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Abstract
We present mid-IR spectroscopic characterization of the low-phonon chalcogenide glass, Ga2Ge5S13 (GGS) doped with Er3+ ions. Under the excitation at ∼800 nm, Er3+:GGS exhibited broad mid-IR emission bands centered at ∼2.7, ∼3.5, and ∼4.5 µm at room temperature. The emission lifetime of the 4I9/2 level of Er3+ ions in GGS glass was found to be millisecond-long at room temperature. The measured fluorescence lifetimes were nearly independent of temperature, indicating negligibly small nonradiative decay rate for the 4I9/2 state, as can be expected for a low-maximum-phonon energy host. The transition line-strengths, radiative lifetimes, fluorescence branching ratios were calculated by using the Judd-Ofelt method. The peak stimulated emission cross-section of the 4I9/2 → 4I11/2 transition of Er3+ ion was determined to be ∼0.10×10−20 cm2 at room temperature.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
A great number of rare-earth (RE) activated gain materials have been investigated in the development of new solid-state infrared (IR) sources for a variety of applications including laser remote sensing, environmental monitoring, molecular spectroscopy, free-space communications as well as medicine [1–28]. All material types are being considered, from crystals to ceramics and glasses, with focus on those RE3+ hosts with low maximum phonon energy. Table 1 shows some examples of laser operation in the mid-IR region from several RE doped crystals such as fluorides and chlorides, as well as chalcopyrite crystals, and chalcogenide glasses [3–15]. The latter (in Table 1) are of particular interest due to the recent demonstration of mid-infrared (mid-IR) gain and lasing from RE3+ (Tb3+, Pr3+, Ce3+) doped low-phonon chalcogenide glasses which has sparked further development of this class of materials for fiber laser applications [13–15].
Table 1. Recent demonstrations of mid-IR rare-earth lasers.
Gallium-germanium (GaGe) based materials have emerged as promising chalcogenide glass hosts for mid-IR laser applications due to their favorable glass-forming ability, chemical and mechanical durability, as well as high rare earth ion solubility [13–15,21–30]. Compared to Arsenic (As) based compositions, GaGe chalcogenide glasses offer the added advantage of being non-toxic. Recent reports on RE3+ doped GaGe-based chalcogenide glasses demonstrated upper laser level lifetimes as long as 3 - 7 ms for mid-IR transitions between the levels with only ∼2000cm−1 spacing [13–15]. As seen from Table 1, many of the GaGe glasses studied thus far for mid-IR lasing have been Selenide (Se) compositions. However, sulfide glasses offer some benefits, such as a wider optical bandgap, which allows for pumping with well-developed high power laser diode sources (∼800 nm, ∼980 nm) without the detrimental effects of two-photon absorption. Moreover, the sulfide glasses have low phonon energies (∼340 - 400 cm−1) for mitigating nonradiative multi-phonon decay rates, a high refractive index allowing for increased probability of spontaneous emissions, and a wide transmission window that extends well beyond 10 µm [21–24]. Recent studies of various GaGe sulfide glasses have explored the effect of varying the Ga & Ge compositions [21–26], and among those studies, Ga2Ge5S13 has emerged as one of the better candidates offering the cleanest Raman spectrum without the high-energy peaks observed in other compositions. Among RE3+ ions, Pr3+, Tb3+, Dy3+, Ho3+, and Er3+, have a wide choice of energy levels with high potential for mid-IR transitions in the 3 - 5 µm spectral range. In this work, we report the findings of our recent spectroscopic investigation of Er3+ doped Ga2Ge5S13 (Er3+:GGS) glass, including absorption and mid-IR emission studies The results of Judd-Ofelt analysis are also presented and discussed.
2. Experimental details
Er3+-doped GGS glasses were prepared using a standard “quench and anneal” procedure wherein a stoichiometric mixture of gallium (6N), germanium (6N), and sulphur (5N), and 1 - 2 wt.% of erbium (3N), were sealed in thin walled (1 mm) quartz ampoules with an inner diameter of 10 mm. The ampoule was sealed under high vacuum (better than 10−6 Torr) before being placed in a rocking furnace. The temperature of the ampoule was gradually increased to 920°C over a period of 5 days, while also being subjected to constant rocking. Then the ampoule was removed quickly from the furnace, quenched in cold water for a few seconds, and then put right back in the annealing furnace at 430°C. The furnace was gradually cooled over a period of 40 hours to room temperature. In this work, one undoped GGS glass and three Er3+ doped GGS glasses were explored. The Er3+ concentrations in the doped glasses were determined to be 0.975 wt.% (1.61 × 1020 cm−3), 1.51 wt.% (2.5 × 1020 cm−3), 2.09 wt.% (3.46 × 1020 cm−3) using inductively coupled plasma optical emission spectroscopy (ICP-OES) by Galbraith Laboratories, Inc.
Room temperature transmission and absorption spectra were recorded using a Cary 6000i UV-Vis-NIR spectrophotometer and a Nicolet 6700 Fourier-transform infrared spectrometer. Mid-IR fluorescence spectra were excited by a continuous-wave Spectra-Physics Tsunami Ti:Sapphire laser. A Princeton Instruments Acton SpectraPro 0.15-m monochromator (λblaze: 4 µm, 150 grooves/mm) was used to acquire the mid-IR emission spectra. The emission signal was recorded by an Infrared Associates liquid-nitrogen-cooled InSb detector in conjunction with a Stanford Research Systems SR830 dual-phase lock-in amplifier. Fluorescence decay measurements were carried out using the output of a pulsed (10-ns pulses, 10 Hz) Nd:YAG pumped Optical Parametric Oscillator system as an excitation source. The decay signal was recorded with a LabVIEW-driven National Instruments (USB-6366 DAQ) data acquisition system. For temperature-dependent emission studies down to 10 K, the sample was mounted on the cold finger of a two-stage closed-cycle CTI Cryodyne cryogenic refrigerator.
Raman spectra, used for assessing the maximum phonon energy of the glass, were measured in the range of 200–600 cm−1 using a Renishaw InVia microscope equipped with 633 nm and 785 nm excitation sources. The Raman measurement result, performed on an undoped Ga2Ge5S13 glass sample, is shown in Fig. 1. It shows the strongest phonon energy peak at ∼340 cm−1, with the maximum phonon energy peak observed at ∼435 cm−1. This result is in a good agreement with other reported Raman measurements performed on similar composition GGS glasses [27,28].
3. Results and discussion
3.1 Transmission, absorption and Judd-Ofelt analysis
Figure 2(a) shows the transmission spectra of undoped and 2wt.% Er3+ doped GGS glasses, and it can be noted that the fundamental absorption edge of GGS is located at ∼ 480 nm. The undoped GGS glass registered a maximum background transmission of ∼80%, which is close to the limit defined by Fresnel reflections. The background transmission of the 2wt.% Er3+:GGS glass is reduced to roughly 70%, most likely due to the incorporation of RE impurities, which decrease the overall optical transparency of the glass, likely due to defect formation. Several intra-4f Er3+ absorption bands are noticeable in the region from ∼0.5 - 1.6 µm, providing evidence of the successful incorporation of Er3+ ions.
Fig. 2. Room temperature (a) transmission (images of undoped GGS and 2%Er:GGS glasses are shown) and (b) absorption spectra of undoped GGS (thickness: ∼3.68 mm) and Er3+ doped GGS (thickness: ∼3.77 mm) glasses. The absorption for the doped glass has been vertically offset to accentuate the differences in the spectra. The Er3+ concentration in this Er3+:GGS glass is determined to be 3.46 × 1020 cm−3. The hydrogen and carbon related impurity absorption bands are indicated.
Figure 2(b) shows the absorption spectra of the undoped and 2wt.% Er3+ doped GGS glasses from 2 to 12 µm. The absorption for the doped glass has been vertically offset to accentuate the differences in the spectra. It is well known that sulphide glasses are prone to contamination caused by hydrogen and carbon impurities, which can lead to formation of the infrared absorption features [21–24]. It can be seen that both undoped and doped glasses showed several impurity absorption bands in the mid-IR region, related to CO2, COS, H2O, OH, and SH contaminants. The presence of these impurities indicates that the studied material is not fully optimized. Further improvements must be made to advance the material preparation, with emphasis on RE doping and the purification processes.
The room temperature absorption coefficient spectrum of 2wt.% Er3+:GGS is shown in Fig. 3(a), with assigned Er3+ intra-4f transitions originating from the ground state in the 0.4 to 1.8 µm region. The absorption spectrum was corrected for background losses due to Fresnel reflection and background loss. As indicated in Fig. 3(b), each Er3+ absorption band originates from the 4I15/2 ground state and terminates at a higher excited state. The characteristic absorption bands of Er3+ were centered at ∼1.532, 0.985, 0.810, 0.662, 0.549, 0.528, and 0.493 µm. It is well known that Er3+ have absorption bands at ∼ 0.805 µm (4I15/2 → 4I9/2) and ∼ 0.983 µm (4I15/2 → 4I11/2), which can be of great utility for laser diode pumping. These bands show peak absorption cross sections of ∼0.24 × 10−20 and 0.42 × 10−20 cm2, respectively.
Fig. 3. (a) Room temperature absorption coefficient spectrum of Er3+:GGS. The Er3+ concentration in this Er3+:GGS glass is determined to be 3.46 × 1020 cm−3. (b) An energy level diagram indicating the absorption transitions corresponding to the absorption bands shown in the spectrum (a).
Six manifolds were selected to determine the three Ωt parameters, known as the Judd-Ofelt (J-O) intensity parameters, for Er3+ transitions in GGS glass. The 2H11/2 absorption band was not considered in the J-O analysis since it is known to be hypersensitive [1,31]. Table 2 shows the average Er3+ transition wavelengths, integrated absorption coefficients, and the line strengths for Er3+:GGS glass. The significant magnetic dipole line strength component in the 4I15/2 → 4I13/2 transition was subtracted from the corresponding experimental line strength value [1,31–34]. The magnetic transitions, which give negligible contribution to the Er3+ transition bands, are not considered in the present work. The measured and calculated line strength values are presented in the right column of the Table 2.
Table 2. Average transition wavelengths, integrated absorption coefficients and the line strengths of Er3+:Ga2Ge5S13 glass.
The three J-O intensity parameters calculated from this analysis were Ω2 = 4.51 × 10−20 cm2, Ω4 = 2.20 × 10−20 cm2, and Ω6 = 0.84 × 10−20 cm2. These J-O parameters are in reasonable agreement with reported values for other Er3+ doped sulphide glasses, examples of which are displayed in Table 3 with their respective Refs. [21,24,35]. The root mean square (rms) error between measured and calculated line strengths was calculated to be ∼0.19 × 10−20 cm2, which indicated good consistency between measured and calculated line strengths and is also comparable to rms values reported for other Er3+ doped glasses [21,24,35].
Table 3. Judd-Ofelt intensity parameters of Er3+ in sulphide glasses.
3.2 Emission and decay time studies
Figure 4(a) depicts the room temperature mid-IR emission bands of Er3+:GGS in the wavelength ranges 2.4 –3.1 µm, 3.1–4.05 µm, 4.05–5.1 µm, which correspond to the 4I11/2 → 4I13/2, 4F9/2 → 4I9/2, and 4I9/2 → 4I11/2 transitions, respectively. Excitation of the upper levels of these transitions was attained by pumping at ∼800 nm, which directly populated the 4I9/2 level, and subsequently the 4I11/2 level through radiative and nonradiative transitions. The 4F9/2 level was excited using excitation at 660 nm. A schematic diagram of the relevant Er3+ energy levels indicating the excitation transitions and observed emission lines is shown in Fig. 4(b)). Mid-IR emissions centered at ∼4.5, ∼3.6, and ∼2.7 µm were observed with spectral bandwidths of 0.32, 0.28, and 0.13 µm at full width at half maximum, respectively. All emissions showed broad spectral features, an indicator of inhomogeneous broadening typical for glass hosts [21,23,35,36]. Such broad features offer the possibility of wide wavelength tunability in these spectral bands.
Fig. 4. (a) Room temperature normalized mid-IR emission spectra of 4I9/2 → 4I11/2, 4I11/2 → 4I13/2, and 4F9/2 → 4I9/2 transitions in Er3+:GGS glass. (b) The partial energy level diagram of Er3+ ions indicating the pump wavelength and corresponding emission transitions. The related emission lifetime values are listed on the right of each excited energy levels.
The emission lifetimes of the first four excited states of Er3+:GGS glass were determined to be ∼4.56 ms (4I13/2), 2.65 ms (4I11/2), 1.15 ms (4I9/2), 0.226 ms (4F9/2) at room temperature. These are longer lifetimes than those observed in other Er3+ doped sulphide glasses [21,35], particularly for the 4I9/2 level which exhibits over a millisecond-long lifetime. Such long lifetimes reflect the lower maximum phonon energy of GGS glass, which leads to reduced nonradiative relaxation rates. There is an estimated ∼10% error in reported decay times, attributed to inaccuracy in interpretation of the measured lifetime data.
In this work, the primary energy level of interest is 4I9/2, which is the upper level of the 4.5 µm emission transition. The 4I9/2 emission decay as a function of Er3+ concentration was performed to determine whether there was a change in lifetime (Fig. 5. (a)). We observed no significant quenching was up to 2wt.% Er3+ concentration. In order to rule out the possibilities that reabsorption was influencing the lifetime results, the pinhole method [37] was employed for during these lifetime measurements. Lengthening of the lifetimes due to reabsorption was found to be negligible. The decay transients were slightly non-exponential, which could possibly be due to energy transfer processes related to impurities (e.g. OH, SH) [21–24]. These processes should be negligible, however, due to the minimal overlap between SH absorption and Er3+ emission in the mid-IR region as shown in Fig. 5(b).
Fig. 5. (a) Room temperature decay transients of Er3+ ions at different concentrations of Er3+ in GGS glass, monitored at ∼1700nm (4I9/2 → 4I13/2). (b) The spectral overlap between the SH-absorption and 4.5-µm emission in the mid-IR region.
The observed long lifetimes have incited an interest in further exploration on temperature dependence of the spectral and decay dynamics. Figure 6(a) depicts the mid-IR emission spectra of the 4I9/2 → 4I11/2 transition for temperatures of 10, 77, 120, 200, and 295 K. The emission bandwidth of the studied Er3+:GGS glass remained relatively broad, even at low temperatures, which can be favorable for tunable laser operation. Note the substantial red-shifting of the emission peak position with decreasing temperature, with a total shift of ∼100 nm between the 10 K and 295 K. This shift can be attributed to the freezing out of the higher-lying Stark levels. Figure 6(b) depicts the 4I9/2 emission lifetime as a function of temperature with the inset showing the 77 K and 295 K decay curves of the 4I9/2 level monitored at ∼1700nm (4I9/2 → 4I13/2). The measured 4I9/2 lifetime is about a millisecond-long and no significant emission lifetime change was observed between 10 K and room temperature. This suggests that nonradiative processes due to a multi-phonon decay are negligible as expected from the energy-gap law for low phonon energy hosts [19,20].
Fig. 6. (a) Temperature dependent mid-IR emission spectra of 4I9/2 → 4I11/2 transition. (b) The 4I9/2 emission lifetimes as a function of temperature. The inset shows the emission decay of 4I9/2 level at 77 K & 295 K.
Using the J-O parameters obtained for Er3+:GGS glass, the radiative transition rates, branching ratios, and radiative and measured lifetimes are presented in Table 4. The calculated radiative rates from the J-O analysis exhibit good agreement with measured rates for all four excited states, despite the estimated ∼ 10% error for the radiative lifetimes calculated from the J-O analysis. This agreement is important because it confirms theoretically, what is seen in experiments: the excited states decay predominantly by the radiative route. The radiative emission lifetimes calculated from the investigated glass are comparable to the results obtained by others for Er3+ doped sulphide glasses [21,35]. The radiative lifetime estimated from J-O analysis is ∼1.49 ms for the 4I9/2 state, which leads to a radiative quantum efficiency of ∼77% at room temperature.
Table 4. Er3+ transitions, average emission wavelengths, calculated radiative rates, branching ratios, radiative lifetimes (τrad) and measured lifetimes (τexp) of Er3+:Ga2Ge5S13 glass.
3.3 Stimulated emission cross-section
The stimulated emission cross section σemiss of the 4I9/2 → 4I11/2 mid-IR transition (∼4.5 µm) was calculated using the Füchtbauer-Ladenburg equation [38]:
Fig. 7. (a) Emission cross-section spectrum for the 4I9/2 → 4I11/2 transition in Er3+:GGS glass at room temperature and 77 K. (b) The partial energy level diagram of Er3+ ions indicating the excitation transition and the emission transition of interest 4I9/2 → 4I11/2.
Table 5. Branching ratios, radiative lifetimes (τrad) and measured lifetimes (τexp), quantum efficiency (ηQE), emission cross-sections (σemiss), and sigma-tau (στ) product of the 4I9/2 excited state for Er3+:Ga2Ge5S13 glass and other Er3+ doped sulphide glasses.
4. Conclusions
In summary, we have presented what is believed to be the first spectroscopic analysis of Er3+ doped Ga2Ge5S13 glass, as it pertains to its potential for mid-IR lasing. The absorption spectrum of Er3+:GGS glass displayed the characteristic Er3+ transitions in the visible and IR spectral region, which were used to perform Judd-Ofelt analysis. Optical excitation into the 4I15/2 → 4I9/2 absorption band at ∼800 nm resulted in observation of mid-IR Er3+ fluorescence bands, corresponding to the 4I11/2 → 4I13/2 (∼2.7 µm), and 4I9/2 → 4I11/2 (∼4.5 µm) transitions, respectively. The room temperature fluorescence lifetimes of the first four excited states of Er3+:GGS glass were determined to be ∼ 4.56 ms (4I13/2), 2.65 ms (4I11/2), 1.15 ms (4I9/2), and 0.226 ms (4F9/2). The emission decay of the 4I9/2 level, which is of particular interest due to its being the upper level for the 4.5 µm emission, was nearly temperature independent. This observation is consistent with low-phonon energy hosts, despite the small energy gap (∼2200 cm−1) between the 4I9/2 level and the next lower 4I11/2 level. The peak mid-IR emission cross section of the 4I9/2 → 4I11/2 transition at ∼4.52 µm was determined to be ∼0.10 × 10−20 cm2 at room temperature. Overall, the promising spectral properties of the Er3+ doped Ga2Ge5S13 glass make it an attractive gain material for mid-IR lasing.
Funding
National Science Foundation (NSF-DMR 1827820 (PREM)); Army Research Office (W911NF2120181, W911NF66020).
Acknowledgements
Brimrose Technology Corporation gratefully acknowledges the support of this work by Army Research Office through Cooperative Agreement W911NF2120181.The work at Hampton University was supported by the Army Research Office through Cooperative Agreement W911NF66020 and National Science Foundation through grant NSF-DMR 1827820 (PREM).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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