With the development of society, two-dimensional displays can no longer meet people's needs. Three-dimensional displays can completely reproduce the scene of world, showing the depth of the object, the sense of hierarchy, and reality because the real world is also three-dimensional. At present, there are a lot of techniques to realize three-dimensional displays, including two-dimensional analog three-dimensional displays, binocular parallax stereoscopic displays, and three-dimensional stereoscopic displays. Nevertheless, two-dimensional simulation three-dimensional display does not produce physical depth of field; Binocular parallax stereoscopic display is limited because of the disadvantages of small angle zoom, causing visual fatigue, and needing to assistant equipment. In contrast, three-dimensional stereoscopic display technology has attracted attention due to its realistic display effect, real-time interaction, and multi-angle observation. Among the related technologies, the static volume display technology based on transparent upconversion luminescent materials shows the most significant advantages.

Appropriate composition design is a necessary condition for the nucleation and growth of fluoride nanocrystals in glass. In this study, rare earth doped pure red upconversion luminescent oxyfluoride transparent glass ceramics were obtained via the conventional melt-quenching-thermal annealing method. The glass compositions (in mole) are 60SiO₂-6Al₂O₃-7Na₂CO₃-19NaF-xErF₃-yTmF₃-(8-x-y)YF₃ (x=0-1.6%, y =0-0.4%). The raw materials are SiO₂, Al₂O₃, Na₂CO₃(Sinopharm Chemical Reagents Co., Ltd.), and high-purity NaF, ErF₃, TmF₃, YF₃(Shanghai Macklin Biochemical Technology Co., Ltd), which were weighed, mixed and ground in an agate mortar. The obtained mixtures were then placed in a sealed alumina crucible and melted at 1 550 ℃ for 45 min under ambient atmosphere. Subsequently, the glass melt was poured into a 200 ℃ preheated copper mould and annealed at 400 ℃ for 5 h to relinquish the internal stress. Finally, the obtained bulk glass was heat-treated at 650 ℃ for 2 h to induce in-situ fluoride crystallization inside the glass to obtain oxyfluoride glass ceramics containing NaYF₄ nanocrystals (NaYF₄@glass).

For Er³⁺/Tm³⁺ co-doped NaYF₄@glass, the influence of Er³⁺ doped content was explored and an optimal Er³⁺ concentration of 15% was confirmed. Meanwhile, with the increase of co-doped Tm³⁺ content, the red emission of Er³⁺ gradually enhanced due to the energy transfer process between Er³⁺ and Tm³⁺. Correspondingly, the color purity of red emission increased to realize the pure red emission upon the 980 nm near-infrared laser excitation. In addition, the temperature dependent spectra (from 80 K to 470 K) of 15%Er-2%Tm∶NaYF₄@glass were tested. Owing to the electrons at the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels of Er³⁺ were populated independently, there was no direct temperatures-dependent electron correlation between the green (²H_11/2, ⁴S_3/2) and red (⁴F_9/2) levels of Er³⁺ ions. With the increase of temperature, the shape of the emission peak remained basically unvaried, indicating excellent chromatic stability. Ultimately, the Er³⁺/Tm³⁺ co-doped NaYF₄@glass-based three-dimensional prototype display device showed three-dimensional static/dynamic designed red patterns with high color purity and high stability, providing its potential use in new-generation three-dimensional stereoscopic display technology.

In summary, Er³⁺/Tm³⁺ co-doped NaYF₄@glass can be successfully prepared via conventional melt-quenching-thermal annealing method. The optimized 15%Er-2%Tm co-doped NaYF₄@glass can achieve pure red upconversion luminescence due to the energy transfer process between Er³⁺ and Tm³⁺. Owing to that the electrons in the ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels of Er³⁺ are independently populated, there is not direct temperature-dependent electron correlation between the green energy level (²H_11/2, ⁴S_3/2) and the red energy level (⁴F_9/2) of Er³⁺. Moreover, the Er³⁺/Tm³⁺ co-doped NaYF₄@glass-based three-dimensional prototype display device is constructured. This study can provide a new idea for the development of pure color transparent upconversion materials for three-dimensional stereoscopic display.