Fluoroindate glasses co-doped with Pr³⁺/Er³⁺ for near-infrared luminescence applications

Abstract

Fluoroindate glasses co-doped with Pr³⁺/Er³⁺ ions were synthesized and their near-infrared luminescence properties have been examined under selective excitation wavelengths. For the Pr³⁺/Er³⁺ co-doped glass samples several radiative and nonradiative relaxation channels and their mechanisms are proposed under direct excitation of Pr³⁺ and/or Er³⁺. The energy transfer processes between Pr³⁺ and Er³⁺ ions in fluoroindate glasses were identified. In particular, broadband near-infrared luminescence (FWHM = 278 nm) associated to the ¹G₄ → ³H₅ (Pr³⁺), ¹D₂ → ¹G₄ (Pr³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions of rare earth ions in fluoroindate glass is successfully observed under direct excitation at 483 nm. Near-infrared luminescence spectra and their decays for glass samples co-doped with Pr³⁺/Er³⁺ are compared to the experimental results obtained for fluoroindate glasses singly doped with rare earth ions.

Introduction

Fluoroindate glasses belong to the low-phonon Heavy Metal Fluoride Glass (HMFG) family, which have been extensively studied for their numerous optical applications. From literature data it is well-known that fluoroindate glasses containing rare earth ions with their lower phonon energies close to 510 cm⁻¹ are promising materials for up-conversion luminescence applications. For example, the laser based on the orange-to-ultraviolet conversion in a Nd³⁺ doped fluoroindate glass powder has been well demonstrated¹. These effects are practically not possible to obtain in Nd³⁺ doped oxide or oxyfluoride glass host matrices. Generally, the efficient up-conversion luminescence of Pr³⁺, Tm³⁺ and Ho³⁺ ions in fluoroindate glasses^2,3,4 was successfully observed. Moreover, the up-conversion luminescence spectra of Pr³⁺, Ho³⁺ and Tm³⁺ ions in fluoroindate glasses^5,6,7,8,9 are enhanced drastically in the presence of Yb³⁺. In the later system⁹, i.e. Tm³⁺/Yb³⁺ co-doped fluoroindate glass, the blue up-conversion luminescence of Tm³⁺ ions is highly increased with Yb³⁺ concentration. For the optimum Tm³⁺ concentration (0.5 mol%), the up-conversion emission intensity was increased by a factor about 100 by co-doping with 2.25 mol% of Yb³⁺. In particular, fluoroindate glasses singly doped with Er³⁺ ions and doubly doped with Er³⁺/Yb³⁺ ions have been examined for up-conversion luminescence^10,11,12,13, which was measured under 790 nm, 980 nm or 1480 nm laser excitation. The up-conversion luminescence processes of Er³⁺ were analyzed with activator concentration, temperature, pumping wavelength and power of diode laser used as the excitation source. In general, the up-conversion results in a strong green emission and weaker blue and red emissions and the Yb³⁺ co-doping will certainly increase the efficiency of an up-conversion-based optical device. Further investigations confirmed this hypothesis. An intensity enhancement of 4.5 for the green up-conversion luminescence in Er³⁺/Yb³⁺ co-doped fluoroindate glass has been obtained by focusing the incoming beam with a 3.8 μm silica microsphere¹⁴. Also, these experimental results open a new method to improve the up-conversion emission intensity in biological samples with rare earth doped nanoparticles that can be used as nano-sensors. In another case, the generation of photocurrent in a commercial solar cell has been achieved under excitation at 1480 nm in fluoroindate glass samples co-doped with Er³⁺/Yb³⁺ ions¹⁵.

Fluoroindate glasses with their excellent spectroscopic properties offer the possibility of using these materials not only in the operation of erbium up-conversion lasers. Also, they belong to promising glass materials emitting near-infrared radiation. Nowadays there is a great interest in compact lasers operating in the near-infrared (1.5 μm) and mid-infrared (2.8 μm) for optical communications, medical and eye-safe light detecting and ranging applications. However, the near-infrared luminescence studies were limited practically to fluoroindate glasses containing Er³⁺ or Er³⁺/Yb³⁺ ions^10,13. Luminescence spectra exhibit a highly intense signal at 1.5 μm due to ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺. Decay measurements indicate that luminescence lifetime for the upper ⁴I_13/2 laser state of Er³⁺ ions in fluoroindate glass is quite long and its value is close nearly to 10 ms at room temperature¹⁰. The mid-infrared luminescence results obtained for Er³⁺/Yb³⁺ ions in fluoroindate glass suggest that co-doping with Yb³⁺ favors only the up-conversion processes which depopulate efficiently the ⁴I_11/2 state, reducing the emission intensity related to ⁴I_11/2 → ⁴I_13/2 transition of Er³⁺ ions at 2.8 μm¹³. These phenomena are important for diode-pumped solid-state lasers, which could provide a compact and efficient device with the advantage of easy coupling with fiber integrated optical systems. For diode-pumped lasers luminescence at near-infrared (1.5 μm) and mid-infrared (2.8 μm) spectral ranges, the trivalent Er³⁺ seems to be an excellent candidate due to ⁴I_13/2 → ⁴I_15/2 and ⁴I_11/2 → ⁴I_13/2 electronic transitions, respectively. From accumulated data it is quite well-known that the transmission capacity of WDM systems (wavelength division multiplexing) should be improved. Many research groups are looking for a new glass matrices and their fibers in order to obtain signal amplification beyond the conventional optical NIR window between 1530 and 1565 nm, commonly known as the C-band. In fact, spectral bandwidth for the commercial EDFA system (Erbium Doped Fiber Amplifiers) based on silicate glass is equal to 40 nm at C-band and broadband near-infrared transmission is limited, unfortunately¹⁶. For that reason, several glass matrices containing rare earth ions are still tested in order to obtain excellent materials with broader emission bands and longer lifetimes. These improved spectroscopic parameters are necessary for signal amplification in telecommunication and laser technology. Some recently published works for fluoroindate glass clearly indicate that a logical extension of Er³⁺-doped systems would be the addition of other rare earth ions such as Tm³⁺ or Ho³⁺ ions^17,18. The interesting results for fluoroindate glasses co-doped with Ho³⁺/Tm³⁺ and Ho³⁺/Nd³⁺ ions^19,20 emitting near-infrared and mid-infrared radiation at 2 μm and 3.9 μm under direct excitation (888 nm or 808 nm) have been also well presented and discussed in details. Among rare earths, the trivalent Pr³⁺ ions exhibit broadband near-infrared luminescence covering a wavelength range from 1.2 μm to 1.7 μm, which is really important for optical fiber amplifiers operating at O-band (1260–1360 nm), E-band (1360–1460 nm), S-band (1460–1530 nm), C-band (1530–1565 nm), and L-band (1565–1625 nm)²¹. The recent results indicate that glasses^22,23,24,25 and crystals²⁶ co-doped with Pr³⁺/Er³⁺ seems to be quite good candidates for broadband near-IR luminescence. The superbroadband near-IR luminescence is contributed mainly by the ¹D₂ → ¹G₄ (Pr³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions which lead to emission lines located at about 1.48 and 1.53 μm²⁵. To the best of our knowledge, these phenomena were not yet examined for fluoroindate glasses co-doped with Pr³⁺/Er³⁺.

Results and discussion

Fluoroindate glasses singly doped with Pr³⁺ and Er³⁺

The optical absorption spectra of fluoroindate glasses singly doped with Pr³⁺ and Er³⁺ ions were recorded at room temperature. They are presented in Fig. 1.

Figure 1

Absorption spectra of fluoroindate glasses singly doped with Pr³⁺ (top) and Er³⁺ (bottom).

In general, the absorption bands are inhomogeneously broadened and clearly resolved, characteristic for 4f² (Pr³⁺) and 4f¹¹ (Er³⁺) transitions of rare earths. They are attributed to the electronic transitions originating from the ³H₄ (Pr³⁺) and ⁴I_15/2 (Er³⁺) ground states to the higher-lying excited states of rare earths. From the optical absorption spectra the experimental oscillator strengths have been determined. The band intensities are estimated by measuring the areas under the absorption lines using the equations:

$$P{}_{meas} = 4.318 \times 10^{ - 9} \int {\varepsilon (\nu )d\nu }$$

(1)

$$\varepsilon ({\text{n}}) \, = {\text{ A }}/{\text{ c l}}$$

(2)

where: ∫ε(ν) represents the area under the absorption line, A indicates the absorbance, c is the concentration of the Ln³⁺ ion (Ln = Pr or Er) in mol l⁻¹ and l denotes the optical path length. In the next step, the theoretical oscillator strengths for each transition of Pr³⁺ and Er³⁺ ions were calculated using the Judd–Ofelt theory^27,28. The theoretical oscillator strength is defined as follows:

$$P_{calc} = \frac{{8\pi^{2} mc(n^{2} + 2)^{2} }}{3h\lambda (2J + 1) \cdot 9n} \times \sum\limits_{t = 2,4,6} {\Omega_{t} } ( < 4f^{N} J\left\| {U^{t} } \right\|4f^{N} J^{\prime} > )^{2}$$

(3)

where m is the mass of the electron, c is the velocity of light, h is the Planck constant and λ is the mean wavelength of the each transition. In order to perform the analysis, the refractive index of the medium (n = 1.48 for fluoroindate glass) and U^t2 from Ref.²⁹ representing the square of the matrix elements of the unit tensor operator U^t (connecting the initial J and final J' states) were used for calculations. Next, the experimental and theoretical oscillator strengths have been compared. They are listed in Table 1.

Table 1 Measured and calculated oscillator strengths for Pr³⁺ and Er³⁺ ions in fluoroindate glasses.

Transitions are from the ³H₄ (Pr³⁺) and ⁴I_15/2 (Er³⁺) ground states to the levels indicated. The three phenomenological intensity parameters Ω_t (t = 2, 4, 6) are found to be (in 10^–20 cm² units): Ω₂ = 2.01 ± 0.81, Ω₄ = 5.25 ± 0.90, Ω₆ = 5.10 ± 0.35, rms = 1.4 × 10^–6 (for Pr³⁺) and Ω₂ = 1.47 ± 0.22, Ω₄ = 1.51 ± 0.30, Ω₆ = 1.69 ± 0.11, rms = 0.7 × 10^–6 (for Er³⁺), respectively.

The three phenomenological Judd–Ofelt intensity parameters Ω_t (t = 2, 4, 6) for rare earth ions in fluoroindate glasses are found to be (in 10^–20 cm² units); Ω₂ = 2.01 ± 0.81, Ω₄ = 5.25 ± 0.90, Ω₆ = 5.10 ± 0.35, rms = 1.4 × 10^–6 (for Pr³⁺) and Ω₂ = 1.47 ± 0.22, Ω₄ = 1.51 ± 0.30, Ω₆ = 1.69 ± 0.11, rms = 0.7 × 10^–6 (for Er³⁺). The fit quality was expressed by the magnitude of the root-mean-square deviation, which is defined by rms = Σ (P_meas – P_calc)².

In the next step, the intensity parameters Ω_t (t = 2, 4, 6) were applied to calculate the radiative transition rates, luminescence branching ratios and radiative lifetimes. The radiative transition rates A_J for excited levels of Pr³⁺ and Er³⁺ ions from an initial level J to a final ground level J’ were calculated using the following relation:

$$A_{J} = \frac{{64\pi ^{4} e^{2} }}{{3h(2J + 1)\lambda ^{3} }} \times \frac{{n(n^{2} + 2)^{2} }}{9} \times \sum\limits_{{t = 2,4,6}} {\Omega _{t} } ( < 4f^{N} J\left\| {U^{t} } \right\|4f^{N} J' > )^{2}$$

(4)

The total radiative emission rate A_T involving all the intermediate terms is the sum of the A_J terms. The radiative lifetime τ_rad of an excited level is the inverse of the total radiative emission rate (Eq. 5), whereas the luminescence branching ratio β is due to the relative intensities of transitions from excited level to all terminal levels (Eq. 6).

$$\tau _{{rad}} = \frac{1}{{\sum\limits_{i} {A_{{Ji}} } }} = \frac{1}{{A_{T} }}$$

(5)

$$\beta = \frac{{A_{J} }}{{\sum\limits_{i} {A_{Ji} } }}$$

(6)

The calculated radiative transition rates A_J, luminescence branching ratios β and corresponding radiative lifetimes τ_rad for Pr³⁺ and Er³⁺ ions in fluoroindate glasses are presented in Table 2. The calculation results are limited to transitions originating from the ¹D₂ and ¹G₄ excited levels of Pr³⁺ as well as the ⁴I_11/2 and ⁴I_13/2 excited levels of Er³⁺, from which the main luminescence lines in the near-infrared spectral range (950–1650 nm) occur.

Table 2 Calculated radiative transition rates A_J, luminescence branching ratios β and corresponding radiative lifetimes τ_rad for Pr³⁺ and Er³⁺ ions in fluoroindate glasses.

Measured lifetimes τ_m for the ¹D₂ and ¹G₄ (Pr³⁺) and the ⁴I_11/2 and ⁴I_13/2 (Er³⁺) excited levels are also given in Table 2 in order to determine quantum efficiencies η of rare earth ions in fluoroindate glasses using the following equation:

$$\eta = \frac{{\tau_{m} }}{{\tau_{rad} }} \times 100\%$$

(7)

Figure 2 presents near-infrared luminescence spectra of fluoroindate glasses singly doped with Er³⁺ and Pr³⁺. The spectra were measured for glass samples in the 950–1650 nm ranges. For both Er³⁺ and Pr³⁺ doped glass samples, the activator concentration is equal to 0.1 mol%.

Figure 2

Near-infrared luminescence spectra of fluoroindate glasses doped with Pr³⁺ (top) and Er³⁺ (bottom).

For Er³⁺ singly doped glass, the spectra consist of luminescence bands centered at 980 nm and 1535 nm, which correspond to transitions originating from the ⁴I_11/2 and ⁴I_13/2 excited levels to the ⁴I_15/2 ground level, respectively. When glass sample is excited at higher-lying ⁴F_7/2 (488 nm) or ²H_11/2 (523 nm) level, luminescence band near 1230 nm associated to the ⁴S_3/2 → ⁴I_11/2 (Er³⁺) transition is also well observed.

The most intense luminescence band is related to the main ⁴I_13/2 → ⁴I_15/2 near-infrared laser transition of Er³⁺. Its luminescence intensity depends strongly on excitation wavelengths. Several spectroscopic parameters for the ⁴I_13/2 → ⁴I_15/2 near-infrared transition of Er³⁺ ions in fluoroindate glass at 1535 nm, necessary for optical and laser characteristics, were determined. The luminescence linewidth defined as the full width at half maximum (FWHM) for the ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transition is equal to 72 nm. The measured luminescence lifetime for the upper ⁴I_13/2 laser level of Er³⁺ ions in fluoroindate glass is close to 8.6 ms, whereas radiative lifetime calculated from the Judd–Ofelt framework seems to be 8.62 ms (Table 2). Thus, the quantum efficiency for the upper ⁴I_13/2 laser level of Er³⁺ ions in fluoroindate glass is nearly ~ 100%. The same situation is also observed for the higher-lying ⁴I_11/2 level of Er³⁺, for which the measured and calculated radiative lifetimes and the quantum efficiency are close to τ_m = 6.65 ms, τ_rad = 6.71 ms and η =  ~ 99%. The quantum efficiency for the main ⁴I_13/2 → ⁴I_15/2 (Er³⁺) laser transition at 1535 nm in fluoroindate glass is significantly higher in comparison to the values obtained earlier for borate (η = 3%) and germanate (η = 71%) glasses³⁰. This phenomenon is related directly to the lower phonon energy (hω = 510 cm⁻¹) of the fluoroindate glass-host¹⁷.

The peak stimulated emission cross-section σ_em is one of the most important radiative parameters and should be also examined because the strong luminescence band at 1535 nm due to the ⁴I_13/2 → ⁴I_15/2 transition has been considered as a potential near-infrared laser emission of Er³⁺ ions in fluoroindate glass. It is generally accepted that an efficient laser transition is characterized by relatively large value of σ_em. The peak stimulated emission cross-section σ_em can be obtained from the calculated radiative transition rate A_J using the following relation:

$$\sigma _{{em}} = \frac{{\lambda _{p}^{4} }}{{8\pi cn^{2} \Delta \lambda }}A_{J}$$

(8)

where λ_p is the peak luminescence wavelength, Δλ is the effective linewidth (FWHM), n is the refractive index and c is the velocity of light. The maximum peak stimulated emission cross-section is close to nearly 5.4 × 10^–21 cm² at 1535 nm for fluoroindate glass and its value is typical for fluoride laser glasses containing Er³⁺ ions³¹.

Near-infrared luminescence spectra of Pr³⁺ ions in fluoroindate glasses are also presented on Fig. 2 (bottom). In general, three near-infrared luminescence bands are quite well observed under different excitation wavelengths. They correspond to ¹D₂ → ³F_3,4 (1050 nm), ¹G₄ → ³H₅ (1335 nm) and ¹D₂ → ¹G₄ (1450 nm) electronic transitions of Pr³⁺. However, the intensities of luminescence bands of Pr³⁺ are extremely low, when glass sample was excited at higher lying ³P₂ (445 nm), ³P₁ (469 nm) or ³P₀ (480 nm) level, respectively. In this case, the ¹G₄ → ³H₅ transition with its FWHM value equal to about 195 nm is dominant transition of trivalent Pr³⁺. The ¹G₄ measured lifetime of Pr³⁺ (τ_m = 0.365 ms) is considerably lower than corresponding calculated radiative lifetime (τ_m = 2.8 ms). Although the luminescence branching ratio for the ¹G₄ → ³H₅ transition at 1335 nm is relatively high and its value is close to β = 65%, the quantum efficiency (η = 13%) for the ¹G₄ excited level of Pr³⁺ ions is rather low (Table 2). Completely different situation is observed for Pr³⁺ ions in fluoroindate glass under direct excitation of ¹D₂ state at 590 nm. The near-infrared luminescence bands originating to transitions from the ¹D₂ level of Pr³⁺ ions are highly intense compared to the ¹G₄ → ³H₅ transition at 1335 nm. In particular, broadband near-infrared spectra covering a wavelength range from 1300 nm to about 1650 nm are great of interest and really important for optical telecommunication²¹. The main most intense near-infrared luminescence band near 1450 nm is assigned to the ¹D₂ → ¹G₄ transition of Pr³⁺ and its value of FWHM is close to 130 nm. Although the luminescence branching ratio (β = 9%), the measured (τ_m = 0.308 ms) and calculated radiative (τ_m = 0.410 ms) lifetimes are not too high, the quantum efficiency for the ¹D₂ level of Pr³⁺ is close to η = 75% (Table 2). Furthermore, the peak stimulated emission cross-section for the ¹D₂ → ¹G₄ transition of Pr³⁺ ions in fluoroindate glass was determined. From literature data it is well-known that inorganic glasses singly doped with praseodymium exhibit fascinating prospects in broadband near-infrared fiber amplifier, when the emission cross-section profile is large²¹. In our case, the peak stimulated emission cross-section for the ¹D₂ → ¹G₄ transition of Pr³⁺ ions is also relatively large. Its value is close to 0.5 × 10⁻²⁰cm². Finally, the stimulated emission cross-section (σ_em), the emission linewidth (FWHM) and the measured lifetime (τ_m) were applied to calculate the gain bandwidth (σ_em × FWHM product) and the figure of merit (FOM) given by σ_em × τ_m, respectively. It is interesting to see that the σ_em × FWHM product for the ¹D₂ → ¹G₄ transition of Pr³⁺ ions in fluoroindate glass is equal to 65 × 10⁻²⁷ cm³, which is rather low compared to the following values: 176.4 × 10⁻²⁷ cm³ for fluorotellurite glass³² and 203.5 × 10⁻²⁷ cm³ for gallo-germanate glass with BaF₂³³. This behavior is mainly due to the fact that luminescence linewidth for the ¹D₂ → ¹G₄ transition of Pr³⁺ ions in fluoroindate glass (FWHM = 130 nm) is considerably lower than values obtained for fluorotellurite glass (FWHM = 196 nm) and gallo-germanate glass (FWHM = 208.5 nm). On the other hand, the figure of merit (FOM) for the ¹D₂ → ¹G₄ transition of Pr³⁺ ions in fluoroindate glass is relatively larger (σ_em x τ_m = 154 × 10⁻²⁶ cm²s) in comparison to the values 83.1 × 10^-26cm²s for fluorotellurite glass³² and 107.5 × 10⁻²⁶ cm²s for gallo-germanate glass modified by BaF₂³³. Our experimental studies and theoretical calculations demonstrate that Pr³⁺ singly doped fluoroindate glasses are promising for broadband near-infrared amplifiers.

Fluoroindate glasses co-doped with Pr³⁺/Er³⁺

Figure 3 present absorption (top) and excitation (bottom) spectra of fluoroindate glasses co-doped with Pr³⁺/Er³⁺. The results are compared to the absorption spectra, which were measured for rare earth singly doped glass samples (inset).

Figure 3

Absorption (top) and excitation (bottom) spectra of fluoroindate glasses co-doped with Pr³⁺/Er³⁺. The results are compared to the spectra, which were measured for rare earth singly doped glass samples.

The absorption spectrum for Pr³⁺/Er³⁺ co-doped glass sample consists of several bands corresponding to both electronic transitions of rare earths originating from the ground levels ³H₄ (Pr³⁺) and ⁴I_15/2 (Er³⁺) to the excited levels. In particular, two spectral ranges near 590 nm and between 480 and 488 nm are important from the spectroscopic point of view. In the first spectral range near 590 nm, the absorption band associated to ³H₄ → ¹D₂ transition of Pr³⁺ is located. In the second spectral range, two absorption bands related to ³H₄ → ³P₀ transition of Pr³⁺ (480 nm) and ⁴I_15/2 → ⁴F_7/2 transition of Er³⁺ (488 nm) are quite well overlapped.

The excitation spectra measured for Pr³⁺/Er³⁺ co-doped glass samples under monitoring emission wavelength 1530 nm (⁴I_13/2 Er³⁺) give interesting results. It is clearly shown in these two spectral ranges denoted as (*) that the intensity of band at 590 nm assigned to ³H₄ → ³P₀ transition of Pr³⁺ ions increase, whereas the relative band intensities of ³H₄ → ³P₀ (Pr³⁺) and ⁴I_15/2 → ⁴F_7/2 (Er³⁺) transitions located between 480 and 488 nm are drastically changed with increasing Pr³⁺ concentration. It also suggests that the energy transfer process between Pr³⁺ and Er³⁺ ions in fluoroindate glass occurs.

In the next step, luminescence spectra of fluoroindate glasses co-doped with Pr³⁺/Er³⁺ have been examined under different excitation wavelengths. In general, Pr³⁺/Er³⁺ co-doped glasses belong to amorphous systems emitting visible light and near-infrared radiation. It is quite well-known that several visible emission bands from the ³P₀, ¹D₂ (Pr³⁺) and the (²H_11/2,⁴S_3/2), ⁴F_9/2 excited levels of rare earth ions can be well observed for Pr³⁺/Er³⁺ co-doped glasses³⁴. These aspects for fluoroindate glass are not presented and discussed here. Our investigations are limited to Pr³⁺/Er³⁺ co-doped fluoroindate glass emitting near-infrared radiation. Figure 4 shows near-infrared luminescence spectra of fluoroindate glasses co-doped with Pr³⁺/Er³⁺ under direct 488 nm (top) and 590 nm (bottom) excitation. The results are compared to the spectra, which were measured for rare earth singly doped glass samples.

Figure 4

Near-infrared luminescence spectra of fluoroindate glasses co-doped with Pr³⁺/Er³⁺ ions under direct 488 nm (top) and 590 nm (bottom) excitation. The results are compared to the spectra, which were measured for rare earth singly doped glass samples.

Four emission bands at 980 nm, 1230 nm, 1335 nm and 1535 nm are observed for Pr³⁺/Er³⁺ co-doped fluoroindate glasses under direct excitation of ⁴F_7/2 level of Er³⁺ at 488 nm. They correspond to ⁴I_11/2 → ⁴I_15/2 (Er³⁺), ⁴S_3/2 → ⁴I_11/2 (Er³⁺), ¹G₄ → ³H₅ (Pr³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions, respectively. In contrast to Er³⁺ singly doped glass, the intensity of emission band at 1535 nm due to the main ⁴I_13/2 → ⁴I_15/2 (Er³⁺) near-infrared laser transition is reduced, whereas the emission band intensities related to ⁴I_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_11/2 (Er³⁺) transitions increase with increasing Pr³⁺ concentration in samples co-doped with Pr³⁺/Er³⁺. The emission linewidth for ⁴I_13/2 → ⁴I_15/2 (Er³⁺) near-infrared laser transition is nearly independent on Pr³⁺ concentration and its FWHM value is close to 75 ± 3 nm. The intensity of emission band at 1335 nm associated to the ¹G₄ → ³H₅ (Pr³⁺) transition increase with increasing activator (Pr³⁺) concentration. The experimental results for Pr³⁺/Er³⁺ co-doped fluoroindate glasses clearly suggest (a) the presence of the energy transfer process from Er³⁺ to Pr³⁺ and (b) near-infrared emission of Pr³⁺ at 1335 nm under direct excitation of Er³⁺. Further investigations indicate that five near-infrared luminescence bands at about 980 nm, 1050 nm, 1335 nm, 1450 nm and 1535 nm are quite well observed for Pr³⁺/Er³⁺ co-doped fluoroindate glasses under direct excitation of ¹D₂ level of Pr³⁺ at 590 nm. Emission bands are due to the ⁴I_11/2 → ⁴I_15/2 (Er³⁺), ¹D₂ → ³F_3,4 (Pr³⁺), ¹G₄ → ³H₅ (Pr³⁺), ¹D₂ → ¹G₄ (Pr³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺), respectively. The presence of ⁴I_11/2 → ⁴I_15/2 and ⁴I_13/2 → ⁴I_15/2 transitions of Er³⁺ in the luminescence spectra measured under direct excitation of Pr³⁺ confirms the energy transfer process from Pr³⁺ to Er³⁺ ions in fluoroindate glasses co-doped with Pr³⁺/Er³⁺. Firstly, broadband near-infrared luminescence at around 1000 nm in Pr³⁺/Er³⁺ co-doped glass samples is due to two overlapped ⁴I_11/2 → ⁴I_15/2 (Er³⁺) and ¹D₂ → ³F_3,4 (Pr³⁺) transitions. These phenomena were studied previously for fluorotellurite glass co-doped with Pr³⁺/Er³⁺ ions³⁵. Secondly, the near-infrared emission band at 1535 nm due to ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transition is overlapped with ¹G₄ → ³H₅ transition of Pr³⁺ in glass samples co-doped with Pr³⁺/Er³⁺. These effects are not observed for Pr³⁺ singly doped glass sample. Furthermore, the intensity of emission band at 1335 nm due to the ¹G₄ → ³H₅ (Pr³⁺) transition enhance, whereas the intensity of emission band at 1450 nm corresponding to ¹D₂ → ¹G₄ (Pr³⁺) is reduced with increasing Pr³⁺ concentration in glass composition. Interestingly, luminescence linewidth for broadband near-infrared luminescence covering a wavelength range from 1300 nm to about 1650 nm and associated to two overlapped ¹G₄ → ³H₅ and ¹D₂ → ¹G₄ transitions of Pr³⁺ enhance drastically in fluoroindate glasses co-doped with Pr³⁺/Er³⁺. In contrast to Pr³⁺ singly doped glass (FWHM = 130 nm), emission linewidth for glass samples co-doped with Pr³⁺/Er³⁺ increase to nearly 225 ± 5 nm and its value slightly depends on Pr³⁺ concentration. All near-infrared transitions indicated for Pr³⁺/Er³⁺ co-doped fluoroindate glasses excited directly at 488 nm or 590 nm are schematized on the energy level diagram (Fig. 5).

Figure 5

Energy level scheme for fluoroindate glass co-doped with Pr³⁺/Er³⁺. All transitions are also indicated for glass samples excited at 488 nm (top) and 590 nm (bottom).

The mechanisms for Pr³⁺/Er³⁺ co-doped glass system involving possible energy transfer and radiative/nonradiative relaxation channels, i.e. energy transfer routes (ET), multiphonon relaxation (MPR) and cross-relaxation (CR) processes, have been proposed and discussed in details by Zhou et al.²⁵.

When glass sample co-doped with Pr³⁺/Er³⁺ is directly pumped at 488 nm, the excited level ⁴F_7/2 (Er³⁺) is well populated by the ground-state absorption process (GSA). The energy transfer process (ET) from the ⁴F_7/2 (Er³⁺) level to the ³P₀ (Pr³⁺) level is not observed and it is rather impossible, because the ³P₀ level of Pr³⁺ is higher-lying than the ⁴F_7/2 level of Er³⁺. There is also the main reason that near-infrared luminescence bands from the higher-lying ¹D₂ level of Pr³⁺ ions in fluoroindate glasses are not observed under 488 nm excitation of Er³⁺. The excitation energy transfers very fast nonradiatively to the ⁴S_3/2 level by multiphonon relaxation (MPR) and then relaxes radiatively generating near-infrared emission at 1230 nm associated to the ⁴S_3/2 → ⁴I_11/2 transition of Er³⁺. The another possible way to depopulate ⁴F_7/2 level is nonradiative relaxation to the ⁴I_11/2 level by MPR process owing to small energy gaps between the ⁴F_7/2 excited level and lower-lying levels or cross-relaxation process (CR): [⁴F_7/2, ⁴I_15/2 → ⁴I_11/2, ⁴I_11/2], when concentration of Er³⁺ ions in glass sample is relatively high. Next, part of excitation energy is transferred nonradiatively from the ⁴I_11/2 level to the lower-lying ⁴I_13/2 level by MPR process, and consequently two near-infrared luminescence bands centered at 980 nm and 1535 nm due to ⁴I_11/2 → ⁴I_15/2 and ⁴I_13/2 → ⁴I_15/2 transitions are well observed. Finally, the excitation energy is successfully transferred from the ⁴I_11/2 (Er³⁺) level to the ¹G₄ (Pr³⁺) level (ET1) and the ⁴I_13/2 (Er³⁺) level to the ³F_3,4 (Pr³⁺) level (ET2) by energy transfer processes. In particular, the ET1 process plays the important role, leading to near-infrared luminescence at 1335 nm due to ¹G₄ → ³H₅ transition of Pr³⁺ under direct excitation of Er³⁺.

When glass sample co-doped with Pr³⁺/Er³⁺ is directly pumped at 590 nm, the excited level ¹D₂ (Pr³⁺) is well populated by the ground-state absorption process (GSA). Part of excitation energy relaxes radiatively and two near-infrared luminescence bands at about 1050 nm and 1450 nm are observed, which correspond to transitions originating from the ¹D₂ excited level to the lower-lying ³F_3,4 and ¹G₄ levels of Pr³⁺. Another part of excitation energy is transferred nonradiatively by well-known CR process: [¹D₂, ³H₄ → ¹G₄, ³F_3,4]³⁶. At this moment, it should be also pointed that the second CR process [¹D₂, ³H₄ → ³F_3,4, ¹G₄] was also proposed, but it is still not sure which one is more dominant. For Pr³⁺/Er³⁺ co-doped samples, another cross-relaxation route that depopulate efficiently the ¹D₂ (Pr³⁺) level is also possible. This can be attributed to the following CR process: [¹D₂ (Pr³⁺), ⁴I_15/2 (Er³⁺) → ¹G₄ (Pr³⁺), ⁴I_13/2 (Er³⁺)]. Thus, part of the excitation energy is transferred from Pr³⁺ to Er³⁺ ions, giving important contribution to near-infrared emission at 1535 nm related to ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺. However, the near-infrared luminescence at 980 nm due to ⁴I_11/2 → ⁴I_15/2 transition of Er³⁺ is also successfully observed under excitation of Pr³⁺ ions at 590 nm (Fig. 4). The back energy transfer process from the ¹G₄ (Pr³⁺) level to the ⁴I_11/2 (Er³⁺) level is rather impossible, because the ⁴I_11/2 level of Er³⁺ is higher-lying than the ¹G₄ level of Pr³⁺.

Based on the energy level diagram, the ⁴I_11/2 level of Er³⁺ ions in Pr³⁺/Er³⁺ co-doped glass samples can be populated in two ways. First, the presence of the phonon-assisted energy transfer process from the ¹D₂ (Pr³⁺) level to the ⁴F_9/2 (Er³⁺) is proposed but it should be ignored considering the relatively large energy gap between them (~ 1500 cm⁻¹) and the absence of luminescence lines from the ⁴F_9/2 level²⁵, for example well-known ⁴F_9/2 → ⁴I_15/2 red transition of Er³⁺ ions at 670 nm. The second way is the possible cross-relaxation process: [¹D₂ (Pr³⁺), ⁴I_15/2 (Er³⁺) → ³F_3,4 (Pr³⁺), ⁴I_11/2 (Er³⁺)]. The question how higher-lying ⁴I_11/2 level of Er³⁺ is populated under direct 590 nm excitation of Pr³⁺ is still open and actual, and further experiments are necessary to understand the population mechanism and multichannel relaxation in Pr³⁺/Er³⁺ co-doped glass systems. The presence of cross-relaxation processes from the ¹D₂ (Pr³⁺) level in fluoroindate glasses co-doped with Pr³⁺/Er³⁺ was confirmed by luminescence spectra and decay measurements.

Luminescence decays from the ¹D₂ level of Pr³⁺ ions in fluoroindate glasses co-doped with Pr³⁺/Er³⁺ are presented in Fig. 6 (top). The luminescence decay curves for the ¹D₂ level of Pr³⁺ were measured under monitoring emission wavelength 1450 nm.

Figure 6

Luminescence decays from ¹D₂ (Pr³⁺) excited state (top) and visible-NIR emission spectra (bottom) of fluoroindate glasses containing Pr³⁺, Er³⁺ and Pr³⁺/Er³⁺.

Based on decays, luminescence lifetimes for the ¹D₂ level of Pr³⁺ were determined. For Pr³⁺/Er³⁺ co-doped glass samples, the ¹D₂ luminescence lifetime is reduced from 197 µs (0.1Er-0.1Pr) to 127 µs (0.1Er-0.3Pr) and 92 µs (0.1Er-0.5Pr) with increasing Pr³⁺ concentration. This behavior may be attributed to the presence of cross-relaxation processes between neighboring pairs Pr³⁺–Pr³⁺ and Pr³⁺–Er³⁺ in fluoroindate glasses. Also, we compare results for Pr³⁺-doped fluoroindate glasses in the absence and presence of Er³⁺. Thus, the measured ¹D₂ lifetime is reduced from 308 µs (0.1Pr) to 197 µs (0.1Er–0.1Pr), whereas the quantum efficiency for the ¹D₂ excited level of Pr³⁺ decreases from 75% (0.1Pr) to 48% (0.1Er–0.1Pr).

Figure 6 (bottom) shows the visible-NIR emission spectra of fluoroindate glasses singly doped with Pr³⁺ and Er³⁺ and doubly doped with Pr³⁺/Er³⁺. In the 630–880 nm spectral range, two emission bands are well observed for Er³⁺ singly doped glass under 488 nm excitation. They correspond to the ⁴F_9/2 → ⁴I_15/2 red transition near 670 nm and the ⁴S_3/2 → ⁴I_13/2 NIR transition at about 840 nm. The ⁴F_9/2 → ⁴I_15/2 red transition at 670 nm is not observed for Pr³⁺/Er³⁺ co-doped glass samples under direct excitation of Pr³⁺ at 590 nm. It was also verified for fluorotellurite glass co-doped with Pr³⁺/Er³⁺ excited at 594 nm²⁵. In comparison to the results obtained for Pr³⁺ singly doped glass, the emission band at about 690 nm related to the ¹D₂ → ³H₅ (Pr³⁺) transition was registered in this spectral range for Pr³⁺/Er³⁺ co-doped glass samples. Unexpectedly, the additional NIR luminescence band at 800 nm corresponding to the ⁴I_9/2 → ⁴I_15/2 transition of Er³⁺ ions was also detected in Pr³⁺/Er³⁺ co-doped fluoroindate glasses under direct excitation ¹D₂ (Pr³⁺) level at 590 nm. For that reason, it is interesting to explain how the ⁴I_9/2 (Er³⁺) level is populated in Pr³⁺/Er³⁺ co-doped glass samples excited at the ¹D₂ (Pr³⁺) level. We postulate the following mechanisms involving cross-relaxation (CR) and energy transfer up-conversion (ETU) processes applied to populate ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2 excited levels of Er³⁺. The excited level ¹D₂ (Pr³⁺) is depopulated through the cross-relaxation processes: [¹D₂ (Pr³⁺), ⁴I_15/2 (Er³⁺) → ¹G₄ (Pr³⁺), ⁴I_13/2 (Er³⁺)] and [¹D₂ (Pr³⁺), ⁴I_15/2 (Er³⁺) → ³F_3,4 (Pr³⁺), ⁴I_11/2 (Er³⁺)]. Thus, both ⁴I_11/2 and ⁴I_13/2 levels are well populated and near-infrared emission bands at 980 nm and 1535 nm due to the ⁴I_11/2 → ⁴I_15/2 and ⁴I_13/2 → ⁴I_15/2 transitions of Er³⁺ occur. Additionally, other processes play significant role in changing the population of ⁴I_11/2 and ⁴I_13/2 excited levels of Er³⁺ ions. The energy transfer up-conversion process (ETU): [⁴I_13/2, ⁴I_13/2 → ⁴I_9/2, ⁴I_15/2] gives important contribution to efficient population of higher-lying ⁴I_9/2 level of Er³⁺ ions³⁷. Further studies for the Er³⁺/Tm³⁺/Pr³⁺ triply doped fluoride glass indicate that the ETU process [⁴I_13/2, ⁴I_13/2 → ⁴I_9/2, ⁴I_15/2] increases population of the ⁴I_9/2 level and then the lower-lying ⁴I_11/2 level can be populated by a fast multiphonon relaxation from the ⁴I_9/2 level, which leads to an increase of mid-infrared emission at 2700 nm³⁸. Owing to presence of ETU process, it is possible to detect near-infrared emission peaking at 800 nm for Pr³⁺/Er³⁺ co-doped fluoroindate glass, which corresponds to the ⁴I_9/2 → ⁴I_15/2 transition of Er³⁺. The origin of near-infrared luminescence of Er³⁺ at 800 nm has been examined in an excellent work published recently³⁹. It was stated that there is no NIR emission found at 800 nm, when the samples are excited to the higher-lying levels (²H_11/2 or ⁴F_9/2) than the ⁴I_9/2 level, and the nonradiative relaxation from the upper excited levels to the ⁴I_9/2 emitting level of Er³⁺ ions is extremely inefficient. The ⁴I_9/2 level of Er³⁺ is mainly populated by the adjacent lower-lying ⁴I_11/2 level upon excitation at 980 nm³⁹. It was also confirmed for Er³⁺ doped chalcogenide glasses and fibers, where the conversion of incoherent infrared light around 4400 nm into a near-infrared signal at 810 nm was obtained by simultaneously 982 nm pumping⁴⁰.

Further spectroscopic analysis of Pr³⁺/Er³⁺ co-doped fluoroindate glasses excited at 488 nm (Fig. 4) clearly indicate that the intensity of near-infrared luminescence band at 1535 nm due to the main ⁴I_13/2 → ⁴I_15/2 (Er³⁺) laser transition is decreased with increasing Pr³⁺ concentration in comparison to the intensities of ⁴I_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_11/2 (Er³⁺) transitions of Er³⁺. Zhou et al.²⁵ suggested that the relative increase of the near-infrared luminescence of Er³⁺ at 1230 nm compared with of 1530 nm can be ascribed to the improved population inversion between the upper and lower levels. Moreover, the decrease of near-infrared luminescence at 1530 nm is due to the energy transfer process from the ⁴I_13/2 (Er³⁺) level to the ³F_3,4 (Pr³⁺) levels. At a consequence, near-infrared emission bands related to transitions originating from the ⁴I_11/2 level to the lower-lying ⁴I_13/2 (2700 nm) and ⁴I_15/2 (980 nm) levels can be enhanced. The luminescence decay analysis for the ⁴I_11/2 and ⁴I_13/2 excited levels of Er³⁺ ions in Pr³⁺/Er³⁺ co-doped fluoroindate glasses confirms this hypothesis.

Figure 7 shows luminescence decay curves for the ⁴I_13/2 and ⁴I_11/2 excited states of Er³⁺ ions in fluoroindate glasses co-doped with Pr³⁺/Er³⁺. The decay curves were measured under monitoring emission wavelength 980 nm and 1530 nm, respectively. In both cases, luminescence decay curves measured for Pr³⁺/Er³⁺ co-doped glass samples are shortened with increasing Pr³⁺ concentration in comparison to glasses singly doped with Er³⁺. Based on decay curves, luminescence lifetimes for the ⁴I_13/2 and ⁴I_11/2 excited levels of Er³⁺ were determined. Based on measured lifetimes for glass samples with the absence and presence of Pr³⁺, the energy transfer efficiencies for the ⁴I_13/2 and ⁴I_11/2 levels of Er³⁺ ions were also calculated. Luminescence lifetimes and energy transfer efficiencies varying with Pr³⁺ concentration are schematically presented on Fig. 8.

Figure 7

Luminescence decays from ⁴I_13/2 (top) and ⁴I_11/2 (bottom) excited states of Er³⁺ ions in fluoroindate glasses co-doped with Pr³⁺/Er³⁺.

Figure 8

Luminescence lifetimes and energy transfer efficiencies varying with Pr³⁺ concentration.

The luminescence lifetime for the ⁴I_13/2 level of Er³⁺ is decreased from 8.60 ms (0.1Er) to 3.40 ms (0.1Er–0.1Pr), 1.96 ms (0.1Er–0.3Pr) and 1.36 ms (0.1Er–0.5Pr) with increasing Pr³⁺ concentration. The reduction of luminescence lifetime for the ⁴I_11/2 level of Er³⁺ ions is also observed. The measured lifetime is changed from 6.65 ms (0.1Er) to 5.91 ms (0.1Er–0.1Pr), 4.92 ms (0.1Er–0.3Pr) and 4.14 ms (0.1Er–0.5Pr) for glass samples with the presence of Pr³⁺. Trends in luminescence lifetimes ⁴I_11/2 and ⁴I_13/2 (Er³⁺) varying with Pr³⁺ concentration are similar, but changes in lifetimes and their corresponding values for glasses singly doped with Er³⁺ and co-doped with Pr³⁺/Er³⁺ are significant from the spectroscopic point of view. It is worthy to notice that luminescence lifetimes obtained for Pr³⁺/Er³⁺ co-doped glasses are generally lower for the ⁴I_13/2 level than ⁴I_11/2 level, in contrast to measured lifetime for Er³⁺ singly doped glass which is higher for the ⁴I_13/2 level (8.60 ms) than ⁴I_11/2 level (6.65 ms). This is also the experimental proof that the intensity of near-infrared emission band at 980 nm due to the ⁴I_11/2 → ⁴I_15/2 transition increase, whereas the intensity luminescence band at 1535 nm related to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ is reduced with increasing Pr³⁺ concentration in fluoroindate glasses co-doped with Pr³⁺/Er³⁺. It was confirmed by luminescence decay curve measurements for similar fluoride ZBLAN glasses co-doped with Pr³⁺/Er³⁺ ions⁴¹, where the influence of the Pr³⁺ codopant on the ⁴I_13/2 and ⁴I_11/2 emission lifetimes has been also studied. For ZBLAN glasses, a significant reduction in the ⁴I_13/2 lifetime was observed following the addition of a small amount of Pr³⁺, while the ⁴I_11/2 lifetime of Er³⁺ was affected to a lesser degree. Thus, the degree of energy transfer process from Er³⁺ to Pr³⁺ ions is found to be much higher for the ⁴I_13/2 level (ET2) than for the ⁴I_11/2 level (Fig. 5). It was also confirmed by calculation of the energy transfer efficiencies in fluoroindate glasses co-doped with Pr³⁺/Er³⁺, where the values of η_ET are higher for the ⁴I_13/2 level than for the ⁴I_11/2 level. The energy transfer efficiency for the ⁴I_13/2 level of Er³⁺ is increased from 60.5% (0.1Er-0.1Pr) to 77.2% (0.1Er-0.3Pr) and 84.2% (0.1Er-0.5Pr), whereas the value of η_ET for the ⁴I_11/2 level of Er³⁺ is changed from 13.3% (0.1Er-0.1Pr) to 27.9% (0.1Er-0.3Pr) and 39.3% (0.1Er-0.5Pr) with increasing Pr³⁺ concentration.

Figure 9 present near-infrared luminescence spectra of fluoroindate glasses co-doped with Pr³⁺/Er³⁺ under selective excitation wavelengths in the spectral range from 480 to 488 nm. This experiment motivated us to demonstrate unambiguously how the excitation wavelengths playing the crucial role in Pr³⁺/Er³⁺ co-doped system influence on broadband near-infrared luminescence covering a spectral range from 1250 to 1650 nm. The inset shows schematically values of emission linewidth (FWHM) depending on the selective excitation wavelengths.

Figure 9

Near-infrared luminescence spectra of fluoroindate glasses co-doped with Pr³⁺/Er³⁺ under selective blue excitation. The inset shows values of FWHM depending on the excitation wavelength.

The mechanisms for Pr³⁺/Er³⁺ co-doped glass system upon selective excitation wavelength at 488 nm (⁴F_7/2 Er³⁺) involving several relaxation routes and two energy transfer processes ET1 (from the ⁴I_11/2 level) and ET2 (from the ⁴I_13/2 level) have been already discussed here.

Upon selective pumping at 480 nm, the ³P₀ level of Pr³⁺ is well populated by the ground state absorption process (GSA) and then the excitation energy transfers by means of two ways. Part of the excitation energy is transferred from the ³P₀ level of Pr³⁺ to the ⁴F_7/2 level of Er³⁺ ions by the energy transfer process ET3. The second way to depopulate the ³P₀ level is very fast non-radiative relaxation to the next lower-lying ¹D₂ level of Pr³⁺ ions through multiphonon relaxation (MPR) and cross-relaxation process (CR): ³P₀, ³H₄ → ¹D₂, ³H₆. In the next step, several near-infrared emission bands originating to radiative transitions from the ¹D₂ and ¹G₄ levels to the lower-lying levels of Pr³⁺ are quite well observed and these aspects have been examined by us in this work. In particular, three near-infrared emission bands of Pr³⁺ and Er³⁺ located at spectral range denoted as (*) are the most interesting and important from the optical point of view. They are due to the ¹G₄ → ³H₅ (Pr³⁺), ¹D₂ → ¹G₄ (Pr³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions of rare earths in fluoroindate glass. Their relative intensities are changed drastically when the excitation wavelength varies from 480 to 488 nm. It is evidently to see that the luminescence band intensities of Pr³⁺ ions are reduced drastically, while the intensity of band due to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions is enhanced when the excitation wavelength is selectively changed from 480 to 488 nm. The inset of Fig. 9 shows values of FWHM depending on the excitation wavelength. When sample co-doped with Pr³⁺/Er³⁺ is excited at 480, 481 or 482 nm, the ¹G₄ → ³H₅ transition of Pr³⁺ ions is dominant transition and emission linewidth seems to be 120–130 nm, which is similar to the value obtained for Pr³⁺ singly doped glass (FWHM = 130 nm). When glass sample co-doped with Pr³⁺/Er³⁺ is excited at spectral range 484/488 nm, the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions is dominant transition and values of FWHM are close to about 75–85 nm, similar to the Er³⁺ singly doped glass (72 nm). Super-broadband near-infrared luminescence with corresponding value of FWHM = 278 nm is successfully observed, when Pr³⁺/Er³⁺ co-doped sample was excited selectively at 483 nm. The luminescence linewidth for Pr³⁺/Er³⁺ co-doped fluoroindate glass (278 nm) is similar to the values of FWHM equal to 236 nm and 296 nm, which were obtained for tellurite glasses doubly doped with Pr³⁺/Er³⁺ ions⁴² and triply doped with Pr³⁺/Er³⁺/Nd³⁺ ions⁴³. Our experimental investigations confirm that the selective excitation wavelength 483 nm is an optimal pump source to obtain super-broadband luminescence in fluoroindate glass co-doped with Pr³⁺/Er³⁺. The previous results suggest that the Pr³⁺/Er³⁺ co-doped phosphate glasses with a 483 nm pump source exciting simultaneously both ³P₀ (Pr³⁺) and ⁴F_7/2 (Er³⁺) levels are promising amorphous materials for broadband optical amplifiers⁴⁴. The energy level diagram and all transitions for fluoroindate glass co-doped with Pr³⁺/Er³⁺ excited selectively at blue spectral region are schematized on Fig. 10.

Figure 10

Energy level scheme and all transitions for fluoroindate glasses co-doped with Pr³⁺/Er³⁺ excited at blue spectral region.

Based on previous scientific reports we confirm that our glass with Pr³⁺/Er³⁺ is a potential material for broadband fiber amplifiers⁴⁵ and its near-IR emission property depends critically on the excitation wavelengths⁴⁶. Similar to oxyfluoroaluminate and fluorozirconate systems⁴⁷, fluoroindate glasses doped with rare earths have potential applications as laser materials.

To summarize, near-infrared luminescence spectra of fluoroindate glasses co-doped with Pr³⁺/Er³⁺ at 1200–1650 nm range have been examined under different excitation wavelengths and the results are compared to glass samples singly doped with Pr³⁺ and Er³⁺ (Fig. 11).

Figure 11

Normalized near-infrared emission spectra corresponding to the ¹G₄ → ³H₅ (Pr³⁺), ¹D₂ → ¹G₄ (Pr³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions of rare earth ions in fluoroindate glasses.

Pr³⁺-singly doped fluoroindate glass shows near-infrared emission centered at 1335 nm and 1450 nm corresponding to the ¹G₄ → ³H₅ and ¹D₂ → ¹G₄ transitions and their relative emission band intensities depend critically on the excitation wavelengths (480 and 590 nm). Er³⁺-singly doped fluoroindate glass under 488 nm excitation shows near-infrared emission near 1535 nm associated to the main ⁴I_13/2 → ⁴I_15/2 laser transition. When fluoroindate glass co-doped with Pr³⁺/Er³⁺ is excited at 590 nm, the intensity of emission band at 1335 nm due to the ¹G₄ → ³H₅ transition is increased in comparison to the ¹D₂ → ¹G₄ transition at 1450 nm, and the spectral linewidth is larger for Pr³⁺/Er³⁺ co-doped glass sample (FWHM = 225 nm) than Pr³⁺ singly doped glass (FWHM = 130 nm).

Superbroadband near-infrared luminescence in Pr³⁺/Er³⁺ co-doped fluoroindate glass under selective excitation wavelength (483 nm) is successfully observed. Broadband emission with its linewidth equal to FWHM = 278 nm covering a wavelength range from 1200 to 1650 nm corresponds to the overlapped ¹G₄ → ³H₅ (Pr³⁺), ¹D₂ → ¹G₄ (Pr³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions of rare earth ions. However, we cannot unambiguously exclude the presence of ³F₃,₄ → ³H₄ (Pr³⁺) transition at about 1600 nm, which was quite well observed in selenide glasses doped with Pr³⁺ and co-doped with Pr³⁺/Er³⁺ ions⁴⁸. Further luminescent studies for Pr³⁺/Er³⁺ co-doped fluoroindate glasses excited at 488 nm suggest that they are promising candidates as dual-wavelength fiber-optic amplifiers for 1335 nm and 1535 nm windows similar to the previous excellent results published for Ge-As-Ga-S glasses co-doped with Pr³⁺/Er³⁺ ions⁴⁹. Moreover reported emission properties of Pr³⁺/Er³⁺ and Er³⁺⁵⁰ doped glasses inclined to use them in optical fiber construction as a fluoroindate fibers started to be valuable for SC generation⁵¹ and lasers^52,53,54 beyond 3 μm. Future research will be devoted for fiber development and its length optimization as we believe to obtain superbroadband emission.

Conclusions

Fluoroindate glasses co-doped with Pr³⁺/Er³⁺ have been investigated for near-infrared luminescence applications. Near-infrared luminescence properties have been examined under selective excitation wavelengths. The radiative and nonradiative relaxation channels involving several processes like multiphonon relaxation (MPR), cross-relaxation (CR), energy transfer up-conversion (ETU) and their mechanisms are proposed in Pr³⁺/Er³⁺ co-doped glass samples under direct excitation of Pr³⁺ and/or Er³⁺ ions and the energy transfer processes (ET) from Pr³⁺ to Er³⁺ and from Er³⁺ to Pr³⁺ were identified. In particular, near-infrared luminescence covering a wavelength range from 1200 to 1650 nm is really important for broadband optical amplifiers and these aspects for fluoroindate glasses have been analyzed in details. Broadband near-infrared emission (FWHM = 278 nm) corresponding to the ¹G₄ → ³H₅ (Pr³⁺), ¹D₂ → ¹G₄ (Pr³⁺) and ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transitions in fluoroindate glass co-doped with Pr³⁺/Er³⁺ is successfully observed under direct 483 nm excitation. Based on luminescence decay measurements, the measured lifetimes for the excited levels of rare earth ions and the energy transfer efficiencies were determined. Near-infrared luminescence spectra and their decays for glass samples co-doped with Pr³⁺/Er³⁺ are compared to the experimental results obtained for fluoroindate glasses singly doped with rare earth ions and theoretical calculations using the Judd–Ofelt framework.

Methods

Synthesis

Fluoroindate glasses singly and doubly doped with rare earth ions (Ln³⁺) with the following molar composition: 37.9InF₃–20ZnF₂–20SrF₂–16BaF₂–4GaF₃–2LaF₃–0.1LnF₃, where Ln = Pr or Er (referred as 0.1Pr and 0.1Er), and (38–x–y)InF₃–20ZnF₂–20SrF₂–16BaF₂–4GaF₃–2LaF₃–xErF₃–yPrF₃, where x = 0.1; y = 0.1, 0.3, 0.5 (referred as 0.1Er–0.1Pr, 0.1Er–0.3Pr and 0.1Er–0.5Pr), were prepared by melting (covered platinum crucible) and quenching method in glove box in a nitrogen atmosphere (O₂, H₂O < 0.5 ppm). Ammonium bifluoride (NH₄HF₂) was added as afluorinating agentto the batch before melting. For the studied glass system, the concentration of ErF₃ is relatively low (0.1 mol%). Thus, the energy transfer processes between Er³⁺ ions are negligibly small. With increasing ErF₃ concentration the thermal stability reduces and strong luminescence quenching is observed due to nonradiative processes corresponding to the Er³⁺–Er³⁺ interaction increase. Also, previous investigations for fluoroindate glasses clearly indicate that the higher activator (Er³⁺) concentrations lead to partial crystallization of fluoroindate glasses⁵⁰. The glass batches were firstly fluorinated at 270 °C for 2 h and then melted at 900 °C for 1 h. Finally, the glass was cast into a stainless steel plate and then annealed at 290 °C for 2 h slowly cooled to room temperature to minimize internal stress during the quenching process. Transparent glassy plates of 10 × 10 mm dimension were obtained. Each glass sample of 1 mm in thickness was polished for optical measurements.

Measurement and characterization

Similar to our previous works for fluoroindate glasses^17,18, the appropriate laser equipment was used for measurements of luminescence spectra and decay curves. The laser system consists of PTI QuantaMaster QM40 spectrofluorimeter, optical parametric oscillator (OPO), Nd:YAG laser (Opotek Opolette 355 LD), double 200 mm monochromators and multimode UVVIS PMT (R928) and Hamamatsu H10330B-75 detectors, and PTI ASOC-10 [USB-2500] oscilloscope. The Varian Cary 5000 UV–VIS–NIR spectrophotometer was used for the absorption spectra measurements and the Metricon 2010 prism coupler for the refractive index at a wavelength of 632.8 nm. Resolution for spectral measurements was 0.1 nm, whereas an accuracy for decay curves was ± 0.5 μs.