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Tb3+ doped Ga5Ge20Sb10Se65-xTex (x = 0-375) chalcogenide glasses and fibers for MWIR and LWIR emissions

Chalcogenide glasses with a nominal composition of Ga5Ge20Sb10Se65-xTex (x = 0, 10, 20, 25, 30, 32.5, 35, 37.5) were synthesized. Their physico-chemical properties, glass network structure and optical properties are clearly modified via the substitution of selenium by tellurium. Based on a detailed study of the Ga5Ge20Sb10Se65-xTexTex bulk glasses properties, the Ga5Ge20Sb10Se45Te20 seleno-telluride glass optimal composition has been selected for fiber drawing. The luminescence properties of Tb3+ (500 ppm) doped Ga5Ge20Sb10Se65 and Ga5Ge20Sb10Se45Te20 bulk glasses and fibers were studied. Radiative transitions parameters calculated from the Judd-Ofelt theory are compared to the experimental values. Mid-wavelength infrared emission in the range of 4.3-6.0 μm is attributed to the 7F5→7F6 transition of Tb3+ ions with a corresponding experimental lifetime of 8.9 and 7.8 ms for the selenide and seleno-telluride matrix, respectively. The 7F4→7F6 
emission was recorded at 3.1 μm with a good signal-to-noise ratio, evidencing a rather strong emission from the 7F4 manifold. Finally, although it was expected that the phonon energy will be lower for telluride glasses, selenide glasses are still more suitable for mid-wavelength infrared and long wavelength infrared emissions with well-defined emissions from 3.1 to 8 μm.


Introduction
Due to their wide infrared transparency and the possibility of incorporation of rare earth ions active in mid-wavelength infrared spectral range, chalcogenide glasses are good candidates to build all optical gas sensors. To detect and quantify gases, one way is to develop chalcogenide glasses presenting transparency compatible with the absorption band frequency of these molecules. Here, two domains of interest can be distinguished: mid-wavelength infrared (MWIR) and long wavelength infrared (LWIR) corresponding to the 3-5 and 8-12 μm spectral ranges, respectively. The expanded wavelength range will enable a variety of commercial and military applications, such as allowing sensors to be tuned to detect atmospheric trace gases for air quality evaluation or hazard alerts. Selenide-and sulfide-based chalcogenide glasses are known for their excellent infrared transmission within the 1-15 µm region with satisfactory thermo-mechanical properties [1]. Doped with Dy 3+ , sulfide glass fibers have been used as MWIR source for gas sensor applied to CO 2 detection [2]. To probe the infrared region beyond 12 μm, telluride glasses appear as an attractive material due to its low phonon energy and broad transparency window (up to 25 µm) [1,3,4]. Despite their interesting optical properties, especially for LWIR, there are only few studies focused on the incorporation of rare earths into telluride glasses. This can be explained by the difficulties to obtain a vitreous network because the Te-based glasses have strong tendency to crystallize which may penalize the incorporation of rare earths and their luminescence properties [5]. To overcome the tendency to crystallization of telluride glasses, one possible way is to add selenium. Indeed, for each glassy system, it is required to determine the best compromise between suitable transparency domain for optical excitation, low phonon energy and glass state stability by optimizing the ratio between selenium and tellurium content. For example, it has been previously shown that with appropriate ratio between Se and Te in As-Se-Te glasses, it is possible to draw fiber with optical attenuation below 1dB/m and a transparency domain between 1.5 and 18 µm [6]. The doping of telluride glass with Tb 3+ ions [7] and Gamodified As 30 Se 50 Te 20 glass with Pr 3+ ions [8] has been reported; however, it only concerned the emissions in MWIR. Following the energy level diagram of Tb 3+ ion, one can expect to have several radiative emissions from 3.1 up to 8 µm. Theoretical work demonstrated that Tb 3+ doped chalcogenide glass is an excellent candidate for laser application at 7.5 µm [9]. For gas sensor applications, this is a wavelength range of interest due to the LWIR absorption band of some hazardous gases [10]. To the best of our knowledge, only two papers report the measurement of the photoluminescence originating from the 7 F 4 → 7 F 5 transition of Tb 3+ ions. The first observation of this radiative emission was confirmed at 7.5 µm for Ge-As-Ga-Se glass doped with 1000 ppm of Tb 3+ [11]. More recently, Starecki et al. published a comprehensive study of the 8 µm fluorescence of Tb 3+ ions incorporated in Ga-Ge-Sb-Se fiber [12]. It is worthy to mention the work of Churbanov et al., who investigated Tb 3+ luminescence (4-5 µm region) in chalcogenide glasses based on As-Se and As-S-Se [13]. They fabricated Tb 3+ doped optical fibers with 1.5 dB/m optical loses at 6-9 µm which implies a low level of impurities. However the Tb 3+ emissions from the 7 F 4 level at 3.1 µm ( 7 F 4 → 7 F 6 ) and at 7.5 µm ( 7 F 4 → 7 F 5 ) were not observed. The highest lifetime of 16.3 ms for the 7 F 5 level was reached with the glass presenting the minimum content of Se-H and S-H entities (43 ppm and 4 ppm respectively). Contrary, for the glass containing the highest impurities concentration (226 ppm for Se-H and 235 ppm for S-H) the lifetime falls to 1.5 ms. Lately, Sojka et al studied the MWIR emission behavior of Tb 3+ doped As-Ga-Ge-Se bulk glasses and conventional fibers. A broad emission band corresponding to the 7 F 5 → 7 F 6 transition was observed at 4.7 µm with a measured lifetime of 12.9 ms (τ rad-JO = 13.1 ms) [14].
In this study we characterize the vitreous network of Ga 5 Ge 20 Sb 10 Se (65-x) Te x (x = 0, 10, 20, 25, 30, 32.5, 35, 37.5) system and investigate the physico-chemical properties of these glasses. First, the influence of substitution of tellurium instead of selenium on the glass properties is evaluated (IR transmission properties, phonon energy, refractive index, density, T g and T x ). Second, we discuss the photoluminescence properties of Tb 3+ (500 ppm) doped Ga 5 Ge 20 Sb 10 Se 65 and Ga 5 Ge 20 Sb 10 Se 45 Te 20 bulk glasses and optical fibers. The experimental luminescence data are compared to radiative parameters calculated via Judd-Ofelt (J-O) theory.

Glass synthesis and basic characterizations
Tb 3+ doped chalcogenide glasses studied in this paper belong to the Ga-Ge-Sb-Se-Te system. The fabricated compositions are Ga 5 Ge 20 Sb 10 Se 65-x Te x with x ranging from 0 to 37.5% undoped or doped with Tb 3+ (500 ppm) for x = 0 and 20. These glasses were prepared by means of conventional melting and quenching method. High purity raw materials were used for glass preparation, e.g. Ge (5N, Umicore), Ga (7N, Alpha), Sb (5N, Alpha), Tb 2 Se 3 (3N, non-commercial); Se (5N, Umicore) and Te (5N, JGI) were further purified by successive distillations (dynamic as well as static distillation) to remove carbon and hydrates impurities. Then, the required amounts of chemical reagents were introduced into silica ampoules inside the gloved box and pumped under vacuum for a few hours. After the pumping, the silica tubes were sealed and heated up to850°C during a total heating process time of 17 hours in a rocking furnace to ensure the homogenization of the melt. After water quenching, the glass rods were annealed near their glass transition temperatures for 3h. The preform of 7 mm in diameter and approximately 10 cm length was drawn in single refractive index fibers with 350 µm diameter.
For the physico-chemical characterizations of bulk glass samples, the remainders of the initial preform after fiber drawing were cut and polished to 5 mm thick disks. Characteristic temperatures (glass transition temperature T g and crystallization temperature T x ) for each glass sample were determined with an accuracy of ± 2 °C using a differential scanning calorimeter DSC 2010 (TA Instruments), with a heating rate of 10 °C/min between room temperature and 380°C. The density of the glasses was measured using Archimedes' principle. The composition of the different samples was checked by using scanning electron microscopy with an energy-dispersive X-ray analyzer (SEM-EDS, JSM 6400 -Oxford Link INCA). The Se-H content in the glasses was determined by IR-spectroscopy exploiting the measured absorption coefficient and known value of the extinction coefficient at 4.5 µm (ε = 1000 dB/km/ppm) [15].

Optical measurements
The ground state absorption measurements were performed with a double-beam Perkin-Elmer spectrophotometer in the wavelength range of 800-3200 nm and a resolution of 1 nm. For bulk glasses, transmission spectra in the IR region were recorded with a Bruker Vector 22 Fourier transformed infrared spectrometer (FTIR) from 1.5 to 22 µm. Fiber attenuation measurements were performed for unclad Tb 3+ doped Ga 5 Ge 20 Sb 10 Se 65-x Te x (x = 0, 20) fibers and undoped Ga 5 Ge 20 Sb 10 Se 65-x Te x (x = 0, 20) by using the cut-back technique [16] with a Bruker FTIR spectrometer modified with fiber coupling ports.
For room-temperature photoluminescence measurements, the pump laser light was either focused into a bulk glass sample with a silica lens or coupled into the terbium doped Ga 5 Ge 20 Sb 10 Se (65-x) Te x (x = 0, 20) fiber. The pump source at 2.05 µm for bulk and fiber experiments was a homemade laser with a Tm 3+ :YAG crystal pumped by a commercial diode at 785 nm. The mid-IR light focused on the monochromator slit was detected by a nitrogen cooled HgCdTe detector. Fluorescence decays were measured using 30 ms pump pulses with a 9 Hz repetition rate. Emission spectra were measured with appropriate long-pass filters and corrected using an Arcoptix MWIR-2.0-9.0 blackbody source.
The structure of Ga 5 Ge 20 Sb 10 Se 65-x Te x (x ranging from 0 to 37.5%) glasses was investigated using a LabRamHR800 (Horiba-Jovin-Yvon) confocal micro-Raman spectrophotometer with 785 nm laser diode coupled to Olympus × 100 microscope. To avoid photoinduced phenomena, optical density filters have been selected to reduce the laser power focused on the glass samples. Infrared measurements were also performed with a FTIR vacuum spectrometer (Bruker Vertex 70V, equipped with Hg arc source) to cover the frequency range from 30 to 1000 cm −1 . The infrared spectra of bulk glasses were recorded in the reflectance mode at quasi normal incidence (11°), and the complex refractive index of each sample was obtained through Kramers-Krönig analysis of its specular reflectance spectrum. The infrared spectra reported in this work are in the form of absorption coefficient spectra, α(ν), calculated from the relation α(ν) = 4πνk(ν) = 2πνε′′(ν)/n(ν), where n(ν) and k(ν) are the real and imaginary parts, respectively, of the complex refractive index, ε′′(ν) is the imaginary part of the dielectric function and ν is the infrared frequency (in cm −1 ).
Linear refractive indices were obtained from the analysis of variable angle spectroscopic ellipsometry (VASE) data measured in NWIR-LWIR spectral range (1-12 µm) (J. A. Woollam Co., Inc., Lincoln, NE, USA). The VASE measurements parameters are as follows: angles of incidence of 65°, 70° and 75°. To derive refractive indices in NWIR-LWIR spectral range, Sellmeier dispersion relation was exploited, setting extinction coefficient in first approximation to zero. X-ray diffraction (XRD) patterns were recorded at room temperature in the 2θ range 15°-120° with a step size of 0.026° and a scan time per step of 400 s using a PANalytical X'Pert Pro diffractometer (PANalytical, Almelo, The Netherlands, Cu K-L2,3 radiation, λ = 1.5418 Å, PIXcel 1D detector). Data Collector and HighScore Plus software packages were used, respectively, for recording and analyzing the patterns.  s, the experim ase of the Te c number) and to g. 1(b)). This ncreasing cont ntent of Te > ns [4]. It is int 0 to 32.5 at.%, C) which con w the absorptio ely. The cut-of e atoms which rgies of the gl Fig. 2(b).) and d-gap in the ne ted in Table 1  The structure of Ga 5 Ge 20 Sb 10 Se 65 glasscan be described by the chemically ordered network model: the Ge-Se, Ga-Se and Sb-Se bonds are the most prominent while Ge-Ge and Ge-Sb bonds were proposed to be formed especially in Se-poor compositions [17].Considering the [GeSe 4/2 ] entities, the IR bands peaking at ~262 and 305 cm −1 are assigned to their antisymmetric stretching modes while the main Raman bands located at 200 and 215 cm −1 have been attributed, respectively, to the localized modes of A 1 symmetric stretching vibration mode of corner-sharing [GeSe 4/2 ] tetrahedra (Td-CS) and A c1 often called "companion" mode, corresponding to the vibration in edge-sharing tetrahedral (Td-ES) [18][19][20]. This attribution was also confirmed by calculation of vibrational normal modes of cluster models by means of first-principles method based on the density-functional theory (DFT) [21][22][23]. Given the closeness of gallium and germanium in terms of atomic weights (Ga: 69.72, Ge: 72.59) and structural units ([Ge(Ga)Se 4/2 ] tetrahedra) involved in selenide glass matrix, it is quite challenging to discern [GeSe 4/2 ] and [GaSe 4/2 ] specific vibrational modes since the concentration of gallium is limited to only 5 at.%. The presence of antimony in Ga 5 Ge 20 Sb 10 Se 65 glass has also a widening effect on the main band compared to the Ge-Se binary glass system. At lower energy side, the [SbSe 3/2 ] pyramids have a symmetric stretching vibration mode at 190 cm −1 . This assignment was also proposed for an IR spectra contribution at 180 cm −1 in Ge-Sb-Se system [24,25]; similarly, the IR band at 200 cm −1 can be attributed to [SbSe 3/2 ] entities vibrations. In Raman spectra, the 235-270 cm −1 spectral range includes the stretching vibration of Se-Se bonds with a wide distribution of vibration modes depending on the nature or length of Se-Se chains. The vibration modes were attributed to (i) Se-Se at the outrigger-raft cluster and/or long Se n -Se chain vibrations (~235 cm −1 ), (ii) A 1 stretching mode of Se 8 ring molecules (245-250 cm −1 ) and (iii) Se n -Se small short chains of [Ge(Ga)Se 4/2 ] or [SbSe 3/2 ] structural units where at least one of the selenium at the tetrahedron or pyramid corner is linked to another selenium(~265 cm −1 ) [20]. The specific stoichiometry of the Ga 5 Ge 20 Sb 10 Se 65 glass results in a relatively low proportion of Se-Se bonds which are involved in short chains as evidenced by the moderate amplitude of the broad band at 265 cm −1 .Polymeric chains of Se can also be observed with a minor band at 135 cm −1 for IR and 138 cm −1 for Raman active modes, according to Lucovsky [26]. The presence of homopolar bonds in this stoichiometric glass is related to atomic disorder and some "wrong" Ge-Ge and Se-Se bonds can still be found in low concentrations. At low-energy side (150-175cm −1 ), the vibration modes are usually connected with the stretching vibration modes of the M-M bonds (M = Ge, Ga, Sb) presumably present in such glass system specially for Se-deficit glasses but also in order to compensate the presence of Se in short chains between two units among [Ge(Ga)Se 4/2 ] and [SbSe 3/2 ] entities [20,27] which is the case in studied selenide glass with the presence of Ge-Ge bonds leading to a contribution at 170 cm −1 . Considering the Ge-Ge bonds vibration modes in [Ge-Ge m Se 4−m ] -including Se 3 Ge-GeSe 3 ethane like -structural units, already observed in amorphous Ge-rich composition, a small wide band centered at ∼270 cm −1 can be expected. Finally, the small Raman band spreading from 285 to 310 cm −1 can be related to the asymmetric stretching modes of Td-CS (305 cm −1 ), more clearly observed in IR spectrum as previously mentioned, with a contribution due to the presence of [GaSe 4/2 ] tetrahedra. In this spectral region, theoretical vibrational modes calculated by DFT are present at 300 cm −1 for asymmetric stretching mode of [GeSe 4/2 ] Td-CS mostly active in IR [21][22][23], but also at 310 and 288 cm −1 for active Raman mode of Td-ES and Se 3 Ge-GeSe 3 ethanelike entities, respectively proposed in [21] or at 311 cm −1 and 285 cm −1 for asymmetric stretching mode of Td-ES more active in IR spectrum [22].

Glass composition and optical properties
The introduction of tellurium instead of selenium with a [Te/(Se + Te)] ratio of between 15.4% and 57.7% progressively affects the IR spectra while a drastic change of the Raman spectra is visible. The latter are shifted very significantly towards lower wavenumbers with a sharp decrease in the intensity of dominant band peaking at 190-215 cm −1 and the disappearance of the band at 265 cm −1 in favor of a wide band that first appears at 170 cm −1 and slips gradually to stabilize at 150 cm −1 for Ga 5 Ge 20 Sb 10 Se 65-x Te x bulk glass with x ranging from 10 to 37.5%. These changes can be interpreted as follows. The contribution of [Ge(Ga)Se 4 ], [SbSe 3 ] and Se-Se entities seem to disappear quickly in favor of a wide band which is first centered at 170 cm −1 and then moves towards 150 cm −1 . We can reasonably assume that the incorporation of tellurium will lead to the formation of mixed entities such as An important point of the vibrational data analysis concerns the non-radiative relaxations of excited Tb 3+ ions via interactions with the phonons of the vitreous matrix. Although the main vibration modes active in Raman spectra become the ones around 150 cm −1 for a [Te/Se] ratio between 45 and 58%, it is also important to consider the modes active in IR spectroscopy. They are localized around 250-300 cm −1 , which is a phonon energy range remaining relatively high to clearly favor mid-IR luminescence in the 8-9 µm region compare to pure selenium matrix. The Fig. 4 shows the refractive indices dispersion curves determined by the VASE data analysis with Sellmeier model. As the polarizability of the chalcogenide elements increases with the weight and electronic density, the substitution of Te instead of Se induces an increase of the refractive index for the chalcogenide glasses from 2.56 to 2.55 for x = 0 to 3.11-3.07 for x = 35 within λ = 2.1-4.8 µm range.
Based on decided to se Ga 5 Ge 20 Sb 10 S reference for have a materi This composi fiber drawing affect spectro

Fiber att
The fiber opti and 4 dB/m respectively. explained by degrade the op   bsorption coeff Ga 5 Ge 20 Sb 10 Se 6 d to the 7 F 6 → in Fig. 6 logically an e of the seleno elenide fiber w 33 ppm) in con th 500 ppm Tb ed in Fig. 6  Performed J-O calculations show that magnetic and electric dipole strengths of Tb 3+ transitions are very close to those reported for a selenide material where antimony is substituted by arsenic [14]. The Ω 2,4,6 parameters obtained in this work are also comparable, resulting in similar values of calculated radiative lifetimes. The calculated 7 F 5 decay lifetime (13.9 ms) obtained for the pure selenide glass sample is in good agreement with the value calculated for a Ga-Ge-As-Se glass sample (13.1 ms) [14]. In the present work, longer Tb 3+ ions radiative lifetimes are found for seleno-telluride than for selenide glasses ( 7 F 5 level lifetime ~14.8 and ~13.9 ms for Ga 5 Ge 20 Sb 10 Se 45 Te 20 and Ga 5 Ge 20 Sb 10 Se 65 matrix, respectively). On the other hand, the measured fluorescence lifetimes of the 7 F 5 manifold are equal to 7.8 and 8.9 ms for Ga 5 Ge 20 Sb 10 Se 45 Te 20 and Ga 5 Ge 20 Sb 10 Se 65 glass, respectively. Both 7 F 5 manifold fluorescence lifetimes are in good agreement with the experimental fluorescence lifetimes reported in the literature [13]. Presented values enable an estimation of the manifold quantum efficiency η in these materials (η = τ exp /τ rad ). The η is found to be lower for Te containing glasses (∼52%) compare to selenide glass (∼64%), which seems to be a drawback for efficient infrared luminescence when adding Te in the selenide matrix.
Either on bulk or fiber samples, the emitted fluorescence in any studied spectral domain exhibits a higher magnitude for samples without Te. The quenching of the Tb 3+ luminescence is somewhat stronger in Ga 5 Ge 20 Sb 10 Se 45 Te 20 than in Ga 5 Ge 20 Sb 10 Se 65 glass composition likely related to dipole-dipole due to clustering of Tb 3+ ions or electron-dipole interactions with mainly impurities like [OH] or [SeH]. For the same incident power, the MWIR luminescence is brighter with a Tb 3+ :Ga 5 Ge 20 Sb 10 Se 65 fiber, and exhibits a larger signal-tonoise ratio (Fig. 7(a), 7(b)) indicating a greater radiation quenching in the Tb 3+ : Ga 5 Ge 20 Sb 10 Se 45 Te 20 material. The reabs 4.5 μm in th less intense in selenium in luminescence result is the o glasses. As de ratio was reco radiative and the chalcogen 7 F 6 radiative t → 7 F 5 transit luminescence this result is c The LWIR same trend as case of the Te the substitutio the phonon e (Fig. 4) and However, the Moreover, th Finally, when energy of sele described con 7 s is clearly ob sorption pheno of a lower prop bsorption resu de-based glasse ide and seleno h a good signal anifold. Obviou e low phonon e t fully impair t .0 μm originati a well-define a 5 Ge 20 Sb 10 Se 65 ide glass. Fig. 7(c). Follo intensity is low pectations com r typically a de of the refracti ission spectra leno-telluride m purification p IR techniques elenides glasse Te-containing 0 ) g served at omenon is portion of ults in a es. A key o-telluride l-to-noise usly nonenergy of the 7 F 4 → ing in 7 F 4 d ~8 µm [12], and owing the wer in the ming from ecrease of ive index (Fig. 2). materials. processes. s, phonon es. Above g glasses.
Nevertheless, the radiative emissions observed especially at ~8.0 µm remain remarkable for seleno-telluride glasses. To obtain optimal luminescent properties for the highly sought luminescence at 8 μm, the next challenge is to decrease the impurity contain of such glasses by purification steps. Thus replacing Se by Te in the chalcogenide glass matrix remains a promising route to obtain better quantum efficiency of LWIR radiative transitions of rare earth ions incorporated in chalcogenide glasses.

Conclusions
Bulk Ga 5 Ge 20 Sb 10 Se 65-x Te x with x = 0, 10, 20, 25, 30, 32.5, 35, 37.5 chalcogenide glasses and 500 ppm Tb 3+ doped Ga 5 Ge 20 Sb 10 Se 65 or Ga 5 Ge 20 Sb 10 Se 45 Te 20 chalcogenide glasses/fibers were synthesized with a deep control of their purity. Tb 3+ ions are efficiently introduced into Ga 5 Ge 20 Sb 10 Se 65 or Ga 5 Ge 20 Sb 10 Se 45 Te 20 glasses. We observe MWIR emission in the range of 4.3-6.0 µm attributed to the 7 F 5 → 7 F 6 transition with a corresponding experimental lifetime of 8.9 and 7.8 ms for the selenide and seleno-telluride matrix, respectively. An emission from the 7 F 4 level is measured in the spectral range of 2.8-3.4 µm which is a first step towards the challenge to observe the 8.0 µm luminescence from studied Tb 3+ :Ga 5 Ge 20 Sb 10 Se 65-x Te x materials. This LWIR emission was also measured for the Tb 3+ :Ga 5 Ge 20 Sb 10 Se 45 Te 20 fiber. Although a lower intensity than that of the selenide matrix has been observed, it still remains very appreciable. The unambiguous observation of the 8.0 μm emission band in Tb 3+ : Ga 5 Ge 20 Sb 10 Se 65-x Te x (x = 0 and 20) opens up new prospects for the mid-IR sensors, especially in the field of the gas remote sensors.