1. Introduction

Orange-red laser (from 585 to 640 nm) can be used in a variety of scientific and technical applications, including in atmospheric monitoring, environment, biomedicine and display. In addition, orange-red laser source emitting around 610 nm is required for quantum information processing applications [1–5]. However, there is still a lack of efficient and stable orange-red laser source whose continuous-wave output wavelength is around 585–640 nm. At present, the reliable technique for generating continuous-wave orange-red laser is to use frequency mixing method from two single-frequency Nd: YVO₄ lasers at 1085 nm and 1342 nm by nonlinear frequency-conversion technique [6]. However, such multiple-cavity systems are still complex to use. It motivates researchers to study how to directly emit orange-red laser. The Eu³⁺ ion, due to the ⁵D₀ → ⁷F₂ transition, would be the potential candidate rare-earth ion for producing orange-red laser [7]. Moreover, the energy gap between Eu³⁺: ⁵D₀ and the lowest ⁷F_J (J = 0 … 6) multiplet with the ⁷F₀ ground-state is large (about 12000 cm⁻¹), which means that multi-phonon relaxation is weak, leading to long fluorescence lifetimes and high luminescence quantum efficiency [8–11]. Besides, with the development of GaN/InGaN laser diode (LD), the new blue-violet emitting laser diodes are more and more accessible to be applied as optical pumping source. Therefore, potential orange-red laser from Eu³⁺-doped crystals excited by blue-violet LD pumping are more realizable [12,13]. However, because the absorption band in the range of ∼400 nm of Eu³⁺ ion is relatively narrow and weak, it cannot be better matched with the wavelength of the blue-violet light pump source. As a result, it will lead to the low absorption efficiency of the crystal. On the other hand, it will further increase the difficulty of crystal thermal management in laser experiments. In order to obtain effective orange-red fluorescence emission in Eu³⁺ ion doped crystals, as we all know, by increasing the doping concentration of Eu³⁺ ion, the absorption coefficient of Eu³⁺ ion around 400 nm can be effectively increased. However, high-concentration doping will cause difficulty in crystal growth, and it is difficult to obtain crystals with higher optical quality. Therefore, it is meaningful to study the co-doping of other rare-earth ions to increase the absorption coefficient and broaden the wavelength range of excitation light source. Co-doping of suitable sensitized ions can improve the absorption coefficient of the crystal and thus reduce the dependence on the wavelength thermal stability of the blue-violet light pump source in laser experiments. Past studies have shown that co-doping of Sm³⁺ ion can effectively expand and strengthen the absorption of the GaN/InGaN LD pumping [14–17]. It is well known that Sm³⁺/Eu³⁺ co-doped ions exhibit strong absorption at about 405 nm in many host lattices, which is close to the emission wavelength 405 nm of GaN/InGaN LD pumping [18]. In addition, Sm³⁺ ion is used as sensitizer to increase the number of Eu³⁺: ⁵D₀ energy level particles under certain excitation condition, leading to the expansion and enhancement of Eu³⁺ ion orange-red fluorescence emission range (from 585 to 640 nm) [19]. Due to the energy transfer (ET) from Sm³⁺: ⁴G_5/2 to Eu³⁺: ⁵D₀, the fluorescence emission of at ∼610 nm is effectively broadened and enhanced, corresponding to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ion, which provides the possibility to realize broadband orange-red tunable lasers [20,21].

The host material for orange-red visible lasers is expected to possess low phonon energy, which can decrease the non-radiative losses efficiently and increasing the quantum efficiency of ⁵D₀ → ⁷F₂ transition of Eu³⁺ ion. However, materials with low phonon energy will enhance the up-conversion luminescence, which has a negative effect on the emission of visible laser light [22]. Therefore, host materials with relative higher phonon energy may be beneficial to the Sm³⁺ and Eu³⁺ ions 4f–4f transition, and sometimes it can accelerate the relaxation processes, which is necessary and beneficial for visible emissions [23]. 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, the maximum phonon energy of Yttrium Aluminum Perovskite (YAP) is only ∼570 cm⁻¹. YAP crystal has excellent thermal and mechanic properties similar to Y₃Al₅O₁₂ (YAG) crystal, and can be used as a suitable host for orange-red visible lasers. Furthermore, since the YAP crystal has structural anisotropy and natural birefringence, it can output linearly-polarized laser, which would favor the nonlinear conversion process [24,25].

YAP crystal is a biaxial crystal with the orthorhombic system, the yttrium ions in sites of Cs (monoclinic) symmetry [26]. The cell parameters of the YAP crystal are a = 5.330 Å, b = 7.375 Å, c = 5.180 Å, and the density is 5.35 g/cm³ [27]. Compared to the original YAG crystal, YAP crystal is also derived from the binary Y₂O₃-Al₂O₃ system. They have similar physical characteristics, such as high mechanical strength, sufficient hardness and significant thermal conductivity [28]. In this work, the effective enhanced emission at ∼610 nm in Sm³⁺/Eu³⁺ co-doped YAP single crystal is reported for the first time. Eu³⁺ single doped, Sm³⁺ single doped and Sm³⁺/Eu³⁺ co-doped YAP single crystals were fabricated by the Czochralski (Cz) method. The orange-red fluorescence emission wavelength range of Eu³⁺ ion is widened due to the transition of Sm³⁺: ⁴G_5/2 → ⁶H_7/2. The spectroscopic investigation of orange-red fluorescence in single crystal also provides help for future applications in orange-red lasers.

2. Experimental section

The Eu³⁺ single doped, Sm³⁺ single doped and Sm³⁺/Eu³⁺ co-doped YAP single crystals can be obtained by the Cz technique with a 30-kW intermediate frequency induction heating system. Oxide powders of Eu₂O₃ (4N), Sm₂O₃ (4N), Y₂O₃ (4N) and Al₂O₃ (4N) were used as starting materials. The raw powders were mixed in the mixer for 24 hours for sufficient reaction, then pressed into disks and followed by heating in air at 1360°C for 30 h for synthesizing rare-earth doped YAP polycrystalline material. After that, YAP obtained polycrystalline material was loaded into a cylindrical iridium crucible 60 mm in diameter for single crystal growth in an atmosphere of nitrogen (5N). A $<100>$-oriented YAP single crystal bar with dimensions of 4×4×30 mm³ was used for crystal growth as the seed. The pulling rate and the rotation rate of the seed were 0.8-1.4 mm/h, and 8–16 rpm, respectively. To prevent the crystal from cracking, it was cooled to room temperature very slowly with a rate of 30∼40 °C/h after growth. After crystal growth, crystals were placed in the muffle furnace and annealed at 1200 °C in air for 28 hours in order to eliminate the residual thermal stress in the crystal.

Figure 1 shows the as-grown Sm³⁺/Eu³⁺: YAP single crystal with size of Ф (25–35) mm × (50-60) mm and spectral measurement sample with dimensions of 3×4×5 mm³. The concentrations of Sm³⁺ and Eu³⁺ ions were detected by the inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis. The doping concentrations of Sm³⁺ and Eu³⁺ in the Sm³⁺/Eu³⁺: YAP single crystal were measured to be 3.13 at. % (6.39×10²⁰ ions/cm³) and 0.44 at. % (8.98×10¹⁹ ions/cm³), respectively. The doping concentration of Sm³⁺ in the Sm³⁺: YAP single crystal was 3.21 at. % (6.42×10²⁰ ions/cm³), while the doping concentration of Eu³⁺ in the Eu³⁺: YAP single crystals was 0.45 at. % (9.15×10¹⁹ ions/cm³).

 

Fig. 1. As-grown Sm³⁺/Eu³⁺: YAP single crystal and spectral measurement sample.

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The partially of processed spectral measurement samples were ground to powder in an agate mortar in order to identify the as-grown crystal structure. The powder X-ray diffraction (XRD) patterns of the three samples were measured by an X-ray powder diffractometer (D/max2550) using Cu Kα radiation. The range of the scanning mode is from 20° to 90° with a step scanning rate of 0.02°. The room temperature absorption spectra of the as-grown crystals were measured by using a UV-3150 UV-vis-NIR spectrophotometer (Shimadzu, Japan) in the range of 350–600 nm with a resolution of 1 nm. The fluorescence spectra in the range of 585–640 nm and fluorescence decay profiles of the Sm³⁺: ⁴G_5/2 state and Eu³⁺: ⁵D₀ state in as-grown crystals were acquired by Edinburgh Instruments FLS920 and FSP920 spectrophotometers under excitation of 405 nm. All the measurements were taken at room temperature.

3. Results and discussion

Figure 2 shows the XRD patterns of the Sm³⁺: YAP, Eu³⁺: YAP, Sm³⁺/Eu³⁺: YAP single crystals and the standard pattern of YAP (JCPDS 70-1677) single crystal. There is no much difference in the position of the diffraction peaks of the as-grown crystals compared to the standard PDF card of YAP crystal, and there is no formation of other impurity diffraction peaks. The results confirmed that the introduction of Sm³⁺ and Eu³⁺ ions into the YAP host lattice had little impact on the crystal structure.

 

Fig. 2. X-ray diffraction patterns of the Sm³⁺: YAP, Eu³⁺: YAP, Sm³⁺/Eu³⁺: YAP single crystals and the database JCPDS 70-1677 (YAP).

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Figure 3 shows the room temperature absorption spectra of Eu³⁺: YAP, Sm³⁺: YAP, and Sm³⁺/Eu³⁺: YAP single crystals in the range from 350 to 600 nm. The absorption peaks corresponding to the transitions of Sm³⁺ and Eu³⁺ from the ground state to the excited ones are marked in Fig. 3. The five characteristic absorption peaks centered at approximately 363, 380, 396, 418, and 466 nm are associated with Eu³⁺ transitions from the ⁷F₀ ground state to ⁵D₄, ⁵L₇, ⁵L₆, ⁵D₃, and ⁵D₂ excited states, respectively. The seven characteristic absorption peaks centered at approximately 364, 378, 407, 419, 441, 473, and 499 nm are associated with Sm³⁺ transitions from the ⁶H_5/2 ground state to ⁴D_3/2, ⁶P_7/2, ⁴K_11/2+⁴F_7/2, ⁶P_5/2, ⁴G_9/2+⁴I_15/2, ⁴I_13/2+⁴I_11/2+⁴M_15/2+⁴I_9/2 and ⁴G_7/2 excited states, respectively. In Sm³⁺/Eu³⁺ co-doped YAP single crystal, the position of the Sm³⁺ and Eu³⁺ ions absorption peaks do not change significantly by comparing with the absorption spectra of Sm³⁺ and Eu³⁺ singly doped YAP crystals, indicating that Sm³⁺ and Eu³⁺ ions replace the lattice sites of Y³⁺ ion and uniformly doped into the YAP matrix. However, the partial absorption peaks of Eu³⁺ ion is weak [29]. On the one hand, it may be due to the spin forbidden transitions of Eu³⁺: ⁷F_j → ⁵D_j. On the other hand, it may be due to the low doping concentrations of Eu³⁺ ion [30]. The main difference between the absorption spectrum of Sm³⁺/Eu³⁺: YAP crystal and the absorption spectrum of Eu³⁺: YAP crystal is the absorption peak at 405 nm, which corresponds to the ⁶H_5/2 → ⁴K_11/2 transition of Sm³⁺ ion. In Sm³⁺/Eu³⁺: YAP crystal, the effective broadening of the absorption band in the range from 395 to 415 nm is due to the effective overlap between the absorption peak of Sm³⁺ ion at 405 nm and the absorption peak of Eu³⁺ ion at 396 nm, which is helpful for the crystal to be pumped by GaN/InGaN laser diode. The absorption cross-section of Sm³⁺/Eu³⁺: YAP is calculated to be 1.64×10⁻²⁰ cm² with peak at 407 nm, and the fitting value of full width at half maxim (FWHM) is 8.3 nm.

 

Fig. 3. Absorption spectra at room temperature for Eu³⁺: YAP, Sm³⁺: YAP, and Sm³⁺/Eu³⁺: YAP single crystals from 350 to 600 nm.

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According to the Judd–Ofelt (J-O) theory, the Judd-Ofelt intensity parameters Ω_2,4,6 of Eu³⁺ (shown in Table 1) was calculated from the room-temperature absorption spectrum [31,32]. The parameter Ω₂ is dependent on the local environments and the covalent chemical bonding, the high value of Ω₂ means that the low asymmetry and the strong covalence characteristics of the crystal [33–35]. It can be seen that the Ω₂ of Eu³⁺ in the Sm³⁺/Eu³⁺ co-doped YAP crystal is larger than that of Eu³⁺ single-doped YAP crystal. It indicates that the co-doping of Sm³⁺ ion would bring about a lower symmetry surrounding Eu³⁺ ion in YAP crystal [36]. Based on the intensity parameters Ω_2,4,6 of Eu³⁺, the fluorescence branching ratio (β) of the ⁵D₀ → ⁷F₂ transition in Sm³⁺/Eu³⁺: YAP crystal was calculated to be 46.4%, which is larger than that of the Eu³⁺: YAP crystal (40.1%). It is well known that the larger fluorescence branching ratio represents the higher possibility of fluorescence emission. The results show that the co-doping of Sm³⁺ ion in the Eu³⁺: YAP crystal, on the one hand, can effectively improve the fluorescence branching ratio of the Eu³⁺: ⁵D₀ → ⁷F₂ transition. On the other hand, it can enhance the absorption intensity of Eu³⁺ ion at 405 nm, which is beneficial to induce the 585-640 nm fluorescence emission and realize the orange-red laser.

Table 1. Judd–Ofelt parameters Ω_2,4,6, calculated branching ratio of Sm³⁺/Eu³⁺: YAP and Eu³⁺: YAP crystals.

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The emission spectra of Eu³⁺: YAP, Sm³⁺: YAP and Sm³⁺/Eu³⁺: YAP single crystals from 585 to 640 nm are shown in Fig. 4. It can be clearly seen that the emission intensity of the crystal co-doped with Sm³⁺ is higher than the crystal without Sm³⁺ co-doped in the range of 585–640 nm. Particularly, the emission intensity of the crystal co-doped with Sm³⁺ at 604 nm is almost 15 times that of the crystal without Sm³⁺ co-doping. This enhanced fluorescence emission should be attributed to the Sm³⁺: ⁴G_5/2 → ⁶H_7/2 transition, which can effectively broaden and enhance the emission of Sm³⁺/Eu³⁺: YAP crystal in orange-red band. The broad emission band from 585 to 640 nm provides the possibility for a tunable visible laser operation. At the same time, it can be seen that the emission intensity of Sm³⁺/Eu³⁺ co-doped YAP crystal at 604 nm is weaker than that of Sm³⁺ single-doped YAP crystal. This weaker fluorescence emission justifies that there is an efficient energy transfer from Sm³⁺ to Eu³⁺ in the Sm³⁺/Eu³⁺: YAP crystal. According to the simplified energy level scheme of Sm³⁺ and Eu³⁺ ions (shown in Fig. 5), firstly, the ions of Sm³⁺: ⁶H_5/2 state are excited to Sm³⁺: ⁴K_11/2 state under the pump of 405 nm LD, then decay non-radiatively to Sm³⁺: ⁴G_5/2 level by the multi-phonon relaxation. One part of the Sm³⁺ ions on the ⁴G_5/2 level will mainly decay radiatively to ⁶H_11/2 state, ⁶H_9/2 state, ⁶H_7/2 state, and ⁶H_5/2 state with ∼711 nm, ∼650 nm, ∼604 nm, and ∼569 nm emissions, respectively. Other particles on the ⁴G_5/2 level of Sm³⁺ ion will be transferred to the ⁵D₀ level of Eu³⁺ ion through energy transfer process (Sm³⁺: ⁴G_5/2 → Eu³⁺: ⁵D₀), because the ⁴G_5/2 energy level of Sm³⁺ ion is about 600 cm⁻¹ higher than the ⁵D₀ energy level of Eu³⁺ ion. As the result, the energy transfer from Sm³⁺: ⁴G_5/2 to Eu³⁺: ⁵D₀ is almost irreversible. The energy transfer process would enhance the population of Eu³⁺: ⁵D₀ energy level. Meantime, the ions in the Eu³⁺: ⁵L₆ level will decay non-radiatively to Eu³⁺: ⁵D₀ level. Eventually, the enhanced emission of orange-red light at ∼610 nm was observed in the Sm³⁺/Eu³⁺: YAP crystal.

 

Fig. 4. Emission spectra of Eu³⁺: YAP, Sm³⁺: YAP, and Sm³⁺/Eu³⁺: YAP single crystals from 585 to 640 nm.

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Fig. 5. Simplified energy level diagram of Sm³⁺ and Eu³⁺ co-doped system, NT: nonradiative transition, ET: energy transfer from Sm³⁺: ⁴G_5/2 to Eu³⁺: ⁵D₀ level.

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The emission cross sections are calculated by the Fuchtbauer–Ladenburg equation [37].

 

Fig. 6. Fluorescence decay curves of the Eu³⁺: ⁵D₀ level of Eu³⁺: YAP, and Sm³⁺/Eu³⁺: YAP single crystals. (excited at 396 nm and monitored at 629 nm)

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Table 2. Emission cross sections σ_e, lifetime of ⁵D₀ level for Eu³⁺ ion (τ_rad and τ_meas are the measured and calculated radiative lifetime), and quantum efficiency η of Sm³⁺/Eu³⁺: YAP and Eu³⁺: YAP crystals.

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To further confirm the energy interaction mechanism, the time-resolved decays of the Eu³⁺: ⁵D₀ of the Eu³⁺ single-doped YAP and Sm³⁺/Eu³⁺ co-doped YAP crystals were measured, and the results are shown in Fig. 7. The fluorescence lifetime was fitted to be 169.29 µs for the Sm³⁺/Eu³⁺ co-doped YAP crystal, which is shorter than that of Sm³⁺ single-doped YAP crystal (321.27 µs). Moreover, the energy transfer efficiency of ET can be estimated by the following equation: η_ET=1-τ_Sm/Eu/τ_Sm, where τ_Sm/Eu and τ_Sm are the Sm³⁺ lifetimes monitored with and without Eu³⁺ ions, respectively. The energy transfer efficiency from Sm³⁺: ⁴G_5/2 level to Eu³⁺: ⁵D₀ level was calculated to be as high as 47.31%, which makes a contribution to the population of Eu³⁺: ⁵D₀ level and enhances the orange-red light emission at ∼610 nm, indicating that the Sm³⁺ ion can be used as an efficient sensitizer to enhance the ∼610 nm fluorescence emission of Sm³⁺/Eu³⁺codoped YAP crystal. Optimizing the concentration of Sm³⁺ and Eu³⁺ ions is a very meaningful work, which is important for further enhancing and broadening the spectrum. This improvement will be undertaken in future research work.

 

Fig. 7. Fluorescence decay curves of the Sm³⁺: ⁴G_5/2 level of Sm³⁺: YAP, and Sm³⁺/Eu³⁺: YAP single crystals. (excited at 405 nm and monitored at 604 nm)

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4. Conclusion

In conclusion, Eu³⁺ single doped, Sm³⁺ single doped and Sm³⁺/Eu³⁺ co-doped YAP single crystals were successfully grown by the Czochralski method. An effective enhanced emission at around 610 nm was observed in the Sm³⁺/Eu³⁺ co-doped YAP crystal under the excitation of a commercial 405 nm LD. Compared with the Eu³⁺ single-doped YAP crystal, the Sm³⁺/Eu³⁺ co-doped YAP crystal has higher quantum efficiency (78.4%), and larger emission cross section (0.90×10⁻²¹ cm²) corresponding to the stimulated emission of Eu³⁺: ⁵D₀ → ⁷F₂ transition. It was also demonstrated that the co-doping of Sm³⁺ ion makes a contribution to the population of the Eu³⁺: ⁵D₀ level. What is more, the energy transition efficiency of ET from Sm³⁺: ⁴G_5/2 level to Eu³⁺: ⁵D₀ level was calculated to be as high as 47.31%, indicating that Sm³⁺ can be used as an efficient sensitizer to enhance the ∼610 nm fluorescence emission and lead to the expansion of orange-red emission range (585-640 nm) in Sm³⁺/Eu³⁺ co-doped YAP crystal.