Evolutive Optical Fibers Combining Oxide and Chalcogenide Glasses with Submicronic Polymer Structures

Multimaterial optical fibers combining tellurite with chalcogenide glasses and featuring thin polymer structures are fabricated via the thermal drawing process. It is demonstrated that micrometric polyethersulfone films can be embedded within larger elongated tellurite/chalcogenide glass architectures. Taking advantage of the strong chemical reactivity contrasts which exist in the considered fiber geometries, a quasi‐exposed‐core waveguide is obtained by selective etching of the glass cladding. The potential of the postprocessed fiber structure is then assessed through evanescent‐wave probing of liquids and numerical investigations are carried out to establish the device performances as function of selected optogeometric parameters. Those results open the way for the development of evolutive photonic objects benefiting from postdrawing processing of multimaterial fibers.


Introduction
In recent years, multimaterial fibers have enabled the development of a large number of novel photonic devices with tremendous innovative potential in numerous domains, such as biophotonic, environmental science, optoelectronics, remote sensing, medicine, and so on. [1][2][3][4][5] The elaboration of such small-scale objects relies on the ability to combine through the thermal drawing process several materials (glasses, metals, polymers, semi-conductors, etc.) and, by extension, several functionalities within the same elongated structure. [6][7][8] From a fabrication standpoint, new materials associations are always sought to produce ever more original fiber architectures, thus solving modern technological challenges. [9][10][11] Glass/polymer combinations offer, for instance, great opportunities for the design and fabrication of functional optical fibers. In particular, the elaboration of fibers in which organic and inorganic materials are shaped together at the micro-or even at the nanometer scale provides promising prospects for infrared light-transmission, sensing, energy harvesting, and others. [12,13] Another strong advantage of the combination of those diverse materials is the creation of highly heterogenous fiber geometries composed of domains with significant physicochemical property disparities. Thermoplastics and inorganic amorphous materials have, for instance, very specific chemical reactivities toward solvents or acids, and different processes such as redox reactions, dissolutions, etching, spatial segregation, and so on could be selectively triggered in fibered structures possessing strong compositional contrasts. Glass-polymer assemblies appear therefore useful for the implementation of simple and targeted postprocessing procedures that would provide additional purpose to the fibers.
To this end, we here demonstrate the development of evolutive glass fibers integrating thin polymer structures. First, a simple multimaterial sandwich-like architecture is produced to assess the fiber drawing ability of the proposed polymer/oxide glass preform profiles. Our approach appears original, especially when only few works in the literature report on the association of thermoplastics with nonchalcogenide glasses for fiber development. [14] It is found that large surface area polymer films with micrometric thickness can be successfully embedded within larger oxide-glass structures through the thermal drawing process and without compromising the organic material physicochemical integrity. This enables the production of elongated geometries in which specific domains are delimited by a polymer inclusion, which greatly simplifies postdrawing functionalization procedures. As a next step, a more sophisticated and functional fiber architecture which integrates a waveguiding core is considered. The fiber profile consists in a regular cylindrical core/cladding structure, except that the central chalcogenide core is wrapped within a thin polyethersulfone (PES) film with DOI: 10.1002/adpr.202200234 Multimaterial optical fibers combining tellurite with chalcogenide glasses and featuring thin polymer structures are fabricated via the thermal drawing process. It is demonstrated that micrometric polyethersulfone films can be embedded within larger elongated tellurite/chalcogenide glass architectures. Taking advantage of the strong chemical reactivity contrasts which exist in the considered fiber geometries, a quasi-exposed-core waveguide is obtained by selective etching of the glass cladding. The potential of the postprocessed fiber structure is then assessed through evanescent-wave probing of liquids and numerical investigations are carried out to establish the device performances as function of selected optogeometric parameters. Those results open the way for the development of evolutive photonic objects benefiting from postdrawing processing of multimaterial fibers.
submicromter thickness and embedded in a tellurite clad. Selective etching of the cladding is performed to produce quasi-exposed-core fibers. The purpose of the polymer is then twofold: 1) it preserves the chalcogenide glass core from the hydrochloric acid treatment carried out to remove the oxide glass cladding, and 2) it provides mechanical protection for the etched portion of the fiber. The resulting fiber device is then tested in a proof-of-principle sensing experiment based on evanescent-wave spectroscopy of liquids. Numerical investigations of modal properties are also carried out to further optimize the proposed fiber geometry and improve its sensing performances through interactions between the evanescent field and an analyte. We believe that small footprint optical detection systems could benefit from the use of such unique and evolutive photonic objects.

Materials Selection
The different materials considered are listed in Table 1. A heavy oxide glass of composition (in mol%) 60TeO 2 -20ZnO-20Na 2 O (TZN) is selected as cladding material, while a chalcogenide Ge 15 Sb 20 S 65 glass (GeSbS) with higher refractive index is selected as core material. Both glasses possess comparable glass transition temperatures (respectively 260 and 250°C) and do not exhibit any crystallization event below 400°C. This attests for their good thermal stability, which is essential for thermal drawing. Additionally, a polymer material, namely, PES, is considered here because of its relatively high T g of %225°C, which is one of the highest among amorphous polymers suitable for fiber drawing and makes it appropriate for codrawing with the selected glasses. The glass transition temperature T g , as well as the crystallization temperature T x and the drawing temperature range T draw of the different materials involved in the composite fibered structures are given in Table 1. [13,[15][16][17][18]

Thin Polymer Film Embedded within a Glass Fiber Structure
As a first experiment, a glass/polymer elongated arrangement is fabricated by the preform-to-fiber drawing method, as depicted in Figure 1a. It consists of a thin polymer film embedded within a rectangular glass structure. First, glass and polymer parts are selected and shaped (cutting and polishing) adequately. A 0.1 Â 10 Â 75 mm PES film is placed between two 2.9 Â 10 Â 75 mm TZN glass plates to form the preform.
The assembly is then consolidated at 280°C for 1 h to ensure good adhesion between the different polymer and glass parts. A moderate pressure (few kPa) is applied on the stack by means of ceramic weights during this procedure. Following, the preform is thermally stretched down into meters-long composite fiber. Cross-sectional view of the fabricated structure is shown in Figure 1b. The rectangular TZN glass frame has outer dimensions of %175 Â 300 μm (see Figure 1b), while the PES sheet possesses a %3.0 μm thickness (see Figure 1c). For the sake of clarity, a diagram describing the fabricated fiber is given in Figure 1h. A thin polymer inclusion has been successfully embedded within the TZN glass structure. Remarkably, the organic film possesses a constant thickness across the whole fiber width (i.e., no significant deformation undergone during thermal drawing). Additionally, the initial preform profile is fully transferred to the fiber as the ratio between the polymer thickness and the glass external dimensions is preserved during the stretching operation. Glass-to-polymer thickness ratio is 5.9/0.1 ¼ 59 on the preform and 175/3.0 ¼ 58.3 on the fiber. Conservation of the initial profile is ensured by finely controlling the materials viscosity during thermal drawing of the preform which is elongated in a homothetic fashion.
Further compositional investigations were carried out through energy-dispersive X-ray (EDX) spectroscopy analysis. Sulfur is targeted for the mapping of zones related to PES inclusions while tellurium helps to identify regions which corresponds to the TZN glass. For clarity, spatial distributions of sodium, zinc, and carbon are not presented here. The 2D distributions at the fiber cross section of tellurium and sulfur are shown in Figure 2c,d, respectively. No contamination nor interdiffusion potentially occurring at the vicinity of the glass/polymer interface could be evidenced from those measurements. Additionally, the distributions of Te and S atoms along a line perpendicular to the glass/polymer/glass structure are plotted in Figure 2e. The repartition of tellurium and sulfur is symmetrical with respect to the median plane of the polymer film. This confirms that no reaction occurs between the glass and the polymer during the temperature-based elongation of the preform. This original approach demonstrates the possibility to manufacture complex fiber architectures which involve for the first time low T g oxide glasses and polymers. Glass/polymer associations are generally restricted to the combination of high-performance polymers (such as polysulfones) with chalcogenide glasses, mainly due to thermal compatibility issues. [12,13] With few exceptions, [14] oxide-based glasses have either too high glass transitions temperatures or too poor thermal stability to be thermally stretched with organic materials. As the TZN parts represent %97% of the volume of the composite preform considered here, its thermal stretching is performed at the glass drawing temperature (300-325°C), which is approximately 25°C higher than that of PES (see Table 1). To ensure that the polymer did not suffer any degradation during its stretching, postdrawing integrity of PES is assessed through differential scanning calorimetry (DSC). DSC measurements are carried out on bare polymer ribbons which are obtained by immerging %10 cm segments of the composite fiber in concentrated hydrochloric acid (HCl % 6 mol L À1 ) for few minutes. This step helps to dissolve the glassy parts of the fiber, as shown in Figure 2f. The bare polymer samples are then thoroughly rinsed in clean deionized water to get rid of HCl and subsequently dried in a hoven set at 120°C for 30 min. Images at of the obtained polymer ribbons are provided in Figure 2g,h. DSC measurements are then carried out on those samples and compared to the initial PES film involved in the preform fabrication. The results are recapped in Figure 2i. Both DSC curves of PES, prior and after thermal drawing, exhibit an endothermic event around 222°C (223.1 and 220.0°C for the initial and the drawn PES film, respectively) corresponding to their glass transition temperature. The comparative differential scanning calorimetry does not reveal any deviation of the PES glass transition temperature. Consequently, the PES sheet did not undergo any degradation during the fiber drawing and its physicochemical integrity is preserved.

Fiber Design
A more sophisticated fiber architecture is now produced using a similar fabrication method as the one described in the previous section. The preform consists this time in two TZN half-cylinders (9 mm outer diameter) with a groove on their internal surface. The glass parts are processed so that the groove is parallel to the main axis of the half-cylinders. A 0.98 mm chalcogenide cylindrical glass cane wrapped within two 50 μm-thick PES sheets is placed between those TZN parts. This means that the chalcogenide glass core is not in direct contact with the tellurite glass cladding. The stack is then annealed at 280°C for 1 h for consolidation purposes, as previously mentioned. Description of the different preform preparation steps is depicted in Figure 3a. The preform is then thermally elongated using a dedicated 3 m-high draw tower. Optical micrographs of the fabricated fibers are shown in Figure 3b.
The overall initial profile is again well preserved throughout the thermal elongation process. The thin polymer structure completely encapsulates the core, keeping it isolated from the external oxide glass cladding. Slight variations of the PES film can, however, be observed. Again, further compositional investigations are carried out using EDX spectroscopy analysis. Here, carbon is targeted for the mapping of zones related to PES www.advancedsciencenews.com www.adpr-journal.com www.advancedsciencenews.com www.adpr-journal.com inclusions while tellurium and germanium help to identify regions which correspond to the TZN and GeSbS glasses, respectively. The 2D distributions at the fiber cross section of germanium, tellurium, and carbon are measured and shown in Figure 4c-e, respectively. Minor deformations of the structure are evidenced, especially in the bottom right part near the GeSbS core. A portion of the polymer film is missing (see Figure 4e), thus provoking a distortion of the core (see Figure 4c) as well as the cladding (see Figure 4d). It is worth noting that the different organic and inorganic compositional domains are well separated and no contamination nor interdiffusion at the vicinity of the different glass/polymer interfaces are detected. EDX distribution profiles along a line running across the core center are given in Figure 4f,g for, respectively, tellurium and sulfur, and for carbon and germanium. The repartition of the different species is symmetrical with respect to the center of the germanium-rich core, which further confirms that the glass/polymer interfaces are not altered during the elongation process. Aside from where the two PES films recombine on each side of the 14.2 μm GeSbS core, the thickness of the organic coating is approximately 0.7 μm and remains fairly unchanged around the chalcogenide glass. Conservation of the initial preform profile is confirmed, namely, core diameter-to-PES film thickness ratios are 0.98/0.05 ¼ 19.60 and 14.2/0.7 ¼ 20.03 for the preform and the fiber, respectively.
This unique combination of diverse materials within a single elongated structure offers interesting opportunities for the development of postdrawing evolutive fiber components.
Here, by taking advantage of the strong chemical reactivity contrast which exists in the present composite fiber geometry, selective etching is performed to produce a quasi-exposed-core waveguide suitable for evanescent-wave probing of liquids. In the literature, exposed core geometries are produced from step index or microstructured fibers which are processed in particular manners. All solid architectures are generally side-polished to reduce the cladding thickness and expose the central core. [19] Air/glass microstructured fibers with exposed core are prepared either from the drawing of specially prepared preforms (with one opened air channel [20] ) or from various postdrawing processing methods of the as-drawn fibers (etching, micromachining, etc. [21][22][23][24] ). Here, the initial fiber is exposed to a solution of %6 mol L À1 hydrochloric acid for few minutes as described in Figure 5b, which completely dissolves the oxide glass used as cladding material. Because of its good resistance toward HCl, the PES thin film remains intact after the acidic treatment and protects the chalcogenide glass core. A 3D diagram of the produced structure is shown in Figure 5a. Additionally, the polymer membrane provides mechanical strength to the etched part of the device, which helps to preserve the light-guiding core from breaking. Pictures of a stripped portion of the present fiber are shown in Figure 5c,d. The GeSbS core is stripped from its oxide glass cladding but is still wrapped within a thin polymer coating, i.e., a quasi-exposed-core fiber. When the polymer film is thin enough, interactions can occur between the evanescent field of the guided mode from the fiber core and an analyte present at the vicinity of the etched part.   Figure 5. a) 3D diagram of a core-cladding composite fiber with a portion of the structure stripped from its TZN glass cladding. b) Description of how the quasi-exposed-core portions of the fiber are obtained from the initial composite structure: fiber segments are exposed few minutes to concentrated hydrochloric acid (HCl % 6 mol L À1 ) on a %2-6 cm zone in order to remove the TZN glass parts. The etched zone is then thoroughly rinsed with clean deionized water to get rid of any excess HCl. c,d) Longitudinal views of the chemically etched zone of the fiber. e) Picture of a bent portion of the chemically etched fiber.
(a) (b) (c) Figure 6. a) 3D diagram of the setup used for the evanescent-wave probing of liquids. b) Transmission spectra of the quasi-exposed-core fiber in air (red line) and in deionized water (blue symbols). Inset: image of the output section of the fiber taken with an IR camera. c) Comparison of the measured absorption spectrum (blue symbols) and the reference absorption spectrum of water (red line). [28] .

Evanescent-Wave Probing of Liquids
In this section, the potential of the above fiber device for evanescent-wave spectroscopy of liquids is assessed. More precisely, transmission of the quasi-exposed-core fiber is tested when the stripped portion is exposed to liquid deionized water. This detection configuration is similar to fiber evanescent-wave spectroscopy (FEWS) sensors, which are based on the interactions between an analyte and the evanescent field of the guided mode from a straight or bent fiber or taper. [25][26][27] The experimental setup used for our proof-of-principle experiment is depicted in Figure 6a. A home-made fiber supercontinuum (SC) source is used as a wideband infrared light source for spectroscopic investigations. The SC is generated through nonlinear propagation of 70 fs pulses at 1560 nm, delivered by a compact fiber laser with nJ-level energy and 80 MHz repetition rate, in a single-mode GeO 2 -doped silica fiber (HLNF). The SC source operates in the 1300-2100 nm range with an average output power of %90 mW. The generated light is then buttcoupled into the glass/polymer fiber using a precision translation stage while the output signal is collected using an InF 3 multimode fiber and sent to an optical spectrum analyzer (OSA). It is worth mentioning that a polarization controller is inserted in the experimental setup to check for any polarization-dependent effects.
In practice, evanescent-wave probing experiments are performed as follows: a reference spectrum is first recorded when the fiber is not exposed to any analyte. Then, the fiber is dipped in a liquid and a second spectrum is recorded. As a proof-of-principle experiment, measurement of liquid deionized water absorption is carried out using the above setup. The related reference and measurement spectra are plotted in Figure 6b (red line and blue symbols, respectively). A strong water-related absorption centered at %1950 nm can be observed. The corresponding absorption spectrum is shown in Figure 6c (blue symbols) and compared to the literature (red line). [28] The measured and reference spectra both exhibit similar shape over the full spectral range, which validates the present system for optical probing of liquid samples.
Numerical investigations are then carried out to analyze the impact of the main physical parameters on the potential of our quasi-exposed-core fibers for evanescent-wave probing of liquids. We considered an ideal cross section of the quasiexposed-core encapsulated by the thin polymer film. Our detailed analysis of modal field and power distribution is only performed for the fundamental mode of our fiber device by means a commercial solver. Despite the multimode nature of the large-core waveguide, we checked that we coupled SC light into the fundamental mode. It is worth mentioning that a multimode analysis could provide further insights into sensitivity enhancement or www.advancedsciencenews.com www.adpr-journal.com limitation of the fiber sensor. [29] In the present glass/polymer fiber architecture, sensitivity of the sensing device is directly related to the portion of evanescent field which interacts with the liquid analyte during light propagation in the quasiexposed-core. The fraction of power outside the waveguiding structure (core þ polymer thin film) η is calculated for the fundamental mode as function of different parameters, as shown in Figure 7. First, evolution of η as function of the wavelength is plotted in Figure 7a. As expected, the evanescent field extends further outside the core at longer wavelengths, for which the mode area is larger. In the meantime, the fraction of power outside the waveguiding structure decreases in a similar fashion when either the polymer film thickness or the core diameter increases, as depicted in Figure 7b,c, respectively. Indeed, reducing the core size is the usual way to maximize the evanescentwave energy fraction (i.e., fiber tapering). [30] In the case of the fiber used for the evanescent-wave probing of deionized water, which has a core diameter d c ¼ 14.2 μm and a PES film thickness e ¼ 0.7 μm, the portion of power probing the analyte is approximately 10 À5 % at 2.0 μm, close to the considered water absorption band. Despite this low value, it was still possible to detect the absorption peak by propagating light through a %7 cm long etched portion of the fiber. Further investigations are needed to fully grasp the potential of the proposed fiber geometry, and reach sensitivity and reusability of state-of-the-art sensors based on multimaterial fibers. [31][32][33] The present numerical investigations provide interesting insight regarding this matter. Significant optimization of the device can be achieved by reducing the core diameter as well as the polymer thickness, as long as capillary break-up conditions are not met. [12,34] For instance, a reduction of the dimensions of the fiber by a factor of two, i.e., producing waveguides with a %7 μm core and a % 0.3 μm-thick PES film (which is feasible via the thermal drawing process), would improve the evanescent power fraction by two orders of magnitudes. The ratio between the core diameter and polymer thickness could also be changed on the initial preform to produce fibers with optimized geometry. The sensitivity could also be improved over several orders of magnitudes when operating the device at longer wavelengths, which is enabled by the considered materials (transmission up to 4.5 and 7 μm on, respectively, tellurite and GeSbS fibers). [17,35]

Conclusion
Multimaterial fiber geometries featuring thin polymer structures have been investigated in this work. The fabrication methodology relies on the simultaneous drawing of tellurite glasses together with thin PES sheets around a chalcogenide glass core. The fiber profile is designed so that portion of the fibered structure can be processed into quasi-exposed-core waveguides through selective etching of the cladding glass. The device is then used in a proof-of-principle experiment which takes advantage of interactions between the evanescent field and a liquid analyte. Absorption spectroscopy measurement is carried out on deionized water from which the 1950 nm absorption band is detected. The efficiency of the exposed-core waveguide as function of wavelength, core diameter, and polymer thickness is numerically investigated for future optimization of the fiber geometry.
Our results confirm that the postthermal-drawing processing of multimaterial fibers which possess high chemical reactivity contrast is promising for the development of all-in-one lab-on fiber solutions. Other glass/polymer associations could be considered for the development of similar structures, as long as the selected materials possess comparable drawing temperature ranges and distinct reactivities toward specific solvents. Besides the improvement of the sensitivity of the system elaborated here, tailoring of the wave-guiding properties (dispersion, effective cross-sectional area, etc.) of the fiber could enable the monolithic implementation of a wideband MIR source based on SC generation. Given the optical assets of the selected materials (transmission window, nonlinear properties, etc.), [36] the spectral broadening of femtosecond pulses would occur within the first few centimeters of the waveguide and the generated light would serve as the incident source. [37] Surface functionalization of the polymer in which the glass core is wrapped could also be looked at, [38] and would provide selectivity toward targeted compounds contained in body fluids, waste water, or other liquid analytes of interest. Ultimately, extremely compact monolithic biosensors and innovative photonic devices could be produced from the assemblies proposed here.

Experimental Section
Glass Synthesis: Oxide-based glass of composition (in mol%) 60TeO 2 -20ZnO-20Na 2 O (TZN) was prepared using the standard meltquenching technique. High purity precursors (99.99%) were weighed in adequate ratios and were melted inside a platinum crucible placed in an electric furnace. Melting was carried out under room atmosphere for 30 min at 800°C. The hot liquid was then poured into a brass mold preheated at 210°C. The shape of the mold was chosen to produce either cylindrical (9 mm diameter and %6 cm long) or parallelepipedal preforms (with dimensions of %3 Â 10 Â 75 mm). Following, the glass was annealed at 250°C for 5 h to relieve any residual mechanical stress caused by the quenching. Finally, the temperature was gradually ramped down (2°C min À1 ) to room temperature. Chalcogenide glass samples of composition Ge 15 Sb 20 Se 65 were prepared by the melt-quenching technique. High purity (99.999%) elemental raw materials were weighed according to the desired stoichiometry and placed inside a silica setup previously cleaned using hydrofluoric acid (HF). The batch was then placed under secondary vacuum (10 À8 bar) for 24 h to remove any moisture adsorbed at the surface of the precursors or the silica recipient. The ampoule was then sealed and introduced in a rocking furnace and heated up to the refining temperature of 850°C for 12 h. After this, the molten batch was rapidly cooled down in a 25°C water bath to obtain the glass. Finally, the glass was annealed at 240°C for 5 h after which the temperature was slowly ramped down (2°C min À1 ) to room temperature.
Preform Preparation and Fiber Drawing: The preforms were fabricated by stacking different glass and polymer parts which were manufactured through the following procedures. TZN glass parallelepipedic parts were straight forwardly obtained by carefully polishing rough glass pieces produced using a parallelepiped mold for the quenching. Grooved half-cylinders were manufactured from 9 mm diameter and 60 mm-long cylindrical TZN glass rods in which a 1 mm diameter and 30 mm long hole was mechanically drilled at their center, forming a tube. After that, the tube was longitudinally polished to half its volume to produce a half cylinder with a central groove. %1 mm. GeSbS glass canes were also used in the preform fabrication procedure. They were obtained from the thermal elongation of cylindrical GeSbS glass preforms with a diameter of 12 mm. PES films of chosen dimensions were cut from an initial 50 μm-thick sheet. The desired thickness of polymer in the preform was obtained by stacking the adequate number of the initial PES films (one layer for 50 μm, two layers for 100 μm, etc.). The adequate glass and polymer parts were selected depending on the targeted profile and the different elements were then stacked to form the preform. After that, a 2-h consolidation treatment of the preform was performed at 280°C. During this procedure, a few kPa pressure was applied to the macroscopic arrangement by means of ceramic weights. Finally, the preform was placed inside the furnace of a draw tower of which the temperature is slowly ramped up (10°C min À1 ). To avoid any contamination, a 3 L min À1 He gas flow was applied during the procedure. After the initiation of the elongation process, the preform was slowly fed into the furnace while the drawing parameters were continuously monitored to produce the fiber with the desired dimensions.