Recently, rare-earths (RE) ions doped optical materials with the characteristic of efficient upconversion luminescence from infrared (IR) to visible radiation^[1,2] attract considerable attention due to their potential applications in solid-state lasers, optical communications, solar cell, biological fluorescence labels, and so on.^[3,4] Upconversion emission is usually obtained in bulk materials with relatively low phonon energy,^[5] which decreases multiphonon relaxation rate and increases the lifetimes of the intermediate metastable states.^[6] LiNbO₃ crystals with low phonon energy are promising matrices for upconversion luminescence due to their good mechanical performance, high stability, excellent electro-optic, acousto-optic, nonlinear optic, and piezoelectric performances.^[7–9] To obtain efficient upconversion luminescence, RE ions such as Er³⁺, Tm³⁺, Nd³⁺, and Ho³⁺ doped LiNbO₃ crystals have been studied for many years. Among them, Tm³⁺ ion is well known as an activator of blue (¹G₄ →³H₆ transition), red (¹G₄ →³F₄ and ³F₃ →³H₆ transitions), and NIR (³H₄ →³H₆ and ³F₄ →³H₆ transitions) emissions.^[1,10,11] It is noteworthy that Tm³⁺ ions can be applied to blue laser which exhibits applications in optical communication, color display, and so on. Although upconversion luminescence of Tm³⁺ ions can be obtained under 980 nm excitation, the absorption of Tm³⁺ ions at the pumping wavelength is weak. While, as a sensitizer, Yb³⁺ ion can strongly absorb the pumping light and transfer energy to Tm³⁺ ions, which increases the pumping light absorption efficiency and lowers the laser threshold.^[10,12] The upconversion luminescence of Yb³⁺ and Tm³⁺ codoped LiNbO₃ crystals has been reported by Xing et al.^[13]

As we all know, the application of Yb:Tm:LiNbO₃ crystal in frequency doubler and laser medium material is limited by the photorefractive damage and the problem can be overcome by doping anti-photorefractive ions such as Mg²⁺, Zn²⁺, Sc³⁺, In³⁺, Hf⁴⁺, and Zr⁴⁺ ions.^[14–19] It is worthy of notice that luminescence behavior can be improved by doping anti-photorefractive ions.^[20] Recently, a few reports have researched the influence of anti-photorefractive ions on upconversion emission in RE³⁺ ions doped LiNbO₃ crystals.^[20–22] Shur et al. found that the addition of ZrO₂ enhanced the 470 nm emission of Tm:LiNbO₃ in blue wavelength region.^[21] Sun et al. reported that 1.5 mol% In₂O₃ enhanced the 1.5 μm emission but suppressed 550 nm green upconversion emission of Er:LiNbO₃.^[22] Hf⁴⁺ has been reported to be an effective ion to improve the anti-photorefractive property of LiNbO₃, while the effect of Hf⁴⁺ on the optical properties of Yb:Tm:LiNbO₃ crystals have not been reported.

In this paper, Hf:Yb:Tm:LiNbO₃ crystals with varying Hf⁴⁺ concentrations (2 mol%, 4 mol%, and 6 mol%) were successfully grown by the Czochralski technique. The defect structure was analyzed by x-ray diffraction (XRD) and infrared (IR) transmission spectra. The UV–VIS–NIR absorption spectra of Hf:Yb:Tm:LiNbO₃ crystals were discussed. The effect of Hf⁴⁺ ions on the upconversion luminescence of Hf:Yb:Tm:LiNbO₃ crystals was investigated under 980 nm excitation. A new strategy to improve the properties of red and blue upconversion luminescence of Yb:Tm:LiNbO₃ crystals is given in this paper.

XRD patterns of the Hf:Yb:Tm:LiNbO₃ crystals and the grown pure LiNbO₃ crystals are displayed in Figs. 3 and 4. It is obvious that there is no new diffraction peak compared with the pure LiNbO₃ structure. Hence, the diffraction patterns of the samples are indexed as hexagonal phase of LiNbO₃ crystal, which reflects that all grown crystals are single phase. According to the data of XRD, the lattice parameters were calculated by the least square method and listed in Table 1. The unit cell volumes were computed by the formula V = a²c cos30°. The lattice constants of the pure congruent LiNbO₃ crystal is shown in Table 1 for comparison.

Fig. 3.

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	Fig. 3. (color online) XRD patterns of Hf:Yb:Tm:LiNbO3 crystals.

Fig. 4.

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	Fig. 4. (color online) XRD pattern of the pure congruent LiNbO3 crystal.

Table 1.

Table 1.

Table 1. Lattice constants of Hf:Yb:Tm:LiNbO3 crystals. . Sample a = b / nm Δ a/nm c/nm Δc/nm V/nm3 LiNbO3 0.5146 0 1.3852 0 0.31767 Hf2 0.5148 0.0002 1.3821 –0.0031 0.31721 Hf4 0.5155 0.0009 1.3834 –0.0018 0.31837 Hf6 0.5157 0.0011 1.3966 0.0114 0.32166

Table 1. Lattice constants of Hf:Yb:Tm:LiNbO3 crystals. .

It is clear that the unit cell volumes of the LiNbO₃ crystals change with the concentration of Hf⁴⁺ ions. Since the concentration of Li⁺ is less than that of Nb⁵⁺ in pure congruent LiNbO₃ crystal, the anti-site Nb ( ) and Li vacancy ( ) exist. As is known to us, three factors affect the unit cell volume: the concentration of , the polarization ability and ionic radius of the doping ions. When doping 2 mol% Hf⁴⁺, 1 mol% Yb³⁺, and 1 mol% Tm³⁺ ions, the unit cell volumes of LiNbO₃ crystals decrease. The reasons are as follows. Hf⁴⁺, Yb³⁺, and Tm³⁺ ions substitute and in the form of , , and . Based on charge balance, , , , and defects need defects to form – , – , – , and – defects. The doping ions reduce the concentration of and increase the concentration of , , and , which leads to the decrease of concentration followed by the decrease of unit cell volume. Another reason is that the polarization ability of is higher than Nb⁵⁺ and . When the doping concentration of Hf⁴⁺ increases up to 4 mol%, Hf⁴⁺ ions replace the normal Li and Nb sites to constitute its own charge balance in the form of – . Theoretically, the unit cell volume of LiNbO₃ crystal should also decrease. However, the unit cell volume increases gradually in the experiment. The reason may be that ionic radius becomes the main factor affecting the unit cell volume with the doping concentration of Hf⁴⁺ increased and the ionic radius of Hf⁴⁺ (0.81 Å^[23]) ions is larger than that of Li (0.69 Å^[24]) and Nb (0.68 Å^[24]) ions.

IR transmission spectra of Hf:Yb:Tm:LiNbO₃ crystals with various Hf⁴⁺ concentrations are shown in Fig. 5, in which the position of OH⁻ absorption peak shifts to the lager wavenumber with the increase of Hf⁴⁺ concentration. The OH⁻ absorption peak of sample Hf2 is located at 3484 cm⁻¹. While the OH⁻ absorption peaks of samples Hf4 and Hf6 are shifted to 3495 cm⁻¹ and 3498 cm⁻¹, respectively. It is known that there are many intrinsic defects including anti-site Nb ( ) and Li vacancy ( ) in congruent LiNbO₃ crystals and the OH⁻ absorption peak of congruent LiNbO₃ crystals is located at 3482 cm⁻¹ owing to the OH⁻ stretching vibration of complex defect ( – –OH⁻).^[25] Based on a large number of reports, the OH⁻ absorption peak of doped LiNbO₃ crystals will obvious shifts from about 3482 cm⁻¹ of pure congruent LiNbO₃ crystals to lager wavenumber when the doping concentration exceed its thresholds.^[25–27] The OH⁻ absorption peak at 3495 cm⁻¹ indicates that the 4 mol% Hf⁴⁺ doping in Yb:Tm:LiNbO₃ crystal is up to the threshold concentration. This is similar to the previously reported threshold concentration in Hf:LiNbO₃ crystals.^[26] In sample Hf2, the Hf⁴⁺, Yb³⁺, and Tm³⁺ ions enter into LiNbO₃ crystal by replacing defects to form , , and defects, which repel H⁺ ions. As a result, the OH⁻ absorption peak of sample Hf2 still reflects the OH⁻ vibration around defects and is located at 3484 cm⁻¹. The slight shift is caused by the reducing of and defects. When the doping level of Hf⁴⁺ exceeds its threshold in samples Hf4 and Hf6, defects are replaced completely and partial Hf⁴⁺ ions begin enter the normal Li and Nb sites in the form of and defects. Since defects have a stronger force to attract H⁺ than , H⁺ ions aggregate around to form –OH⁻ defects. Meanwhile more energy is required for OH⁻ stretching vibration. So the absorption peak that reflects the OH vibration around shows blue shift.

Fig. 5.

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	Fig. 5. (color online) IR transmission spectra of Hf:Yb:Tm:LiNbO3 crystals.

The UV–VIS–NIR absorption spectra of Hf:Yb:Tm:LiNbO₃ crystals are shown in Fig. 6. Besides the absorption band centered at 980 nm caused by ²F_7/2 →²F^5/2 transition of Yb³⁺ ions, the other absorption bands owe to the electronic transition of Tm³⁺ ions. The absorption bands centered at 703 nm, 795 nm, and 1215 nm are attributed to the transitions from the ground state ³H₆ to the excited states ³F_{2, 3}, ³H₄, and ³H₅. And the absorption bands centered at 1666 nm and 1757 nm both correspond to the transition from ³H₆ to ³F₄ due to the stark sublevel absorption caused by the crystal-field splitting in Hf:Yb:Tm:LiNbO₃ crystals. In addition, the absorption band centered at 2352 nm is attributed to the ³H₅ →³H₄ transition of Tm³⁺ ions. Among these absorption bands, 980 nm which is well adapted for emission wavelength of commercial diode laser can be selected as the pump wavelength. By comparison of the absorption spectra, the following characteristics are identified. Firstly, the absorption spectra of samples Hf2, Hf4, and Hf6 are similar in the shape and band position, except disappearance of absorption band centered at 2352 nm in sample Hf6. Since the Tm³⁺ doped crystal has a potential application for 2–2.5 μm laser, the high doping Hf⁴⁺ ions will eliminate the absorption at this wavelength and be benefit for the application. Secondly, the intensities of the absorption bands first increase and then decrease with the increase of Hf⁴⁺ ions concentration.

Fig. 6.

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	Fig. 6. (color online) The UV–VIS–NIR absorption spectra of the Hf:Yb:Tm:LiNbO3 crystals.

Upconversion luminescence spectra of Hf:Yb:Tm:LiNbO₃ crystals in the visible wavelength range excited by 980 nm laser diode are shown in Fig. 7. The strongest intensity peak is centered at 475 nm in the blue wavelength range. This peak is attributed to the ¹G₄ →³H₆ transition of Tm³⁺ ions. Another peak is centered at 651 nm in the red wavelength range and attributed to the ¹G₄ →³F₄ transition. Compared with sample Hf2, the intensity of blue and red emissions increases obviously in samples Hf4 and Hf6. Especially, in sample Hf6, the blue emission is about 4.7 times and the red emission is about 1.9 times more than that in sample Hf2. It is obvious that the increase of Hf⁴⁺ doping concentration is beneficial for upconversion luminescence of Hf:Yb:Tm:LiNbO₃ crystals.

Fig. 7.

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	Fig. 7. (color online) Room-temperature upconversion luminescence spectra of Hf:Yb:Tm:LiNbO3 crystals: (a) 400–550 nm, (b) 550–700 nm.

As is well known, the intensity of upconversion luminescence is closely related to the local environment, the doping concentration, and the distribution of active ions in a host material.^[20] In consideration of the fixed Tm³⁺ and Yb³⁺ ions concentration, the change of the intensity of upconversion luminescence is caused by the local environment of Tm³⁺ and Yb³⁺ ions adjusted by Hf⁴⁺ ions. One reason for the enhancement is the minimization of structural defects in crystals. The atomic radius difference of Li (1.54 Å) and Nb (1.48 Å) is more remarkable than Hf (1.59 Å). Hence, doping Hf⁴⁺ ions that reduce defects and replace defects to form will minimize the structural defects of the crystal. Another reason is explained as follows. As we have known, three and two photons are involved in the blue and red upconversion emissions, respectively. The energy transfer mechanism of upconversion process in Hf:Yb:Tm:LiNbO₃ crystals is shown in Fig. 8. In the upconversion process, Yb³⁺ ions are excited from the ground state ²F_7/2 to the ²F_5/2 state by absorbing 980 nm NIR photon via ground state absorption (GSA). Then the energy transfers from Yb³⁺ to Tm³⁺ ions followed by populating ³H₅, ³F₂, and ¹G₄ intermediate states of Tm³⁺ ions. The ³H₅ →³F₄ and ³F₂ →³H₄ transitions of Tm³⁺ ions are achieved by nonradiative relaxation. Finally, blue and red upconversion emissions are obtained though the ¹G₄ →³H₆ and ¹G₄ →³F₄ transitions respectively in radiative relaxation progress. However, the existence of cross relaxation (CR) processes (³H₄+³H₆ →³F₄+³F₄) will suppress the population of ³H₄.^[28] Because the excited state absorption (ESA) which is an inherent result of a given pump choice is hard to change and CR processes can be suppressed by decrease of Tm³⁺ cluster sites.^[29–31] The enhancement of upconversion emission indicates the suppression of CR processes and reduction of Tm³⁺ cluster sites, which is caused by high doping Hf⁴⁺ ions. In conclusion, Hf⁴⁺ ions can improve the luminescence environment and enhance the intensity of blue and red emissions in Hf:Yb:Tm:LiNbO₃ crystals.

Fig. 8.

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	Fig. 8. (color online) The energy levels of Yb3+ and Tm3+ ions as well as the upconversion luminescence mechanism of Hf:Yb:Tm:LiNbO3 crystals.

In this work, Hf:Yb:Tm:LiNbO₃ crystals with variable Hf⁴⁺ concentrations have been successfully grown by the Czochralski method. The IR transmission spectrum indicates that defects are replaced completely and Hf⁴⁺ ions begin enter the normal Li and Nb sites when the Hf⁴⁺ doping concentration is up to 4 mol%. The UV–VIS–NIR absorption spectra first increase and then decrease with the increase of Hf⁴⁺ ions concentration and display strong absorption peaks at 980 nm. Under excitation at this wavelength, the blue (475 nm) and red (651 nm) upconversion luminescences are achieved though the ¹G₄ →³H₆ and ¹G₄ →³F₄ transitions of the Tm ions. The blue emissions intensity of 6 mol% Hf⁴⁺ tridoped Yb:Tm:LiNbO₃ crystal is about 4.7 times larger than that of 2 mol% Hf⁴⁺ tridoped Yb:Tm:LiNbO₃ crystal and the red emission intensity enhances by 1.9 times. The upconversion luminescence spectra remarkably increase with the increase of Hf⁴⁺ doping concentration. The generation and enhancement of the blue and red upconversion luminescence in Hf:Yb:Tm:LiNbO₃ crystals are interesting for solid-state lasers and color display.