1. Introduction

In recent years, visible laser, especially yellow lasers of 560–590 nm, are fascinated by researchers, because it can be widely applied in medical cosmetology, telecommunication, and detection [1–3]. At present, the most reliable technique to generate a continuous wave yellow lasers is using frequency mixing method from two single-frequency Nd:YAG lasers at 1064 nm and 1319 nm by nonlinear frequency-conversion techniques [4, 5]. However, such multiple-cavity systems are still complex to use. Dy³⁺-doped materials are attractive for directly emitting yellow lasers due to the ⁴F_9/2→⁶H_13/2 transition (∼570 nm) [6–8]. As is shown in Fig. 1, the energy gap between ⁴F_9/2 and ⁶F_1/2 is large (about 7500 cm⁻¹), which means that multi-phonon relaxation is weak, and hence a high-yield visible emission of Dy³⁺ is generally expected. Besides, with the development of InGaN laser diodes, the new blue-violet emitting laser diodes are more and more accessible to be applied as optical pumping source. Therefore, potential yellow laser from Dy³⁺-doped crystal excited by blue laser diode are feasible. For example, yellow laser oscillations in Dy³⁺:Y₃Al₅O₁₂ [9], Dy³⁺:LiLuF₄ [10], and Dy³⁺:LiNbO₃ [11] crystals have also been reported in recent years. Moreover, co-doping of deactivation ion, such as Tb³⁺ ion, has been demonstrated to be an effective method to quench the lower level ⁶H_13/2 of Dy³⁺, which leads to a fast depopulation of the population in the terminal level, and reduces the pumping threshold [12, 13]. However, as far as we know, there is only one report about Dy³⁺, Tb³⁺ co-doped crystal as the gain material for yellow lasers, and limited to fluoride crystal. In 2014, a yellow laser performance with a highest output power of 55 mW was obtained in Dy³⁺, Tb³⁺ co-doped LiLuF₄ crystal [10].

 

Fig. 1 Simplified energy level diagram of Dy³⁺ and Tb³⁺ co-doped system. ET1: energy transfer from Dy³⁺:⁴F_9/2 to Tb³⁺:⁵D₄ level; ET2: energy transfer from Dy³⁺:⁶H_13/2 to Tb³⁺:⁷F₄ level.

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Moreover, the ⁴F_9/2→⁶H_13/2 transition for yellow fluorescence is a non-ground-state transition, the energy gap between ⁶H_13/2 and ⁶H_15/2 is about 3300 cm⁻¹. Therefore, host materials with higher phonon energies may be beneficial to quench the lower level ⁶H_13/2 by nonradiative transition from ⁶H_13/2 to ⁶H_15/2 based on the multiphonon relaxation [14, 15]. As we know, oxide crystals have phonon frequency of 600–1000 cm⁻¹, which is about two times larger than that in fluoride crystals. Among the different oxide crystals, we are now focusing our scientific program on a potential gain medium Na₂Gd₄(MoO₄)₇ (NGM) crystal for efficient yellow laser operation.

Na₂Gd₄(MoO₄)₇ crystal, whose chemical formula can be rewritten to Na_2/7Gd_4/7∅_1/7MoO₄ (∅ stand for vacancies), offering two nonequivalent cationic sites for optically active Re³⁺ substitution [16]. Consequently, NGM crystal is characterized by a high degree of structural disorder. Thus, the high degree of disorder of NGM crystal inevitably contributes to inhomogeneous broadening of the absorption and fluorescence spectra, bringing a large amount of advantages from the standpoint of applications as active media in LD-pumped solid-state lasers.

To the best of our knowledge, there are no reports on the yellow fluorescence properties in Dy³⁺, Tb³⁺ co-doped oxide crystals up to now. In this work, Dy³⁺, Tb³⁺ co-doped NGM crystal was successfully grown. Tb³⁺ was demonstrated to greatly facilitate the Dy³⁺: ⁴F_9/2→⁶H_13/2 emission by efficient energy transfer (ET) from Dy³⁺:⁶H_13/2 to Tb³⁺: ⁷F₄. The spectroscopic investigation of yellow fluorescence has been also made to future applications in yellow lasers.

2. Experimental section

The Dy³⁺ single doped and Dy³⁺/Tb³⁺ co-doped NGM crystals were grown by the Czochralski method in air. The raw materials were prepared from MoO₃(99.99%), Na₂CO₃(99.99%), Gd₂O₃(99.99%), Dy₂O₃(99.99%), and Tb₄O₇(99.99%). The raw powders were mixed together, grinded and extruded to form chunk, and then placed in the corundum crucible and heated up to 900 ^◦C. Then kept for 8 h to make the reaction completely. After the solid-state reaction, these compounds were put into a platinum crucible with diameter of 50 mm and height of 60 mm, and slowly heated up to 1250 °C. The pulling rates and rotating rates were 0.8–2 mm/h and 5–15 r/min. When the pulling procedure ended, the crystal was cooled down to room temperature slowly and annealed at 900 °C in air atmosphere to reduce the color centers inside the crystal. A 1 mm thick crystal cut from the medium and polished was prepared to spectral measurement. The concentrations of Dy³⁺ and Tb³⁺ were measured by the inductively coupled plasma-atomic emission spectrometry (ICP–AES) method. The single-doped crystal was measured to be 2.08 at.% (2.65×10²⁰ ions/cm³) of Dy³⁺. The co-doped crystal was measured to be 2.07 at.% (2.64×10²⁰ ions/cm³) of Dy³⁺, and 2.0 at.% (2.55×10²⁰ ions/cm³) of Tb³⁺, respectively.

The room temperature absorption spectra were measured by spectrophotometer (UV-3150, Shimadzu, Japan) in a range 400–1800 nm. Fluorescence spectra were measured by spectrometer (FL920, Edinburgh) under excitation at 454 nm in a range of 500–800 nm. The fluorescence decay curves of the as grown crystals were measured at 573 nm under pulse excitation of 454 nm. All the measurements were taken at room temperature.

3. Experimental results and discussion

The absorption spectrum of Dy³⁺/Tb³⁺ co-doped NGM crystal is shown in Fig. 2. It is obvious to see that there are many typical absorption peaks of Dy³⁺ ion, such as the wavelength near 1703, 1300, 1100, 907, 808, 759 and 454 nm, corresponding to the transitions from the ground state ⁶H_15/2 to the excited state ⁶H_11/2, ⁶H_9/2+⁶F_11/2, ⁶H_7/2+⁶F_9/2, ⁶H_5/2+⁶F_7/2, ⁶F_5/2 and ⁴I_15/2+⁴F_9/2, respectively. In the range of 440–470 nm, the highest absorption of 0.15 cm⁻¹ has been obtained at 454 nm with full width at half maximum (FWHM) of 9.2 nm, which is suitable for being pumped by blue-emitting InGaN laser diodes.

 

Fig. 2 Absorption spectrum of Dy³⁺/Tb³⁺ co-doped NGM crystal in the range of 400–1800 nm.

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According to the Judd–Ofelt theory [17, 18], the intensity parameters Ω_2,4,6 of Dy³⁺ (shown in Table 1) was calculated from the room-temperature absorption spectrum. It can be seen that the Ω₂ of Dy³⁺ in the Dy³⁺/Tb³⁺ co-doped NGM crystal is larger than that of Dy³⁺ single-doped NGM crystal, and much larger than that of other Dy³⁺ doped crystals. It is well known that the value of Ω₂ increases with the decrease in the symmetry [19]. The larger Ω₂ of Dy³⁺ in the Dy³⁺/Tb³⁺ co-doped NGM crystal indicates that the codoping of Tb³⁺ ions would bring about a lower symmetry surrounding Dy³⁺ ions in NGM crystal.

Table 1. Judd-Ofelt parameters Ω_2,4,6, branching ratio β, emission cross sections σ, life-time of ⁴F_9/2 level for Dy³⁺ ions (τ_M and τ_R are the measured and calculated radiative lifetime), and quantum efficiency η of different Dy³⁺ doped materials.

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Based on the intensity parameters Ω_2,4,6 of Dy³⁺, the spontaneous emission probabilities (A), radiative lifetime (τ_R) of Dy³⁺:⁴F_9/2 level, and the fluorescence branching ratio β of Dy³⁺:⁴F_9/2→⁶H_13/2 transition can be also calculated, and the results are shown in Table 1. It is clear to see that the fluorescence branching ratio β of Dy³⁺:⁴F_9/2→⁶H_13/2 transition for yellow fluorescence emission around 573 nm of Dy³⁺ single-doped NGM crystal is as high as 75.2%, which is larger than that of the Dy³⁺ doped fluoride crystals, such as Dy:LiLuF₄ (65.4%) [20], and Dy:KYF (59.8%) [21]. Moreover, compared to the Dy³⁺ single-doped NGM crystal, the Dy³⁺/Tb³⁺ co-doped NGM crystal possesses a larger fluorescence branching ratio β of Dy³⁺:⁴F_9/2→⁶H_13/2 transition, which is as high as 80.2%. It is well known that the larger fluorescence branching ratio represents the higher possibility of fluorescence emission. The larger β of Dy³⁺:⁴F_9/2→⁶H_13/2 transition in the Dy³⁺/Tb³⁺ co-doped NGM crystal indicates that the codoping of Tb³⁺ ions would bring about a more efficient yellow fluorescence emission.

To further study the effect of Tb³⁺ codoping on the yellow photoluminescence of Dy³⁺, the corresponding emission cross sections are calculated by the Fuchtbauer–Ladenburg formula [24]:

 

Fig. 3 Yellow emission cross-sections of Dy³⁺ single-doped and Dy³⁺/Tb³⁺ co-doped NGM crystals.

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The time-resolved decays of the Dy³⁺:⁴F_9/2 level for the Dy³⁺ single-doped and Dy³⁺/Tb³⁺ co-doped NGM crystals were measured, and shown in Fig. 4. The measured lifetime is 114 μs for the Dy³⁺/Tb³⁺ co-doped NGM crystal, which is a little shorter than that of Dy³⁺ single-doped NGM crystal (125 μs). As shown in Fig. 1, firstly ions of Dy³⁺:⁶H_15/2 state are excited to Dy³⁺:⁴I_15/2 state by a 450 nm LD, then decay nonradiatively to Dy³⁺:⁴F_9/2 level by the multiphonon relaxation. One part of the Dy³⁺ ions on the ⁴F_9/2 level will mainly decay radiatively to ⁶H_15/2 state and ⁶H_13/2 state with ~480 nm and the ∼573 nm emissions, respectively. Other Dy³⁺ ions on ⁴F_9/2 level will undergo the energy transfer 1 (ET1) to the Tb³⁺ ion on the ⁵D₄ level, which would go against the yellow fluorescence emission of Dy³⁺. The ET1 efficiency η_ET from Dy³⁺ to Tb³⁺ can be described as η_ET =1-τ_Dy−−Tb/τ_Dy, where τ_Dy−−Tb and τ_Dy are lifetime of Dy³⁺:⁴F_9/2 level of Dy³⁺ doped NGM crystal with and without Tb³⁺, and the calculated result is as low as 8.8%. This confirms that the codoping of Tb³⁺ ions has little influence on the Dy³⁺:⁴F_9/2 level. Moreover, the quantum efficiency, evaluated by η=τ_M/τ_R, where τ_M and τ_R are the measured and calculated (by Judd–Ofelt theory) radiative lifetime of ⁴F_9/2 level for Dy³⁺ ions in Dy³⁺ doped crystals. Therefore, the value of η in the Dy³⁺/Tb³⁺ co-doped NGM crystal was calculated to be 52.7%, which is larger than that of Dy³⁺ single-doped NGM crystal (48.9%), and larger than that of the other Dy³⁺ doped crystals (shown in Table 1), indicating that Dy³⁺/Tb³⁺ co-doped NGM crystal is a promising candidate for yellow laser operation.

 

Fig. 4 Fluorescence decay curves of Dy³⁺ single-doped and Dy³⁺/Tb³⁺ co-doped NGM crystals for the ⁴F_9/2 mainfold.

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As shown in Fig. 1, after the yellow fluorescence emission based on the decay radiatively from Dy³⁺:⁴F_9/2 level to Dy³⁺:⁶H_13/2 level, the ions in the Dy³⁺:⁶H_13/2 level will undergo the ET2 process to Tb³⁺:⁷F₄ level, leading to an accelerated depletion of the population in the laser terminal level ⁶H_13/2. In this way, the lifetime of the lower laser level ⁶H_13/2 can be effectively decreased, which makes population inversion for the Dy³⁺:⁴F_9/2→⁶H_13/2 transition much easier and enhances the yellow fluorescence emission. To further confirm the energy interaction mechanism, the time-resolved decays of the ⁶H_13/2 level of the Dy³⁺ single-doped and Dy³⁺/Tb³⁺ co-doped NGM crystals were measured, and the results are shown in Fig. 5. It is obvious to see that the measured lifetime value of the ⁶H_13/2 level in the crystal co-doped with Tb³⁺ is 0.96 ms, which is 33.3% shorter when compared with that of the single-doped crystal (1.44 ms). This shortening of the measured lifetime indicates that there is a strong interaction between the Dy³⁺ and Tb³⁺, and confirms that Tb³⁺ ions are able to effectively depopulate the Dy³⁺:⁶H_13/2 for ∼573 nm emission in NGM crystal by energy transition from Dy³⁺:⁶H_13/2 to Tb³⁺:⁷F₄, which may induce the population inversion and facilitate laser operation.

 

Fig. 5 Fluorescence decay curves of Dy³⁺ single-doped and Dy³⁺/Tb³⁺ co-doped NGM crystals for the ⁶H_13/2 mainfold.

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4. Conclusion

In conclusion, Dy³⁺ single- and Dy³⁺/Tb³⁺ co-doped Na₂Gd₄(MoO₄)₇ crystals were successfully grown. Compared with the Dy³⁺ single-doped Na₂Gd₄(MoO₄)₇ crystal, the Dy³⁺/Tb³⁺ co-doped Na₂Gd₄(MoO₄)₇ crystal has higher fluorescence branching ratio (80.2%), transition probability (3704 s⁻¹), and fluorescence emission cross section (1.34×10⁻²⁰ cm²) corresponding to the laser transition ⁴F_9/2→⁶H_13/2 of Dy³⁺ around the yellow bands. It was also demonstrated that the codoping of Tb³⁺ depopulates the population of the laser lower level Dy³⁺: ⁶H_13/2, and has little influence on the laser upper level Dy³⁺: ⁴F_9/2 at the same time, which is benefited to the possible population inversion for the Dy³⁺:⁴F_9/2→⁶H_13/2 transition, and enhances the fluorescence emission. These results suggests that the Dy³⁺/Tb³⁺ co-doped Na₂Gd₄(MoO₄)₇ crystal is a promising material for yellow laser applications under being pumped by a conventional 450 nm LD.