Growth, structure, and spectroscopic properties of Yb,Ho,Pr:GYTO single crystal （Invited）

Previously, rare-earth orthotantalate (RETaO₄) had been attracted as scintillator crystals and X-ray phosphors, due to its high chemical stability, high density, rich physical properties, and so on^[1-2]. RETaO₄ belongs to the fergusonite structure and exhibits excellent luminescent properties^[3-6]. It usually exhibits two modifications, fergusonite M-type structure I2/a ($ {C}_{2h}^{6} $, #15, Z = 4), and fergusonite M’-type structure P2/a ($ {C}_{2h}^{4} $, #13, Z = 2)^[7]. RE ions, with similar ion radius, can be substituted easily by other rare earth ions to realize characteristic emission^[8]. Besides, RETaO₄ belong to monoclinic system and the site of RE ions is C₂ symmetry, which are advantageous to Stark levels splitting of active ions and realization of new emission and tunable wavelength. Therefore, RETaO₄ can be used as new host matrices. Additionally, mixed crystal is an effective method to reduce lattice symmetry further and obtain the absorption and emission spectra with inhomogeneous broadening^[9-10]. In the past decade, our group have finished a lot of research works on RETaO₄, especially on GdTaO₄ (GTO)^[11]. On the base of the previous works, the GTO crystal field can be effectively regulated by mixing Y₂O₃. Moreover, the position of emission peaks can be regulated by the differentproportion of Y₂O₃ in GdYTaO₄ (GYTO) crystal. Now, rare earth(Ho, Nd)-doped GTO and GYTO have been realized laser output in near infrared band^[12-15].

With the rapid development and application of laser technology, the search for new mid infrared laser materials has always been an important direction^[16-20]. The ⁵I₆→⁵I₇ transition of Ho³⁺ is an effective approach to obtain 2.9 μm lasers^[21-22]. However, the laser efficiency is poor. Because of the long lifetime of ⁵I₇ level and the short lifetime of ⁵I₆, it is hard to realize population inversion, that is self-terminating “bottleneck” effect. To overcome this “bottleneck” effect, Pr³⁺ ions are usually used as deactivators to reduce the lifetime of low laser level, which have been achieved good results in other crystals^[23-24]. In our previous work, the detailed properties of Ho-doped GYTO, Yb,Ho-doped GYTO, and Tm,Ho-doped GYTO are studied^[25-27]. Unfortunately, there is no laser output. Therefore, Pr³⁺ ions are doped into Yb,Ho:GYTO to reduce the lifetime of the laser low level ⁵I₇.

In this study, a Yb,Ho,Pr:GYTO crystal was grown successfully using Czochralski method for the first time. The crystal structure and quality are analyzed. The polarized absorption spectra are investigated. The optical properties, including fluorescence, lifetimes, and energy transfer mechanisms among the ions are measured and analyzed.

According to the chemical formula Yb_0.05Ho_0.01Pr_0.002Gd_0.738Y_0.2TaO₄, the high purity oxides were weighed, mixed, and calcined. The Yb,Ho,Pr:GYTO single crystal was grown by the Czochralski method. The temperature gradient, growth parameter, and growth process are the same as the previous work^[26]. A transparent and crack free crystal with a size of Φ 23 mm × 40 mm was obtained, as shown in Fig.1(a). Under a 1 W 532 nm laser irradiation, no light-scattering points were observed in the as-grown Yb,Ho,Pr:GYTO crystal. The <100>, <010>, and <001>-oriented slice samples were cut with a thickness of 2 mm and polished on both sides for measurements (shown in Fig.1(b)).

Figure 1.  (a) Photograph of the as-grown Yb,Ho,Pr:GYTO crystal; (b) <100>, <010>, and <001>-oriented wafers of Yb,Ho,Pr:GYTO crystal

The X-ray diffraction (XRD) patterns of the as-grown Yb,Ho,Pr:GYTO crystal were measured using a Philip X′pert PRO X-ray diffractometer equipped with Cu Kα radiation. The diffraction peaks were recorded in the 2θ range of 10°-90° with a scan step of 0.033°. A high resolution X′Pert Pro MPD diffractometer equipped with a hybrid Kα₁ monochromator was employed to collect the X-ray rocking curve. The doping concentrations of Yb³⁺, Ho³⁺, Pr³⁺ and Y³⁺ ions in the Yb,Ho,Pr:GYTO crystal were measured by Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS). The analyses of the sample which was cut from the shoulder part of the as-grown crystal were carried out on an Agilent 7900 quadrupole ICP-MS coupled to a Photon Machines equipped with Analyte HE 193 nm ArF Excimer Laser Ablation system. The effective segregation coefficients (k_eff) of the doping ions were obtained by comparing the LA-ICP-MS results with the initial concentrations in the raw materials used for crystal growth. Polarized absorption spectra were recorded at room temperature by using a Perkin-Elmer UV-VIS-NIR spectrometer (Lambda-900). In addition, we used a fluorescence spectrometer (Edinburgh FLSP920) with an exciting source of 940 nm LD and Opolette (OPO) 355I lasers to record the fluorescence spectrum from 2850 to 3000 nm and the fluorescence decay curves.

The XRD patterns of the Yb,Ho,Pr:GYTO is shown in Fig.2. There are strong diffraction peaks,corresponding to (020), (110), (−121), (121), (040), (200), (002), (240), (042), (202), (−321), (−123), and (123) planes. The number and relative intensity of peaks are the same with the standard pattern of the GTO phase (ICSD-109186), which means that they belong to the same monoclinic space group of I2/a (No.15). Taking thestructure parameters of GTO as the initial values, the XRD data of Yb,Ho,Pr:GYTO crystal is fitted using the Rietveld refinement method to obtain the structural parameters. The refinement results of Yb,Ho,Pr:GYTO are shown in Fig.3 and Tab.1. The lattice parameters of Yb,Ho,Pr:GYTO are fitted to be a=5.381 Å(1Å=0.1 nm), b=11.023 Å, c=5.076 Å, β=95.59º, V=299.68 ų, which are slightly smaller than the lattice parameters a=5.411, b=11.049, c=5.073, β=95.59º, V=302.56 ų of GTO. The reason for this is that the sites of Gd³⁺ in GTO are occupied by Yb³⁺, Ho³⁺, Pr³⁺ and Y³⁺, and their ionic radii are smaller than that of Gd³⁺.

Figure 2.  XRD patterns of Yb,Ho,Pr:GYTO single crystal

Figure 3.  Rietveld refinement results from the XRD data of Yb,Ho,Pr:GYTO crystal

In recent years, inductively coupled plasma mass spectrometry (ICP-MS) is an effective detection for element concentrations measurement, especially trace element. However, the tested sample needs to be dissolved fully. Therefore, in this process, there are some shortcomings, such as insufficient dissolution, introduction of new impurities, which will lead to the incorrect results. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has become a preferred method for the measurement of major and trace element concentrations in mineral, gem, steel, ceramic, other synthetic and natural samples. It is a highly sensitive metal analytical technique and can realize microanalysis. Importantly, the tested sample does not need to be processed. In this study, a beam size of 15-40 µm and scan speeds of 15-40 µm/s (equal to beam size) were chosen. The repetition of 193 nm laser was 10 Hz with a constant energy output of 50 mJ, resulting in an energy density of 2-3 J/cm² at the target. Meanwhile, multi-point measurements of samples were carried out. Then the average value was calculated and the concentrations of doping ions Yb³⁺, Ho³⁺, Pr³⁺ and Y³⁺ ions in the as-grown crystal are shown in Tab.2. The k_eff of elements Yb, Ho, Pr, and Y are calculated according to the equation k_eff= C_s/C₀, where C_s and C₀ are the ion concentrations in the crystal and melt, respectively. The k_eff of Yb, Ho, Pr, and Y in Yb,Ho,Pr:GYTO crystal is 0.624, 1.220, 1.350, and 0.977, respectively.

The X-ray rocking curves of the (100), (010), and (001) diffraction planes are shown in Fig.4. The three rocking curves are single diffraction peak with symmetric shape and without splitting, and the full widths at half maximum (FWHM) are 0.036°, 0.013°, and 0.077°, respectively. It indicates that the as-grown Yb, Ho,Pr:GYTO crystal is a single crystal with good crystalline quality.

Figure 4.  X-ray rocking curves of Yb,Ho,Pr:GYTO crystal

The room-temperature polarized absorption spectra of Yb,Ho,Pr:GYTO in the wavelength between 350 nm and 2200 nm are shown in Fig.5(a). There are seven obvious absorption bands centered at around 360, 419, 450, 535, 645, 1175, and 1945 nm, which correspond to the transitions starting from the ⁵I₈ ground state of Ho³⁺ to the excited states ⁵G₅(1)+³H₆, ⁵G₅, ⁵F₁+⁵G₆, ⁵S₂+⁵F₄, ⁵F₅, ⁵I₆, and ⁵I₇ of Ho³⁺, respectively. Importantly, the absorption bands from in the wavelength of 900-1050 nm corresponds to the transition of Yb³⁺ ions from the ground state ²F_7/2 to the excited state ²F_5/2, which matches well with the emission wavelength of commercially available high power InGaAs laser diodes (LD). For comparision, the 900-1050 nm absorption bands of Yb,Ho:GYTO crystal and Yb,Ho,Pr:GYTO crystal are shown in Fig.5(b) and expressed in a, b, c and a’, b’, c’ respectively. The absorption coefficient of E//c and E//c’ are larger than those along the other directions of themselves.Besides, the absorption coefficient of E//c is larger than that of E//c’, which indicated that the crystal absorption coefficient was not influenced by Pr³⁺ doped in Yb,Ho,Pr:GYTO crystal. The dopant of Yb³⁺ ions in Yb,Ho,Pr:GYTO crystal was calculated to be 4.16×10²⁰ cm⁻³. Thus the absorption cross section of Yb,Ho,Pr:GYTO crystal can be calculated by the formula σ_abs=α(λ)/N. Where σ_abs is the absorption cross section, α(λ) is the absorption coefficient, and N is the unit volume concentration of Yb³⁺ ions. The strongest absorption peaks are located at 958 nm, 932 nm, and 1004 nm for E//c, corresponding to the absorption cross sections of 2.07×10⁻²⁰ cm², 1.63×10⁻²⁰ cm², and 1.03×10⁻²⁰ cm². These strong absorption peaks are beneficial to improve pumping efficiency and reduce the dependence on the temperature of pump source.

Figure 5.  (a) Polarized absorption spectra of Yb,Ho,Pr:GYTO; (b) Comparization of Polarized absorption spectra of Yb,Ho,Pr:GYTO and Yb,Ho:GYTO in 850-1100 nm (a, b, c →Yb,Ho,Pr:GYTO; a’, b’, c’ →Yb,Ho:GYTO)

Figure 6 shows the emission spectrum of Yb,Ho,Pr:GYTO crystal in the wavelength range of 2850-3000 nm excited by 940 nm LD. In the 2.9 μm band, there is a strong emission peak, centered at 2908 nm. The FWHM of 2908 nm is about 15 nm. The wide emission peak is helpful to the tunability of laser wavelength. In addition, compared with that of Yb,Ho:GYTO crystal^[27], the position of the strongest emission peak is shifted to the short wave direction by 2 nm, due to the little change of crystal field with the doped of Pr³⁺.

Figure 6.  2.9 μm emission spectrum of Yb,Ho,Pr:GYTO crystal

Furthermore, the stimulated emission cross section is calculated with the measured emission spectrum based on the Fchtbauer-Ladenburg equation:

where I(λ) is the emission intensity, λ is the emission wavelength, c is the speed of light, τ is the radiative lifetime of the upper energy level, and n is the refractive index, which is about 1.9^[28]. The β factor is 16.324%, as reported in reference [25]. The maximum emission cross section at 2908 nm is 1.44 × 10⁻¹⁹ cm². And the 2.9 μm emission cross section of Ho in GYTO and other hosts are presented in Tab.3. By comparison, the Yb,Ho,Pr:GYTO crystal possesses a larger emission cross section,which suggests it is easier to realize laser output. However, the emission cross section of Yb,Ho,Pr:GYTO crystal is smaller than that of Yb,Ho:GYTO crystal, because of the deactivation of Pr³⁺ on the ⁵I₆ level. The details of the regulation of Pr³⁺ on energy level of Ho³⁺ are explained in the following part.

The room temperature fluorescence decay curves of 1204 nm (⁵I₆→⁵I₈) and 2068 nm (⁵I₇→⁵I₈) emission of Yb,Ho,Pr:GYTO crystal excited by OPO pulse lasers are shown in Fig.7. Both of them are single exponential decay behavior. According to the fitted decay curves, the lifetimes of ⁵I₆ and ⁵I₇ level are 0.376 and 0.939 ms, respectively. Compared with the lifetimes of Yb,Ho:GYTO crystal as 0.419 and 7.298 ms, the Yb,Ho,Pr:GYTO crystal exhibits a remarkable attenuation of the ⁵I₇ level lifetime and little influence on the ⁵I₆ level. All these are attributed to the deactivation of Pr³⁺ through energy transfer between Ho-Pr in GYTO crystal. The energy transfer details are shown in Fig.8. The Yb³⁺ ions absorb pumping energy and transfer it to Ho³⁺ through cross-relaxation process. The emission from ⁵I₆→⁵I₇ of Ho³⁺ is located at 2.9 µm. Further doped with Pr³⁺, the energy transfer (ET) between Ho³⁺ and Pr³⁺ are through two processes: ET₁, ⁵I₆→³F₄+³F₃; ET₂, ⁵I₇→³F₂+³H₆. The efficiency of energy transfer ET₁ and ET₂ is directly related to the lifetime of ⁵I₆ and ⁵I₇ levels. The higher the efficiency is, the more the level lifetime is reduced. In addition, the efficiency of energy transfer from the Ho³⁺ to Pr³⁺ ions can be calculated based on the following equation:

Figure 7.  Fluorescence decay curves. (a) 1204 nm (⁵I₆→⁵I₈); (b) 2068 nm(⁵I₇→⁵I₈)

Figure 8.  Schematic of energy transfer processes among Yb³⁺, Ho³⁺, and Pr³⁺ ions

where τ_DA is the level lifetime of Yb,Ho,Pr:GYTO with deactivated ion, and τ_D is the level lifetime of Yb,Ho:GYTO without deactivated ion. According to equation (2) and the aforementioned level lifetimes of the Yb,Ho,Pr:GYTO and Yb,Ho:GYTO crystals, the energy transfer efficiencies of Ho³⁺ → Pr³⁺ in ET₁ and ET₂ processes are calculated to be about 10.26% and 87.13%, respectively. The energy transfer efficiency of ET₂ is greater than that of ET₁. Thus, the doping of Pr³⁺ ions can inhibit the self-termination phenomenon effectively. Population inversions between the ⁵I₆ and ⁵I₇ levels of the Ho³⁺ ions in Yb,Ho,Pr:GYTO crystal are likely to be realized at a lower pumping threshold.

Moreover, the upper and lower laser level lifetimes of other hosts are presented in Tab.4. From the table, we can see that the Yb,Ho,Pr:GYTO crystal possesses a shorter lifetime of the lower level ⁵I₇ and a similar lifetime of the upper level ⁵I₆, which are easier to realizepopulation inversion and laser output.

High-quality Yb,Ho,Pr:GYTO single crystal was successfully grown using Czochralski method. It belongs to the monoclinic space group of I2/a (No.15) and the lattice parameters are fitted to be a=5.381 Å, b=11.023 Å, c=5.076 Å, β=95.59º, V=299.68 ų. The k_eff of Yb, Ho, Pr, and Y in Yb,Ho,Pr:GYTO crystal are 0.624, 1.220, 1.350, and 0.977, respectively. The FWHM of X-ray rocking curves on the (100), (010), and (001) crystalline faces are 0.036˚, 0.013˚, and 0.077˚, respectively, suggesting a high crystalline quality. The polarized absorption spectra indicate that the coefficient of E//c is larger than that of the other direction. The strongest absorption peaks are located at 958 nm, 932 nm, and 1004 nm for E//c, corresponding to the absorption cross sections of 2.07×10⁻²⁰ cm², 1.63×10⁻²⁰ cm², and 1.03×10⁻²⁰ cm². The strongest emission peak is located at 2908 nm, and the FWHM is about 15 nm. Emission cross section at 2908 nm is as large as 1.44 × 10⁻¹⁹ cm². Importantly, the lifetimes of ⁵I₆ and ⁵I₇ level are obtained to be 0.376 and 0.939 ms, 10.26% and 87.13% less than Yb,Ho:GYTO respectively. Therefore, the deactivator Pr³⁺ ions may be conducive to reducing the laser threshold and improving the conversion efficiency of the 2.9 µm laser in the Yb,Ho,Pr:GYTO crystal.