Lithium niobate (LiNbO₃, LN) crystal is widely used as a multifunctional material due to its excellent physical and chemical characteristics such as electro-optic, acoustic-optic, nonlinear optics, piezoelectric, ferroelectric, etc.^[1,2] Especially, rare-earth (RE)-doped engineering has attracted considerable attention due to its enormous potential applications in optical communications, optical gyroscopes, optical data storage, laser technology, and remote sensing technology, etc. The rich emission spectrum and unique chemical characteristics of rare-earth ions have been widely studied as the new darling of the laser material.^[3–7] Among them, Ho which has a special electronic structure has been the most popular activator ion in recent years. Up to now, the up-conversion luminescence properties of Ho ions-doped crystals have been extensively investigated. However, the study of the up-conversion luminescence properties of Ho ions-doped LiNbO crystal is rarely reported.^[8,9] In the present paper, Yb ion can act as a sensitize ion to increase the up-conversion luminescence intensity of the Ho ion because it has a simple energy structure and a long excited state lifetime.^[10–12] Nevertheless, a problem for applications of LiNbO₃ crystal is that the optical damages will appear when the high-power laser irradiates the pure LiNbO₃ crystal or the RE-doped LiNbO₃ crystal. Doping anti-photorefractive ions can solve this problem. Many previous studies and experimental results showed that doping Mg ions can significantly improve the optical damage resistance ability.^[13]

In our experiments, the congruent Yb:Ho:LiNbO₃ crystals co-doping different concentrations of Mg (1, 3, 5, and 7 mol%) are grown by the Czochralski technique, wherein the concentrations of Ho and Yb ions are both 1 mol%. The concentrations of Mg , Yb , and Ho ions are measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES). Internal defect structures are studied via x-ray diffraction (XRD). Finally, the light-induced scattering is as a function of exposure energy, and the influences on the defect structures and the optical damage resistance ability of the Mg:Yb:Ho:LiNbO₃ crystals with different Mg ions doping concentrations are studied. The up-conversion emission spectra, the power pump dependence, and the up-conversion mechanism are measured to understand the influence of Mg ion on the up-conversion luminescence of Ho ion in Mg:Yb:Ho:LiNbO₃ crystal.

Mg:Yb:Ho:LiNbO₃ crystals were successfully grown in air along the c direction by the Czochralski technique. The doping concentrations of Yb and Ho ions were 1 mol%, Mg ions were 1, 3, 5, and 7 mol% in the melt. The chemicals we used were Li₂CO₃, Nb₂O₅, Yb₂O₃, MgO, and Ho₂O (4N purity) to ensure a high optical quality of the grown crystal. The raw materials were heated up to 750 C for 2 h to remove the CO₂, and then heated up to 1150 C for 3 h for a solid reaction. In the growth process, the optimum conditions were chosen as follows: the temperature gradient of the furnace was 30 C cm –40 C cm , the pulling rate was 1 mm/h 2mm/h, and the rotating rate was 20 r/pm 35 r/pm. Finally, the grown crystals were polarized at 1200 C with a current density of the 5 mA/cm². The polarized crystal was orientational and polished to optical grade. Hereafter, the four samples are denoted as 1#, 2#, 3#, and 4# crystals for convenience.

The concentrations of Mg , Yb , and Ho ions in the Mg:Yb:Ho:LiNbO₃ crystals were measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 5300DV). The XRD patterns and the lattice constant of Mg:Yb:Ho:LiNbO crystal were obtained by an XRD-6000 type x-ray diffractometer (SHIMADZU, Japan) to analyze the structural characteristics.^[14,15] Up-conversion emission spectra of Mg:Yb:Ho:LiNbO₃ and microsecond time-resolved spectra were measured under excitation at 980-nm semiconductor diode laser.^[16] Power dependence was measured by using neutral density filters.

Laser irradiation, even the weak laser pumping source, can cause optical damage as long as the exposure time is long enough. For this situation, Zhang et al. proposed a light-induced scattering threshold effect.^[17] In the experiment, we adopted a light-induced scattering exposure energy flux threshold method to study the optical damage resistance ability of the samples.^[18] The light-induced scattering threshold effect can be quantitatively described by the light intensity value because light-induced scattering is a cumulative effect of the incident light intensity versus time. Therefore, the light-induced scattering threshold effect was proposed to investigate the optical damage resistance ability of the samples. A schematic diagram of the light-induced scattering measurement setup is depicted in Fig. 1. The diode-pumped laser (532 nm) was used as a source, the sample was horizontally polarized, the spot diameter incident on the crystal was 1 mm, and the incident laser power can be adjusted by using an adjustable attenuator. The main role of the diaphragm is to filter scattered light to pass only the central portion of the transmitted light. The diameter of the diaphragm was 1 mm, namely the transmitted beam diameter was equal to the beam diameter incident on the crystal.

Fig. 1.

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	Fig. 1. (color online) Experimental setup for exposure energy flux.

As is well known, the properties of crystalline material are decided by the defect structure. To study the performance, the first thing is to determine the actual concentration of the doping ions in the crystal. The concentrations of Mg , Yb , and Ho ions in the Mg:Yb:Ho:LiNbO₃ crystals and the values of effective distribution coefficient are listed in Table 1. The values of effective distribution coefficient of Mg , Yb , and Ho ions are the concentration percentages in the crystals and melts. The dependence of distribution coefficients of Mg , Yb , and Ho ions on MgO concentration in the melt is shown in Fig. 2.

Fig. 2.

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	Fig. 2. (color online) Plots of segregation coefficients of Mg , Yb , and Ho ions versus MgO concentration in the melt.

Table 1.

Table 1.

Table 1. Compositions of Mg:Yb:Ho:LiNbO3 crystals and the optimum growth parameters. . Sample 1# 2# 3# 4# Mg in the crystal/mol% 0.793 2.433 4.215 6.055 of Mg 0.793 0.811 0.843 0.865 Yb in the crystal/mol% 0.802 0.778 0.743 0.736 of Yb 0.802 0.778 0.743 0.736 Ho in the crystal/mol % 0.841 0.835 0.820 0.809 of Ho 0.841 0.835 0.820 0.809 Crystal size/mm2 Axial temperature gradient/( C/cm) 30–40 30–35 30–35 25–30 Radial temperature gradient/( C/cm) 4.5–6.0 4.5–5.0 3.5–4.5 3.0–4.0

Table 1. Compositions of Mg:Yb:Ho:LiNbO3 crystals and the optimum growth parameters. .

It can be clearly seen that the effective distribution coefficient of Mg ions increases with increasing MgO concentration in the melt. In addition, the values of effective distribution coefficient of Yb and Ho ions decrease as the doping concentration of Mg increases in their coresponding melts. When Mg ion enters into LiNbO₃ crystal, it could readily occupy intrinsic defect Nb site and the Nb concentration could be reduced. At this time, Ho and Yb ions have always occupied Nb-bits. When the doping concentration of Mg ions reaches or exceeds the threshold concentration, anti-site Nb is completely substituted. Mg ions begin to substitute Li-site and Nb-site of the normal lattice, to form Mg charge balance so that the effective distribution coefficient of Mg increases continuously. Nevertheless, the effective distribution coefficients of Yb and Ho ions decrease sharply, which is ascribed to Mg , Yb , and Ho ions replacing Nb-bits to form Mg , Ho , and Yb which are negative charge excluding each other.

The x-ray diffraction technique is an important means to study the structure of matter. In addition, it can also accurately determine the lattice parameter to perform phase analysis, then study the crystal integrity. Therefore, the lattice constants and lattice structure are the important parameters of x-ray diffraction analysis. The x-ray diffraction patterns of Mg:Yb:Ho:LiNbO₃ crystals and experimental results are shown in Fig. 3.

Fig. 3.

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	Fig. 3. (color online) The x-ray diffraction patterns of Mg:Yb:Ho:LiNbO3 crystals.

It can be seen from Fig. 3 that the positions of the diffraction peak of four samples are basically similar, and no new diffraction peaks appear. It indicates that the doping Mg , Yb , and Ho ions cause none of their corresponding crystals to significantly change its internal structure, and they are still trigonal systems, like pure lithium niobate crystal. There is no new phase produced, but the radii of doping Mg (0.66 Å), Yb (0.85 Å), and Ho (0.90 Å) ions are different from those of Li (0.68 Å) and Nb (0.69 Å) ions. As is well known, the doping ion radius and defect structure in the crystal will affect the lattice constants and thus the unit cell volume. Therefore, we believe that the doping ions used to substitute Li and Nb ions enter into crystal lattices. According to experimental data, the least squares method is used to calculate the lattice constants of each sample, and the results are shown in Table 2. The lattice constants of congruent LiNbO₃ crystal are also listed in Table 2 for comparison, marked as sample 0#. Meanwhile, the unit cell volume V, , , and are obtained by the formula . The , , and can change values of the lattice constant and lattice volume, correspondingly.

Table 2.

Table 2.

Table 2. Lattice constants of Mg:Yb:Ho: LiNbO3 crystals. . Sample a/Å /Å c/Å /Å V/Å3 /Å3 0# 5.1484 0 13.8382 0 317.65 0 1# 5.1513 0.0029 13.8245 –0.0137 317.69 0.04 2# 5.1522 0.0038 13.8509 0.0127 318.41 0.76 3# 5.1551 0.0067 13.8732 0.035 319.28 1.63 4# 5.1519 0.0035 13.8475 0.0093 318.29 0.64

Table 2. Lattice constants of Mg:Yb:Ho: LiNbO3 crystals. .

In conjunction with Fig. 3 and Table 2, it is determined that the change of lattice constant is caused by the different Mg ions doping concentrations. For samples 1# and 2#, when the Mg ions doping concentration is below the threshold concentration, the lattice constants increase slightly; it may be caused by Mg ions replacing the anti-site Nb defects and then occupying Li-sites. In order to make the crystal keep neutral, the reduced is re-occupied by the same number of Li ions. The oxygen octahedron is expanded because the polarization ability of Nb is higher than that of Mg so that the volume of the unit cell is expanded. In each of samples 3# and 4#, the doping concentration of Mg ions exceeds the threshold concentration (the threshold concentration of Mg ions in LiNbO₃ crystal is 4.6 mol%), the lattice constants and cell volume both decline. It may be implied that the substitution site of Mg changes when the doping concentration of Mg ions exceeds the threshold concentration.

In the light-induced scattering test, the intensity irradiated onto the crystal is divided into three parts. Among them, the transmitted intensity received by photo-detector decreases with the increase of exposure time. The relationship between them is shown in Fig. 4. The scattered intensity depending on the transmitted intensity can be expressed by the following equation

Fig. 4.

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	Fig. 4. (color online) Exposure time-dependent transmitted intensities of Mg:Yb:Ho:LiNbO3 crystals.

The laser light intensity through a lens ( is divided into two parts, one is the effective intensity incident on the crystal ( , and the other is the reflection of the crystals ( . The relationship between the two is as follows:

According to the definition of exposure energy flow, the effective exposure energy flow irradiating the crystal is as follows:

Fig. 5.

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	Fig. 5. (color online) Exposure time-dependent square roots of scattering ratio of Mg:Yb:Ho:LiNbO3 crystals.

Table 3.

Table 3.

Table 3. Values of light-induced scattering parameters of Mg:Yb:Ho:LiNbO crystals, illumination light intensity (I), effective incident intensity ( , exposure time (τ), and total exposure energy . . Crystals τ/s I/(mW/cm /(mW/cm /(J/cm 1# 13.71 142.66 121.93 1.67 2# 26.73 401.87 343.48 9.18 3# 570.80 445.63 380.88 217.41 4# 1611.38 514.92 440.10 709.17

Table 3. Values of light-induced scattering parameters of Mg:Yb:Ho:LiNbO crystals, illumination light intensity (I), effective incident intensity ( , exposure time (τ), and total exposure energy . .

The effective exposure energy flux is used to quantitatively describe the optical damage resistance ability of Mg:Yb:Ho:LiNbO₃ crystal. The optical damage resistance ability improves with the value of increasing. As can be seen from Table 3, the exposure energy of sample 4# is biggest, which is about 425 times higher than that of sample 1#. Under the comprehensive comparison, it can be concluded that the Mg ions doping concentration of sample 4# is optimal to suppress the light-induced scattering effect.

Up-conversion luminescence wavelengths of the Mg:Yb:Ho:LiNbO₃ crystals in a range from 400 nm to 760 nm excited with 980-nm LD are shown in Fig. 6. The spectra exhibit five up-conversion luminescence bands centered at approximately 426 nm (violet, medium), 486 nm (blue, medium), 556 nm (kelly, very strong), 654 nm (red, weak) and 727 nm (red, strong), which are attributed to , , ( , , and transitions of Ho ions, respectively.^[18]

Fig. 6.

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	Fig. 6. (color online) Up-conversion emission spectra (400 nm–760 nm, nm) recorded under the same experimental conditions for Mg:Yb:Ho:LiNbO3 samples.

Figure 7 displays the plots of logarithmic up-conversion intensity versus logarithmic excitation power at 980 nm of Mg:Yb:Ho:LiNbO₃ sample. In order to understand the UC mechanism, the pump power dependence of the fluorescent radiation is investigated. The number of photons that are responsible for the upper emitting state can be obtained from the following relation:^[20]

Fig. 7.

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	Fig. 7. (color online) Plots of the logarithemic up-conversion intensity versus logarithmic excitation power at 980 nm of Mg:Yb:Ho:LiNbO3 sample.

Schematic energy-level diagrams of Ho and Yb ions and the up-conversion luminescence mechanism for Mg:Yb:Ho:LiNbO₃ crystals are illustrated in Fig. 8. The up-conversion process in Mg:Yb:Ho:LiNbO₃ crystal is briefly described as follows. First, the sensitizer Yb ions are excited from the to level upon the absorption of NIR photons (980 nm). The ions then transfer energy to the Yb ions, excited Yb ions populate , , , and energy levels of Ho . The energy levels can drop to , , and levels via nonradioactive relaxation and via multi phonon relaxation. The state relaxes to the and states in cascade, emitting blue 486 nm and green 561 nm radiations. The partial populations at the level directly relax to states in cascade, emitting the violet 426 nm radiation.

Fig. 8.

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	Fig. 8. (color online) Schematic energy-level diagrams of Ho and Yb ions and the up-conversion luminescence mechanism for Mg:Yb:Ho:LiNbO3 crystals.

Figure 9 shows the dependence of up-converted intensity of the sample on Mg concentration under excitation at 980 nm. We can see the up-conversion luminescence intensity of the crystal increases with the increase of Mg doping concentration. Our results suggest that the increasing of the doping concentration of Mg may be favorable to the electrons located at the , , , , and energy levels, and thereby significantly increases the spectrum transitions from them to the level. This is because the incorporation of Mg takes the place of the former electron trap, further reduces the rate of non-radiative transition of electrons, improves the luminescence environment of Ho , and strengthens the radiation transition.

Fig. 9.

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	Fig. 9. (color online) Mg concentration dependent up-converted intensity of Mg:Yb:Ho:LiNbO3 crystals.

In this work, a series of LiNbO₃ crystals tri-doped with Mg , Yb , and Ho is grown by the Czochralski technique. The effective distribution coefficients of Yb and Ho ions in the Mg:Yb:Ho:LiNbO₃ crystals decrease with the increase of Mg ions doping concentration, but Mg ions behave oppositely. The XRD results show that no new structural phase is gained, and the lattice constants of samples 1# and 2# slightly increase, and that of sample 3# increases a little more, while that of sample 4# decreases. This suggests that substitution site of Mg changes when the doping concentration of Mg ions exceeds the threshold concentration. The optical damage resistance ability of Mg:Yb:Ho:LiNbO₃ crystal is measured by the light scattering threshold energy flux method. The Mg(7 mol):YbHo:LiNbO₃ is the best choice for suppressing the light-induced scattering effect, which has the largest value 709.17 J/cm of , about 400 times higher than that of Mg(1 mol):Yb:Ho:LiNbO₃. Five up-converted emission bands are assigned to the transitions , , , , and , respectively. We believe that this paper can promote the understanding of the relations between the Mg concentration and the optical properties of Mg:Yb:Ho:LiNbO₃ crystal.