Growth and Spectral Properties of Er³⁺ and Yb³⁺ Co-Doped Bismuth Silicate Single Crystal

Abstract

Rare-earth-doped bismuth silicate (Bi₄Si₃O₁₂, BSO) crystal is a multifunctional material for scintillation, LED, and laser applications. In the present study, Er³⁺ and Yb³⁺ ions co-doped bismuth silicate crystals were grown by a modified vertical Bridgman method, and their spectral properties were investigated for the first time. Transparent Er/Yb: BSO single crystal up to Φ 25 mm × 30 mm was obtained. The segregation coefficient of the Er/Yb: BSO crystal was measured to be 0.96 for Er³⁺ ions and 0.91 for Yb³⁺ ions. Absorption and fluorescence spectra had been recorded in the range of 200–1700 nm. The absorption cross section was calculated to be 6.96 × 10⁻²⁰ cm² at 976 nm with the full width at half maximum (FWHM) of 8 nm, and the emission cross section was 0.9771 × 10⁻²⁰ cm² at 1543 nm with FWHM of 16 nm. The fluorescence decay curve was measured at 976 nm excitation. By linear fitting, the fluorescence lifetime of the upper ⁴I_13/2 level of Er³⁺ was 8.464 ms at room temperature. Compared with Er³⁺ ion-doped bismuth silicate crystal (Er: BSO), the Er/Yb: BSO crystal has a wider FWHM and larger absorption cross section. The results indicate that the Er/Yb: BSO crystal is a potential lasing crystal.

1. Introduction

In recent years, the development of diode-pumped solid-state lasers has become increasingly significant, and the lasing crystal is one of their most important parts [1]. Among them, erbium-doped solid-state lasers with emission wavelengths of 1.5~1.6 μm can be utilized in the security scope of natural eyes and are widely used in remote sensing, medical treatment, source imaging, optical communication systems, and ranging [2,3], thus attracting much attention. Er³⁺ is a significant laser ion with a complex energy level structure, and the emission wavelengths of 1.5~1.6 μm correspond to the ⁴I_13/2 → ⁴I_15/2 transition. However, the Er³⁺ ion-doped materials have a drawback (weaker absorption peaks near 800 nm and 980 nm), which is unfavorable for solid-state laser diode pumping. It is found that Yb³⁺ and Er³⁺ co-doped systems can solve this problem by sequentially improving the pumping efficiency of the materials. Yb³⁺ has a relatively simple energy level structure: a ²F_7/2 ground state and a ²F_5/2 excited state. In addition, Yb³⁺ shows a strong and wide absorption band near 980 nm, and the absorption cross section in this region is much larger than that of Er³⁺, so Yb³⁺ can act as a sensitizer and effectively transfer the absorbed energy to Er³⁺. In previous work, Er³⁺ and Yb³⁺ ions co-doped crystals have been reported, such as Y₃Al₅O₁₂ [4], GdCa₄O(BO₃)₃ [5], CaMoO₄ [6], YCa₄O(BO₃)₃ [7], Lu₂Si₂O₇ [8], and YVO₄ [9]. It is well known that laser emission is related to the host materials as well as lasing emitting ions. The host materials should have higher thermal stability, more stable mechanical properties, and stronger absorption. BSO crystals as scintillation materials have been investigated in our lab [10,11]. This shows high thermal and mechanical properties and are expected to be a potential lasing host crystal.

BSO crystal belongs to the cubic crystal system with the space group $Td-I\overline{4}3d$ and the lattice parameters a = 10.2867(5) Å, V = 1088.5 ų, Z = 4 [12]. Due to its high hardness, short decay time, and low cost, it has become a new scintillation crystal of great interest for applications in nuclear physics; high-energy physics; and, especially, the dual-readout calorimeter [13]. BSO crystals have the advantages of stable chemical properties, good mechanical properties, a low thermal expansion coefficient, and good optical properties. The thermal conductivity of BSO crystals is 3.29 W∙m⁻¹K⁻¹ at 298 K [14], which is greater than that of isomorphic BGO crystals (2.59 W∙m⁻¹K⁻¹) [15] and comparable to Lu₂SiO₅ (3.67 W∙m⁻¹K⁻¹) [16]. Based on excellent properties, BSO crystals were considered as lasing host materials as well as scintillation materials. Huang et al. reported the Czochralski growth and laser output of Yb: BSO crystal [17]. Lira et al. reported the photoluminescence of the Er: BSO crystal [18]. Previously, we have reported rare-earth ions and Ge⁴⁺- and U⁴⁺-doped BSO laser crystals, and laser output was realized in Ho³⁺: BSO and Tm³⁺: BSO crystals [19,20,21,22]. In the present work, the growth and spectral properties of the Er/Yb: BSO crystal are reported.

2. Experimental Procedures

Er/Yb: BSO single crystal was grown using a modified vertical Bridgman method. Highly pure Bi₂O₃ (5N), SiO₂ (4N), Er₂O₃ (4N), and Yb₂O₃ (4N) were used as raw materials, as listed in

Table 1. Experimental drugs used to grow pure BSO, Er: BSO, and Er/Yb: BSO crystals.

Name	Purity	Company Name	City, Country
Bi2O3	5N	Jincheng Reagent Co., Ltd.	Kunshan, China
SiO2	4N	Shanghai Boer Chemical Reagent Co., Ltd.	Shanghai, China
Er2O3	4N	Sinopharm Chemical Reagent Co., Ltd.	Shanghai, China
Yb2O3	4N	Sinopharm Chemical Reagent Co., Ltd.	Shanghai, China

. They were dried at 500 °C in an oven and accurately weighed according to the stoichiometric proportion of Bi₄Si₃O₁₂ with 1 mol% dopants (0.1 mol% Er₂O₃ and 0.4 mol% Yb₂O₃). They were mixed uniformly in a mortar and then sintered in a muffle furnace at 750 °C for 8 h. In order to obtain pure phase Er/Yb: BSO polycrystalline, the sintered mixture needed secondary sintering at 820 °C for 3 h for crystal growth. The sintered Er/Yb: BSO raw materials were checked by XRD using a Philips XPERT-MED X-ray diffractometer and conventional CuKα radiation in the 2θ range from 10° to 70°. The Er/Yb: BSO polycrystalline powders were loaded into a platinum crucible, and single crystal was grown in a vertical Bridgman furnace. The Φ 25 mm platinum crucible is cylindrical in shape, with a seed well of 10 mm diameter at the bottom. A (001)-oriented BSO crystal was used as seed. The furnace temperature was controlled at 1080 °C (about 50 °C above the melting point of the BSO crystals) to ensure complete melting of the raw materials. The growth rate of the crystals was controlled at 0.2~0.6 mm/h. Pure BSO, Er: BSO, and Er/Yb: BSO crystals were grown in the furnace by the same preparation method at the same time. Figure 1 shows as-grown pure BSO, Er: BSO, and Er/Yb: BSO crystals.

The brown circle outside of as-grown crystals was observed, which was attributed to the phase segregation during the crystal growth [23]. The transparent crystals were obtained when as-grown crystals were cut perpendicular to the growth direction, as shown in Figure 2. The crystals were cut into several pieces and polished for optical and spectral measurement. We chose two pieces for the measurement of dopant concentrations. One was in the shouldering part, and the other was in cylindrical part. They were 10 mm apart. The real concentrations of rare earth ions in BSO crystals were measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES) (Agilent 725, Santa Clara, CA, USA). The transmission and absorption spectra of Er/Yb: BSO crystals were recorded at room temperature with a dual-beam UV–VIS-IR spectrophotometer (Agilent Cary5000, CA, USA). The fluorescence spectra and fluorescence lifetimes of Er/Yb: BSO crystals were recorded with a fluorescence spectrometer (Edinburgh FLS-920, Edinburgh, UK), with a xenon lamp as the excitation light source.

3. Results and Discussion

3.1. Crystal Growth

The preparation of pure-phase polycrystalline is the key to ensure the successful growth of the BSO crystal because of the great difference between SiO₂ (the melting point of 1750 °C) and the Bi₂O₃ melting point (850 °C) [24]. The segregation occurred easily during the growth, resulting in brown, circular Bi₂O₃-rich precipitates and white SiO₂ on the top [25]. The excess of Bi₂O₃ or SiO₂ in polycrystalline raw materials may aggravate phase separation during the growth. The secondary sintering and sol-gel methods both allow for the synthesis of pure BSO polycrystalline [24,26]. In the present work, pure phase polycrystalline was prepared by secondary sintering with the previous conditions. Figure 3 shows the XRD patterns of pure BSO, Er: BSO, and Er/Yb: BSO powders, which match well with the standard card PDF#33-0215, and no second phase was observed. This indicated that the pure-phase polycrystalline had been obtained by the secondary sintering. We also measured the powder XRD of Er/Yb: BSO crystal. The lattice parameter and cell volume of Er/Yb: BSO crystals were 10.291 Å and 1089.9 ų. The results were same with the pure BSO crystal. This means that the change of lattice constant was negligible due to low doping concentrations.

The doping concentrations of Er³⁺ and Yb³⁺ were measured by ICP-AES spectrometer. The concentration of Er³⁺ in the Er: BSO crystal was 1.26 at%, and the concentrations of Er³⁺ and Yb³⁺ in the Er/Yb: BSO crystal were 0.19 at% and 0.73 at%, respectively. According to the following equation for the segregation coefficient:

$$K_{eff}=\raisebox{1ex}{$C_t$}\!\left/ \!\raisebox{-1ex}{$C_0$}\right.$$

(1)

where $K_{eff}$ is the segregation coefficient, $C_t$ is the concentration of Er³⁺ or Yb³⁺ in the crystal, and $C_0$ is the concentration of Er³⁺ or Yb³⁺ in the melt. The segregation coefficient of Er³⁺ in the Er: BSO crystal was calculated to be 1.26, and 0.96 for Er³⁺ and 0.91 for Yb³⁺ in Er/Yb: BSO crystal. Therefore, it can be seen from the result that the segregation coefficients of both Er³⁺ and Yb³⁺ in the BSO crystal are close to unity.

Although there are brown Bi-rich precipitates in the outside of as-grown crystal, the polished sample looks transparent. The transmittance spectra of pure BSO, Er: BSO, and Er/Yb: BSO crystals slices were measured at room temperature from 200 nm to 800 nm, as shown in Figure 4, where the inset shows the transmission spectrum of pure BSO crystal. The transmittance of doped BSO crystal is near 80%, which is approximately the same as that of the pure BSO crystal. However, there are several absorption peaks at 365, 378, 408, 452, 489, 520, 545, and 652 nm in the transmittance spectra of Er: BSO, and the Er/Yb: BSO crystals are attributed to the presence of Er³⁺ ion’s 4f electron energy level transition, which corresponds to the transition of 4f electrons from the ground state ⁴I_15/2 to the excited states ⁴G_9/2 + ²K_15/2, ⁴G_11/2, ²H_9/2, ⁴F_5/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, and ⁴F_9/2, respectively [27]. The absorption peaks of the Er/Yb: BSO crystals were found to be weaker than those of Er: BSO, which should be attributed to a slight decrease in the number of particles in the excited state of the Er³⁺ ions transferred to the energy level of Yb³⁺.

3.2. Spectral Properties

3.2.1. Absorption Spectrum

The absorption spectra of Er: BSO and Er/Yb: BSO crystals at room temperature from 200–1700 nm are shown in Figure 5. We observed seven characteristic emission peaks in Figure 5a: 379, 488, 520, 652, 800, 976, and 1542 nm, which corresponds to Er³⁺ from the ground state ⁴I_15/2 to ⁴G_11/2 + ²K_15/2, ⁴F_7/2, ²H_11/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 transition [27]. After co-doping, the absorption peaks decreased, while the intensity of the peak at 976 nm in Figure 5b was increased remarkably compared with that in Figure 5a. The absorption peak at 976 nm was attributed to the ⁴I_15/2 → ⁴I_11/2 transition of Er³⁺ ions in Figure 5a. However, the strong absorption peak at 976 nm in Figure 5b was attributed to ⁴I_15/2 → ⁴I_11/2 transition of Er³⁺ ions and the ²F_7/2 → ²F_5/2 transition of Yb³⁺ ions [28]. Thus, Er³⁺/Yb³⁺ co-doping increased the absorption at 976 nm, which is beneficial to the diode laser pumping.

The absorption cross section is defined by the equation

$${\sigma }_{\alpha }=\frac{\alpha }{N_m}$$

(2)

where α is the absorption coefficient and $N_m$ is the doping concentration of rare earth ions, expressed as ions/cm³. According to the absorption spectrum of Figure 5b and the above absorption cross section equation, the absorption cross section ${\sigma }_{\alpha }$ of Er³⁺ at 976 nm in the Er/Yb: BSO crystal was calculated to be 6.96 × 10⁻²⁰ cm². In addition, the FWHM at 976 nm is 8 nm for diode laser pumping. We also calculated the absorption cross section and FWHM of Er³⁺ in Er: BSO crystal, and the values are 6.64 × 10⁻²¹ cm² and 3 nm. The results confirmed that the absorption cross section and FWHM of the Er/Yb: BSO crystal are higher than those of Er: BSO crystal.

3.2.2. Fluorescence Spectrum

Figure 6 shows the 1450–1650 nm emission spectra of Er: BSO and Er/Yb: BSO crystals recorded at room temperature under excitation at 976 nm. The 1450–1650 nm band corresponds to the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺. The results show that the emission peak intensity of the Er: BSO crystal is weaker than that of Er/Yb: BSO crystal, which indicates that the energy transfer from the sensitizer Yb³⁺ to the activator Er³⁺ is effective, and a cross relaxation energy transfer between them may occur [29]. Under the excitation of 976 nm radiation, the sensitizer Yb³⁺ absorbs energy to generate the ⁴F_7/2 → ²F_5/2 transition, and the dipole–dipole interaction force transferred the energy of the Yb³⁺ upper level ²F_5/2 to the Er³⁺ upper level ⁴I_11/2. Then, the ⁴I_11/2 energy level is depopulated to the ⁴I_13/2 energy level, and finally the energy level ⁴I_13/2 radiates to the ground state ⁴I_15/2 [30]. The Er/Yb: BSO fluorescence emission spectrum also shows a series of emission peaks at 1475, 1488, 1525, 1532, 1535, 1543, 1558, 1569, and 1626 nm, which corresponds to different Stark sub-level transitions between ⁴I_13/2 and ⁴I_15/2 of Er³⁺. However, there are two strong emission peaks at 1532 and 1543 nm, corresponding to the potential laser output wavelength of the crystal.

The FWHM of the emission spectrum of the Er/Yb: BSO crystal is measured to be 16 nm. Based on the emission spectrum, the excited emission cross section of the ⁴I_13/2 → ⁴I_15/2 transition is calculated according to the F-L formula

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

(3)

where λ is the wavelength, I(λ) is the emission intensity at λ wavelength, c is the speed of light, n is the refractive index of the crystal (BSO crystal, n = 2.06 [13]), and ${\tau }_f$ is the fluorescence lifetime of the emission energy level. After the fitting calculation of Equation (3), the functional relationship between the emission cross section and the wavelength can be obtained, as shown in Figure 7.

Figure 7 shows the excited emission cross sections of the Er: BSO and Er/Yb: BSO crystals. As can be seen from the figure, both crystals have a maximum emission cross section at 1543 nm, and the Er/Yb: BSO crystal has a larger emission cross section than the Er: BSO crystal. In addition, the emission cross section corresponding to each emission peak of the Er/Yb: BSO crystal is larger than that of Er: BSO crystal. The peak emission cross section for the ⁴I_13/2 → ⁴I_15/2 transition calculated by the F-L formula is about 0.9771 × 10⁻²⁰ cm² at 1543 nm. This is slightly larger than the excitation emission cross sections of well-studied Er/Yb co-doped crystals, such as YAG (0.6 × 10⁻²⁰ cm²) [31] and YVO₄ (0.7 × 10⁻²⁰ cm²) [32].

3.3. Fluorescence Decay Curve and Fluorescence Lifetime

The luminescence decay curves of the ⁴I_13/2 → ⁴I_15/2 transition of Er: BSO and Er/Yb: BSO crystals at 1543 nm were recorded under 976 nm excitation, as shown in Figure 8. A typical fluorescence decay curve showing multi-exponential decay behavior is revealed in the figure. The fluorescence lifetime can be fitted by the following equation:

$$R\left(t\right)=B_1\exp \left(\frac{-t}{{\tau }_1}\right)+B_2\exp \left(\frac{-t}{{\tau }_2}\right)$$

(4)

$$\tau =\frac{B_1{\tau }_1^2+B_2{\tau }_2^2}{B_1{\tau }_1+B_2{\tau }_2}$$

(5)

where τ is the fluorescence lifetime. Therefore, the fluorescence lifetime of the ⁴I_13/2 energy level (1543 nm) of the fitted the Er/Yb: BSO crystal is 8.464 ms, and the fluorescence lifetime of the ⁴I_13/2 energy level of the Er: BSO crystal is 13.381 ms. Obviously, the decay time of the ⁴I_13/2 energy level of the Er/Yb: BSO crystal is faster due to the effective energy transfer from the Yb³⁺ to the Er³⁺ ion.

Table 2. Comparison of spectral parameters of Er/Yb co-doped laser crystals and glass [33].

	K (W∙m−1K−1)	σα (10−20 cm2) at 980 nm	σem (10−20 cm2) at 1.5 μm	τ (ms) at 1.5 μm
BSO	3.29	6.96	0.9771	8.464
Lu2Si2O7 [8]	14.28	0.8 E//X 1.12 E//Y 0.76 E//Z	0.85 E//X 1.2 E//Y 0.81 E//Z	8.6
YCOB [34]	2.7	0.9	0.6	1.25
YAB [35]	4.7	2.75	2.5	0.33
KYW [36,37]	2.7	3.2	2.5	4.8
YAG [31]	14	0.9	1.5	7.7
YVO4 [9,32]	9.6	8	0.7	2.3
Glass [38]	0.8	1.4	0.8	8

listed the main parameters of several laser crystals and glass. Similar to excellent laser crystals Er/Yb: YAG and Er/Yb: YVO₄, the Er/Yb: BSO crystal shows good thermal conductivity, larger absorption cross section, and comparable decay time. Er/Yb: BSO crystals are expected to be potential candidates for diode solid-state lasers.

4. Conclusions

We reported the Bridgman growth of the Er/Yb: BSO crystal with a size of Φ 25 mm × 30 mm. The transmittance of the 1 mol% Er/Yb co-doped BSO crystal is about 80% in the range of visible light. The absorption spectra in the 200–1700 nm range show a strong absorption peak at 976 nm, corresponding to the transition of Er³⁺: ⁴I_15/2 → ⁴I_11/2 and Yb³⁺: ²F_7/2 → ²F_5/2. The corresponding absorption cross section is about 6.96 × 10⁻²⁰ cm², and the FWHM is about 8 nm. The fluorescence spectrum of 1450–1650 nm was fitted, the maximum emission cross section was 0.9771 × 10⁻²⁰ cm², and the FWHM was 16 nm. The fluorescence lifetime of the upper energy level ⁴I_13/2 was 8.464 ms. By comparing the spectra of the Er: BSO and Er/Yb: BSO crystals, it was concluded that Yb³⁺ ion co-doping the Er: BSO crystal can increase the absorption cross section at 976 nm. Therefore, Er³⁺ and Yb³⁺ co-doped BSO crystals can be considered as potential materials, especially for 1.5~1.6 μm solid-state laser applications.