1. Introduction

Materials with luminescent characteristics in infrared region attract great interest for potential applications in the fields of optical communication, fluorescence-based biological imaging, photovoltaic systems and so on. [1–6] Lanthanide-doped luminescence materials have the advantages of superior photo-stability and low toxicity compared with quantum dots and fluorescent dyes. Glasses and glass ceramics should be considered because the hosts for dopants are cost-effective and easy to produce in large scale, and rare-earth materials are abundant in nature. Rare-earth doped silicate glasses, phosphate glasses, chalcohalide glasses and telluride glasses, have been reported for near- and mid-infrared luminescence. [7–10] However, nearly all the literatures concentrated on the emissions at a single band or multiband instead of superbroadband seamless luminescence spectra. It is a very valuable work to develop a kind of material or technology for high-power and superbroadband seamless infrared emissions, which can be used in various fields. The possibility for increasing quantum efficiency in the new materials depends on the host and energy transfer (ET).

Tellurite glasses can be a candidate for their various unique properties such as low melting temperature, good chemical resistance, high dielectric constant, low crystallization ability, good transmission for infrared radiation with a wide range of wavelengths, high thermal stability and low phonon energy. [10–16] Because of the above advantages, tellurite glasses have potential applications in medical, civil, photonic and military areas such as in thermal imaging, fiber laser, optical amplifier, and so on. Pure tellurium oxide (TeO₂) is an incipient glass former and does not have glass forming ability under normal quenching rates, thus it requires the presence of other network modifiers to make a glass.

In this work, we choose TeO₂–B₂O₃–MgO (TBM), TeO₂–ZnO–CaO (TZC) and TeO₂–ZnO–CaO–Nd₂O₃ (TZN) glasses as hosts of uninterrupted multiband near-infrared (NIR) emission material upon 808 & 980 nm dual excitation. We propose Er³⁺-Tm³⁺-Nd³⁺ tri-doping scheme to achieve the seamless NIR emission for the first time to our best knowledge. Simultaneous dual-lasers excitation is a facile strategy that has been used in the fields of white light emission, thermometry and other research. [17–19] More efficient utilization of excited photons by dual laser excitation can result in an enhancement of NIR emission intensity and then greatly weaken the negative effect of these rare earth narrow emission band from the f-f transition. Thus, dual-wavelength lasers excitation can be an effective strategy to realize the seamless NIR emission. We have also analysis of optical properties and emission mechanism in TBM, TZN and TZC glasses. Tm³⁺: ³F₄→³H₆ emission around 1800 nm can be enhanced in TZN and TZC glasses in two different effects compared to the TBM glass. 808 nm and 980 nm laser diode low-cost and frequently-used, making the glasses have the ability of using for NIR region applications.

2. Sample preparation and measurement

The three types of tellurite glass samples were prepared by high temperature melting method and subsequent heat treatment with the following composition in a mole percentage:

The absorption spectra of glass in the UV–Vis–NIR region were recorded by a UV–Vis–NIR spectrometer (CARY5000). The NIR emission spectrum was acquired from Zolix SBP300 spectrofluorometer (Zolix Corp., Beijing, China) and detected by InGaAs photo-detector excited by 808 nm, 980 nm and dual 808 & 980 nm laser diode (LD), respectively. The cut-off wavelength in the NIR range of the scanning spectrometer was 2200 nm. And we did the measurement in the region of 1000-2100 nm.

3. Results and discussion

To obtain the typical absorption of Er³⁺, Tm³⁺ and Nd³⁺ around 808 nm and 980 nm region, we do the absorption measurement of the TBM, TZN and TZC glasses. Herein, the absorption spectrum of Er³⁺-Tm³⁺-Nd³⁺ tri-doped tellurite glass sample is shown in Fig. 1. The band positions for Er³⁺ and Tm³⁺ are similar in tellurite glass system, and the absorption spectra are similar for all the samples used in this study. Thus, we show the absorption spectrum of TZN glasses alone. It is known that Tm³⁺ ion is used as proper acceptor and efficient NIR emitting center at ∼1.8 µm attributed to the transition of Tm³⁺:³F₄ to Tm³⁺:³H₆ level. In this system, all the Er³⁺, Tm³⁺ and Nd³⁺ ions have absorption around 808 nm, assigned to the Er³⁺:⁴I_15/2→⁴I_9/2, Tm³⁺: ³H₆→³H₄ and Nd³⁺: ⁴I_9/2→⁴F_5/2 transitions. Moreover, Er³⁺ and Nd³⁺ ions have absorption around 980nm, owing to Er³⁺:⁴I_15/2→⁴I_11/2 and Nd³⁺: ⁴I_9/2→⁴F_5/2 transitions. Thus, the 808 nm and 980nm lights can be excited by all the Er³⁺, Tm³⁺ and Nd³⁺ ions.

 

Fig. 1. Absorption spectrum of Er³⁺-Tm³⁺-Nd³⁺ tri-doped telluride glass.

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Figure 2(a) presents emission spectra of Er³⁺-Tm³⁺ co-doped TBM glasses upon excitation at 808 nm, 980 nm and 808 & 980 nm, respectively. To analysis the Tm³⁺: ³F₄→³H₆ emission around 1.8 µm, the schematic energy level diagrams with the involved ET process are depicted in Fig. 4. Under the excitation of 808 nm LD, the ET processes between Er³⁺ and Tm³⁺. The emission at 1806 nm is observed, but the intensity of Tm³⁺: ³F₄→³H₆ is much lower than intensity of Er³⁺: ⁴I_13/2→⁴I_15/2 peaking at 1540 nm. Upon 808 nm and 980 nm two wavelength excitations, the intensity of Tm³⁺: ³F₄→³H₆ emission is higher than that of the 808 nm wavelength excitation, meaning that 980 nm excitation also makes contribution to the emission of Tm³⁺: ³F₄→³H₆ around 1.8 µm.

 

Fig. 2. (a) Emission spectra of the Er³⁺-Tm³⁺-codoped TBM glasses with 808 nm, 980 nm and 808 & 980nm two wavelength excitations, respectively. (b) Emission spectra of Er³⁺-Tm³⁺-codoped TZC glasses with 808 nm, 980nm and 808 & 980 nm two wavelength excitations, respectively. (c) Emission spectra of Er³⁺-Tm³⁺-Nd³⁺ tri-doped TZN glasses with 808 nm, 980 nm and 808 & 980 nm two wavelength excitations, respectively. (d) Emission spectra of TZN glasses upon excitation at 808 nm.

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Figure 2(b) shows emission spectra of Er³⁺-Tm³⁺ co-doped TZC glasses upon excitation at 808 nm, 980 nm and 808 & 980 nm, respectively. Under the excitation of 808 nm LD, the emission at 1770 nm is observed due to the ET processes between Er³⁺ and Tm³⁺. The blue shift of Tm³⁺: ³F₄→³H₆ around 1.8 µm is possibly caused by the higher Tm³⁺ concentration which can be assigned to the short wavelength band-edge emission and corresponds to the electron transition of Tm³⁺ from the ³F₄ multiplet to the ³H₆ multiplet. [20–21] Moreover, the intensity of Tm³⁺: ³F₄→³H₆ in TZC glass is higher than that of in TBM glass. This phenomenon suggests that ET between Nd³⁺ and Tm³⁺ ions is more efficient in TZC glasses than in TBM glasses. Thus, using higher Tm³⁺ concentration is another effective way for enhancing the emission of Tm³⁺: ³F₄→³H₆ around 1.8 µm. Upon 808 nm and 980 nm two wavelength excitations, the intensity of Tm³⁺: ³F₄→³H₆ emission is higher than that of the 808 nm wavelength excitation, meaning that 980 nm excitation also makes contribution to the emission of Tm³⁺: ³F₄→³H₆ around 1.8 µm.

The emission spectra of Er³⁺-Tm³⁺-Nd³⁺ tri-doped TZN glasses are shown in Fig. 2(c), upon excitation at 808 nm, 980nm and 808 & 980 nm, respectively. Under the excitation of 808 nm LD, the emission at 1806 nm is observed due to the ET process between Er³⁺ and Tm³⁺ together with Nd³⁺ and Tm³⁺. Moreover, the intensity of Tm³⁺: ³F₄→³H₆ is higher than that of Er³⁺: ⁴I_13/2→⁴I_15/2 peaking at 1550 nm. This phenomenon suggests that ET in Nd-Tm is efficient. Thus, adding Nd³⁺ is an effective way for enhancing the emission of Tm³⁺: ³F₄→³H₆ around 1.8 µm. Upon 808 and 980 nm two wavelength excitations, the intensity of Tm³⁺: ³F₄→³H₆ emission is higher than that of the 808 nm wavelength excitation, indicating that 980 nm excitation also makes contribution to the emission of Tm³⁺: ³F₄→³H₆ around 1.8 µm.

To obtain more information about the photoluminescence behavior, the emission spectrum of TZN glass has also been measured in the range 1000-2100 nm, as shown in Fig. 2(d). The emission in the 1000-1400 nm region can be owing to typical emission from Nd³⁺:F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2, which can be obviously observed from Fig. 2(d), meaning that the ET from Nd³⁺ to Er³⁺ and Tm³⁺ in not efficient.

Figure 3 exhibits the comparison of the NIR emission spectra of TBM, TZN and TZC glasses under 808 & 980 nm two wavelength excitation. It is obviously observed that the intensity of emission of Tm³⁺: ³F₄→³H₆ around 1.8 µm is enhanced to a much higher level. For further clarity, the emission in the 1400∼2100 nm wavelength region is magnified several folds, as shown in Fig. 3(b). The emission of the Er³⁺-Tm³⁺-Nd³⁺ tri-doped telluride glass sample can cover bands Nd³⁺: ⁴F_5/2 to ⁴I_13/2, ⁴I_11/2 and ⁴I_9/2 levels at 1000-1400 nm, Er³⁺: ⁴I_13/2 to ⁴I_15/2 level at 1400-1600 nm, and Tm³⁺: ³F₄ to ³H₆ level at 1600-2100 nm, respectively, as shown in Fig. 3(a). More significantly, all the value of photoluminescence in the 1200-2100 nm region can above zero, when pumped upon 808 & 980 nm. All these results indicate the seamless emission feature of this material. Note that the emission intensity of TZC and TBM samples at 1.8 µm is different, which is assigned to the Tm³⁺: ³F₄→³H₆ transition. The phonon energy parameters of tellurite and borate glasses are around 750 cm⁻¹ and 1300 cm⁻¹, respectively. Compared to the TBM glass, TZC glass have lower phonon energy, the addition ZnO and CaO do not participate the formation of the main glass network, whereas the B₂O₃ addition of the TBM glass can be involved in glass network. The phonon energy can influence the ET between the NIR active centers. Commonly, the lower phonon energy glass system possesses, the more effective ET can occur. Since the TZC glass possesses lower phonon energy, non-radiative process is weaker than that of TZC glass, the ET from Er³⁺ to Tm³⁺ is more efficient, as shown in Fig. 3(b).

 

Fig. 3. (a) Emission spectra of TBM, TZN and TZC glasses upon simultaneous 980nm & 808nm wavelength excitation. (b) Emission spectra of TBM, TZN and TZC glasses upon simultaneous 808 & 980 nm wavelength excitation in the wavelength region of 1400-2100 nm.

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The NIR luminescence mechanism for Er³⁺-Tm³⁺-Nd³⁺ tri-doped tellurite glasses could be explained based on the energy-level diagram of Er³⁺, Tm³⁺ and Nd³⁺ ions, as shown in Fig. 4. ET is a universal phenomenon among the luminescence centers. Commonly, resonant ET process can arise within one ion or among two or more different ions. And a good overlap between emission spectrum of the donor and the excitation spectrum of the acceptor is indispensable. Non-resonant ET can also occur in this case when there is proper mismatch. First, the 808 nm laser excitation of Tm³⁺, Er³⁺, Nd³⁺ populates the ³H₄, ⁴I_9/2 and ⁴F_5/2 levels from the ground states Tm³⁺: ³H₆, Er³⁺: ⁴I_15/2 and Nd³⁺: ⁴I_9/2 levels, respectively, which is consistent with the absorption spectrum in Fig. 1. Upon 980 nm laser excitation, Er³⁺ populates the Er³⁺: ⁴I_11/2 level from the ground state Er³⁺: ⁴I_15/2 level, which can also be shown in Fig. 1. Then the relaxation in Tm³⁺ from ³H₄→³F₄ level yields 1480 nm emission, whereas the Er³⁺ de-excites non-radiatively to ⁴I_11/2, then to ⁴I_13/2. Finally, Tm³⁺ and Er³⁺ relax to the respective ground states ³H₆ and ⁴I_15/2, generating the ∼1860 nm and 1540 nm emissions. Meanwhile, Nd³⁺ relax to the ⁴I_13/2, ⁴I_11/2 and ground state ⁴I_9/2, generating the ∼1337 nm, 1063 nm and 901 nm emissions. ET1, ET3 and ET4 are near-resonant energy transfer, which depopulates ³H₄ level and populates the respective ⁴I_9/2 and ⁴I_13/2 level of Er³⁺ and ⁴F_5/2 level of Nd³⁺. While ET2 is a non-resonant process in which one Er³⁺ ion relaxes from ⁴I_11/2 level to ground state and transfers its energy to a Tm³⁺ ion in the ground state. Then it will be promoted to the level ³H₅ from which it decays non-radiatively to level ³F₄. This process needs at least 2 phonons assisted to make it happen due to the large energy mismatch around 2900 cm⁻¹. Therefore, the non-resonant process is not efficient in the system. It should be mentioned that ET2 and ET3 processes can decrease the emission intensity of the 1540 nm of Er³⁺ and in turn increase the 1.8 µm emission from Tm³⁺ ions. From the above analysis, we can propose that the main ET mechanism is resonant ET among the NIR active centers.

 

Fig. 4. Schematic energy-level diagram and possible mechanism of Nd³⁺, Er³⁺ and Tm³⁺ for the multiband NIR emission.

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4. Summary

In conclusion, three types of tellurite glasses have been synthesized by conventional melt-quenching method. We propose Nd³⁺-Er³⁺-Tm³⁺ tri-doping scheme to achieve the superbroadband seamless NIR emission for the first time to our best knowledge. A broad emission covering the wavelength range of 1400–1900 nm corresponding to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ and the ³H₄→³F₆ transition of Tm³⁺ can be observed in TBM glass with the dual excitation of 800 nm and 980 nm LD. Yet the Tm³⁺ emission in ∼1.8 µm is not efficient. For the TZC glass, a relative stronger emission peaking at ∼1.8 µm emerges due to the lower phonon energy compared with the TBM glass. When adding Nd³⁺ in the TZN glass, which has strong 808 nm absorption, the intensity of Tm³⁺: ³F₄→³H₆ emission centered at 2 µm exceeds the Er³⁺:⁴I_15/2→⁴I_9/2 centered at ∼1540 nm. The same result is observed by 808 nm and 980 nm two-wavelength excitations. Moreover, it is experimentally demonstrated that to obtain Tm³⁺: ³F₄→³H₆ emission, the 808 nm laser is more useful than 980 nm laser. The ET processes among Nd³⁺, Er³⁺ and Tm³⁺ ions play important roles in the luminescence mechanism. The results indicate simultaneous dual-lasers excitations can enhance the emission intensity of tellurite glass compared to the single laser excitation. The seamless multiband NIR emission tellurite glass material can be optimized for an amplified spontaneous emission source and broadband NIR applications.