Mid-infrared (MIR) lasers are quite useful light sources for gas sensing, chemical process monitoring, military and medical applications.^[1–5] Various active gain media operating in 2–5 μm wavelength region have been extensively studied including rare earth (RE) ions doped glass fibers^[1,6,7] and crystals,^[8–10] transition metal (TM) doped crystals,^[4] and so on. For example, Pr³⁺ ions doped LaCl₃ crystal has exhibited the longest wavelength of 7.2 μm laser output at room temperature so far; laser outputs in the wavelength ranges of 1.8–3.1 μm and 3.9–5.1 μm have been achieved in Cr²⁺: ZnSe and Fe²⁺: ZnSe crystals, respectively. However, the crystals suffer from the long growth cycle, complex preparation techniques, and relatively high cost, which restrict their practical applications. In comparison, glass media are known for ease of large-scale and complex shape production, wide tuning range of compositions, as well as superior chemical and mechanical durability. Continuous wave (CW) lasings at 2.8 μm, 2.9 μm, and 3.5 μm have been obtained in Er³⁺ and Ho³⁺ ions doped ZBLAN glass fibers at room temperature, but it remains highly challenging for lasing at longer wavelengths.

Due to the outstanding MIR transmitting property and low phonon energy of chalcogenide glass (ChG), RE ions doped ChGs are considered to be ideal candidates as MIR laser gain materials. Recently, an increasing number of studies focused on the MIR emission characteristics of RE ions in ChGs.^[11–18] Among the RE ions, Dy³⁺ represents a typical three-level lasing system with an emission peak locating at ∼ 3 μm that can be pumped to the ⁶H_11/2 excited state by ∼ 1.7 μm light. The simple and effective energy level structure of Dy³⁺ facilitates the development of amplifiers or lasers operating at ∼ 3 μm.^[19] Moreover, Dy³⁺ ions are also potentially feasible for the ∼ 4.4 μm MIR emission, which is extremely difficult if not entirely impossible to be realized in other glass hosts because of the strong luminescence quenching related to the high multi-phonon relaxation (MPR) rate. Recently, Ga₂S₃–Sb₂S₃–CsI chalcohalide glasses^[20] stand out as promising MIR laser gain media owing to the outstanding properties including low phonon energy, large solubility of RE ions, and good thermal stability. In this work, the thermal stability, structure and MIR luminescent properties of Dy³⁺-doped Ga₂S₃–Sb₂S₃–CsI chalcohalide glasses were studied, and their suitability for MIR fiber lasers was assessed.

In this study, the composition 20 Ga₂S₃–60 Sb₂S₃–20 CsI,^[20] which has optimized thermal stability against crystallization in the chalcohalide system, was chosen as the host glass. The RE doped glasses, (20 − x) Ga₂S₃–60 Sb₂S₃–20 CsI–x Dy₂S₃ (x = 0, 0.05, 0.20, 0.40, 0.80, 1.00, 1.20), were synthesized through a melt-quenching method. High purity Sb (6N), Ga (6N), S (6N), CsI (5N), and Dy₂S₃ (3N) were weighed and loaded in a silica tube with an inner diameter of 9 mm. The tube containing the mixture was subsequently evacuated down to less than 10⁻⁵ mbar. The mixture was then baked at 100 °C for more than 2 h to reduce possible moisture. After that, the tube was sealed and heated to 950 °C in a rocking furnace. The mixture was homogenized at this temperature for more than 12 h. In the end, the tube was quenched in water, and the formed glass was annealed to reduce the internal stress.

The differential scanning calorimetry (DSC) curves were measured by a TA Q2000 calorimeter. About 5–15 mg glass was used for each measurement, and the heating rate was 10 °C/min. The transmission spectra in 0.5–3.3 μm were measured using a Perkin–Elmer Lambda 950 spectrophotometer and those in 3.3–20 μm were recorded with a Bruker Tensor 27 IR spectrophotometer. The refractive indices were tested by a Woollam IR-VASE ellipsometer.^[21,22] The MIR emission spectra and corresponding lifetimes were recorded with an Edinburgh FS980 fluorescence spectrometer. The measurement details are similar to those described in Ref. [11]. To measure the Raman spectra, a customized BWTek BWS415 Raman spectrometer with an excitation wavelength of 785 nm was used.

To investigate the impact of Dy³⁺ dopant on the thermal stability of the Ga₂S₃–Sb₂S₃–CsI chalcohalide glasses, the glass transition temperature (T_g) and the onset crystallization temperature (T_x) are determined based on the measured DSC curves and listed in Table 1. The T_g increases slightly with increasing amount of Dy³⁺ dopant. The ΔT (= T_x–T_g), a measure of the thermal stability against crystallization, is larger than 100 °C, pointing to the good thermal stability of the prepared glasses.

Table 1.

Table 1.

Table 1. Characteristic temperatures of (20 – x) Ga2S3–60 Sb2S3–20 CsI–x Dy2S3 glasses. . x Tg/°C Tx/°C ΔT/°C 0 228 ± 1 361 ± 1 133 0.05 229 ± 1 357 ± 1 128 0.20 231 ± 1 343 ± 1 112 0.40 232 ± 1 348 ± 1 116 0.80 233 ± 1 340 ± 1 107 1.00 234 ± 1 337 ± 1 103 1.20 235 ± 1 350 ± 1 115

Table 1. Characteristic temperatures of (20 – x) Ga2S3–60 Sb2S3–20 CsI–x Dy2S3 glasses. .

Figure 1 displays the absorption spectra of the Dy³⁺-doped Ga₂S₃–Sb₂S₃–CsI chalcohalide glasses in the wavelength range of 0.5–3.3 μm. Six absorption bands centered at 0.81 μm, 0.92 μm, 1.11 μm, 1.30 μm, 1.70 μm, and 2.85 μm are observed. These bands are attributed to the 4f–4f electron transitions of Dy³⁺ from its ground level ⁶H_15/2 to the excited ones, which are displayed in the figure. In the absorption spectra, the intensity at 1.30 μm is the strongest, indicating that the glasses can be efficiently excited by ∼ 1.30 μm light.

Fig. 1.

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	Fig. 1. (color online) Absorption spectra of (20 – x) Ga2S3–60 Sb2S3–20 CsI–x Dy2S3 glasses.

Based on the absorption spectra, the emission spectra of the (20 – x) Ga₂S₃–60 Sb₂S₃–20 CsI–x Dy₂S₃ chalcohalide glasses in the 2–5 μm MIR range were measured using a 1.32 μm CW semiconductor laser as the excitation light source (see Fig. 2). Three strong emission bands centered at 2.45 μm, 2.96 μm, and 4.41 μm are observed. These bands correspond to the transitions ⁶H_9/2, ⁶F_11/2 → ⁶H_13/2, ⁶H_13/2 → ⁶H_15/2, and ⁶H_11/2 → ⁶H_13/2, respectively. The emission intensities (EIs) increase with the increasing amount of Dy³⁺ dopant until the doping level reaches ∼ 0.36 mol% (x = 0.8), where the doped glass shows the strongest emissions. When the Dy³⁺ concentration is higher than 0.36 mol%, the EIs reduce with increasing doping content. This is associated with the concentration quenching effect,^[23] which usually happens at high doping concentrations and tends to overwhelm the radiative transitions.

Fig. 2.

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	Fig. 2. (color online) MIR emission spectra of (20 – x) Ga2S3–60 Sb2S3–20 CsI–x Dy2S3 glasses when excited at 1.32 μm, the inset shows possible transitions between energy levels of Dy3+.

To reveal the underlying structural influence on the luminescence properties of the dopant, the structure of the host glass was studied by Raman spectroscopy. Figure 3 displays the reduced Raman^[24] spectrum of 20 Ga₂S₃–60 Sb₂S₃–20 CsI (Ga–Sb–S–CsI or GSSC) glass. As a comparison, the Raman spectrum of 20 Ga₂S₃–60 Ge₂S₃–20 CsI (Ga–Ge–S–CsI or GGSC) glass was also measured and shown in Fig.3. For the GSSC glass, the main Raman band peaking at 330 cm⁻¹ can be deconvoluted into three Gaussian sub-bands corresponding to the vibrations of S₃Ga–GaS₃ (∼ 265 cm⁻¹),^[25,26] [SbS₃] (∼ 314 cm⁻¹),^[27,28] and [GaS₄] (∼ 342 cm⁻¹) structural units.^[25,29] As to the GGSC glass, the Raman band consists of vibrations ascribed to S₃Ga(Ge)–Ga(Ge)S₃ (∼ 265 cm⁻¹),^[30] [GaS₃I] (∼ 296 cm⁻¹),^[31] corner-sharing [Ga(Ge)S₄] (∼ 342 cm⁻¹, 390 cm⁻¹, 425 cm⁻¹),^[25,29,32] edge-sharing [Ga(Ge)S₄] (∼ 370 cm⁻¹) and S–S (438 cm⁻¹) structural units (see Fig. 3). Based on the Raman shifts and integral areas of the sub-bands, the effective phonon energies (EPEs) are estimated to be 307 cm⁻¹ and 352 cm⁻¹ for the GSSC and GGSC glasses, respectively. The lower EPE of the GSSC glass would result in smaller MPR rates for the doping RE ions, and therefore improve the MIR luminescent properties of the RE ions in the glass.

Fig. 3.

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	Fig. 3. (color online) Reduced Raman spectra of (a) 20 Ga2S3–60 Sb2S3–20 CsI and (b) 20 Ga2S3–60 GeS2–20 CsI glasses.

To assess the suitability of the Dy³⁺-doped GSSC glasses as MIR gain media, we conducted a Judd–Ofelt (J–O)^[33,34] analysis on the glasses according to the absorption spectra (see Fig. 1) and calcluated associated radiative parameters. Table 2 displays the experimental (f_exp) and calculated (f_cal) oscillator strengths of the considered transitions. The absorption band at 2.85 μm was excluded in the calculation because the OH impurity also has contrubition to this band. The J–O intensity parameters obtained by fitting the f_exp are Ω₂ = 10.73 × 10⁻²⁰ cm², Ω₄ = 3.18 × 10⁻²⁰ cm², and Ω₆ = 1.53 × 10⁻²⁰ cm². Using the J–O intensity parameters, we calculated the radiative parameters related to the MIR emissions of Dy³⁺ ions (see Table 3). The equations used in the calculations can be found in Refs. [35] and [36]. The 2.96 μm and 4.41 μm emissions possess quantum efficiencies (η) of ∼ 61.0% and ∼ 58.0%; and the corresponding stimulated emission cross sections (σ_em) are ∼ 1.04 × 10²⁰ cm² and ∼ 0.27 × 10²⁰ cm², respectively. As a comparison, the η and corresponding σ_em for the emissions of Dy³⁺ in a few other ChGs are also shown in Table 3. The η of the MIR emissions in Dy³⁺-doped GSSC glass is obviously larger than that of the corresponding emissions in Dy³⁺-doped GGSC, Ga–Ge–S, Ga–Ge–S–CdI, and Ga–La–S glasses. The high η is associated with the low EPE of the GSSC glass. It is worth mentioning that the η of the MIR emissions in Dy³⁺-doped GSSC glass is lower than that in Dy³⁺-doped Ga–Sb–S glass, although the two hosts have almost the same EPE.^[37] This may be caused by the higher OH content in the former glass. CsI is moisture sensitive and tends to bring additional OH impurity into the glass. Figure 4 shows the IR transmission spectra of 20 Ga₂S₃–60 Sb₂S₃–20 CsI and 20 Ga₂S₃–80 Sb₂S₃ glasses prepared at the same condition. It is clear that the addition of CsI indeed brings more OH impurity into the glass as reflected by the stronger absorption band around 2.9 μm. The OH impurity has been known to have a significant quenching effect on the MIR fluorescence.^[38,39] This situation can also explain the remarkable η difference between Dy³⁺-doped Ga–Ge–S and Ga–Ge–S–CsI glasses (see Table 3). The OH content might be greatly reduced if appropriate purification processes are applied during the glass synthesis. Thus, the η of the MIR emissions could be further improved. The σ_em of the MIR emissions of Dy³⁺ in GSSC glass is also relatively large, which is beneficial to the material for achieving high gain factor.^[40] Figure 5 further compares the 2–5 μm emissions of 0.36 mol% Dy³⁺ doped GSSC and GGSC glasses. The former shows much stronger emissions than the latter, which is consistent with the lower-phonon-energy feature of the former. The high η and large σ_em for the MIR emissions in the Dy³⁺-doped GSSC glasses demonstrate their suitability for MIR fiber amplifiers or lasers.

Fig. 4.

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	Fig. 4. (color online) Transmission spectra of 20 Ga2S3–80 Sb2S3 and 20 Ga2S3–60 Sb2S3–20 CsI glasses (thickness = 2.1 mm).

Fig. 5.

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	Fig. 5. (color online) The 2.5–5 μm emission spectra of Ga–Sb–S–CsI and Ga–Ge–S–CsI glasses doped with 0.36 mol% Dy3+ ions.

Table 2.

Table 2.

Table 2. Experimental (fexp) and calculated (fcal) oscillator strengths of electron transitions for 19.2 Ga2S3–60 Sb2S3–20 CsI–0.8 Dy2S3 glass. λ and δ are wavelength and deviation of fexp from fcal, respectively. . Transition λ/nm fexp/10−6 fcal/10−6 δ/10−6 6H15/2 → 6F7/2 918 3.75 3.23 0.52 6H15/2 → 6H7/2 + 6F9/2 1113 5.31 5.39 –0.08 6H15/2 → 6H9/2 + 6F11/2 1302 20.00 19.92 0.08 6H15/2 → 6H11/2 1710 1.84 2.06 –0.22

Table 2. Experimental (fexp) and calculated (fcal) oscillator strengths of electron transitions for 19.2 Ga2S3–60 Sb2S3–20 CsI–0.8 Dy2S3 glass. λ and δ are wavelength and deviation of fexp from fcal, respectively. .

Table 3.

Table 3.

Table 3. Radiative parameters of Dy3+ions in various hosts. λ, n, β, τrad, τmea, η, and σem are the wavelength, refractive index, fluorescence branch ratio, radiative lifetime, measured fluorescence lifetime, quantum efficiency, and stimulated emission cross section, respectively. . Host glass Transition λ/μm n β/% τrad/ms τmea/ms η/% σem/1020 cm2 Ref. Ga–Sb–S–CsI 6H11/2 → 6H13/2 4.41 2.444 ± 0.001 16 2.50 1.45 ± 0.02 58.0 0.27 this work 6H13/2 → 6H15/2 2.96 2.454 ± 0.001 100 5.19 3.17 ± 0.02 61.0 1.04 this work Ga–Ge–S–CsI 6H11/2 → 6H13/2 4.41 1.991 ± 0.001 16 3.94 1.02 ± 0.02 26.0 this work 6H13/2 → 6H15/2 2.96 1.996 ± 0.001 100 8.18 2.31 ± 0.02 28.0 this work Ga–Sb–S 6H11/2 → 6H13/2 4.40 2.69 14 1.87 1.42 75.9 0.38 [13] 6H13/2 → 6H15/2 2.95 2.72 100 4.11 3.62 88.1 1.11 [13] Ga–Ge–S 6H11/2 → 6H13/2 4.40 2.08 14.4 3.17 1.31 41.3 0.36 [13] 6H13/2 → 6H15/2 2.95 2.10 100 6.99 3.53 50.5 0.86 [13] Ga–Ge–S–CdI 6H11/2 → 6H13/2 4.40 – 15 3.63 – – 0.19a) [15] 6H13/2 → 6H15/2 2.95 100 8.20 0.86 10.5 0.84 [15] Ga–La–S 6H11/2 → 6H13/2 4.40 14 2.53 1.3 51.4 [41] 6H13/2 → 6H15/2 2.95 2.4 100 6.29 3.6 57.2 0.92 [41] a) this value is recalculated by taking β into account.

Table 3. Radiative parameters of Dy3+ions in various hosts. λ, n, β, τrad, τmea, η, and σem are the wavelength, refractive index, fluorescence branch ratio, radiative lifetime, measured fluorescence lifetime, quantum efficiency, and stimulated emission cross section, respectively. .

The (20 – x) Ga₂S₃–60 Sb₂S₃–20 CsI–x Dy₂S₃ (x = 0, 0.05, 0.20, 0.40, 0.80, 1.00, 1.20) chalcohalide glasses have good thermal stability and show intense emissions centered at 2.96 μm and 4.41 μm. The strongest MIR emissions are observed when the concentration of Dy³⁺ ions is 0.36 mol%. The η of the 2.96 μm and 4.41 μm emissions are about 61.0% and 58.0%, and the corresponding σ_em are 1.04 × 10⁻²⁰ cm² and 0.27 × 10⁻²⁰ cm², respectively. The high η coupled with the relatively large σ_em bode well for the potential of the Dy³⁺-doped Ga₂S₃–Sb₂S₃–CsI glasses as MIR gain media.