Structure and spectral properties of Dy³⁺ doped CaYAlO₄ single crystal

Abstract

A 2 at.% Dy³⁺: CaYAlO₄ single crystal was grown successfully. The electronic structures of Ca²⁺/Y³⁺ mixed sites in CaYAlO₄ were investigated using first-principles based on density functional theory. The effects of Dy³⁺ doping on the structural parameters of host crystal were studied using XRD pattern. The optical properties including absorption spectrum, excitation spectrum, emission spectra and fluorescence decay curves were thoroughly investigated. The results show that the Dy³⁺: CaYAlO₄ crystal could be pumped by the blue InGaN and AlGaAs or 1281 nm laser diodes. Furthermore, an intense 578 nm yellow emission was obtained directly under excitation at 453 nm, meanwhile, evident mid-infrared light emitting was observed by 808 or 1281 nm laser excitation. The fitted fluorescence lifetimes of ⁴F_9/2 and ⁶H_13/2 levels were about 0.316 ms and 0.038 ms, respectively. It can be concluded that this Dy³⁺: CaYAlO₄ crystal could simultaneously act as a promising medium for both solid-state yellow and mid-infrared laser outputs.

Introduction

570–590 nm visible lasers and 3–5 µm mid-infrared (MIR) lasers have attracted much attention due to their important applications in various fields, especially in the medical field, such as the treatment of skin and eye diseases^1–5. Therefore, the development of spectral 570–590 nm yellow lasers and 3–5 µm MIR lasers is of great significance. Especially for yellow lasers, nonlinear frequency conversion is still the most popular technique for obtaining yellow lasers^6,7, which belongs to indirect method. However, the above-mentioned technology for indirectly obtaining yellow laser is expensive, inefficient and complex. These drawbacks inspire researchers to develop a new method for obtaining yellow lasers. Recently, due to the rapid development of laser diode (LD) technology, a simpler and more reliable system for the direct generation of yellow lasers has been proposed, which is achieved by LD-pumping rare-earths doped laser materials^8,9. Therefore, materials capable of directly emitting yellow lasers have received great attention.

Trivalent dysprosium (Dy³⁺) ions are widely used in W-LEDs and MIR lasers due to their unique luminescence characteristics in the visible light regions of 460–500 nm (⁴F_9/2 → ⁶H_15/2), 550–600 nm (⁴F_9/2 → ⁶H_13/2), and the MIR regions of 2400–3500 nm (⁶H_13/2 → ⁶H_15/2) and 3700–4800 nm (⁶H_11/2 → ⁶H_13/2)^10–16. However, the concentration quenching effect (CQE) of Dy³⁺ is the major issue¹⁷. Despite this, Dy³⁺-doped laser materials, such as Dy: YAG^18,19, Dy, Tb: Y₃Al₅O₁₂ ²⁰, Dy: LiLuF₄²¹, Dy: ZnWO₄⁹, Dy: LaF₃²², Dy: CaF₂²³, have yielded promising results in yellow lasers and 3–5 µm MIR lasers. Also, more meaningful achievements in this field are expected in the near future.

CaYAlO₄ (CYA) crystal is regarded as a promising laser host material due to its spectral inhomogeneous broadening property, which is mainly due to its hybrid structure²⁴. The CYA crystal belongs to the tetragonal system with space group I4/mmm. The crystal parameters are a = b = 3.6451 Å, c = 11.8743 Ų⁵. The Yb³⁺, Nd³⁺, Pr³⁺, Ho³⁺ and Er³⁺-doped CYA crystals have been reported for its excellent physical and chemical properties^26–30. However, reports on the use of Dy³⁺-doped CYA crystals to simultaneously produce yellow and MIR lasers are scarce so far.

As the CYA crystal melts congruently, a Dy³⁺-doped single crystal was successfully grown using the Czochralski technique. The crystal structure and electronic structures of the grown crystal were investigated. The luminescence properties of the grown crystal were also discussed using the measured spectral parameters.

Experimental

A classical solid-phase sintering process was used to synthesize polycrystalline powder with the chemical formula of Dy_0.02CaY_0.98AlO₄ (Dy: CYA). The chemical raw materials used were CaCO₃, Al₂O₃ (AR grade) and Y₂O₃, Dy₂O₃ (4 N purity) powders. All of them were purchased from Changchun Heprui Rare Earth Materials Technology Co., Ltd. The crystal grown was carried out in a DGL-400 furnace (NCIREO, China). The crystal growth process and parameter settings are similar to those described in the Ref.³⁰. Finally, a high-quality Dy: CYA crystal for this experiment was obtained.

The concentration of Dy³⁺ in the singly-doped CYA crystal was measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Ultima 2, Jobin-Yvon). The result was 3.59 × 10²⁰ ions cm⁻³. The segregation coefficient k of Dy³⁺ in the Dy: CYA crystal was calculated by

$$k=c_s/c_o$$ 1

where c_s and c₀ were the concentrations of Dy³⁺ in the crystal and initial raw materials, respectively. The value of k was 1.42.

The CYA single crystal structure data used for theoretical calculations were obtained from the ICSD database (No. 1001545). The approximate process and parameter settings of the theoretical calculations are similar to those described in the Ref.²⁴. Here, the cutoff energy is set to 380 eV. The k-point grids used for the unit cell geometry optimization and electronic structure calculation of the Brillouin zone are 3 × 3 × 1 and 7 × 7 × 2, respectively. Interactions between ionic cores and valence electrons are described, including Ca-3s²3p⁶4s², Y-4s²4p⁶5s²4d, Al-3s²3p, and O-2s²2p⁴ electrons.

The powder XRD pattern of the grown crystal was performed on a Miniflex600 with a diffracted beam monochromator set for Cu-Kα radiation (λ = 1.54056 Å). The range of 2θ was 10°–80°, and the scan step size was 0.02°.

In addition, the absorption spectrum was recorded using a Perkin-Elmer UV–Vis–NIR Spectrometer (Lambda-900) in a range of 400–2000 nm. The fluorescence spectra and emission decay curves with different pump sources were measured using an Edinburgh Instruments FLS920 Spectrophotometer. All spectra were measured at room temperature (RT). The dimensions of the experimental sample used for spectral testing were 10.0 × 8.0 × 1.0 mm³.

Results and discussion

Crystal structure and electronic structures

As a member of the ABCO₄ family, CYA has a perovskite-type structure with Al ions occupying octahedral symmetry sites and divalent Ca and trivalent Y randomly distributed in a 1:1 ratio at C_4v symmetry sites, as shown in Fig. 1a. Due to the unique properties of Ca²⁺ and Y³⁺, the perfect unit cell can be divided into three non-equivalent configurations, as shown in Fig. 1b. In addition, due to their similarity in radius and valence, Y³⁺ are easily replaced by Dy³⁺, which further increases the disorder of the crystal. This can cause non-uniform broadening of absorption and emission lines. The broad absorption spectrum is beneficial for increasing the absorption of pump light, and the broad emission spectrum is conducive to obtaining a tunable or ultrafast laser³¹.

Figure 1.

(a) The crystal structure model and (b) three nonequivalent configurations of the CYA unit cell (denoted as “I”, “II” and “III”).

To understand the bonding interactions in CYA, theoretical calculations based on the DFT method were performed. Figure 2a demonstrates the band structure of CYA (configuration “I”), which shows that the CYA is a kind of direct band gap material with a bandgap of 3.000 eV. For the unit cell of configurations “II” and “III”, the band structures of CYA are also calculated and shown in Fig. S1(a) and Fig. S2(a), respectively.

Figure 2.

(a) Calculated band structure of CYA; (b) electronic DOS of CYA: (1)–(4) the partial DOS of Ca, Y, Al, O and (5) the total DOS.

Figure 2b shows the full and partial density of states (DOS) (configuration “I”), which contributes to the bands. Since the CYA single crystal is composed of AlO₆ octahedron, the valence band is mainly provided by the Al-O bond. For the conduction band, from 7.8 to 0 eV, the O-2p⁴, Al-3s², 3p, Y-4d, and Ca-3d⁰ states are mainly involved. For the other two configurations, the corresponding densities of states are shown in Fig. S1(b) and Fig. S2(b), respectively.

X-ray and Rietveld refinement analysis

The measured XRD pattern confirmed the phase purity and crystallinity of Dy³⁺-doped CYA crystal, as shown in Fig. 3a. The diffraction peaks of the sample are consistent with those of the standard JCPDF file [No. 24–0221], which indicates that Dy³⁺ were successfully introduced into the CYA host lattice. The structure of the Dy: CYA crystal is further refined using XRD data. The related refinement results and structural parameters are shown in Fig. 3b and Table 1, respectively. As shown in Fig. 3b, the observed and calculated diffraction patterns are consistent, indicating that the synthesized Dy: CYA crystal still has a tetragonal phase with space group I4/mmm. Furthermore, as can be seen in Table 1, the parameters of the Dy: CYA crystal become slightly smaller compared to JCPDF #24-0221 (a = b = 3.648 Å, c = 11.890 Å), which is mainly caused by the inconsistent radius between doped ions and Y. The radius of the dopant ion Dy³⁺ (0.908 Å) is smaller than that of Y³⁺ (1.075 Å).

Figure 3.

(a) XRD pattern of Dy: CYA crystal; (b) the refined results of Dy: CYA crystal (the "bkg" refers to the background diffraction peak intensity. The "obs" refers to the experimentally measured raw data, "cal" refers to theoretical simulation data, and "obs-calc" refers to the difference between the two).

Table 1.

The parameters of refined structure.

Parameters	Dy: CYA
Crystal structure	Tetragonal
Space group	I4/mmm
a = b (Å)	3.64730 ± 0.00029
c (Å)	11.88342 ± 0.00029
α = β = γ	90°
RP	6.01%
RWP	9.04%

The absorption spectrum and Judd–Ofelt analysis

Figure 4 illustrates the absorption spectrum of Dy: CYA crystal in the 400–2000 nm range. There are seven prime absorption bands located at 453, 758, 806, 909, 1076, 1251, and 1633 nm, that correspond to the transitions of the Dy³⁺ from the ground state energy level ⁶H_15/2 to various excited states, as marked in Fig. 4. The absorption bands corresponding to the transitions of ⁶H_15/2 → ⁴I_15/2, ⁶H_15/2 → ⁶F_5/2, and ⁶H_15/2 → ⁶F_11/2 + ⁶H_9/2 piqued our interest, because their peaks at 453, 808 and 1281 nm coincide with the output wavelengths of the commercial LDs. As shown in Fig. 5, the peaks of Dy³⁺: ⁴I_15/2 energy level overlaps with the emission of blue InGaN LD. Therefore, yellow solid-state laser pumped by blue LD can be made via the transition of Dy³⁺: ⁴F_9/2 → ⁶H_13/2. Furthermore, the peaks of Dy³⁺: ⁶F_5/2 and ⁶H_9/2&⁶F_11/2 energy levels overlaps with the emission of AlGaAs and 1280 nm LDs, respectively. Consequently, the 808 nm and 1281 nm LD pumped MIR laser can be made based on Dy³⁺: ⁶H_13/2 → ⁶H_15/2 transition (as shown in Fig. 9).

Figure 4.

Absorption spectrum of Dy: CYA crystal.

Figure 5.

The excitation spectrum of Dy: CYA crystal.

Figure 9.

The MIR emission spectra and fluorescence decay curves of Dy³⁺: ⁶H_13/2 in CYA crystal pumped by 1281 nm (upper panel) and 808 nm (lower panel).

As an important spectral parameter, known as the absorption cross-section (σ_a) can be determined by the following formula:

$${\sigma }_a\left(\lambda \right)=\frac{2.303}{N_0\times l}OD\left(\lambda \right)$$ 2

where λ is the wavelength, N₀ is the Dy³⁺ concentration, l is the thickness of the crystal and OD is the optical density. The calculated σ_a values are 1.86 × 10^–21 cm², 3.14 × 10⁻²¹ cm², and 4.32 × 10^–21 cm² for 453 nm, 808 nm, and 1281 nm, respectively.

The radiative transition of the 4f configuration of Dy³⁺ in Dy: CYA crystal was analyzed using Judd–Ofelt (J–O) theory^32,33 and measured absorption spectrum. The details of the J–O calculation method can be found in the literature³⁴. The results of the calculation are listed in Table 2. It shows that the calculated oscillators are consistent with the measured oscillators. The reality and validity of the results can be evaluated by the root mean square deviation (RMS Δf). Here, the RMS Δf is calculated to be 0.08801 × 10^–6, indicating that the calculated results have a very high reference value. Furthermore, three intensity parameters Ω_t (t = 2, 4, 6) are fitted to be 1.97 × 10^–20 cm², 1.56 × 10^–20 cm², and 2.51 × 10^–20 cm², respectively. In general, the Ω₂ can reflect the coordination symmetry of matrix materials and the orderliness of structures, which is sensitive to component changes, while Ω₄/Ω₆ is the spectroscopic quality factor³⁵. In comparison to other Dy³⁺-doped crystals, the value of Ω₄/Ω₆ in Dy: CYA crystal is 0.62, which is larger than that in PbF₂, Lu₂SiO₅, YAG, GSAG and LiYF₄ crystals, as shown in Table 3.

Table 2.

Oscillator strengths of Dy: CYA crystal.

Transition 6H15/2 →	λ (nm)	n	fexp (× 10–6)	fcal (× 10–6)
4I15/2	453	1.931	0.534	0.510
6F3/2	760	1.896	0.200	0.251
6F5/2	807	1.893	1.217	1.333
6F7/2	911	1.889	2.564	2.723
6F9/2, 6H7/2	1098	1.884	3.187	3.143
6F11/2, 6H9/2	1293	1.880	3.952	3.965
6H11/2	1693	1.874	1.432	1.334
			RMS Δf = 0.08801 × 10–6

Table 3.

J–O parameters of Dy³⁺-doped laser crystals.

Crystals	Ω2 (× 10–20 cm2)	Ω4 (× 10–20 cm2)	Ω6 (× 10–2 cm2)	Ω4/Ω6	References
PbF2	3.18	1.16	2.27	0.51	36
Lu2SiO5	4.31	1.28	3.49	0.37	37
YAl3(BO3)4	10.81	2.05	3.28	0.63	38
BaY2F8	1.52	2.33	3.67	0.63	39
YAG	1.49	0.94	3.20	0.29	40
GSAG	2.17	1.06	2.32	0.46	41
LiYF4	2.01	1.34	2.39	0.56	42
CYA	1.97	1.56	2.51	0.62	This work

The radiative transition rate A, fluorescence branching ratio β, and radiative lifetime τ_r of Dy³⁺ transits from ⁴F_9/2 or ⁶H_13/2 to different lower levels were calculated using the obtained Ω_t parameters and are listed in Table 4. For the ⁴F_9/2 level, the ⁴F_9/2 → ⁶H_13/2 transition has the largest radiative transition rate and the fluorescence branching ratio (51%). The calculated results indicate that the Dy: CYA crystal has great potential for producing yellow and MIR laser outputs.

Table 4.

Calculated spontaneous radiation probabilities, branching ratios, and radiative lifetime of Dy: CYA crystal.

Transition		Aed	Amd	A (s−1)	β	τr (ms)
4F9/2 →	4F5/2	4.321	0		0.003	0.758
	6F7/2	6.827	9.038		0.012
	6H5/2	5.662	0		0.004
	6H7/2, 6F9/2	34.957	14.666		0.038
	6F9/2, 6F11/2	39.57	62.250		0.100
	6H11/2	48.878	18.961		0.051
	6H13/2	672.486	0		0.510
	6H15/2	370.951	0	1319	0.281
6H13/2 →	6H15/2	36.486	13.038	49.523	1	20.192

Yellow fluorescence spectra

Figure 5 shows the RT excitation spectrum for the emission at 582 nm. Eight main excitation peaks centered at 296, 326, 353, 368, 389, 429, 453, and 467 nm were observed, which corresponds to the transition from the ground level ⁶H_15/2 to upper levels ⁶H_11/2, ⁶P_3/2, ⁶P_7/2, ⁶P_5/2, ⁴I_13/2 + ⁴F_7/2, ⁴G_11/2, ⁴I_15/2, and ⁴F_9/2, respectively. Although the most intense peak is at 353 nm, the output power of 350 nm LDs is lower. Therefore, the blue GaInN LD at 453 nm was used as the pumping source.

Then the emission spectrum in the visible band excited by 453 nm was measured and presented in Fig. 6a. The emission bands centered at 484, 582, 670, and 755 nm can be seen in Fig. 6a, and the corresponding transition processes are shown in Fig. 6b. The strongest emission is concentrated at 582 nm, which is consistent with the fluorescence branching rate calculated by the J–O theory. The corresponding FWHM of ⁴F_9/2 → ⁶H_13/2 emission is 14.3 nm.

Figure 6.

(a) The visible emission spectrum of Dy: CYA crystal; (b) the diagram of the corresponding energy level transitions.

The stimulated emission cross-section can be calculated by the Füchtbauere–Ladenburg (F–L) formula^43,44:

$${\sigma }_{\mathit{em}}=\frac{{\lambda }^5{A}_{J{J}^{\prime }}}{8\pi c{n}^2}\frac{I\left(\lambda \right)}{\int \lambda I\left(\lambda \right)d\lambda }$$ 3

where I(λ) is the experimental fluorescence intensity at wavelength λ. The value of σ_em at 582 nm is 0.24 × 10^–20 cm².

Moreover, the chromaticity coordinate CIE 1931 for Dy: CYA crystal was calculated and shown in Fig. 7. The obtained chromaticity coordinate is (x = 0.4946, y = 0.5044), which is in the yellow area. The correlated color temperature (CCT) can be calculated by the following formula⁴⁵:

$$\text{CCT}=-449{\text{n}}^3+3525{\text{n}}^2-6823.3\text{n}+5520.33$$ 4

here $\text{n}=(\text{x}-{\text{x}}_{\text{e}})/(\text{y}-{\text{y}}_{\text{e}})$ and ${\text{(x}}_{\text{e}}{\text{, y}}_{\text{e}})=(0\text{.332, 0}.186)$. The value of CCT was 2928 K. The above results indicate that the Dy: CYA can be used as a new material for yellow light emission.

Figure 7.

The chromaticity coordinate CIE 1931 of Dy: CYA crystal.

Figure 8 shows the fluorescence decay curve of the Dy³⁺: ⁴F_9/2 level pumped at 453 nm. This decay curve exhibits an exponential decay behavior. Therefore, the corresponding fluorescence lifetime can be fitted by the following formula:

$$\text{I}(\text{t})={\text{I}}_{0}exp\left[-\frac{\text{t}}{\mathrm{\tau }}\right]$$ 5

where τ is the fluorescence lifetime. The fluorescence lifetime obtained by fitting is 0.316 ms. According to the equation: ${\eta }_T={\tau }_f/{\tau }_r,$ where ${\tau }_r$ is the radiative lifetime (as listed in Table 4), the quantum efficiency is about 41.6%.

Figure 8.

Fluorescence decay curve of the Dy³⁺: ⁴F_9/2 multiplet.

Table 5 lists the main optical parameters of Dy: CYA and other Dy³⁺-doped crystals. For Dy: CYA crystal, the absorption cross-section at 453 nm is 1.86 × 10^–21 cm², which is larger than that of Dy: CaGdAlO₄⁴⁶, Dy: LiNbO₃⁴⁷, Dy: YAG⁴⁸ and Dy: Li₂Gd₄(MoO₄)₇⁴⁹. The emission cross-section at 582 nm is 0.24 × 10^–20 cm², which is larger than that of Dy: Li₂Gd₄(MoO₄)₇⁴⁹. The fluorescence lifetime of Dy³⁺: ⁴F_9/2 level is 0.316 ms, which is much longer than that of Dy: CaGdAlO₄⁴⁶ and Dy: LiNbO₃⁴⁷. The quantum efficiency is 48.90%, which is larger than that of Dy: CaGdAlO₄⁴⁶ and Dy: YAG⁴⁸. The above-mentioned advantages indicate that the 2 at.% Dy: CYA crystal is a potential material for a yellow solid-state laser.

Table 5.

The main optical parameters of some Dy³⁺-doped crystals.

Crystals	σabs (10–21 cm2)	σem (10–20 cm2)	τf (ms)	τr (ms)	η (%)	References
CaGdAlO4	2.43 (σ)	0.51 (σ)	0.222	0.501	44.31	46
	1.28 (π)	0.55 (π)
LiNbO3	0.95 (σ)	0.03 (σ)	0.298	0.387	77.0	47
	0.6 (π)	0.32 (π)
YAG	1.6	0.3	0.376	2.020	18.61	48
Li2Gd4 (MoO4)7	1.0 (σ)	0.17 (σ)	0.865	–	–	49
	1.4 (π)	0.16 (π)
CYA	1.86	0.24	0.316	0.758	41.69	This work

MIR emission spectra

The measured MIR emission spectra of Dy³⁺: ⁶H_13/2 → ⁶H_15/2, which were pumped by 1281 nm and 808 nm, are shown in Fig. 9. As shown in Fig. 4, it could be seen that the absorption band of ⁶H_15/2 → ⁶H_9/2/⁶F_11/2 transition is very strong, which matches the emission band of the 1281 nm LD, so 1281 nm was chose for excitation wavelength, as shown in Fig. 9 (upper panel). In addition, the MIR emission spectrum with the pump wavelength of 808 nm was also analyzed and is shown in Fig. 9 (lower panel). According to the formula (3), when the crystal was excited by 1281 nm, the emission cross-section was 5.84 × 10^–21 cm² at the peak with an FWHM of 297 nm, otherwise it was 3.72 × 10^–21 cm² with an FWHM of 342 nm. Furthermore, the fluorescence lifetime τf of the Dy³⁺: ⁶H_13/2 level was fitted to be about 0.032 ms for 1281 nm LD pumping and 0.038 ms for 808 nm LD pumping.

Conclusion

Dy³⁺-doped CYA singly crystal was successfully grown. The electronic structures of CYA were presented and analyzed using first-principles calculations. The crystal lattice parameters were obtained by Rietveld refinement. The spectroscopic properties of the Dy: CYA crystal were investigated. According to the J–O theory, the evaluated intensity parameters are Ω₂ = 1.97 × 10^–20 cm², Ω₄ = 1.56 × 10^–20 cm² and Ω₆ = 2.51 × 10^–20 cm². The main spectral parameters of the crystal are obtained and compared. An intense yellow emission was observed at 582 nm when 453 nm was used as a pump wavelength. The stimulated emission cross-section at the peak was 0.24 × 10^–20 cm². The fitted fluorescence lifetime of the ⁴F_9/2 state was 0.316 ms. The color coordinate (CIE 1931) was also calculated (x = 0.4946, y = 0.5044), which was in the yellow area. When pumped by 1251 or 808 nm, a strong MIR emission was also observed at about 2920 nm and the stimulated emission cross-sections at peak were also calculated. The findings of this research indicate that the 2 at.% Dy: CYA crystal is not only a potential candidate for a blue InGaN LD directly pumped yellow laser, but it also has promising applications in the field of MIR luminescence.

Author contributions

Y.L., M.W. and H.S. contributed to the crystal’s growth. Y.W., X.W. and C.H. contributed to the data analysis. Y.L. conceived the experiments and wrote the manuscript. C.T. and J.G. supervised the project and reviewed the manuscript. All authors assisted in manuscript preparation.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-023-33366-x.