Sm3+/Eu3+: YAlO3 (Sm3+/Eu3+: YAP) single crystal with a size of Ф (25-35) mm × (50-60) mm was successfully grown and analyzed. The use of Sm3+ co-doping to enhance Eu3+: 5D0 → 7F2 orange-red emission has been investigated in the YAP crystal for the first time. Compared with the Eu3+ single-doped YAP crystal, the Sm3+/Eu3+ co-doped YAP crystal possessed a larger fluorescence emission cross section (0.90×10−21 cm2), higher quantum efficiency (78.4%), and comparative fluorescence lifetime (1.74 ms), corresponding to the stimulated emission of Eu3+: 5D0 → 7F2 transition. Moreover, the energy transition efficiency of the energy transition process from the Sm3+: 4G5/2 level to the Eu3+: 5D0 level was calculated to be as high as 47.31%. These results suggest that Sm3+ ion can be used as an efficient sensitizer to enhance the orange-red fluorescence emission and can lead to the expansion of the emission range from 585–640 nm in Sm3+/Eu3+ co-doped YAP crystal.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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 . 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 . Moreover, the energy gap between Eu3+: 5D0 and the lowest 7FJ (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 . 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) . Due to the energy transfer (ET) from Sm3+: 4G5/2 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 . 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 . 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 . 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 . 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 . 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+: 4G5/2 → 6H7/2. 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).
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+: 4G5/2 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.
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 6H5/2 ground state to 4D3/2, 6P7/2, 4K11/2+4F7/2, 6P5/2, 4G9/2+4I15/2, 4I13/2+4I11/2+4M15/2+4I9/2 and 4G7/2 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 . On the one hand, it may be due to the spin forbidden transitions of Eu3+: 7Fj → 5Dj. On the other hand, it may be due to the low doping concentrations of Eu3+ ion . 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 6H5/2 → 4K11/2 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.
According to the Judd–Ofelt (J-O) theory, the Judd-Ofelt intensity parameters Ω2,4,6 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 . Based on the intensity parameters Ω2,4,6 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.
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+: 4G5/2 → 6H7/2 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+: 6H5/2 state are excited to Sm3+: 4K11/2 state under the pump of 405 nm LD, then decay non-radiatively to Sm3+: 4G5/2 level by the multi-phonon relaxation. One part of the Sm3+ ions on the 4G5/2 level will mainly decay radiatively to 6H11/2 state, 6H9/2 state, 6H7/2 state, and 6H5/2 state with ∼711 nm, ∼650 nm, ∼604 nm, and ∼569 nm emissions, respectively. Other particles on the 4G5/2 level of Sm3+ ion will be transferred to the 5D0 level of Eu3+ ion through energy transfer process (Sm3+: 4G5/2 → Eu3+: 5D0), because the 4G5/2 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+: 4G5/2 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.
The emission cross sections are calculated by the Fuchtbauer–Ladenburg equation .
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: ηET=1-τSm/Eu/τSm, where τSm/Eu and τSm are the Sm3+ lifetimes monitored with and without Eu3+ ions, respectively. The energy transfer efficiency from Sm3+: 4G5/2 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.
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+: 4G5/2 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.
National Natural Science Foundation of China (51702124, 51872307, 51972149, 61935010); Key-Area Research and Development Program of Guangdong Province (2020B090922006); Guangdong Project of Science and Technology Grants (2018B010114002, 2018B030323017); Guangzhou science and technology project (201904010385); Fundamental Research Funds for the Central Universities (21620445).
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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