Effect of gallium environment on infrared emission in Er3+-doped gallium– antimony– sulfur glasses

Gallium-based Ga–Sb–S sulfide glasses was elaborated and studied. A relationship between the structure, composition, and optical properties of the glass has been established. The effects of the introduction of Ga on the structure using infrared and Raman spectroscopies and on the Er3+-doped IR emission have been discussed. The results show that incorporation of Ga induced the dissociation of [SbS3] pyramids units and the formation of tetrahedral [GaS4] units. The dissolved rare earth ions are separated around the Ga–S bonding and the infrared emission quenching are controlled. Moreover, continuous introduction of Er ions into the glass forms more Er–S bonds through the further aggregation surrounding the [GaS4] units. In return, the infrared emission intensity decreased with excessive Er ion addition. This phenomenon is correlated with the recurrence concentration quenching effect induced by the increase of [GaS4] units.

Scientific RepoRts | 7:41168 | DOI: 10.1038/srep41168 induced by structure evolution was investigated hoping to explore the way of improve RE infrared emission in chalcogenide glass systems. Figure 1 shows the absorption spectra of Er 3+ -doped Ga-Sb-S (GSS) glasses in the range of 400-2500 nm at room temperature and the absorption bands of Er 3+ corresponding to the transitions labeled starting from the ground state to the higher levels. The electronic intra-4f transitions from the ground energy level 4 I 15/2 to the higher energy state 2S+ 1 L J are located at 1540 nm ( 4 I 15/2 → 4 I 13/2 ), 990 nm ( 4 I 15/2 -4 I 11/2 ), and 810 nm ( 4 I 15/2 -4 I 9/2 ), respectively. The results indicate that the doped Er 3+ ions are successfully introduced into the glass matrix. The obvious absorption around 980 nm suggests that this glass can be pumped efficiently with a 980 nm laser diode. In the inset of the undoped GGS glasses, one can see that a shift of absorption edge to the short wavelength occurred with increase of Ga concentration, which means the microstructure change inside the glass network.

Results and Discussion
The infrared emission spectra of Er 3+ -doped glass samples are demonstrated ranging from 1000 nm to 3000 nm under 980 nm light excitation in Fig. 2. Two significant emission bands centered at 1.5 and 2.7 μ m are attributed to the transitions 4 I 13/2 → 4 I 15/2 and 4 I 11/2 → 4 I 13/2 , respectively. Moreover, the emission intensity at 1.5 and 2.7 μ m increased with addition of gallium component as shown in Fig. 2(a), in which the maximum emission was obtained in sample containing the most Ga component. The results implied that the ability of the Er 3+ radiative transition was promoted with gallium element by inducing a structured environment. According to the reports, the solubility of RE ions can be significantly improved by adding gallium in the chalcogenide glass matrix 15,25,26 . Ga was proposed to be associated with the formation of [GaS 4 ] tetrahedra in the vitreous network 27,28,29 . The increased [GaS 4 ] tetrahedra along with introduction of Ga element may have provided the optimization of the energy transition environment around RE ions, such as the chemical bonds between Ga and RE ions, which compensate for the negative charge of free S 2− ions. Most results focused on the increased solubility of RE ions in the presence of gallium since the first report of the incorporation of Ga in As 2 S 3 glasses by Kolomiets et al. in 1960 30 . We need to ensure that the effect of Ga addition induced structure linkage on the RE emissions in glasses.   shows the infrared emissions in Ga8 glass sample as a function of Er 3+ concentration. Notable maximum emission intensity at 1.5 and 2.7 μ m was obtained until 0.5 mol% Er 3+ was introduced into the glass matrix. The concentration quenching effect is observed in 1.0 mol% RE-doped sample. Comparing with Tm 3+ doped sol-gel glass, the optimum concentration of Tm 3+ was improved to 0.8 mol%, owing to the La/Al co-doping as network modifiers to decrease the clustering quenching effect of RE ions 31 . As glass modifiers, La, Ga, Al, Y and P were usually used to optimize the microstructure environment around RE ions in glass materials. In this work, Ga was introduced as a modifier to improve the solubility of RE ions in GSS glasses. The RE ions seemed to be surrounded by 6 or 7 sulfur atoms in chalcogenide glassy matrix 29,32,33 and incorporated on sites close to the gallium atoms to balance the partial negative charge of tetrahedral [GaS 4 ] units 34 . The introduction of Ga into Ge-As-S glasses greatly enhanced RE solubility and dispersal, particularly for Ga:RE ratios ≥ 10:1 35 . The peak position exhibited an obvious red-shift in the 1 mol% Er 3+ -doped glass sample, which coincide with the ratio exceeding the critical point. As reported by Petr Nemec through mass spectra measurement, this phenomenon was attributed to the fact that the two erbium-containing species contain gallium atoms (GaSb 2 SEr + and GaS 6 Er 2+ ) 36 . The second cluster corresponds to the structural fragment (suggested in the literature for high concentration of erbium and gallium), which could be involved in the clustering effect of the higher addition of Ga and Er samples. Therefore, the RE site transformation was the possible reason for the emission performance. Figure 3(a) shows the decay curves of Er 3+ :1.5 μ m emission in GSS glasses at room temperature. The lifetime of Er 3+ : 4 I 13/2 level was shortened from 3.37 ms to 2.75 ms with increase of Ga into the glass host. According to the radiative transition theory of Einstein, the radiative transition probability is proportional to the reciprocal of fluorescent lifetime, which is 297 sec −1 , 308 sec −1 , 345 sec −1 , 364 sec −1 and 382 sec −1 respectively with introduction of Ga component in this work. Comparing with Dy 3+ doped glass, the lifetimes obtained by the single exponent fitting method are 3.62 ms and 1.42 ms, which is measured at 2.95 and 4.40 μ m. The radiative transition probability is calculated as 276 and 704 sec −1 , respectively 37 . While in Tm 3+ doped present GaSbS glass, the radiative transition probability of emission at 3.8 μ m is about 500 sec −1 21 . It shows that different rare earth ions in the same glass system presented on a different radiative transition probability. In Er 3+ -doped GeGaSbS glasses, the radiative transition probability corresponding to 1500 nm is about 714 and 588 sec −1 when the dopant concentration is 500 ppm and 10000 ppm, respectively 38 . The comparative results further showed that the radiative transition probability of rare earth ions is influenced by multiple factors. The result identified that the Ga-based environment promoted the energy transition of doped RE 3+ ions, therefore leading to the increase of the radiative transition probability of electrons on the upper energy. The more electrons were expensed for generation of infrared emission photos, the stronger emission intensity was obtained. Similarly, the emission at 2.7 μ m was increased simultaneously, although the relative increase was much lower. As can be explained by the energy diagram in Fig. 3(b), the 1.5 μ m energy transition efficiency was higher than that of 2.7 μ m emission. The reason was that 2.7 μ m emission belongs to the energy transition between the excitation states, which need much lower phonon energy. The non-radiative transition with phonon assisted participated in the lower excitation energy transition, thus generating more 1.5 μ m emissions. The phonon energy is a key factor for the mid and far infrared emissions in chalcogenide glasses, which is determined by the glass compositions and the type of network connections. Therefore, we need to ensure the structure units of the glass host, which is always a research spot and a problem.
To explore the structure details of gallium-based samples of antimony sulfide matrix, the Raman spectra is displayed in Fig. 4. For comparison, the vibration spectra of Ga 2 S 3 , Sb 2 S 3 crystals, and GeGaS, GeGaSbS glasses were added as a reference in the picture. Three vibration bands were dominated in GSS glasses, including peak at ~56 cm −1 and broad bands centering at ~138 and ~300 cm −1 . The origin of the band in the lower frequency at ~56 cm −1 is not totally clear. Compared with the Raman spectra of Sb 2 S 3 crystals and two referenced glasses, this band may be associated with the vibrations of Sb-S bonds because no similar vibrations band are detected in    With the above information in mind and on the basis of the results, the structure of GSS glasses can be visualized as follows. The basic structural units forming the backbone of the network structure are [SbS 3 ] pyramids and [GaS 4 ] tetrahedra connected through a bridging sulfur. Little fraction of metal-metal bonds appeared, which are most probably Ga-Ga with increasing Ga 2 S 3 concentration. Simultaneously, the vibration strengthen of [GaS 4 ] units was improved and the vibration mode developed from V 2 to V 4 indicating the systematic to asymmetrical bending vibrations. Not only that, variation of pyramid [SbS 3 ] vibration was observed in the V 3 asymmetric stretching and V 1 symmetric stretching mode with addition of Ga 2 S 3 proportion. The vibration mode changes implied the alteration of the microstructure inside the glass host. The details of the structure bonding transformation with addition of Ga element are depicted in Fig. 7.
In the lower Ga-containing sample, the glass structure was predominated by the Sb-S linkage framed [SbS 3 ] pyramid three-dimensional network. The doped RE 3+ ions are dispersed in the Sb-S covalent network, tending to aggregate together owing to the mismatch coordination environment. The network directly induced the limited RE solubility and the fast fluorescence quenched effect. Interestingly, with addition of Ga component, the framework of the [SbS 3 ] units was broken and bonded with [GaS 4 ] tetrahedron. Owing to the negative charge of the [GaS 4 ] units, the doped RE 3+ ions connected with the tetrahedron structure to balance the electric neutrality and the matrix stability. Therefore, RE 3+ ions were dispersed evenly, and the quenching effect was further prevented. The corresponding infrared emission was then improved with addition of Ga element. After more Er 3+ ions were doped in the matrix, the obvious red shift of the peak position in the emission spectra was observed. The reason is that the network bonding transformation, such as GaSb 2 SEr + to GaS 6 Er 2+ , occurred when more Er 3+ ions connected with [GaS 4 ] units, in which the latter cluster is inclined to generate concentration quenching. Thus, the balanced amount of glass modifier mechanism, including the effect of the group vibration mode changes, needs to be established for the optimization of the infrared emissions in chalcogenide glasses.
In summary, chalcogenide glasses of Ga-Sb-S systems were successfully prepared, and the effect of Ga addition on the glass structure evolution and doped RE 3+ ions emission was better modified. The FTIR and Raman results demonstrated that the rare earth first dispersed in the Sb-S bonding [SbS 3 ] pyramid units consists of glass network. With increase of Ga concentration, [GaS 4 ] tetrahedron units were connected with Er ions, leading to the decrease of the aggregation effect. The radiative transition probability of the rare earth ions was raised efficiently. The Ga atoms induced network arrangement is a potential way to obtain the enhancement of infrared quantum efficiency in RE-doped chalcogenide glasses.  (5N). The samples were named as Ga4, Ga6, Ga8, Ga10 and Ga12 dependent of Ga concentrations. The glass forming ability decreased a lot when more Ga component such as 15 mol% was employed. The elements were weighted into silica glassy ampoules in a glove box with dry argon atmosphere, evacuated to residual pressure of ~10 −3 Pa, sealed before the ampoule was placed into a rocking furnace, and heated at 980 °С for 12 h. The melts of the glasses were quenched into water, and the glasses were annealed at T = T g − 20 K for 5 h to relax the mechanical strain. The furnace was switched off after this time, and the samples were left there for slow cooling to room temperature. The bulk glasses were cut into discs with parallel faces, diameter of ~9 mm, and thickness of ~1.5 mm. The discs were both polished into rectangular shape with submicrometer-sized diamond particles to the optical quality.

Synthesis of glass materials. Chalcogenide glasses of Ga
Characterization of samples. The optical absorption spectra were recorded by using a double-beam UV-Vis-NIR spectrometer (JASCO 570) in the spectral range of 350-2500 nm to find 4f-4 f electronic transitions of Er 3+ ions as well as the fundamental optical absorption of the glassy host matrix. Experiments of FTIR absorption spectra were carried out in the 100-450 cm −1 range by using an Equinox55 spectrometer equipped with a KBr detector. Photoluminescence was measured in the spectral laser pumped with diodes lasers operating at wavelength of 980 nm with a maximum power of 2 W as excitation source. The near-and mid-infrared fluorescence emission spectra were measured with FLS 980 (Edinburg Co., England) and detected with a liquid-nitrogen-cooled PbS detector. The fluorescence decaying curve was recorded with a digital oscilloscope. The Raman spectrum was measured by a Renishaw Micro-Raman instrument. A 300 kV field emission analytical transmission electron microscope (HRTEM, FEI tecnai G2F30) equipped with an energy dispersive X-ray spectroscope (Oxford X-MAX) was used to observe the structure clustering phenomenon and made element analysis. All the measurements were carried out at room temperature.