuriya, D. Furniss, Z. Q. Tang, Ł Sojka, C. Markos, S. Sujecki, A. B. Seddon, and T. M. Benson, “Determining the refractive index dispersion and thickness of hot-pressed chalcogenide thin films from an improved Swanepoel method,” Opt. Quantum Electron. 49(7), 237 (2017). [CrossRef]
30. T. Wang, W. H. Wei, X. Shen, R. P. Wang, B. Luther-Davies, and I. Jackson, “Elastic transition thresholds in Ge–As(Sb)–Se glasses,” J. Phys. D: Appl. Phys. 46(16), 165302 (2013). [CrossRef]
31. J. A. Harrington, Infrared Fibers and Their Applications (SPIE Press, 2004).
32. B. Zhang, W. Guo, Y. Yu, C. Zhai, S. Qi, A. Yang, L. Li, Z. Yang, R. Wang, D. Tang D, G. Tao, and B. Luther-Davies, “Low loss, high NA chalcogenide glass fibers for broadband mid-infrared supercontinuum generation,” J. Am. Ceram. Soc. 98(5), 1389–1392 (2015). [CrossRef]
33. G. E. Snopatin, M. F. Churbanov, A. A. Pushkin, V. Gerasimenko, E. Dianov, and V. Plotnichenko, “High purity arsenic-sulfide glasses and fibers with minimum attenuation of 12 dB/km,” Optoelectronics and advanced materials-rapid communications 3(7), 669–671 (2009).