Spectroscopy of thulium and holmium co-doped silicate glasses

Ruijie Cao; Yu Lu; Ying Tian; Feifei Huang; Shiqing Xu; Junjie Zhang

doi:10.1364/OME.6.002252

1. Introduction

In order to achieve fiber lasers emitting in the near infrared region (NIR), numerous of researchers focus their interests on the emission and interaction features of rare earth (RE) ions such as Er³⁺, Nd³⁺, Pr³⁺, Ho³⁺, Tm³⁺, and Yb³⁺. Among these ions, Ho³⁺ has been widely used in medicine, range monitoring, and sensing [1–3] based on the energy level transition ⁵I₇→⁵I₈ lying in the 1.95-2.15 μm wavelength range. Nevertheless, exciting this transition directly is infeasible owing to the mismatching of absorption peak with the wavelength of available commercial diode lasers. Thus sensitized by other ions (Tm³⁺, Yb³⁺ et al.) is an alternative way to achieve 2 μm emissions [4, 5]. In Tm³⁺/Ho³⁺ co-doped system, pumping the photon of Tm³⁺ at ~790 nm to ³H₄ level offers highly efficient energy transfer (ET) to activator ions and several transition processes [6]. The cross-relaxation process is conducive to the ions gather of level ³F₄, a portion of energy of ³F₄ can be transfer to neighboring Ho³⁺, then according to the ⁵I₇→⁵I₈ transition, emit the ~2050 nm laser [7]. Not only traditional rare earth ions but also other activators are excited to emit in mid infrared. For activators such as Bi⁺ or Cr⁺, although they are able to achieve ultra-broadband luminescence in 1000-1700 nm [8], they are sensitive to coordination environment. When coordination number is changed, the valence state of luminescence center is affected [9]. In Bi⁺ doped glass, the Bi⁺ cannot be dispersed to the glass structure uniformly, which will induced cluster or quenching in glass, which is harmful to luminescence [10]. Besides, the Bi⁺ fluorescence peak is located in 1300, 1330, 1470 nm, which is different with the peak of Ho³⁺ and the energy transfer process is not clearly, the most doping concentration is lower than Ho³⁺, it is difficult to achieve high power laser emission, which limited the further development of Bi⁺. For glass fiber, the laser efficiency is decreased obviously with the rising of temperature [11]. Thus, the Ho³⁺ doped glass sensitized by other ions is still a promising candidate to realize 2 μm laser output. The difficulties to emit earth doped fiber laser are selecting the optimal population, high efficient pumping source and reduction of depletion processes (energy transfer up-conversion from the upper energy level and back energy transfer processes).

Several studies related to Tm³⁺/Ho³⁺ co-doped fiber lasers in silica or heavy metal glass hosts have been reported in recent years. 2 μm Tm³⁺/Ho³⁺ co-doped silica fiber laser have achieved [12]. Enhanced 2 μm emission of bismuth germanate glasses doped with Ho³⁺/Tm³⁺ ions are published in 2015 [13]. Besides, Tm³⁺/Ho³⁺ co-doped are applied to α-NaYF₄ single crystals in order to produce 2 μm emissions [14]. Compare with these matrixes, silicate glasses are able to dissolve much higher concentration of rare earth ions, which is helpful to efficient cross relaxation energy transfer, improving the quantum efficiency and reducing device length [15], own better ability of anti-crystallization, higher refractive index, and relatively low thermal expansion coefficient [16]. Besides, the maximum phonon energy of silicate (~1050 cm⁻¹) is much lower compared to those of borate glasses (~1350 cm⁻¹) and phosphate glasses (~1300 cm⁻¹) [15], so quantum efficiency can be less influenced by the multiphonon relaxation process. Thus silicate glass possesses incomparable advantages in glass fiber field. Recently, the luminescence properties of Er³⁺/Yb³⁺ silicate glasses [17] is demonstrated and some researchers have interest in lithium iron silicate [18]. Emission of Co²⁺ ions with Bi³⁺ ions in lead silicate glasses has been discussed [19].

From all we know, there is few studies focus on the silicate glass co-doped with Tm³⁺/Ho³⁺. In our previous work [20], we reported highly Ho³⁺/Yb³⁺ co-doped silicate glasses and discussed the optimal dopant concentration for ~2 μm lasers, the results shown that it is a potential way to achieve ~2 μm laser. In this study, we extend the previous work by introducing Tm³⁺ in the same silicate glass host to investigate the spectroscopy properties of Ho³⁺ and Tm³⁺.

2. Experimental techniques

2.1 Material synthesis

In this study, the silicate glass samples are composed (in mol %) of (99-x) (SiO₂-Al₂O₃-CaO-BaO- BaF₂- La₂O₃)-1Tm₂O₃-xHo₂O₃ (where x = 0.1, 0.2, 0.3, 0.4), with denoted as STH-1, STH-2, STH-3, STH-4, respectively. The mixed row powders are melted at 1400°C for 1h, and then poured onto a preheated (750°C) steel plate, and annealed for 4h at 600°C. Annealed glasses are fabricated and polished to 10 × 10 × 1 mm³ sizes for optical property measurements.

2.2. Measurement

According to the Archimedes principle, the densities of STH-3 (3.78 g/cm³) had been measured using distilled water as immersion liquid. The absorption spectrum had been obtained by JASCOV 570UV/VIS spectrophotometer. The emission spectrum had been tested by pumping the sample with 808 nm laser diode. The lifetime had been determined by a combined fluorescence lifetime and steady state spectrometer (FLSP-920) (Edingburg Co.). All measurements were at room temperature.

3. Experimental results and discussion

3.1 Absorption spectrum and Judd-Ofelt analysis

The absorption spectrum of Tm³⁺, Ho³⁺ singly doped and co-doped glass samples in the wavelength region of 370-2000 nm are shown in Fig. 1. The absorption bands of Tm³⁺/Ho³⁺ co-doped sample corresponding to the transitions starting from the ⁵I₈ ground state to the higher levels ⁵I₇, ⁵I₆, ⁵F₅, (⁵F₄, ⁵S₂), ⁵F₃, (⁵G₆, ⁵F₁), the absorption peak of these energy levels are centered at the wavelengths of 1932, 1140, 642, 540, 452, and 420 nm, respectively. Contrasting the absorption curve in Fig. 1, it is worth noting that for Tm³⁺ singly and Tm³⁺/Ho³⁺ co-doped doped silicate glass, the shape and peak positions of each transition are extremely similar to glasses with other matrix doped Tm³⁺ and Ho³⁺ [21, 22]. Besides, although Ho³⁺ ions do not have any absorption bands near 808 nm or 980 nm, Tm³⁺ has a strong absorption band around 808 nm, which can be excited by commercial laser diode. At the same time, the absorption band of ³F₄ (Tm³⁺) is close to that of ⁵I₇ (Ho³⁺), means that it is likely to exist efficient energy transfer between them. It is obvious that for different concentrations rate of Tm³⁺/Ho³⁺ co-doped glass samples, there is no obvious distinguish in the position of the absorption peaks, only the intensity grows stronger with Ho³⁺ concentration increasing.

Fig. 1 Absorption spectrum of silicate glass doped Tm³⁺, Ho³⁺ singly and Tm³⁺/Ho³⁺ co-doped.

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The Judd-Ofelt theory [23, 24] is comprehensively accepted to evaluate the spectroscopic properties of rare-earth doped glasses. Based on the absorption spectra, the Judd-Ofelt parameters represent the local environment and bond property of rare-earth ions. The parameters Ω₂, Ω₄, and Ω₆ of Ho³⁺ in STH-3 are 4.87 × 10⁻²⁰, 2.05 × 10⁻²⁰, and 1.39 × 10⁻²⁰ cm², respectively, and the comparison with other glasses is listed in Table 1. In this table, we can find that the ZBLAN glass host possesses comparatively small Ω₂, where as germanate, tellurite, and silicate glasses show relatively the larger ones, because Ω₂ is relative to covalent bonding in micromechanics, heavily metal and oxide glasses have a stronger covalent bonding than that of fluoride glass. At the same time, Ω₂ of Ho³⁺ in STH-3 is larger than in a previously reported silicate glass [15], which is due to the variation in the local environment of Ho³⁺ in different glasses. Diverse kinds of modifying elements tend to be distributed in modifier regions, thus the modifier ions affect Judd-Ofelt parameters directly. There are no alkali ions in this silicate glasses component, the distortion around Ho³⁺ ion is larger than that with alkali ions, that is the reason STH-3 owns a larger Ω₂ [18].

Table 1. Judd-Ofelt parameters Ω_t of Ho³⁺ in various glasses

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The transition probability A and radiative lifetime τ_rad can be calculated from Judd-Ofelt parameters. These two parameters can be determined by the following expressions:

A = \frac{1}{τ_{rad}} = \frac{64 π^{4} e^{2}}{3 h (2 J + 1) λ^{3}} [\frac{n {(n^{2} + 1)}^{2}}{9} S_{ED} + n^{2} S_{MD}]

S_{ED} = \sum_{λ = 2, 4, 6}^{} Ω_{λ} | <S, L, J ‖ U^{(λ)} ‖ {S^{'}, L^{'}, J^{'} > |}^{2}

S_{MD} = (\frac{ħ}{2mc}) | <S, L, J ‖ L + 2 S ‖ {S^{'}, L^{'}, J^{'} > |}^{2}

In above formula, n is the refractive index (Here is 1.649 in STH-3 glass sample), m and e are the mass and charge of the electron, J is the total angular momentum at the ground state, |U(λ)| is the reduced matrix element in sensitive to the host environment, and S_ED and S_MD are the line strengths for electric and magnetic dipole transitions, respectively. The transition probability (A) of the transition from ⁵I₇ to ⁵I₈ is 129.89s⁻¹ and the calculated lifetime (τ_rad) of STH-3 is 7.70 ms, respectively. These values are larger than those in two other previously reported silicate glasses (71.64s⁻¹ [15] and 61.65s⁻¹ [22]). Higher transition probability provides a better opportunity for lasers, thus we infer STH glasses maybe show better emission performances.

3.2 Fluorescence spectra and hydroxyl analysis

Fluorescence spectra of the STH samples are measured from 1600 nm to 2200 nm and shown in Fig. 2. In this figure, the emission peaks of Tm³⁺ and Ho³⁺ are located around 1838 and 2018 nm, respectively. With the increasing of Ho³⁺, the emission of Ho³⁺ from ⁵I₇ is enhanced, which indicates existence effective ET from Tm: ³F₄ to Ho: ⁵I₇ in glass samples. However, when the concentration of Ho³⁺ as high as 0.4 mol%, it appears the decline of the fluorescence intensity, which led by concentration quenching. STH-3 has the maximum emission intensity of Ho³⁺: ⁵I₇→⁵I₈. That is the reason why we choose the STH-3 as the object of study. A broad band spectrum with a full width at half-maximum (FWHM) of 189 nm are presented, which is little smaller than that in tellurite glass [26].

Fig. 2 the fluorescence spectra of STH glass.

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The factors that affect the emission of silicate glass in MIR are mainly concentration quench and hydroxyl quench. The concentration quench have occurred measured by fluorescence spectrum. Microscopically, when the critical distances of rare earth ions are broken, it will happen cascade effect, energy are transferred from one center to another, emission is weaken. That is the reason why the fluorescence intensity of STH-4 is lower than that of STH-3. Besides, OH^- is the mainly quench center in laser glass, the vibration of OH^- groups (2450-3700 cm⁻¹) matches with the maximum phonon energy of silicate glass (~1080 cm⁻¹), only two or three phonon can weak the luminance of rare earth ions. However, with conventional melting processes, the glasses containing little OH^- groups cannot be prepared without an elaborate setup. A feasible way of removing the OH^- groups from the glass during melting process is to add a fluoride to liberate hydrogen species from the melt. The added fluorine ions will react with OH^- groups according to [24]:

{OH}^{-} + F^{-} = O^{2 -} + HF ↑

Figure 3 is the transmittance spectra when alkali earth metal oxide is replaced by fluoride. It is obvious that with the increasing of F^-, the content of OH^- is decreased sharply. The content of OH^- groups in the glass can be evaluated by the absorption coefficient α_OH^- of the OH^- vibration band at 3 μm, which can be given by α_OH^- = ln(T₀/T)/l where l is the thickness of the sample equal to 0.15 cm, and the T₀ and T are the incident and transmitted transmittance intensities, respectively. The value of α_OH- is 1.401, 1.025, 0.545, it can be demonstrated by the effect of introduced fluorine ion on the structure of glass: a small amount introduction of fluorine ions will destroy the stable structure of silicate glass and the equilibrium between fluorine and oxygen ions were achieve with the increase of fluorine ions. Then, the glasses’ structure tends to be much more stable and homogeneous, which is registered as better thermal stability and transmission. The introduced fluorine ions can eliminate the OH^- content and the BaF₂/CaF₂ sample possesses the lowest α_OH^- in all samples. The excellent properties hints that the STH glass matrix might possesses better emission properties especially 2 μm emission.

Fig. 3 The transmittance spectrum of silicate glass samples with different fluoride content.

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3.3 Cross-sections and gain coefficient of Ho³⁺

To evaluate the possibility of achieving laser effects, the simulated emission cross-section (σ_em) defined as the intensity gain of a laser beam per unit of population inversion is calculated. Based on the obtained fluorescence spectra, the emission cross-section can be calculated using the Füchtbauer-Ladenburg formula [28, 29]:

σ_{em} (λ) = \frac{λ^{4} A_{rad}}{8 {πcn}^{2}} \frac{λI (λ)}{\int^{​} λI (λ) dλ}

Where λ is the wavelength of the measured fluorescence spectra, A_rad is the transition probability calculated by the Judd-Ofelt theory (here is 129.89 s⁻¹), I (λ) is the fluorescence intensity, n and c are the light speed and the index of refraction, respectively. The absorption cross-section (σ_abs) can be calculated according to the Beer-Lambert theory and displayed as [28, 30]:

σ_{a} = \frac{2.303 log (\frac{I_{0}}{I})}{N l}

Where I₀ (λ) and I (λ) are the incident and outgoing light intensities of the samples respectively, l is the sample thickness and N is the rare-earth ion concentration. Absorption and emission cross-sections of transitions from Tm³⁺: ³F₄ to ³H₆ and from Ho³⁺: ⁵I₇ to ⁵I₈ are calculated and shown in the same figure (Fig. 4). The maximum emission cross section of the transition from Ho: ⁵I₇ to ⁵I₈ is 7.59 × 10⁻²¹ cm² at 2069 nm. This value is much larger than the reported value (3.07 × 10⁻²¹ cm²) in silicate glass [15], and slightly larger than that (7.00 × 10⁻²¹ cm²) in another glass [22]. Energy transfer process also can be evaluated by calculating the absorption and emission cross sections. From Fig. 4, we notice there is a large overlap between the cross sections of Tm³⁺ emission and Ho³⁺ absorption, the bigger overlap area, the more possibility to occur the effective ET process from Tm³⁺: ³F₄ to Ho³⁺: ⁵I₇ [31].

Fig. 4 Calculated absorption cross sections and emission cross sections corresponding to the ⁵I₇→⁵I₈ transition of the Ho³⁺ doped and to the ³F₄→³H₆ transition of the Tm³⁺ doped glasses.

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The gain spectrum can be obtained using the absorption and emission cross sections of Ho³⁺ according to the following expression [31]:

G (λ) = N [P σ_{e} (λ) - (1 - P) σ_{a} (λ)]

Where P is the population at the upper laser level, and N is the total rare earth ion concentration. We choose the P is 0, 0.2, 0.4, 0.6, 0.8 and 1, the results is shown in Fig. 5. When P>0.4, the gain coefficient is positive at wavelengths >1983 nm. When P>0.6, the gain coefficient is positive from 1850 nm to 2100 nm.

Fig. 5 Gain coefficient with various population inversion values P ranging from 0 to1 for Ho^{3+ 5}I₇→⁵I₈ transition.

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3.4 Energy transfer and lifetimes

To further understand the mechanism of energy transfer between Tm³⁺ and Ho³⁺ when pumped by 808 nm LD, the energy level diagram for silicate glass samples is shown in Fig. 6. We have known that at 808 nm LD excitation, Tm³⁺ ions can be excited to ³H₄ state from the ³H₆ ground state. When the qualities of Tm³⁺ ions are accumulated to a certain degree, a part of the Tm³⁺ ions in ³H₄ states is transferred to ³H₆, while most of the ions decay from the ³H₄ state to the ³F₄ state producing 1.47 μm emissions. Meanwhile, other ions are excited from the ³H₆ ground state to the ³F₄ state, which is the cross relaxation between Tm³⁺: state ³H₄ and Tm³⁺: ³H₆ state (³H₄ + ³H₆→³F₄ + ³F₄). Once Tm³⁺ ions are saturated in ³F₄ level, on one aspect, they decay to ³H₆ ground state with strong 1.8 μm emission via ³F₄→³H₆ transition; furthermore, Tm³⁺ ions in ³F₄ state transfer their energy to Ho³⁺: ⁵I₇ state via energy transferring (ET) (³F₄ + ⁵I₈→³H₆ + ⁵I₇). When the ions are populated in ⁵I₇ state, the Ho³⁺ ions decay to ⁵I₈ ground state producing strong 2 μm emissions. Besides, ESA in Tm³⁺ and Ho³⁺ also produce fluorescence in weak visible at 480 nm and 550 nm from the Tm³⁺: ¹G₄→³H₆ and Ho³⁺: ⁵S₂, ⁵F₄→⁵I₈ transitions, respectively [32].

Fig. 6 Energy level diagram of Tm³⁺ and Ho³⁺ in silicate glass samples.

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The energy transfer efficiency η from Tm³⁺ to Ho³⁺ ions can be measured by the donor lifetime values according to the formula [33]

η = 1 - \frac{τ_{Tm}}{τ_{Tm}^{0}}

Where τ_Tm and

τ_{Tm}^{0}

are the lifetimes of glasses that co-doped with Tm³⁺/Ho³⁺ and doped Tm³⁺ singly. The lifetimes of Tm³⁺ for 1 mol% Tm₂O₃ singly silicate glass sample and 1mol% Tm₂O₃/0.3 mol% Ho₂O₃ co-doped silicate glass samples are 470 ms and 155 ms, respectively. Then the energy transfer efficiency from Tm³⁺ to Ho³⁺ ions is calculated to be 67.02%, which is higher than the values in previous reported. Though both donor excited levels are same (³F₄ of Tm³⁺ ions), the intrinsic lifetimes differ, which also proves that there are interactions in Ho³⁺ and Tm³⁺ indirectly [25, 30]. In the Tm³⁺/Ho³⁺ co-doped silicate glass, the higher energy transfer efficient is, the more opportunities that energy of Tm³⁺ transfer to Ho³⁺, which is beneficial to obtain 2 μm emission.

It is common to use extended integral method analyze ET processes. The energy transfer probability rate between donor and acceptor (here is Tm³⁺ ions and Ho³⁺ ions respectively) can be valued as [34]:

W_{d - a} = C_{d - a} / R_{6}

Where R is the distance between the donor and acceptor, and C_d-a is the transfer constant calculated by [34]:

C_{D - A} = \frac{6 {cg}_{low}^{D}}{{(2 π)}^{4} n^{2} g_{up}^{D}} \sum_{m = 0}^{\infty} e^{- (2 \bar{n} + 1) S_{0}} \frac{S_{0}^{m}}{m!} {(n + 1)}^{m} \int^{​} σ_{ems}^{D} λ_{m}^{+} σ_{abs}^{A} (λ) dλ

where

\bar{n}

is the average occupancy of the phonon mode at temperature T, c is the light speed, n is the refractive index,

g_{d}^{up}

and

g_{d}^{low}

are the degeneracies of the upper and lower levels of the donor, respectively, and

λ_{m}^{+} = 1 / (1 / λ - m ħ ω_{0})

is the wavelength with m phonon creation. The critical radius of each interaction can be obtained by [34]:

R_{c}^{6} = C_{D - A} τ_{D}

τ_D is the intrinsic lifetime of the donor without the acceptor. The calculated transfer constants and critical radii of energy migration (EM) are listed in Table 2. These transfer constants are larger than those in silicate glass with alkali ion and fluoride glass but smaller than those in germanate glass [15, 35, 25]. The critical radii of these ET processes are smaller than those in fluoride and germanate glasses [35, 25]. The intrinsic lifetimes are shorter in silicate glass than in fluoride and germanate glasses because of the larger phonon energy of silicate glass. The energy gap between Tm³⁺ and Ho³⁺ is nearly ~700 cm⁻¹. As is known the phonon energy of the silicate glasses is ~1050 cm⁻¹, so only about one or less phonon is required to bridge the energy gap.

Table 2. energy transfer constant and critical radius of each energy transfer process

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Figure 7 is the decay curves obtained under excitation at 808 nm LD. And the radiative lifetimes of the ⁵I₇ level (τ_m) in STH glasses samples are valued by a single exponential function to a good approximation. Since a long lifetime is beneficial high population inversion under steady state, τ_m of ⁵I₇ is a critical factor to achieve the 2 μm laser [36]. As is shown in Fig. 6, the τ_m of present glass is larger than those of hosts [15, 22], which makes this Ho³⁺/Tm³⁺ co-doped silicate glass possess more probably to develop 2 μm lasers. In decay curves, the lifetimes of Ho³⁺ are increased with the sensitizer concentration increment. Due to the influence of concentration quenching, the lifetime of STH-4 is much smaller than that of STH-3, which is in keeping with the result of fluorescence spectra. In this process, Tm³⁺ ions are transferred from ³H₄ to ³H₅, and then relax to ³F₄ through multiphonon relaxation. The energy of there are transferred to ⁵I₇ (Ho³⁺) state.

Fig. 7 The decay curves of ⁵I₇ level in the STH glass samples.

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In the view of the above results, the prepared Tm³⁺/Ho³⁺ co-doped silicate glass is a promising candidate to achieve mid-infrared laser emission around the wavelength of 2 μm with high energy transfer.

3.5 Raman spectra and thermal stability analysis

The spectroscopic technique is used to analyze the glass structure, as well as the vibrational modes. In addition, the corresponding Raman spectra of silicate glass samples are shown in Fig. 8. As is shown in this spectrum, three obvious Raman peaks are labeled. Peak A occurring around 566 cm⁻¹ in a low-frequency region may be attributed to breathing modes of the three-ring structures of SiO₄ tetrahedron. Peak B around 660 cm⁻¹ observed in a high-frequency region are related to the convolution of bands related to flexural vibrations of Si-O units [37]. With the appearance of F^-, the bridging oxide is disappeared and appears non-bridging oxide. Peak C is ascribed to Si-O-Si symmetric stretching [38]. In addition, the Si-O-Si asymmetric stretching intensity in this glass matrix is stronger than that in other glass, indicating the further destruction of the asymmetric stretching. Due to the appearance of fluoride, the phonon energy of present silicate glass is reduced to 958 cm⁻¹, which is lower the average value (1080 cm⁻¹) [39] of silicate glass. As we known, the smaller phonon energy is the smaller non-radiative relation rate, which means this glass possesses excellent luminescence productiveness.

Fig. 8 The Raman spectra of silicate glass.

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Combing the spectrum prosperities, STH-3 glass sample is considered to be the promising candidate to achieve laser emission. For drawing fiber, the thermal stability is a significant factor that will affect the quality of glass fibers. Just as Fig. 9 shows, the values of T_g, T_x, are found to be 619, 786 °C, respectively. The glass criterion, ΔT = T_x-T_g [40], introduced by Dietzel, is often regarded as an important parameter for evaluating the glass-forming ability. In this glass, the ΔT is 167°C. The glass formation factor of the materials is given by the formula k_gl = (T_x-T_g)/(T_m-T_g), which is more suitable for estimating the glass thermal stability than ΔT. Larger k_gl represents better forming ability of the glass. The existing ability criterion parameters k_gl is 0.214. The values reveal that the STH-3 glass possesses good forming ability. Obviously, the STH-3 glass with a high glass transition temperature T_g is desired and has a potential for high power laser application.

Fig. 9 DSC curve of silicate glass (STH-3).

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4. Conclusion

In summary, we have prepared samples of Tm³⁺/Ho³⁺ co-doped silicate glass by the melt-quenching method. The infrared emission of Tm³⁺/Ho³⁺ co-doped silicate glass is obtained under 808 nm excitation. The Judd-Ofelt theory has been used to calculate the intensity parameters and emission cross sections. In consideration of the emission property, fluorescence lifetimes, the content of hydroxyl and other factors, we select the STH-3 as the best candidate. The predicted spontaneous transition probability and emission cross section of Ho³⁺: ⁵I₇→⁵I₈ transition in STH-3 reach 128.89 s⁻¹ and 7.59 × 10⁻²¹ cm² at the wavelength of 2065 nm, respectively. It is found that Tm³⁺ ions can transfer their energy to Ho³⁺ with high efficiency (67.02%), which can enhance the pump absorption. The energy transfer constant from Tm³⁺ to Ho³⁺ can reach as high as 63.2 × 10⁻⁴⁰ cm⁶/s and fluorescence lifetime of STH-3 is 0.637 ms. Besides, the thermal stable is nice. The above results indicate that the present glasses should have potential technological applications in 2 μm laser materials.

Acknowledgments

This research was financially supported by the Chinese National Natural Science Foundation (No. 51372235, 51272243, 51472225 and 61308090), Zhejiang Provincial Natural Science Foundation of China (No.LR14E020003), the International Science & Technology Cooperation Program of China (Grant no. 2013DFE63070), and Public Technical International Cooperation project of Science Technology Department of Zhejiang Province(2015c340009)

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Ω_t (10⁻²⁰cm²)	Ω₂	Ω₄	Ω₆	Reference
Germanate	3.35	2.16	0.94	[25]
Tellurite	4.98	0.99	2.96	[26]
ZBLAN	2.98	1.40	1.04	[27]
Silicate	3.14	3.04	0.92	[15]
Silicate STH-3	3.60 4.87	2.30 2.05	0.65 1.39	[22] This work

Ω_t (10⁻²⁰cm²)	Ω₂	Ω₄	Ω₆	Reference
Germanate	3.35	2.16	0.94	[25]
Tellurite	4.98	0.99	2.96	[26]
ZBLAN	2.98	1.40	1.04	[27]
Silicate	3.14	3.04	0.92	[15]
Silicate STH-3	3.60 4.87	2.30 2.05	0.65 1.39	[22] This work

Spectroscopy of thulium and holmium co-doped silicate glasses

Abstract

1. Introduction