Optical properties of Dy:YCa4O(BO3)3 crystal grown by the Czochralski technique

Dy3+-doped YCa4O(BO3)3 (Dy:YCOB) crystal was successfully grown by the Czochralski technique. The absorption cross section at 453 nm was 0.28 × 10–21 cm2, that is related to the 6H15/2 → 4I15/2 transition. The Judd-Ofelt parameters Ωt (t = 2, 4, 6) were 1.62 × 10–20, 0.10 × 10–20, 0.41 × 10–20 cm2, respectively. The emission cross section assigned to the transition 4F9/2 → 6H13/2 was 1.02 × 10–21 cm2. The 4F9/2 energy level’s fluorescence lifetime was 900 μs.

In this work, we succeeded in synthesizing Dy:YCOB crystal via Czochralski technique. To discuss the spectral properties, the absorbance spectrum, emission spectrum and fluorescence decay curve were detected.

Crystal synthesis
We synthesized Dy:YCOB crystal via Czochralski technique. The starting materials were composed of Dy 2 O 3 (99.999%), CaCO 3 (99.999%), Y 2 O 3 (99.999%) and H 3 BO 3 (99.999%) powders. The weighing process was based on stoichiometric ratio and to compensate the loss during the synthesis process, we added 1 wt% additional H 3 BO 3 . The powders were first mixed (3 h) and sintered (900°C, 7 h, air). Then they were sufficiently ground, mixed, compressed into pellets and sintered (1100°C, 10 h, air). The weighing and sintering processes followed the chemical equation: The synthesis parameters were shown as follows: growth direction, growth atmosphere, pulling speed, rotation rate, cooling rate, the grown crystal length and diameter. The related parameter values are Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. crystalline b-axis, nitrogen, 0.5∼1 mm h −1 , 10∼20 rpm, 10∼30°C h −1 , 50 mm and 15 mm. Figure 1 demonstrates the polished Dy:YCOB sample. The crystal was cut along the plane (−2 0 1) with the size of 10 mm × 5 mm × 2 mm.

Structure and spectral examinations
We utilized the x-ray powder diffractometer to obtain the phase calibration of Dy:YCOB crystal. The element content of Dy 3+ ions was detected by ICP-AES. The absorbance spectra were obtained by a Spectrophotometer     figure 3. We mainly concentrate on the transition 6 H 15/2 → 4 I 15/2 around 450 nm, that is appropriate to be pumped by blue-emitting LDs. At 453 nm, the absorption cross section σ abs was 0.28×10 -21 cm 2 . What's more, the full width at half maximum (FWHM) was 2.4 nm.

J-O theory calculations
The Judd-Ofelt theory is generally utilized for analyzing the spectral performance in RE ions doped crystals as well as glasses [19][20][21].
The experimental line strength S exp (J, J′), calculated line strength S cal (J, J′) as well as the mean wavelength l were gathered in table 1, that were calculated from the absorbance spectra. The Root-Mean-Square (RMS) deviation in S exp (J, J′) and S cal (J, J′) is 0.050×10 -20 cm 2 , manifesting a good consistance between them.
The emission cross section σ em is worked out by: where I(λ) is the measured fluorescence intensity. In Dy:YCOB, σ em at around 585 nm was 1.02×10 -21 cm 2 corresponding to the yellow emission and the FWHM was 15.14 nm, that is beneficial to mode-locked and tunable laser generation. Figure 5 presents 4 F 9/2 level's fluorescence lifetime curve. Using the single exponential fitting, the fluorescent lifetime τ f was 900 μs. The lifetime is higher than most of the oxide host materials, for example, Dy:YAG (400 μs) [2], Dy:NaGd(WO 4 ) 2 (177 μs) [31], Dy:YAP (185 μs) [3], suggesting a higher probability of energy storage  capability. The fluorescence quantum efficiency η (η = τ f /τ rad ) was 40.0%, which attributes to the cross relaxation caused by high doping concentration. The above-mentioned data denotes that Dy:YCOB is encouraging for realizing yellow laser performance.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).