Spectra properties of Yb3+, Er3+: Sc2SiO5 crystal

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Xue Yanyan, Zheng Lihe, Jiang Dapeng, Sai Qinglin, Su Liangbi, Xu Jun. Spectra properties of Yb³⁺, Er³⁺: Sc₂SiO₅ crystal. Chinese Physics B, 2019, 28(3): 037802 Copy to clipboard

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Spectra properties of Yb³⁺, Er³⁺: Sc₂SiO₅ crystal

Xue Yanyan¹, Zheng Lihe^{2, †}, Jiang Dapeng², Sai Qinglin³, Su Liangbi², Xu Jun^{1, 4, ‡}

1School of Physics Science and Engineering, Institute for Advanced Study, Tongji University, Shanghai 200092, China

2Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201899, China

3Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China

4Shanghai Engineering Research Center for Sapphire Crystals, Shanghai 201899, China

† Corresponding author. E-mail: zhenglihe@gmail.com

xujun@mail.shcnc.ac.cn

Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0701002), the Joint Fund of the National Natural Science Foundation of China and China Academy of Engineering Physics (NSAF) (Grant No. U1530152), the National Natural Science Foundation of China (Grant Nos. 91222112 and 61475177), and Shanghai Rising-Star Program (B type), China (Grant No. 14QB1401600).

Abstract

The influence of Er³⁺ ions on the spectra of Yb³⁺, Er³⁺: Sc₂SiO₅ (SSO) single crystal, which was obtained by Czochralski (Cz) method, is discussed. The absorption coefficient at 980 nm was 13.36 cm⁻¹ with a peak absorption cross-section of 1.46 × 10⁻²⁰ cm². The emission cross-sections at 1034 nm and 1062 nm were 5.5 × 10⁻²¹ cm² and 4.9 × 10⁻²¹ cm², respectively. The fluorescence lifetime was estimated as 1.24 ms at 1061 nm. The mechanical properties of SSO single crystal were also presented.

PACS: 78.55.-m;42.70.Hj;78.45.+h

Keyword:Yb³⁺;Er³⁺: Sc₂SiO₅;spectra properties;mechanical properties

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1. Introduction

High-performance InGaAs diode laser operating in the range of 900–1100 nm raises the development of Yb³⁺ doped materials for all-solid-state ultrafast laser, high power laser, and thin disk laser.^[1–5] Yb³⁺ doped laser materials possess advantages over Nd³⁺ doped ones, including long radiative lifetime, small quantum defect between absorption and emission, and absence of deleterious effects such as excited state absorption, up-conversion, and concentration quenching. The combination of Er³⁺ and Yb³⁺ in up-conversion is well studied. Upon excitation at the ⁴I_11/2 level, the cross-relaxation process of [Er³⁺ (⁴I_11/2 → ⁴I_15/2); Yb³⁺ (²F_7/2 → ²F_5/2)] could occur.^[6] Therefore, in this study, we choose Er³⁺ as the sensitizer in Yb³⁺ activated laser crystal.

However, the three-energy-level scheme of Yb³⁺ laser could lead to a high laser threshold due to the high thermal population of the terminal level. Quasi-four energy levels of Yb³⁺ could be introduced by selecting a crystal host with low symmetry and multi-crystallographic sites that would improve the emission band broadening and energy level splitting.^[7,8] In the silicate crystal family, scandium silicate crystal (Sc₂SiO₅, SSO) possesses the low-symmetry crystal structure of monoclinic C2/c space group and two non-equivalent crystallographic sites.^[9–11] In addition, SSO crystal possesses high thermal conductivity (κ = 7.5 W·m⁻¹ · K⁻¹) and negative thermo-optical coefficient with the dn/dT value of −6.3 × 10⁻⁶ K⁻¹.^[8] With the benefits from strong coupling between ytterbium ions and crystal field, quasi-four-level Yb³⁺: SSO single crystal exhibits large energy stark splitting of 1027 cm⁻¹, which is beneficial for generating short pulse laser^[12–14] and reducing heat generated in high power laser system.^[3,15,16] Li et al. reported a diode-pumped Yb³⁺: SSO chirped pulse amplifier with 1 ps pulse duration.^[17] SSO crystal host has been widely used in the fields of high power lasers, ultra-fast lasers, and amplifiers.

In this paper, the mechanical properties of pure Sc₂SiO₅ bulk crystal will be demonstrated for the first time. The influence of Er³⁺ ions on the near infrared spectroscopy of Yb³⁺, Er³⁺: SSO will also be discussed.

2. Experiments

2.1. Crystal growth

Undoped SSO crystal was grown for the measurement of fracture toughness and flexure strength. The raw materials of Sc₂O₃ (4N), SiO₂ (4N), Yb₂O₃ (4N5), and Er₂O₃ (4N5) were used for SSO and Yb³⁺, Er³⁺: SSO single crystals growth. The mixture powders were compressed and loaded into an alumina container before sintering at 1250 °C for 12 h in a muffle furnace. The crystals were grown by the Czochralski method in an inductively heated iridium crucible under nitrogen ambient atmosphere. SSO crystal seed oriented in (010) direction was used. The pulling rate was set at 1–2 mm/h and the rotation rate was 15–20 rpm. After pulling apart from the melting, the crystals were cooled down to room temperature in 40 h. SSO and Yb³⁺, Er³⁺: SSO crystals with diameters of 25 mm and lengths of 65 mm were obtained as shown in Fig. 1. The transparent Yb³⁺, Er³⁺: SSO crystal showed a light pink color.

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	Fig. 1. Photographs of the as-grown SSO and Yb³⁺, Er³⁺: SSO single crystals.

2.2. Spectra measurements

The concentrations of Yb³⁺ and Er³⁺ in Yb³⁺, Er³⁺: SSO crystal detected by inductively coupled plasma atomic emission spectrometry (ICP-AES) were 7.07 wt.% and 0.065 wt.%, respectively. The segregation coefficient of Yb³⁺ in the Yb³⁺, Er³⁺: SSO crystal was calculated to be 0.85, while the segregation coefficient was 0.96 in Yb³⁺: SSO crystal.^[3] The solubility of Yb³⁺ in the Yb³⁺, Er³⁺: SSO host lattice was reduced due to the competition of Er³⁺ ions with Yb³⁺ while substituting in the Sc³⁺ lattice. The influence of Er³⁺ on the spectra will be further discussed in Section 3.

Crystal structure was confirmed by x-ray diffraction (XRD, Cu target, Kα, X-pert, Holland). Figure 2 presents the x-ray powder diffraction pattern of Yb³⁺, Er³⁺: SSO crystal. The diffraction peak positions match well with those of standard Sc₂SiO₅ (PDF#40-0035). The XRD indicates that Yb³⁺, Er³⁺: SSO crystal maintains the primitive monoclinic symmetry with a space group of C2/c. The cell parameters were calculated to be a = 0.9960 nm, b = 0.6428 nm, c = 1.2058 nm, β = 103.7° and V = 0.7500 nm³. Due to a larger ionic radius of Yb³⁺ than that of Sc³⁺, the cell volume is slightly larger than V = 0.7498 nm³ of the SSO single crystal.

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	Fig. 2. XRD pattern of Yb³⁺, Er³⁺: SSO crystal.

3. Results and discussion

3.1. Mechanical properties

To apply laser crystals in high power laser system, it is important to evaluate mechanical properties that could predict the energy storage capability of the laser host.

The Vickers hardness (H_V) and fracture toughness of SSO crystal were determined by indentation technique in a digital Vickers hardness tester (Wilson-WolpertTukon2100B) at room temperature for the first time. Mechanical properties of Nd: YAG crystal were also tested for comparison.

Figure 3 presents the image of indentation on the mechanically polished surface of the SSO crystal. The Vickers hardness can be estimated by

where F is the force applied to the indenter and A is the surface area of the resulting indentation. The fracture toughness can be experimentally determined from the linear sizes of radial cracks (C) arising near the point of load application and estimated by

where β is the correction angle of 68°. The flexure strength was determined by a three-point bending test on 3 mm × 5 mm × 20 mm bars which were polished after cutting using a universal testing machine (Instron-1195). Five specimens for each sample were tested.

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	Fig. 3. Microscopic image of indentation in mechanically polished SSO crystal.

The fracture toughness and flexure strength of SSO and Nd: YAG crystals are listed in Table 1. Vickers hardnesses of SSO and Nd: YAG crystals are 908 and 1325 under the load of 0.5 kg, respectively. It can be seen that the fracture toughness of SSO is close to that of Nd: YAG crystal but higher than that of Y₂O₃ and YSO crystals. The flexure strength of SSO is inferior to that of Y₂O₃ and Nd: YAG crystals. All obtained data indicate that the SSO crystal is suitable for mechanical processing and high power operation.

Table 1.

Fracture toughness and flexure strength of SSO, Nd: YAG, and other reported crystals.

3.2. Absorption and emission spectra

The absorption and emission spectra at room temperature of Yb³⁺, Er³⁺: SSO crystal in the wavelength region of 850–1150 nm are shown in Fig. 4. The unpolarized absorption spectrum was measured by using a UV/Vis/NIR spectrophotometer (Model Cary 5000, Agilent) in the wavelength region from 400 nm to 2000 nm. The absorption spectrum is mainly composed of four strong bands around 915 nm, 958 nm, 980 nm, and 990 nm corresponding to the transitions from the ground state ²F_7/2 to the sublevels of ²F_5/2 of Yb³⁺. The peak at 980 nm shows the highest absorption coefficient of 13.36 cm⁻¹. The absorption cross-sections of Yb³⁺ were calculated according to

where D_o is the optical density as a function of wavelength, N is the Yb³⁺ concentration with the value of 9.16 × 10²⁰ ions/cm³, and L is the sample thickness. The absorption cross-section σ_abs is shown in Fig. 4. The absorption cross section at 980 nm is 1.46 × 10⁻²⁰ cm², which is larger than that of Yb³⁺: SSO crystal.^[3] It is indicated that Er³⁺ ions make contribution to the absorption intensity.

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	Fig. 4. The absorption and emission spectra of Yb³⁺, Er³⁺: SSO crystal.

The emission spectrum was investigated under the excitation of Xe lamp (Jobin-Yvon TRIAX 550 spectrophotometer) in the range of 900–1200 nm. The fluorescence spectrum is mainly composed of three bands around 998 nm, 1034 nm, and 1062 nm in addition to the zero-line at 981 nm. The fluorescence intensities at 1034 nm and 1062 nm are stronger than that at 998 nm. The shapes of absorption and fluorescence spectra are broad due to the thermal spreading of the spectra at room temperature.

A fluorescence lifetime measurement was carried out by a computer-controlled transient digitizer decay curve of emission. The fluorescence lifetime at 1061 nm was estimated to be 1.24 ms, as shown in Fig. 5. The emission cross-sections of Yb³⁺ for the transition between ²F_5/2 and ²F_7/2 can be calculated by Füchtbauer–Ladenburg formula^[20]

where I(λ) is the emission intensity, τ_rad is the radiative lifetime of the excited manifold ²F_5/2 of Yb³⁺, c is the light velocity, and n is the refractive index. The emission cross-section σ_em is shown in Fig. 4.

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	Fig. 5. The fluorescence decay curve of Yb³⁺, Er³⁺: SSO crystal at 1061 nm.

Table 2 shows the spectroscopic parameters of Yb³⁺, Er³⁺: SSO crystal. The σ_abs at 915 nm, 958 nm, 980 nm, and 990 nm were estimated to be 7.8 × 10⁻²¹ cm², 5.2 × 10⁻²¹ cm², 14.6 × 10⁻²¹ cm², and 7.4 × 10⁻²¹ cm², and the absorption bandwidths were around 16 nm, 19 nm, 10 nm, and 12 nm, respectively. Broad and strong absorption bands are necessary to improve the diode-pumping efficiency, because laser diodes typically emit in a 5 nm wide spectral range while presenting a thermal shift of the peak wavelength. Therefore, the absorption bands of Yb³⁺, Er³⁺: SSO crystal are favorable for matching the emission wavelength of InGaAs laser diodes for highly efficient pumping. Two strong fluorescence bands around 1034 nm and 1062 nm could be served as the efficient laser output with σ_em of 5.5 × 10⁻²¹ cm² and 4.9 × 10⁻²¹ cm², respectively. Particularly, in addition to the smallest thermal populating of the terminal laser level which brings about the smallest re-absorption losses, the emission band around 1034 nm possesses the largest emission cross-section, which is promising for low threshold and high efficient laser operation. It should be noted that, the particularly broad emission bandwidth (FWHM) of Yb³⁺, Er³⁺: SSO crystal reaches 72 nm, which is broader than that of Yb: YSO (48 nm).^[21]

Table 2.

Spectroscopic parameters of Yb³⁺, Er³⁺: SSO crystal.

In order to better understand the effect of Er³⁺ in Yb³⁺, Er³⁺: SSO crystal, the emission spectrum and fluorescence decay curve of Er³⁺ around 1548 nm are presented in Fig. 6. The fluorescence lifetime of Er³⁺ in Yb³⁺, Er³⁺: SSO crystal was fitted to be 5.7 ms. Energy transfer process of the Er³⁺ and Yb³⁺ in Sc₂SiO₅ host is shown in Fig. 7. The Er³⁺ in Yb³⁺, Er³⁺: SSO crystal could also be pumped directly to the ⁴I_11/2 level at the excitation wavelength of 896 nm from Xe lamp. Yb³⁺ is acceptor and the energy is transferred from the ⁴I_11/2 level of Er³⁺ to the ²F_5/2 level of Yb³⁺ and the cross-relaxation process of [Er³⁺ (⁴I_11/2 → ⁴I_15/2); Yb³⁺ (²F_7/2 → ²F_5/2)] can occur, which explains the increase of σ_em from 4.4 × 10⁻²¹ cm² in Yb³⁺: SSO^[3] to 5.5 × 10⁻²¹ cm² in Yb³⁺, Er³⁺: SSO with the co-doping of 0.065 wt.% Er³⁺.

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	Fig. 6. The emission spectrum and fluorescence decay curve of Er³⁺ around 1548 nm.

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	Fig. 7. Energy transfer process between Er³⁺ and Yb³⁺.

4. Conclusion

In summary, the fracture toughness was obtained to be 0.83 MPa·m^1/2 and flexure strength was 135.5 MPa in SSO crystal. Yb³⁺, Er³⁺: SSO crystal with Er³⁺ content of 0.065 wt.% shows broader emission bandwidth of 72 nm. The emission band at 1034 nm possesses the largest emission cross-section of 5.5 × 10⁻²¹ cm², which is favorable for low threshold and high efficient laser operation. The effective regulation of optical parameters obtained in Yb³⁺, Er³⁺: SSO crystal demonstrated that cations-interaction is promising in investigating new laser candidates with higher emission cross-section for high power laser systems.

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