Study of the Optical Features of Tb³⁺:CaYAlO₄ and Tb³⁺/Pr³⁺:CaYAlO₄ Crystals for Visible Laser Applications

Abstract

Single crystals of Tb³⁺ single-doped and Tb³⁺/Pr³⁺ co-doped CaYAlO₄ were produced by the Czochralski method. The room-temperature polarized absorption spectra, emission spectra, and decay curves were recorded and analyzed in detail. The absorption cross-section around 487 nm was found to be 1.53 × 10⁻²² cm² for the π polarization in the Tb³⁺:CaYAlO₄ crystal and increased to 5.23 × 10⁻²² cm² in the Tb³⁺/Pr³⁺:CaYAlO₄ crystal. The spectroscopic parameters were calculated through the Judd–Ofelt theory. For the Tb³⁺:CaYAlO₄ crystal, the emission bands of green light at 546 nm and yellow light at 587 nm had fluorescence branching ratios of 64.7% and 6.65% with cross-sections of 8.82 × 10⁻²² cm² (σ-polarization) and 0.44 × 10⁻²² cm² (π-polarization), respectively. The decay lifetimes of ⁵D₄ multiplets were measured to be 1.41 ms and 1.1 ms for Tb³⁺:CaYAlO₄ and Tb³⁺/Pr³⁺:CaYAlO₄ crystals, respectively. The energy transfer mechanisms of Tb³⁺ and Pr³⁺ and their emission spectral intensities at different temperatures were analyzed. As the temperature increased, the luminescence intensity of the Tb³⁺:CaYAlO₄ and Tb³⁺/Pr³⁺:CaYAlO₄ crystals decreased almost linearly with the CIE coordinate variation, from (0.370, 0.621) to (0.343, 0.636) and from (0.345, 0.638) to (0.246, 0.698), respectively. The results indicate the potential of Tb³⁺:CaYAlO₄ and Tb³⁺/Pr³⁺:CaYAlO₄ crystals as visible laser materials with a wide temperature range.

1. Introduction

Solid-state lasers in the visible band have a variety of applications, including biomedical instrumentation, visual displays, and remote sensing [1,2,3]. There are several reports about the operation of visible solid-state lasers. One example is the 589 nm laser produced by 1064 and 1319 nm lasers through sum-frequency mixing from a Nd:YAG crystal [4,5]. Second-harmonic generation (SHG) or sum-frequency generation takes place in lithium triborate crystals, producing visible outputs at any of the following three wavelengths: 537 nm, 546 nm, and 556 nm [6]. Moreover, the appropriate configuration of a He-Ne laser can emit laser beams at 594 nm and 612 nm. Although nonlinear optical technology is used in practice, the adoption of these methods may lead to a complex and expensive optical system, complicated operation, and poor beam quality, restricting their further development and application. Thus, it is of great scientific significance to discover other routes to produce visible lasers. Today, thanks to the rapid development of laser diodes (LDs) in the blue region [7,8,9], the output of green and yellow lasers has been achieved by LD-pumped visible laser gain media. For example, in 2020, 622 nm, 662 nm, and 747 nm lasers were produced with a YAlO₃:Pr³⁺ crystal pumped by a 488 nm semiconductor laser [10]. Chen et al. reported a Tb³⁺:LiYF₄ laser with maximum output power of 1.17 W (@544 nm) and 0.5 W (@587 nm) [11,12]. This method avoids the complex nonlinear frequency conversion and has the properties of compact structure, high stability, good beam quality, etc., playing an increasingly vital role in visible laser techniques.

As is widely known, based on the energy level of Tb³⁺, the emission bands around 546 nm and 578 nm are located in the green and yellow ranges, respectively, corresponding to the ⁵D₄→⁷F_J (5, 4) transition [13]. According to previous investigations, Tb³⁺ was introduced to some fluoride host materials, such as CaF₂, CdF₂, and LiYF₄ [14,15,16], which commonly suffer much energy waste and poor physical and chemical properties. As an alternative choice, oxides have higher mechanical strength and better chemical stability for lasing operations. The structure of CaYAlO₄ (abbreviated as CYA) crystal is highly disordered, and its lattice parameters are a = b = 3.6451 Å and c = 11.8743 Å [17].

However, the transition of Tb³⁺:⁷F₆→⁵D₄ is a spin-forbidden process, resulting in a relatively small absorption cross-section around 487 nm, at a magnitude of 10⁻²² cm² [14]. Higher Tb³⁺ concentrations or co-doping with rare-earth ions are the commonly used methods to overcome its weak absorption in practical applications. The energy level of Pr³⁺:³P₀ is very similar to that of Tb³⁺:⁵D₄ (as shown in Figure 1); the energy migration between these two states may be able help improve the small absorption cross-section of Tb³⁺.

In our work, a Tb³⁺ single-doped CYA crystal was produced via the Czochralski method. The spectral characteristics of the crystal were measured. In order to explore the effect of Pr³⁺ on the low absorption cross-section of Tb³⁺ around 487 nm, a Tb³⁺/Pr³⁺ co-doped CYA crystal was grown through the same growth technique. The energy migration route between Tb³⁺ and Pr³⁺, as along with the effect of temperature on the fluorescence emission, was displayed and studied for the exploration of their laser prospects.

2. Materials and Methods

Single crystals of 10 at.% Tb³⁺ single-doped and 10% Tb³⁺/0.6 at.% Pr³⁺ co-doped CYA were produced by the Czochralski method. The Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA polycrystalline materials with formulae of CaY_0.9Tb_0.1AlO₄ and CaY_0.84Pr_0.06Tb_0.1AlO₄, respectively, were prepared using high-temperature solid-state technology. The original materials used were CaCO₃ (AR grade, Sinopharm, Beijing, China), Al₂O₃ (AR grade, Sinopharm, Beijing, China), Y₂O₃ (99.99%, Changchun, China), Tb₄O₇ (99.99%, Changchun, China), and Pr₆O₁₁ (99.99%, Changchun, China) powders. The specific experimental process for the crystal growth was as described in [18]. Dark green Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals with almost the same size of Φ18 × 18 × 25 mm³ were obtained, as shown in Figure 2. The as-grown crystals were reheated in a flowing N₂(95%)–H₂(5%) mixture atmosphere at 1000 °C for 48 h to remove their intrinsic color center. The concentrations of Tb³⁺ and Tb³⁺/Pr³⁺ in the single- and co-doped as-grown crystals were determined to be 13.87 at.% (1.87 × 10²¹ cm⁻³) and 13.71 at.% (1.75 × 10²¹ cm⁻³)/0.38 at.% (0.477 × 10²⁰ cm⁻³), respectively, by the inductively coupled plasma atomic emission spectrometry method (ICP-AES).

The XRD patterns of the two obtained crystals were studied by X-ray diffraction (Miniflex600, Rigaku, Tokyo, Japan). Samples with dimensions of 5 × 5 × 2 mm³ were cut from the annealed crystals and optically polished for spectral measurement. The room-temperature polarized absorption spectra in the range of 300 nm–2500 nm were recorded using a PerkinElmer UV-VIS-NIR Spectrometer (Lambda-900, PerkinElmer, Woltam, MA, USA). The fluorescence spectra and the appropriate lifetime decay curves were recorded at room temperature using FLS920 and FSP980 (Edinburg, UK) spectrophotometers, respectively. The measurement conditions for the spectra remained the same for both samples to enable data comparisons.

3. Results and Discussion

3.1. X-ray Diffraction Analysis

The X-ray diffraction patterns of the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals were studied and are shown in Figure 3. The diffraction peaks of the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals were in good agreement with those of pure CYA crystal (PDF#24-0221). No other impurity peaks were detected, indicating that the as-grown crystals had a K₂NiF₄-type structure with an I⁴/_mmm space group.

3.2. Absorption Spectra

The room-temperature polarized absorption spectra of the Tb³⁺ single-doped and Tb³⁺/Pr³⁺ co-doped CYA crystals are shown in Figure 4. There are eight distinct absorption bands related to transitions from the ground multiplet ⁷F₆ to the excited multiplets of the Tb³⁺, which are also indicated in Figure 4, as are the transitions of the Pr³⁺ absorption band from its ground state ³H₄ to its excited state. In Figure 4, one can see that the weak absorption peaks of Tb³⁺ are located at 320 nm, 340 nm, 351 nm, 370 nm, 380 nm, and 487 nm, corresponding to the ⁷F₆→⁵H₇ + ⁵D_0,1, ⁵L₆ + ⁵L_7,8 + ⁵G₃, ⁵L₉ + ⁵G₄ + ⁵D₂, ⁵L₁₀, ⁵D₃ + ⁵G₆, and ⁵D₄ transitions in the visible range, respectively. We can see two strong absorption peaks located around 1984 nm and 2293 nm in the near-infrared region, corresponding to transitions from ⁷F₆ to the higher multiplets ⁷F_J (J = 0,1,2,3), respectively. In those absorption bands, the weak peak around 487 nm in the ⁷F₆→⁵D₄ transition is consistent with commercial semiconductor lasers, which are commonly used as the pump source of Tb³⁺ lasers. The π and σ polarization absorption cross-sections of Tb³⁺:CYA at 487 nm were 1.53 × 10⁻²² cm² and 1.55 × 10⁻²² cm², which are smaller than those of Tb³⁺:YAlO₃ (3.0 × 10⁻²² cm² @ 489 nm) but much larger than the value of Tb³⁺:CaF₂ (0.6 × 10⁻²² cm² @ 485 nm) [14,19]. The π and σ polarization absorption cross-sections of Tb³⁺/Pr³⁺:CYA at 492 nm and 489 nm were 5.23 × 10⁻²² cm² and 4.04 × 10⁻²² cm², respectively, which are much larger than that of Tb³⁺:CYA. The full widths at half-maximum (FWHMs) of the Tb³⁺:CYA crystal around 487 nm were measured to be 9.39 nm and 8.93 nm for σ and π polarization, respectively, which are larger than the values for Sr₃Tb₂(BO₃)₄ (8.5 nm at 486 nm) and Tb³⁺:YAlO₃ (3.64 nm at 486 nm) [13,20]. The absorption cross-sections were strengthened, meaning that the co-doped Pr³⁺ could be used to solve the problem of the weak absorption cross-section of the ⁷F₆→⁵D₄ transition in Tb³⁺.

3.3. Judd–Ofelt Analysis

The spectral characteristics of the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals were analyzed by the Judd–Ofelt (J–O) theory. The calculation process of the J–O theory is similar to that described in Ref. [20]. The mean wavelength ($\overline{\mathsf{\lambda }}$) and the experimental and calculated line strengths for the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals in both polarizations are listed in

Table 1. Mean wavelength$\overline{\mathsf{\lambda }}$ and experimental and calculated absorption line strengths of ED transitions of the Tb³⁺:CYA crystal.

Transitions	π-Polarization, S(10−20 cm2)			σ-Polarization, S(10−20 cm2)
6F7→	$\overline{\mathsf{\lambda }}\left(\mathbf{n}\mathbf{m}\right)$	${\mathit{S}}_{\mathit{e}\mathit{x}\mathit{p}}^{\mathit{E}\mathit{D}}$	${\mathit{S}}_{\mathit{c}\mathit{a}\mathit{l}}^{\mathit{E}\mathit{D}}$	$\overline{\mathsf{\lambda }}\left(\mathbf{n}\mathbf{m}\right)$	${\mathit{S}}_{\mathit{e}\mathit{x}\mathit{p}}^{\mathit{E}\mathit{D}}$	${\mathit{S}}_{\mathit{c}\mathit{a}\mathit{l}}^{\mathit{E}\mathit{D}}$
5H7 + 5D0,1	320	0.046	0.042	320	0.047	0.05
5L6 + 5L7,8 + 5G3	340	0.058	0.056	341	0.054	0.063
5L9 + 5G4 + 5D2	351	0.085	0.082	352	0.106	0.092
5L10	370	0.084	0.083	371	0.092	0.091
5D3 + 5G6	380	0.037	0.028	381	0.029	0.038
5D4	487	0.0077	0.015	487	0.022	0.0081
7F0,1,2	1984	1.506	2.37	1984	2.339	1.61
7F3	2293	1.175	1.347	2290	1.348	1.162

and

Table 2. Mean wavelength $\overline{\mathsf{\lambda }}$ and experimental and calculated absorption line strengths of ED transitions of Tb³⁺ in the Tb³⁺/Pr³⁺:CYA crystal.

Transitions	π-Polarization, S(10−20 cm2)			σ-Polarization, S(10−20 cm2)
6F7→	$\overline{\mathsf{\lambda }}\left(\mathbf{n}\mathbf{m}\right)$	${\mathit{S}}_{\mathit{e}\mathit{x}\mathit{p}}^{\mathit{E}\mathit{D}}$	${\mathit{S}}_{\mathit{c}\mathit{a}\mathit{l}}^{\mathit{E}\mathit{D}}$	$\overline{\mathsf{\lambda }}\left(\mathbf{n}\mathbf{m}\right)$	${\mathit{S}}_{\mathit{e}\mathit{x}\mathit{p}}^{\mathit{E}\mathit{D}}$	${\mathit{S}}_{\mathit{c}\mathit{a}\mathit{l}}^{\mathit{E}\mathit{D}}$
5H7 + 5D0,1	320	0.051	0.061	320	0.036	0.048
5L6 + 5L7,8 + 5G3	340	0.096	0.092	341	0.024	0.032
5L9 + 5G4 + 5D2	351	0.16	0.129	352	0.055	0.049
5L10	370	0.151	0.171	371	0.085	0.046
5D3 + 5G6	380	0.046	0.0033	381	0.018	0.025
5D4	487	0.029	0.012	487	0.0052	0.0051
7F0,1,2	1984	2.292	2.081	1984	2.776	0.869
7F3	2293	0.962	0.962	2290	1.807	0.859

, respectively. In

Table 3. J–O intensity parameters of different crystals doped with Tb³⁺.

Crystal		Ω2(10−20 cm2)	Ω4(10−20 cm2)	Ω6(10−20 cm2)	Ω4/Ω6	Reference
Tb3+:LiYF4		28.30	1.65	2.15	0.77	[16]
Tb3+:KYb(WO4)2		1.91	2.41	4.91	0.49	[23]
Tb3+:CaF2		1.71	2.65	2.25	1.18	[14]
Tb3+:YAG		2.75	0.12	3.37	0.03	[24]
Tb3+:YAP		3.49	5.87	2.55	2.30	[19]
Tb3+:CYA	Ωπ	3.79	2.58	1.4		This work
	Ωσ	4.25	2.31	1.51
	Ωeff	4.1	2.4	1.47	1.63
Tb3+/Pr3+:CYA	Ωπ	4.42	1.17	1.79
	Ωσ	3.98	3.19	1.05
	Ωeff	4.13	2.52	1.30	1.94

, the calculated J–O intensity parameters of Tb³⁺ in CYA and other crystals are listed. On account of the polarized absorption, the effective J–O intensity parameters can be obtained by Ω_eff = ($\mathsf{\Omega }\mathsf{\pi }$ + $2\mathsf{\Omega }\mathsf{\sigma }$)/3. According to some previous works, Ω₂ is a covalency-dependent parameter, while Ω₄ and Ω₆ are structure-dependent ones, and the former depends on covalent bonding between coordination ions and rare-earth ions [21,22]. The Ω_eff,2 of Tb³⁺ in the CYA crystal was much greater than that in CaF₂ and YAG, showing that a higher Ω_eff,2 value means a higher covalency of the metal–ligand bond, along with low symmetry of the coordination structure around Tb³⁺. The value of Ω_eff,4/Ω_eff,6 was 1.61 and 1.94 in the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals, respectively, which are higher than the values in LiYF₄, YAG, and CGA, but smaller than that in YAP.

The ED spontaneous transition rate (A^ED) was calculated on the basis of the obtained J–O parameters. The mean spontaneous transition rate (A) was obtained by A = ($\mathrm{A}\mathsf{\pi }$ + $2\mathrm{A}\mathsf{\sigma }$)/3 with A = ${\mathrm{A}}_{\mathrm{q}}^{\mathrm{ED}}$ + ${\mathrm{A}}_{\mathrm{q}}^{\mathrm{MD}}$. Then, the fluorescence branching ratio ($\mathsf{\beta }$) and radiation lifetime (${\mathsf{\tau }}_{\mathrm{rad}}$) were assessed and tabulated, as shown in

Table 4. Spontaneous transition rates (A), fluorescence branching ratios (β), and radiation lifetime (${\mathsf{\tau }}_{\mathrm{rad}}$) of the Tb³⁺:CYA crystal.

Transition	${\mathbf{A}}_{\mathsf{\pi }}^{\mathbf{ED}}\left({\mathbf{S}}^{-1}\right)$	${\mathbf{A}}_{\mathsf{\pi }}^{\mathbf{MD}}\left({\mathbf{S}}^{-1}\right)$	${\mathbf{A}}_{\mathsf{\sigma }}^{\mathbf{ED}}\left({\mathbf{S}}^{-1}\right)$	${\mathbf{A}}_{\mathsf{\sigma }}^{\mathbf{MD}}\left({\mathbf{S}}^{-1}\right)$	A (S−1)	β(%)	τr (ms)
5D4→							1.805
7F0	16.907	0	15.166	0	15.746	2.84
7F1	10.661	0	9.563	0	9.929	1.79
7F2	14.184	0	15.192	0	14.856	2.68
7F3	43.888	0.288	47.899	0.286	46.848	8.45
7F4	36.483	0.215	35.898	0.215	36.308	6.55
7F5	331.998	2.631	368.493	2.631	358.959	64.7
7F6	69.889	0.907	70.83	0.907	71.42	12.8

and

Table 5. Spontaneous transition rates (A), fluorescence branching ratios (β), and radiation lifetime (${\mathsf{\tau }}_{\mathrm{rad}}$) of the Tb³⁺/Pr³⁺:CYA crystal.

Transition	${\mathbf{A}}_{\mathsf{\pi }}^{\mathbf{ED}}\left({\mathbf{S}}^{-1}\right)$	${\mathbf{A}}_{\mathsf{\pi }}^{\mathbf{MD}}\left({\mathbf{S}}^{-1}\right)$	${\mathbf{A}}_{\mathsf{\sigma }}^{\mathbf{ED}}\left({\mathbf{S}}^{-1}\right)$	${\mathbf{A}}_{\mathsf{\sigma }}^{\mathbf{MD}}\left({\mathbf{S}}^{-1}\right)$	A (S−1)	β(%)	τr (ms)
5D4→							1.86
7F0	2.756	0	14.634	0	10.67	2.95
7F1	1.783	0	9.23	0	6.75	1.86
7F2	15.196	0	8.777	0	10.92	2.71
7F3	55.856	0.288	25.42	0.277	35.85	8.48
7F4	27.734	0.215	26.317	0.207	27	6.64
7F5	429.988	2.631	182.682	2.54	270.35	64.8
7F6	62.88	0.907	47.885	0.878	53.77	12.74

, respectively, indicating that the transition ⁵D₄→⁷F₅ of Tb³⁺ had the greatest $\mathsf{\beta }$ in both Tb³⁺- and Tb³⁺/Pr³⁺-doped CYA crystals, with values of 64.7% and 64.8%, respectively. The ${\mathsf{\tau }}_{\mathrm{rad}}$ for the ⁵D₄ multiplets of the Tb³⁺- and Tb³⁺/Pr³⁺-doped CYA crystals was calculated to be 1.805 ms and 1.86 ms, respectively—higher than the 1.7 ms recorded for Tb³⁺:YAP [19]. Compared with the Tb³⁺:CYA, the value of Ω₂ was slightly larger in Tb³⁺/Pr³⁺:CYA, indicating a more disordered local symmetry of Tb³⁺ in the co-doped crystal. This result was similar to that reported for a Tb³⁺/Pr³⁺:CaF₂ crystal by Liu [14].

3.4. Fluorescence Spectra

The polarized fluorescence spectra of the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals under the excitation of 487 nm and 492 nm, respectively, were recorded in the range of 500–725 nm, as shown in Figure 5. According to the energy level structure of Tb³⁺, the visual-range emission bands located around 546 nm, 587 nm, 623 nm, 648 nm, 673 nm, and 683 nm correspond to the transitions of ⁵D₄→⁷F_J (J = 5, 4, 3, 2, 1, 0), respectively, as indicated in Figure 5.

As shown in Figure 5, the emission band shape of Tb³⁺/Pr³⁺:CYA was highly consistent with that of Tb³⁺:CYA, due to the substantial coincidence of the fluorescence emission peaks of Tb³⁺(⁵D₄→) and Pr³⁺(³P₀→), and the emission of an ultralow concentration of Pr³⁺ was compensated for by the high concentration of Tb³⁺ [25]. Meanwhile, the intensities of the Tb³⁺/Pr³⁺ co-doped sample were weaker than those of the single-doped one. In the Tb³⁺/Pr³⁺:CYA crystal, the adjacent energy positions of Tb³⁺:⁵D₄ and Pr³⁺:³P₀, provide possible paths for energy transfer. As a result of the huge concentration difference between Tb³⁺ (13.87 at.%) and Pr³⁺ (0.38 at.%), the energy transfer process of Tb³⁺(⁵D₄)→Pr³⁺(³P₀) was more efficient than the backward one, leading to a weaker fluorescence intensity than the single-doped sample.

Based on the following Füchtbauer–Ladenburg (F–L) formula [18], the stimulated emission cross-sections for the ⁵D₄→⁷F_J (J = 5, 4) translations can be obtained from polarized fluorescence spectra:

$${\mathsf{\sigma }}_{\mathrm{em}}=\frac{{\mathsf{\beta }\mathsf{\lambda }}^5\mathrm{I}\left(\mathsf{\lambda }\right)}{8{\mathsf{\pi }\mathrm{cn}}^2{\mathsf{\tau }}_{\mathrm{rad}}\int \mathsf{\lambda }\mathrm{I}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda } }$$

(1)

where λ, β, c, and I(λ) refer to the fluorescence wavelength, branching ratio, speed of light, and fluorescence intensity, respectively. The peak emission wavelengths, FWHMs, and emission cross-sections ${\mathsf{\sigma }}^{\mathrm{em}}$ of the transitions starting from the ⁵D₄ multiplets of the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals are listed in

Table 6. Peak emission wavelengths, FWHMs, and emission cross-sections ${\mathsf{\sigma }}^{\mathrm{em}}$ of the transitions starting from the ⁵D₄ multiplets of Tb³⁺:CYA and Pr³⁺/Tb³⁺:CYA crystals.

Crystal	Transition	Polarization	Peak Wavelength (nm)	FWHM (nm)	${\displaystyle {\sigma }^{\mathbf{e}\mathbf{m}}\left({10}^{-22}{\mathbf{cm}}^2\right)}$
	5D4→
Tb3+:CYA	7F5	π	546	9.41	7.57
		σ	546	7.79	8.82
	7F4	π	587	8.43	0.44
		σ	587	13.3	0.34
Tb3+/Pr3+:CYA	7F5	π	546	9.31	6.99
		σ	546	6.28	8.55
	7F4	π	587	8.24	0.35
		σ	587	10.54	0.21

. According to Table 6, the FWHMs of the 546 nm emission band in the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals (π polarization) were 9.41 nm, 9.31 nm, respectively. The π and σ polarization emission cross-sections at 546 nm in the green light region were 7.57 × 10⁻²² cm² and 8.82 × 10⁻²² cm², respectively, for the Tb³⁺:CYA crystal—slightly larger than the π and σ polarization emission cross-sections of the Pr³⁺/Tb³⁺:CYA crystal (6.99 × 10⁻²² cm² and 8.55 × 10⁻²² cm², respectively). These results were also greater than those for Tb³⁺:CaF₂ (5.56 × 10⁻²² cm²) [14] and Ba₃TbPO₄ (5.9 × 10⁻²² cm²) [26]. The emission cross-sections of the ⁵D₄→⁷F₄ transition for Tb³⁺:CYA were calculated to be 0.44 × 10⁻²² cm² (π) and 0.34 × 10⁻²² cm² (σ), while the results for Tb³⁺/Pr³⁺:CYA were determined to be 0.35 × 10⁻²² cm²(π) and 0.21 × 10⁻²² cm² (σ). The maximum emission cross-section of Tb³⁺:CYA at 587 nm (0.44 × 10⁻²² cm²) was of the same order of magnitude as that of Tb³⁺:STB crystal (0.61 × 10⁻²² cm² E//Z) [13].

In order to explore effects of the doping concentration ratio of Tb³⁺ and Pr³⁺ on the energy transfer process between those two ions, we produced 10at.% Tb³⁺/0.6at.% Pr³⁺, 10at.% Tb³⁺/1at.% Pr³⁺, and 10at.% Tb³⁺/3at.% Pr³⁺ co-doped CYA single-crystal fibers through the micro-pulling-down method. The room-temperature fluorescence spectra in the 530–680 nm range were recorded, as shown in Figure 6. Based on these results, the luminescence intensity of the main bands responsible for Tb³⁺ ions decreased with the increase in the Pr³⁺ ions. This phenomenon can be explained through the differences in the electron shell structures of Pr³⁺ and Tb³⁺. Non-radiative processes were the main energy transfer routes between Tb³⁺ and Pr³⁺. It is therefore assumed that non-radiative energy transfer is carried out with high energy levels from the Tb³⁺ to the Pr³⁺. For the co-doped samples, Tb³⁺ is the dominant luminescence center, as its concentration is as high as 10 at.%. With the increase in the Pr³⁺ concentration, the distance between Tb³⁺ and Pr³⁺ shortened accordingly, and the non-radiative processes between Tb³⁺ and Pr³⁺ intensified, causing a reduction in the luminescence intensity. Similar experimental results were observed in Tb³⁺/Pr³⁺ co-doped scintillation glass [27]. The large distance between Tb³⁺ and Pr³⁺ might weaken the interaction between them. In the study of Chen et al., energy transfer from Tb³⁺→Pr³⁺, which involved two processes—Tb³⁺:⁵D₄ + Pr³⁺:³H₄→Tb³⁺:⁷F₆ + Pr³⁺:¹I₆ and Tb³⁺:⁵D₄ + Pr³⁺:³H₄→Tb³⁺:⁷F₄ + Pr³⁺:³P₀—was achieved in 0.3 at.% Tb³⁺/0.5 at.% Pr³⁺:CYA phosphor [28]. This result indicates that the dominant energy transfer channel in CYA is Tb³⁺→Pr³⁺, although the two ions are both at low doping levels.

The energy transfer processes between Tb³⁺ and Pr³⁺ are inefficient, and the metal-to-metal intervalence charge transfer (IVCT) processes between d0 electron-configured transition metal ions in oxide crystals and Pr³⁺/Tb³⁺ have been confirmed to be effective pathways to excite the Pr³⁺/Tb³⁺ [29]. However, no IVCT process takes place in Tb³⁺/Pr³⁺:CYA. According to the experimental results of Liu et al., the effective absorption of 5 at.% Tb³⁺:CYA was improved by co-doping with 5 at.% Pr³⁺. Due to the concentration quenching of Pr³⁺, the fluorescence intensity for the main Tb³⁺ emission bands did not decrease, but the corresponding fluorescence lifetime reduced greatly [14]. Thus, in our work, the problem of weak absorption of Tb³⁺ around 487 nm was slightly improved by co-doping with Pr³⁺. However, due to the inefficient energy transfer between Tb³⁺ and Pr³⁺ in compounds with no IVCT processes, the emission of Tb³⁺ in the visible band was slightly weakened by co-doping with Pr³⁺.

3.5. Fluorescence Lifetime

Figure 7 shows the fluorescence decay curves of the ⁵D₄→⁷F₅ transitions in Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals excited at 487 nm and 492 nm, respectively. After being fitted, the fluorescence lifetime can be obtained through the following function [30]:

$$I(t)=A_1{e}^{\frac{t}{{\tau }_1}}+A_2{e}^{\frac{t}{{\tau }_2}}+B_1$$

(2)

$${\tau }_f=\frac{A_1{\tau }_1^2+A_2{\tau }_2^2}{A_1{\tau }_1+A_2{\tau }_2}$$

(3)

where $I\left(t\right)$refers to the fluorescence intensity as a function of time. The experimental lifetimes τ_f of the ⁵D₄→⁷F₅ transitions for the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals were calculated to be 1.43 ms and 1.1ms, respectively, and the quantum efficiency $\eta$ ($\eta =\frac{{\tau }_f}{{\tau }_r}$) was estimated to be 79.2% and 59.14%, respectively. Compared with Tb³⁺:CYA, the shorter fluorescence lifetime of ⁵D₄→⁷F₅ in the Tb³⁺/Pr³⁺:CYA crystal may be attributed to the energy transfer process of Tb³⁺(⁵D₄)→Pr³⁺(³P₀). The energy transfer efficiency from Tb³⁺(⁵D₄) to Pr³⁺(³P₀) was calculated to be η = 1 − (1.1/1.43) = 23.07%. The energy transfer process decreased the population of Tb³⁺ in the ⁵D₄ state, which had a negative effect on the fluorescence and led to the weakening of the fluorescence lifetime of Tb³⁺. Unfortunately, the energy transfer efficiency value was slightly too large; hence, the impact on the Tb³⁺ fluorescence. Although the emission spectral intensity, emission cross-section, and fluorescence lifetime of Tb³⁺ were decreased through co-doping with Pr³⁺, the absorption cross-section around 487 nm was increased.

3.6. Effects of Temperature on Fluorescence Emission

Since the laser crystals suffer as a result of high temperatures during long-term operation, the exploration of the thermal stability of the optical properties of the crystals is an important task. Figure 8 shows the relative peak intensity curves of the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals under 487 nm and 492 nm excitation, respectively, with the increase in the temperature from 298 K to 548 K. The relative peak intensity of the two samples decreased almost linearly the increase in temperature. With the increase in the temperature from 298 to 398 K, the intensities of three bright lights at 546 nm (green), 588 nm (yellow), and 623 nm (red) dropped by 24%, 26%, and 27%, respectively, for Tb³⁺:CYA and by 36%, 38%, and 36%, respectively, for Tb³⁺/Pr³⁺:CYA. Additionally, the chromaticity coordinates of the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals at various temperatures under 487 nm and 492 nm excitation, respectively, are listed in

Table 7. The chromaticity coordinates of the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals at various temperatures.

Temperature (K)	Tb3+:CYA(CIE)			Tb3+/Pr3+:CYA(CIE)
	$\mathbf{X}$	$\mathit{Y}$	CCT (K)	$\mathbf{X}$	$\mathit{Y}$	CCT (K)
298 K	0.370	0.621	4951	0.345	0.638	5327
348 K	0.367	0.622	4995	0.313	0.658	5800
398 K	0.366	0.623	5010	0.289	0.673	6150
448 K	0.363	0.625	5056	0.282	0.678	6250
498 K	0.358	0.627	5130	0.268	0.687	6450
548 K	0.343	0.636	5355	0.246	0.698	6767

. The correlated color temperatures (CCTs) were calculated using McCamy’s empirical formula [31]:

$$\mathrm{CCT}=-449{\mathrm{n}}^3+3523{\mathrm{n}}^2-6823.8\mathrm{n}+5520.33$$

(4)

$$\mathrm{n}=\left(\mathrm{x}-0.3320\right)/\left(\mathrm{y}-0.1858\right)$$

(5)

With the increase in temperature, the chromaticity coordinates of Tb³⁺:CYA varied from (0.370, 0.621) at 298 K to (0.343, 0.636) at 548 K, and the values of Tb³⁺/Pr³⁺:CYA varied from (0.345, 0.638) at 298 K to (0.246, 0.698) at 548 K; the decrease in the x value and the increase in the y value of the CIE coordinates resulted in all of the coordinates (x,y) invariably being located in the green color region, as shown in Figure 9. This was nothing like the occurrence in the Tb³⁺/Pr³⁺:Na₅Gd(WO₄)₄ phosphors, in which the most prominent transition was an 648 nm with (0.541, 0.378) coordinates in the orange–yellow region. This was most likely caused by the IVCT processes between Tb³⁺ or Pr³⁺ and transition metal ions (i.e., Ti⁴⁺, V⁵⁺, Nb⁵⁺, Mo⁶⁺, or W⁶⁺) with d0 electrons configured in oxide crystals [32]. The results indicated that the Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals possessed good thermal stability of their optical properties, as well as potential for green laser applications with a wide temperature range.

4. Conclusions

Single crystals of 13.87 at.% Tb³⁺ single^-doped and 13.71 at.% Tb³⁺/0.38 at.% Pr³⁺ co-doped CYA were produced by the Czochralski method. The polarized spectra and fluorescence decay curves were studied in detail. Through the incorporation of Pr³⁺, the absorption cross-section around 487 nm was increased from 1.53 × 10⁻²² cm² to 5.53 × 10⁻²² cm² for the π polarization. The J–O intensity parameters Ω_t (2, 4, 6), fluorescence branch ratios (β), and radiation lifetimes (τ_rad) were calculated. For the Tb³⁺:CYA crystal, the stimulated emission cross-sections of the ⁵D₄→⁷F₅ and ⁷F₄ transitions were calculated to be 7.57 × 10⁻²² cm² and 0.44 × 10⁻²² cm² for π polarization, respectively, which were larger than the values for the Tb³⁺/Pr³⁺:CYA crystal. The fluorescence lifetime of the ⁵D₄ level was measured to be 1.41 ms and 1.1ms with quantum efficiency of 79.2% and 59.14% for Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA, respectively. All of the results show that Tb³⁺:CYA and Tb³⁺/Pr³⁺:CYA crystals may be potential media for the operation of visible-range lasers. However, Pr³⁺ may not be a good candidate for use as a sensitizing ion for Tb³⁺ to strengthen the visible emission in CYA crystals.